





Internet Engineering Task Force (IETF)                 R. Moskowitz, Ed.
Request for Comments: 7401                                HTT Consulting
Obsoletes: 5201                                                  T. Heer
Category: Standards Track              Hirschmann Automation and Control
ISSN: 2070-1721                                                P. Jokela
                                            Ericsson Research NomadicLab
                                                            T. Henderson
                                                University of Washington
                                                              April 2015


                Host Identity Protocol Version 2 (HIPv2)

Abstract

   This document specifies the details of the Host Identity Protocol
   (HIP).  HIP allows consenting hosts to securely establish and
   maintain shared IP-layer state, allowing separation of the identifier
   and locator roles of IP addresses, thereby enabling continuity of
   communications across IP address changes.  HIP is based on a Diffie-
   Hellman key exchange, using public key identifiers from a new Host
   Identity namespace for mutual peer authentication.  The protocol is
   designed to be resistant to denial-of-service (DoS) and man-in-the-
   middle (MitM) attacks.  When used together with another suitable
   security protocol, such as the Encapsulating Security Payload (ESP),
   it provides integrity protection and optional encryption for upper-
   layer protocols, such as TCP and UDP.

   This document obsoletes RFC 5201 and addresses the concerns raised by
   the IESG, particularly that of crypto agility.  It also incorporates
   lessons learned from the implementations of RFC 5201.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7401.






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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................5
      1.1. A New Namespace and Identifiers ............................6
      1.2. The HIP Base Exchange (BEX) ................................6
      1.3. Memo Structure .............................................7
   2. Terms and Definitions ...........................................7
      2.1. Requirements Terminology ...................................7
      2.2. Notation ...................................................8
      2.3. Definitions ................................................8
   3. Host Identity (HI) and Its Structure ............................9
      3.1. Host Identity Tag (HIT) ...................................10
      3.2. Generating a HIT from an HI ...............................11
   4. Protocol Overview ..............................................12
      4.1. Creating a HIP Association ................................12
           4.1.1. HIP Puzzle Mechanism ...............................14
           4.1.2. Puzzle Exchange ....................................15
           4.1.3. Authenticated Diffie-Hellman Protocol with
                  DH Group Negotiation ...............................17
           4.1.4. HIP Replay Protection ..............................18
           4.1.5. Refusing a HIP Base Exchange .......................19
           4.1.6. Aborting a HIP Base Exchange .......................20
           4.1.7. HIP Downgrade Protection ...........................20
           4.1.8. HIP Opportunistic Mode .............................21
      4.2. Updating a HIP Association ................................24
      4.3. Error Processing ..........................................24
      4.4. HIP State Machine .........................................25
           4.4.1. State Machine Terminology ..........................26
           4.4.2. HIP States .........................................27
           4.4.3. HIP State Processes ................................28
           4.4.4. Simplified HIP State Diagram .......................35





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      4.5. User Data Considerations ..................................37
           4.5.1. TCP and UDP Pseudo Header Computation for
                  User Data ..........................................37
           4.5.2. Sending Data on HIP Packets ........................37
           4.5.3. Transport Formats ..................................37
           4.5.4. Reboot, Timeout, and Restart of HIP ................37
      4.6. Certificate Distribution ..................................38
   5. Packet Formats .................................................38
      5.1. Payload Format ............................................38
           5.1.1. Checksum ...........................................40
           5.1.2. HIP Controls .......................................40
           5.1.3. HIP Fragmentation Support ..........................40
      5.2. HIP Parameters ............................................41
           5.2.1. TLV Format .........................................44
           5.2.2. Defining New Parameters ............................46
           5.2.3. R1_COUNTER .........................................47
           5.2.4. PUZZLE .............................................48
           5.2.5. SOLUTION ...........................................49
           5.2.6. DH_GROUP_LIST ......................................50
           5.2.7. DIFFIE_HELLMAN .....................................51
           5.2.8. HIP_CIPHER .........................................52
           5.2.9. HOST_ID ............................................54
           5.2.10. HIT_SUITE_LIST ....................................56
           5.2.11. TRANSPORT_FORMAT_LIST .............................58
           5.2.12. HIP_MAC ...........................................59
           5.2.13. HIP_MAC_2 .........................................59
           5.2.14. HIP_SIGNATURE .....................................60
           5.2.15. HIP_SIGNATURE_2 ...................................61
           5.2.16. SEQ ...............................................61
           5.2.17. ACK ...............................................62
           5.2.18. ENCRYPTED .........................................62
           5.2.19. NOTIFICATION ......................................64
           5.2.20. ECHO_REQUEST_SIGNED ...............................67
           5.2.21. ECHO_REQUEST_UNSIGNED .............................68
           5.2.22. ECHO_RESPONSE_SIGNED ..............................69
           5.2.23. ECHO_RESPONSE_UNSIGNED ............................69
      5.3. HIP Packets ...............................................70
           5.3.1. I1 - the HIP Initiator Packet ......................71
           5.3.2. R1 - the HIP Responder Packet ......................72
           5.3.3. I2 - the Second HIP Initiator Packet ...............75
           5.3.4. R2 - the Second HIP Responder Packet ...............76
           5.3.5. UPDATE - the HIP Update Packet .....................77
           5.3.6. NOTIFY - the HIP Notify Packet .....................78
           5.3.7. CLOSE - the HIP Association Closing Packet .........78
           5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet ..79






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      5.4. ICMP Messages .............................................79
           5.4.1. Invalid Version ....................................79
           5.4.2. Other Problems with the HIP Header and
                  Packet Structure ...................................80
           5.4.3. Invalid Puzzle Solution ............................80
           5.4.4. Non-existing HIP Association .......................80
   6. Packet Processing ..............................................80
      6.1. Processing Outgoing Application Data ......................81
      6.2. Processing Incoming Application Data ......................82
      6.3. Solving the Puzzle ........................................83
      6.4. HIP_MAC and SIGNATURE Calculation and Verification ........84
           6.4.1. HMAC Calculation ...................................84
           6.4.2. Signature Calculation ..............................87
      6.5. HIP KEYMAT Generation .....................................89
      6.6. Initiation of a HIP Base Exchange .........................90
           6.6.1. Sending Multiple I1 Packets in Parallel ............91
           6.6.2. Processing Incoming ICMP Protocol
                  Unreachable Messages ...............................92
      6.7. Processing of Incoming I1 Packets .........................92
           6.7.1. R1 Management ......................................94
           6.7.2. Handling of Malformed Messages .....................94
      6.8. Processing of Incoming R1 Packets .........................94
           6.8.1. Handling of Malformed Messages .....................97
      6.9. Processing of Incoming I2 Packets .........................97
           6.9.1. Handling of Malformed Messages ....................100
      6.10. Processing of Incoming R2 Packets .......................101
      6.11. Sending UPDATE Packets ..................................101
      6.12. Receiving UPDATE Packets ................................102
           6.12.1. Handling a SEQ Parameter in a Received
                   UPDATE Message ...................................103
           6.12.2. Handling an ACK Parameter in a Received
                   UPDATE Packet ....................................104
      6.13. Processing of NOTIFY Packets ............................104
      6.14. Processing of CLOSE Packets .............................105
      6.15. Processing of CLOSE_ACK Packets .........................105
      6.16. Handling State Loss .....................................105
   7. HIP Policies ..................................................106
   8. Security Considerations .......................................106
   9. IANA Considerations ...........................................109
   10. Differences from RFC 5201 ....................................113
   11. References ...................................................117
      11.1. Normative References ....................................117
      11.2. Informative References ..................................119
   Appendix A. Using Responder Puzzles ..............................122
   Appendix B. Generating a Public Key Encoding from an HI ..........123






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   Appendix C. Example Checksums for HIP Packets ....................123
     C.1. IPv6 HIP Example (I1 Packet) ..............................124
     C.2. IPv4 HIP Packet (I1 Packet) ...............................124
     C.3. TCP Segment ...............................................125
   Appendix D. ECDH and ECDSA 160-Bit Groups ........................125
   Appendix E. HIT Suites and HIT Generation ........................125
   Acknowledgments ..................................................127
   Authors' Addresses ...............................................128

1.  Introduction

   This document specifies the details of the Host Identity Protocol
   (HIP).  A high-level description of the protocol and the underlying
   architectural thinking is available in the separate HIP architecture
   description [HIP-ARCH].  Briefly, the HIP architecture proposes an
   alternative to the dual use of IP addresses as "locators" (routing
   labels) and "identifiers" (endpoint, or host, identifiers).  In HIP,
   public cryptographic keys, of a public/private key pair, are used as
   host identifiers, to which higher-layer protocols are bound instead
   of an IP address.  By using public keys (and their representations)
   as host identifiers, dynamic changes to IP address sets can be
   directly authenticated between hosts, and if desired, strong
   authentication between hosts at the TCP/IP stack level can be
   obtained.

   This memo specifies the base HIP protocol ("base exchange") used
   between hosts to establish an IP-layer communications context, called
   a HIP association, prior to communications.  It also defines a packet
   format and procedures for updating and terminating an active HIP
   association.  Other elements of the HIP architecture are specified in
   other documents, such as:

   o  "Using the Encapsulating Security Payload (ESP) Transport Format
      with the Host Identity Protocol (HIP)" [RFC7402]: how to use the
      Encapsulating Security Payload (ESP) for integrity protection and
      optional encryption

   o  "Host Mobility with the Host Identity Protocol" [HIP-HOST-MOB]:
      how to support host mobility in HIP

   o  "Host Identity Protocol (HIP) Domain Name System (DNS) Extension"
      [HIP-DNS-EXT]: how to extend DNS to contain Host Identity
      information

   o  "Host Identity Protocol (HIP) Rendezvous Extension"
      [HIP-REND-EXT]: using a rendezvous mechanism to contact mobile HIP
      hosts




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   Since the HIP base exchange was first developed, there have been a
   few advances in cryptography and attacks against cryptographic
   systems.  As a result, all cryptographic protocols need to be agile.
   That is, the ability to switch from one cryptographic primitive to
   another should be a part of such protocols.  It is important to
   support a reasonable set of mainstream algorithms to cater to
   different use cases and allow moving away from algorithms that are
   later discovered to be vulnerable.  This update to the base exchange
   includes this needed cryptographic agility while addressing the
   downgrade attacks that such flexibility introduces.  In addition,
   Elliptic Curve support via Elliptic Curve DSA (ECDSA) and Elliptic
   Curve Diffie-Hellman (ECDH) has been added.

1.1.  A New Namespace and Identifiers

   The Host Identity Protocol introduces a new namespace, the Host
   Identity namespace.  Some ramifications of this new namespace are
   explained in the HIP architecture description [HIP-ARCH].

   There are two main representations of the Host Identity, the full
   Host Identity (HI) and the Host Identity Tag (HIT).  The HI is a
   public key and directly represents the Identity of a host.  Since
   there are different public key algorithms that can be used with
   different key lengths, the HI, as such, is unsuitable for use as a
   packet identifier, or as an index into the various state-related
   implementation structures needed to support HIP.  Consequently, a
   hash of the HI, the Host Identity Tag (HIT), is used as the
   operational representation.  The HIT is 128 bits long and is used
   in the HIP headers and to index the corresponding state in the
   end hosts.  The HIT has an important security property in that it
   is self-certifying (see Section 3).

1.2.  The HIP Base Exchange (BEX)

   The HIP base exchange is a two-party cryptographic protocol used to
   establish communications context between hosts.  The base exchange is
   a SIGMA-compliant [KRA03] four-packet exchange.  The first party is
   called the Initiator and the second party the Responder.  The
   protocol exchanges Diffie-Hellman [DIF76] keys in the 2nd and 3rd
   packets, and authenticates the parties in the 3rd and 4th packets.
   The four-packet design helps to make HIP resistant to DoS attacks.
   It allows the Responder to stay stateless until the IP address and
   the cryptographic puzzle are verified.  The Responder starts the
   puzzle exchange in the 2nd packet, with the Initiator completing it
   in the 3rd packet before the Responder stores any state from the
   exchange.





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   The exchange can use the Diffie-Hellman output to encrypt the Host
   Identity of the Initiator in the 3rd packet (although Aura, et al.
   [AUR05] note that such operation may interfere with packet-inspecting
   middleboxes), or the Host Identity may instead be sent unencrypted.
   The Responder's Host Identity is not protected.  It should be noted,
   however, that both the Initiator's and the Responder's HITs are
   transported as such (in cleartext) in the packets, allowing an
   eavesdropper with a priori knowledge about the parties to identify
   them by their HITs.  Hence, encrypting the HI of any party does not
   provide privacy against such an attacker.

   Data packets start to flow after the 4th packet.  The 3rd and 4th HIP
   packets may carry a data payload in the future.  However, the details
   of this may be defined later.

   An existing HIP association can be updated using the update mechanism
   defined in this document, and when the association is no longer
   needed, it can be closed using the defined closing mechanism.

   Finally, HIP is designed as an end-to-end authentication and key
   establishment protocol, to be used with Encapsulating Security
   Payload (ESP) [RFC7402] and other end-to-end security protocols.  The
   base protocol does not cover all the fine-grained policy control
   found in Internet Key Exchange (IKE) [RFC7296] that allows IKE to
   support complex gateway policies.  Thus, HIP is not a complete
   replacement for IKE.

1.3.  Memo Structure

   The rest of this memo is structured as follows.  Section 2 defines
   the central keywords, notation, and terms used throughout the rest of
   the document.  Section 3 defines the structure of the Host Identity
   and its various representations.  Section 4 gives an overview of the
   HIP base exchange protocol.  Sections 5 and 6 define the detailed
   packet formats and rules for packet processing.  Finally, Sections 7,
   8, and 9 discuss policy, security, and IANA considerations,
   respectively.

2.  Terms and Definitions

2.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].






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2.2.  Notation

   [x]    indicates that x is optional.

   {x}    indicates that x is encrypted.

   X(y)   indicates that y is a parameter of X.

   <x>i   indicates that x exists i times.

   -->    signifies "Initiator to Responder" communication (requests).

   <--    signifies "Responder to Initiator" communication (replies).

   |      signifies concatenation of information (e.g., X | Y is the
          concatenation of X with Y).

   Ltrunc (H(x), #K)
          denotes the lowest-order #K bits of the result of the
          hash function H on the input x.

2.3.  Definitions

   HIP base exchange (BEX):  The handshake for establishing a new HIP
      association.

   Host Identity (HI):  The public key of the signature algorithm that
      represents the identity of the host.  In HIP, a host proves its
      identity by creating a signature with the private key belonging to
      its HI (cf. Section 3).

   Host Identity Tag (HIT):  A shorthand for the HI in IPv6 format.  It
      is generated by hashing the HI (cf. Section 3.1).

   HIT Suite:  A HIT Suite groups all cryptographic algorithms that are
      required to generate and use an HI and its HIT.  In particular,
      these algorithms are 1) the public key signature algorithm, 2) the
      hash function, and 3) the truncation (cf. Appendix E).

   HIP association:  The shared state between two peers after completion
      of the BEX.

   HIP packet:  A control packet carrying a HIP packet header relating
      to the establishment, maintenance, or termination of the HIP
      association.

   Initiator:  The host that initiates the BEX.  This role is typically
      forgotten once the BEX is completed.



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   Responder:  The host that responds to the Initiator in the BEX.  This
      role is typically forgotten once the BEX is completed.

   Responder's HIT hash algorithm (RHASH):  The hash algorithm used for
      various hash calculations in this document.  The algorithm is the
      same as is used to generate the Responder's HIT.  The RHASH is the
      hash function defined by the HIT Suite of the Responder's HIT
      (cf. Section 5.2.10).

   Length of the Responder's HIT hash algorithm (RHASH_len):  The
      natural output length of RHASH in bits.

   Signed data:  Data that is signed is protected by a digital signature
      that was created by the sender of the data by using the private
      key of its HI.

   KDF:  The Key Derivation Function (KDF) is used for deriving the
      symmetric keys from the Diffie-Hellman key exchange.

   KEYMAT:  The keying material derived from the Diffie-Hellman key
      exchange by using the KDF.  Symmetric keys for encryption and
      integrity protection of HIP packets and encrypted user data
      packets are drawn from this keying material.

3.  Host Identity (HI) and Its Structure

   In this section, the properties of the Host Identity and Host
   Identity Tag are discussed, and the exact format for them is defined.
   In HIP, the public key of an asymmetric key pair is used as the Host
   Identity (HI).  Correspondingly, the host itself is defined as the
   entity that holds the private key of the key pair.  See the HIP
   architecture specification [HIP-ARCH] for more details on the
   difference between an identity and the corresponding identifier.

   HIP implementations MUST support the Rivest Shamir Adleman [RSA]
   public key algorithm and the Elliptic Curve Digital Signature
   Algorithm (ECDSA) for generating the HI as defined in Section 5.2.9.
   Additional algorithms MAY be supported.

   A hashed encoding of the HI, the Host Identity Tag (HIT), is used in
   protocols to represent the Host Identity.  The HIT is 128 bits long
   and has the following three key properties: i) it is the same length
   as an IPv6 address and can be used in fixed address-sized fields in
   APIs and protocols; ii) it is self-certifying (i.e., given a HIT, it
   is computationally hard to find a Host Identity key that matches the
   HIT); and iii) the probability of a HIT collision between two hosts





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   is very low; hence, it is infeasible for an attacker to find a
   collision with a HIT that is in use.  For details on the security
   properties of the HIT, see [HIP-ARCH].

   The structure of the HIT is defined in [RFC7343].  The HIT is an
   Overlay Routable Cryptographic Hash Identifier (ORCHID) and consists
   of three parts: first, an IANA-assigned prefix to distinguish it from
   other IPv6 addresses; second, a four-bit encoding of the algorithms
   that were used for generating the HI and the hashed representation of
   HI; third, a 96-bit hashed representation of the Host Identity.  The
   encoding of the ORCHID generation algorithm and the exact algorithm
   for generating the hashed representation are specified in Appendix E
   and [RFC7343].

   Carrying HIs and HITs in the header of user data packets would
   increase the overhead of packets.  Thus, it is not expected that they
   are carried in every packet, but other methods are used to map the
   data packets to the corresponding HIs.  In some cases, this makes it
   possible to use HIP without any additional headers in the user data
   packets.  For example, if ESP is used to protect data traffic, the
   Security Parameter Index (SPI) carried in the ESP header can be used
   to map the encrypted data packet to the correct HIP association.

3.1.  Host Identity Tag (HIT)

   The Host Identity Tag is a 128-bit value -- a hashed encoding of the
   Host Identifier.  There are two advantages of using a hashed encoding
   over the actual variable-sized Host Identity public key in protocols.
   First, the fixed length of the HIT keeps packet sizes manageable and
   eases protocol coding.  Second, it presents a consistent format for
   the protocol, independent of the underlying identity technology
   in use.

   RFC 7343 [RFC7343] specifies 128-bit hash-based identifiers, called
   ORCHIDs.  Their prefix, allocated from the IPv6 address block, is
   defined in [RFC7343].  The Host Identity Tag is one type of ORCHID.

   This document extends the original, experimental HIP specification
   [RFC5201] with measures to support crypto agility.  One of these
   measures allows different hash functions for creating a HIT.  HIT
   Suites group the sets of algorithms that are required to generate and
   use a particular HIT.  The Suites are encoded in HIT Suite IDs.
   These HIT Suite IDs are transmitted in the ORCHID Generation
   Algorithm (OGA) field in the ORCHID.  With the HIT Suite ID in the
   OGA ID field, a host can tell, from another host's HIT, whether it
   supports the necessary hash and signature algorithms to establish a
   HIP association with that host.




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3.2.  Generating a HIT from an HI

   The HIT MUST be generated according to the ORCHID generation method
   described in [RFC7343] using a context ID value of 0xF0EF F02F BFF4
   3D0F E793 0C3C 6E61 74EA (this tag value has been generated randomly
   by the editor of this specification), and an input that encodes the
   Host Identity field (see Section 5.2.9) present in a HIP payload
   packet.  The set of hash function, signature algorithm, and the
   algorithm used for generating the HIT from the HI depends on the HIT
   Suite (see Section 5.2.10) and is indicated by the four bits of the
   OGA ID field in the ORCHID.  Currently, truncated SHA-1, truncated
   SHA-384, and truncated SHA-256 [FIPS.180-4.2012] are defined as
   hashes for generating a HIT.

   For identities that are either RSA, Digital Signature Algorithm (DSA)
   [FIPS.186-4.2013], or Elliptic Curve DSA (ECDSA) public keys, the
   ORCHID input consists of the public key encoding as specified for the
   Host Identity field of the HOST_ID parameter (see Section 5.2.9).
   This document defines four algorithm profiles: RSA, DSA, ECDSA, and
   ECDSA_LOW.  The ECDSA_LOW profile is meant for devices with low
   computational capabilities.  Hence, one of the following applies:

      The RSA public key is encoded as defined in [RFC3110], Section 2,
      taking the exponent length (e_len), exponent (e), and modulus (n)
      fields concatenated.  The length (n_len) of the modulus (n) can be
      determined from the total HI Length and the preceding HI fields
      including the exponent (e).  Thus, the data that serves as input
      for the HIT generation has the same length as the HI.  The fields
      MUST be encoded in network byte order, as defined in [RFC3110].

      The DSA public key is encoded as defined in [RFC2536], Section 2,
      taking the fields T, Q, P, G, and Y, concatenated as input.  Thus,
      the data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long,
      where T is the size parameter as defined in [RFC2536].  The size
      parameter T, affecting the field lengths, MUST be selected as the
      minimum value that is long enough to accommodate P, G, and Y.  The
      fields MUST be encoded in network byte order, as defined in
      [RFC2536].

      The ECDSA public keys are encoded as defined in Sections 4.2 and 6
      of [RFC6090].

   In Appendix B, the public key encoding process is illustrated using
   pseudo-code.







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4.  Protocol Overview

   This section is a simplified overview of the HIP protocol operation,
   and does not contain all the details of the packet formats or the
   packet processing steps.  Sections 5 and 6 describe in more detail
   the packet formats and packet processing steps, respectively, and are
   normative in case of any conflicts with this section.

   The protocol number 139 has been assigned by IANA to the Host
   Identity Protocol.

   The HIP payload (Section 5.1) header could be carried in every IP
   datagram.  However, since HIP headers are relatively large
   (40 bytes), it is desirable to 'compress' the HIP header so that the
   HIP header only occurs in control packets used to establish or change
   HIP association state.  The actual method for header 'compression'
   and for matching data packets with existing HIP associations (if any)
   is defined in separate documents, describing transport formats and
   methods.  All HIP implementations MUST implement, at minimum, the ESP
   transport format for HIP [RFC7402].

4.1.  Creating a HIP Association

   By definition, the system initiating a HIP base exchange is the
   Initiator, and the peer is the Responder.  This distinction is
   typically forgotten once the base exchange completes, and either
   party can become the Initiator in future communications.

   The HIP base exchange serves to manage the establishment of state
   between an Initiator and a Responder.  The first packet, I1,
   initiates the exchange, and the last three packets, R1, I2, and R2,
   constitute an authenticated Diffie-Hellman [DIF76] key exchange for
   session-key generation.  In the first two packets, the hosts agree on
   a set of cryptographic identifiers and algorithms that are then used
   in and after the exchange.  During the Diffie-Hellman key exchange, a
   piece of keying material is generated.  The HIP association keys are
   drawn from this keying material by using a Key Derivation Function
   (KDF).  If other cryptographic keys are needed, e.g., to be used with
   ESP, they are expected to be drawn from the same keying material by
   using the KDF.

   The Initiator first sends a trigger packet, I1, to the Responder.
   The packet contains the HIT of the Initiator and possibly the HIT of
   the Responder, if it is known.  Moreover, the I1 packet initializes
   the negotiation of the Diffie-Hellman group that is used for
   generating the keying material.  Therefore, the I1 packet contains a
   list of Diffie-Hellman Group IDs supported by the Initiator.  Note
   that in some cases it may be possible to replace this trigger packet



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   with some other form of a trigger, in which case the protocol starts
   with the Responder sending the R1 packet.  In such cases, another
   mechanism to convey the Initiator's supported DH groups (e.g., by
   using a default group) must be specified.

   The second packet, R1, starts the actual authenticated Diffie-Hellman
   exchange.  It contains a puzzle -- a cryptographic challenge that the
   Initiator must solve before continuing the exchange.  The level of
   difficulty of the puzzle can be adjusted based on the level of trust
   with the Initiator, the current load, or other factors.  In addition,
   the R1 contains the Responder's Diffie-Hellman parameter and lists of
   cryptographic algorithms supported by the Responder.  Based on these
   lists, the Initiator can continue, abort, or restart the base
   exchange with a different selection of cryptographic algorithms.
   Also, the R1 packet contains a signature that covers selected parts
   of the message.  Some fields are left outside the signature to
   support pre-created R1s.

   In the I2 packet, the Initiator MUST display the solution to the
   received puzzle.  Without a correct solution, the I2 message is
   discarded.  The I2 packet also contains a Diffie-Hellman parameter
   that carries needed information for the Responder.  The I2 packet is
   signed by the Initiator.

   The R2 packet acknowledges the receipt of the I2 packet and completes
   the base exchange.  The packet is signed by the Responder.

























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   The base exchange is illustrated below in Figure 1.  The term "key"
   refers to the Host Identity public key, and "sig" represents a
   signature using such a key.  The packets contain other parameters not
   shown in this figure.

      Initiator                              Responder

                   I1: DH list
                 -------------------------->
                                             select precomputed R1
                   R1: puzzle, DH, key, sig
                 <-------------------------
   check sig                                 remain stateless
   solve puzzle
                 I2: solution, DH, {key}, sig
                 -------------------------->
   compute DH                                check puzzle
                                             check sig
                           R2: sig
                 <--------------------------
   check sig                                 compute DH

                                 Figure 1

4.1.1.  HIP Puzzle Mechanism

   The purpose of the HIP puzzle mechanism is to protect the Responder
   from a number of denial-of-service threats.  It allows the Responder
   to delay state creation until receiving the I2 packet.  Furthermore,
   the puzzle allows the Responder to use a fairly cheap calculation to
   check that the Initiator is "sincere" in the sense that it has
   churned enough CPU cycles in solving the puzzle.

   The puzzle allows a Responder implementation to completely delay
   association-specific state creation until a valid I2 packet is
   received.  An I2 packet without a valid puzzle solution can be
   rejected immediately once the Responder has checked the solution.
   The solution can be checked by computing only one hash function, and
   invalid solutions can be rejected before state is created, and before
   CPU-intensive public-key signature verification and Diffie-Hellman
   key generation are performed.  By varying the difficulty of the
   puzzle, the Responder can frustrate CPU- or memory-targeted DoS
   attacks.

   The Responder can remain stateless and drop most spoofed I2 packets
   because puzzle calculation is based on the Initiator's Host Identity
   Tag.  The idea is that the Responder has a (perhaps varying) number
   of pre-calculated R1 packets, and it selects one of these based on



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   the information carried in the I1 packet.  When the Responder then
   later receives the I2 packet, it can verify that the puzzle has been
   solved using the Initiator's HIT.  This makes it impractical for the
   attacker to first exchange one I1/R1 packet, and then generate a
   large number of spoofed I2 packets that seemingly come from different
   HITs.  This method does not protect the Responder from an attacker
   that uses fixed HITs, though.  Against such an attacker, a viable
   approach may be to create a piece of local state, and remember that
   the puzzle check has previously failed.  See Appendix A for one
   possible implementation.  Responder implementations SHOULD include
   sufficient randomness in the puzzle values so that algorithmic
   complexity attacks become impossible [CRO03].

