





Independent Submission                                       D. Ovsienko
Request for Comments: 7298                                        Yandex
Updates: 6126                                                  July 2014
Category: Experimental
ISSN: 2070-1721


            Babel Hashed Message Authentication Code (HMAC)
                      Cryptographic Authentication

Abstract

   This document describes a cryptographic authentication mechanism for
   the Babel routing protocol.  This document updates RFC 6126.  The
   mechanism allocates two new TLV types for the authentication data,
   uses Hashed Message Authentication Code (HMAC), and is both optional
   and backward compatible.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This is a contribution to the RFC Series, independently
   of any other RFC stream.  The RFC Editor has chosen to publish this
   document at its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see 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/rfc7298.

Copyright Notice

   Copyright (c) 2014 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.




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Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................5
   2. Cryptographic Aspects ...........................................5
      2.1. Mandatory-to-Implement and Optional Hash Algorithms ........5
      2.2. Definition of Padding ......................................6
      2.3. Cryptographic Sequence Number Specifics ....................8
      2.4. Definition of HMAC .........................................9
   3. Updates to Protocol Data Structures ............................11
      3.1. RxAuthRequired ............................................11
      3.2. LocalTS ...................................................11
      3.3. LocalPC ...................................................11
      3.4. MaxDigestsIn ..............................................11
      3.5. MaxDigestsOut .............................................12
      3.6. ANM Table .................................................12
      3.7. ANM Timeout ...............................................13
      3.8. Configured Security Associations ..........................14
      3.9. Effective Security Associations ...........................16
   4. Updates to Protocol Encoding ...................................17
      4.1. Justification .............................................17
      4.2. TS/PC TLV .................................................19
      4.3. HMAC TLV ..................................................20
   5. Updates to Protocol Operation ..................................21
      5.1. Per-Interface TS/PC Number Updates ........................21
      5.2. Deriving ESAs from CSAs ...................................23
      5.3. Updates to Packet Sending .................................25
      5.4. Updates to Packet Receiving ...............................28
      5.5. Authentication-Specific Statistics Maintenance ............30
   6. Implementation Notes ...........................................31
      6.1. Source Address Selection for Sending ......................31
      6.2. Output Buffer Management ..................................31
      6.3. Optimizations of Deriving Procedure for ESAs ..............32
      6.4. Duplication of Security Associations ......................33
   7. Network Management Aspects .....................................34
      7.1. Backward Compatibility ....................................34
      7.2. Multi-Domain Authentication ...............................35
      7.3. Migration to and from Authenticated Exchange ..............36
      7.4. Handling of Authentication Key Exhaustion .................37
   8. Security Considerations ........................................38
   9. IANA Considerations ............................................43
   10. Acknowledgements ..............................................43
   11. References ....................................................44
      11.1. Normative References .....................................44
      11.2. Informative References ...................................44
   Appendix A. Figures and Tables ....................................47
   Appendix B. Test Vectors ..........................................52




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1.  Introduction

   Authentication of routing protocol exchanges is a common means of
   securing computer networks.  The use of protocol authentication
   mechanisms helps in ascertaining that only the intended routers
   participate in routing information exchange and that the exchanged
   routing information is not modified by a third party.

   [BABEL] ("the original specification") defines data structures,
   encoding, and the operation of a basic Babel routing protocol
   instance ("instance of the original protocol").  This document ("this
   specification") defines data structures, encoding, and the operation
   of an extension to the Babel protocol -- an authentication mechanism
   ("this mechanism").  Both the instance of the original protocol and
   this mechanism are mostly self-contained and interact only at
   coupling points defined in this specification.

   A major design goal of this mechanism is transparency to operators
   that is not affected by implementation and configuration specifics.
   A complying implementation makes all meaningful details of
   authentication-specific processing clear to the operator, even when
   some of the operational parameters cannot be changed.

   The currently established (see [RIP2-AUTH], [OSPF2-AUTH],
   [ISIS-AUTH-A], [RFC6039], and [OSPF3-AUTH-BIS]) approach to an
   authentication mechanism design for datagram-based routing protocols
   such as Babel relies on two principal data items embedded into
   protocol packets, typically as two integral parts of a single data
   structure:

   o  A fixed-length unsigned integer, typically called a cryptographic
      sequence number, used in replay attack protection.

   o  A variable-length sequence of octets, a result of the Hashed
      Message Authentication Code (HMAC) construction (see [RFC2104])
      computed on meaningful data items of the packet (including the
      cryptographic sequence number) on one hand and a secret key on the
      other, used in proving that both the sender and the receiver share
      the same secret key and that the meaningful data was not changed
      in transmission.

   Depending on the design specifics, either all protocol packets or
   only those packets protecting the integrity of protocol exchange are
   authenticated.  This mechanism authenticates all protocol packets.

   Although the HMAC construction is just one of many possible
   approaches to cryptographic authentication of packets, this mechanism
   makes use of relevant prior experience by using HMAC as well, and its



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   solution space correlates with the solution spaces of the mechanisms
   above.  At the same time, it allows for a future extension that
   treats HMAC as a particular case of a more generic mechanism.
   Practical experience with the mechanism defined herein should be
   useful in designing such a future extension.

   This specification defines the use of the cryptographic sequence
   number in detail sufficient to make replay attack protection strength
   predictable.  That is, an operator can tell the strength from the
   declared characteristics of an implementation and, if the
   implementation allows the changing of relevant parameters, the effect
   of a reconfiguration as well.

   This mechanism explicitly allows for multiple HMAC results per
   authenticated packet.  Since meaningful data items of a given packet
   remain the same, each such HMAC result stands for a different secret
   key and/or a different hash algorithm.  This enables a simultaneous,
   independent authentication within multiple domains.  This
   specification is not novel in this regard; for example, the Layer 2
   Tunneling Protocol (L2TPv3) allows for one or two results per
   authenticated packet ([RFC3931] Section 5.4.1), and Mobile Ad Hoc
   Network (MANET) protocols allow for several ([RFC7183] Section 6.1).

   An important concern addressed by this mechanism is limiting the
   amount of HMAC computations done per authenticated packet,
   independently for sending and receiving.  Without these limits, the
   number of computations per packet could be as high as the number of
   configured authentication keys (in the sending case) or as high as
   the number of keys multiplied by the number of supplied HMAC results
   (in the receiving case).

   These limits establish a basic competition between the configured
   keys and (in the receiving case) an additional competition between
   the supplied HMAC results.  This specification defines related data
   structures and procedures in a way to make such competition
   transparent and predictable for an operator.

   Wherever this specification mentions the operator reading or changing
   a particular data structure, variable, parameter, or event counter
   "at runtime", it is up to the implementor how this is to be done.
   For example, the implementation can employ an interactive command
   line interface (CLI), a management protocol such as the Simple
   Network Management Protocol (SNMP), a means of inter-process
   communication such as a local socket, or a combination of these.







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1.1.  Requirements Language

   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 BCP 14 [RFC2119].

2.  Cryptographic Aspects

2.1.  Mandatory-to-Implement and Optional Hash Algorithms

   [RFC2104] defines HMAC as a construction that can use any
   cryptographic hash algorithm with a known digest length and internal
   block size.  This specification preserves this property of HMAC by
   defining data processing that itself does not depend on any
   particular hash algorithm either.  However, since this mechanism is a
   protocol extension case, there are relevant design considerations to
   take into account.

   Section 4.5 of [RFC6709] suggests selecting one hash algorithm as
   mandatory to implement for the purpose of global interoperability
   (Section 3.2 of [RFC6709]) and selecting another of distinct lineage
   as recommended for implementation for the purpose of cryptographic
   agility.  This specification makes the latter property guaranteed,
   rather than probable, through an elevation of the requirement level.
   There are two mandatory-to-implement hash algorithms; each is
   unambiguously defined and generally available in multiple
   implementations.

   An implementation of this mechanism MUST include support for two hash
   algorithms:

   o  RIPEMD-160 (160-bit digest)

   o  SHA-1 (160-bit digest)

   Besides that, an implementation of this mechanism MAY include support
   for additional hash algorithms, provided each such algorithm is
   publicly and openly specified and its digest length is 128 bits or
   more (to meet the constraint implied in Section 2.2).  Implementors
   SHOULD consider strong, well-known hash algorithms as additional
   implementation options and MUST NOT consider a hash algorithm if
   meaningful attacks exist for it or it is commonly viewed as
   deprecated.








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   In the latter case, it is important to take into account
   considerations both common (such as those made in [RFC4270]) and
   specific to the HMAC application of the hash algorithm.  For example,
   [RFC6151] considers MD5 collisions and concludes that new protocol
   designs should not use HMAC-MD5, while [RFC6194] includes a
   comparable analysis of SHA-1 that finds HMAC-SHA-1 secure for the
   same purpose.

   For example, the following hash algorithms meet these requirements at
   the time of this writing (in alphabetical order):

   o  GOST R 34.11-94 (256-bit digest)

   o  SHA-224 (224-bit digest, SHA-2 family)

   o  SHA-256 (256-bit digest, SHA-2 family)

   o  SHA-384 (384-bit digest, SHA-2 family)

   o  SHA-512 (512-bit digest, SHA-2 family)

   o  Tiger (192-bit digest)

   o  Whirlpool (512-bit digest, 2nd rev., 2003)

   The set of hash algorithms available in an implementation MUST be
   clearly stated.  When known weak authentication keys exist for a hash
   algorithm used in the HMAC construction, an implementation MUST deny
   the use of such keys.

2.2.  Definition of Padding

   Many practical applications of HMAC for authentication of datagram-
   based network protocols (including routing protocols) involve the
   padding procedure, a design-specific conditioning of the message that
   both the sender and the receiver perform before the HMAC computation.
   The specific padding procedure of this mechanism addresses the
   following needs:

   o  Data Initialization

      A design that places the HMAC result(s) computed for a message
      inside that same message after the computation has to have
      previously (i.e., before the computation) allocated in that
      message some data unit(s) purposed specifically for those HMAC






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      result(s) (in this mechanism, it is the HMAC TLV(s); see
      Section 4.3).  The padding procedure sets the respective octets of
      the data unit(s), in the simplest case to a fixed value known as
      the padding constant.

      The particular value of the constant is specific to each design.
      For instance, in [RIP2-AUTH] as well as works derived from it
      ([ISIS-AUTH-B], [OSPF2-AUTH], and [OSPF3-AUTH-BIS]), the value is
      0x878FE1F3.  In many other designs (for instance, [RFC3315],
      [RFC3931], [RFC4030], [RFC4302], [RFC5176], and [ISIS-AUTH-A]),
      the value is 0x00.

      However, the HMAC construction is defined on the basis of a
      cryptographic hash algorithm, that is, an algorithm meeting a
      particular set of requirements made for any input message.  Thus,
      any padding constant values, whether single- or multiple-octet, as
      well as any other message-conditioning methods, don't affect
      cryptographic characteristics of the hash algorithm and the HMAC
      construction, respectively.

   o  Source Address Protection

      In the specific case of datagram-based routing protocols, the
      protocol packet (that is, the message being authenticated) often
      does not include network-layer addresses, although the source and
      (to a lesser extent) the destination address of the datagram may
      be meaningful in the scope of the protocol instance.

      In Babel, the source address may be used as a prefix next hop (see
      Section 3.5.3 of [BABEL]).  A well-known (see Section 2.3 of
      [OSPF3-AUTH-BIS]) solution to the source address protection
      problem is to set the first respective octets of the data unit(s)
      above to the source address (yet setting the rest of the octets to
      the padding constant).  This procedure adapts this solution to the
      specifics of Babel, which allows for the exchange of protocol
      packets using both IPv4 and IPv6 datagrams (see Section 4 of
      [BABEL]).  Even though in the case of IPv6 exchange a Babel
      speaker currently uses only link-local source addresses
      (Section 3.1 of [BABEL]), this procedure protects all octets of an
      arbitrary given source address for the reasons of future
      extensibility.  The procedure implies that future Babel extensions
      will never use an IPv4-mapped IPv6 address as a packet source
      address.








