Subject: [6lo] Thomas' review of draft-ietf-6lo-routing-dispatch-00

From: Thomas Watteyne <[email-masked]>
Date: Thu, 10 Dec 2015 08:30:00 +0100

All,

Following the adoption of draft-ietf-6lo-routing-dispatch-00, please find
my detailed review of the draft below.
On top of questions and suggestions which I prefixed with TW>, I made
inline changes for tiny editorial nits. Authors, please use diff to see
those.

Thomas

---

TW> I've done some editing directly inline, so please use a diff tool to
see what I did
TW> for questions and suggestions, I used the TW> prefix

6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     Cisco
TW> That's strange, would it really update it, or add to it in the same way
RFC6282 does?
Updates: 4944 (if approved)                                   C. Bormann
Intended status: Standards Track                          Uni Bremen TZI
Expires: June 6, 2016                                         L. Toutain
                                                    IMT-TELECOM Bretagne
                                                               R. Cragie
                                                                     ARM
                                                        December 4, 2015

              6LoWPAN Routing Header And Paging Dispatches
                   draft-ietf-6lo-routing-dispatch-00

Abstract

   This specification introduces a new context switch mechanism for
   6LoWPAN compression, expressed in terms of Pages and signaled by a
   new Paging Dispatch.  A new 6LoWPAN dispatch type is proposed in a
   new Page 1 for use in 6LoWPAN Route-Over topologies, that initially
   covers the needs of RPL (RFC6550) data packets compression.  This
   specification defines a method to compress RPL Option (RFC6553)
   information and Routing Header type 3 (RFC6554), an efficient IP-in-
   IP technique and is extensible for more applications.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 6, 2016.

Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  New Page 1 Paging Dispatch {#Page 1}  . . . . . . . . . .   6
     3.2.  New Routing Header Dispatch (6LoRH) . . . . . . . . . . .   6
   4.  Placement Of The New Dispatch Types . . . . . . . . . . . . .   7
     4.1.  Placement Of The Page 1 Paging Dispatch . . . . . . . . .   7
     4.2.  Placement Of The 6LoRH  . . . . . . . . . . . . . . . . .   7
   5.  6LoWPAN Routing Header General Format . . . . . . . . . . . .   7
     5.1.  Elective Format . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Critical Format . . . . . . . . . . . . . . . . . . . . .   8
   6.  The Routing Header Type 3 (RH3) 6LoRH . . . . . . . . . . . .   9
   7.  The RPL Packet Information 6LoRH  . . . . . . . . . . . . . .  10
     7.1.  Compressing the RPLInstanceID . . . . . . . . . . . . . .  12
     7.2.  Compressing the SenderRank  . . . . . . . . . . . . . . .  12
     7.3.  The Overall RPI-6LoRH encoding  . . . . . . . . . . . . .  12
   8.  The IP-in-IP 6LoRH  . . . . . . . . . . . . . . . . . . . . .  15
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The design of Low Power and Lossy Networks (LLNs) is generally
   focused on saving energy, a very constrained resource in most cases.
   The other constraints, such as the memory capacity and the duty
   cycling of the LLN devices, derive from that primary concern.  Energy
   is often available from primary batteries that are expected to last
   for years, or is scavenged from the environment in very limited
   quantities.  Any protocol that is intended for use in LLNs must be
   designed with the primary concern of saving energy as a strict
   requirement.

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   Controlling the amount of data transmission is one possible venue to
   save energy.  In a number of LLN standards, the frame size is limited
   to much smaller values than the IPv6 maximum transmission unit (MTU)
   of 1280 bytes.  In particular, an LLN that relies on the classical
   Physical Layer (PHY) of IEEE 802.15.4 [IEEE802154] is limited to 127
   bytes per frame.  The need to compress IPv6 packets over IEEE
   802.15.4 led to the 6LoWPAN Header Compression [RFC6282] work
   (6LoWPAN-HC).

   Innovative Route-over techniques have been and are still being
   developed for routing inside a LLN.  In a general fashion, such
   techniques require additional information in the packet to provide
   loop prevention and to indicate information such as flow
   identification, source routing information, etc.