   The Responder can set the puzzle difficulty for the Initiator, based
   on its level of trust of the Initiator.  Because the puzzle is not
   included in the signature calculation, the Responder can use
   pre-calculated R1 packets and include the puzzle just before sending
   the R1 to the Initiator.  The Responder SHOULD use heuristics to
   determine when it is under a denial-of-service attack, and set the
   puzzle difficulty value #K appropriately, as explained later.

4.1.2.  Puzzle Exchange

   The Responder starts the puzzle exchange when it receives an I1
   packet.  The Responder supplies a random number #I, and requires the
   Initiator to find a number #J.  To select a proper #J, the Initiator
   must create the concatenation of #I, the HITs of the parties, and #J,
   and calculate a hash over this concatenation using the RHASH
   algorithm.  The lowest-order #K bits of the result MUST be zeros.
   The value #K sets the difficulty of the puzzle.

   To generate a proper number #J, the Initiator will have to generate a
   number of #Js until one produces the hash target of zeros.  The
   Initiator SHOULD give up after exceeding the puzzle Lifetime in the
   PUZZLE parameter (as described in Section 5.2.4).  The Responder
   needs to re-create the concatenation of #I, the HITs, and the
   provided #J, and compute the hash once to prove that the Initiator
   completed its assigned task.

   To prevent precomputation attacks, the Responder MUST select the
   number #I in such a way that the Initiator cannot guess it.
   Furthermore, the construction MUST allow the Responder to verify that
   the value #I was indeed selected by it and not by the Initiator.  See
   Appendix A for an example on how to implement this.

   Using the Opaque data field in the PUZZLE (see Section 5.2.4) in an
   ECHO_REQUEST_SIGNED (see Section 5.2.20) or in an
   ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21), the Responder



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   can include some data in R1 that the Initiator MUST copy unmodified
   in the corresponding I2 packet.  The Responder can use the opaque
   data to transfer a piece of local state information to the Initiator
   and back -- for example, to recognize that the I2 is a response to a
   previously sent R1.  The Responder can generate the opaque data in
   various ways, e.g., using encryption or hashing with some secret, the
   sent #I, and possibly using other related data.  With the same
   secret, the received #I (from the I2 packet), and the other related
   data (if any), the Responder can verify that it has itself sent the
   #I to the Initiator.  The Responder MUST periodically change such a
   secret.

   It is RECOMMENDED that the Responder generates new secrets for the
   puzzle and new R1s once every few minutes.  Furthermore, it is
   RECOMMENDED that the Responder is able to verify a valid puzzle
   solution at least Lifetime seconds after the puzzle secret has been
   deprecated.  This time value guarantees that the puzzle is valid for
   at least Lifetime and at most 2 * Lifetime seconds.  This limits the
   usability that an old, solved puzzle has to an attacker.  Moreover,
   it avoids problems with the validity of puzzles if the lifetime is
   relatively short compared to the network delay and the time for
   solving the puzzle.

   The puzzle value #I and the solution #J are inputs for deriving the
   keying material from the Diffie-Hellman key exchange (see
   Section 6.5).  Therefore, to ensure that the derived keying material
   differs, a Responder SHOULD NOT use the same puzzle #I with the same
   DH keys for the same Initiator twice.  Such uniqueness can be
   achieved, for example, by using a counter as an additional input for
   generating #I.  This counter can be increased for each processed I1
   packet.  The state of the counter can be transmitted in the Opaque
   data field in the PUZZLE (see Section 5.2.4), in an
   ECHO_REQUEST_SIGNED parameter (see Section 5.2.20), or in an
   ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21) without the need
   to establish state.

   NOTE: The protocol developers explicitly considered whether R1 should
   include a timestamp in order to protect the Initiator from replay
   attacks.  The decision was to NOT include a timestamp, to avoid
   problems with global time synchronization.

   NOTE: The protocol developers explicitly considered whether a memory-
   bound function should be used for the puzzle instead of a CPU-bound
   function.  The decision was to not use memory-bound functions.







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4.1.3.  Authenticated Diffie-Hellman Protocol with DH Group Negotiation

   The packets R1, I2, and R2 implement a standard authenticated
   Diffie-Hellman exchange.  The Responder sends one of its public
   Diffie-Hellman keys and its public authentication key, i.e., its Host
   Identity, in R1.  The signature in the R1 packet allows the Initiator
   to verify that the R1 has been once generated by the Responder.
   However, since the R1 is precomputed and therefore does not cover
   association-specific information in the I1 packet, it does not
   protect against replay attacks.

   Before the actual authenticated Diffie-Hellman exchange, the
   Initiator expresses its preference regarding its choice of the DH
   groups in the I1 packet.  The preference is expressed as a sorted
   list of DH Group IDs.  The I1 packet is not protected by a signature.
   Therefore, this list is sent in an unauthenticated way to avoid
   costly computations for processing the I1 packet at the Responder
   side.  Based on the preferences of the Initiator, the Responder sends
   an R1 packet containing its most suitable public DH value.  The
   Responder also attaches a list of its own preferences to the R1 to
   convey the basis for the DH group selection to the Initiator.  This
   list is carried in the signed part of the R1 packet.  If the choice
   of the DH group value in the R1 does not match the preferences of the
   Initiator and the Responder, the Initiator can detect that the list
   of DH Group IDs in the I1 was manipulated (see below for details).

   If none of the DH Group IDs in the I1 packet are supported by the
   Responder, the Responder selects the DH group most suitable for it,
   regardless of the Initiator's preference.  It then sends the R1
   containing this DH group and its list of supported DH Group IDs to
   the Initiator.

   When the Initiator receives an R1, it receives one of the Responder's
   public Diffie-Hellman values and the list of DH Group IDs supported
   by the Responder.  This list is covered by the signature in the R1
   packet to avoid forgery.  The Initiator compares the Group ID of the
   public DH value in the R1 packet to the list of supported DH Group
   IDs in the R1 packets and to its own preferences expressed in the
   list of supported DH Group IDs.  The Initiator continues the BEX only
   if the Group ID of the public DH value of the Responder is the most
   preferred of the IDs supported by both the Initiator and Responder.
   Otherwise, the communication is subject to a downgrade attack, and
   the Initiator MUST either restart the base exchange with a new I1
   packet or abort the base exchange.  If the Responder's choice of the
   DH group is not supported by the Initiator, the Initiator MAY abort
   the handshake or send a new I1 packet with a different list of
   supported DH groups.  However, the Initiator MUST verify the




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   signature of the R1 packet before restarting or aborting the
   handshake.  It MUST silently ignore the R1 packet if the signature is
   not valid.

   If the preferences regarding the DH Group ID match, the Initiator
   computes the Diffie-Hellman session key (Kij).  The Initiator creates
   a HIP association using keying material from the session key (see
   Section 6.5) and may use the HIP association to encrypt its public
   authentication key, i.e., the Host Identity.  The resulting I2 packet
   contains the Initiator's Diffie-Hellman key and its (optionally
   encrypted) public authentication key.  The signature of the I2
   message covers all parameters of the signed parameter ranges (see
   Section 5.2) in the packet without exceptions, as in the R1.

   The Responder extracts the Initiator's Diffie-Hellman public key from
   the I2 packet, computes the Diffie-Hellman session key, creates a
   corresponding HIP association, and decrypts the Initiator's public
   authentication key.  It can then verify the signature using the
   authentication key.

   The final message, R2, completes the BEX and protects the Initiator
   against replay attacks, because the Responder uses the shared key
   from the Diffie-Hellman exchange to create a Hashed Message
   Authentication Code (HMAC) and also uses the private key of its Host
   Identity to sign the packet contents.

4.1.4.  HIP Replay Protection

   HIP includes the following mechanisms to protect against malicious
   packet replays.  Responders are protected against replays of I1
   packets by virtue of the stateless response to I1 packets with
   pre-signed R1 messages.  Initiators are protected against R1 replays
   by a monotonically increasing "R1 generation counter" included in
   the R1.  Responders are protected against replays of forged I2
   packets by the puzzle mechanism (see Section 4.1.1 above), and
   optional use of opaque data.  Hosts are protected against replays of
   R2 packets and UPDATEs by use of a less expensive HMAC verification
   preceding the HIP signature verification.

   The R1 generation counter is a monotonically increasing 64-bit
   counter that may be initialized to any value.  The scope of the
   counter MAY be system-wide, but there SHOULD be a separate counter
   for each Host Identity, if there is more than one local Host
   Identity.  The value of this counter SHOULD be preserved across
   system reboots and invocations of the HIP base exchange.  This
   counter indicates the current generation of puzzles.  Implementations
   MUST accept puzzles from the current generation and MAY accept
   puzzles from earlier generations.  A system's local counter MUST be



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   incremented at least as often as every time old R1s cease to be
   valid.  The local counter SHOULD never be decremented; otherwise, the
   host exposes its peers to the replay of previously generated, higher-
   numbered R1s.

   A host may receive more than one R1, either due to sending multiple
   I1 packets (see Section 6.6.1) or due to a replay of an old R1.  When
   sending multiple I1 packets to the same host, an Initiator SHOULD
   wait for a small amount of time (a reasonable time may be
   2 * expected RTT) after the first R1 reception to allow possibly
   multiple R1s to arrive, and it SHOULD respond to an R1 among the set
   with the largest R1 generation counter.  If an Initiator is
   processing an R1 or has already sent an I2 packet (still waiting for
   the R2 packet) and it receives another R1 with a larger R1 generation
   counter, it MAY elect to restart R1 processing with the fresher R1,
   as if it were the first R1 to arrive.

   The R1 generation counter may roll over or may become reset.  It is
   important for an Initiator to be robust to the loss of state about
   the R1 generation counter of a peer or to a reset of the peer's
   counter.  It is recommended that, when choosing between multiple R1s,
   the Initiator prefer to use the R1 that corresponds to the current R1
   generation counter, but that if it is unable to make progress with
   that R1, the Initiator may try the other R1s, beginning with the R1
   packet with the highest counter.

4.1.5.  Refusing a HIP Base Exchange

   A HIP-aware host may choose not to accept a HIP base exchange.  If
   the host's policy is to only be an Initiator and policy allows the
   establishment of a HIP association with the original Initiator, it
   should begin its own HIP base exchange.  A host MAY choose to have
   such a policy since only the privacy of the Initiator's HI is
   protected in the exchange.  It should be noted that such behavior can
   introduce the risk of a race condition if each host's policy is to
   only be an Initiator, at which point the HIP base exchange will fail.

   If the host's policy does not permit it to enter into a HIP exchange
   with the Initiator, it should send an ICMP 'Destination Unreachable,
   Administratively Prohibited' message.  A more complex HIP packet is
   not used here as it actually opens up more potential DoS attacks than
   a simple ICMP message.  A HIP NOTIFY message is not used because no
   HIP association exists between the two hosts at that time.








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4.1.6.  Aborting a HIP Base Exchange

   Two HIP hosts may encounter situations in which they cannot complete
   a HIP base exchange because of insufficient support for cryptographic
   algorithms, in particular the HIT Suites and DH groups.  After
   receiving the R1 packet, the Initiator can determine whether the
   Responder supports the required cryptographic operations to
   successfully establish a HIP association.  The Initiator can abort
   the BEX silently after receiving an R1 packet that indicates an
   unsupported set of algorithms.  The specific conditions are described
   below.

   The R1 packet contains a signed list of HIT Suite IDs as supported by
   the Responder.  Therefore, the Initiator can determine whether its
   source HIT is supported by the Responder.  If the HIT Suite ID of the
   Initiator's HIT is not contained in the list of HIT Suites in the R1,
   the Initiator MAY abort the handshake silently or MAY restart the
   handshake with a new I1 packet that contains a source HIT supported
   by the Responder.

   During the handshake, the Initiator and the Responder agree on a
   single DH group.  The Responder selects the DH group and its DH
   public value in the R1 based on the list of DH Group IDs in the I1
   packet.  If the Responder supports none of the DH groups requested by
   the Initiator, the Responder selects an arbitrary DH and replies with
   an R1 containing its list of supported DH Group IDs.  In such a case,
   the Initiator receives an R1 packet containing the DH public value
   for an unrequested DH group and also the Responder's DH group list in
   the signed part of the R1 packet.  At this point, the Initiator MAY
   abort the handshake or MAY restart the handshake by sending a new I1
   packet containing a selection of DH Group IDs that is supported by
   the Responder.

4.1.7.  HIP Downgrade Protection

   In a downgrade attack, an attacker attempts to unnoticeably
   manipulate the packets of an Initiator and/or a Responder to
   influence the result of the cryptographic negotiations in the BEX in
   its favor.  As a result, the victims select weaker cryptographic
   algorithms than they would otherwise have selected without the
   attacker's interference.  Downgrade attacks can only be successful if
   they remain undetected by the victims and the victims falsely assume
   a secure communication channel.

   In HIP, almost all packet parameters related to cryptographic
   negotiations are covered by signatures.  These parameters cannot be
   directly manipulated in a downgrade attack without invalidating the
   signature.  However, signed packets can be subject to replay attacks.



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   In such a replay attack, the attacker could use an old BEX packet
   with an outdated and weak selection of cryptographic algorithms and
   replay it instead of a more recent packet with a collection of
   stronger cryptographic algorithms.  Signed packets that could be
   subject to this replay attack are the R1 and I2 packet.  However,
   replayed R1 and I2 packets cannot be used to successfully establish a
   HIP BEX because these packets also contain the public DH values of
   the Initiator and the Responder.  Old DH values from replayed packets
   lead to invalid keying material and mismatching shared secrets
   because the attacker is unable to derive valid keying material from
   the DH public keys in the R1 and cannot generate a valid HMAC and
   signature for a replayed I2.

   In contrast to the first version of HIP [RFC5201], version 2 of HIP
   as defined in this document begins the negotiation of the DH groups
   already in the first BEX packet, the I1.  The I1 packet is, by
   intention, not protected by a signature, to avoid CPU-intensive
   cryptographic operations processing floods of I1 packets targeted at
   the Responder.  Hence, the list of DH Group IDs in the I1 packet is
   vulnerable to forgery and manipulation.  To thwart an unnoticed
   manipulation of the I1 packet, the Responder chooses the DH group
   deterministically and includes its own list of DH Group IDs in the
   signed part of the R1 packet.  The Initiator can detect an attempted
   downgrade attack by comparing the list of DH Group IDs in the R1
   packet to its own preferences in the I1 packet.  If the choice of the
   DH group in the R1 packet does not equal the best match of the two
   lists (the highest-priority DH ID of the Responder that is present in
   the Initiator's DH list), the Initiator can conclude that its list in
   the I1 packet was altered by an attacker.  In this case, the
   Initiator can restart or abort the BEX.  As mentioned before, the
   detection of the downgrade attack is sufficient to prevent it.

4.1.8.  HIP Opportunistic Mode

   It is possible to initiate a HIP BEX even if the Responder's HI (and
   HIT) is unknown.  In this case, the initial I1 packet contains all
   zeros as the destination HIT.  This kind of connection setup is
   called opportunistic mode.

   The Responder may have multiple HITs due to multiple supported HIT
   Suites.  Since the Responder's HIT Suite in the opportunistic mode is
   not determined by the destination HIT of the I1 packet, the Responder
   can freely select a HIT of any HIT Suite.  The complete set of HIT
   Suites supported by the Initiator is not known to the Responder.
   Therefore, the Responder SHOULD select its HIT from the same HIT
   Suite as the Initiator's HIT (indicated by the HIT Suite information
   in the OGA ID field of the Initiator's HIT) because this HIT Suite is
   obviously supported by the Initiator.  If the Responder selects a



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   different HIT that is not supported by the Initiator, the Initiator
   MAY restart the BEX with an I1 packet with a source HIT that is
   contained in the list of the Responder's HIT Suites in the R1 packet.

   Note that the Initiator cannot verify the signature of the R1 packet
   if the Responder's HIT Suite is not supported.  Therefore, the
   Initiator MUST treat R1 packets with unsupported Responder HITs as
   potentially forged and MUST NOT use any parameters from the
   unverified R1 besides the HIT_SUITE_LIST.  Moreover, an Initiator
   that uses an unverified HIT_SUITE_LIST from an R1 packet to determine
   a possible source HIT MUST verify that the HIT_SUITE_LIST in the
   first unverified R1 packet matches the HIT_SUITE_LIST in the second
   R1 packet for which the Initiator supports the signature algorithm.
   The Initiator MUST restart the BEX with a new I1 packet for which the
   algorithm was mentioned in the verifiable R1 if the two lists do not
   match.  This procedure is necessary to mitigate downgrade attacks.

   There are both security and API issues involved with the
   opportunistic mode.  These issues are described in the remainder of
   this section.

   Given that the Responder's HI is not known by the Initiator, there
   must be suitable API calls that allow the Initiator to request,
   directly or indirectly, that the underlying system initiates the HIP
   base exchange solely based on locators.  The Responder's HI will be
   tentatively available in the R1 packet, and in an authenticated form
   once the R2 packet has been received and verified.  Hence, the
   Responder's HIT could be communicated to the application via new API
   mechanisms.  However, with a backwards-compatible API the application
   sees only the locators used for the initial contact.  Depending on
   the desired semantics of the API, this can raise the following
   issues:

   o  The actual locators may later change if an UPDATE message is used,
      even if from the API perspective the association still appears to
      be between two specific locators.  However, the locator update is
      still secure, and the association is still between the same nodes.

   o  Different associations between the same two locators may result in
      connections to different nodes, if the implementation no longer
      remembers which identifier the peer had in an earlier association.
      This is possible when the peer's locator has changed for
      legitimate reasons or when an attacker pretends to be a node that
      has the peer's locator.  Therefore, when using opportunistic mode,
      HIP implementations MUST NOT place any expectation that the peer's
      HI returned in the R1 message matches any HI previously seen from
      that address.




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      If the HIP implementation and application do not have the same
      understanding of what constitutes an association, this may even
      happen within the same association.  For instance, an
      implementation may not know when HIP state can be purged for
      UDP-based applications.

   In addition, the following security considerations apply.  The
   generation counter mechanism will be less efficient in protecting
   against replays of the R1 packet, given that the Responder can choose
   a replay that uses an arbitrary HI, not just the one given in the I1
   packet.

   More importantly, the opportunistic exchange is vulnerable to
   man-in-the-middle attacks, because the Initiator does not have any
   public key information about the peer.  To assess the impacts of this
   vulnerability, we compare it to vulnerabilities in current,
   non-HIP-capable communications.

   An attacker on the path between the two peers can insert itself as a
   man-in-the-middle by providing its own identifier to the Initiator
   and then initiating another HIP association towards the Responder.
   For this to be possible, the Initiator must employ opportunistic
   mode, and the Responder must be configured to accept a connection
   from any HIP-enabled node.

   An attacker outside the path will be unable to do so, given that it
   cannot respond to the messages in the base exchange.

   These security properties are characteristic also of communications
   in the current Internet.  A client contacting a server without
   employing end-to-end security may find itself talking to the server
   via a man-in-the-middle, assuming again that the server is willing to
   talk to anyone.

   If end-to-end security is in place, then the worst that can happen in
   both the opportunistic HIP and non-HIP (normal IP) cases is denial-
   of-service; an entity on the path can disrupt communications, but
   will be unable to successfully insert itself as a man-in-the-middle.

   However, once the opportunistic exchange has successfully completed,
   HIP provides confidentiality and integrity protection for the
   communications, and can securely change the locators of the
   endpoints.

   As a result, opportunistic mode in HIP offers a "better than nothing"
   security model.  Initially, a base exchange authenticated in the
   opportunistic mode involves a leap of faith subject to man-in-the-
   middle attacks, but subsequent datagrams related to the same HIP



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   association cannot be compromised by a new man-in-the-middle attack.
   Further, if the man-in-the-middle moves away from the path of the
   active association, the attack would be exposed after the fact.
   Thus, it can be stated that opportunistic mode in HIP is at least as
   secure as unprotected IP-based communications.

4.2.  Updating a HIP Association

   A HIP association between two hosts may need to be updated over time.
   Examples include the need to rekey expiring security associations,
   add new security associations, or change IP addresses associated with
   hosts.  The UPDATE packet is used for those and other similar
   purposes.  This document only specifies the UPDATE packet format and
   basic processing rules, with mandatory parameters.  The actual usage
   is defined in separate specifications.

   HIP provides a general-purpose UPDATE packet, which can carry
   multiple HIP parameters, for updating the HIP state between two
   peers.  The UPDATE mechanism has the following properties:

      UPDATE messages carry a monotonically increasing sequence number
      and are explicitly acknowledged by the peer.  Lost UPDATEs or
      acknowledgments may be recovered via retransmission.  Multiple
      UPDATE messages may be outstanding under certain circumstances.

      UPDATE is protected by both HIP_MAC and HIP_SIGNATURE parameters,
      since processing UPDATE signatures alone is a potential DoS attack
      against intermediate systems.

      UPDATE packets are explicitly acknowledged by the use of an
      acknowledgment parameter that echoes an individual sequence number
      received from the peer.  A single UPDATE packet may contain both a
      sequence number and one or more acknowledgment numbers (i.e.,
      piggybacked acknowledgment(s) for the peer's UPDATE).

   The UPDATE packet is defined in Section 5.3.5.

4.3.  Error Processing

   HIP error processing behavior depends on whether or not there exists
   an active HIP association.  In general, if a HIP association exists
   between the sender and receiver of a packet causing an error
   condition, the receiver SHOULD respond with a NOTIFY packet.  On the
   other hand, if there are no existing HIP associations between the
   sender and receiver, or the receiver cannot reasonably determine the
   identity of the sender, the receiver MAY respond with a suitable ICMP
   message; see Section 5.4 for more details.




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RFC 7401                          HIPv2                       April 2015


   The HIP protocol and state machine are designed to recover from one
   of the parties crashing and losing its state.  The following
   scenarios describe the main use cases covered by the design.

      No prior state between the two systems.

         The system with data to send is the Initiator.  The process
         follows the standard four-packet base exchange, establishing
         the HIP association.

      The system with data to send has no state with the receiver, but
      the receiver has a residual HIP association.

         The system with data to send is the Initiator.  The Initiator
         acts as in no prior state, sending an I1 packet and receiving
         an R1 packet.  When the Responder receives a valid I2 packet,
         the old association is 'discovered' and deleted, and the new
         association is established.

      The system with data to send has a HIP association, but the
      receiver does not.

         The system sends data on the outbound user data security
         association.  The receiver 'detects' the situation when it
         receives a user data packet that it cannot match to any HIP
         association.  The receiving host MUST discard this packet.

         The receiving host SHOULD send an ICMP packet, with the type
         Parameter Problem, to inform the sender that the HIP
         association does not exist (see Section 5.4), and it MAY
         initiate a new HIP BEX.  However, responding with these
         optional mechanisms is implementation or policy dependent.  If
         the sending application doesn't expect a response, the system
         could possibly send a large number of packets in this state, so
         for this reason, the sending of one or more ICMP packets is
         RECOMMENDED.  However, any such responses MUST be rate-limited
         to prevent abuse (see Section 5.4).

4.4.  HIP State Machine

   HIP itself has little state.  In the HIP base exchange, there is an
   Initiator and a Responder.  Once the security associations (SAs) are
   established, this distinction is lost.  If the HIP state needs to be
   re-established, the controlling parameters are which peer still has
   state and which has a datagram to send to its peer.  The following
   state machine attempts to capture these processes.





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RFC 7401                          HIPv2                       April 2015


   The state machine is symmetric and is presented in a single system
   view, representing either an Initiator or a Responder.  The state
   machine is not a full representation of the processing logic.
   Additional processing rules are presented in the packet definitions.
   Hence, both are needed to completely implement HIP.

   This document extends the state machine as defined in [RFC5201] and
   introduces a restart option to allow for the negotiation of
   cryptographic algorithms.  The extension to the previous state
   machine in [RFC5201] is a transition from state I1-SENT back again to
   I1-SENT; namely, the restart option.  An Initiator is required to
   restart the HIP base exchange if the Responder does not support the
   HIT Suite of the Initiator.  In this case, the Initiator restarts the
   HIP base exchange by sending a new I1 packet with a source HIT
   supported by the Responder.

   Implementors must understand that the state machine, as described
   here, is informational.  Specific implementations are free to
   implement the actual processing logic differently.  Section 6
   describes the packet processing rules in more detail.  This state
   machine focuses on the HIP I1, R1, I2, and R2 packets only.  New
   states and state transitions may be introduced by mechanisms in other
   specifications (such as mobility and multihoming).

4.4.1.  State Machine Terminology

   Unused Association Lifetime (UAL):  Implementation-specific time for
      which, if no packet is sent or received for this time interval, a
      host MAY begin to tear down an active HIP association.

   Maximum Segment Lifetime (MSL):  Maximum time that a HIP packet is
      expected to spend in the network.  A default value of 2 minutes
      has been borrowed from [RFC0793] because it is a prevailing
      assumption for packet lifetimes.

   Exchange Complete (EC):  Time that the host spends at the R2-SENT
      state before it moves to the ESTABLISHED state.  The time is n *
      I2 retransmission timeout, where n is about I2_RETRIES_MAX.

   Receive ANYOTHER:  Any received packet for which no state transitions
      or processing rules are defined for a given state.










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RFC 7401                          HIPv2                       April 2015


4.4.2.  HIP States

   +---------------------+---------------------------------------------+
   | State               | Explanation                                 |
   +---------------------+---------------------------------------------+
   | UNASSOCIATED        | State machine start                         |
   |                     |                                             |
   | I1-SENT             | Initiating base exchange                    |
   |                     |                                             |
   | I2-SENT             | Waiting to complete base exchange           |
   |                     |                                             |
   | R2-SENT             | Waiting to complete base exchange           |
   |                     |                                             |
   | ESTABLISHED         | HIP association established                 |
   |                     |                                             |
   | CLOSING             | HIP association closing, no data can be     |
   |                     | sent                                        |
   |                     |                                             |
   | CLOSED              | HIP association closed, no data can be sent |
   |                     |                                             |
   | E-FAILED            | HIP base exchange failed                    |
   +---------------------+---------------------------------------------+

                            Table 1: HIP States



























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4.4.3.  HIP State Processes

   System behavior in state UNASSOCIATED, Table 2.