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      This procedure does not protect the destination address, which is
      currently considered meaningless (Section 3.1 of [BABEL]) in the
      same scope.  A future extension that looks to add such protection
      would likely use a new TLV or sub-TLV to include the destination
      address in the protocol packet (see Section 4.1).

   Description of the padding procedure:

   1.  Set the first 16 octets of the Digest field of the given HMAC
       TLV to:

       *  the given source address, if it is an IPv6 address, or

       *  the IPv4-mapped IPv6 address (per Section 2.5.5.2 of
          [RFC4291]) holding the given source address, if it is an IPv4
          address.

   2.  Set the remaining (TLV Length - 18) octets of the Digest field of
       the given HMAC TLV to 0x00 each.

   For an example of a Babel packet with padded HMAC TLVs, see Table 3
   in Appendix A.

2.3.  Cryptographic Sequence Number Specifics

   The operation of this mechanism may involve multiple local and
   multiple remote cryptographic sequence numbers, each essentially
   being a 48-bit unsigned integer.  This specification uses the term
   "TS/PC number" to avoid confusion with the route's (Section 2.5 of
   [BABEL]) or node's (Section 3.2.1 of [BABEL]) sequence numbers of the
   original Babel specification and to stress the fact that there are
   two distinguished parts of this 48-bit number, each handled in its
   specific way (see Section 5.1):

    0                   1     2 3                   4
    0 1 2 3 4 5 6 7 8 9 0 //  9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         TS         //         |              PC               |
   +-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      //

   The high-order 32 bits are called "timestamp" (TS), and the low-order
   16 bits are called "packet counter" (PC).








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   This mechanism stores, updates, compares, and encodes each TS/PC
   number as two independent unsigned integers -- TS and PC,
   respectively.  Such a comparison of TS/PC numbers, as performed in
   item 3 of Section 5.4, is algebraically equivalent to a comparison of
   the respective 48-bit unsigned integers.  Any byte order conversion,
   when required, is performed on TS and PC parts independently.

2.4.  Definition of HMAC

   The algorithm description below uses the following nomenclature,
   which is consistent with [FIPS-198]:

   Text   The data on which the HMAC is calculated (note item (b) of
          Section 8).  In this specification, it is the contents of a
          Babel packet ranging from the beginning of the Magic field of
          the Babel packet header to the end of the last octet of the
          Packet Body field, as defined in Section 4.2 of [BABEL] (see
          Figure 2 in Appendix A).

   H      The specific hash algorithm (see Section 2.1).

   K      A sequence of octets of an arbitrary, known length.

   Ko     The cryptographic key used with the hash algorithm.

   B      The block size of H, measured in octets rather than bits.
          Note that B is the internal block size, not the digest length.

   L      The digest length of H, measured in octets rather than bits.

   XOR    The bitwise exclusive-or operation.

   Opad   The hexadecimal value 0x5C repeated B times.

   Ipad   The hexadecimal value 0x36 repeated B times.

   The algorithm below is the original, unmodified HMAC construction as
   defined in both [RFC2104] and [FIPS-198]; hence, it is different from
   the algorithms defined in [RIP2-AUTH], [ISIS-AUTH-B], [OSPF2-AUTH],
   and [OSPF3-AUTH-BIS] in exactly two regards:

   o  The algorithm below sets the size of Ko to B, not to L (L is not
      greater than B).  This resolves both ambiguity in XOR expressions
      and incompatibility in the handling of keys that have length
      greater than L but not greater than B.






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   o  The algorithm below does not change the value of Text before or
      after the computation.  Padding a Babel packet before the
      computation and placing the result inside the packet are both
      performed elsewhere.

   The intent of this is to enable the most straightforward use of
   cryptographic libraries by implementations of this specification.  At
   the time of this writing, implementations of the original HMAC
   construction coupled with hash algorithms of choice are generally
   available.

   Description of the algorithm:

   1.  Preparation of the Key

       In this application, Ko is always B octets long.  If K is B
       octets long, then Ko is set to K.  If K is more than B octets
       long, then Ko is set to H(K) with the necessary amount of zeroes
       appended to the end of H(K), such that Ko is B octets long.  If K
       is less than B octets long, then Ko is set to K with zeroes
       appended to the end of K, such that Ko is B octets long.

   2.  First-Hash

       A First-Hash, also known as the inner hash, is computed
       as follows:

                    First-Hash = H(Ko XOR Ipad || Text)

   3.  Second-Hash

       A Second-Hash, also known as the outer hash, is computed
       as follows:

                 Second-Hash = H(Ko XOR Opad || First-Hash)

   4.  Result

       The resulting Second-Hash becomes the authentication data that is
       returned as the result of HMAC calculation.

   Note that in the case of Babel the Text parameter will never exceed a
   few thousand octets in length.  In this specific case, the
   optimization discussed in Section 6 of [FIPS-198] applies, namely,
   for a given K that is more than B octets long, the following
   associated intermediate results may be precomputed only once:
   Ko, (Ko XOR Ipad), and (Ko XOR Opad).




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3.  Updates to Protocol Data Structures

3.1.  RxAuthRequired

   RxAuthRequired is a boolean parameter.  Its default value MUST be
   TRUE.  An implementation SHOULD make RxAuthRequired a per-interface
   parameter but MAY make it specific to the whole protocol instance.
   The conceptual purpose of RxAuthRequired is to enable a smooth
   migration from an unauthenticated Babel packet exchange to an
   authenticated Babel packet exchange and back (see Section 7.3).  The
   current value of RxAuthRequired directly affects the receiving
   procedure defined in Section 5.4.  An implementation SHOULD allow the
   operator to change the RxAuthRequired value at runtime or by means of
   a Babel speaker restart.  An implementation MUST allow the operator
   to discover the effective value of RxAuthRequired at runtime or from
   the system documentation.

3.2.  LocalTS

   LocalTS is a 32-bit unsigned integer variable.  It is the TS part of
   a per-interface TS/PC number.  LocalTS is a strictly per-interface
   variable not intended to be changed by the operator.  Its
   initialization is explained in Section 5.1.

3.3.  LocalPC

   LocalPC is a 16-bit unsigned integer variable.  It is the PC part of
   a per-interface TS/PC number.  LocalPC is a strictly per-interface
   variable not intended to be changed by the operator.  Its
   initialization is explained in Section 5.1.

3.4.  MaxDigestsIn

   MaxDigestsIn is an unsigned integer parameter conceptually purposed
   for limiting the amount of CPU time spent processing a received
   authenticated packet.  The receiving procedure performs the most
   CPU-intensive operation -- the HMAC computation -- only at most
   MaxDigestsIn (Section 5.4 item 7) times for a given packet.

   The MaxDigestsIn value MUST be at least 2.  An implementation SHOULD
   make MaxDigestsIn a per-interface parameter but MAY make it specific
   to the whole protocol instance.  An implementation SHOULD allow the
   operator to change the value of MaxDigestsIn at runtime or by means
   of a Babel speaker restart.  An implementation MUST allow the
   operator to discover the effective value of MaxDigestsIn at runtime
   or from the system documentation.





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3.5.  MaxDigestsOut

   MaxDigestsOut is an unsigned integer parameter conceptually purposed
   for limiting the amount of a sent authenticated packet's space spent
   on authentication data.  The sending procedure adds at most
   MaxDigestsOut (Section 5.3 item 5) HMAC results to a given packet.

   The MaxDigestsOut value MUST be at least 2.  An implementation SHOULD
   make MaxDigestsOut a per-interface parameter but MAY make it specific
   to the whole protocol instance.  An implementation SHOULD allow the
   operator to change the value of MaxDigestsOut at runtime or by means
   of a Babel speaker restart, in a safe range.  The maximum safe value
   of MaxDigestsOut is implementation specific (see Section 6.2).  An
   implementation MUST allow the operator to discover the effective
   value of MaxDigestsOut at runtime or from the system documentation.

3.6.  ANM Table

   The ANM (Authentic Neighbours Memory) table resembles the neighbour
   table defined in Section 3.2.3 of [BABEL].  Note that the term
   "neighbour table" means the neighbour table of the original Babel
   specification, and the term "ANM table" means the table defined
   herein.  Indexing of the ANM table is done in exactly the same way as
   indexing of the neighbour table, but its purpose, field set, and
   associated procedures are different.

   The conceptual purpose of the ANM table is to provide longer-term
   replay attack protection than would be possible using the neighbour
   table.  Expiry of an inactive entry in the neighbour table depends on
   the last received Hello Interval of the neighbour and typically
   stands for tens to hundreds of seconds (see Appendixes A and B of
   [BABEL]).  Expiry of an inactive entry in the ANM table depends only
   on the local speaker's configuration.  The ANM table retains (for at
   least the amount of seconds set by the ANM timeout parameter as
   defined in Section 3.7) a copy of the TS/PC number advertised in
   authentic packets by each remote Babel speaker.















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   The ANM table is indexed by pairs of the form (Interface, Source).
   Every table entry consists of the following fields:

   o  Interface

      An implementation-specific reference to the local node's interface
      through which the authentic packet was received.

   o  Source

      The source address of the Babel speaker from which the authentic
      packet was received.

   o  LastTS

      A 32-bit unsigned integer -- the TS part of a remote TS/PC number.

   o  LastPC

      A 16-bit unsigned integer -- the PC part of a remote TS/PC number.

   Each ANM table entry has an associated aging timer, which is reset by
   the receiving procedure (Section 5.4 item 9).  If the timer expires,
   the entry is deleted from the ANM table.

   An implementation SHOULD use persistent memory (NVRAM) to retain the
   contents of the ANM table across restarts of the Babel speaker, but
   only as long as both the Interface field reference and expiry of the
   aging timer remain correct.  An implementation MUST be clear
   regarding if and how persistent memory is used for the ANM table.  An
   implementation SHOULD allow the operator to retrieve the current
   contents of the ANM table at runtime.  An implementation SHOULD allow
   the operator to remove some or all ANM table entries at runtime or by
   means of a Babel speaker restart.

3.7.  ANM Timeout

   ANM timeout is an unsigned integer parameter.  An implementation
   SHOULD make ANM timeout a per-interface parameter but MAY make it
   specific to the whole protocol instance.  ANM timeout is conceptually
   purposed for limiting the maximum age (in seconds) of entries in the
   ANM table that stand for inactive Babel speakers.  The maximum age is
   immediately related to replay attack protection strength.  The
   strongest protection is achieved with the maximum possible value of
   ANM timeout set, but it may not provide the best overall result for
   specific network segments and implementations of this mechanism.





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   Specifically, implementations unable to maintain the local TS/PC
   number strictly increasing across Babel speaker restarts will reuse
   the advertised TS/PC numbers after each restart (see Section 5.1).
   The neighbouring speakers will treat the new packets as replayed and
   discard them until the aging timer of the respective ANM table entry
   expires or the new TS/PC number exceeds the one stored in the entry.

   Another possible, but less probable, case could be an environment
   that uses IPv6 for the exchange of Babel datagrams and that involves
   physical moves of network-interface hardware between Babel speakers.
   Even when performed without restarting the speakers, these physical
   moves would cause random drops of the TS/PC number advertised for a
   given (Interface, Source) index, as viewed by neighbouring speakers,
   since IPv6 link-local addresses are typically derived from interface
   hardware addresses.

   Assuming that in such cases the operators would prefer to use a lower
   ANM timeout value to let the entries expire on their own rather than
   having to manually remove them from the ANM table each time, an
   implementation SHOULD set the default value of ANM timeout to a value
   between 30 and 300 seconds.

   At the same time, network segments may exist with every Babel speaker
   having its advertised TS/PC number strictly increasing over the
   deployed lifetime.  Assuming that in such cases the operators would
   prefer using a much higher ANM timeout value, an implementation
   SHOULD allow the operator to change the value of ANM timeout at
   runtime or by means of a Babel speaker restart.  An implementation
   MUST allow the operator to discover the effective value of ANM
   timeout at runtime or from the system documentation.