   For reasons such as security and the capability to send ICMP errors
   back to the source, an original packet must not be tampered with, and
   any information that must be inserted in or removed from an IPv6
   packet must be placed in an extra IP-in-IP encapsulation.  This is
   the case when additional routing information is inserted by a
   router on the path of a packet, for instance a mesh root, as opposed
   to the source node.  This is also the case when some routing
   information must be removed from a packet that flows outside the
   LLN.

   As an example, the Routing Protocol for Low Power and Lossy Networks
   [RFC6550] (RPL) is designed to optimize the routing operations in
   constrained LLNs.  As part of this optimization, RPL requires the
   addition of RPL Packet Information (RPI) in every packet, as defined
   in Section 11.2 of [RFC6550].

   The RPL Option for Carrying RPL Information in Data-Plane Datagrams
   [RFC6553] specification indicates how the RPI can be placed in a RPL
   Option for use in an IPv6 Hop-by-Hop header.  This representation
   demands a total of 8 bytes when in most cases the actual RPI payload
   requires only 19 bits.  Since the Hop-by-Hop header must not flow
   outside of the RPL domain, it must be removed from packets that leave
   the domain, and be inserted in packets entering the domain.  In both
   cases, this operation implies an IP-in-IP encapsulation.
TW> It would be good to indicate here what IP-in-IP encapsulation looks
like when using 6LoWPAN.
TW> This doc talks about 8 bytes later on, it would be good to see where
this comes from (yes, I could take RFC6282, but I'm lazy)

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              ------+---------                            ^
                    |          Internet                   |
                    |                                     | Native IPv6
                 +-----+                                  |
                 |     | Border Router (RPL Root)    ^    |    ^
                 |     |                             |    |    |
                 +-----+                             |    |    | IPv6 in
                    |                                |    |    | IPv6
              o    o   o    o                        |    |    | + RPI
          o o   o  o   o  o  o o   o                 |    |    |  or RH3
         o  o o  o o    o   o   o  o  o              |    |    |
         o   o    o  o     o  o    o  o  o           |    |    |
        o  o   o  o   o         o   o o              v    v    v
        o          o             o     o
                          LLN

              Figure 1: IP-in-IP Encapsulation within the LLN.

   Additionally, in the case of the Non-Storing Mode of Operation (MOP),
   RPL requires a Routing Header type 3 (RH3) as defined in the IPv6
   Routing Header for Source Routes with RPL [RFC6554] specification,
   for all packets that are routed down a RPL graph (from the Internet into
the LLN, see Fig. 1).  With Non-Storing
   RPL, even if the source is a node in the same LLN, the packet must
   first reach up the graph to the root so that the root can insert the
   RH3 to go down the graph.  Regardless of whether the packet was
   originated in a node in the LLN or outside the LLN, and regardless of
   whether the packet stays within the LLN or not, as long as the source
   of the packet is not the root itself, the source-routing operation
   also implies IP-in-IP encapsulation at the root in order to insert the
RH3.

   6TiSCH [I-D.ietf-6tisch-architecture] specifies the operation of IPv6
   over the TimeSlotted Channel Hopping [I-D.ietf-6tisch-tsch]
TW> outdate ref. Replace by RFC number
   (TSCH)
   mode of operation of IEEE 802.15.4.  The architecture requires the
   use of both RPL and the 6lo adaptation layer framework ([RFC4944],
   [RFC6282]) over IEEE 802.15.4.  Because it inherits the constraints
   on the frame size from the MAC layer, 6TiSCH cannot afford to spend 8
   bytes per packet on the RPI.  Hence the requirement for a 6LoWPAN
   header compression of the RPI.

   There a need for an extensible compression technique
   that simplifies IP-in-IP encapsulation, when needed, and
   optimally compresses existing routing artifacts found in LLNs.

   This specification extends the 6lo adaptation layer framework
TW> add references to 4944 and 6282?
   so as
   to carry routing information for Route-over
TW> what casing is used in 6550 for route over?
   use cases.  The

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   specification includes the formats necessary for RPL, and is
   extensible for additional formats.

2.  Terminology

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

   The Terminology used in this document is consistent with and
   incorporates that described in `Terminology in Low power And Lossy
   Networks' [RFC7102] and [RFC6550].

   The terms Route-over and Mesh-under are defined in [RFC6775].

   Other terms in use in LLNs are found in [RFC7228].

   The term "byte" is used in its now customary sense as a synonym for
   "octet".