   +----------------------------+--------------------------------------+
   | Trigger                    | Action                               |
   +----------------------------+--------------------------------------+
   | User data to send,         | Send I1 and go to I1-SENT            |
   | requiring a new HIP        |                                      |
   | association                |                                      |
   |                            |                                      |
   | Receive I1                 | Send R1 and stay at UNASSOCIATED     |
   |                            |                                      |
   | Receive I2, process        | If successful, send R2 and go to     |
   |                            | R2-SENT                              |
   |                            |                                      |
   |                            | If fail, stay at UNASSOCIATED        |
   |                            |                                      |
   | Receive user data for an   | Optionally send ICMP as defined in   |
   | unknown HIP association    | Section 5.4 and stay at UNASSOCIATED |
   |                            |                                      |
   | Receive CLOSE              | Optionally send ICMP Parameter       |
   |                            | Problem and stay at UNASSOCIATED     |
   |                            |                                      |
   | Receive ANYOTHER           | Drop and stay at UNASSOCIATED        |
   +----------------------------+--------------------------------------+

                    Table 2: UNASSOCIATED - Start State























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RFC 7401                          HIPv2                       April 2015


   System behavior in state I1-SENT, Table 3.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1 from     | If the local HIT is smaller than the peer   |
   | Responder           | HIT, drop I1 and stay at I1-SENT (see       |
   |                     | Section 6.5 for HIT comparison)             |
   |                     |                                             |
   |                     | If the local HIT is greater than the peer   |
   |                     | HIT, send R1 and stay at I1-SENT            |
   |                     |                                             |
   | Receive I2, process | If successful, send R2 and go to R2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at I1-SENT                    |
   |                     |                                             |
   | Receive R1, process | If the HIT Suite of the local HIT is not    |
   |                     | supported by the peer, select supported     |
   |                     | local HIT, send I1, and stay at I1-SENT     |
   |                     |                                             |
   |                     | If successful, send I2 and go to I2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at I1-SENT                    |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at I1-SENT                    |
   |                     |                                             |
   | Timeout             | Increment trial counter                     |
   |                     |                                             |
   |                     | If counter is less than I1_RETRIES_MAX,     |
   |                     | send I1 and stay at I1-SENT                 |
   |                     |                                             |
   |                     | If counter is greater than I1_RETRIES_MAX,  |
   |                     | go to E-FAILED                              |
   +---------------------+---------------------------------------------+

            Table 3: I1-SENT - Initiating the HIP Base Exchange















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RFC 7401                          HIPv2                       April 2015


   System behavior in state I2-SENT, Table 4.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | Send R1 and stay at I2-SENT                 |
   |                     |                                             |
   | Receive R1, process | If successful, send I2 and stay at I2-SENT  |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive I2, process | If successful and local HIT is smaller than |
   |                     | the peer HIT, drop I2 and stay at I2-SENT   |
   |                     |                                             |
   |                     | If successful and local HIT is greater than |
   |                     | the peer HIT, send R2 and go to R2-SENT     |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive R2, process | If successful, go to ESTABLISHED            |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK and go to     |
   | process             | CLOSED                                      |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at I2-SENT                    |
   |                     |                                             |
   | Timeout             | Increment trial counter                     |
   |                     |                                             |
   |                     | If counter is less than I2_RETRIES_MAX,     |
   |                     | send I2 and stay at I2-SENT                 |
   |                     |                                             |
   |                     | If counter is greater than I2_RETRIES_MAX,  |
   |                     | go to E-FAILED                              |
   +---------------------+---------------------------------------------+

        Table 4: I2-SENT - Waiting to Finish the HIP Base Exchange











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   System behavior in state R2-SENT, Table 5.

   +------------------------+------------------------------------------+
   | Trigger                | Action                                   |
   +------------------------+------------------------------------------+
   | Receive I1             | Send R1 and stay at R2-SENT              |
   |                        |                                          |
   | Receive I2, process    | If successful, send R2 and stay at       |
   |                        | R2-SENT                                  |
   |                        |                                          |
   |                        | If fail, stay at R2-SENT                 |
   |                        |                                          |
   | Receive R1             | Drop and stay at R2-SENT                 |
   |                        |                                          |
   | Receive R2             | Drop and stay at R2-SENT                 |
   |                        |                                          |
   | Receive data or UPDATE | Move to ESTABLISHED                      |
   |                        |                                          |
   | Exchange Complete      | Move to ESTABLISHED                      |
   | Timeout                |                                          |
   |                        |                                          |
   | Receive CLOSE, process | If successful, send CLOSE_ACK and go to  |
   |                        | CLOSED                                   |
   |                        |                                          |
   |                        | If fail, stay at ESTABLISHED             |
   |                        |                                          |
   | Receive CLOSE_ACK      | Drop and stay at R2-SENT                 |
   |                        |                                          |
   | Receive NOTIFY         | Process and stay at R2-SENT              |
   +------------------------+------------------------------------------+

                 Table 5: R2-SENT - Waiting to Finish HIP



















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RFC 7401                          HIPv2                       April 2015


   System behavior in state ESTABLISHED, Table 6.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | Send R1 and stay at ESTABLISHED             |
   |                     |                                             |
   | Receive I2          | Process with puzzle and possible Opaque     |
   |                     | data verification                           |
   |                     |                                             |
   |                     | If successful, send R2, drop old HIP        |
   |                     | association, establish a new HIP            |
   |                     | association, and go to R2-SENT              |
   |                     |                                             |
   |                     | If fail, stay at ESTABLISHED                |
   |                     |                                             |
   | Receive R1          | Drop and stay at ESTABLISHED                |
   |                     |                                             |
   | Receive R2          | Drop and stay at ESTABLISHED                |
   |                     |                                             |
   | Receive user data   | Process and stay at ESTABLISHED             |
   | for HIP association |                                             |
   |                     |                                             |
   | No packet           | Send CLOSE and go to CLOSING                |
   | sent/received       |                                             |
   | during UAL minutes  |                                             |
   |                     |                                             |
   | Receive UPDATE      | Process and stay at ESTABLISHED             |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK and go to     |
   | process             | CLOSED                                      |
   |                     |                                             |
   |                     | If fail, stay at ESTABLISHED                |
   |                     |                                             |
   | Receive CLOSE_ACK   | Drop and stay at ESTABLISHED                |
   |                     |                                             |
   | Receive NOTIFY      | Process and stay at ESTABLISHED             |
   +---------------------+---------------------------------------------+

            Table 6: ESTABLISHED - HIP Association Established











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RFC 7401                          HIPv2                       April 2015


   System behavior in state CLOSING, Table 7.

   +----------------------------+--------------------------------------+
   | Trigger                    | Action                               |
   +----------------------------+--------------------------------------+
   | User data to send,         | Send I1 and go to I1-SENT            |
   | requires the creation of   |                                      |
   | another incarnation of the |                                      |
   | HIP association            |                                      |
   |                            |                                      |
   | Receive I1                 | Send R1 and stay at CLOSING          |
   |                            |                                      |
   | Receive I2, process        | If successful, send R2 and go to     |
   |                            | R2-SENT                              |
   |                            |                                      |
   |                            | If fail, stay at CLOSING             |
   |                            |                                      |
   | Receive R1, process        | If successful, send I2 and go to     |
   |                            | I2-SENT                              |
   |                            |                                      |
   |                            | If fail, stay at CLOSING             |
   |                            |                                      |
   | Receive CLOSE, process     | If successful, send CLOSE_ACK,       |
   |                            | discard state, and go to CLOSED      |
   |                            |                                      |
   |                            | If fail, stay at CLOSING             |
   |                            |                                      |
   | Receive CLOSE_ACK, process | If successful, discard state and go  |
   |                            | to UNASSOCIATED                      |
   |                            |                                      |
   |                            | If fail, stay at CLOSING             |
   |                            |                                      |
   | Receive ANYOTHER           | Drop and stay at CLOSING             |
   |                            |                                      |
   | Timeout                    | Increment timeout sum and reset      |
   |                            | timer.  If timeout sum is less than  |
   |                            | UAL+MSL minutes, retransmit CLOSE    |
   |                            | and stay at CLOSING.                 |
   |                            |                                      |
   |                            | If timeout sum is greater than       |
   |                            | UAL+MSL minutes, go to UNASSOCIATED  |
   +----------------------------+--------------------------------------+

   Table 7: CLOSING - HIP Association Has Not Been Used for UAL Minutes







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RFC 7401                          HIPv2                       April 2015


   System behavior in state CLOSED, Table 8.

   +----------------------------------------+--------------------------+
   | Trigger                                | Action                   |
   +----------------------------------------+--------------------------+
   | Datagram to send, requires the         | Send I1 and stay at      |
   | creation of another incarnation of the | CLOSED                   |
   | HIP association                        |                          |
   |                                        |                          |
   | Receive I1                             | Send R1 and stay at      |
   |                                        | CLOSED                   |
   |                                        |                          |
   | Receive I2, process                    | If successful, send R2   |
   |                                        | and go to R2-SENT        |
   |                                        |                          |
   |                                        | If fail, stay at CLOSED  |
   |                                        |                          |
   | Receive R1, process                    | If successful, send I2   |
   |                                        | and go to I2-SENT        |
   |                                        |                          |
   |                                        | If fail, stay at CLOSED  |
   |                                        |                          |
   | Receive CLOSE, process                 | If successful, send      |
   |                                        | CLOSE_ACK and stay at    |
   |                                        | CLOSED                   |
   |                                        |                          |
   |                                        | If fail, stay at CLOSED  |
   |                                        |                          |
   | Receive CLOSE_ACK, process             | If successful, discard   |
   |                                        | state and go to          |
   |                                        | UNASSOCIATED             |
   |                                        |                          |
   |                                        | If fail, stay at CLOSED  |
   |                                        |                          |
   | Receive ANYOTHER                       | Drop and stay at CLOSED  |
   |                                        |                          |
   | Timeout (UAL+2MSL)                     | Discard state and go to  |
   |                                        | UNASSOCIATED             |
   +----------------------------------------+--------------------------+

    Table 8: CLOSED - CLOSE_ACK Sent, Resending CLOSE_ACK if Necessary










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RFC 7401                          HIPv2                       April 2015


   System behavior in state E-FAILED, Table 9.

   +-------------------------+-----------------------------------------+
   | Trigger                 | Action                                  |
   +-------------------------+-----------------------------------------+
   | Wait for                | Go to UNASSOCIATED.  Renegotiation is   |
   | implementation-specific | possible after moving to UNASSOCIATED   |
   | time                    | state.                                  |
   +-------------------------+-----------------------------------------+

     Table 9: E-FAILED - HIP Failed to Establish Association with Peer

4.4.4.  Simplified HIP State Diagram

   The following diagram (Figure 2) shows the major state transitions.
   Transitions based on received packets implicitly assume that the
   packets are successfully authenticated or processed.


































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RFC 7401                          HIPv2                       April 2015


                               +--+       +----------------------------+
              recv I1, send R1 |  |       |                            |
                               |  v       v                            |
                             +--------------+  recv I2, send R2        |
            +----------------| UNASSOCIATED |----------------+         |
   datagram |  +--+          +--------------+                |         |
   to send, |  |  | Alg. not supported,                      |         |
    send I1 |  |  | send I1                                  |         |
     .      v  |  v                                          |         |
     .   +---------+  recv I2, send R2                       |         |
   +---->| I1-SENT |--------------------------------------+  |         |
   |     +---------+            +----------------------+  |  |         |
   |          | recv R2,        | recv I2, send R2     |  |  |         |
   |          v send I2         |                      v  v  v         |
   |       +---------+          |                    +---------+       |
   |  +--->| I2-SENT |----------+     +--------------| R2-SENT |<---+  |
   |  |    +---------+                |              +---------+    |  |
   |  |          |  |recv R2          |        data or|             |  |
   |  |recv R1,  |  |                 |     EC timeout|             |  |
   |  |send I2   +--|-----------------+               |  receive I2,|  |
   |  |          |  |       +-------------+           |      send R2|  |
   |  |          |  +------>| ESTABLISHED |<----------+             |  |
   |  |          |          +-------------+                         |  |
   |  |          |            |  |  |      receive I2, send R2      |  |
   |  |          +------------+  |  +-------------------------------+  |
   |  |          |               +-----------+                      |  |
   |  |          |    no packet sent/received|    +---+             |  |
   |  |          |    for UAL min, send CLOSE|    |   |timeout      |  |
   |  |          |                           v    v   |(UAL+MSL)    |  |
   |  |          |                        +---------+ |retransmit   |  |
   +--|----------|------------------------| CLOSING |-+CLOSE        |  |
      |          |                        +---------+               |  |
      |          |                         | |   | |                |  |
      +----------|-------------------------+ |   | +----------------+  |
      |          |               +-----------+   +------------------|--+
      |          |               |recv CLOSE,      recv CLOSE_ACK   |  |
      |          +-------------+ |send CLOSE_ACK   or timeout       |  |
      |     recv CLOSE,        | |                 (UAL+MSL)        |  |
      |     send CLOSE_ACK     v v                                  |  |
      |                     +--------+  receive I2, send R2         |  |
      +---------------------| CLOSED |------------------------------+  |
                            +--------+                                 |
                             ^ |  |                                    |
   recv CLOSE, send CLOSE_ACK| |  |              timeout (UAL+2MSL)    |
                             +-+  +------------------------------------+

                                 Figure 2




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RFC 7401                          HIPv2                       April 2015


4.5.  User Data Considerations

4.5.1.  TCP and UDP Pseudo Header Computation for User Data

   When computing TCP and UDP checksums on user data packets that flow
   through sockets bound to HITs, the IPv6 pseudo header format
   [RFC2460] MUST be used, even if the actual addresses in the header of
   the packet are IPv4 addresses.  Additionally, the HITs MUST be used
   in place of the IPv6 addresses in the IPv6 pseudo header.  Note that
   the pseudo header for actual HIP payloads is computed differently;
   see Section 5.1.1.

4.5.2.  Sending Data on HIP Packets

   Other documents may define how to include user data in various HIP
   packets.  However, currently the HIP header is a terminal header, and
   not followed by any other headers.

4.5.3.  Transport Formats

   The actual data transmission format, used for user data after the HIP
   base exchange, is not defined in this document.  Such transport
   formats and methods are described in separate specifications.  All
   HIP implementations MUST implement, at minimum, the ESP transport
   format for HIP [RFC7402].  The transport format to be chosen is
   negotiated in the base exchange.  The Responder expresses its
   preference regarding the transport format in the
   TRANSPORT_FORMAT_LIST in the R1 packet, and the Initiator selects one
   transport format and adds the respective HIP parameter to the I2
   packet.

4.5.4.  Reboot, Timeout, and Restart of HIP

   Simulating a loss of state is a potential DoS attack.  The following
   process has been crafted to manage state recovery without presenting
   a DoS opportunity.

   If a host reboots or the HIP association times out, it has lost its
   HIP state.  If the host that lost state has a datagram to send to the
   peer, it simply restarts the HIP base exchange.  After the base
   exchange has completed, the Initiator can create a new payload
   association and start sending data.  The peer does not reset its
   state until it receives a valid I2 packet.

   If a system receives a user data packet that cannot be matched to any
   existing HIP association, it is possible that it has lost the state
   and its peer has not.  It MAY send an ICMP packet with the Parameter
   Problem type, and with the Pointer pointing to the referred



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RFC 7401                          HIPv2                       April 2015


   HIP-related association information.  Reacting to such traffic
   depends on the implementation and the environment where the
   implementation is used.

   If the host that apparently has lost its state decides to restart the
   HIP base exchange, it sends an I1 packet to the peer.  After the base
   exchange has been completed successfully, the Initiator can create a
   new HIP association, and the peer drops its old payload associations
   and creates a new one.

4.6.  Certificate Distribution

   This document does not define how to use certificates or how to
   transfer them between hosts.  These functions are expected to be
   defined in a future specification, as was done for HIP version 1 (see
   [RFC6253]).  A parameter type value, meant to be used for carrying
   certificates, is reserved, though: CERT, Type 768; see Section 5.2.

5.  Packet Formats

5.1.  Payload Format

   All HIP packets start with a fixed header.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Header   | Header Length |0| Packet Type |Version| RES.|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum             |           Controls            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Sender's Host Identity Tag (HIT)               |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Receiver's Host Identity Tag (HIT)              |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                        HIP Parameters                         /
   /                                                               /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The HIP header is logically an IPv6 extension header.  However, this
   document does not describe processing for Next Header values other
   than decimal 59, IPPROTO_NONE, the IPv6 'no next header' value.
   Future documents MAY define behavior for other values.  However,
   current implementations MUST ignore trailing data if an unimplemented
   Next Header value is received.

   The Header Length field contains the combined length of the HIP
   Header and HIP parameters in 8-byte units, excluding the first
   8 bytes.  Since all HIP headers MUST contain the sender's and
   receiver's HIT fields, the minimum value for this field is 4, and
   conversely, the maximum length of the HIP Parameters field is
   (255 * 8) - 32 = 2008 bytes (see Section 5.1.3 regarding HIP
   fragmentation).  Note: this sets an additional limit for sizes of
   parameters included in the Parameters field, independent of the
   individual parameter maximum lengths.

   The Packet Type indicates the HIP packet type.  The individual packet
   types are defined in the relevant sections.  If a HIP host receives a
   HIP packet that contains an unrecognized packet type, it MUST drop
   the packet.

   The HIP Version field is four bits.  The version defined in this
   document is 2.  The version number is expected to be incremented only
   if there are incompatible changes to the protocol.  Most extensions
   can be handled by defining new packet types, new parameter types, or
   new Controls (see Section 5.1.2).

   The following three bits are reserved for future use.  They MUST be
   zero when sent, and they MUST be ignored when handling a received
   packet.

   The two fixed bits in the header are reserved for SHIM6 compatibility
   [RFC5533], Section 5.3.  For implementations adhering (only) to this
   specification, they MUST be set as shown when sending and MUST be
   ignored when receiving.  This is to ensure optimal forward
   compatibility.  Note that for implementations that implement other
   compatible specifications in addition to this specification, the
   corresponding rules may well be different.  For example, an
   implementation that implements both this specification and the SHIM6
   protocol may need to check these bits in order to determine how to
   handle the packet.

   The HIT fields are always 128 bits (16 bytes) long.







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5.1.1.  Checksum

   Since the checksum covers the source and destination addresses in the
   IP header, it MUST be recomputed on HIP-aware NAT devices.

   If IPv6 is used to carry the HIP packet, the pseudo header [RFC2460]
   contains the source and destination IPv6 addresses, HIP packet length
   in the pseudo header Length field, a zero field, and the HIP protocol
   number (see Section 5.1) in the Next Header field.  The Length field
   is in bytes and can be calculated from the HIP Header Length field:

   (HIP Header Length + 1) * 8.

   In case of using IPv4, the IPv4 UDP pseudo header format [RFC0768] is
   used.  In the pseudo header, the source and destination addresses are
   those used in the IP header, the zero field is obviously zero, the
   protocol is the HIP protocol number (see Section 4), and the length
   is calculated as in the IPv6 case.

5.1.2.  HIP Controls

   The HIP Controls field conveys information about the structure of the
   packet and capabilities of the host.

   The following fields have been defined:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | | | | | | | | | | | | | | | |A|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A - Anonymous:  If this is set, the sender's HI in this packet is
      anonymous, i.e., one not listed in a directory.  Anonymous HIs
      SHOULD NOT be stored.  This control is set in packets using
      anonymous sender HIs.  The peer receiving an anonymous HI in an R1
      or I2 may choose to refuse it.

   The rest of the fields are reserved for future use, and MUST be set
   to zero in sent packets and MUST be ignored in received packets.

5.1.3.  HIP Fragmentation Support

   A HIP implementation MUST support IP fragmentation/reassembly.
   Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
   fragment generation is REQUIRED to be implemented in IPv4 (IPv4
   stacks and networks will usually do this by default) and RECOMMENDED
   to be implemented in IPv6.  In IPv6 networks, the minimum MTU is
   larger, 1280 bytes, than in IPv4 networks.  The larger MTU size is
   usually sufficient for most HIP packets, and therefore fragment



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   generation may not be needed.  If it is expected that a host will
   send HIP packets that are larger than the minimum IPv6 MTU, the
   implementation MUST implement fragment generation even for IPv6.

   In IPv4 networks, HIP packets may encounter low MTUs along their
   routed path.  Since basic HIP, as defined in this document, does not
   provide a mechanism to use multiple IP datagrams for a single HIP
   packet, support for path MTU discovery does not bring any value to
   HIP in IPv4 networks.  HIP-aware NAT devices SHOULD perform IPv4
   reassembly/fragmentation for HIP packets.

   All HIP implementations have to be careful while employing a
   reassembly algorithm so that the algorithm is sufficiently resistant
   to DoS attacks.

   Certificate chains can cause the packet to be fragmented, and
   fragmentation can open implementations to denial-of-service attacks
   [KAU03].  "Hash and URL" schemes as defined in [RFC6253] for HIP
   version 1 may be used to avoid fragmentation and mitigate resulting
   DoS attacks.

5.2.  HIP Parameters

   The HIP parameters carry information that is necessary for
   establishing and maintaining a HIP association.  For example, the
   peer's public keys as well as the signaling for negotiating ciphers
   and payload handling are encapsulated in HIP parameters.  Additional
   information, meaningful for end hosts or middleboxes, may also be
   included in HIP parameters.  The specification of the HIP parameters
   and their mapping to HIP packets and packet types is flexible to
   allow HIP extensions to define new parameters and new protocol
   behavior.

   In HIP packets, HIP parameters are ordered according to their numeric
   type number and encoded in TLV format.
















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   The following parameter types are currently defined.

   +------------------------+-------+-----------+----------------------+
   | TLV                    | Type  | Length    | Data                 |
   +------------------------+-------+-----------+----------------------+
   | R1_COUNTER             | 129   | 12        | Puzzle generation    |
   |                        |       |           | counter              |
   |                        |       |           |                      |
   | PUZZLE                 | 257   | 12        | #K and Random #I     |
   |                        |       |           |                      |
   | SOLUTION               | 321   | 20        | #K, Random #I and    |
   |                        |       |           | puzzle solution #J   |
   |                        |       |           |                      |
   | SEQ                    | 385   | 4         | UPDATE packet ID     |
   |                        |       |           | number               |
   |                        |       |           |                      |
   | ACK                    | 449   | variable  | UPDATE packet ID     |
   |                        |       |           | number               |
   |                        |       |           |                      |
   | DH_GROUP_LIST          | 511   | variable  | Ordered list of DH   |
   |                        |       |           | Group IDs supported  |
   |                        |       |           | by a host            |
   |                        |       |           |                      |
   | DIFFIE_HELLMAN         | 513   | variable  | public key           |
   |                        |       |           |                      |
   | HIP_CIPHER             | 579   | variable  | List of HIP          |
   |                        |       |           | encryption           |
   |                        |       |           | algorithms           |
   |                        |       |           |                      |
   | ENCRYPTED              | 641   | variable  | Encrypted part of a  |
   |                        |       |           | HIP packet           |
   |                        |       |           |                      |
   | HOST_ID                | 705   | variable  | Host Identity with   |
   |                        |       |           | Fully Qualified      |
   |                        |       |           | Domain Name (FQDN)   |
   |                        |       |           | or Network Access    |
   |                        |       |           | Identifier (NAI)     |
   |                        |       |           |                      |
   | HIT_SUITE_LIST         | 715   | variable  | Ordered list of the  |
   |                        |       |           | HIT Suites supported |
   |                        |       |           | by the Responder     |
   |                        |       |           |                      |
   | CERT                   | 768   | variable  | HI Certificate; used |
   |                        |       |           | to transfer          |
   |                        |       |           | certificates.        |
   |                        |       |           | Specified in a       |
   |                        |       |           | separate document.   |
   |                        |       |           |                      |



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   | NOTIFICATION           | 832   | variable  | Informational data   |
   |                        |       |           |                      |
   | ECHO_REQUEST_SIGNED    | 897   | variable  | Opaque data to be    |
   |                        |       |           | echoed back; signed  |
   |                        |       |           |                      |
   | ECHO_RESPONSE_SIGNED   | 961   | variable  | Opaque data echoed   |
   |                        |       |           | back by request;     |
   |                        |       |           | signed               |
   |                        |       |           |                      |
   | TRANSPORT_FORMAT_LIST  | 2049  | Ordered   | variable             |
   |                        |       | list of   |                      |
   |                        |       | preferred |                      |
   |                        |       | HIP       |                      |
   |                        |       | transport |                      |
   |                        |       | type      |                      |
   |                        |       | numbers   |                      |
   |                        |       |           |                      |
   | HIP_MAC                | 61505 | variable  | HMAC-based message   |
   |                        |       |           | authentication code, |
   |                        |       |           | with key material    |
   |                        |       |           | from KEYMAT          |
   |                        |       |           |                      |
   | HIP_MAC_2              | 61569 | variable  | HMAC-based message   |
   |                        |       |           | authentication code, |
   |                        |       |           | with key material    |
   |                        |       |           | from KEYMAT.  Unlike |
   |                        |       |           | HIP_MAC, the HOST_ID |
   |                        |       |           | parameter is         |
   |                        |       |           | included in          |
   |                        |       |           | HIP_MAC_2            |
   |                        |       |           | calculation.         |
   |                        |       |           |                      |
   | HIP_SIGNATURE_2        | 61633 | variable  | Signature used in R1 |
   |                        |       |           | packet               |
   |                        |       |           |                      |
   | HIP_SIGNATURE          | 61697 | variable  | Signature of the     |
   |                        |       |           | packet               |
   |                        |       |           |                      |
   | ECHO_REQUEST_UNSIGNED  | 63661 | variable  | Opaque data to be    |
   |                        |       |           | echoed back; after   |
   |                        |       |           | signature            |
   |                        |       |           |                      |
   | ECHO_RESPONSE_UNSIGNED | 63425 | variable  | Opaque data echoed   |
   |                        |       |           | back by request;     |
   |                        |       |           | after signature      |
   +------------------------+-------+-----------+----------------------+





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   As the ordering (from lowest to highest) of HIP parameters is
   strictly enforced (see Section 5.2.1), the parameter type values for
   existing parameters have been spaced to allow for future protocol
   extensions.

   The following parameter type number ranges are defined.

   +---------------+---------------------------------------------------+
   | Type Range    | Purpose                                           |
   +---------------+---------------------------------------------------+
   | 0 -  1023     | Handshake                                         |
   |               |                                                   |
   | 1024 -   2047 | Reserved                                          |
   |               |                                                   |
   | 2048 -   4095 | Parameters related to HIP transport formats       |
   |               |                                                   |
   | 4096 -   8191 | Signed parameters allocated through specification |
   |               | documents                                         |
   |               |                                                   |
   | 8192 -  32767 | Reserved                                          |
   |               |                                                   |
   | 32768 - 49151 | Reserved for Private Use.  Signed parameters.     |
   |               |                                                   |
   | 49152 - 61439 | Reserved                                          |
   |               |                                                   |
   | 61440 - 62463 | Signatures and (signed) MACs                      |
   |               |                                                   |
   | 62464 - 63487 | Parameters that are neither signed nor MACed      |
   |               |                                                   |
   | 63488 - 64511 | Rendezvous and relaying                           |
   |               |                                                   |
   | 64512 - 65023 | Parameters that are neither signed nor MACed      |
   |               |                                                   |
   | 65024 - 65535 | Reserved                                          |
   +---------------+---------------------------------------------------+

   The process for defining new parameters is described in Section 5.2.2
   of this document.

   The range between 32768 (2^15) and 49151 (2^15 + 2^14) is Reserved
   for Private Use.  Types from this range SHOULD be selected in a
   random fashion to reduce the probability of collisions.

5.2.1.  TLV Format

   The TLV-encoded parameters are described in the following
   subsections.  The Type field value also describes the order of these
   fields in the packet.  The parameters MUST be included in the packet



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   so that their types form an increasing order.  If multiple parameters
   with the same type number are in one packet, the parameters with the
   same type MUST be consecutive in the packet.  If the order does not
   follow this rule, the packet is considered to be malformed and it
   MUST be discarded.

   Parameters using type values from 2048 up to 4095 are related to
   transport formats.  Currently, one transport format is defined: the
   ESP transport format [RFC7402].

   All of the encoded TLV parameters have a length (that includes the
   Type and Length fields), which is a multiple of 8 bytes.  When
   needed, padding MUST be added to the end of the parameter so that the
   total length is a multiple of 8 bytes.  This rule ensures proper
   alignment of data.  Any added padding bytes MUST be zeroed by the
   sender, and their values SHOULD NOT be checked by the receiver.

   The Length field indicates the length of the Contents field (in
   bytes).  Consequently, the total length of the TLV parameter
   (including Type, Length, Contents, and Padding) is related to the
   Length field according to the following formula:

   Total Length = 11 + Length - (Length + 3) % 8;

   where % is the modulo operator.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type            |C|             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     /                          Contents                             /
     /                                               +-+-+-+-+-+-+-+-+
     |                                               |    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type         Type code for the parameter.  16 bits long, C-bit
                  being part of the Type code.
     C            Critical.  One if this parameter is critical and
                  MUST be recognized by the recipient, zero otherwise.
                  The C-bit is considered to be a part of the Type
                  field.  Consequently, critical parameters are always
                  odd, and non-critical ones have an even value.
     Length       Length of the Contents, in bytes, excluding Type,
                  Length, and Padding
     Contents     Parameter specific, defined by Type
     Padding      Padding, 0-7 bytes, added if needed



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   Critical parameters (indicated by the odd type number value) MUST be
   recognized by the recipient.  If a recipient encounters a critical
   parameter that it does not recognize, it MUST NOT process the packet
   any further.  It MAY send an ICMP or NOTIFY, as defined in
   Section 4.3.