3.8.  Configured Security Associations

   A Configured Security Association (CSA) is a data structure
   conceptually purposed for associating authentication keys and hash
   algorithms with Babel interfaces.  All CSAs are managed in finite
   sequences, one sequence per interface (hereafter referred to as
   "interface's sequence of CSAs").  Each interface's sequence of CSAs,
   as an integral part of the Babel speaker configuration, MAY be
   intended for persistent storage as long as this conforms with the
   implementation's key-management policy.  The default state of an
   interface's sequence of CSAs is empty, which has a special meaning of
   no authentication configured for the interface.  The sending
   (Section 5.3 item 1) and the receiving (Section 5.4 item 1)
   procedures address this convention accordingly.






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   A single CSA structure consists of the following fields:

   o  HashAlgo

      An implementation-specific reference to one of the hash algorithms
      supported by this implementation (see Section 2.1).

   o  KeyChain

      A finite sequence of elements (hereafter referred to as "KeyChain
      sequence") representing authentication keys, each element being a
      structure consisting of the following fields:

      *  LocalKeyID

         An unsigned integer of an implementation-specific bit length.

      *  AuthKeyOctets

         A sequence of octets of an arbitrary, known length to be used
         as the authentication key.

      *  KeyStartAccept

         The time that this Babel speaker will begin considering this
         authentication key for accepting packets with authentication
         data.

      *  KeyStartGenerate

         The time that this Babel speaker will begin considering this
         authentication key for generating packet authentication data.

      *  KeyStopGenerate

         The time that this Babel speaker will stop considering this
         authentication key for generating packet authentication data.

      *  KeyStopAccept

         The time that this Babel speaker will stop considering this
         authentication key for accepting packets with authentication
         data.

   Since there is no limit imposed on the number of CSAs per interface,
   but the number of HMAC computations per sent/received packet is
   limited (through MaxDigestsOut and MaxDigestsIn, respectively), it
   may appear that only a fraction of the associated keys and hash



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   algorithms are used in the process.  The ordering of elements within
   a sequence of CSAs and within a KeyChain sequence is important to
   make the association selection process deterministic and transparent.
   Once this ordering is deterministic at the Babel interface level, the
   intermediate data derived by the procedure defined in Section 5.2
   will be deterministically ordered as well.

   An implementation SHOULD allow an operator to set any arbitrary order
   of elements within a given interface's sequence of CSAs and within
   the KeyChain sequence of a given CSA.  Regardless of whether this
   requirement is or isn't met, the implementation MUST provide a means
   to discover the actual element order used.  Whichever order is used
   by an implementation, it MUST be preserved across Babel speaker
   restarts.

   Note that none of the CSA structure fields is constrained to contain
   unique values.  Section 6.4 explains this in more detail.  It is
   possible for the KeyChain sequence to be empty, although this is not
   the intended manner of using CSAs.

   The KeyChain sequence has a direct prototype, which is the "key
   chain" syntax item of some existing router configuration languages.
   If an implementation already implements this syntax item, it is
   suggested that the implementation reuse it, that is, implement a CSA
   syntax item that refers to a key chain item rather than reimplement
   the latter in full.

3.9.  Effective Security Associations

   An Effective Security Association (ESA) is a data structure
   immediately used in sending (Section 5.3) and receiving (Section 5.4)
   procedures.  Its conceptual purpose is to determine a runtime
   interface between those procedures and the deriving procedure defined
   in Section 5.2.  All ESAs are temporary data units managed as
   elements of finite sequences that are not intended for persistent
   storage.  Element ordering within each such finite sequence
   (hereafter referred to as "sequence of ESAs") MUST be preserved as
   long as the sequence exists.













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   A single ESA structure consists of the following fields:

   o  HashAlgo

      An implementation-specific reference to one of the hash algorithms
      supported by this implementation (see Section 2.1).

   o  KeyID

      A 16-bit unsigned integer.

   o  AuthKeyOctets

      A sequence of octets of an arbitrary, known length to be used as
      the authentication key.

   Note that among the protocol data structures introduced by this
   mechanism, the ESA structure is the only one not directly interfaced
   with the system operator (see Figure 1 in Appendix A); it is not
   immediately present in the protocol encoding, either.  However, the
   ESA structure is not just a possible implementation technique but an
   integral part of this specification: the deriving (Section 5.2), the
   sending (Section 5.3), and the receiving (Section 5.4) procedures are
   defined in terms of the ESA structure and its semantics provided
   herein.  The ESA structure is as meaningful for a correct
   implementation as the other protocol data structures.

4.  Updates to Protocol Encoding

4.1.  Justification

   The choice of encoding is very important in the long term.  The
   protocol encoding limits various authentication mechanism designs and
   encodings, which in turn limit future developments of the protocol.

   Considering existing implementations of the Babel protocol instance
   itself and related modules of packet analysers, the current encoding
   of Babel allows for compact and robust decoders.  At the same time,
   this encoding allows for future extensions of Babel by three (not
   excluding each other) principal means as defined in Sections 4.2 and
   4.3 of [BABEL] and further discussed in [BABEL-EXTENSION]:

   a.  A Babel packet consists of a four-octet header followed by a
       packet body, that is, a sequence of TLVs (see Figure 2 in
       Appendix A).  Besides the header and the body, an actual Babel






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       datagram may have an arbitrary amount of trailing data between
       the end of the packet body and the end of the datagram.  An
       instance of the original protocol silently ignores such trailing
       data.

   b.  The packet body uses a binary format allowing for 256 TLV types
       and imposing no requirements on TLV ordering or number of TLVs of
       a given type in a packet.  [BABEL] allocates TLV types 0 through
       10 (see Table 1 in Appendix A), defines the TLV body structure
       for each, and establishes the requirement for a Babel protocol
       instance to ignore any unknown TLV types silently.  This makes it
       possible to examine a packet body (to validate the framing and/or
       to pick particular TLVs for further processing), taking into
       account only the type (to distinguish between a Pad1 TLV and any
       other TLV) and the length of each TLV, regardless of whether any
       additional TLV types are eventually deployed (and if so, how
       many).

   c.  Within each TLV of the packet body, there may be some extra data
       after the expected length of the TLV body.  An instance of the
       original protocol silently ignores any such extra data.  Note
       that any TLV types without the expected length defined (such as
       the PadN TLV) cannot be extended with the extra data.

   Considering each of these three principal extension means for the
   specific purpose of adding authentication data items to each protocol
   packet, the following arguments can be made:

   o  The use of the TLV extra data of some existing TLV type would not
      be a solution, since no particular TLV type is guaranteed to be
      present in a Babel packet.

   o  The use of the TLV extra data could also conflict with future
      developments of the protocol encoding.

   o  Since the packet trailing data is currently unstructured, using it
      would involve defining an encoding structure and associated
      procedures; this would add to the complexity of both specification
      and implementation and would increase exposure to protocol attacks
      such as fuzzing.

   o  A naive use of the packet trailing data would make it unavailable
      to any future extension of Babel.  Since this mechanism is
      possibly not the last extension and since some other extensions
      may allow no other embedding means except the packet trailing
      data, the defined encoding structure would have to enable the
      multiplexing of data items belonging to different extensions.
      Such a definition is out of the scope of this work.



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   o  Deprecating an extension (or only its protocol encoding) that uses
      purely purpose-allocated TLVs is as simple as deprecating the
      TLVs.

   o  The use of purpose-allocated TLVs is transparent for both the
      original protocol and any its future extensions, regardless of the
      embedding technique(s) used by the latter.

   Considering all of the above, this mechanism uses neither the packet
   trailing data nor the TLV extra data but uses two new TLV types:
   type 11 for a TS/PC number and type 12 for an HMAC result (see
   Table 1 in Appendix A).

4.2.  TS/PC TLV

   The purpose of a TS/PC TLV is to store a single TS/PC number.  There
   is exactly one TS/PC TLV in an authenticated Babel 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 = 11   |     Length    |         PacketCounter         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type            Set to 11 to indicate a TS/PC TLV.

   Length          The length, in octets, of the body, exclusive of the
                   Type and Length fields.

   PacketCounter   A 16-bit unsigned integer in network byte order --
                   the PC part of a TS/PC number stored in this TLV.

   Timestamp       A 32-bit unsigned integer in network byte order --
                   the TS part of a TS/PC number stored in this TLV.

   Note that the ordering of PacketCounter and Timestamp in the TLV
   structure is the opposite of the ordering of TS and PC in the TS/PC
   number and the 48-bit equivalent (see Section 2.3).

   Considering the expected length and the extra data as mentioned in
   Section 4.3 of [BABEL], the expected length of a TS/PC TLV body is
   unambiguously defined as 6 octets.  The receiving procedure would
   correctly process any TS/PC TLV with body length not less than the
   expected length, ignoring any extra data (Section 5.4 items 3 and 9).



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   The sending procedure produces a TS/PC TLV with body length equal to
   the expected length and the Length field, respectively, set as
   described in Section 5.3 item 3.

   Future Babel extensions (such as sub-TLVs) MAY modify the sending
   procedure to include the extra data after the fixed-size TS/PC TLV
   body defined herein, making adjustments to the Length TLV field, the
   "Body length" packet header field, and output buffer management (as
   explained in Section 6.2) necessary.

4.3.  HMAC TLV

   The purpose of an HMAC TLV is to store a single HMAC result.  To
   assist a receiver in reproducing the HMAC computation, LocalKeyID
   modulo 2^16 of the authentication key is also provided in the TLV.
   There is at least one HMAC TLV in an authenticated Babel 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 = 12   |    Length     |             KeyID             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Digest...
   +-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type            Set to 12 to indicate an HMAC TLV.

   Length          The length, in octets, of the body, exclusive of the
                   Type and Length fields.

   KeyID           A 16-bit unsigned integer in network byte order.

   Digest          A variable-length sequence of octets that is at least
                   16 octets long (see Section 2.2).

   Considering the expected length and the extra data as mentioned in
   Section 4.3 of [BABEL], the expected length of an HMAC TLV body is
   not defined.  The receiving and padding procedures process every
   octet of the Digest field, deriving the field boundary from the
   Length field value (Section 5.4 item 7 and Section 2.2,
   respectively).  The sending procedure produces HMAC TLVs with the
   Length field precisely sizing the Digest field to match the digest
   length of the hash algorithm used (Section 5.3 items 5 and 8).

   The HMAC TLV structure defined herein is final.  Future Babel
   extensions MUST NOT extend it with any extra data.



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5.  Updates to Protocol Operation

5.1.  Per-Interface TS/PC Number Updates

   The LocalTS and LocalPC interface-specific variables constitute the
   TS/PC number of a Babel interface.  This number is advertised in the
   TS/PC TLV of authenticated Babel packets sent from that interface.
   There is only one property that is mandatory for the advertised TS/PC
   number: its 48-bit equivalent (see Section 2.3) MUST be strictly
   increasing within the scope of a given interface of a Babel speaker
   as long as the protocol instance is continuously operating.  This
   property, combined with ANM tables of neighbouring Babel speakers,
   provides them with the most basic replay attack protection.

   Initialization and increment are two principal updates performed on
   an interface TS/PC number.  The initialization is performed when a
   new interface becomes a part of a Babel protocol instance.  The
   increment is performed by the sending procedure (Section 5.3 item 2)
   before advertising the TS/PC number in a TS/PC TLV.

   Depending on the particular implementation method of these two
   updates, the advertised TS/PC number may possess additional
   properties that improve the replay attack protection strength.  This
   includes, but is not limited to, the methods below.

   a.  The most straightforward implementation would use LocalTS as a
       plain wrap counter, defining the updates as follows:

       initialization  Set LocalPC to 0, and set LocalTS to 0.

       increment       Increment LocalPC by 1.  If LocalPC wraps
                       (0xFFFF + 1 = 0x0000), increment LocalTS by 1.

       In this case, the advertised TS/PC numbers would be reused after
       each Babel protocol instance restart, making neighbouring
       speakers reject authenticated packets until the respective ANM
       table entries expire or the new TS/PC number exceeds the old (see
       Sections 3.6 and 3.7).