3.  Updating RFC4944

   This draft adapts 6LoWPAN while maintaining backward compatibility
   with IPv6 over IEEE 802.15.4 [RFC4944] by introducing a concept of
   "context" in the 6LoWPAN parser, a context being identified by a Page
   number. This specification defines 16 Pages.

   Pages are delimited in a 6LoWPAN packet by a Paging Dispatch value
   that indicates the next current Page.  The Page number is encoded in
   a Paging Dispatch with the Value Bit Pattern of 1111xxxx where xxxx
   is the Page number, 0 to 15, as follows:

                            0
                            0 1 2 3 4 5 6 7
                           +-+-+-+-+-+-+-+-+
                           |1|1|1|1|Page Nb|
                           +-+-+-+-+-+-+-+-+

            Figure 2: Paging Dispatch with Page Number Encoding.

   Values of the Dispatch byte defined in [RFC4944] are considered as
   belonging to the Page 0 parsing context, which is the default and does
   not need to be signaled explicitly at the beginning of a 6LoWPAN
   packet.  This ensures backward compatibility with existing
   implementations of 6LoWPAN.

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   Note: This specification does not use the Escape Dispatch, which
   extends Page 0 to more values, but rather allocates another Dispatch
   Bit Pattern (1111xxxx) for a new Paging Dispatch, that is present in
   all Pages, including Page 0 and Pages defined in future
   specifications, to indicate the next parsing context represented by
   its Page number.
TW> maybe add a sentence which indicates that using the Escape Dispath
would add bytes?

3.1.  New Page 1 Paging Dispatch {#Page 1}

   This draft defines a new Page 1 Paging Dispatch (Dispatch Value
   of 11110001) which indicates a context switch in the 6LoWPAN parser to
   a Page 1.

   The Dispatch bits defined in Page 0 by [RFC4944] are free to be
   reused in the Pages 1 to 15.

   On the other hand, the Dispatch bits defined in Page 0 for the
   Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based
   Networks [RFC6282] are defined with the same values in Page 1 so
   there is no need to switch context back from Page 1 to Page 0 to
   address LOWPAN_IPHC and LOWPAN_NHC.
TW> this is unclear. What do you mean by "switch back"? Maybe a couple of
examples would help here, or a reference to an Appendix containining such
examples.

3.2.  New Routing Header Dispatch (6LoRH)

   This specification introduces a new 6LoWPAN Routing Header (6LoRH) to
   carry IPv6 routing information.  The 6LoRH may contain source routing
   information such as a compressed form of RH3, as well as other sorts
   of routing information such as the RPL Packet Information and IP-in-
   IP encapsulation.

   The 6LoRH is expressed in a 6loWPAN packet as a Type-Length-Value
   (TLV) field, which is extensible for future use.

   This specification uses the bit pattern 10xxxxxx in Page 1 for the
   new 6LoRH Dispatch.

TW> I don't see the need for this paragraph.
   The 6LoRH uses on a 1/4th of the Dispatch space in Page 1, and this
   specification only uses a limited portion of the TLV space in the
   6LoRH to encode RPL artifacts as detailed in Section 5.

   It is expected that in the future, other specification with extend
   the 6LoRH for other features related to packet routing and forwarding
   in 6LoWPAN networks.

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4.  Placement Of The New Dispatch Types

4.1.  Placement Of The Page 1 Paging Dispatch

   In a zone of a packet where Page 1 is active, which means once a Page
   1 Paging Dispatch is parsed, and as long as no other Paging Dispatch
   is parsed, the parsing of the packet MUST follow this specification
   if the 6LoRH Bit Pattern [Section 5] is found.

   Mesh Headers represent Layer-2 information and are processed before
   any Layer-3 information that is encoded in Page 1.  If a 6LoWPAN
   packet requires a Mesh header, the Mesh Header MUST always be placed
   in the packet before the first Page 1 Paging Dispatch, if any.

   For the same reason, Fragment Headers as defined in [RFC4944] MUST
   always be placed in the packet before the first Page 1 Paging
   Dispatch, if any.

   The NALP Dispatch Bit Pattern as defined in
   [RFC4944] is only defined for the first octet in the packet.
   Switching back to Page 0 for NALP inside a 6LoWPAN packet does not make
sense.

   It results that there is no need for restoring the Page 0
   parsing context after a context was switched to Page 1, so the value
   for the Page 0 Paging Dispatch of 11110000 may not actually be seen
   in packets following the 6LoWPAN specifications that are available at
   the time of writing.