   Non-critical parameters MAY be safely ignored.  If a recipient
   encounters a non-critical parameter that it does not recognize, it
   SHOULD proceed as if the parameter was not present in the received
   packet.

5.2.2.  Defining New Parameters

   Future specifications may define new parameters as needed.  When
   defining new parameters, care must be taken to ensure that the
   parameter type values are appropriate and leave suitable space for
   other future extensions.  One must remember that the parameters MUST
   always be arranged in numerically increasing order by Type code,
   thereby limiting the order of parameters (see Section 5.2.1).

   The following rules MUST be followed when defining new parameters.

   1.  The low-order bit C of the Type code is used to distinguish
       between critical and non-critical parameters.  Hence, even
       parameter type numbers indicate non-critical parameters while odd
       parameter type numbers indicate critical parameters.

   2.  A new parameter MAY be critical only if an old implementation
       that ignored it would cause security problems.  In general, new
       parameters SHOULD be defined as non-critical, and expect a reply
       from the recipient.

   3.  If a system implements a new critical parameter, it MUST provide
       the ability to set the associated feature off, such that the
       critical parameter is not sent at all.  The configuration option
       MUST be well documented.  Implementations operating in a mode
       adhering to this specification MUST disable the sending of new
       critical parameters by default.  In other words, the management
       interface MUST allow vanilla standards-only mode as a default
       configuration setting, and MAY allow new critical payloads to be
       configured on (and off).

   4.  See Section 9 for allocation rules regarding Type codes.








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5.2.3.  R1_COUNTER

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Reserved, 4 bytes                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                R1 generation counter, 8 bytes                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           129
     Length         12
     R1 generation
       counter      The current generation of valid puzzles

   The R1_COUNTER parameter contains a 64-bit unsigned integer in
   network byte order, indicating the current generation of valid
   puzzles.  The sender SHOULD increment this counter periodically.  It
   is RECOMMENDED that the counter value is incremented at least as
   often as old PUZZLE values are deprecated so that SOLUTIONs to them
   are no longer accepted.

   Support for the R1_COUNTER parameter is mandatory, although its
   inclusion in the R1 packet is optional.  It SHOULD be included in the
   R1 (in which case it is covered by the signature), and if present in
   the R1, it MUST be echoed (including the Reserved field verbatim) by
   the Initiator in the I2 packet.





















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5.2.4.  PUZZLE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  #K, 1 byte   |    Lifetime   |        Opaque, 2 bytes        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Random #I, RHASH_len / 8 bytes           |
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           257
     Length         4 + RHASH_len / 8
     #K             #K is the number of verified bits
     Lifetime       puzzle lifetime 2^(value - 32) seconds
     Opaque         data set by the Responder, indexing the puzzle
     Random #I      random number of size RHASH_len bits

   Random #I is represented as an n-bit integer (where n is RHASH_len),
   and #K and Lifetime as 8-bit integers, all in network byte order.

   The PUZZLE parameter contains the puzzle difficulty #K and an n-bit
   random integer #I.  The Puzzle Lifetime indicates the time during
   which the puzzle solution is valid, and sets a time limit that should
   not be exceeded by the Initiator while it attempts to solve the
   puzzle.  The lifetime is indicated as a power of 2 using the formula
   2^(Lifetime - 32) seconds.  A puzzle MAY be augmented with an
   ECHO_REQUEST_SIGNED or an ECHO_REQUEST_UNSIGNED parameter included in
   the R1; the contents of the echo request are then echoed back in the
   ECHO_RESPONSE_SIGNED or in the ECHO_RESPONSE_UNSIGNED parameter,
   allowing the Responder to use the included information as a part of
   its puzzle processing.

   The Opaque and Random #I fields are not covered by the
   HIP_SIGNATURE_2 parameter.














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5.2.5.  SOLUTION

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  #K, 1 byte   |   Reserved    |        Opaque, 2 bytes        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Random #I, n bytes                       |
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Puzzle solution #J, RHASH_len / 8 bytes            |
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type                321
     Length              4 + RHASH_len / 4
     #K                  #K is the number of verified bits
     Reserved            zero when sent, ignored when received
     Opaque              copied unmodified from the received PUZZLE
                         parameter
     Random #I           random number of size RHASH_len bits
     Puzzle solution #J  random number of size RHASH_len bits

   Random #I and Random #J are represented as n-bit unsigned integers
   (where n is RHASH_len), and #K as an 8-bit unsigned integer, all in
   network byte order.

   The SOLUTION parameter contains a solution to a puzzle.  It also
   echoes back the random difficulty #K, the Opaque field, and the
   puzzle integer #I.



















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5.2.6.  DH_GROUP_LIST

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | DH GROUP ID #1| DH GROUP ID #2| DH GROUP ID #3| DH GROUP ID #4|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | DH GROUP ID #n|                Padding                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           511
     Length         number of DH Group IDs
     DH GROUP ID    identifies a DH GROUP ID supported by the host.
                    The list of IDs is ordered by preference of the
                    host.  The possible DH Group IDs are defined
                    in the DIFFIE_HELLMAN parameter.  Each DH
                    Group ID is one octet long.

   The DH_GROUP_LIST parameter contains the list of supported DH Group
   IDs of a host.  The Initiator sends the DH_GROUP_LIST in the I1
   packet, and the Responder sends its own list in the signed part of
   the R1 packet.  The DH Group IDs in the DH_GROUP_LIST are listed in
   the order of their preference of the host sending the list.  DH Group
   IDs that are listed first are preferred over the DH Group IDs listed
   later.  The information in the DH_GROUP_LIST allows the Responder to
   select the DH group preferred by itself and supported by the
   Initiator.  Based on the DH_GROUP_LIST in the R1 packet, the
   Initiator can determine if the Responder has selected the best
   possible choice based on the Initiator's and Responder's preferences.
   If the Responder's choice differs from the best choice, the Initiator
   can conclude that there was an attempted downgrade attack (see
   Section 4.1.7).

   When selecting the DH group for the DIFFIE_HELLMAN parameter in the
   R1 packet, the Responder MUST select the first DH Group ID in its
   DH_GROUP_LIST in the R1 packet that is compatible with one of the
   Suite IDs in the Initiator's DH_GROUP_LIST in the I1 packet.  The
   Responder MUST NOT select any other DH Group ID that is contained in
   both lists, because then a downgrade attack cannot be detected.

   In general, hosts SHOULD prefer stronger groups over weaker ones if
   the computation overhead is not prohibitively high for the intended
   application.






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5.2.7.  DIFFIE_HELLMAN

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Group ID    |      Public Value Length      | Public Value  /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           513
     Length         length in octets, excluding Type, Length, and
                    Padding
     Group ID       identifies values for p and g as well as the KDF
     Public Value   length of the following Public Value in octets
       Length
     Public Value   the sender's public Diffie-Hellman key

   A single DIFFIE_HELLMAN parameter may be included in selected HIP
   packets based on the DH Group ID selected (Section 5.2.6).  The
   following Group IDs have been defined; values are assigned by this
   document:

    Group                              KDF              Value

    Reserved                                            0
    DEPRECATED                                          1
    DEPRECATED                                          2
    1536-bit MODP group  [RFC3526]     HKDF [RFC5869]   3
    3072-bit MODP group  [RFC3526]     HKDF [RFC5869]   4
    DEPRECATED                                          5
    DEPRECATED                                          6
    NIST P-256 [RFC5903]               HKDF [RFC5869]   7
    NIST P-384 [RFC5903]               HKDF [RFC5869]   8
    NIST P-521 [RFC5903]               HKDF [RFC5869]   9
    SECP160R1  [SECG]                  HKDF [RFC5869]  10
    2048-bit MODP group  [RFC3526]     HKDF [RFC5869]  11

   The MODP Diffie-Hellman groups are defined in [RFC3526].  ECDH
   groups 7-9 are defined in [RFC5903] and [RFC6090].  ECDH group 10
   is covered in Appendix D.  Any ECDH used with HIP MUST have a
   co-factor of 1.





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   The Group ID also defines the key derivation function that is to be
   used for deriving the symmetric keys for the HMAC and symmetric
   encryption from the keying material from the Diffie-Hellman key
   exchange (see Section 6.5).

   A HIP implementation MUST implement Group ID 3.  The 160-bit
   SECP160R1 group can be used when lower security is enough (e.g., web
   surfing) and when the equipment is not powerful enough (e.g., some
   PDAs).  Implementations SHOULD implement Group IDs 4 and 8.

   To avoid unnecessary failures during the base exchange, the rest of
   the groups SHOULD be implemented in hosts where resources are
   adequate.

5.2.8.  HIP_CIPHER

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Cipher ID #1         |          Cipher ID #2         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Cipher ID #n         |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           579
     Length         length in octets, excluding Type, Length, and
                    Padding
     Cipher ID      identifies the cipher algorithm to be used for
                    encrypting the contents of the ENCRYPTED parameter




















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   The following Cipher IDs are defined:

        Suite ID           Value

        RESERVED           0
        NULL-ENCRYPT       1     ([RFC2410])
        AES-128-CBC        2     ([RFC3602])
        RESERVED           3     (unused value)
        AES-256-CBC        4     ([RFC3602])

   The sender of a HIP_CIPHER parameter MUST make sure that there are no
   more than six (6) Cipher IDs in one HIP_CIPHER parameter.

   Conversely, a recipient MUST be prepared to handle received transport
   parameters that contain more than six Cipher IDs by accepting the
   first six Cipher IDs and dropping the rest.  The limited number of
   Cipher IDs sets the maximum size of the HIP_CIPHER parameter.  As the
   default configuration, the HIP_CIPHER parameter MUST contain at least
   one of the mandatory Cipher IDs.  There MAY be a configuration option
   that allows the administrator to override this default.

   The Responder lists supported and desired Cipher IDs in order of
   preference in the R1, up to the maximum of six Cipher IDs.  The
   Initiator MUST choose only one of the corresponding Cipher IDs.  This
   Cipher ID will be used for generating the ENCRYPTED parameter.

   Mandatory implementation: AES-128-CBC.  Implementors SHOULD support
   NULL-ENCRYPT for testing/debugging purposes but MUST NOT offer or
   accept this value unless explicitly configured for testing/debugging
   of HIP.





















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5.2.9.  HOST_ID

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          HI Length            |DI-Type|      DI Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Algorithm            |         Host Identity         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |       Domain Identifier       /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                               |    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type               705
     Length             length in octets, excluding Type, Length, and
                        Padding
     HI Length          length of the Host Identity in octets
     DI-Type            type of the following Domain Identifier field
     DI Length          length of the Domain Identifier field in octets
     Algorithm          index to the employed algorithm
     Host Identity      actual Host Identity
     Domain Identifier  the identifier of the sender

   The following DI-Types have been defined:

         Type                    Value

         none included           0
         FQDN                    1
         NAI                     2

         FQDN            Fully Qualified Domain Name, in binary format
         NAI             Network Access Identifier

   The format for the FQDN is defined in RFC 1035 [RFC1035],
   Section 3.1.  The format for the NAI is defined in [RFC4282].

   A host MAY optionally associate the Host Identity with a single
   Domain Identifier in the HOST_ID parameter.  If there is no Domain
   Identifier, i.e., the DI-Type field is zero, the DI Length field is
   set to zero as well.







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   The following HI Algorithms have been defined:

        Algorithm profiles   Values

        RESERVED             0
        DSA                  3 [FIPS.186-4.2013]  (RECOMMENDED)
        RSA                  5 [RFC3447]          (REQUIRED)
        ECDSA                7 [RFC4754]          (REQUIRED)
        ECDSA_LOW            9 [SECG]             (RECOMMENDED)

   For DSA, RSA, and ECDSA key types, profiles containing at least
   112 bits of security strength (as defined by [NIST.800-131A.2011])
   should be used.  For RSA signature padding, the Probabilistic
   Signature Scheme (PSS) method of padding [RFC3447] MUST be used.

   The Host Identity is derived from the DNSKEY format for RSA and DSA.
   For these, the Public Key field of the RDATA part from RFC 4034
   [RFC4034] is used.  For Elliptic Curve Cryptography (ECC), we
   distinguish two different profiles: ECDSA and ECDSA_LOW.  ECC
   contains curves approved by NIST and defined in RFC 4754 [RFC4754].
   ECDSA_LOW is defined for devices with low computational capabilities
   and uses shorter curves from the Standards for Efficient Cryptography
   Group [SECG].  Any ECDSA used with HIP MUST have a co-factor of 1.

   For ECDSA and ECDSA_LOW, Host Identities are represented by the
   following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          ECC Curve            |                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                         Public Key                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     ECC Curve     Curve label
     Public Key    Represented in octet-string format [RFC6090]

   For hosts that implement ECDSA as the algorithm, the following ECC
   curves are required:

        Algorithm    Curve            Values

        ECDSA        RESERVED         0
        ECDSA        NIST P-256       1 [RFC4754]
        ECDSA        NIST P-384       2 [RFC4754]





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   For hosts that implement the ECDSA_LOW algorithm profile, the
   following curve is required:

        Algorithm    Curve            Values

        ECDSA_LOW    RESERVED         0
        ECDSA_LOW    SECP160R1        1 [SECG]

5.2.10.  HIT_SUITE_LIST

   The HIT_SUITE_LIST parameter contains a list of the supported HIT
   Suite IDs of the Responder.  The Responder sends the HIT_SUITE_LIST
   in the signed part of the R1 packet.  Based on the HIT_SUITE_LIST,
   the Initiator can determine which source HIT Suite IDs are supported
   by the Responder.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     ID #1     |     ID #2     |     ID #3     |     ID #4     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     ID #n     |                Padding                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           715
     Length         number of HIT Suite IDs
     ID             identifies a HIT Suite ID supported by the host.
                    The list of IDs is ordered by preference of the
                    host.  Each HIT Suite ID is one octet long.  The
                    four higher-order bits of the ID field correspond
                    to the HIT Suite ID in the ORCHID OGA ID field.  The
                    four lower-order bits are reserved and set to 0
                    by the sender.  The reception of an ID with the
                    four lower-order bits not set to 0 SHOULD be
                    considered as an error that MAY result in a
                    NOTIFICATION of type UNSUPPORTED_HIT_SUITE.

   The HIT Suite ID indexes a HIT Suite.  HIT Suites are composed of
   signature algorithms as defined in Section 5.2.9, and hash functions.

   The ID field in the HIT_SUITE_LIST is defined as an eight-bit field,
   as opposed to the four-bit HIT Suite ID and OGA ID field in the
   ORCHID.  This difference is a measure to accommodate larger HIT Suite
   IDs if the 16 available values prove insufficient.  In that case, one
   of the 16 values, zero, will be used to indicate that four additional
   bits of the ORCHID will be used to encode the HIT Suite ID.  Hence,



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   the current four-bit HIT Suite IDs only use the four higher-order
   bits in the ID field.  Future documents may define the use of the
   four lower-order bits in the ID field.

   The following HIT Suite IDs are defined, and the relationship between
   the four-bit ID value used in the OGA ID field and the eight-bit
   encoding within the HIT_SUITE_LIST ID field is clarified:

        HIT Suite       Four-bit ID    Eight-bit encoding

        RESERVED            0             0x00
        RSA,DSA/SHA-256     1             0x10           (REQUIRED)
        ECDSA/SHA-384       2             0x20           (RECOMMENDED)
        ECDSA_LOW/SHA-1     3             0x30           (RECOMMENDED)

   The following table provides more detail on the above HIT Suite
   combinations.  The input for each generation algorithm is the
   encoding of the HI as defined in Section 3.2.  The output is 96 bits
   long and is directly used in the ORCHID.

   +-------+----------+--------------+------------+--------------------+
   | Index | Hash     | HMAC         | Signature  | Description        |
   |       | function |              | algorithm  |                    |
   |       |          |              | family     |                    |
   +-------+----------+--------------+------------+--------------------+
   |     0 |          |              |            | Reserved           |
   |       |          |              |            |                    |
   |     1 | SHA-256  | HMAC-SHA-256 | RSA, DSA   | RSA or DSA HI      |
   |       |          |              |            | hashed with        |
   |       |          |              |            | SHA-256, truncated |
   |       |          |              |            | to 96 bits         |
   |       |          |              |            |                    |
   |     2 | SHA-384  | HMAC-SHA-384 | ECDSA      | ECDSA HI hashed    |
   |       |          |              |            | with SHA-384,      |
   |       |          |              |            | truncated to 96    |
   |       |          |              |            | bits               |
   |       |          |              |            |                    |
   |     3 | SHA-1    | HMAC-SHA-1   | ECDSA_LOW  | ECDSA_LOW HI       |
   |       |          |              |            | hashed with SHA-1, |
   |       |          |              |            | truncated to 96    |
   |       |          |              |            | bits               |
   +-------+----------+--------------+------------+--------------------+

                           Table 10: HIT Suites







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   The hash of the Responder as defined in the HIT Suite determines the
   HMAC to be used for the RHASH function.  The HMACs currently defined
   here are HMAC-SHA-256 [RFC4868], HMAC-SHA-384 [RFC4868], and
   HMAC-SHA-1 [RFC2404].

5.2.11.  TRANSPORT_FORMAT_LIST

   The TRANSPORT_FORMAT_LIST parameter contains a list of the supported
   HIP transport formats (TFs) of the Responder.  The Responder sends
   the TRANSPORT_FORMAT_LIST in the signed part of the R1 packet.  Based
   on the TRANSPORT_FORMAT_LIST, the Initiator chooses one suitable
   transport format and includes the respective HIP transport format
   parameter in its response packet.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          TF type #1           |           TF type #2          /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /          TF type #n           |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           2049
     Length         2x number of TF types
     TF Type        identifies a transport format (TF) type supported
                    by the host.  The TF type numbers correspond to
                    the HIP parameter type numbers of the respective
                    transport format parameters.  The list of TF types
                    is ordered by preference of the sender.

   The TF type numbers index the respective HIP parameters for the
   transport formats in the type number range between 2050 and 4095.
   The parameters and their use are defined in separate documents.
   Currently, the only transport format defined is IPsec ESP [RFC7402].

   For each listed TF type, the sender of the TRANSPORT_FORMAT_LIST
   parameter MUST include the respective transport format parameter in
   the HIP packet.  The receiver MUST ignore the TF type in the
   TRANSPORT_FORMAT_LIST if no matching transport format parameter is
   present in the packet.









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5.2.12.  HIP_MAC

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                             HMAC                              |
     /                                                               /
     /                               +-------------------------------+
     |                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           61505
     Length         length in octets, excluding Type, Length, and
                    Padding
     HMAC           HMAC computed over the HIP packet, excluding the
                    HIP_MAC parameter and any following parameters,
                    such as HIP_SIGNATURE, HIP_SIGNATURE_2,
                    ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED.
                    The Checksum field MUST be set to zero, and the
                    HIP header length in the HIP common header MUST be
                    calculated not to cover any excluded parameters
                    when the HMAC is calculated.  The size of the
                    HMAC is the natural size of the hash computation
                    output depending on the used hash function.

   The HMAC uses RHASH as the hash algorithm.  The calculation and
   verification process is presented in Section 6.4.1.

5.2.13.  HIP_MAC_2

   HIP_MAC_2 is a MAC of the packet and the HI of the sender in the form
   of a HOST_ID parameter when that parameter is not actually included
   in the packet.  The parameter structure is the same as the structure
   shown in Section 5.2.12.  The fields are as follows:

     Type           61569
     Length         length in octets, excluding Type, Length, and
                    Padding
     HMAC           HMAC computed over the HIP packet, excluding the
                    HIP_MAC_2 parameter and any following parameters
                    such as HIP_SIGNATURE, HIP_SIGNATURE_2,
                    ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED,
                    and including an additional sender's HOST_ID
                    parameter during the HMAC calculation.  The
                    Checksum field MUST be set to zero, and the HIP



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                    header length in the HIP common header MUST be
                    calculated not to cover any excluded parameters
                    when the HMAC is calculated.  The size of the
                    HMAC is the natural size of the hash computation
                    output depending on the used hash function.

   The HMAC uses RHASH as the hash algorithm.  The calculation and
   verification process is presented in Section 6.4.1.

5.2.14.  HIP_SIGNATURE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SIG alg                    |            Signature          /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           61697
     Length         length in octets, excluding Type, Length, and
                    Padding
     SIG alg        signature algorithm
     Signature      the signature is calculated over the HIP packet,
                    excluding the HIP_SIGNATURE parameter and any
                    parameters that follow the HIP_SIGNATURE
                    parameter.  When the signature is calculated, the
                    Checksum field MUST be set to zero, and the HIP
                    header length in the HIP common header MUST be
                    calculated only up to the beginning of the
                    HIP_SIGNATURE parameter.

   The signature algorithms are defined in Section 5.2.9.  The signature
   in the Signature field is encoded using the method depending on the
   signature algorithm (e.g., according to [RFC3110] in the case of RSA/
   SHA-1, [RFC5702] in the case of RSA/SHA-256, [RFC2536] in the case of
   DSA, or [RFC6090] in the case of ECDSA).

   HIP_SIGNATURE calculation and verification follow the process defined
   in Section 6.4.2.









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5.2.15.  HIP_SIGNATURE_2

   HIP_SIGNATURE_2 excludes the variable parameters in the R1 packet to
   allow R1 pre-creation.  The parameter structure is the same as the
   structure shown in Section 5.2.14.  The fields are as follows:

     Type           61633
     Length         length in octets, excluding Type, Length, and
                    Padding
     SIG alg        signature algorithm
     Signature      Within the R1 packet that contains the
                    HIP_SIGNATURE_2 parameter, the Initiator's HIT, the
                    Checksum field, and the Opaque and Random #I fields
                    in the PUZZLE parameter MUST be set to zero while
                    computing the HIP_SIGNATURE_2 signature.  Further,
                    the HIP packet length in the HIP header MUST be
                    adjusted as if the HIP_SIGNATURE_2 was not in the
                    packet during the signature calculation, i.e., the
                    HIP packet length points to the beginning of
                    the HIP_SIGNATURE_2 parameter during signing and
                    verification.

   Zeroing the Initiator's HIT makes it possible to create R1 packets
   beforehand, to minimize the effects of possible DoS attacks.  Zeroing
   the Random #I and Opaque fields within the PUZZLE parameter allows
   these fields to be populated dynamically on precomputed R1s.

   Signature calculation and verification follow the process defined in
   Section 6.4.2.

5.2.16.  SEQ

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Update ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type            385
     Length          4
     Update ID       32-bit sequence number

   The Update ID is an unsigned number in network byte order,
   initialized by a host to zero upon moving to ESTABLISHED state.  The
   Update ID has scope within a single HIP association, and not across




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   multiple associations or multiple hosts.  The Update ID is
   incremented by one before each new UPDATE that is sent by the host;
   the first UPDATE packet originated by a host has an Update ID of 0.

5.2.17.  ACK

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       peer Update ID 1                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                       peer Update ID n                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type             449
     Length           length in octets, excluding Type and Length
     peer Update ID   32-bit sequence number corresponding to the
                      Update ID being ACKed

   The ACK parameter includes one or more Update IDs that have been
   received from the peer.  The number of peer Update IDs can be
   inferred from the length by dividing it by 4.

5.2.18.  ENCRYPTED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              IV                               /
     /                                                               /
     /                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
     /                        Encrypted data                         /
     /                                                               /
     /                               +-------------------------------+
     /                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           641
     Length         length in octets, excluding Type, Length, and
                    Padding
     Reserved       zero when sent, ignored when received



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     IV             Initialization vector, if needed, otherwise
                    nonexistent.  The length of the IV is inferred from
                    the HIP_CIPHER.
     Encrypted      The data is encrypted using the encryption algorithm
       data         defined in the HIP_CIPHER parameter.

   The ENCRYPTED parameter encapsulates other parameters, the encrypted
   data, which holds one or more HIP parameters in block encrypted form.

   Consequently, the first fields in the encapsulated parameter(s) are
   Type and Length of the first such parameter, allowing the contents to
   be easily parsed after decryption.

   The field labeled "Encrypted data" consists of the output of one or
   more HIP parameters concatenated together that have been passed
   through an encryption algorithm.  Each of these inner parameters is
   padded according to the rules of Section 5.2.1 for padding individual
   parameters.  As a result, the concatenated parameters will be a block
   of data that is 8-byte aligned.

   Some encryption algorithms require that the data to be encrypted must
   be a multiple of the cipher algorithm block size.  In this case, the
   above block of data MUST include additional padding, as specified by
   the encryption algorithm.  The size of the extra padding is selected
   so that the length of the unencrypted data block is a multiple of the
   cipher block size.  The encryption algorithm may specify padding
   bytes other than zero; for example, AES [FIPS.197.2001] uses the
   PKCS5 padding scheme (see Section 6.1.1 of [RFC2898]) where the
   remaining n bytes to fill the block each have the value of n.  This
   yields an "unencrypted data" block that is transformed to an
   "encrypted data" block by the cipher suite.  This extra padding added
   to the set of parameters to satisfy the cipher block alignment rules
   is not counted in HIP TLV Length fields, and this extra padding
   should be removed by the cipher suite upon decryption.

   Note that the length of the cipher suite output may be smaller or
   larger than the length of the set of parameters to be encrypted,
   since the encryption process may compress the data or add additional
   padding to the data.

   Once this encryption process is completed, the Encrypted data field
   is ready for inclusion in the parameter.  If necessary, additional
   Padding for 8-byte alignment is then added according to the rules of
   Section 5.2.1.







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5.2.19.  NOTIFICATION

   The NOTIFICATION parameter is used to transmit informational data,
   such as error conditions and state transitions, to a HIP peer.  A
   NOTIFICATION parameter may appear in NOTIFY packets.  The use of the
   NOTIFICATION parameter in other packet types is for further study.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Reserved             |      Notify Message Type      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               /
     /                   Notification Data                           /
     /                                               +---------------+
     /                                               |     Padding   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type             832
     Length           length in octets, excluding Type, Length, and
                      Padding
     Reserved         zero when sent, ignored when received
     Notify Message   specifies the type of notification
       Type
     Notification     informational or error data transmitted in
       Data           addition to the Notify Message Type.  Values
                      for this field are type specific (see below).

   Notification information can be error messages specifying why a HIP
   Security Association could not be established.  It can also be status
   data that a HIP implementation wishes to communicate with a peer
   process.  The table below lists the notification messages and their
   Notify Message Types.  HIP packets MAY contain multiple NOTIFICATION
   parameters if several problems exist or several independent pieces of
   information must be transmitted.

   To avoid certain types of attacks, a Responder SHOULD avoid sending a
   NOTIFICATION to any host with which it has not successfully verified
   a puzzle solution.

   Notify Message Types in the range 0-16383 are intended for reporting
   errors, and those in the range 16384-65535 are for other status
   information.  An implementation that receives a NOTIFY packet with a
   Notify Message Type that indicates an error in response to a request





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   packet (e.g., I1, I2, UPDATE) SHOULD assume that the corresponding
   request has failed entirely.  Unrecognized error types MUST be
   ignored, except that they SHOULD be logged.

   As currently defined, Notify Message Type values 1-10 are used for
   informing about errors in packet structures, and values 11-20 for
   informing about problems in parameters.

   Notification Data in NOTIFICATION parameters where the Notify Message
   Type is in the status range MUST be ignored if not recognized.

     Notify Message Types - Errors             Value
     -----------------------------             -----

     UNSUPPORTED_CRITICAL_PARAMETER_TYPE        1

       Sent if the parameter type has the "critical" bit set and the
       parameter type is not recognized.  Notification Data contains the
       two-octet parameter type.