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   b.  A more advanced implementation could make use of any 32-bit
       unsigned integer timestamp (number of time units since an
       arbitrary epoch), such as the UNIX timestamp, if the timestamp
       itself spans a reasonable time range and is guaranteed against a
       decrease (such as one resulting from network time use).  The
       updates would be defined as follows:

       initialization  Set LocalPC to 0, and set LocalTS to 0.

       increment       If the current timestamp is greater than LocalTS,
                       set LocalTS to the current timestamp and LocalPC
                       to 0, then consider the update complete.
                       Otherwise, increment LocalPC by 1, and if LocalPC
                       wraps, increment LocalTS by 1.

       In this case, the advertised TS/PC number would remain unique
       across the speaker's deployed lifetime without the need for any
       persistent storage.  However, a suitable timestamp source is not
       available in every implementation case.

   c.  Another advanced implementation could use LocalTS in a way
       similar to the "wrap/boot count" suggested in Section 4.1 of
       [OSPF3-AUTH-BIS], defining the updates as follows:

       initialization  Set LocalPC to 0.  If there is a TS value stored
                       in NVRAM for the current interface, set LocalTS
                       to the stored TS value, then increment the stored
                       TS value by 1.  Otherwise, set LocalTS to 0, and
                       set the stored TS value to 1.

       increment       Increment LocalPC by 1.  If LocalPC wraps, set
                       LocalTS to the TS value stored in NVRAM for the
                       current interface, then increment the stored TS
                       value by 1.

       In this case, the advertised TS/PC number would also remain
       unique across the speaker's deployed lifetime, relying on NVRAM
       for storing multiple TS numbers, one per interface.

   As long as the TS/PC number retains its mandatory property stated
   above, it is up to the implementor to determine which methods of TS/
   PC number updates are available and whether the operator can
   configure the method per interface and/or at runtime.  However, an
   implementation MUST disclose the essence of each update method it
   includes, in a comprehensible form such as natural language
   description, pseudocode, or source code.  An implementation MUST
   allow the operator to discover which update method is effective for
   any given interface, either at runtime or from the system



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   documentation.  These requirements are necessary to enable the
   optimal (see Section 3.7) management of ANM timeout in a network
   segment.

   Note that wrapping (0xFFFFFFFF + 1 = 0x00000000) of LastTS is
   unlikely, but possible, causing the advertised TS/PC number to be
   reused.  Resolving this situation requires replacing all
   authentication keys of the involved interface.  In addition to that,
   if the wrap was caused by a timestamp reaching its end of epoch,
   using this mechanism will be impossible for the involved interface
   until some different timestamp or update implementation method is
   used.

5.2.  Deriving ESAs from CSAs

   Neither receiving nor sending procedures work with the contents of an
   interface's sequence of CSAs directly; both (Section 5.4 item 4 and
   Section 5.3 item 4, respectively) derive a sequence of ESAs from the
   sequence of CSAs and use the derived sequence (see Figure 1 in
   Appendix A).  There are two main goals achieved through this
   indirection:

   o  Elimination of expired authentication keys and deduplication of
      security associations.  This is done as early as possible to keep
      subsequent procedures focused on their respective tasks.

   o  Maintenance of particular ordering within the derived sequence of
      ESAs.  The ordering deterministically depends on the ordering
      within the interface's sequence of CSAs and the ordering within
      the KeyChain sequence of each CSA.  The particular correlation
      maintained by this procedure implements a concept of fair
      (independent of the number of keys contained by each) competition
      between CSAs.

   The deriving procedure uses the following input arguments:

   o  input sequence of CSAs

   o  direction (sending or receiving)

   o  current time (CT)










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   The processing of input arguments begins with an empty output
   sequence of ESAs and consists of the following steps:

   1.  Make a temporary copy of the input sequence of CSAs.

   2.  Remove all expired authentication keys from each KeyChain
       sequence of the copy, that is, any keys such that:

       *  for receiving: KeyStartAccept is greater than CT or
          KeyStopAccept is less than CT

       *  for sending: KeyStartGenerate is greater than CT or
          KeyStopGenerate is less than CT

       Note well that there are no special exceptions.  Remove all
       expired keys, even if there are no keys left after that (see
       Section 7.4).

   3.  Use the copy to populate the output sequence of ESAs as follows:

       3.1.  When the KeyChain sequence of the first CSA contains at
             least one key, use its first key to produce an ESA with
             fields set as follows:

             HashAlgo       Set to HashAlgo of the current CSA.

             KeyID          Set to LocalKeyID modulo 2^16 of the current
                            key of the current CSA.

             AuthKeyOctets  Set to AuthKeyOctets of the current key of
                            the current CSA.

             Append this ESA to the end of the output sequence.

       3.2.  When the KeyChain sequence of the second CSA contains at
             least one key, use its first key the same way, and so forth
             until all first keys of the copy are processed.

       3.3.  When the KeyChain sequence of the first CSA contains at
             least two keys, use its second key the same way.

       3.4.  When the KeyChain sequence of the second CSA contains at
             least two keys, use its second key the same way, and so
             forth until all second keys of the copy are processed.

       3.5.  ...and so forth, until all keys of all CSAs of the copy are
             processed, exactly once each.




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       In the description above, the ordinals ("first", "second", and so
       on) with regard to keys stand for an element position after the
       removal of expired keys, not before.  For example, if a KeyChain
       sequence was { Ka, Kb, Kc, Kd } before the removal and became
       { Ka, Kd } after, then Ka would be the "first" element and Kd
       would be the "second".

   4.  Deduplicate the ESAs in the output sequence; that is, wherever
       two or more ESAs exist that share the same (HashAlgo, KeyID,
       AuthKeyOctets) triplet value, remove all of these ESAs except the
       one closest to the beginning of the sequence.

   The resulting sequence will contain zero or more unique ESAs, ordered
   in a way deterministically correlated with the ordering of CSAs
   within the original input sequence of CSAs and the ordering of keys
   within each KeyChain sequence.  This ordering maximizes the
   probability of having an equal amount of keys per original CSA in any
   N first elements of the resulting sequence.  Possible optimizations
   of this deriving procedure are outlined in Section 6.3.

5.3.  Updates to Packet Sending

   Perform the following authentication-specific processing after the
   instance of the original protocol considers an outgoing Babel packet
   ready for sending, but before the packet is actually sent (see
   Figure 1 in Appendix A).  After that, send the packet, regardless of
   whether the authentication-specific processing modified the outgoing
   packet or left it intact.

   1.  If the current outgoing interface's sequence of CSAs is empty,
       finish authentication-specific processing and consider the packet
       ready for sending.

   2.  Increment the TS/PC number of the current outgoing interface, as
       explained in Section 5.1.

   3.  Add to the packet body (see the note at the end of this section)
       a TS/PC TLV with fields set as follows:

       Type            Set to 11.

       Length          Set to 6.

       PacketCounter   Set to the current value of the LocalPC variable
                       of the current outgoing interface.






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       Timestamp       Set to the current value of the LocalTS variable
                       of the current outgoing interface.

       Note that the current step may involve byte order conversion.

   4.  Derive a sequence of ESAs, using the procedure defined in
       Section 5.2, with the current interface's sequence of CSAs as the
       input sequence of CSAs, the current time as CT, and "sending" as
       the direction.  Proceed to the next step even if the derived
       sequence is empty.

   5.  Iterate over the derived sequence, using its ordering.  For each
       ESA, add to the packet body (see the note at the end of this
       section) an HMAC TLV with fields set as follows:

       Type     Set to 12.

       Length   Set to 2 plus the digest length of HashAlgo of the
                current ESA.

       KeyID    Set to KeyID of the current ESA.

       Digest   Size exactly equal to the digest length of HashAlgo of
                the current ESA.  Pad (see Section 2.2), using the
                source address of the current packet (see Section 6.1).

       As soon as there are MaxDigestsOut HMAC TLVs added to the current
       packet body, immediately proceed to the next step.

       Note that the current step may involve byte order conversion.

   6.  Increment the "Body length" field value of the current packet
       header by the total length of TS/PC and HMAC TLVs appended to the
       current packet body so far.

       Note that the current step may involve byte order conversion.

   7.  Make a temporary copy of the current packet.













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   8.  Iterate over the derived sequence again, using the same order and
       number of elements.  For each ESA (and, respectively, for each
       HMAC TLV recently appended to the current packet body), compute
       an HMAC result (see Section 2.4), using the temporary copy (not
       the original packet) as Text, HashAlgo of the current ESA as H,
       and AuthKeyOctets of the current ESA as K.  Write the HMAC result
       to the Digest field of the current HMAC TLV (see Table 4 in
       Appendix A) of the current packet (not the copy).

   9.  After this point, allow no more changes to the current packet
       header and body, and consider it ready for sending.

   Note that even when the derived sequence of ESAs is empty, the packet
   is sent anyway, with only a TS/PC TLV appended to its body.  Although
   such a packet would not be authenticated, the presence of the sole
   TS/PC TLV would indicate authentication key exhaustion to operators
   of neighbouring Babel speakers.  See also Section 7.4.

   Also note that it is possible to place the authentication-specific
   TLVs in the packet's sequence of TLVs in a number of different valid
   ways so long as there is exactly one TS/PC TLV in the sequence and
   the ordering of HMAC TLVs relative to each other, as produced in
   step 5 above, is preserved.

   For example, see Figure 2 in Appendix A.  The diagrams represent a
   Babel packet without (D1) and with (D2, D3, D4) authentication-
   specific TLVs.  The optional trailing data block that is present in
   D1 is preserved in D2, D3, and D4.  Indexing (1, 2, ..., n) of the
   HMAC TLVs means the order in which the sending procedure produced
   them (and, respectively, the HMAC results).  In D2, the added TLVs
   are appended: the previously existing TLVs are followed by the TS/PC
   TLV, which is followed by the HMAC TLVs.  In D3, the added TLVs are
   prepended: the TS/PC TLV is the first and is followed by the HMAC
   TLVs, which are followed by the previously existing TLVs.  In D4, the
   added TLVs are intermixed with the previously existing TLVs and the
   TS/PC TLV is placed after the HMAC TLVs.  All three packets meet the
   requirements above.

   Implementors SHOULD use appending (D2) for adding the authentication-
   specific TLVs to the sequence; this is expected to result in more
   straightforward implementation and troubleshooting in most use cases.










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5.4.  Updates to Packet Receiving

   Perform the following authentication-specific processing after an
   incoming Babel packet is received from the local network stack but
   before it is acted upon by the Babel protocol instance (see Figure 1
   in Appendix A).  The final action conceptually depends not only upon
   the result of the authentication-specific processing but also on the
   current value of the RxAuthRequired parameter.  Immediately after any
   processing step below accepts or refuses the packet, either deliver
   the packet to the instance of the original protocol (when the packet
   is accepted or RxAuthRequired is FALSE) or discard it (when the
   packet is refused and RxAuthRequired is TRUE).

   1.   If the current incoming interface's sequence of CSAs is empty,
        accept the packet.

   2.   If the current packet does not contain exactly one TS/PC TLV,
        refuse it.

   3.   Perform a lookup in the ANM table for an entry having Interface
        equal to the current incoming interface and Source equal to the
        source address of the current packet.  If such an entry does not
        exist, immediately proceed to the next step.  Otherwise, compare
        the entry's LastTS and LastPC field values with the Timestamp
        and PacketCounter values, respectively, of the TS/PC TLV of the
        packet.  That is, refuse the packet if at least one of the
        following two conditions is true:

        *  Timestamp is less than LastTS

        *  Timestamp is equal to LastTS and PacketCounter is not greater
           than LastPC

        Note that the current step may involve byte order conversion.

   4.   Derive a sequence of ESAs, using the procedure defined in
        Section 5.2, with the current interface's sequence of CSAs as
        the input sequence of CSAs, current time as CT, and "receiving"
        as the direction.  If the derived sequence is empty, refuse the
        packet.