4.2.  Placement Of The 6LoRH

   With this specification, the 6LoRH [Section 5] is only defined in
   Page 1, so it MUST be placed in the packet in a zone where the Page 1
   context is active.

   One or more 6LoRHs
TW> 6LoRH is a not a noun. Please say something like "6LoRH headers" (maybe
not the right term?)
   MAY be placed in a 6LoWPAN packet and MUST always
   be placed before the LOWPAN_IPHC [RFC6282].

TW> words missing?
   A 6LoRH being placed in a Page 1 context, it MUST always be placed
   after any Fragmentation Header and/or Mesh Header [RFC4944], even if
   a sur-compression mechanism
TW> what on Earth is sur-compression?
   is used that elides the Paging
   Dispatches.

5.  6LoWPAN Routing Header General Format

TW> why link to Page 0 at all? Why not simply stay "6LoRH uses Dispatch
Value Bit Pattern 10xxxxxx in Page 1"?
   In its canonical form, the 6LoRH reuses in Page 1 the Dispatch Value
   Bit Pattern of 10xxxxxx that is defined in Page 0 for the Mesh Header
   in [RFC4944].

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   The Dispatch Value Bit Pattern is split in two forms of 6LoRH:

      Elective (6LoRHE) that may skipped if not understood

      Critical (6LoRHC) that may not be ignored

5.1.  Elective Format

TW> "In its canonical form" doesn't mean anything. This is a spec, it's
either a format or it's not.
   In its canonical form, the 6LoRHE uses the Dispatch Value Bit Pattern
   of 101xxxxx.  A 6LoRHE may be ignored and skipped in parsing.  If it
   is ignored, the 6LoRHE is forwarded with no change inside the LLN.

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-       ...        -+
      |1|0|1| Length  |      Type     |                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-       ...        -+
                                       <--    Length    -->

                 Figure 3: Elective 6LoWPAN Routing Header.

   Length:
      Length of the 6LoRHE expressed in bytes, excluding the first 2
      bytes.  This enablesa node to skip a 6LoRHE that it does
      not support and/or cannot parse, for instance if the Type is not
      known.

   Type:
      Type of the 6LoRHE

5.2.  Critical Format

TW> Aaaaarrg, stop referring to canonical forms.
   In its canonical form, the 6LoRHC uses the Dispatch Value Bit Pattern
   of 100xxxxx.

   A node which does not support the 6LoRHC Type MUST silently discard
   the packet.

   Note: there is no provision for the exchange of error messages; such
   a situation should be avoided by judicious use of administrative
   control and/or capability indications.
TW> please add a sentence to indicate why using ICMPv6 here doesn't make
sense

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     0                   1
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-              ...               -+
    |1|0|0|   TSE   |      Type     |                                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-              ...               -+
                                     <-- Length implied by Type/TSE -->

                 Figure 4: Critical 6LoWPAN Routing Header.

   TSE:
      Type Specific Extension.  The meaning depends on the Type, which
      must be known in all of the nodes.  The interpretation of the TSE
      depends on the Type field that follows.  For instance, it may be
      used to transport control bits, the number of elements in an
      array, or the length of the remainder of the 6LoRHC expressed in a
      unit other than bytes.

   Type:
      Type of the 6LoRHC

6.  The Routing Header Type 3 (RH3) 6LoRH

   The Routing Header type 3 (RH3) 6LoRH (RH3-6LoRH)
TW> why not call it 6LoRH-RH3? It's first a 6LoRH, with subtype RH3
   is a Critical
   6LoWPAN Routing Header that provides a compressed form for the RH3,
   as defined in [RFC6554] for use by RPL routers.  Routers that need to
   forward a packet with a RH3-6LoRH are expected to be RPL routers and
   are expected to support this specification.  If a non-RPL router receives
   a packet with a RPI-6LoRH, this means that there was a routing error
TW> RPI-6LoRH is not defined at this point?
   and the packet should be dropped so the Type cannot be ignored.

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    -+-    -+ ... +-    -+
      |1|0|0|  Size   |6LoRH Type 0..4| Hop1 | Hop2 |     | HopN |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    -+-    -+ ... +-    -+

            Size indicates the number of compressed addresses

                          Figure 5: The RH3-6LoRH.