     INVALID_SYNTAX                             7

       Indicates that the HIP message received was invalid because some
       type, length, or value was out of range or because the request
       was otherwise malformed.  To avoid a denial-of-service
       attack using forged messages, this status may only be returned
       for packets whose HIP_MAC (if present) and SIGNATURE have been
       verified.  This status MUST be sent in response to any error not
       covered by one of the other status types and SHOULD NOT contain
       details, to avoid leaking information to someone probing a node.
       To aid debugging, more detailed error information SHOULD be
       written to a console or log.

     NO_DH_PROPOSAL_CHOSEN                     14

       None of the proposed Group IDs were acceptable.

     INVALID_DH_CHOSEN                         15

       The DH Group ID field does not correspond to one offered
       by the Responder.

     NO_HIP_PROPOSAL_CHOSEN                    16

       None of the proposed HIT Suites or HIP Encryption Algorithms were
       acceptable.





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     INVALID_HIP_CIPHER_CHOSEN                 17

       The HIP_CIPHER Crypto ID does not correspond to one offered by
       the Responder.

     UNSUPPORTED_HIT_SUITE                     20

       Sent in response to an I1 or R1 packet for which the HIT Suite
       is not supported.

     AUTHENTICATION_FAILED                     24

       Sent in response to a HIP signature failure, except when
       the signature verification fails in a NOTIFY message.

     CHECKSUM_FAILED                           26

       Sent in response to a HIP checksum failure.

     HIP_MAC_FAILED                            28

       Sent in response to a HIP HMAC failure.

     ENCRYPTION_FAILED                         32

       The Responder could not successfully decrypt the
       ENCRYPTED parameter.

     INVALID_HIT                               40

       Sent in response to a failure to validate the peer's
       HIT from the corresponding HI.

     BLOCKED_BY_POLICY                         42

       The Responder is unwilling to set up an association
       for some policy reason (e.g., the received HIT is NULL
       and the policy does not allow opportunistic mode).

     RESPONDER_BUSY_PLEASE_RETRY               44

       The Responder is unwilling to set up an association, as it is
       suffering under some kind of overload and has chosen to shed load
       by rejecting the Initiator's request.  The Initiator may retry;
       however, the Initiator MUST find another (different) puzzle
       solution for any such retries.  Note that the Initiator may need
       to obtain a new puzzle with a new I1/R1 exchange.




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     Notify Message Types - Status            Value
     -----------------------------            -----

     I2_ACKNOWLEDGEMENT                       16384

       The Responder has an I2 packet from the Initiator but had to
       queue the I2 packet for processing.  The puzzle was correctly
       solved, and the Responder is willing to set up an association but
       currently has a number of I2 packets in the processing queue.
       The R2 packet is sent after the I2 packet was processed.

5.2.20.  ECHO_REQUEST_SIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type          897
     Length        length of the opaque data in octets
     Opaque data   opaque data, supposed to be meaningful only to
                   the node that sends ECHO_REQUEST_SIGNED and
                   receives a corresponding ECHO_RESPONSE_SIGNED or
                   ECHO_RESPONSE_UNSIGNED

   The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data
   that the sender wants to get echoed back in the corresponding reply
   packet.

   The ECHO_REQUEST_SIGNED and corresponding echo response parameters
   MAY be used for any purpose where a node wants to carry some state in
   a request packet and get it back in a response packet.  The
   ECHO_REQUEST_SIGNED is covered by the HIP_MAC and SIGNATURE.  A HIP
   packet can contain only one ECHO_REQUEST_SIGNED parameter and MAY
   contain multiple ECHO_REQUEST_UNSIGNED parameters.  The
   ECHO_REQUEST_SIGNED parameter MUST be responded to with an
   ECHO_RESPONSE_SIGNED.











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5.2.21.  ECHO_REQUEST_UNSIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type          63661
     Length        length of the opaque data in octets
     Opaque data   opaque data, supposed to be meaningful only to
                   the node that sends ECHO_REQUEST_UNSIGNED and
                   receives a corresponding ECHO_RESPONSE_UNSIGNED

   The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data
   that the sender wants to get echoed back in the corresponding reply
   packet.

   The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters
   MAY be used for any purpose where a node wants to carry some state in
   a request packet and get it back in a response packet.  The
   ECHO_REQUEST_UNSIGNED is not covered by the HIP_MAC and SIGNATURE.  A
   HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters.
   It is possible that middleboxes add ECHO_REQUEST_UNSIGNED parameters
   in HIP packets passing by.  The creator of the ECHO_REQUEST_UNSIGNED
   (end host or middlebox) has to create the Opaque field so that it can
   later identify and remove the corresponding ECHO_RESPONSE_UNSIGNED
   parameter.

   The ECHO_REQUEST_UNSIGNED parameter MUST be responded to with an
   ECHO_RESPONSE_UNSIGNED parameter.


















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5.2.22.  ECHO_RESPONSE_SIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type          961
     Length        length of the opaque data in octets
     Opaque data   opaque data, copied unmodified from the
                   ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                   parameter that triggered this response

   The ECHO_RESPONSE_SIGNED parameter contains an opaque blob of data
   that the sender of the ECHO_REQUEST_SIGNED wants to get echoed back.
   The opaque data is copied unmodified from the ECHO_REQUEST_SIGNED
   parameter.

   The ECHO_REQUEST_SIGNED and ECHO_RESPONSE_SIGNED parameters MAY be
   used for any purpose where a node wants to carry some state in a
   request packet and get it back in a response packet.  The
   ECHO_RESPONSE_SIGNED is covered by the HIP_MAC and SIGNATURE.

5.2.23.  ECHO_RESPONSE_UNSIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type          63425
     Length        length of the opaque data in octets
     Opaque data   opaque data, copied unmodified from the
                   ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                   parameter that triggered this response

   The ECHO_RESPONSE_UNSIGNED parameter contains an opaque blob of data
   that the sender of the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
   wants to get echoed back.  The opaque data is copied unmodified from
   the corresponding echo request parameter.





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   The echo request and ECHO_RESPONSE_UNSIGNED parameters MAY be used
   for any purpose where a node wants to carry some state in a request
   packet and get it back in a response packet.  The
   ECHO_RESPONSE_UNSIGNED is not covered by the HIP_MAC and SIGNATURE.

5.3.  HIP Packets

   There are eight basic HIP packets (see Table 11).  Four are for the
   HIP base exchange, one is for updating, one is for sending
   notifications, and two are for closing a HIP association.  Support
   for the NOTIFY packet type is optional, but support for all other HIP
   packet types listed below is mandatory.

   +------------------+------------------------------------------------+
   |   Packet type    | Packet name                                    |
   +------------------+------------------------------------------------+
   |        1         | I1 - the HIP Initiator Packet                  |
   |                  |                                                |
   |        2         | R1 - the HIP Responder Packet                  |
   |                  |                                                |
   |        3         | I2 - the Second HIP Initiator Packet           |
   |                  |                                                |
   |        4         | R2 - the Second HIP Responder Packet           |
   |                  |                                                |
   |        16        | UPDATE - the HIP Update Packet                 |
   |                  |                                                |
   |        17        | NOTIFY - the HIP Notify Packet                 |
   |                  |                                                |
   |        18        | CLOSE - the HIP Association Closing Packet     |
   |                  |                                                |
   |        19        | CLOSE_ACK - the HIP Closing Acknowledgment     |
   |                  | Packet                                         |
   +------------------+------------------------------------------------+

               Table 11: HIP Packets and Packet Type Values

   Packets consist of the fixed header as described in Section 5.1,
   followed by the parameters.  The parameter part, in turn, consists of
   zero or more TLV-coded parameters.

   In addition to the base packets, other packet types may be defined
   later in separate specifications.  For example, support for mobility
   and multihoming is not included in this specification.

   See "Notation" (Section 2.2) for the notation used in the operations.






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   In the future, an optional upper-layer payload MAY follow the HIP
   header.  The Next Header field in the header indicates if there is
   additional data following the HIP header.  The HIP packet, however,
   MUST NOT be fragmented into multiple extension headers by setting the
   Next Header field in a HIP header to the HIP protocol number.  This
   limits the size of the possible additional data in the packet.

5.3.1.  I1 - the HIP Initiator Packet

   The HIP header values for the I1 packet:

     Header:
       Packet Type = 1
       SRC HIT = Initiator's HIT
       DST HIT = Responder's HIT, or NULL

     IP ( HIP ( DH_GROUP_LIST ) )

   The I1 packet contains the fixed HIP header and the Initiator's
   DH_GROUP_LIST.

   Valid control bits: None

   The Initiator receives the Responder's HIT from either a DNS lookup
   of the Responder's FQDN (see [HIP-DNS-EXT]), some other repository,
   or a local table.  If the Initiator does not know the Responder's
   HIT, it may attempt to use opportunistic mode by using NULL (all
   zeros) as the Responder's HIT.  See also "HIP Opportunistic Mode"
   (Section 4.1.8).

   Since the I1 packet is so easy to spoof even if it were signed, no
   attempt is made to add to its generation or processing cost.

   The Initiator includes a DH_GROUP_LIST parameter in the I1 packet to
   inform the Responder of its preferred DH Group IDs.  Note that the
   DH_GROUP_LIST in the I1 packet is not protected by a signature.

   Implementations MUST be able to handle a storm of received I1
   packets, discarding those with common content that arrive within a
   small time delta.











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5.3.2.  R1 - the HIP Responder Packet

   The HIP header values for the R1 packet:

     Header:
       Packet Type = 2
       SRC HIT = Responder's HIT
       DST HIT = Initiator's HIT

     IP ( HIP ( [ R1_COUNTER, ]
                PUZZLE,
                DIFFIE_HELLMAN,
                HIP_CIPHER,
                HOST_ID,
                HIT_SUITE_LIST,
                DH_GROUP_LIST,
                [ ECHO_REQUEST_SIGNED, ]
                TRANSPORT_FORMAT_LIST,
                HIP_SIGNATURE_2 )
                <, ECHO_REQUEST_UNSIGNED >i)

   Valid control bits: A

   If the Responder's HI is an anonymous one, the A control MUST be set.

   The Initiator's HIT MUST match the one received in the I1 packet if
   the R1 is a response to an I1.  If the Responder has multiple HIs,
   the Responder's HIT used MUST match the Initiator's request.  If the
   Initiator used opportunistic mode, the Responder may select freely
   among its HIs.  See also "HIP Opportunistic Mode" (Section 4.1.8).

   The R1 packet generation counter is used to determine the currently
   valid generation of puzzles.  The value is increased periodically,
   and it is RECOMMENDED that it is increased at least as often as
   solutions to old puzzles are no longer accepted.

   The puzzle contains a Random #I and the difficulty #K.  The
   difficulty #K indicates the number of lower-order bits, in the puzzle
   hash result, that must be zeros; see Section 4.1.2.  The Random #I is
   not covered by the signature and must be zeroed during the signature
   calculation, allowing the sender to select and set the #I into a
   precomputed R1 packet just prior to sending it to the peer.

   The Responder selects the DIFFIE_HELLMAN Group ID and Public Value
   based on the Initiator's preference expressed in the DH_GROUP_LIST
   parameter in the I1 packet.  The Responder sends back its own
   preference based on which it chose the DH public value as




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   DH_GROUP_LIST.  This allows the Initiator to determine whether its
   own DH_GROUP_LIST in the sent I1 packet was manipulated by an
   attacker.

   The Diffie-Hellman public value is ephemeral, and values SHOULD NOT
   be reused across different HIP associations.  Once the Responder has
   received a valid response to an R1 packet, that Diffie-Hellman value
   SHOULD be deprecated.  It is possible that the Responder has sent the
   same Diffie-Hellman value to different hosts simultaneously in
   corresponding R1 packets, and those responses should also be
   accepted.  However, as a defense against I1 packet storms, an
   implementation MAY propose, and reuse unless avoidable, the same
   Diffie-Hellman value for a period of time -- for example, 15 minutes.
   By using a small number of different puzzles for a given
   Diffie-Hellman value, the R1 packets can be precomputed and delivered
   as quickly as I1 packets arrive.  A scavenger process should clean up
   unused Diffie-Hellman values and puzzles.

   Reusing Diffie-Hellman public values opens up the potential security
   risk of more than one Initiator ending up with the same keying
   material (due to faulty random number generators).  Also, more than
   one Initiator using the same Responder public key half may lead to
   potentially easier cryptographic attacks and to imperfect forward
   security.

   However, these risks involved in reusing the same public value are
   statistical; that is, the authors are not aware of any mechanism that
   would allow manipulation of the protocol so that the risk of the
   reuse of any given Responder Diffie-Hellman public key would differ
   from the base probability.  Consequently, it is RECOMMENDED that
   Responders avoid reusing the same DH key with multiple Initiators,
   but because the risk is considered statistical and not known to be
   manipulable, the implementations MAY reuse a key in order to ease
   resource-constrained implementations and to increase the probability
   of successful communication with legitimate clients even under an I1
   packet storm.  In particular, when it is too expensive to generate
   enough precomputed R1 packets to supply each potential Initiator with
   a different DH key, the Responder MAY send the same DH key to several
   Initiators, thereby creating the possibility of multiple legitimate
   Initiators ending up using the same Responder-side public key.
   However, as soon as the Responder knows that it will use a particular
   DH key, it SHOULD stop offering it.  This design is aimed to allow
   resource-constrained Responders to offer services under I1 packet
   storms and to simultaneously make the probability of DH key reuse
   both statistical and as low as possible.






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   If the Responder uses the same DH key pair for multiple handshakes,
   it must take care to avoid small subgroup attacks [RFC2785].  To
   avoid these attacks, when receiving the I2 message, the Responder
   SHOULD validate the Initiator's DH public key as described in
   [RFC2785], Section 3.1.  If the validation fails, the Responder MUST
   NOT generate a DH shared key and MUST silently abort the HIP BEX.

   The HIP_CIPHER parameter contains the encryption algorithms supported
   by the Responder to encrypt the contents of the ENCRYPTED parameter,
   in the order of preference.  All implementations MUST support AES
   [RFC3602].

   The HIT_SUITE_LIST parameter is an ordered list of the Responder's
   preferred and supported HIT Suites.  The list allows the Initiator to
   determine whether its own source HIT matches any suite supported by
   the Responder.

   The ECHO_REQUEST_SIGNED and ECHO_REQUEST_UNSIGNED parameters contain
   data that the sender wants to receive unmodified in the corresponding
   response packet in the ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED
   parameter.  The R1 packet may contain zero or more
   ECHO_REQUEST_UNSIGNED parameters as described in Section 5.2.21.

   The TRANSPORT_FORMAT_LIST parameter is an ordered list of the
   Responder's preferred and supported transport format types.  The list
   allows the Initiator and the Responder to agree on a common type for
   payload protection.  This parameter is described in Section 5.2.11.

   The signature is calculated over the whole HIP packet as described in
   Section 5.2.15.  This allows the Responder to use precomputed R1s.
   The Initiator SHOULD validate this signature.  It MUST check that the
   Responder's HI matches with the one expected, if any.



















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5.3.3.  I2 - the Second HIP Initiator Packet

   The HIP header values for the I2 packet:

     Header:
       Packet Type = 3
       SRC HIT = Initiator's HIT
       DST HIT = Responder's HIT

     IP ( HIP ( [R1_COUNTER,]
                SOLUTION,
                DIFFIE_HELLMAN,
                HIP_CIPHER,
                ENCRYPTED { HOST_ID } or HOST_ID,
                [ ECHO_RESPONSE_SIGNED, ]
                TRANSPORT_FORMAT_LIST,
                HIP_MAC,
                HIP_SIGNATURE
                <, ECHO_RESPONSE_UNSIGNED>i ) )

   Valid control bits: A

   The HITs used MUST match the ones used in the R1.

   If the Initiator's HI is an anonymous one, the A control bit MUST
   be set.

   If present in the I1 packet, the Initiator MUST include an unmodified
   copy of the R1_COUNTER parameter received in the corresponding R1
   packet into the I2 packet.

   The Solution contains the Random #I from R1 and the computed #J.  The
   low-order #K bits of the RHASH( #I | ... | #J ) MUST be zero.

   The Diffie-Hellman value is ephemeral.  If precomputed, a scavenger
   process should clean up unused Diffie-Hellman values.  The Responder
   MAY reuse Diffie-Hellman values under some conditions as specified in
   Section 5.3.2.

   The HIP_CIPHER contains the single encryption suite selected by the
   Initiator, that it uses to encrypt the ENCRYPTED parameters.  The
   chosen cipher MUST correspond to one of the ciphers offered by the
   Responder in the R1.  All implementations MUST support AES [RFC3602].

   The Initiator's HI MAY be encrypted using the HIP_CIPHER encryption
   algorithm.  The keying material is derived from the Diffie-Hellman
   exchange as defined in Section 6.5.




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   The ECHO_RESPONSE_SIGNED and ECHO_RESPONSE_UNSIGNED contain the
   unmodified opaque data copied from the corresponding echo request
   parameter(s).

   The TRANSPORT_FORMAT_LIST contains the single transport format type
   selected by the Initiator.  The chosen type MUST correspond to one of
   the types offered by the Responder in the R1.  Currently, the only
   transport format defined is the ESP transport format ([RFC7402]).

   The HMAC value in the HIP_MAC parameter is calculated over the whole
   HIP packet, excluding any parameters after the HIP_MAC, as described
   in Section 6.4.1.  The Responder MUST validate the HIP_MAC.

   The signature is calculated over the whole HIP packet, excluding any
   parameters after the HIP_SIGNATURE, as described in Section 5.2.14.
   The Responder MUST validate this signature.  The Responder uses the
   HI in the packet or an HI acquired by some other means for verifying
   the signature.

5.3.4.  R2 - the Second HIP Responder Packet

   The HIP header values for the R2 packet:

     Header:
       Packet Type = 4
       SRC HIT = Responder's HIT
       DST HIT = Initiator's HIT

     IP ( HIP ( HIP_MAC_2, HIP_SIGNATURE ) )

   Valid control bits: None

   The HIP_MAC_2 is calculated over the whole HIP packet, with the
   Responder's HOST_ID parameter concatenated with the HIP packet.  The
   HOST_ID parameter is removed after the HMAC calculation.  The
   procedure is described in Section 6.4.1.

   The signature is calculated over the whole HIP packet.

   The Initiator MUST validate both the HIP_MAC and the signature.











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5.3.5.  UPDATE - the HIP Update Packet

   The HIP header values for the UPDATE packet:

     Header:
       Packet Type = 16
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT

     IP ( HIP ( [SEQ, ACK, ] HIP_MAC, HIP_SIGNATURE ) )

   Valid control bits: None

   The UPDATE packet contains mandatory HIP_MAC and HIP_SIGNATURE
   parameters, and other optional parameters.

   The UPDATE packet contains zero or one SEQ parameter.  The presence
   of a SEQ parameter indicates that the receiver MUST acknowledge the
   UPDATE.  An UPDATE that does not contain a SEQ but only an ACK
   parameter is simply an acknowledgment of a previous UPDATE and itself
   MUST NOT be acknowledged by a separate ACK parameter.  Such UPDATE
   packets containing only an ACK parameter do not require processing in
   relative order to other UPDATE packets.  An UPDATE packet without
   either a SEQ or an ACK parameter is invalid; such unacknowledged
   updates MUST instead use a NOTIFY packet.

   An UPDATE packet contains zero or one ACK parameter.  The ACK
   parameter echoes the SEQ sequence number of the UPDATE packet being
   ACKed.  A host MAY choose to acknowledge more than one UPDATE packet
   at a time; e.g., the ACK parameter may contain the last two SEQ
   values received, for resilience against packet loss.  ACK values are
   not cumulative; each received unique SEQ value requires at least one
   corresponding ACK value in reply.  Received ACK parameters that are
   redundant are ignored.  Hosts MUST implement the processing of ACK
   parameters with multiple SEQ sequence numbers even if they do not
   implement sending ACK parameters with multiple SEQ sequence numbers.

   The UPDATE packet may contain both a SEQ and an ACK parameter.  In
   this case, the ACK parameter is being piggybacked on an outgoing
   UPDATE.  In general, UPDATEs carrying SEQ SHOULD be ACKed upon
   completion of the processing of the UPDATE.  A host MAY choose to
   hold the UPDATE carrying an ACK parameter for a short period of time
   to allow for the possibility of piggybacking the ACK parameter, in a
   manner similar to TCP delayed acknowledgments.







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   A sender MAY choose to forego reliable transmission of a particular
   UPDATE (e.g., it becomes overcome by events).  The semantics are such
   that the receiver MUST acknowledge the UPDATE, but the sender MAY
   choose to not care about receiving the ACK parameter.

   UPDATEs MAY be retransmitted without incrementing SEQ.  If the same
   subset of parameters is included in multiple UPDATEs with different
   SEQs, the host MUST ensure that the receiver's processing of the
   parameters multiple times will not result in a protocol error.

5.3.6.  NOTIFY - the HIP Notify Packet

   The NOTIFY packet MAY be used to provide information to a peer.
   Typically, NOTIFY is used to indicate some type of protocol error or
   negotiation failure.  NOTIFY packets are unacknowledged.  The
   receiver can handle the packet only as informational, and SHOULD NOT
   change its HIP state (see Section 4.4.2) based purely on a received
   NOTIFY packet.

   The HIP header values for the NOTIFY packet:

     Header:
       Packet Type = 17
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT, or zero if unknown

     IP ( HIP (<NOTIFICATION>i, [HOST_ID, ] HIP_SIGNATURE) )

   Valid control bits: None

   The NOTIFY packet is used to carry one or more NOTIFICATION
   parameters.

5.3.7.  CLOSE - the HIP Association Closing Packet

   The HIP header values for the CLOSE packet:

     Header:
       Packet Type = 18
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT

     IP ( HIP ( ECHO_REQUEST_SIGNED, HIP_MAC, HIP_SIGNATURE ) )

   Valid control bits: None






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   The sender MUST include an ECHO_REQUEST_SIGNED used to validate
   CLOSE_ACK received in response, and both a HIP_MAC and a signature
   (calculated over the whole HIP packet).

   The receiver peer MUST reply with a CLOSE_ACK containing an
   ECHO_RESPONSE_SIGNED corresponding to the received
   ECHO_REQUEST_SIGNED.

5.3.8.  CLOSE_ACK - the HIP Closing Acknowledgment Packet

   The HIP header values for the CLOSE_ACK packet:

     Header:
       Packet Type = 19
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT

     IP ( HIP ( ECHO_RESPONSE_SIGNED, HIP_MAC, HIP_SIGNATURE ) )

   Valid control bits: None

   The sender MUST include both an HMAC and signature (calculated over
   the whole HIP packet).

   The receiver peer MUST validate the ECHO_RESPONSE_SIGNED and validate
   both the HIP_MAC and the signature if the receiver has state for a
   HIP association.

5.4.  ICMP Messages

   When a HIP implementation detects a problem with an incoming packet,
   and it either cannot determine the identity of the sender of the
   packet or does not have any existing HIP association with the sender
   of the packet, it MAY respond with an ICMP packet.  Any such replies
   MUST be rate-limited as described in [RFC4443].  In most cases, the
   ICMP packet has the Parameter Problem type (12 for ICMPv4, 4 for
   ICMPv6), with the Pointer pointing to the field that caused the ICMP
   message to be generated.

5.4.1.  Invalid Version

   If a HIP implementation receives a HIP packet that has an
   unrecognized HIP version number, it SHOULD respond, rate-limited,
   with an ICMP packet with type Parameter Problem, with the Pointer
   pointing to the Version/RES. byte in the HIP header.






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5.4.2.  Other Problems with the HIP Header and Packet Structure

   If a HIP implementation receives a HIP packet that has other
   unrecoverable problems in the header or packet format, it MAY
   respond, rate-limited, with an ICMP packet with type Parameter
   Problem, with the Pointer pointing to the field that failed to pass
   the format checks.  However, an implementation MUST NOT send an ICMP
   message if the checksum fails; instead, it MUST silently drop the
   packet.

5.4.3.  Invalid Puzzle Solution

   If a HIP implementation receives an I2 packet that has an invalid
   puzzle solution, the behavior depends on the underlying version of
   IP.  If IPv6 is used, the implementation SHOULD respond with an ICMP
   packet with type Parameter Problem, with the Pointer pointing to the
   beginning of the Puzzle solution #J field in the SOLUTION payload in
   the HIP message.

   If IPv4 is used, the implementation MAY respond with an ICMP packet
   with the type Parameter Problem, copying enough bytes from the I2
   message so that the SOLUTION parameter fits into the ICMP message,
   with the Pointer pointing to the beginning of the Puzzle solution #J
   field, as in the IPv6 case.  Note, however, that the resulting ICMPv4
   message exceeds the typical ICMPv4 message size as defined in
   [RFC0792].

5.4.4.  Non-existing HIP Association

   If a HIP implementation receives a CLOSE or UPDATE packet, or any
   other packet whose handling requires an existing association, that
   has either a Receiver or Sender HIT that does not match with any
   existing HIP association, the implementation MAY respond, rate-
   limited, with an ICMP packet with the type Parameter Problem.  The
   Pointer of the ICMP Parameter Problem packet is set pointing to the
   beginning of the first HIT that does not match.

   A host MUST NOT reply with such an ICMP if it receives any of the
   following messages: I1, R2, I2, R2, and NOTIFY packet.  When
   introducing new packet types, a specification SHOULD define the
   appropriate rules for sending or not sending this kind of ICMP reply.

6.  Packet Processing

   Each host is assumed to have a single HIP implementation that manages
   the host's HIP associations and handles requests for new ones.  Each
   HIP association is governed by a conceptual state machine, with
   states defined above in Section 4.4.  The HIP implementation can



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   simultaneously maintain HIP associations with more than one host.
   Furthermore, the HIP implementation may have more than one active HIP
   association with another host; in this case, HIP associations are
   distinguished by their respective HITs.  It is not possible to have
   more than one HIP association between any given pair of HITs.
   Consequently, the only way for two hosts to have more than one
   parallel association is to use different HITs, at least at one end.

   The processing of packets depends on the state of the HIP
   association(s) with respect to the authenticated or apparent
   originator of the packet.  A HIP implementation determines whether it
   has an active association with the originator of the packet based on
   the HITs.  In the case of user data carried in a specific transport
   format, the transport format document specifies how the incoming
   packets are matched with the active associations.

6.1.  Processing Outgoing Application Data

   In a HIP host, an application can send application-level data using
   an identifier specified via the underlying API.  The API can be a
   backwards-compatible API (see [RFC5338]), using identifiers that look
   similar to IP addresses, or a completely new API, providing enhanced
   services related to Host Identities.  Depending on the HIP
   implementation, the identifier provided to the application may be
   different; for example, it can be a HIT or an IP address.

   The exact format and method for transferring the user data from the
   source HIP host to the destination HIP host are defined in the
   corresponding transport format document.  The actual data is
   transferred in the network using the appropriate source and
   destination IP addresses.

   In this document, conceptual processing rules are defined only for
   the base case where both hosts have only single usable IP addresses;
   the multi-address multihoming case is specified separately.

   The following conceptual algorithm describes the steps that are
   required for handling outgoing datagrams destined to a HIT.

   1.  If the datagram has a specified source address, it MUST be a HIT.
       If it is not, the implementation MAY replace the source address
       with a HIT.  Otherwise, it MUST drop the packet.

   2.  If the datagram has an unspecified source address, the
       implementation MUST choose a suitable source HIT for the
       datagram.  Selecting the source HIT is subject to local policy.





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   3.  If there is no active HIP association with the given <source,
       destination> HIT pair, one MUST be created by running the base
       exchange.  While waiting for the base exchange to complete, the
       implementation SHOULD queue at least one user data packet per HIP
       association to be formed, and it MAY queue more than one.