   5.   Make a temporary copy of the current packet.

   6.   Pad (see Section 2.2) every HMAC TLV present in the temporary
        copy (not the original packet), using the source address of the
        original packet.





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   7.   Iterate over all the HMAC TLVs of the original input packet (not
        the copy), using their order of appearance in the packet.  For
        each HMAC TLV, look up all ESAs in the derived sequence such
        that 2 plus the digest length of HashAlgo of the ESA is equal to
        Length of the TLV and KeyID of the ESA is equal to the value of
        KeyID of the TLV.  Iterate over these ESAs in the relative order
        of their appearance on the full sequence of ESAs.  Note that
        nesting the iterations the opposite way (over ESAs, then over
        HMAC TLVs) would be wrong.

        For each of these ESAs, compute an HMAC result (see
        Section 2.4), using the temporary copy (not the original packet)
        as Text, HashAlgo of the current ESA as H, and AuthKeyOctets of
        the current ESA as K.  If the current HMAC result exactly
        matches the contents of the Digest field of the current HMAC
        TLV, immediately proceed to the next step.  Otherwise, if the
        number of HMAC computations done for the current packet so far
        is equal to MaxDigestsIn, immediately proceed to the next step.
        Otherwise, follow the normal order of iterations.

        Note that the current step may involve byte order conversion.

   8.   Refuse the input packet unless there was a matching HMAC result
        in the previous step.

   9.   Modify the ANM table, using the same index as for the entry
        lookup above, to contain an entry with LastTS set to the value
        of Timestamp and LastPC set to the value of PacketCounter fields
        of the TS/PC TLV of the current packet.  That is, either add a
        new ANM table entry or update the existing one, depending on the
        result of the entry lookup above.  Reset the entry's aging timer
        to the current value of ANM timeout.

        Note that the current step may involve byte order conversion.

   10.  Accept the input packet.

   Before performing the authentication-specific processing above, an
   implementation SHOULD perform those basic procedures of the original
   protocol that don't take any protocol actions on the contents of the
   packet but that will discard the packet if it is not sufficiently
   well formed for further processing.  Although the exact composition
   of such procedures belongs to the scope of the original protocol, it
   seems reasonable to state that a packet SHOULD be discarded early,
   regardless of whether any authentication-specific processing is due,
   unless its source address conforms to Section 3.1 of [BABEL] and is
   not the receiving speaker's own address (see item (e) of Section 8).




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   Note that RxAuthRequired affects only the final action but not the
   defined flow of authentication-specific processing.  The purpose of
   this is to preserve authentication-specific processing feedback (such
   as log messages and event-counter updates), even with RxAuthRequired
   set to FALSE.  This allows an operator to predict the effect of
   changing RxAuthRequired from FALSE to TRUE during a migration
   scenario (Section 7.3) implementation.

5.5.  Authentication-Specific Statistics Maintenance

   A Babel speaker implementing this mechanism SHOULD maintain a set of
   counters for the following events, per protocol instance and per
   interface:

   a.  Sending an unauthenticated Babel packet through an interface
       having an empty sequence of CSAs (Section 5.3 item 1).

   b.  Sending an unauthenticated Babel packet with a TS/PC TLV but
       without any HMAC TLVs, due to an empty derived sequence of ESAs
       (Section 5.3 item 4).

   c.  Sending an authenticated Babel packet containing both TS/PC and
       HMAC TLVs (Section 5.3 item 9).

   d.  Accepting a Babel packet received through an interface having an
       empty sequence of CSAs (Section 5.4 item 1).

   e.  Refusing a received Babel packet due to an empty derived sequence
       of ESAs (Section 5.4 item 4).

   f.  Refusing a received Babel packet that does not contain exactly
       one TS/PC TLV (Section 5.4 item 2).

   g.  Refusing a received Babel packet due to the TS/PC TLV failing the
       ANM table check (Section 5.4 item 3).  With possible future
       extensions in mind, in implementations of this mechanism, this
       event SHOULD leave out some small amount, per current (Interface,
       Source, LastTS, LastPC) tuple, of the packets refused due to the
       Timestamp value being equal to LastTS and the PacketCounter value
       being equal to LastPC.

   h.  Refusing a received Babel packet missing any HMAC TLVs
       (Section 5.4 item 8).

   i.  Refusing a received Babel packet due to none of the processed
       HMAC TLVs passing the ESA check (Section 5.4 item 8).





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   j.  Accepting a received Babel packet having both TS/PC and HMAC TLVs
       (Section 5.4 item 10).

   k.  Delivery of a refused packet to the instance of the original
       protocol due to the RxAuthRequired parameter being set to FALSE.

   Note that the terms "accepting" and "refusing" are used in the sense
   of the receiving procedure; that is, "accepting" does not mean a
   packet delivered to the instance of the original protocol purely
   because the RxAuthRequired parameter is set to FALSE.  Event-counter
   readings SHOULD be available to the operator at runtime.

6.  Implementation Notes

6.1.  Source Address Selection for Sending

   Section 3.1 of [BABEL] allows for the exchange of protocol datagrams,
   using IPv4, IPv6, or both.  The source address of the datagram is a
   unicast (link-local in the case of IPv6) address.  Within an address
   family used by a Babel speaker, there may be more than one address
   eligible for the exchange and assigned to the same network interface.
   The original specification considers this case out of scope and
   leaves it up to the speaker's network stack to select one particular
   address as the datagram source address, but the sending procedure
   requires (Section 5.3 item 5) exact knowledge of the packet source
   address for proper padding of HMAC TLVs.

   As long as a network interface has more than one address eligible for
   the exchange within the same address family, the Babel speaker SHOULD
   internally choose one of those addresses for Babel packet sending
   purposes and then indicate this choice to both the sending procedure
   and the network stack (see Figure 1 in Appendix A).  Wherever this
   requirement cannot be met, this limitation MUST be clearly stated in
   the system documentation to allow an operator to plan network address
   management accordingly.

6.2.  Output Buffer Management

   An instance of the original protocol will buffer produced TLVs until
   the buffer becomes full or a delay timer has expired.  This is
   performed independently for each Babel interface, with each buffer
   sized according to the interface MTU (see Sections 3.1 and 4 of
   [BABEL]).








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   Since TS/PC TLVs, HMAC TLVs, and any other TLVs -- and most likely
   the TLVs of the original protocol -- share the same packet space (see
   Figure 2 in Appendix A) and, respectively, the same buffer space, a
   particular portion of each interface buffer needs to be reserved for
   one TS/PC TLV and up to MaxDigestsOut HMAC TLVs.  The amount (R) of
   this reserved buffer space is calculated as follows:

                    R = St + MaxDigestsOut * Sh
                    R = 8  + MaxDigestsOut * (4 + Lmax)

   St      The size of a TS/PC TLV.

   Sh      The size of an HMAC TLV.

   Lmax    The maximum possible digest length in octets for a particular
           interface.  It SHOULD be calculated based on the particular
           interface's sequence of CSAs but MAY be taken as the maximum
           digest length supported by a particular implementation.

   An implementation allowing for a per-interface value of MaxDigestsOut
   or Lmax has to account for a different value of R across different
   interfaces, even interfaces having the same MTU.  An implementation
   allowing for a runtime change to the value of R (due to MaxDigestsOut
   or Lmax) has to take care of the TLVs already buffered by the time of
   the change -- specifically, when the value of R increases.

   The maximum safe value of the MaxDigestsOut parameter depends on the
   interface MTU and maximum digest length used.  In general, at least
   200-300 octets of a Babel packet should always be available to data
   other than TS/PC and HMAC TLVs.  An implementation following the
   requirements of Section 4 of [BABEL] would send packets of 512 octets
   or larger.  If, for example, the maximum digest length is 64 octets
   and the MaxDigestsOut value is 4, the value of R would be 280,
   leaving less than half of a 512-octet packet for any other TLVs.  As
   long as the interface MTU is larger or the digest length is smaller,
   higher values of MaxDigestsOut can be used safely.

6.3.  Optimizations of Deriving Procedure for ESAs

   The following optimizations of the deriving procedure for ESAs can
   reduce the amount of CPU time consumed by authentication-specific
   processing, preserving an implementation's effective behaviour.

   a.  The most straightforward implementation would treat the deriving
       procedure as a per-packet action, but since the procedure is
       deterministic (its output depends on its input only), it is
       possible to significantly reduce the number of times the
       procedure is performed.



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       The procedure would obviously return the same result for the same
       input arguments (sequence of CSAs, direction, CT) values.
       However, it is possible to predict when the result will remain
       the same, even for a different input.  That is, when the input
       sequence of CSAs and the direction both remain the same but CT
       changes, the result will remain the same as long as CT's order on
       the time axis (relative to all critical points of the sequence of
       CSAs) remains unchanged.  Here, the critical points are
       KeyStartAccept and KeyStopAccept (for the receiving direction),
       and KeyStartGenerate and KeyStopGenerate (for the sending
       direction), of all keys of all CSAs of the input sequence.  In
       other words, in this case the result will remain the same as long
       as (1) none of the active keys expire and (2) none of the
       inactive keys enter into operation.

       An implementation optimized in this way would perform the full
       deriving procedure for a given (interface, direction) pair only
       after an operator's change to the interface's sequence of CSAs or
       after reaching one of the critical points mentioned above.

   b.  Considering that the sending procedure iterates over at most
       MaxDigestsOut elements of the derived sequence of ESAs
       (Section 5.3 item 5), there would be little sense, in the case of
       the sending direction, in returning more than MaxDigestsOut ESAs
       in the derived sequence.  Note that a similar optimization would
       be relatively difficult in the case of the receiving direction,
       since the number of ESAs actually used in examining a particular
       received packet (not to be confused with the number of HMAC
       computations) depends on additional factors besides just
       MaxDigestsIn.

6.4.  Duplication of Security Associations

   This specification defines three data structures as finite sequences:
   a KeyChain sequence, an interface's sequence of CSAs, and a sequence
   of ESAs.  There are associated semantics to take into account during
   implementation, in that the same element can appear multiple times at
   different positions of the sequence.  In particular, none of the CSA
   structure fields (including HashAlgo, LocalKeyID, and AuthKeyOctets),
   alone or in a combination, have to be unique within a given CSA, or
   within a given sequence of CSAs, or within all sequences of CSAs of a
   Babel speaker.

   In the CSA space defined in this way, for any two authentication
   keys, their one field (in)equality would not imply another field
   (in)equality.  In other words, it is acceptable to have more than one
   authentication key with the same LocalKeyID or the same
   AuthKeyOctets, or both at a time.  It is a conscious design decision



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   that CSA semantics allow for duplication of security associations.
   Consequently, ESA semantics allow for duplication of intermediate
   ESAs in the sequence until the explicit deduplication (Section 5.2
   item 4).

   One of the intentions of this is to define the security association
   management in a way that allows the addressing of some specifics of
   Babel as a mesh routing protocol.  For example, a system operator
   configuring a Babel speaker to participate in more than one
   administrative domain could find each domain using its own
   authentication key (AuthKeyOctets) under the same LocalKeyID value,
   e.g., a "well-known" or "default" value like 0 or 1.  Since
   reconfiguring the domains to use distinct LocalKeyID values isn't
   always feasible, the multi-domain Babel speaker, using several
   distinct authentication keys under the same LocalKeyID, would make a
   valid use case for such duplication.

   Furthermore, if the operator decided in this situation to migrate one
   of the domains to a different LocalKeyID value in a seamless way, the
   respective Babel speakers would use the same authentication key
   (AuthKeyOctets) under two different LocalKeyID values for the time of
   the transition (see also item (f) of Section 8).  This would make a
   similar use case.

   Another intention of this design decision is to decouple security
   association management from authentication key management as much as
   possible, so that the latter, be it manual keying or a key-management
   protocol, could be designed and implemented independently (as the
   respective reasoning made in Section 3.1 of [RIP2-AUTH] still
   applies).  This way, the additional key-management constraints, if
   any, would remain out of the scope of this authentication mechanism.
   A similar thinking justifies the LocalKeyID field having a bit length
   in an ESA structure definition, but not in that of the CSA.