   The values for the RH3-6LoRH Type are an enumeration, 0 to 4.  The
   form of compression is indicated by the Type as follows:
TW> please reference the figures by their number.

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     +-----------+-----------+
     |   Type    | Size Unit |
     +-----------+-----------+
     |    0      |      1    |
     |    1      |      2    |
     |    2      |      4    |
     |    3      |      8    |
     |    4      |     16    |
     +-----------+-----------+

                       Figure 6: The RH3-6LoRH Types.

TW> what does the size unit mean? Is there a paragraph missing here?

   In the case of a RH3-6LoRH, the TSE field is used as a Size, which
   encodes the number of hops minus 1; so a Size of 0 means one hop, and
   the maximum that can be encoded is 32 hops.  (If more than 32 hops
   need to be expressed, a sequence of RH3-6LoRH can be employed.)

   The next Hop
TW> please use consistent casing. next hop? Next Hop?
   is indicated in the first entry of the first RH3-6LoRH.
   Upon reception, the entry is checked whether it refers to the
   processing router itself.
TW> I don't understand "whether it refers to the processing router itself"
   If it so, the entry is removed from the
   RH3-6LoRH and the Size is decremented.  If the Size is now zero, the
   whole RH3-6LoRH is removed.  If there is no more RH3-6LoRH, the
   processing node is the last router on the way, which may or may not
   be collocated with the final destination.
TW> I have a proposal: why not leave one more address in the source route?
this would enable piecewise source routing. Please ping me if you want me
to make this suggestion in the ML.

TW> this paragraph contradicts the previous one, which states that the last
address in the RH is not per-se the final destination
   The last hop in the last RH3-6LoRH is the last router prior to the
   destination in the LLN.  So even when there is a RH3-6LoRH in the
   frame,
TW> what do you mean by frame?
   the address of the final destination is in the LoWPAN_IPHC
   [RFC6282].

   If some bits of the first address in the RH3-6LoRH can be derived
   from the final destination is
TW> remove "is"?
   in the LoWPAN_IPHC, then that address
   may be compressed, otherwise is is expressed in full.
TW> full IPv6 I assume?
   Next addresses
   only need to express the delta from the previous address.

   All addresses in a RH3-6LoRH are compressed in a same fashion, down
   to the same number of bytes per address.  In order to get different
   forms of compression, multiple consecutive RH3-6LoRH must be used.

TW> a couple of example would be very welcome here, or a link to an
appendix with examples

7.  The RPL Packet Information 6LoRH

   [RFC6550], Section 11.2, specifies the RPL Packet Information (RPI)
   as a set of fields that are to be
TW> is that a MUST in 6550? if yes, please write MUST
   added to the IP packets for the
   purpose of Instance Identification, as well as Loop Avoidance and
   Detection.

   In particular, the SenderRank, which is the scalar metric computed by
   an specialized Objective Function such as [RFC6552], indicates the

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   Rank of the sender and is modified at each hop.  The SenderRank
   field is used to validate that the packet progresses in the expected
   direction, either upwards or downwards, along the DODAG.

   RPL defines the RPL Option for Carrying RPL Information in Data-Plane
   Datagrams [RFC6553] to transport the RPI, which is carried in an IPv6
   Hop-by-Hop Options Header [RFC2460], typically consuming eight bytes
   per packet.

   With [RFC6553], the RPL option is encoded as six Octets; it must be
   placed in a Hop-by-Hop header that consumes two additional octets for
   a total of eight.
TW> a couple of examples here might be beneficial, as I'm confused by the
8, 2 and 6 numbers
   In order to limit its range to the inside the RPL
   domain, the Hop-by-Hop header must be added to (or removed from)
   packets that cross the border of the RPL domain.

   The 8-byte overhead is detrimental to the LLN operation, in
   particular with regards to bandwidth and battery constraints.  These
   bytes may cause a containing frame to grow above maximum frame size,
   leading to Layer 2 or 6LoWPAN [RFC4944] fragmentation, which in turn
   causes even more energy spending and issues discussed in the LLN
   Fragment Forwarding and Recovery
   [I-D.thubert-6lo-forwarding-fragments].