   4.  Once there is an active HIP association for the given <source,
       destination> HIT pair, the outgoing datagram is passed to
       transport handling.  The possible transport formats are defined
       in separate documents, of which the ESP transport format for HIP
       is mandatory for all HIP implementations.

   5.  Before sending the packet, the HITs in the datagram are replaced
       with suitable IP addresses.  For IPv6, the rules defined in
       [RFC6724] SHOULD be followed.  Note that this HIT-to-IP-address
       conversion step MAY also be performed at some other point in the
       stack, e.g., before wrapping the packet into the output format.

6.2.  Processing Incoming Application Data

   The following conceptual algorithm describes the incoming datagram
   handling when HITs are used at the receiving host as application-
   level identifiers.  More detailed steps for processing packets are
   defined in corresponding transport format documents.

   1.  The incoming datagram is mapped to an existing HIP association,
       typically using some information from the packet.  For example,
       such mapping may be based on the ESP Security Parameter Index
       (SPI).

   2.  The specific transport format is unwrapped, in a way depending on
       the transport format, yielding a packet that looks like a
       standard (unencrypted) IP packet.  If possible, this step SHOULD
       also verify that the packet was indeed (once) sent by the remote
       HIP host, as identified by the HIP association.

       Depending on the used transport mode, the verification method can
       vary.  While the HI (as well as the HIT) is used as the higher-
       layer identifier, the verification method has to verify that the
       data packet was sent by the correct node identity and that the
       actual identity maps to this particular HIT.  When using the ESP
       transport format [RFC7402], the verification is done using the
       SPI value in the data packet to find the corresponding SA with
       associated HIT and key, and decrypting the packet with that
       associated key.






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   3.  The IP addresses in the datagram are replaced with the HITs
       associated with the HIP association.  Note that this IP-address-
       to-HIT conversion step MAY also be performed at some other point
       in the stack.

   4.  The datagram is delivered to the upper layer (e.g., UDP or TCP).
       When demultiplexing the datagram, the right upper-layer socket is
       selected based on the HITs.

6.3.  Solving the Puzzle

   This subsection describes the details for solving the puzzle.

   In the R1 packet, the values #I and #K are sent in network byte
   order.  Similarly, in the I2 packet, the values #I and #J are sent in
   network byte order.  The hash is created by concatenating, in network
   byte order, the following data, in the following order and using the
   RHASH algorithm:

      n-bit random value #I (where n is RHASH_len), in network byte
      order, as appearing in the R1 and I2 packets.

      128-bit Initiator's HIT, in network byte order, as appearing in
      the HIP Payload in the R1 and I2 packets.

      128-bit Responder's HIT, in network byte order, as appearing in
      the HIP Payload in the R1 and I2 packets.

      n-bit random value #J (where n is RHASH_len), in network byte
      order, as appearing in the I2 packet.

   In a valid response puzzle, the #K low-order bits of the resulting
   RHASH digest MUST be zero.

   Notes:

        i) The length of the data to be hashed is variable, depending on
           the output length of the Responder's hash function RHASH.

       ii) All the data in the hash input MUST be in network byte order.

      iii) The orderings of the Initiator's and Responder's HITs are
           different in the R1 and I2 packets; see Section 5.1.  Care
           must be taken to copy the values in the right order to the
           hash input.

       iv) For a puzzle #I, there may exist multiple valid puzzle
           solutions #J.



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   The following procedure describes the processing steps involved,
   assuming that the Responder chooses to precompute the R1 packets:

   Precomputation by the Responder:
      Sets up the puzzle difficulty #K.
      Creates a signed R1 and caches it.

   Responder:
      Selects a suitable cached R1.
      Generates a random number #I.
      Sends #I and #K in an R1.
      Saves #I and #K for a delta time.

   Initiator:
      Generates repeated attempts to solve the puzzle until a matching
      #J is found:
      Ltrunc( RHASH( #I | HIT-I | HIT-R | #J ), #K ) == 0
      Sends #I and #J in an I2.

   Responder:
      Verifies that the received #I is a saved one.
      Finds the right #K based on #I.
      Computes V := Ltrunc( RHASH( #I | HIT-I | HIT-R | #J ), #K )
      Rejects if V != 0
      Accepts if V == 0

6.4.  HIP_MAC and SIGNATURE Calculation and Verification

   The following subsections define the actions for processing HIP_MAC,
   HIP_MAC_2, HIP_SIGNATURE, and HIP_SIGNATURE_2 parameters.  The
   HIP_MAC_2 parameter is contained in the R2 packet.  The
   HIP_SIGNATURE_2 parameter is contained in the R1 packet.  The
   HIP_SIGNATURE and HIP_MAC parameters are contained in other HIP
   packets.

6.4.1.  HMAC Calculation

   The HMAC uses RHASH as the underlying hash function.  The type of
   RHASH depends on the HIT Suite of the Responder.  Hence, HMAC-SHA-256
   [RFC4868] is used for HIT Suite RSA/DSA/SHA-256, HMAC-SHA-1 [RFC2404]
   is used for HIT Suite ECDSA_LOW/SHA-1, and HMAC-SHA-384 [RFC4868] is
   used for HIT Suite ECDSA/SHA-384.

   The following process applies both to the HIP_MAC and HIP_MAC_2
   parameters.  When processing HIP_MAC_2, the difference is that the
   HIP_MAC calculation includes a pseudo HOST_ID field containing the
   Responder's information as sent in the R1 packet earlier.




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   Both the Initiator and the Responder should take some care when
   verifying or calculating the HIP_MAC_2.  Specifically, the Initiator
   has to preserve the HOST_ID exactly as it was received in the R1
   packet until it receives the HIP_MAC_2 in the R2 packet.

   The scope of the calculation for HIP_MAC is as follows:

   HMAC: { HIP header | [ Parameters ] }

   where Parameters include all of the packet's HIP parameters with type
   values ranging from 1 to (HIP_MAC's type value - 1), and excluding
   those parameters with type values greater than or equal to HIP_MAC's
   type value.

   During HIP_MAC calculation, the following apply:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_MAC parameter.

   Parameter order is described in Section 5.2.1.

   The scope of the calculation for HIP_MAC_2 is as follows:

   HIP_MAC_2: { HIP header | [ Parameters ] | HOST_ID }

   where Parameters include all of the packet's HIP parameters with type
   values from 1 to (HIP_MAC_2's type value - 1), and excluding those
   parameters with type values greater than or equal to HIP_MAC_2's type
   value.

   During HIP_MAC_2 calculation, the following apply:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_MAC_2 parameter and increased by the
      length of the concatenated HOST_ID parameter length (including the
      Type and Length fields).

   o  The HOST_ID parameter is exactly in the form it was received in
      the R1 packet from the Responder.

   Parameter order is described in Section 5.2.1, except that the
   HOST_ID parameter in this calculation is added to the end.





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   The HIP_MAC parameter is defined in Section 5.2.12 and the HIP_MAC_2
   parameter in Section 5.2.13.  The HMAC calculation and verification
   process (the process applies both to HIP_MAC and HIP_MAC_2, except
   where HIP_MAC_2 is mentioned separately) is as follows:

   Packet sender:

   1.  Create the HIP packet, without the HIP_MAC, HIP_SIGNATURE,
       HIP_SIGNATURE_2, or any other parameter with greater type value
       than the HIP_MAC parameter has.

   2.  In case of HIP_MAC_2 calculation, add a HOST_ID (Responder)
       parameter to the end of the packet.

   3.  Calculate the Header Length field in the HIP header, including
       the added HOST_ID parameter in case of HIP_MAC_2.

   4.  Compute the HMAC using either the HIP-gl or HIP-lg integrity key
       retrieved from KEYMAT as defined in Section 6.5.

   5.  In case of HIP_MAC_2, remove the HOST_ID parameter from the
       packet.

   6.  Add the HIP_MAC parameter to the packet and any parameter with
       greater type value than the HIP_MAC's (HIP_MAC_2's) that may
       follow, including possible HIP_SIGNATURE or HIP_SIGNATURE_2
       parameters.

   7.  Recalculate the Length field in the HIP header.

   Packet receiver:

   1.  Verify the HIP Header Length field.

   2.  Remove the HIP_MAC or HIP_MAC_2 parameter, as well as all other
       parameters that follow it with greater type value including
       possible HIP_SIGNATURE or HIP_SIGNATURE_2 fields, saving the
       contents if they are needed later.

   3.  In case of HIP_MAC_2, build and add a HOST_ID parameter (with
       Responder information) to the packet.  The HOST_ID parameter
       should be identical to the one previously received from the
       Responder.

   4.  Recalculate the HIP packet length in the HIP header and clear the
       Checksum field (set it to all zeros).  In case of HIP_MAC_2, the
       length is calculated with the added HOST_ID parameter.




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   5.  Compute the HMAC using either the HIP-gl or HIP-lg integrity key
       as defined in Section 6.5 and verify it against the received
       HMAC.

   6.  Set the Checksum and Header Length fields in the HIP header to
       original values.  Note that the Checksum and Length fields
       contain incorrect values after this step.

   7.  In case of HIP_MAC_2, remove the HOST_ID parameter from the
       packet before further processing.

6.4.2.  Signature Calculation

   The following process applies both to the HIP_SIGNATURE and
   HIP_SIGNATURE_2 parameters.  When processing the HIP_SIGNATURE_2
   parameter, the only difference is that instead of the HIP_SIGNATURE
   parameter, the HIP_SIGNATURE_2 parameter is used, and the Initiator's
   HIT and PUZZLE Opaque and Random #I fields are cleared (set to all
   zeros) before computing the signature.  The HIP_SIGNATURE parameter
   is defined in Section 5.2.14 and the HIP_SIGNATURE_2 parameter in
   Section 5.2.15.

   The scope of the calculation for HIP_SIGNATURE and HIP_SIGNATURE_2 is
   as follows:

   HIP_SIGNATURE: { HIP header | [ Parameters ] }

   where Parameters include all of the packet's HIP parameters with type
   values from 1 to (HIP_SIGNATURE's type value - 1).

   During signature calculation, the following apply:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_SIGNATURE parameter.

   Parameter order is described in Section 5.2.1.

   HIP_SIGNATURE_2: { HIP header | [ Parameters ] }

   where Parameters include all of the packet's HIP parameters with type
   values ranging from 1 to (HIP_SIGNATURE_2's type value - 1).








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   During signature calculation, the following apply:

   o  In the HIP header, both the Checksum and the Receiver's HIT fields
      are set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_SIGNATURE_2 parameter.

   o  The PUZZLE parameter's Opaque and Random #I fields are set to
      zero.

   Parameter order is described in Section 5.2.1.

   The signature calculation and verification process (the process
   applies both to HIP_SIGNATURE and HIP_SIGNATURE_2, except in the case
   where HIP_SIGNATURE_2 is separately mentioned) is as follows:

   Packet sender:

   1.  Create the HIP packet without the HIP_SIGNATURE parameter or any
       other parameters that follow the HIP_SIGNATURE parameter.

   2.  Calculate the Length field and zero the Checksum field in the HIP
       header.  In case of HIP_SIGNATURE_2, set the Initiator's HIT
       field in the HIP header as well as the PUZZLE parameter's Opaque
       and Random #I fields to zero.

   3.  Compute the signature using the private key corresponding to the
       Host Identifier (public key).

   4.  Add the HIP_SIGNATURE parameter to the packet.

   5.  Add any parameters that follow the HIP_SIGNATURE parameter.

   6.  Recalculate the Length field in the HIP header, and calculate the
       Checksum field.

   Packet receiver:

   1.  Verify the HIP Header Length field and checksum.

   2.  Save the contents of the HIP_SIGNATURE parameter and any other
       parameters following the HIP_SIGNATURE parameter, and remove them
       from the packet.







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   3.  Recalculate the HIP packet Length in the HIP header and clear the
       Checksum field (set it to all zeros).  In case of
       HIP_SIGNATURE_2, set the Initiator's HIT field in the HIP header
       as well as the PUZZLE parameter's Opaque and Random #I fields
       to zero.

   4.  Compute the signature and verify it against the received
       signature using the packet sender's Host Identity (public key).

   5.  Restore the original packet by adding removed parameters (in
       step 2) and resetting the values that were set to zero (in
       step 3).

   The verification can use either the HI received from a HIP packet;
   the HI retrieved from a DNS query, if the FQDN has been received in
   the HOST_ID parameter; or an HI received by some other means.

6.5.  HIP KEYMAT Generation

   HIP keying material is derived from the Diffie-Hellman session key,
   Kij, produced during the HIP base exchange (see Section 4.1.3).  The
   Initiator has Kij during the creation of the I2 packet, and the
   Responder has Kij once it receives the I2 packet.  This is why I2 can
   already contain encrypted information.

   The KEYMAT is derived by feeding Kij into the key derivation function
   defined by the DH Group ID.  Currently, the only key derivation
   function defined in this document is the Hash-based Key Derivation
   Function (HKDF) [RFC5869] using the RHASH hash function.  Other
   documents may define new DH Group IDs and corresponding key
   distribution functions.

   In the following, we provide the details for deriving the keying
   material using HKDF.

   where

   info    = sort(HIT-I | HIT-R)
   salt    =  #I | #J

   Sort(HIT-I | HIT-R) is defined as the network byte order
   concatenation of the two HITs, with the smaller HIT preceding the
   larger HIT, resulting from the numeric comparison of the two HITs
   interpreted as positive (unsigned) 128-bit integers in network byte
   order.  The #I and #J values are from the puzzle and its solution
   that were exchanged in R1 and I2 messages when this HIP association
   was set up.  Both hosts have to store #I and #J values for the HIP
   association for future use.



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   The initial keys are drawn sequentially in the order that is
   determined by the numeric comparison of the two HITs, with the
   comparison method described in the previous paragraph.  HOST_g
   denotes the host with the greater HIT value, and HOST_l the host with
   the lower HIT value.

   The drawing order for the four initial keys is as follows:

      HIP-gl encryption key for HOST_g's ENCRYPTED parameter

      HIP-gl integrity (HMAC) key for HOST_g's outgoing HIP packets

      HIP-lg encryption key for HOST_l's ENCRYPTED parameter

      HIP-lg integrity (HMAC) key for HOST_l's outgoing HIP packets

   The number of bits drawn for a given algorithm is the "natural" size
   of the keys.  For the mandatory algorithms, the following sizes
   apply:

      AES       128 or 256 bits

      SHA-1     160 bits

      SHA-256   256 bits

      SHA-384   384 bits

      NULL      0 bits

   If other key sizes are used, they MUST be treated as different
   encryption algorithms and defined separately.

6.6.  Initiation of a HIP Base Exchange

   An implementation may originate a HIP base exchange to another host
   based on a local policy decision, usually triggered by an application
   datagram, in much the same way that an IPsec IKE key exchange can
   dynamically create a Security Association.  Alternatively, a system
   may initiate a HIP exchange if it has rebooted or timed out, or
   otherwise lost its HIP state, as described in Section 4.5.4.

   The implementation prepares an I1 packet and sends it to the IP
   address that corresponds to the peer host.  The IP address of the
   peer host may be obtained via conventional mechanisms, such as DNS
   lookup.  The I1 packet contents are specified in Section 5.3.1.  The





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   selection of which source or destination Host Identity to use, if an
   Initiator or Responder has more than one to choose from, is typically
   a policy decision.

   The following steps define the conceptual processing rules for
   initiating a HIP base exchange:

   1.  The Initiator receives one or more of the Responder's HITs and
       one or more addresses from either a DNS lookup of the Responder's
       FQDN, some other repository, or a local database.  If the
       Initiator does not know the Responder's HIT, it may attempt
       opportunistic mode by using NULL (all zeros) as the Responder's
       HIT (see also "HIP Opportunistic Mode" (Section 4.1.8)).  If the
       Initiator can choose from multiple Responder HITs, it selects a
       HIT for which the Initiator supports the HIT Suite.

   2.  The Initiator sends an I1 packet to one of the Responder's
       addresses.  The selection of which address to use is a local
       policy decision.

   3.  The Initiator includes the DH_GROUP_LIST in the I1 packet.  The
       selection and order of DH Group IDs in the DH_GROUP_LIST MUST be
       stored by the Initiator, because this list is needed for later R1
       processing.  In most cases, the preferences regarding the DH
       groups will be static, so no per-association storage is
       necessary.

   4.  Upon sending an I1 packet, the sender transitions to state
       I1-SENT and starts a timer for which the timeout value SHOULD be
       larger than the worst-case anticipated RTT.  The sender SHOULD
       also increment the trial counter associated with the I1.

   5.  Upon timeout, the sender SHOULD retransmit the I1 packet and
       restart the timer, up to a maximum of I1_RETRIES_MAX tries.

6.6.1.  Sending Multiple I1 Packets in Parallel

   For the sake of minimizing the association establishment latency, an
   implementation MAY send the same I1 packet to more than one of the
   Responder's addresses.  However, it MUST NOT send to more than three
   (3) Responder addresses in parallel.  Furthermore, upon timeout, the
   implementation MUST refrain from sending the same I1 packet to
   multiple addresses.  That is, if it retries to initialize the
   connection after a timeout, it MUST NOT send the I1 packet to more
   than one destination address.  These limitations are placed in order
   to avoid congestion of the network, and potential DoS attacks that





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   might occur, e.g., because someone's claim to have hundreds or
   thousands of addresses could generate a huge number of I1 packets
   from the Initiator.

   As the Responder is not guaranteed to distinguish the duplicate I1
   packets it receives at several of its addresses (because it avoids
   storing states when it answers back an R1 packet), the Initiator may
   receive several duplicate R1 packets.

   The Initiator SHOULD then select the initial preferred destination
   address using the source address of the selected received R1, and use
   the preferred address as a source address for the I2 packet.
   Processing rules for received R1s are discussed in Section 6.8.

6.6.2.  Processing Incoming ICMP Protocol Unreachable Messages

   A host may receive an ICMP 'Destination Protocol Unreachable' message
   as a response to sending a HIP I1 packet.  Such a packet may be an
   indication that the peer does not support HIP, or it may be an
   attempt to launch an attack by making the Initiator believe that the
   Responder does not support HIP.

   When a system receives an ICMP 'Destination Protocol Unreachable'
   message while it is waiting for an R1 packet, it MUST NOT terminate
   waiting.  It MAY continue as if it had not received the ICMP message,
   and send a few more I1 packets.  Alternatively, it MAY take the ICMP
   message as a hint that the peer most probably does not support HIP,
   and return to state UNASSOCIATED earlier than otherwise.  However, at
   minimum, it MUST continue waiting for an R1 packet for a reasonable
   time before returning to UNASSOCIATED.

6.7.  Processing of Incoming I1 Packets

   An implementation SHOULD reply to an I1 with an R1 packet, unless the
   implementation is unable or unwilling to set up a HIP association.
   If the implementation is unable to set up a HIP association, the host
   SHOULD send an 'ICMP Destination Protocol Unreachable,
   Administratively Prohibited' message to the I1 packet source IP
   address.  If the implementation is unwilling to set up a HIP
   association, the host MAY ignore the I1 packet.  This latter case may
   occur during a DoS attack such as an I1 packet flood.

   The implementation SHOULD be able to handle a storm of received I1
   packets, discarding those with common content that arrive within a
   small time delta.






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   A spoofed I1 packet can result in an R1 attack on a system.  An R1
   packet sender MUST have a mechanism to rate-limit R1 packets sent to
   an address.

   It is RECOMMENDED that the HIP state machine does not transition upon
   sending an R1 packet.

   The following steps define the conceptual processing rules for
   responding to an I1 packet:

   1.  The Responder MUST check that the Responder's HIT in the received
       I1 packet is either one of its own HITs or NULL.  Otherwise, it
       must drop the packet.

   2.  If the Responder is in ESTABLISHED state, the Responder MAY
       respond to this with an R1 packet, prepare to drop an existing
       HIP security association with the peer, and stay at ESTABLISHED
       state.

   3.  If the Responder is in I1-SENT state, it MUST make a comparison
       between the sender's HIT and its own (i.e., the receiver's) HIT.
       If the sender's HIT is greater than its own HIT, it should drop
       the I1 packet and stay at I1-SENT.  If the sender's HIT is
       smaller than its own HIT, it SHOULD send the R1 packet and stay
       at I1-SENT.  The HIT comparison is performed as defined in
       Section 6.5.

   4.  If the implementation chooses to respond to the I1 packet with an
       R1 packet, it creates a new R1 or selects a precomputed R1
       according to the format described in Section 5.3.2.  It creates
       or chooses an R1 that contains its most preferred DH public value
       that is also contained in the DH_GROUP_LIST in the I1 packet.  If
       no suitable DH Group ID was contained in the DH_GROUP_LIST in the
       I1 packet, it sends an R1 with any suitable DH public key.

   5.  If the received Responder's HIT in the I1 is NULL, the Responder
       selects a HIT with the same HIT Suite as the Initiator's HIT.  If
       this HIT Suite is not supported by the Responder, it SHOULD
       select a REQUIRED HIT Suite from Section 5.2.10, which is
       currently RSA/DSA/SHA-256.  Other than that, selecting the HIT is
       a local policy matter.










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   6.  The Responder expresses its supported HIP transport formats in
       the TRANSPORT_FORMAT_LIST as described in Section 5.2.11.  The
       Responder MUST provide at least one payload transport format
       type.

   7.  The Responder sends the R1 packet to the source IP address of the
       I1 packet.

6.7.1.  R1 Management

   All compliant implementations MUST be able to produce R1 packets;
   even if a device is configured by policy to only initiate
   associations, it must be able to process I1s in cases of recovery
   from loss of state or key exhaustion.  An R1 packet MAY be
   precomputed.  An R1 packet MAY be reused for a short time period,
   denoted here as "Delta T", which is implementation dependent, and
   SHOULD be deprecated and not used once a valid response I2 packet has
   been received from an Initiator.  During an I1 message storm, an R1
   packet MAY be reused beyond the normal Delta T.  R1 information MUST
   NOT be discarded until a time period "Delta S" (again, implementation
   dependent) after the R1 packet is no longer being offered.  Delta S
   is the assumed maximum time needed for the last I2 packet in response
   to the R1 packet to arrive back at the Responder.

   Implementations that support multiple DH groups MAY precompute R1
   packets for each supported group so that incoming I1 packets with
   different DH Group IDs in the DH_GROUP_LIST can be served quickly.

   An implementation MAY keep state about received I1 packets and match
   the received I2 packets against the state, as discussed in
   Section 4.1.1.

6.7.2.  Handling of Malformed Messages

   If an implementation receives a malformed I1 packet, it SHOULD NOT
   respond with a NOTIFY message, as such a practice could open up a
   potential denial-of-service threat.  Instead, it MAY respond with an
   ICMP packet, as defined in Section 5.4.

6.8.  Processing of Incoming R1 Packets

   A system receiving an R1 packet MUST first check to see if it has
   sent an I1 packet to the originator of the R1 packet (i.e., it is in
   state I1-SENT).  If so, it SHOULD process the R1 as described below,
   send an I2 packet, and transition to state I2-SENT, setting a timer
   to protect the I2 packet.  If the system is in state I2-SENT, it MAY
   respond to the R1 packet if the R1 packet has a larger R1 generation
   counter; if so, it should drop its state due to processing the



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   previous R1 packet and start over from state I1-SENT.  If the system
   is in any other state with respect to that host, the system SHOULD
   silently drop the R1 packet.

   When sending multiple I1 packets, an Initiator SHOULD wait for a
   small amount of time after the first R1 reception to allow possibly
   multiple R1 packets to arrive, and it SHOULD respond to an R1 packet
   among the set with the largest R1 generation counter.

   The following steps define the conceptual processing rules for
   responding to an R1 packet:

   1.   A system receiving an R1 MUST first check to see if it has sent
        an I1 packet to the originator of the R1 packet (i.e., it has a
        HIP association that is in state I1-SENT and that is associated
        with the HITs in the R1).  Unless the I1 packet was sent in
        opportunistic mode (see Section 4.1.8), the IP addresses in the
        received R1 packet SHOULD be ignored by the R1 processing and,
        when looking up the right HIP association, the received R1
        packet SHOULD be matched against the associations using only the
        HITs.  If a match exists, the system should process the R1
        packet as described below.

   2.   Otherwise, if the system is in any state other than I1-SENT or
        I2-SENT with respect to the HITs included in the R1 packet, it
        SHOULD silently drop the R1 packet and remain in the current
        state.

   3.   If the HIP association state is I1-SENT or I2-SENT, the received
        Initiator's HIT MUST correspond to the HIT used in the original
        I1.  Also, the Responder's HIT MUST correspond to the one used
        in the I1, unless the I1 packet contained a NULL HIT.

   4.   The system SHOULD validate the R1 signature before applying
        further packet processing, according to Section 5.2.15.

   5.   If the HIP association state is I1-SENT, and multiple valid R1
        packets are present, the system MUST select from among the R1
        packets with the largest R1 generation counter.

   6.   The system MUST check that the Initiator's HIT Suite is
        contained in the HIT_SUITE_LIST parameter in the R1 packet
        (i.e., the Initiator's HIT Suite is supported by the Responder).
        If the HIT Suite is supported by the Responder, the system
        proceeds normally.  Otherwise, the system MAY stay in state
        I1-SENT and restart the BEX by sending a new I1 packet with an
        Initiator HIT that is supported by the Responder and hence is
        contained in the HIT_SUITE_LIST in the R1 packet.  The system



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        MAY abort the BEX if no suitable source HIT is available.  The
        system SHOULD wait for an acceptable time span to allow further
        R1 packets with higher R1 generation counters or different HIT
        and HIT Suites to arrive before restarting or aborting the BEX.

   7.   The system MUST check that the DH Group ID in the DIFFIE_HELLMAN
        parameter in the R1 matches the first DH Group ID in the
        Responder's DH_GROUP_LIST in the R1 packet, and also that this
        Group ID corresponds to a value that was included in the
        Initiator's DH_GROUP_LIST in the I1 packet.  If the DH Group ID
        of the DIFFIE_HELLMAN parameter does not express the Responder's
        best choice, the Initiator can conclude that the DH_GROUP_LIST
        in the I1 packet was adversely modified.  In such a case, the
        Initiator MAY send a new I1 packet; however, it SHOULD NOT
        change its preference in the DH_GROUP_LIST in the new I1 packet.
        Alternatively, the Initiator MAY abort the HIP base exchange.

   8.   If the HIP association state is I2-SENT, the system MAY re-enter
        state I1-SENT and process the received R1 packet if it has a
        larger R1 generation counter than the R1 packet responded to
        previously.

   9.   The R1 packet may have the A-bit set -- in this case, the system
        MAY choose to refuse it by dropping the R1 packet and returning
        to state UNASSOCIATED.  The system SHOULD consider dropping the
        R1 packet only if it used a NULL HIT in the I1 packet.  If the
        A-bit is set, the Responder's HIT is anonymous and SHOULD NOT be
        stored permanently.

   10.  The system SHOULD attempt to validate the HIT against the
        received Host Identity by using the received Host Identity to
        construct a HIT and verify that it matches the Sender's HIT.

   11.  The system MUST store the received R1 generation counter for
        future reference.

   12.  The system attempts to solve the puzzle in the R1 packet.  The
        system MUST terminate the search after exceeding the remaining
        lifetime of the puzzle.  If the puzzle is not successfully
        solved, the implementation MAY either resend the I1 packet
        within the retry bounds or abandon the HIP base exchange.

   13.  The system computes standard Diffie-Hellman keying material
        according to the public value and Group ID provided in the
        DIFFIE_HELLMAN parameter.  The Diffie-Hellman keying material
        Kij is used for key extraction as specified in Section 6.5.