7.  Network Management Aspects

7.1.  Backward Compatibility

   Support of this mechanism is optional.  It does not change the
   default behaviour of a Babel speaker and causes no compatibility
   issues with speakers properly implementing the original Babel
   specification.  Given two Babel speakers -- one implementing this
   mechanism and configured for authenticated exchange (A) and another
   not implementing it (B) -- these speakers would not distribute
   routing information unidirectionally, form a routing loop, or
   experience other protocol logic issues specific purely to the use of
   this mechanism.




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   The Babel design requires a bidirectional neighbour reachability
   condition between two given speakers for a successful exchange of
   routing information.  Apparently, neighbour reachability would be
   unidirectional in the case above.  The presence of TS/PC and HMAC
   TLVs in Babel packets sent by A would be transparent to B, but a lack
   of authentication data in Babel packets sent by B would make them
   effectively invisible to the instance of the original protocol of A.
   Unidirectional links are not specific to the use of this mechanism;
   they naturally exist on their own and are properly detected and coped
   with by the original protocol (see Section 3.4.2 of [BABEL]).

7.2.  Multi-Domain Authentication

   The receiving procedure treats a packet as authentic as soon as one
   of its HMAC TLVs passes the check against the derived sequence of
   ESAs.  This allows for packet exchange authenticated with multiple
   (hash algorithm, authentication key) pairs simultaneously, in
   combinations as arbitrary as permitted by MaxDigestsIn and
   MaxDigestsOut.

   For example, consider three Babel speakers with one interface each,
   configured with the following CSAs:

   o  speaker A: (hash algorithm H1; key SK1), (hash algorithm H1;
      key SK2)

   o  speaker B: (hash algorithm H1; key SK1)

   o  speaker C: (hash algorithm H1; key SK2)

   Packets sent by A would contain two HMAC TLVs each.  Packets sent by
   B and C would contain one HMAC TLV each.  A and B would authenticate
   the exchange between themselves, using H1 and SK1; A and C would use
   H1 and SK2; B and C would discard each other's packets.

   Consider a similar set of speakers configured with different CSAs:

   o  speaker D: (hash algorithm H2; key SK3), (hash algorithm H3;
      key SK4)

   o  speaker E: (hash algorithm H2; key SK3), (hash algorithm H4;
      keys SK5 and SK6)

   o  speaker F: (hash algorithm H3; keys SK4 and SK7), (hash
      algorithm H5; key SK8)






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   Packets sent by D would contain two HMAC TLVs each.  Packets sent by
   E and F would contain three HMAC TLVs each.  D and E would
   authenticate the exchange between themselves, using H2 and SK3; D and
   F would use H3 and SK4; E and F would discard each other's packets.
   The simultaneous use of H4, SK5, and SK6 by E, as well as the use of
   SK7, H5, and SK8 by F (for their own purposes), would remain
   insignificant to D.

   An operator implementing multi-domain authentication should keep in
   mind that values of MaxDigestsIn and MaxDigestsOut may be different
   both within the same Babel speaker and across different speakers.
   Since the minimum value of both parameters is 2 (see Sections 3.4 and
   3.5), when more than two authentication domains are configured
   simultaneously it is advisable to confirm that every involved speaker
   can handle a sufficient number of HMAC results for both sending and
   receiving.

   The recommended method of Babel speaker configuration for
   multi-domain authentication is to not only use a different
   authentication key for each domain but also a separate CSA for each
   domain, even when hash algorithms are the same.  This allows for fair
   competition between CSAs and sometimes limits the consequences of a
   possible misconfiguration to the scope of one CSA.  See also item (f)
   of Section 8.

7.3.  Migration to and from Authenticated Exchange

   It is common in practice to consider a migration to the authenticated
   exchange of routing information only after the network has already
   been deployed and put into active use.  Performing the migration in a
   way without regular traffic interruption is typically demanded, and
   this specification allows a smooth migration using the RxAuthRequired
   interface parameter defined in Section 3.1.  This measure is similar
   to the "transition mode" suggested in Section 5 of [OSPF3-AUTH-BIS].

   An operator performing the migration needs to arrange configuration
   changes as follows:

   1.  Decide on particular hash algorithm(s) and key(s) to be used.

   2.  Identify all speakers and their involved interfaces that need to
       be migrated to authenticated exchange.

   3.  For each of the speakers and the interfaces to be reconfigured,
       first set the RxAuthRequired parameter to FALSE, then configure
       necessary CSA(s).





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   4.  Examine the speakers to confirm that Babel packets are
       successfully authenticated according to the configuration (for
       instance, by examining ANM table entries and authentication-
       specific statistics; see Figure 1 in Appendix A), and address any
       discrepancies before proceeding further.

   5.  For each of the speakers and the reconfigured interfaces, set the
       RxAuthRequired parameter to TRUE.

   Likewise, temporarily setting RxAuthRequired to FALSE can be used to
   migrate smoothly from an authenticated packet exchange back to an
   unauthenticated one.

7.4.  Handling of Authentication Key Exhaustion

   This specification employs a common concept of multiple
   authentication keys coexisting for a given interface, with two
   independent lifetime ranges associated with each key (one for sending
   and another for receiving).  It is typically recommended that the
   keys be configured using finite lifetimes, adding new keys before the
   old keys expire.  However, it is obviously possible for all keys to
   expire for a given interface (for sending, receiving, or both).
   Possible ways of addressing this situation raise their own concerns:

   o  Automatic switching to unauthenticated protocol exchange.  This
      behaviour invalidates the initial purposes of authentication and
      is commonly viewed as unacceptable ([RIP2-AUTH] Section 5.1,
      [OSPF2-AUTH] Section 3.2, and [OSPF3-AUTH-BIS] Section 3).

   o  Stopping routing information exchange over the interface.  This
      behaviour is likely to impact regular traffic routing and is
      commonly viewed as "not advisable" ([RIP2-AUTH], [OSPF2-AUTH], and
      [OSPF3-AUTH]), although [OSPF3-AUTH-BIS] is different in this
      regard.

   o  The use of the "most recently expired" key over its intended
      lifetime range.  This behaviour is recommended for implementation
      in [RIP2-AUTH], [OSPF2-AUTH], and [OSPF3-AUTH] but not in
      [OSPF3-AUTH-BIS].  Such use of this key may become a problem, due
      to an offline cryptographic attack (see item (f) of Section 8) or
      a compromise of the key.  In addition, distinguishing a recently
      expired key from a key that has never been used may be impossible
      after a router restart.

   The design of this mechanism prevents automatic switching to
   unauthenticated exchange and is consistent with similar
   authentication mechanisms in this regard, but since the best choice
   between two other options depends on local site policy, this decision



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   is left up to the operator rather than the implementor (in a way
   resembling the "fail secure" configuration knob described in
   Section 5.1 of [RIP2-AUTH]).

   Although the deriving procedure does not allow for any exceptions in
   the filtering of expired keys (Section 5.2 item 2), the operator can
   trivially enforce one of the two remaining behaviour options through
   local key-management procedures.  In particular, when using the key
   over its intended lifetime is preferable to regular traffic
   disruption, the operator would explicitly leave the old key expiry
   time open until the new key is added to the router configuration.  In
   the opposite case, the operator would always configure the old key
   with a finite lifetime and bear associated risks.

8.  Security Considerations

   The use of this mechanism implies requirements common to the use of
   shared authentication keys, including, but not limited to:

   o  holding the keys secret,

   o  including sufficient amounts of random bits into each key,

   o  rekeying on a regular basis, and

   o  never reusing a used key for a different purpose.

   That said, proper design and implementation of a key-management
   policy are out of the scope of this work.  Many publications on this
   subject exist and should be used for this purpose (BCP 107 [RFC4107],
   BCP 132 [RFC4962], and [RFC6039] are suggested as starting points).

   It is possible for a network that exercises rollover of
   authentication keys to experience accidental expiration of all the
   keys for a network interface, as discussed at greater length in
   Section 7.4.  With that and the guidance of Section 5.1 of
   [RIP2-AUTH] in mind, in such an event the Babel speaker MUST send a
   "last key expired" notification to the operator (e.g., via syslog,
   SNMP, and/or other implementation-specific means), most likely in
   relation to item (b) of Section 5.5.  Also, any actual occurrence of
   an authentication key expiration MUST cause a security event to be
   logged by the implementation.  The log item MUST include at least a
   note that the authentication key has expired, the Babel routing
   protocol instance(s) affected, the network interface(s) affected, the
   LocalKeyID that is affected, and the current date/time.  Operators
   are encouraged to check such logs as an operational security
   practice.




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   Considering particular attacks being in scope or out of scope on one
   hand and measures taken to protect against particular in-scope
   attacks on the other, the original Babel protocol and this
   authentication mechanism are in line with similar datagram-based
   routing protocols and their respective mechanisms.  In particular,
   the primary concerns addressed are:

   a.  Peer Entity Authentication

       The Babel speaker authentication mechanism defined herein is
       believed to be as strong as the class itself to which it belongs.
       This specification is built on fundamental concepts implemented
       for authentication of similar routing protocols: per-packet
       authentication, the use of the HMAC construction, and the use of
       shared keys.  Although this design approach does not address all
       possible concerns, it is so far known to be sufficient for most
       practical cases.

   b.  Data Integrity

       Meaningful parts of a Babel datagram are the contents of the
       Babel packet (in the definition of Section 4.2 of [BABEL]) and
       the source address of the datagram (Section 3.5.3 of [BABEL]).
       This mechanism authenticates both parts, using the HMAC
       construction, so that making any meaningful change to an
       authenticated packet after it has been emitted by the sender
       should be as hard as attacking the HMAC construction itself or
       successfully recovering the authentication key.

       Note well that any trailing data of the Babel datagram is not
       meaningful in the scope of the original specification and does
       not belong to the Babel packet.  Integrity of the trailing data
       is thus not protected by this mechanism.  At the same time,
       although any TLV extra data is also not meaningful in the same
       scope, its integrity is protected, since this extra data is a
       part of the Babel packet (see Figure 2 in Appendix A).

   c.  Denial of Service

       Proper deployment of this mechanism in a Babel network
       significantly increases the efforts required for an attacker to
       feed arbitrary Babel packets into a protocol exchange (with the
       intent of attacking a particular Babel speaker or disrupting the
       exchange of regular traffic in a routing domain).  It also
       protects the neighbour table from being flooded with forged
       speaker entries.





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       At the same time, this protection comes with a price of CPU time
       being spent on HMAC computations.  This may be a concern for
       low-performance CPUs combined with high-speed interfaces, as
       sometimes seen in embedded systems and hardware routers.  The
       MaxDigestsIn parameter, which is used to limit the maximum amount
       of CPU time spent on a single received Babel packet, addresses
       this concern to some extent.

   d.  Reflection Attacks

       Given the approach discussed in item (b), the only potential
       reflection attack on this mechanism could be replaying exact
       copies of Babel packets back to the sender from the same source
       address.  The mitigation in this case is straightforward and is
       discussed in Section 5.4.

   The following in-scope concern is only partially addressed:

   e.  Replay Attacks

       This specification establishes a basic replay protection measure
       (see Section 3.6), defines a timeout parameter affecting its
       strength (see Section 3.7), and outlines implementation methods
       also affecting protection strength in several ways (see
       Section 5.1).  The implementor's choice of the timeout value and
       particular implementation methods may be suboptimal due to, for
       example, insufficient hardware resources of the Babel speaker.

       Furthermore, it may be possible that an operator configures the
       timeout and the methods to address particular local specifics,
       and this further weakens the protection.  An operator concerned
       about replay attack protection strength should understand these
       factors and their meaning in a given network segment.

       That said, a particular form of replay attack on this mechanism
       remains possible anyway.  Whether there are two or more network
       segments using the same CSA and there is an adversary that
       captures Babel packets on one segment and replays on another (and
       vice versa, due to the bidirectional reachability requirement for
       neighbourship), some of the speakers on one such segment will
       detect the "virtual" neighbours from another and may prefer them
       for some destinations.  This applies even more so as Babel
       doesn't require a common pre-configured network prefix between
       neighbours.