   An additional overhead comes from the need, in certain cases, to add
   an IP-in-IP encapsulation to carry the Hop-by-Hop header.  This is
   needed when the router that inserts the Hop-by-Hop header is not the
   source of the packet, so that an error can be returned to the router.
   This is also the case when a packet originated by a RPL node must be
   stripped from the Hop-by-Hop header to be routed outside the RPL
   domain.

   This specification defines an IPinIP-6LoRH in Section 8 for that
   purpose, but it must be noted that stripping a 6LoRH does not require
   a manipulation of the packet in the LOWPAN_IPHC, and thus, if the
   source address in the LOWPAN_IPHC is the node that inserted the
   IPinIP-6LoRH then this alone does not mandate an IPinIP-6LoRH.

   As a result, a RPL packet may bear only an RPI-6LoRH and no IPinIP-
   6LoRH.  In that case, the source and destination of the packet are
   located in the LOWPAN_IPHC.
TW> examples examples!

   As with [RFC6553], the fields in the RPI include an 'O', an 'R', and
   an 'F' bit, an 8-bit RPLInstanceID (with some internal structure),
   and a 16-bit SenderRank.

   The remainder of this section defines the RPI-6LoRH, a Critical
   6LoWPAN Routing Header that is designed to transport the RPI in
   6LoWPAN LLNs.

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7.1.  Compressing the RPLInstanceID

   RPL Instances are discussed in [RFC6550], Section 5.  A number of
   simple use cases do not require more than one instance, and in such
   cases, the instance is expected to be the global Instance 0.
   A
   global RPLInstanceID is encoded in a RPLInstanceID field as follows:

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |0|     ID      |  Global RPLInstanceID in 0..127
      +-+-+-+-+-+-+-+-+

         Figure 7: RPLInstanceID Field Format for Global Instances.

   For the particular case of the global Instance 0, the RPLInstanceID
   field is all zeros.  This specification allows to elide a
   RPLInstanceID field that is all zeros, and defines a I flag that,
   when set, signals that the field is elided.

7.2.  Compressing the SenderRank

   The SenderRank is the result of the DAGRank operation on the rank of
   the sender; here the DAGRank operation is defined in [RFC6550],
   Section 3.5.1, as:

      DAGRank(rank) = floor(rank/MinHopRankIncrease)

   If MinHopRankIncrease is set to a multiple of 256, the least
   significant 8 bits of the SenderRank will be all zeroes; by eliding
   those, the SenderRank can be compressed into a single byte.  This
   idea is used in [RFC6550] by defining DEFAULT_MIN_HOP_RANK_INCREASE
   as 256 and in [RFC6552] that defaults MinHopRankIncrease to
   DEFAULT_MIN_HOP_RANK_INCREASE.

   This specification allows to encode the SenderRank as either one or
   two bytes, and defines a K flag that, when set, signals that a single
   byte is used.

7.3.  The Overall RPI-6LoRH encoding

   The RPI-6LoRH provides a compressed form for the RPL RPI.  Routers
   that need to forward a packet with a RPI-6LoRH are expected to be RPL
   routers and expected to support this specification.  If a non-RPL
   router receives a packet with a RPI-6LoRH, this means that there was
   a routing error and the packet should be dropped so the Type cannot
   be ignored.

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   Since the I flag is not set, the TSE field does not need to be a
   length expressed in bytes.  The field is fully reused for control
   bits so as to encode the O, R and F flags from the RPI, and the I and
   K flags that indicate the compression that is taking place.

   The Type for the RPI-6LoRH is 5.

   The RPI-6LoRH is immediately followed by the RPLInstanceID field,
   unless that field is fully elided, and then the SenderRank, which is
   either compressed into one byte or fully in-lined as the whole 2
   bytes.  The I and K flags in the RPI-6LoRH indicate whether the
   RPLInstanceID is elided and/or the SenderRank is compressed and
   depending on these bits, the Length of the RPI-6LoRH may vary as
   described hereafter.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ...  -+-+-+
      |1|0|0|O|R|F|I|K| 6LoRH Type=5  |   Compressed fields  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ...  -+-+-+

                  Figure 8: The Generic RPI-6LoRH Format.

   O, R, and F bits:
         The O, R, and F bits as defined in [RFC6550], Section 11.2.

   I bit:
         If it is set, the Instance ID is elided and the RPLInstanceID
         is the Global RPLInstanceID 0.  If it is not set, the octet
         immediately following the type field contains the RPLInstanceID
         as specified in [RFC6550] section 5.1.