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   14.  The system selects the HIP_CIPHER ID from the choices presented
        in the R1 packet and uses the selected values subsequently when
        generating and using encryption keys, and when sending the I2
        packet.  If the proposed alternatives are not acceptable to the
        system, it may either resend an I1 within the retry bounds or
        abandon the HIP base exchange.

   15.  The system chooses one suitable transport format from the
        TRANSPORT_FORMAT_LIST and includes the respective transport
        format parameter in the subsequent I2 packet.

   16.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   17.  The system prepares and sends an I2 packet, as described in
        Section 5.3.3.

   18.  The system SHOULD start a timer whose timeout value SHOULD be
        larger than the worst-case anticipated RTT, and MUST increment a
        trial counter associated with the I2 packet.  The sender SHOULD
        retransmit the I2 packet upon a timeout and restart the timer,
        up to a maximum of I2_RETRIES_MAX tries.

   19.  If the system is in state I1-SENT, it SHALL transition to state
        I2-SENT.  If the system is in any other state, it remains in the
        current state.

6.8.1.  Handling of Malformed Messages

   If an implementation receives a malformed R1 message, it MUST
   silently drop the packet.  Sending a NOTIFY or ICMP would not help,
   as the sender of the R1 packet typically doesn't have any state.  An
   implementation SHOULD wait for some more time for a possibly well-
   formed R1, after which it MAY try again by sending a new I1 packet.

6.9.  Processing of Incoming I2 Packets

   Upon receipt of an I2 packet, the system MAY perform initial checks
   to determine whether the I2 packet corresponds to a recent R1 packet
   that has been sent out, if the Responder keeps such state.  For
   example, the sender could check whether the I2 packet is from an
   address or HIT for which the Responder has recently received an I1.
   The R1 packet may have had opaque data included that was echoed back
   in the I2 packet.  If the I2 packet is considered to be suspect, it
   MAY be silently discarded by the system.






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   Otherwise, the HIP implementation SHOULD process the I2 packet.  This
   includes validation of the puzzle solution, generating the
   Diffie-Hellman key, possibly decrypting the Initiator's Host
   Identity, verifying the signature, creating state, and finally
   sending an R2 packet.

   The following steps define the conceptual processing rules for
   responding to an I2 packet:

   1.   The system MAY perform checks to verify that the I2 packet
        corresponds to a recently sent R1 packet.  Such checks are
        implementation dependent.  See Appendix A for a description of
        an example implementation.

   2.   The system MUST check that the Responder's HIT corresponds to
        one of its own HITs and MUST drop the packet otherwise.

   3.   The system MUST further check that the Initiator's HIT Suite is
        supported.  The Responder SHOULD silently drop I2 packets with
        unsupported Initiator HITs.

   4.   If the system's state machine is in the R2-SENT state, the
        system MAY check to see if the newly received I2 packet is
        similar to the one that triggered moving to R2-SENT.  If so, it
        MAY retransmit a previously sent R2 packet and reset the R2-SENT
        timer, and the state machine stays in R2-SENT.

   5.   If the system's state machine is in the I2-SENT state, the
        system MUST make a comparison between its local and sender's
        HITs (similar to the comparison method described in
        Section 6.5).  If the local HIT is smaller than the sender's
        HIT, it should drop the I2 packet, use the peer Diffie-Hellman
        key and nonce #I from the R1 packet received earlier, and get
        the local Diffie-Hellman key and nonce #J from the I2 packet
        sent to the peer earlier.  Otherwise, the system should process
        the received I2 packet and drop any previously derived
        Diffie-Hellman keying material Kij it might have formed upon
        sending the I2 packet previously.  The peer Diffie-Hellman key
        and the nonce #J are taken from the I2 packet that just arrived.
        The local Diffie-Hellman key and the nonce #I are the ones that
        were sent earlier in the R1 packet.

   6.   If the system's state machine is in the I1-SENT state, and the
        HITs in the I2 packet match those used in the previously sent I1
        packet, the system uses this received I2 packet as the basis for
        the HIP association it was trying to form, and stops
        retransmitting I1 packets (provided that the I2 packet passes
        the additional checks below).



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   7.   If the system's state machine is in any state other than
        R2-SENT, the system SHOULD check that the echoed R1 generation
        counter in the I2 packet is within the acceptable range if the
        counter is included.  Implementations MUST accept puzzles from
        the current generation and MAY accept puzzles from earlier
        generations.  If the generation counter in the newly received I2
        packet is outside the accepted range, the I2 packet is stale
        (and perhaps replayed) and SHOULD be dropped.

   8.   The system MUST validate the solution to the puzzle by computing
        the hash described in Section 5.3.3 using the same RHASH
        algorithm.

   9.   The I2 packet MUST have a single value in the HIP_CIPHER
        parameter, which MUST match one of the values offered to the
        Initiator in the R1 packet.

   10.  The system must derive Diffie-Hellman keying material Kij based
        on the public value and Group ID in the DIFFIE_HELLMAN
        parameter.  This key is used to derive the HIP association keys,
        as described in Section 6.5.  If the Diffie-Hellman Group ID is
        unsupported, the I2 packet is silently dropped.

   11.  The encrypted HOST_ID is decrypted by the Initiator's encryption
        key defined in Section 6.5.  If the decrypted data is not a
        HOST_ID parameter, the I2 packet is silently dropped.

   12.  The implementation SHOULD also verify that the Initiator's HIT
        in the I2 packet corresponds to the Host Identity sent in the I2
        packet.  (Note: some middleboxes may not be able to make this
        verification.)

   13.  The system MUST process the TRANSPORT_FORMAT_LIST parameter.
        Other documents specifying transport formats (e.g., [RFC7402])
        contain specifications for handling any specific transport
        selected.

   14.  The system MUST verify the HIP_MAC according to the procedures
        in Section 5.2.12.

   15.  The system MUST verify the HIP_SIGNATURE according to
        Sections 5.2.14 and 5.3.3.

   16.  If the checks above are valid, then the system proceeds with
        further I2 processing; otherwise, it discards the I2 and its
        state machine remains in the same state.





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   17.  The I2 packet may have the A-bit set -- in this case, the system
        MAY choose to refuse it by dropping the I2 and the state machine
        returns to state UNASSOCIATED.  If the A-bit is set, the
        Initiator's HIT is anonymous and should not be stored
        permanently.

   18.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   19.  Upon successful processing of an I2 message when the system's
        state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or
        R2-SENT, an R2 packet is sent and the system's state machine
        transitions to state R2-SENT.

   20.  Upon successful processing of an I2 packet when the system's
        state machine is in state ESTABLISHED, the old HIP association
        is dropped and a new one is installed, an R2 packet is sent, and
        the system's state machine transitions to R2-SENT.

   21.  Upon the system's state machine transitioning to R2-SENT, the
        system starts a timer.  The state machine transitions to
        ESTABLISHED if some data has been received on the incoming HIP
        association, or an UPDATE packet has been received (or some
        other packet that indicates that the peer system's state machine
        has moved to ESTABLISHED).  If the timer expires (allowing for a
        maximal amount of retransmissions of I2 packets), the state
        machine transitions to ESTABLISHED.

6.9.1.  Handling of Malformed Messages

   If an implementation receives a malformed I2 message, the behavior
   SHOULD depend on how many checks the message has already passed.  If
   the puzzle solution in the message has already been checked, the
   implementation SHOULD report the error by responding with a NOTIFY
   packet.  Otherwise, the implementation MAY respond with an ICMP
   message as defined in Section 5.4.















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6.10.  Processing of Incoming R2 Packets

   An R2 packet received in state UNASSOCIATED, I1-SENT, or ESTABLISHED
   results in the R2 packet being dropped and the state machine staying
   in the same state.  If an R2 packet is received in state I2-SENT, it
   MUST be processed.

   The following steps define the conceptual processing rules for an
   incoming R2 packet:

   1.  If the system is in any state other than I2-SENT, the R2 packet
       is silently dropped.

   2.  The system MUST verify that the HITs in use correspond to the
       HITs that were received in the R1 packet that caused the
       transition to the I1-SENT state.

   3.  The system MUST verify the HIP_MAC_2 according to the procedures
       in Section 5.2.13.

   4.  The system MUST verify the HIP signature according to the
       procedures in Section 5.2.14.

   5.  If any of the checks above fail, there is a high probability of
       an ongoing man-in-the-middle or other security attack.  The
       system SHOULD act accordingly, based on its local policy.

   6.  Upon successful processing of the R2 packet, the state machine
       transitions to state ESTABLISHED.

6.11.  Sending UPDATE Packets

   A host sends an UPDATE packet when it intends to update some
   information related to a HIP association.  There are a number of
   possible scenarios when this can occur, e.g., mobility management and
   rekeying of an existing ESP Security Association.  The following
   paragraphs define the conceptual rules for sending an UPDATE packet
   to the peer.  Additional steps can be defined in other documents
   where the UPDATE packet is used.

   The sequence of UPDATE messages is indicated by their SEQ parameter.
   Before sending an UPDATE message, the system first determines whether
   there are any outstanding UPDATE messages that may conflict with the
   new UPDATE message under consideration.  When multiple UPDATEs are
   outstanding (not yet acknowledged), the sender must assume that such
   UPDATEs may be processed in an arbitrary order by the receiver.
   Therefore, any new UPDATEs that depend on a previous outstanding
   UPDATE being successfully received and acknowledged MUST be postponed



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   until reception of the necessary ACK(s) occurs.  One way to prevent
   any conflicts is to only allow one outstanding UPDATE at a time.
   However, allowing multiple UPDATEs may improve the performance of
   mobility and multihoming protocols.

   The following steps define the conceptual processing rules for
   sending UPDATE packets:

   1.  The first UPDATE packet is sent with an Update ID of zero.
       Otherwise, the system increments its own Update ID value by one
       before continuing the steps below.

   2.  The system creates an UPDATE packet that contains a SEQ parameter
       with the current value of the Update ID.  The UPDATE packet MAY
       also include zero or more ACKs of the peer's Update ID(s) from
       previously received UPDATE SEQ parameter(s).

   3.  The system sends the created UPDATE packet and starts an UPDATE
       timer.  The default value for the timer is 2 * RTT estimate.  If
       multiple UPDATEs are outstanding, multiple timers are in effect.

   4.  If the UPDATE timer expires, the UPDATE is resent.  The UPDATE
       can be resent UPDATE_RETRY_MAX times.  The UPDATE timer SHOULD be
       exponentially backed off for subsequent retransmissions.  If no
       acknowledgment is received from the peer after UPDATE_RETRY_MAX
       times, the HIP association is considered to be broken and the
       state machine SHOULD move from state ESTABLISHED to state CLOSING
       as depicted in Section 4.4.4.  The UPDATE timer is cancelled upon
       receiving an ACK from the peer that acknowledges receipt of the
       UPDATE.

6.12.  Receiving UPDATE Packets

   When a system receives an UPDATE packet, its processing depends on
   the state of the HIP association and the presence and values of the
   SEQ and ACK parameters.  Typically, an UPDATE message also carries
   optional parameters whose handling is defined in separate documents.

   For each association, a host stores the peer's next expected
   in-sequence Update ID ("peer Update ID").  Initially, this value is
   zero.  Update ID comparisons of "less than" and "greater than" are
   performed with respect to a circular sequence number space.  Hence, a
   wraparound after 2^32 updates has to be expected and MUST be handled
   accordingly.

   The sender MAY send multiple outstanding UPDATE messages.  These
   messages are processed in the order in which they are received at the
   receiver (i.e., no resequencing is performed).  When processing



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   UPDATEs out of order, the receiver MUST keep track of which UPDATEs
   were previously processed, so that duplicates or retransmissions are
   ACKed and not reprocessed.  A receiver MAY choose to define a receive
   window of Update IDs that it is willing to process at any given time,
   and discard received UPDATEs falling outside of that window.

   The following steps define the conceptual processing rules for
   receiving UPDATE packets:

   1.  If there is no corresponding HIP association, the implementation
       MAY reply with an ICMP Parameter Problem, as specified in
       Section 5.4.4.

   2.  If the association is in the ESTABLISHED state and the SEQ (but
       not ACK) parameter is present, the UPDATE is processed and
       replied to as described in Section 6.12.1.

   3.  If the association is in the ESTABLISHED state and the ACK (but
       not SEQ) parameter is present, the UPDATE is processed as
       described in Section 6.12.2.

   4.  If the association is in the ESTABLISHED state and there are both
       an ACK and SEQ in the UPDATE, the ACK is first processed as
       described in Section 6.12.2, and then the rest of the UPDATE is
       processed as described in Section 6.12.1.

6.12.1.  Handling a SEQ Parameter in a Received UPDATE Message

   The following steps define the conceptual processing rules for
   handling a SEQ parameter in a received UPDATE packet:

   1.  If the Update ID in the received SEQ is not the next in the
       sequence of Update IDs and is greater than the receiver's window
       for new UPDATEs, the packet MUST be dropped.

   2.  If the Update ID in the received SEQ corresponds to an UPDATE
       that has recently been processed, the packet is treated as a
       retransmission.  The HIP_MAC verification (next step) MUST NOT be
       skipped.  (A byte-by-byte comparison of the received packet and a
       stored packet would be acceptable, though.)  It is recommended
       that a host caches UPDATE packets sent with ACKs to avoid the
       cost of generating a new ACK packet to respond to a replayed
       UPDATE.  The system MUST acknowledge, again, such (apparent)
       UPDATE message retransmissions but SHOULD also consider rate-
       limiting such retransmission responses to guard against replay
       attacks.





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   3.  The system MUST verify the HIP_MAC in the UPDATE packet.  If the
       verification fails, the packet MUST be dropped.

   4.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
       verification fails, the packet SHOULD be dropped and an error
       message logged.

   5.  If a new SEQ parameter is being processed, the parameters in the
       UPDATE are then processed.  The system MUST record the Update ID
       in the received SEQ parameter, for replay protection.

   6.  An UPDATE acknowledgment packet with the ACK parameter is
       prepared and sent to the peer.  This ACK parameter MAY be
       included in a separate UPDATE or piggybacked in an UPDATE with
       the SEQ parameter, as described in Section 5.3.5.  The ACK
       parameter MAY acknowledge more than one of the peer's Update IDs.

6.12.2.  Handling an ACK Parameter in a Received UPDATE Packet

   The following steps define the conceptual processing rules for
   handling an ACK parameter in a received UPDATE packet:

   1.  The sequence number reported in the ACK must match with an UPDATE
       packet sent earlier that has not already been acknowledged.  If
       no match is found or if the ACK does not acknowledge a new
       UPDATE, then either the packet MUST be dropped if no SEQ
       parameter is present, or the processing steps in Section 6.12.1
       are followed.

   2.  The system MUST verify the HIP_MAC in the UPDATE packet.  If the
       verification fails, the packet MUST be dropped.

   3.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
       verification fails, the packet SHOULD be dropped and an error
       message logged.

   4.  The corresponding UPDATE timer is stopped (see Section 6.11) so
       that the now-acknowledged UPDATE is no longer retransmitted.  If
       multiple UPDATEs are acknowledged, multiple timers are stopped.

6.13.  Processing of NOTIFY Packets

   Processing of NOTIFY packets is OPTIONAL.  If processed, any errors
   in a received NOTIFICATION parameter SHOULD be logged.  Received
   errors MUST be considered only as informational, and the receiver
   SHOULD NOT change its HIP state (see Section 4.4.2) purely based on
   the received NOTIFY message.




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6.14.  Processing of CLOSE Packets

   When the host receives a CLOSE message, it responds with a CLOSE_ACK
   message and moves to the CLOSED state.  (The authenticity of the
   CLOSE message is verified using both HIP_MAC and SIGNATURE.)  This
   processing applies whether or not the HIP association state is
   CLOSING, in order to handle simultaneous CLOSE messages from both
   ends that cross in flight.

   The HIP association is not discarded before the host moves to the
   UNASSOCIATED state.

   Once the closing process has started, any new need to send data
   packets triggers the creation and establishment of a new HIP
   association, starting with sending an I1 packet.

   If there is no corresponding HIP association, the CLOSE packet is
   dropped.

6.15.  Processing of CLOSE_ACK Packets

   When a host receives a CLOSE_ACK message, it verifies that it is in
   the CLOSING or CLOSED state and that the CLOSE_ACK was in response to
   the CLOSE.  A host can map CLOSE_ACK messages to CLOSE messages by
   comparing the value of ECHO_REQUEST_SIGNED (in the CLOSE packet) to
   the value of ECHO_RESPONSE_SIGNED (in the CLOSE_ACK packet).

   The CLOSE_ACK contains the HIP_MAC and the SIGNATURE parameters for
   verification.  The state is discarded when the state changes to
   UNASSOCIATED and, after that, the host MAY respond with an ICMP
   Parameter Problem to an incoming CLOSE message (see Section 5.4.4).

6.16.  Handling State Loss

   In the case of a system crash and unanticipated state loss, the
   system SHOULD delete the corresponding HIP state, including the
   keying material.  That is, the state SHOULD NOT be stored in
   long-term storage.  If the implementation does drop the state
   (as RECOMMENDED), it MUST also drop the peer's R1 generation counter
   value, unless a local policy explicitly defines that the value of
   that particular host is stored.  An implementation MUST NOT store a
   peer's R1 generation counters by default, but storing R1 generation
   counter values, if done, MUST be configured by explicit HITs.








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7.  HIP Policies

   There are a number of variables that will influence the HIP base
   exchanges that each host must support.  All HIP implementations MUST
   support more than one simultaneous HI, at least one of which SHOULD
   be reserved for anonymous usage.  Although anonymous HIs will be
   rarely used as Responders' HIs, they will be common for Initiators.
   Support for more than two HIs is RECOMMENDED.

   Initiators MAY use a different HI for different Responders to provide
   basic privacy.  Whether such private HIs are used repeatedly with the
   same Responder, and how long these HIs are used, are decided by local
   policy and depend on the privacy requirements of the Initiator.

   The value of #K used in the HIP R1 must be chosen with care.  Values
   of #K that are too high will exclude clients with weak CPUs because
   these devices cannot solve the puzzle within a reasonable amount of
   time.  #K should only be raised if a Responder is under high load,
   i.e., it cannot process all incoming HIP handshakes any more.  If a
   Responder is not under high load, #K SHOULD be 0.

   Responders that only respond to selected Initiators require an Access
   Control List (ACL), representing for which hosts they accept HIP base
   exchanges, and the preferred transport format and local lifetimes.
   Wildcarding SHOULD be supported for such ACLs, and also for
   Responders that offer public or anonymous services.

8.  Security Considerations

   HIP is designed to provide secure authentication of hosts.  HIP also
   attempts to limit the exposure of the host to various denial-of-
   service and man-in-the-middle (MitM) attacks.  In doing so, HIP
   itself is subject to its own DoS and MitM attacks that potentially
   could be more damaging to a host's ability to conduct business as
   usual.

   Denial-of-service attacks often take advantage of asymmetries in the
   cost of starting an association.  One example of such asymmetry is
   the need of a Responder to store local state while a malicious
   Initiator can stay stateless.  HIP makes no attempt to increase the
   cost of the start of state at the Initiator, but makes an effort to
   reduce the cost for the Responder.  This is accomplished by having
   the Responder start the 3-way exchange instead of the Initiator,
   making the HIP exchange 4 packets long.  In doing this, the first
   packet from the Responder, R1, becomes a 'stock' packet that the
   Responder MAY use many times, until some Initiator has provided a
   valid response to such an R1 packet.  During an I1 packet storm, the
   host may reuse the same DH value also, even if some Initiator has



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   provided a valid response using that particular DH value.  However,
   such behavior is discouraged and should be avoided.  Using the same
   Diffie-Hellman values and random puzzle #I value has some risks.
   This risk needs to be balanced against a potential storm of HIP I1
   packets.

   This shifting of the start of state cost to the Initiator in creating
   the I2 HIP packet presents another DoS attack.  The attacker can
   spoof the I1 packet, and the Responder sends out the R1 HIP packet.
   This could conceivably tie up the 'Initiator' with evaluating the R1
   HIP packet, and creating the I2 packet.  The defense against this
   attack is to simply ignore any R1 packet where a corresponding I1
   packet was not sent (as defined in Section 6.8, step 1).

   The R1 packet is considerably larger than the I1 packet.  This
   asymmetry can be exploited in a reflection attack.  A malicious
   attacker could spoof the IP address of a victim and send a flood of
   I1 messages to a powerful Responder.  For each small I1 packet, the
   Responder would send a larger R1 packet to the victim.  The
   difference in packet sizes can further amplify a flooding attack
   against the victim.  To avoid such reflection attacks, the Responder
   SHOULD rate-limit the sending of R1 packets in general or SHOULD
   rate-limit the sending of R1 packets to a specific IP address.

   Floods of forged I2 packets form a second kind of DoS attack.  Once
   the attacking Initiator has solved the puzzle, it can send packets
   with spoofed IP source addresses with either an invalid HIP signature
   or invalid encrypted HIP payload (in the ENCRYPTED parameter).  This
   would take resources in the Responder's part to reach the point to
   discover that the I2 packet cannot be completely processed.  The
   defense against this attack is that after N bad I2 packets with the
   same puzzle solution, the Responder would discard any I2 packets that
   contain the given solution.  This will shut down the attack.  The
   attacker would have to request another R1 packet and use that to
   launch a new attack.  The Responder could increase the value of #K
   while under attack.  Keeping a list of solutions from malformed
   packets requires that the Responder keeps state for these malformed
   I2 packets.  This state has to be kept until the R1 counter is
   increased.  As malformed packets are generally filtered by their
   checksum before signature verification, only solutions in packets
   that are forged to pass the checksum and puzzle are put into the
   blacklist.  In addition, a valid puzzle is required before a new list
   entry is created.  Hence, attackers that intend to flood the
   blacklist must solve puzzles first.







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   A third form of DoS attack is emulating the restart of state after a
   reboot of one of the peers.  A restarting host would send an I1
   packet to the peers, which would respond with an R1 packet even if it
   were in the ESTABLISHED state.  If the I1 packet were spoofed, the
   resulting R1 packet would be received unexpectedly by the spoofed
   host and would be dropped, as in the first case above.

   A fourth form of DoS attack is emulating the closing of the HIP
   association.  HIP relies on timers and a CLOSE/CLOSE_ACK handshake to
   explicitly signal the end of a HIP association.  Because both CLOSE
   and CLOSE_ACK messages contain a HIP_MAC, an outsider cannot close a
   connection.  The presence of an additional SIGNATURE allows
   middleboxes to inspect these messages and discard the associated
   state (e.g., for firewalling, SPI-based NATing, etc.).  However, the
   optional behavior of replying to CLOSE with an ICMP Parameter Problem
   packet (as described in Section 5.4.4) might allow an attacker
   spoofing the source IP address to send CLOSE messages to launch
   reflection attacks.

   A fifth form of DoS attack is replaying R1s to cause the Initiator to
   solve stale puzzles and become out of synchronization with the
   Responder.  The R1 generation counter is a monotonically increasing
   counter designed to protect against this attack, as described in
   Section 4.1.4.

   Man-in-the-middle attacks are difficult to defend against, without
   third-party authentication.  A skillful MitM could easily handle all
   parts of HIP, but HIP indirectly provides the following protection
   from a MitM attack.  If the Responder's HI is retrieved from a signed
   DNS zone, a certificate, or through some other secure means, the
   Initiator can use this to validate the R1 HIP packet.

   Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
   certificate, or otherwise securely available, the Responder can
   retrieve the HI (after having got the I2 HIP packet) and verify that
   the HI indeed can be trusted.

   The HIP "opportunistic mode" concept has been introduced in this
   document, but this document does not specify what the semantics of
   such a connection setup are for applications.  There are certain
   concerns with opportunistic mode, as discussed in Section 4.1.8.

   NOTIFY messages are used only for informational purposes, and they
   are unacknowledged.  A HIP implementation cannot rely solely on the
   information received in a NOTIFY message because the packet may have
   been replayed.  An implementation SHOULD NOT change any state
   information purely based on a received NOTIFY message.




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   Since not all hosts will ever support HIP, ICMP 'Destination Protocol
   Unreachable' messages are to be expected and may be used for a DoS
   attack.  Against an Initiator, the attack would look like the
   Responder does not support HIP, but shortly after receiving the ICMP
   message, the Initiator would receive a valid R1 HIP packet.  Thus, to
   protect against this attack, an Initiator SHOULD NOT react to an ICMP
   message until a reasonable delta time to get the real Responder's R1
   HIP packet.  A similar attack against the Responder is more involved.
   Normally, if an I1 message received by a Responder was a bogus one
   sent by an attacker, the Responder may receive an ICMP message from
   the IP address the R1 message was sent to.  However, a sophisticated
   attacker can try to take advantage of such behavior and try to break
   up the HIP base exchange by sending such an ICMP message to the
   Responder before the Initiator has a chance to send a valid I2
   message.  Hence, the Responder SHOULD NOT act on such an ICMP
   message.  Especially, it SHOULD NOT remove any minimal state created
   when it sent the R1 HIP packet (if it did create one), but wait for
   either a valid I2 HIP packet or the natural timeout (that is, if R1
   packets are tracked at all).  Likewise, the Initiator SHOULD ignore
   any ICMP message while waiting for an R2 HIP packet, and SHOULD
   delete any pending state only after a natural timeout.

9.  IANA Considerations

   IANA has reserved protocol number 139 for the Host Identity Protocol
   and included it in the "IPv6 Extension Header Types" registry
   [RFC7045] and the "Assigned Internet Protocol Numbers" registry.  The
   reference in both of these registries has been updated from [RFC5201]
   to this specification.

   The reference to the 128-bit value under the CGA Message Type
   namespace [RFC3972] of "0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA"
   has been changed from [RFC5201] to this specification.

   The following changes to the "Host Identity Protocol (HIP)
   Parameters" have been made.  In many cases, the changes involved
   updating the reference from [RFC5201] to this specification, but
   there are some differences as outlined below.  Allocation terminology
   is defined in [RFC5226]; any existing references to "IETF Consensus"
   can be replaced with "IETF Review" as per [RFC5226].











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   HIP Version

      This document adds the value "2" to the existing registry.  The
      value of "1" has been left with a reference to [RFC5201].

   Packet Type

      The 7-bit Packet Type field in a HIP protocol packet describes the
      type of a HIP protocol message.  It is defined in Section 5.1.
      All existing values referring to [RFC5201] have been updated to
      refer to this specification.  Other values have been left
      unchanged.

   HIT Suite ID

      This specification creates a new registry for "HIT Suite ID".
      This is different than the existing registry for "Suite ID", which
      can be left unmodified for version 1 of the protocol ([RFC5201]).
      The registry has been closed to new registrations.

      The four-bit HIT Suite ID uses the OGA ID field in the ORCHID to
      express the type of the HIT.  This document defines three HIT
      Suites (see Section 5.2.10).

      The HIT Suite ID is also carried in the four higher-order bits of
      the ID field in the HIT_SUITE_LIST parameter.  The four
      lower-order bits are reserved for future extensions of the HIT
      Suite ID space beyond 16 values.

      For the time being, the HIT Suite uses only four bits because
      these bits have to be carried in the HIT.  Using more bits for the
      HIT Suite ID reduces the cryptographic strength of the HIT.  HIT
      Suite IDs must be allocated carefully to avoid namespace
      exhaustion.  Moreover, deprecated IDs should be reused after an
      appropriate time span.  If 15 Suite IDs (the zero value is
      initially reserved) prove to be insufficient and more HIT Suite
      IDs are needed concurrently, more bits can be used for the HIT
      Suite ID by using one HIT Suite ID (0) to indicate that more bits
      should be used.  The HIT_SUITE_LIST parameter already supports
      8-bit HIT Suite IDs, should longer IDs be needed.  However,
      RFC 7343 [RFC7343] does not presently support such an extension.
      We suggest trying the rollover approach described in Appendix E
      first.  Possible extensions of the HIT Suite ID space to
      accommodate eight bits and new HIT Suite IDs are defined through
      IETF Review.