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       A reliable solution to this particular problem, which Section 4.5
       of [RFC7186] discusses as well, is not currently known.  It is
       recommended that the operators use distinct CSAs for distinct
       network segments.

   The following in-scope concerns are not addressed:

   f.  Offline Cryptographic Attacks

       This mechanism is obviously subject to offline cryptographic
       attacks.  As soon as an attacker has obtained a copy of an
       authenticated Babel packet of interest (which gets easier to do
       in wireless networks), he has all of the parameters of the
       authentication-specific processing performed by the sender,
       except for authentication key(s) and the choice of particular
       hash algorithm(s).  Since digest lengths of common hash
       algorithms are well known and can be matched with those seen in
       the packet, the complexity of this attack is essentially that of
       the authentication key attack.

       Viewing the cryptographic strength of particular hash algorithms
       as a concern of its own, the main practical means of resisting
       offline cryptographic attacks on this mechanism are periodic
       rekeying and the use of strong keys with a sufficient number of
       random bits.

       It is important to understand that in the case of multiple keys
       being used within a single interface (for multi-domain
       authentication or during a key rollover) the strength of the
       combined configuration would be that of the weakest key, since
       only one successful HMAC test is required for an authentic
       packet.  Operators concerned about offline cryptographic attacks
       should enforce the same strength policy for all keys used for a
       given interface.

       Note that a special pathological case is possible with this
       mechanism.  Whenever two or more authentication keys are
       configured for a given interface such that all keys share the
       same AuthKeyOctets and the same HashAlgo, but LocalKeyID modulo
       2^16 is different for each key, these keys will not be treated as
       duplicate (Section 5.2 item 4), but an HMAC result computed for a
       given packet will be the same for each of these keys.  In the
       case of the sending procedure, this can produce multiple HMAC
       TLVs with exactly the same value of the Digest field but
       different values of the KeyID field.  In this case, the attacker
       will see that the keys are the same, even without knowledge of





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       the key itself.  The reuse of authentication keys is not the
       intended use case of this mechanism and should be strongly
       avoided.

   g.  Non-repudiation

       This specification relies on the use of shared keys.  There is no
       timestamp infrastructure and no key-revocation mechanism defined
       to address the compromise of a shared key.  Establishing the time
       that a particular authentic Babel packet was generated is thus
       not possible.  Proving that a particular Babel speaker had
       actually sent a given authentic packet is also impossible as soon
       as the shared key is claimed compromised.  Even if the shared key
       is not compromised, reliably identifying the speaker that had
       actually sent a given authentic Babel packet is not possible.
       Since any of the speakers sharing a key can impersonate any other
       speaker sharing the same key, it is only possible to prove that
       the speaker belongs to the group sharing the key.

   h.  Confidentiality Violations

       The original Babel protocol does not encrypt any of the
       information contained in its packets.  The contents of a Babel
       packet are trivial to decode and thus can reveal network topology
       details.  This mechanism does not improve this situation in any
       way.  Since routing protocol messages are not the only kind of
       information subject to confidentiality concerns, a complete
       solution to this problem is likely to include measures based on
       the channel security model, such as IPsec and Wi-Fi Protected
       Access 2 (WPA2) at the time of this writing.

   i.  Key Management

       Any authentication key exchange/distribution concerns are out of
       scope.  However, the internal representation of authentication
       keys (see Section 3.8) allows implementations to use such diverse
       key-management techniques as manual configuration, a provisioning
       system, a key-management protocol, or any other means that comply
       with this specification.

   j.  Message Deletion

       Any message deletion attacks are out of scope.  Since a datagram
       deleted by an attacker cannot be distinguished from a datagram
       naturally lost in transmission, and since datagram-based routing
       protocols are designed to withstand a certain loss of packets,





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       the currently established practice is treating authentication
       purely as a per-packet function, without any added detection of
       lost packets.

9.  IANA Considerations

   At the time of publication of this document, the Babel TLV Types
   namespace did not have an IANA registry.  TLV types 11 and 12 were
   assigned (see Table 1 in Appendix A) to the TS/PC and HMAC TLV types
   by Juliusz Chroboczek, designer of the original Babel protocol.
   Therefore, this document has no IANA actions.

10.  Acknowledgements

   Thanks to Randall Atkinson and Matthew Fanto for their comprehensive
   work on [RIP2-AUTH] that initiated a series of publications on
   routing protocol authentication, including this one.  This
   specification adopts many concepts belonging to the whole series.

   Thanks to Juliusz Chroboczek, Gabriel Kerneis, and Matthieu Boutier.
   This document incorporates many technical and editorial corrections
   based on their feedback.  Thanks to all contributors to Babel,
   because this work would not be possible without the prior works.
   Thanks to Dominic Mulligan for editorial proofreading of this
   document.  Thanks to Riku Hietamaki for suggesting the test vectors
   section.

   Thanks to Joel Halpern, Jim Schaad, Randall Atkinson, and Stephen
   Farrell for providing (in chronological order) valuable feedback on
   earlier versions of this document.

   Thanks to Jim Gettys and Dave Taht for developing the CeroWrt
   wireless router project and collaborating on many integration issues.
   A practical need for Babel authentication emerged during research
   based on CeroWrt that eventually became the very first use case of
   this mechanism.

   Thanks to Kunihiro Ishiguro and Paul Jakma for establishing the GNU
   Zebra and Quagga routing software projects, respectively.  Thanks to
   Werner Koch, the author of Libgcrypt.  The very first implementation
   of this mechanism was made on a base of Quagga and Libgcrypt.










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11.  References

11.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
              Keyed-Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [FIPS-198] National Institute of Standards and Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS
              PUB 198-1, July 2008.

   [BABEL]    Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
              April 2011.

11.2.  Informative References

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC4030]  Stapp, M. and T. Lemon, "The Authentication Suboption for
              the Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Option", RFC 4030, March 2005.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

   [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270, November 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RIP2-AUTH]
              Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
              Authentication", RFC 4822, February 2007.





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   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

   [RFC5176]  Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
              Aboba, "Dynamic Authorization Extensions to Remote
              Authentication Dial In User Service (RADIUS)", RFC 5176,
              January 2008.

   [ISIS-AUTH-A]
              Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, October 2008.

   [ISIS-AUTH-B]
              Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.

   [OSPF2-AUTH]
              Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, October 2009.

   [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
              with Existing Cryptographic Protection Methods for Routing
              Protocols", RFC 6039, October 2010.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, March 2011.

   [OSPF3-AUTH]
              Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 6506,
              February 2012.

   [RFC6709]  Carpenter, B., Aboba, B., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              September 2012.

   [BABEL-EXTENSION]
              Chroboczek, J., "Extension Mechanism for the Babel Routing
              Protocol", Work in Progress, June 2014.




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   [OSPF3-AUTH-BIS]
              Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 7166, March 2014.

   [RFC7183]  Herberg, U., Dearlove, C., and T. Clausen, "Integrity
              Protection for the Neighborhood Discovery Protocol (NHDP)
              and Optimized Link State Routing Protocol Version 2
              (OLSRv2)", RFC 7183, April 2014.

   [RFC7186]  Yi, J., Herberg, U., and T. Clausen, "Security Threats for
              the Neighborhood Discovery Protocol (NHDP)", RFC 7186,
              April 2014.







































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Appendix A.  Figures and Tables

      +-------------------------------------------------------------+
      |              authentication-specific statistics             |
      +-------------------------------------------------------------+
        ^                            |                            ^
        |                            v                            |
        |    +-----------------------------------------------+    |
        |    |                system operator                |    |
        |    +-----------------------------------------------+    |
        |        ^ |      ^ |       ^ |       ^ |      ^ |        |
        |        | v      | |       | |       | |      | v        |
      +---+  +---------+  | |       | |       | |  +---------+  +---+
      |   |->|   ANM   |  | |       | |       | |  | LocalTS |->|   |
      | R |<-|  table  |  | |       | |       | |  | LocalPC |<-| T |
      | x |  +---------+  | v       | v       | v  +---------+  | x |
      |   |  +----------------+ +---------+ +----------------+  |   |
      | p |  | MaxDigestsIn   | |         | | MaxDigestsOut  |  | p |
      | r |<-| ANM timeout    | |  CSAs   | |                |->| r |
      | o |  | RxAuthRequired | |         | |                |  | o |
      | c |  +----------------+ +---------+ +----------------+  | c |
      | e |  +-------------+     |       |     +-------------+  | e |
      | s |  |   Rx ESAs   |     |       |     |   Tx ESAs   |  | s |
      | s |<-| (temporary) |<----+       +---->| (temporary) |->| s |
      | i |  +-------------+                   +-------------+  | i |
      | n |  +------------------------------+----------------+  | n |
      | g |  |     instance of              | output buffers |=>| g |
      |   |=>|     the original             +----------------+  |   |
      |   |  |     protocol                 | source address |->|   |
      +---+  +------------------------------+----------------+  +---+
       /\                                            |            ||
       ||                                            v            \/
      +-------------------------------------------------------------+
      |                        network stack                        |
      +-------------------------------------------------------------+
         /\ ||       /\ ||                       /\ ||       /\ ||
         || \/       || \/                       || \/       || \/
      +---------+ +---------+                 +---------+ +---------+
      | speaker | | speaker |       ...       | speaker | | speaker |
      +---------+ +---------+                 +---------+ +---------+


      Flow of control data           : --->
      Flow of Babel datagrams/packets: ===>

                       Figure 1: Interaction Diagram





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                  P
   |<---------------------------->|                                 (D1)
   |                B             |
   |  |<------------------------->|
   |  |                           |
   +--+-----+-----+...+-----+-----+--+   P: Babel packet
   |H |some |some |   |some |some |T |   H: Babel packet header
   |  |TLV  |TLV  |   |TLV  |TLV  |  |   B: Babel packet body
   |  |     |     |   |     |     |  |   T: optional trailing data block
   +--+-----+-----+...+-----+-----+--+

                               P
   |<----------------------------------------------------->|        (D2)
   |                             B                         |
   |  |<-------------------------------------------------->|
   |  |                                                    |
   +--+-----+-----+...+-----+-----+------+------+...+------+--+
   |H |some |some |   |some |some |TS/PC |HMAC  |   |HMAC  |T |
   |  |TLV  |TLV  |   |TLV  |TLV  |TLV   |TLV 1 |   |TLV n |  |
   |  |     |     |   |     |     |      |      |   |      |  |
   +--+-----+-----+...+-----+-----+------+------+...+------+--+

                               P
   |<----------------------------------------------------->|        (D3)
   |                             B                         |
   |  |<-------------------------------------------------->|
   |  |                                                    |
   +--+------+------+...+------+-----+-----+...+-----+-----+--+
   |H |TS/PC |HMAC  |   |HMAC  |some |some |   |some |some |T |
   |  |TLV   |TLV 1 |   |TLV n |TLV  |TLV  |   |TLV  |TLV  |  |
   |  |      |      |   |      |     |     |   |     |     |  |
   +--+------+------+...+------+-----+-----+...+-----+-----+--+


                                  P
   |<------------------------------------------------------------>| (D4)
   |                                B                             |
   |  |<--------------------------------------------------------->|
   |  |                                                           |
   +--+-----+------+-----+------+...+-----+------+...+------+-----+--+
   |H |some |HMAC  |some |HMAC  |   |some |HMAC  |   |TS/PC |some |T |
   |  |TLV  |TLV 1 |TLV  |TLV 2 |   |TLV  |TLV n |   |TLV   |TLV  |  |
   |  |     |      |     |      |   |     |      |   |      |     |  |
   +--+-----+------+-----+------+...+-----+------+...+------+-----+--+