   K bit:
         If it is set, the SenderRank is be compressed into one octet,
         and the lowest significant octet is elided.  If it is not set,
         the SenderRank, is fully inlined as 2 octets.

   In Figure 9, the RPLInstanceID is the Global RPLInstanceID 0, and the
   MinHopRankIncrease is a multiple of 256 so the least significant byte
   is all zeros and can be elided:

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       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0|0|O|R|F|1|1| 6LoRH Type=5  | SenderRank    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                I=1, K=1

                  Figure 9: The most compressed RPI-6LoRH.

   In Figure 10, the RPLInstanceID is the Global RPLInstanceID 0, but
   both bytes of the SenderRank are significant so it can not be
   compressed:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0|0|O|R|F|1|0| 6LoRH Type=5  |        SenderRank             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                I=1, K=0

                   Figure 10: Eliding the RPLInstanceID.

   In Figure 11, the RPLInstanceID is not the Global RPLInstanceID 0,
   and the MinHopRankIncrease is a multiple of 256:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0|0|O|R|F|0|1| 6LoRH Type=5  | RPLInstanceID |  SenderRank   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                I=0, K=1

                     Figure 11: Compressing SenderRank.

   In Figure 12, the RPLInstanceID is not the Global RPLInstanceID 0,
   and both bytes of the SenderRank are significant:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0|0|O|R|F|0|0| 6LoRH Type=5  | RPLInstanceID |    Sender-...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ...-Rank      |
      +-+-+-+-+-+-+-+-+
                I=0, K=0

               Figure 12: Least compressed form of RPI-6LoRH.

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TW> why not put this intro at the beginning of Section 8?
   A typical packet in RPL non-storing mode going down the RPL graph
   requires an IPinIP encapsulating the RH3, whereas the RPI is usually
   omitted, unless it is important to indicate the RPLInstanceID.  To
   match this structure, an optimized IPinIP 6LoRH is defined in
   Section 8.

TW> I don't understand what this table is...
   And the types include the setting of I and K as follows:

     +-----------+-------+-------+
     |   Type    |   I   |   K   |
     +-----------+-------+-------+
     |     5     |   0   |   0   |
     |     6     |   0   |   1   |
     |     7     |   1   |   0   |
     |     8     |   1   |   1   |
     +-----------+-------+-------+

                      Figure 13: The RPI-6LoRH Types.

8.  The IP-in-IP 6LoRH

   The IP-in-IP 6LoRH (IPinIP-6LoRH) is an Elective 6LoWPAN Routing
   Header that provides a compressed form for the encapsulating IPv6
   Header in the case of an IP-in-IP encapsulation.

   An IPinIP encapsulation is used to insert a field such as a Routing
   Header or an RPI at a router that is not the source of the packet.
   In order to send an error back regarding the inserted field, the
   address of the router that performs the insertion must be provided.

   The encapsulation can also enable a router down the path removing a
   field such as the RPI, but this can be done in the compressed form by
   removing the RPI-6LoRH, so an IPinIP-6LoRH encapsulation is not
   required for that sole purpose.
TW> I thought I understood IP-in-IP was needed as well for RPL upstream
traffic?

   This field is not critical for routing so the Type can be ignored,
   and the TSE field contains the Length in bytes.

     0                   1                   2
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-       ...      -+
    |1|0|1| Length  | 6LoRH Type 6  |  Hop Limit    | Encaps. Address  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-       ...      -+

                        Figure 14: The IPinIP-6LoRH.

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   The Length of an IPinIP-6LoRH is expressed in bytes and MUST be at
   least 1, to indicate a Hop Limit (HL), that is decremented at each
   hop.  When the HL reaches 0, the packet is dropped per [RFC2460]

   If the Length of an IPinIP-6LoRH is exactly 1, then the Encapsulator
   Address is elided, which means that the Encapsulator is a well-known
   router, for instance the root in a RPL graph.

   If the Length of an IPinIP-6LoRH is strictly more than 1, then an
   Encapsulator Address is placed in a compressed form after the Hop
   Limit field.  The value of the Length indicates which compression is
   performed on the Encapsulator Address.  For instance, a Size of 3
   indicates that the Encapsulator Address is compressed to 2 bytes.

   When it cannot be elided, the destination IP address of the IP-in-IP
   header is transported in a RH3-6LoRH as the first address of the
   list.