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      Requests to register reused values should include a note that the
      value is being reused after a deprecation period, to ensure
      appropriate IETF review and approval.

   Parameter Type

      The 16-bit Type field in a HIP parameter describes the type of the
      parameter.  It is defined in Section 5.2.1.  The current values
      are defined in Sections 5.2.3 through 5.2.23.  The existing
      "Parameter Types" registry has been updated as follows.

      A new value (129) for R1_COUNTER has been introduced, with a
      reference to this specification, and the existing value (128) for
      R1_COUNTER has been left in place with a reference to [RFC5201].
      This documents the change in value that has occurred in version 2
      of this protocol.  For clarity, the name for the value 128 has
      been changed from "R1_COUNTER" to "R1_Counter (v1 only)".

      A new value (579) for a new Parameter Type HIP_CIPHER has been
      added, with reference to this specification.  This Parameter Type
      functionally replaces the HIP_TRANSFORM Parameter Type
      (value 577), which has been left in the table with the existing
      reference to [RFC5201].  For clarity, the name for the
      value 577 has been changed from "HIP_TRANSFORM" to
      "HIP_TRANSFORM (v1 only)".

      A new value (715) for a new Parameter Type HIT_SUITE_LIST has been
      added, with reference to this specification.

      A new value (2049) for a new Parameter Type TRANSPORT_FORMAT_LIST
      has been added, with reference to this specification.

      The name of the HMAC Parameter Type (value 61505) has been changed
      to HIP_MAC.  The name of the HMAC_2 Parameter Type (value 61569)
      has been changed to HIP_MAC_2.  The reference has been changed to
      this specification.

      All other Parameter Types that reference [RFC5201] have been
      updated to refer to this specification, and Parameter Types that
      reference other RFCs are unchanged.

      The Type codes 32768 through 49151 (not 49141: a value corrected
      from a previous version of this table) have been Reserved for
      Private Use.  Implementors SHOULD select types in a random fashion
      from this range, thereby reducing the probability of collisions.
      A method employing genuine randomness (such as flipping a coin)
      SHOULD be used.




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      Where the existing ranges once stated "First Come First Served
      with Specification Required", this has been changed to
      "Specification Required".

   Group ID

      The eight-bit Group ID values appear in the DIFFIE_HELLMAN
      parameter and the DH_GROUP_LIST parameter and are defined in
      Section 5.2.7.  This registry has been updated based on the new
      values specified in Section 5.2.7; values noted as being
      DEPRECATED can be left in the table with reference to [RFC5201].
      New values are assigned through IETF Review.

   HIP Cipher ID

      The 16-bit Cipher ID values in a HIP_CIPHER parameter are defined
      in Section 5.2.8.  This is a new registry.  New values from either
      the reserved or unassigned space are assigned through IETF Review.

   DI-Type

      The four-bit DI-Type values in a HOST_ID parameter are defined in
      Section 5.2.9.  New values are assigned through IETF Review.  All
      existing values referring to [RFC5201] have been updated to refer
      to this specification.

   HI Algorithm

      The 16-bit Algorithm values in a HOST_ID parameter are defined in
      Section 5.2.9.  This is a new registry.  New values from either
      the reserved or unassigned space are assigned through IETF Review.

   ECC Curve Label

      When the HI Algorithm values in a HOST_ID parameter are defined to
      the values of either "ECDSA" or "ECDSA_LOW", a new registry is
      needed to maintain the values for the ECC Curve Label as defined
      in Section 5.2.9.  This might be handled by specifying two
      algorithm-specific subregistries named "ECDSA Curve Label" and
      "ECDSA_LOW Curve Label".  New values are to be assigned through
      IETF Review.










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   Notify Message Type

      The 16-bit Notify Message Type values in a NOTIFICATION parameter
      are defined in Section 5.2.19.

      Notify Message Type values 1-10 are used for informing about
      errors in packet structures, values 11-20 for informing about
      problems in parameters containing cryptographic related material,
      and values 21-30 for informing about problems in authentication or
      packet integrity verification.  Parameter numbers above 30 can be
      used for informing about other types of errors or events.

      The existing registration procedures have been updated as follows.
      The range from 1-50 can remain as "IETF Review".  The range from
      51-8191 has been marked as "Specification Required".  Values
      8192-16383 remain as "Reserved for Private Use".  Values
      16384-40959 have been marked as "Specification Required".  Values
      40960-65535 remain as "Reserved for Private Use".

      The following updates to the values have been made to the existing
      registry.  All existing values referring to [RFC5201] have been
      updated to refer to this specification.

      INVALID_HIP_TRANSFORM_CHOSEN has been renamed to
      INVALID_HIP_CIPHER_CHOSEN with the same value (17).

      A new value of 20 for the type UNSUPPORTED_HIT_SUITE has been
      added.

      HMAC_FAILED has been renamed to HIP_MAC_FAILED with the same
      value (28).

      SERVER_BUSY_PLEASE_RETRY has been renamed to
      RESPONDER_BUSY_PLEASE_RETRY with the same value (44).

10.  Differences from RFC 5201

   This section summarizes the technical changes made from [RFC5201].
   This section is informational, intended to help implementors of the
   previous protocol version.  If any text in this section contradicts
   text in other portions of this specification, the text found outside
   of this section should be considered normative.

   This document specifies the HIP Version 2 protocol, which is not
   interoperable with the HIP Version 1 protocol specified in [RFC5201].
   The main technical changes are the inclusion of additional
   cryptographic agility features, and an update of the mandatory and
   optional algorithms, including Elliptic Curve support via the



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   Elliptic Curve DSA (ECDSA) and Elliptic Curve Diffie-Hellman (ECDH)
   algorithms.  The mandatory cryptographic algorithm implementations
   have been updated, such as replacing HMAC-SHA-1 with HMAC-SHA-256 and
   the RSA/SHA-1 signature algorithm with RSASSA-PSS, and adding ECDSA
   to RSA as mandatory public key types.  This version of HIP is also
   aligned with the ORCHID revision [RFC7343].

   The following changes have been made to the protocol operation.

   o  Section 4.1.3 describes the new process for Diffie-Hellman group
      negotiation, an aspect of cryptographic agility.  The Initiator
      may express a preference for the choice of a DH group in the I1
      packet and may suggest multiple possible choices.  The Responder
      replies with a preference based on local policy and the options
      provided by the Initiator.  The Initiator may restart the base
      exchange if the option chosen by the Responder is unsuitable
      (unsupported algorithms).

   o  Another aspect of cryptographic agility that has been added is the
      ability to use different cryptographic hash functions to generate
      the HIT.  The Responder's HIT hash algorithm (RHASH) terminology
      was introduced to support this.  In addition, HIT Suites have been
      introduced to group the set of cryptographic algorithms used
      together for public key signature, hash function, and hash
      truncation.  The use of HIT Suites constrains the combinatorial
      possibilities of algorithm selection for different functions.  HIT
      Suite IDs are related to the ORCHID OGA ID field ([RFC7343]).

   o  The puzzle mechanism has been slightly changed, in that the #I
      parameter depends on the HIT hash function (RHASH) selected, and
      the specification now advises against reusing the same #I value to
      the same Initiator; more details are provided in Sections 4.1.2
      and 5.2.4).

   o  Section 4.1.4 was extended to cover details about R1 generation
      counter rollover or reset.

   o  Section 4.1.6 was added to describe procedures for aborting a HIP
      base exchange.

   o  Section 4.1.7 provides guidance on avoiding downgrade attacks on
      the cryptographic algorithms.

   o  Section 4.1.8 on opportunistic mode has been updated to account
      for cryptographic agility by adding HIT selection procedures.






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   o  The HIP KEYMAT generation has been updated as described in
      Section 6.5 to make the key derivation function a negotiable
      aspect of the protocol.

   o  Packet processing for the I1, R1, and I2 packets has been updated
      to account for new parameter processing.

   o  This specification adds a requirement that hosts MUST support
      processing of ACK parameters with several SEQ sequence numbers
      even when they do not support sending such parameters.

   o  This document now clarifies that several ECHO_REQUEST_UNSIGNED
      parameters may be present in an R1 and that several ECHO_RESPONSE
      parameters may be present in an I2.

   o  Procedures for responding to version mismatches with an ICMP
      Parameter Problem have been added.

   o  The security considerations section (Section 8) has been updated
      to remove possible attacks no longer considered applicable.

   o  The use of the Anonymous bit for making the sender's Host Identity
      anonymous is now supported in packets other than the R1 and I2.

   o  Support for the use of a NULL HIP CIPHER is explicitly limited to
      debugging and testing HIP and is no longer a mandatory algorithm
      to support.

   The following changes have been made to the parameter types and
   encodings (Section 5.2).

   o  Four new parameter types have been added: DH_GROUP_LIST,
      HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST.

   o  Two parameter types have been renamed: HMAC has been renamed to
      HIP_MAC, and HMAC2 has been renamed to HIP_MAC_2.

   o  One parameter type is deprecated: HIP_TRANSFORM.  Functionally, it
      has been replaced by the HIP_CIPHER but with slightly different
      semantics (hashes have been removed and are now determined by
      RHASH).

   o  The TRANSPORT_FORMAT_LIST parameter allows transports to be
      negotiated with the list instead of by their order in the
      HIP packet.






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   o  The type code for the R1_COUNTER has been changed from 128 to 129
      to reflect that it is now considered a Critical parameter and must
      be echoed when present in R1.

   o  The PUZZLE and SOLUTION parameter lengths are now variable and
      dependent on the RHASH length.

   o  The Diffie-Hellman Group IDs supported have been updated.

   o  The HOST_ID parameter now requires specification of an Algorithm.

   o  The NOTIFICATION parameter supports new Notify Message Type
      values.

   o  The HIP_SIGNATURE algorithm field has been changed from 8 bits to
      16 bits to achieve alignment with the HOST_ID parameters.

   o  The specification clarifies that the SEQ parameter always contains
      one update ID but that the ACK parameter may acknowledge several
      update IDs.

   o  The restriction that only one ECHO_RESPONSE_UNSIGNED parameter
      must be present in each HIP packet has been removed.

   o  The document creates a new type range allocation for parameters
      that are only covered by a signature if a signature is present and
      applies it to the newly created DH_GROUP_LIST parameter.

   o  The document clarifies that several NOTIFY parameters may be
      present in a packet.

   The following changes have been made to the packet contents
   (Section 5.3).

   o  The I1 packet now carries the Initiator's DH_GROUP_LIST.

   o  The R1 packet now carries the HIP_CIPHER, HIT_SUITE_LIST,
      DH_GROUP_LIST, and TRANSPORT_FORMAT_LIST parameters.

   o  The I2 packet now carries the HIP_CIPHER and TRANSPORT_FORMAT_LIST
      parameters.

   o  This document clarifies that UPDATE packets that do not contain
      either a SEQ or ACK parameter are invalid.







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11.  References

11.1.  Normative References

   [FIPS.180-4.2012]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

   [NIST.800-131A.2011]
              National Institute of Standards and Technology,
              "Transitions: Recommendation for Transitioning the Use of
              Cryptographic Algorithms and Key Lengths", NIST
              SP 800-131A, January 2011, <http://csrc.nist.gov/
              publications/nistpubs/800-131A/sp800-131A.pdf>.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980, <http://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981, <http://www.rfc-editor.org/
              info/rfc793>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987,
              <http://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
              ESP and AH", RFC 2404, November 1998,
              <http://www.rfc-editor.org/info/rfc2404>.

   [RFC2410]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
              Its Use With IPsec", RFC 2410, November 1998,
              <http://www.rfc-editor.org/info/rfc2410>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998,
              <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2536]  Eastlake 3rd, D., "DSA KEYs and SIGs in the Domain Name
              System (DNS)", RFC 2536, March 1999,
              <http://www.rfc-editor.org/info/rfc2536>.




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   [RFC3110]  Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
              Domain Name System (DNS)", RFC 3110, May 2001,
              <http://www.rfc-editor.org/info/rfc3110>.

   [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, May 2003, <http://www.rfc-editor.org/
              info/rfc3526>.

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602,
              September 2003, <http://www.rfc-editor.org/info/rfc3602>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005, <http://www.rfc-editor.org/
              info/rfc3972>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005, <http://www.rfc-editor.org/
              info/rfc4034>.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005,
              <http://www.rfc-editor.org/info/rfc4282>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006,
              <http://www.rfc-editor.org/info/rfc4443>.

   [RFC4754]  Fu, D. and J. Solinas, "IKE and IKEv2 Authentication Using
              the Elliptic Curve Digital Signature Algorithm (ECDSA)",
              RFC 4754, January 2007, <http://www.rfc-editor.org/
              info/rfc4754>.

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
              HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
              May 2007, <http://www.rfc-editor.org/info/rfc4868>.

   [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
              and RRSIG Resource Records for DNSSEC", RFC 5702,
              October 2009, <http://www.rfc-editor.org/info/rfc5702>.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.



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   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, September 2014,
              <http://www.rfc-editor.org/info/rfc7343>.

   [RFC7402]  Jokela, P., Moskowitz, R., and J. Melen, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 7402, April 2015,
              <http://www.rfc-editor.org/info/rfc7402>.

11.2.  Informative References

   [AUR05]    Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of the
              HIP Base Exchange Protocol", in Proceedings of the 10th
              Australasian Conference on Information Security and
              Privacy, July 2005.

   [CRO03]    Crosby, S. and D. Wallach, "Denial of Service via
              Algorithmic Complexity Attacks", in Proceedings of the
              12th USENIX Security Symposium, Washington, D.C.,
              August 2003.

   [DIF76]    Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory
              Volume IT-22, Number 6, pages 644-654, November 1976.

   [FIPS.186-4.2013]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-4, July 2013,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-4.pdf>.

   [FIPS.197.2001]
              National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", FIPS PUB 197, November 2001,
              <http://csrc.nist.gov/publications/fips/fips197/
              fips-197.pdf>.

   [HIP-ARCH] Moskowitz, R., Ed., and M. Komu, "Host Identity Protocol
              Architecture", Work in Progress,
              draft-ietf-hip-rfc4423-bis-09, October 2014.

   [HIP-DNS-EXT]
              Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", Work in Progress,
              draft-ietf-hip-rfc5205-bis-06, January 2015.





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   [HIP-HOST-MOB]
              Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
              with the Host Identity Protocol", Work in Progress,
              draft-ietf-hip-rfc5206-bis-08, January 2015.

   [HIP-REND-EXT]
              Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", Work in Progress,
              draft-ietf-hip-rfc5204-bis-05, December 2014.

   [KAU03]    Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
              protection for UDP-based protocols", in Proceedings of the
              10th ACM Conference on Computer and Communications
              Security, October 2003.

   [KRA03]    Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE
              Protocols", in Proceedings of CRYPTO 2003, pages 400-425,
              August 2003.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981, <http://www.rfc-editor.org/
              info/rfc792>.

   [RFC2785]  Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
              Attacks on the Diffie-Hellman Key Agreement Method for
              S/MIME", RFC 2785, March 2000,
              <http://www.rfc-editor.org/info/rfc2785>.

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898, September 2000,
              <http://www.rfc-editor.org/info/rfc2898>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003,
              <http://www.rfc-editor.org/info/rfc3447>.

   [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
              Reserved for Documentation", RFC 3849, July 2004,
              <http://www.rfc-editor.org/info/rfc3849>.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008,
              <http://www.rfc-editor.org/info/rfc5201>.






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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008, <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5338]  Henderson, T., Nikander, P., and M. Komu, "Using the Host
              Identity Protocol with Legacy Applications", RFC 5338,
              September 2008, <http://www.rfc-editor.org/info/rfc5338>.

   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, June 2009,
              <http://www.rfc-editor.org/info/rfc5533>.

   [RFC5737]  Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
              Reserved for Documentation", RFC 5737, January 2010,
              <http://www.rfc-editor.org/info/rfc5737>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC5903]  Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
              Prime (ECP Groups) for IKE and IKEv2", RFC 5903,
              June 2010, <http://www.rfc-editor.org/info/rfc5903>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011,
              <http://www.rfc-editor.org/info/rfc6090>.

   [RFC6253]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 6253, May 2011,
              <http://www.rfc-editor.org/info/rfc6253>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045, December 2013,
              <http://www.rfc-editor.org/info/rfc7045>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, October 2014,
              <http://www.rfc-editor.org/info/rfc7296>.

   [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
              Obtaining Digital Signatures and Public-Key
              Cryptosystems", Communications of the ACM 21 (2),
              pp. 120-126, February 1978.

   [SECG]     SECG, "Recommended Elliptic Curve Domain Parameters",
              SEC 2 Version 2.0, January 2010, <http://www.secg.org/>.



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Appendix A.  Using Responder Puzzles

   As mentioned in Section 4.1.1, the Responder may delay state creation
   and still reject most spoofed I2 packets by using a number of
   pre-calculated R1 packets and a local selection function.  This
   appendix defines one possible implementation in detail.  The purpose
   of this appendix is to give the implementors an idea of how to
   implement the mechanism.  If the implementation is based on this
   appendix, it MAY contain some local modification that makes an
   attacker's task harder.

   The Responder creates a secret value S, that it regenerates
   periodically.  The Responder needs to remember the two latest values
   of S.  Each time the S is regenerated, the R1 generation counter
   value is incremented by one.

   The Responder generates a pre-signed R1 packet.  The signature for
   pre-generated R1s must be recalculated when the Diffie-Hellman key is
   recomputed or when the R1_COUNTER value changes due to S value
   regeneration.

   When the Initiator sends the I1 packet for initializing a connection,
   the Responder receives the HIT and IP address from the packet, and
   generates an #I value for the puzzle.  The #I value is set to the
   pre-signed R1 packet.

       #I value calculation:
       #I = Ltrunc( RHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), n)
       where n = RHASH_len

   The RHASH algorithm is the same as is used to generate the
   Responder's HIT value.

   From an incoming I2 packet, the Responder receives the required
   information to validate the puzzle: HITs, IP addresses, and the
   information of the used S value from the R1_COUNTER.  Using these
   values, the Responder can regenerate the #I, and verify it against
   the #I received in the I2 packet.  If the #I values match, it can
   verify the solution using #I, #J, and difficulty #K.  If the #I
   values do not match, the I2 is dropped.

       puzzle_check:
       V := Ltrunc( RHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), #K )
       if V != 0, drop the packet

   If the puzzle solution is correct, the #I and #J values are stored
   for later use.  They are used as input material when keying material
   is generated.



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   Keeping state about failed puzzle solutions depends on the
   implementation.  Although it is possible for the Responder not to
   keep any state information, it still may do so to protect itself
   against certain attacks (see Section 4.1.1).

Appendix B.  Generating a Public Key Encoding from an HI

   The following pseudo-code illustrates the process to generate a
   public key encoding from an HI for both RSA and DSA.

   The symbol ":=" denotes assignment; the symbol "+=" denotes
   appending.  The pseudo-function "encode_in_network_byte_order" takes
   two parameters, an integer (bignum) and a length in bytes, and
   returns the integer encoded into a byte string of the given length.

   switch ( HI.algorithm )
   {

   case RSA:
      buffer := encode_in_network_byte_order ( HI.RSA.e_len,
                ( HI.RSA.e_len > 255 ) ? 3 : 1 )
      buffer += encode_in_network_byte_order ( HI.RSA.e, HI.RSA.e_len )
      buffer += encode_in_network_byte_order ( HI.RSA.n, HI.RSA.n_len )

      break;

   case DSA:
      buffer := encode_in_network_byte_order ( HI.DSA.T , 1 )
      buffer += encode_in_network_byte_order ( HI.DSA.Q , 20 )
      buffer += encode_in_network_byte_order ( HI.DSA.P , 64 +
                                               8 * HI.DSA.T )
      buffer += encode_in_network_byte_order ( HI.DSA.G , 64 +
                                               8 * HI.DSA.T )
      buffer += encode_in_network_byte_order ( HI.DSA.Y , 64 +
                                               8 * HI.DSA.T )

      break;

   }

Appendix C.  Example Checksums for HIP Packets

   The HIP checksum for HIP packets is specified in Section 5.1.1.
   Checksums for TCP and UDP packets running over HIP-enabled security
   associations are specified in Section 4.5.1.  The examples below use
   [RFC3849] and [RFC5737] addresses, and HITs with the prefix of
   2001:20 followed by zeros, followed by a decimal 1 or 2,
   respectively.



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   The following example is defined only for testing the checksum
   calculation.

C.1.  IPv6 HIP Example (I1 Packet)

     Source Address:                 2001:db8::1
     Destination Address:            2001:db8::2
     Upper-Layer Packet Length:      48              0x30
     Next Header:                    139             0x8b
     Payload Protocol:               59              0x3b
     Header Length:                  5               0x5
     Packet Type:                    1               0x1
     Version:                        2               0x2
     Reserved:                       1               0x1
     Control:                        0               0x0
     Checksum:                       6750            0x1a5e
     Sender's HIT:                   2001:20::1
     Receiver's HIT:                 2001:20::2
     DH_GROUP_LIST type:             511             0x1ff
     DH_GROUP_LIST length:           3               0x3
     DH_GROUP_LIST Group IDs:        3,4,8

C.2.  IPv4 HIP Packet (I1 Packet)

   The IPv4 checksum value for the example I1 packet is shown below.

     Source Address:                 192.0.2.1
     Destination Address:            192.0.2.2
     Upper-Layer Packet Length:      48              0x30
     Next Header:                    139             0x8b
     Payload Protocol:               59              0x3b
     Header Length:                  5               0x5
     Packet Type:                    1               0x1
     Version:                        2               0x2
     Reserved:                       1               0x1
     Control:                        0               0x0
     Checksum:                       61902           0xf1ce
     Sender's HIT:                   2001:20::1
     Receiver's HIT:                 2001:20::2
     DH_GROUP_LIST type:             511             0x1ff
     DH_GROUP_LIST length:           3               0x3
     DH_GROUP_LIST Group IDs:        3,4,8









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C.3.  TCP Segment

   Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
   use the IPv6 pseudo header format [RFC2460], with the HITs used in
   place of the IPv6 addresses.

     Sender's HIT:                   2001:20::1
     Receiver's HIT:                 2001:20::2
     Upper-Layer Packet Length:      20              0x14
     Next Header:                    6               0x06
     Source port:                    65500           0xffdc
     Destination port:               22              0x0016
     Sequence number:                1               0x00000001
     Acknowledgment number:          0               0x00000000
     Data offset:                    5               0x5
     Flags:                          SYN             0x02
     Window size:                    65535           0xffff
     Checksum:                       28586           0x6faa
     Urgent pointer:                 0               0x0000

Appendix D.  ECDH and ECDSA 160-Bit Groups

   The ECDH and ECDSA 160-bit group SECP160R1 is rated at 80 bits
   symmetric strength.  This was once considered appropriate for one
   year of security.  Today, these groups should be used only when the
   host is not powerful enough (e.g., some embedded devices) and when
   security requirements are low (e.g., long-term confidentiality is not
   required).

Appendix E.  HIT Suites and HIT Generation

   The HIT as an ORCHID [RFC7343] consists of three parts: A 28-bit
   prefix, a 4-bit encoding of the ORCHID generation algorithm (OGA),
   and a hash that includes the Host Identity and a context ID.  The OGA
   is an index pointing to the specific algorithm by which the public
   key and the 96-bit hashed encoding are generated.  The OGA is
   protocol specific and is to be interpreted as defined below for all
   protocols that use the same context ID as HIP.  HIP groups sets of
   valid combinations of signature and hash algorithms into HIT Suites.
   These HIT Suites are addressed by an index, which is transmitted in
   the OGA ID field of the ORCHID.

   The set of used HIT Suites will be extended to counter the progress
   in computation capabilities and vulnerabilities in the employed
   algorithms.  The intended use of the HIT Suites is to introduce a new
   HIT Suite and phase out an old one before it becomes insecure.  Since
   the 4-bit OGA ID field only permits 15 HIT Suites to be used at the
   same time (the HIT Suite with ID 0 is reserved), phased-out HIT



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   Suites must be reused at some point.  In such a case, there will be a
   rollover of the HIT Suite ID and the next newly introduced HIT Suite
   will start with a lower HIT Suite index than the previously
   introduced one.  The rollover effectively deprecates the reused HIT
   Suite.  For a smooth transition, the HIT Suite should be deprecated a
   considerable time before the HIT Suite index is reused.

   Since the number of HIT Suites is tightly limited to 16, the HIT
   Suites must be assigned carefully.  Hence, sets of suitable
   algorithms are grouped in a HIT Suite.

   The HIT Suite of the Responder's HIT determines the RHASH and the
   hash function to be used for the HMAC in HIP packets as well as the
   signature algorithm family used for generating the HI.  The list of
   HIT Suites is defined in Table 10.




































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Acknowledgments

   The drive to create HIP came to being after attending the MALLOC
   meeting at the 43rd IETF meeting.  Baiju Patel and Hilarie Orman
   really gave the original author, Bob Moskowitz, the assist to get HIP
   beyond 5 paragraphs of ideas.  It has matured considerably since the
   early versions thanks to extensive input from IETFers.  Most
   importantly, its design goals are articulated and are different from
   other efforts in this direction.  Particular mention goes to the
   members of the NameSpace Research Group of the IRTF.  Noel Chiappa
   provided valuable input at early stages of discussions about
   identifier handling and Keith Moore the impetus to provide
   resolvability.  Steve Deering provided encouragement to keep working,
   as a solid proposal can act as a proof of ideas for a research group.

   Many others contributed; extensive security tips were provided by
   Steve Bellovin.  Rob Austein kept the DNS parts on track.  Paul
   Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
   for the Initiator to respond, but easy for the Responder to validate.
   Bill Sommerfeld supplied the Birthday concept, which later evolved
   into the R1 generation counter, to simplify reboot management.  Erik
   Nordmark supplied the CLOSE-mechanism for closing connections.
   Rodney Thayer and Hugh Daniels provided extensive feedback.  In the
   early times of this document, John Gilmore kept Bob Moskowitz
   challenged to provide something of value.

   During the later stages of this document, when the editing baton was
   transferred to Pekka Nikander, the input from the early implementors
   was invaluable.  Without having actual implementations, this document
   would not be on the level it is now.

   In the usual IETF fashion, a large number of people have contributed
   to the actual text or ideas.  The list of these people includes Jeff
   Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Xin Gu, Rene
   Hummen, Miika Komu, Mika Kousa, Julien Laganier, Andrew McGregor, Jan
   Melen, Henrik Petander, Michael Richardson, Tim Shepard, Jorma Wall,
   and Jukka Ylitalo.  Our apologies to anyone whose name is missing.

   Once the HIP Working Group was founded in early 2004, a number of
   changes were introduced through the working group process.  Most
   notably, the original document was split in two, one containing the
   base exchange and the other one defining how to use ESP.  Some
   modifications to the protocol proposed by Aura, et al. [AUR05] were
   added at a later stage.







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Authors' Addresses

   Robert Moskowitz (editor)
   HTT Consulting
   Oak Park, MI
   United States

   EMail: rgm@labs.htt-consult.com


   Tobias Heer
   Hirschmann Automation and Control
   Stuttgarter Strasse 45-51
   Neckartenzlingen  72654
   Germany

   EMail: tobias.heer@belden.com


   Petri Jokela
   Ericsson Research NomadicLab
   Jorvas  FIN-02420
   Finland

   Phone: +358 9 299 1
   EMail: petri.jokela@nomadiclab.com


   Thomas R. Henderson
   University of Washington
   Campus Box 352500
   Seattle, WA
   United States

   EMail: tomhend@u.washington.edu
















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