                    Figure 2: Babel Datagram Structure





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            +-------+-------------------------+---------------+
            | Value | Name                    | Reference     |
            +-------+-------------------------+---------------+
            |     0 | Pad1                    | [BABEL]       |
            |     1 | PadN                    | [BABEL]       |
            |     2 | Acknowledgement Request | [BABEL]       |
            |     3 | Acknowledgement         | [BABEL]       |
            |     4 | Hello                   | [BABEL]       |
            |     5 | IHU                     | [BABEL]       |
            |     6 | Router-Id               | [BABEL]       |
            |     7 | Next Hop                | [BABEL]       |
            |     8 | Update                  | [BABEL]       |
            |     9 | Route Request           | [BABEL]       |
            |    10 | Seqno Request           | [BABEL]       |
            |    11 | TS/PC                   | this document |
            |    12 | HMAC                    | this document |
            +-------+-------------------------+---------------+

                   Table 1: Babel TLV Types 0 through 12

    +--------------+-----------------------------+-------------------+
    | Packet field | Packet octets (hexadecimal) | Meaning (decimal) |
    +--------------+-----------------------------+-------------------+
    | Magic        | 2a                          | 42                |
    | Version      | 02                          | version 2         |
    | Body length  | 00:14                       | 20 octets         |
    | [TLV] Type   | 04                          | 4 (Hello)         |
    | [TLV] Length | 06                          | 6 octets          |
    | Reserved     | 00:00                       | no meaning        |
    | Seqno        | 09:25                       | 2341              |
    | Interval     | 01:90                       | 400 (4.00 s)      |
    | [TLV] Type   | 08                          | 8 (Update)        |
    | [TLV] Length | 0a                          | 10 octets         |
    | AE           | 00                          | 0 (wildcard)      |
    | Flags        | 40                          | default router-id |
    | Plen         | 00                          | 0 bits            |
    | Omitted      | 00                          | 0 bits            |
    | Interval     | ff:ff                       | infinity          |
    | Seqno        | 68:21                       | 26657             |
    | Metric       | ff:ff                       | infinity          |
    +--------------+-----------------------------+-------------------+

            Table 2: A Babel Packet without Authentication TLVs








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   +---------------+-------------------------------+-------------------+
   | Packet field  | Packet octets (hexadecimal)   | Meaning (decimal) |
   +---------------+-------------------------------+-------------------+
   | Magic         | 2a                            | 42                |
   | Version       | 02                            | version 2         |
   | Body length   | 00:4c                         | 76 octets         |
   | [TLV] Type    | 04                            | 4 (Hello)         |
   | [TLV] Length  | 06                            | 6 octets          |
   | Reserved      | 00:00                         | no meaning        |
   | Seqno         | 09:25                         | 2341              |
   | Interval      | 01:90                         | 400 (4.00 s)      |
   | [TLV] Type    | 08                            | 8 (Update)        |
   | [TLV] Length  | 0a                            | 10 octets         |
   | AE            | 00                            | 0 (wildcard)      |
   | Flags         | 40                            | default router-id |
   | Plen          | 00                            | 0 bits            |
   | Omitted       | 00                            | 0 bits            |
   | Interval      | ff:ff                         | infinity          |
   | Seqno         | 68:21                         | 26657             |
   | Metric        | ff:ff                         | infinity          |
   | [TLV] Type    | 0b                            | 11 (TS/PC)        |
   | [TLV] Length  | 06                            | 6 octets          |
   | PacketCounter | 00:01                         | 1                 |
   | Timestamp     | 52:1d:7e:8b                   | 1377664651        |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:c8                         | 200               |
   | Digest        | fe:80:00:00:00:00:00:00:0a:11 | padding           |
   |               | 96:ff:fe:1c:10:c8:00:00:00:00 |                   |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:64                         | 100               |
   | Digest        | fe:80:00:00:00:00:00:00:0a:11 | padding           |
   |               | 96:ff:fe:1c:10:c8:00:00:00:00 |                   |
   +---------------+-------------------------------+-------------------+

   Table 3: A Babel Packet with Each HMAC TLV Padded Using IPv6 Address
                         fe80::0a11:96ff:fe1c:10c8













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   +---------------+-------------------------------+-------------------+
   | Packet field  | Packet octets (hexadecimal)   | Meaning (decimal) |
   +---------------+-------------------------------+-------------------+
   | Magic         | 2a                            | 42                |
   | Version       | 02                            | version 2         |
   | Body length   | 00:4c                         | 76 octets         |
   | [TLV] Type    | 04                            | 4 (Hello)         |
   | [TLV] Length  | 06                            | 6 octets          |
   | Reserved      | 00:00                         | no meaning        |
   | Seqno         | 09:25                         | 2341              |
   | Interval      | 01:90                         | 400 (4.00 s)      |
   | [TLV] Type    | 08                            | 8 (Update)        |
   | [TLV] Length  | 0a                            | 10 octets         |
   | AE            | 00                            | 0 (wildcard)      |
   | Flags         | 40                            | default router-id |
   | Plen          | 00                            | 0 bits            |
   | Omitted       | 00                            | 0 bits            |
   | Interval      | ff:ff                         | infinity          |
   | Seqno         | 68:21                         | 26657             |
   | Metric        | ff:ff                         | infinity          |
   | [TLV] Type    | 0b                            | 11 (TS/PC)        |
   | [TLV] Length  | 06                            | 6 octets          |
   | PacketCounter | 00:01                         | 1                 |
   | Timestamp     | 52:1d:7e:8b                   | 1377664651        |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:c8                         | 200               |
   | Digest        | c6:f1:06:13:30:3c:fa:f3:eb:5d | HMAC result       |
   |               | 60:3a:ed:fd:06:55:83:f7:ee:79 |                   |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:64                         | 100               |
   | Digest        | df:32:16:5e:d8:63:16:e5:a6:4d | HMAC result       |
   |               | c7:73:e0:b5:22:82:ce:fe:e2:3c |                   |
   +---------------+-------------------------------+-------------------+

   Table 4: A Babel Packet with Each HMAC TLV Containing an HMAC Result














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Appendix B.  Test Vectors

   The test vectors below may be used to verify the correctness of some
   procedures performed by an implementation of this mechanism, namely:

   o  appending TS/PC and HMAC TLVs to the Babel packet body,

   o  padding the HMAC TLV(s),

   o  computation of the HMAC result(s), and

   o  placement of the result(s) in the TLV(s).

   This verification isn't exhaustive.  There are other important
   implementation aspects that would require testing methods of
   their own.

   The test vectors were produced as follows.

   1.  A Babel speaker with a network interface with IPv6 link-local
       address fe80::0a11:96ff:fe1c:10c8 was configured to use two CSAs
       for the interface:

       *  CSA1={HashAlgo=RIPEMD-160, KeyChain={{LocalKeyID=200,
          AuthKeyOctets=Key26}}}

       *  CSA2={HashAlgo=SHA-1, KeyChain={{LocalKeyId=100,
          AuthKeyOctets=Key70}}}

       The authentication keys above are:

       *  Key26 in ASCII:

          ABCDEFGHIJKLMNOPQRSTUVWXYZ

       *  Key26 in hexadecimal:

          41:42:43:44:45:46:47:48:49:4a:4b:4c:4d:4e:4f:50
          51:52:53:54:55:56:57:58:59:5a

       *  Key70 in ASCII:

  This=key=is=exactly=70=octets=long.=ABCDEFGHIJKLMNOPQRSTUVWXYZ01234567








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       *  Key70 in hexadecimal:

          54:68:69:73:3d:6b:65:79:3d:69:73:3d:65:78:61:63
          74:6c:79:3d:37:30:3d:6f:63:74:65:74:73:3d:6c:6f
          6e:67:2e:3d:41:42:43:44:45:46:47:48:49:4a:4b:4c
          4d:4e:4f:50:51:52:53:54:55:56:57:58:59:5a:30:31
          32:33:34:35:36:37

       The length of each key was picked to relate (using the terms
       listed in Section 2.4) to the properties of its respective hash
       algorithm as follows:

       *  the digest length (L) of both RIPEMD-160 and SHA-1 is 20
          octets,

       *  the internal block size (B) of both RIPEMD-160 and SHA-1 is 64
          octets,

       *  the length of Key26 (26) is greater than L but less than B,
          and

       *  the length of Key70 (70) is greater than B (and thus greater
          than L).

       KeyStartAccept, KeyStopAccept, KeyStartGenerate, and
       KeyStopGenerate were set to make both authentication keys valid.

   2.  The instance of the original protocol of the speaker produced a
       Babel packet (PktO) to be sent from the interface.  Table 2
       provides a decoding of PktO, the contents of which are below:

       2a:02:00:14:04:06:00:00:09:25:01:90:08:0a:00:40
       00:00:ff:ff:68:21:ff:ff

   3.  The authentication mechanism appended one TS/PC TLV and two HMAC
       TLVs to the packet body, updated the "Body length" packet header
       field, and padded the Digest field of the HMAC TLVs, using the
       link-local IPv6 address of the interface and the necessary amount
       of zeroes.  Table 3 provides a decoding of the resulting
       temporary packet (PktT), the contents of which are below:

       2a:02:00:4c:04:06:00:00:09:25:01:90:08:0a:00:40
       00:00:ff:ff:68:21:ff:ff:0b:06:00:01:52:1d:7e:8b
       0c:16:00:c8:fe:80:00:00:00:00:00:00:0a:11:96:ff
       fe:1c:10:c8:00:00:00:00:0c:16:00:64:fe:80:00:00
       00:00:00:00:0a:11:96:ff:fe:1c:10:c8:00:00:00:00





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   4.  The authentication mechanism produced two HMAC results,
       performing the computations as follows:

       *  For H=RIPEMD-160, K=Key26, and Text=PktT, the HMAC result is:

          c6:f1:06:13:30:3c:fa:f3:eb:5d:60:3a:ed:fd:06:55
          83:f7:ee:79

       *  For H=SHA-1, K=Key70, and Text=PktT, the HMAC result is:

          df:32:16:5e:d8:63:16:e5:a6:4d:c7:73:e0:b5:22:82
          ce:fe:e2:3c

   5.  The authentication mechanism placed each HMAC result into its
       respective HMAC TLV, producing the final authenticated Babel
       packet (PktA), which was eventually sent from the interface.

       Table 4 provides a decoding of PktA, the contents of which are
       below:

       2a:02:00:4c:04:06:00:00:09:25:01:90:08:0a:00:40
       00:00:ff:ff:68:21:ff:ff:0b:06:00:01:52:1d:7e:8b
       0c:16:00:c8:c6:f1:06:13:30:3c:fa:f3:eb:5d:60:3a
       ed:fd:06:55:83:f7:ee:79:0c:16:00:64:df:32:16:5e
       d8:63:16:e5:a6:4d:c7:73:e0:b5:22:82:ce:fe:e2:3c

   Interpretation of this process is to be done differently for the
   sending and receiving directions (see Figure 1).

   For the sending direction, given a Babel speaker configured using the
   IPv6 address and the sequence of CSAs as described above, the
   implementation SHOULD (see notes in Section 5.3) produce exactly the
   temporary packet PktT if the original protocol instance produces
   exactly the packet PktO to be sent from the interface.  If the
   temporary packet exactly matches PktT, the HMAC results computed
   afterwards MUST exactly match the respective results above, and the
   final authenticated packet MUST exactly match PktA above.

   For the receiving direction, given a Babel speaker configured using
   the sequence of CSAs as described above (but a different IPv6
   address), the implementation MUST (assuming that the TS/PC check
   didn't fail) produce exactly the temporary packet PktT above if its
   network stack receives through the interface exactly the packet PktA
   above from the source IPv6 address above.  The first HMAC result
   computed afterwards MUST match the first result above.  The receiving
   procedure doesn't compute the second HMAC result in this case, but if
   the implementor decides to compute it anyway for verification
   purposes, it MUST exactly match the second result above.



Ovsienko                      Experimental                     [Page 54]

RFC 7298         Babel HMAC Cryptographic Authentication       July 2014


Author's Address

   Denis Ovsienko
   Yandex
   16, Leo Tolstoy St.
   Moscow  119021
   Russia

   EMail: infrastation@yandex.ru










































Ovsienko                      Experimental                     [Page 55]