   With RPL, the destination address in the IP-in-IP header is
   implicitly the root in the RPL graph for packets going upwards, and
   the destination address in the IPHC for packets going downwards.  If
   the implicit value is correct, the destination IP address of the IP-
   in-IP encapsulation can be elided.

   If the final destination of the packet is a leaf that does not
   support this specification, then the chain of 6LoRH must be stripped
   by the RPL/6LR router to which the leaf is attached.  In that
   example, the destination IP address of the IP-in-IP header cannot be
   elided.

   In the special case where the 6LoRH is used to route 6LoWPAN
   fragments, the destination address is not accessible in the IPHC on
   all fragments and can be elided only for the first fragment and for
   packets going upwards.
TW> I understand the discussion above, but I don't see from the IPinIP-6LoRH
TW> format how you can carry both the Encapsulator address and the
destination address

9.  Security Considerations

   The security considerations of [RFC4944], [RFC6282], and [RFC6553]
   apply.

   Using a compressed format as opposed to the full in-line format is
   logically equivalent and does not create an opening for a new threat
   when compared to [RFC6550], [RFC6553] and [RFC6554].

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10.  IANA Considerations

   This document creates a IANA registry for the 6LoWPAN Routing Header
   Type, and assigns the following values:

      0..4 : RH3-6LoRH [RFCthis]

      5 : RPI-6LoRH [RFCthis]

      6 : IPinIP-6LoRH [RFCthis]

11.  Acknowledgments

TW> If you like my review, consider adding me to this list. It usual also
to order people alphabetically
   The authors wish to thank Martin Turon, James Woodyatt, Samita
   Chakrabarti, Jonathan Hui, Gabriel Montenegro and Ralph Droms for
   constructive reviews to the design in the 6lo Working Group.  The
   overall discussion involved participants to the 6MAN, 6TiSCH and ROLL
   WGs, thank you all.  Special thanks to the chairs of the ROLL WG,
   Michael Richardson and Ines Robles, and Brian Haberman, Internet Area
   A-D, and Adrian Farrel, Routing Area A-D, for driving this complex
   effort across Working Groups and Areas.

12.  References

12.1.  Normative References

   [IEEE802154]
              IEEE standard for Information Technology, "IEEE std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks", 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <http://www.rfc-editor.org/info/rfc4944>.

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   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <http://www.rfc-editor.org/info/rfc6282>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <http://www.rfc-editor.org/info/rfc6550>.

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 2012,
              <http://www.rfc-editor.org/info/rfc6552>.

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <http://www.rfc-editor.org/info/rfc6553>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <http://www.rfc-editor.org/info/rfc6554>.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <http://www.rfc-editor.org/info/rfc7102>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

12.2.  Informative References

   [I-D.bergmann-bier-ccast]
              Bergmann, O., Bormann, C., and S. Gerdes, "Constrained-
              Cast: Source-Routed Multicast for RPL", draft-bergmann-
              bier-ccast-00 (work in progress), November 2014.

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   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-08 (work
              in progress), May 2015.

   [I-D.ietf-6tisch-tsch]
              Watteyne, T., Palattella, M., and L. Grieco, "Using
              IEEE802.15.4e TSCH in an IoT context: Overview, Problem
              Statement and Goals", draft-ietf-6tisch-tsch-06 (work in
              progress), March 2015.

   [I-D.thubert-6lo-forwarding-fragments]
              Thubert, P. and J. Hui, "LLN Fragment Forwarding and
              Recovery", draft-thubert-6lo-forwarding-fragments-02 (work
              in progress), November 2014.

   [I-D.wijnands-bier-architecture]
              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-wijnands-bier-architecture-05 (work in
              progress), March 2015.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <http://www.rfc-editor.org/info/rfc6775>.

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis  06410
   FRANCE

   Phone: +33 4 97 23 26 34
   Email: [email-masked]

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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: [email-masked]

   Laurent Toutain
   Institut MINES TELECOM; TELECOM Bretagne
   2 rue de la Chataigneraie
   CS 17607
   Cesson-Sevigne Cedex  35576
   France

   Email: [email-masked]

   Robert Cragie
   ARM Ltd.
   110 Fulbourn Road
   Cambridge  CB1 9NJ
   UK

   Email: [email-masked]

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