6LoWPAN-GHC: Generic Header Compression for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)

Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 7400                       Universitaet Bremen TZI
Category: Standards Track                                  November 2014
ISSN: 2070-1721


            6LoWPAN-GHC: Generic Header Compression for IPv6
       over Low-Power Wireless Personal Area Networks (6LoWPANs)

Abstract

   RFC 6282 defines header compression in 6LoWPAN packets (where
   "6LoWPAN" refers to "IPv6 over Low-Power Wireless Personal Area
   Network").  The present document specifies a simple addition that
   enables the compression of generic headers and header-like payloads,
   without a need to define a new header compression scheme for each
   such new header or header-like payload.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7400.

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
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. The Header Compression Coupling Problem ....................2
      1.2. Compression Approach .......................................3
      1.3. Terminology ................................................3
      1.4. Notation ...................................................4
   2. 6LoWPAN-GHC .....................................................4
   3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC .........................6
      3.1. Compressing Payloads (UDP and ICMPv6) ......................6
      3.2. Compressing Extension Headers ..............................6
      3.3. Indicating GHC Capability ..................................7
      3.4. Using the 6CIO .............................................8
   4. IANA Considerations .............................................9
   5. Security Considerations ........................................10
   6. References .....................................................11
      6.1. Normative References ......................................11
      6.2. Informative References ....................................12
   Appendix A. Examples ..............................................14
   Acknowledgements ..................................................24
   Author's Address ..................................................24

1.  Introduction

1.1.  The Header Compression Coupling Problem

   [RFC6282] defines a scheme for header compression in 6LoWPAN
   [RFC4944] packets; in this document, we refer to that scheme as
   6LoWPAN Header Compression, or 6LoWPAN-HC (where "6LoWPAN" refers to
   "IPv6 over Low-Power Wireless Personal Area Network").  As with most
   header compression schemes, a new specification is necessary for
   every new kind of header that needs to be compressed.  In addition,
   [RFC6282] does not define an extensibility scheme like the Robust
   Header Compression (ROHC) profiles defined in ROHC [RFC3095]
   [RFC5795].  This leads to the difficult situation in which 6LoWPAN-HC
   tended to be reopened and reexamined each time a new header receives
   consideration (or an old header is changed and reconsidered) in the
   6LoWPAN/roll/CoRE cluster of IETF working groups.  Although [RFC6282]
   was finally completed and published, the underlying problem remains
   unsolved.

   The purpose of the present contribution is to plug into [RFC6282] as
   is, using its Next Header Compression (NHC) concept.  We add a
   slightly less efficient, but vastly more general, form of compression
   for headers of any kind and even for header-like payloads such as
   those exhibited by routing protocols, DHCP, etc.: Generic Header
   Compression (GHC).  The objective is an extremely simple
   specification that can be defined on a single page and implemented in
   a small number of lines of code, as opposed to a general data
   compression scheme such as that defined in [RFC1951].

1.2.  Compression Approach

   The basic approach of GHC's compression function is to define a
   bytecode for LZ77-style compression [LZ77].  The bytecode is a series
   of simple instructions for the decompressor to reconstitute the
   uncompressed payload.  These instructions include:

   o  appending bytes to the reconstituted payload that are literally
      given with the instruction in the compressed data

   o  appending a given number of zero bytes to the reconstituted
      payload

   o  appending bytes to the reconstituted payload by copying a
      contiguous sequence from the payload being reconstituted
      ("backreferencing")

   o  an ancillary instruction for setting up parameters for the
      backreferencing instruction in "decompression variables"

   o  a stop code (optional; see Section 3.2)

   The buffer for the reconstituted payload ("destination buffer") is
   prefixed by a predefined dictionary that can be used in the
   backreferencing as if it were a prefix of the payload.  This
   predefined dictionary is built from the IPv6 addresses of the packet
   being reconstituted, followed by a static component, the "static
   dictionary".

   As usual, this specification defines the decompressor operation in
   detail but leaves the detailed operation of the compressor open to
   implementation.  The compressor can be implemented as with a
   classical LZ77 compressor, or it can be a simple protocol encoder
   that just makes use of known compression opportunities.

1.3.  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
   RFC 2119 [RFC2119].

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

   Terms from [RFC7228] are used in Section 5.

1.4.  Notation

   This specification uses a trivial notation for code bytes and the
   bitfields in them, the meaning of which should be mostly obvious.
   More formally, the meaning of the notation is as follows:

   Potential values for the code bytes themselves are expressed by
   templates that represent 8-bit most-significant-bit-first binary
   numbers (without any special prefix), where 0 stands for 0, 1 for 1,
   and variable segments in these code byte templates are indicated by
   sequences of the same letter, such as kkkkkkk or ssss, the length of
   which indicates the length of the variable segment in bits.

   In the notation of values derived from the code bytes, 0b is used as
   a prefix for expressing binary numbers in most-significant-bit-first
   notation (akin to the use of 0x for most-significant-digit-first
   hexadecimal numbers in the C programming language).  Where the above-
   mentioned sequences of letters are then referenced in such a binary
   number in the text, the intention is that the value from these
   bitfields in the actual code byte be inserted.

   Example: The code byte template

      101nssss

   stands for a byte that starts (most-significant-bit-first) with the
   bits 1, 0, and 1, and continues with five variable bits, the first of
   which is referenced as "n" and the next four of which are referenced
   as "ssss".  Based on this code byte template, a reference to

      0b0ssss000

   means a binary number composed from a zero bit; the four bits that
   are in the "ssss" field (for 101nssss, the four least significant
   bits) in the actual byte encountered, kept in the same order; and
   three more zero bits.

2.  6LoWPAN-GHC

   The format of a GHC-compressed header or payload is a simple
   bytecode.  A compressed header consists of a sequence of pieces, each
   of which begins with a code byte, which may be followed by zero or
   more bytes as its argument.  Some code bytes cause bytes to be laid
   out in the destination buffer, and some simply modify some
   decompression variables.

   At the start of decompressing a header or payload within an L2 packet
   (= fragment), the decompression variables "sa" and "na" are
   initialized as zero.

   The code bytes are defined as follows (Table 1):

   +----------+---------------------------------------------+----------+
   | code     | Action                                      | Argument |
   | byte     |                                             |          |
   +----------+---------------------------------------------+----------+
   | 0kkkkkkk | Append k = 0b0kkkkkkk bytes of data in the  | k bytes  |
   |          | bytecode argument (k < 96)                  | of data  |
   |          |                                             |          |
   | 1000nnnn | Append 0b0000nnnn+2 bytes of zeroes         |          |
   |          |                                             |          |
   | 10010000 | stop code (end of compressed data; see      |          |
   |          | Section 3.2)                                |          |
   |          |                                             |          |
   | 101nssss | Set up extended arguments for a             |          |
   |          | backreference: sa += 0b0ssss000,            |          |
   |          | na += 0b0000n000                            |          |
   |          |                                             |          |
   | 11nnnkkk | Backreference: n = na+0b00000nnn+2;         |          |
   |          | s = 0b00000kkk+sa+n; append n bytes from    |          |
   |          | previously output bytes, starting s bytes   |          |
   |          | to the left of the current output pointer;  |          |
   |          | set sa = 0, na = 0                          |          |
   +----------+---------------------------------------------+----------+

             Table 1: Bytecodes for Generic Header Compression

   Note that the following bit combinations are reserved at this time:

   o  011xxxxx

   o  1001nnnn (where 0b0000nnnn > 0)

   For the purposes of the backreferences, the expansion buffer is
   initialized with a predefined dictionary, at the end of which the
   reconstituted payload begins.  This dictionary is composed of the
   source and destination IPv6 addresses of the packet being
   reconstituted, followed by a 16-byte static dictionary (Figure 1).
   These 48 dictionary bytes are therefore available for backreferencing
   but not copied into the final reconstituted payload.

             16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00

           Figure 1: The 16 Bytes of Static Dictionary (in Hex)

3.  Integrating 6LoWPAN-GHC into 6LoWPAN-HC

   6LoWPAN-GHC plugs in as an NHC format for 6LoWPAN-HC [RFC6282].

3.1.  Compressing Payloads (UDP and ICMPv6)

   By definition, GHC is generic and can be applied to different kinds
   of packets.  Many of the examples given in Appendix A are for ICMPv6
   packets; a single NHC value suffices to define an NHC format for
   ICMPv6 based on GHC (see below).

   In addition, it is useful to include an NHC format for UDP, as many
   header-like payloads (e.g., DHCPv6, Datagram Transport Layer Security
   (DTLS)) are carried in UDP.  [RFC6282] already defines an NHC format
   for UDP (11110CPP).  GHC uses an analogous NHC byte formatted as
   shown in Figure 2.  The difference to the existing UDP NHC
   specification is that for 11010CPP NHC bytes, the UDP payload is not
   supplied literally but compressed by 6LoWPAN-GHC.

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     | 1 | 1 | 0 | 1 | 0 | C |   P   |
                     +---+---+---+---+---+---+---+---+

                 Figure 2: NHC Byte for UDP GHC (11010CPP)

   To stay in the same general numbering space, we use 11011111 as the
   NHC byte for ICMPv6 GHC (Figure 3).

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

               Figure 3: NHC Byte for ICMPv6 GHC (11011111)

3.2.  Compressing Extension Headers

   Compression of specific extension headers is added in a similar way
   (Figure 4) (however, probably only Extension Header ID (EID) 0 to 3
   [RFC6282] need to be assigned).  As there is no easy way to extract
   the Length field from the GHC-encoded header before decoding, this
   would make detecting the end of the extension header somewhat
   complex.  The easiest (and most efficient) approach is to completely
   elide the Length field (in the same way NHC already elides the Next
   Header field in certain cases) and reconstruct it only on
   decompression.  To serve as a terminator for the extension header,
   the bytecode 0b10010000 has been assigned as a stop code.  Note that
   the stop code is only needed for extension headers, not for the final
   payloads discussed in the previous subsection, the decompression of
   which is automatically stopped by the end of the packet.

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     | 1 | 0 | 1 | 1 |    EID    |NH |
                     +---+---+---+---+---+---+---+---+

                Figure 4: NHC Byte for Extension Header GHC

3.3.  Indicating GHC Capability

   The 6LoWPAN baseline includes just [RFC4944], [RFC6282], and
   [RFC6775] (see [Roadmap-6LoWPAN]).  To enable the use of GHC towards
   a neighbor, a 6LoWPAN node needs to know that the neighbor implements
   it.  While this can also simply be administratively required, a
   transition strategy as well as a way to support mixed networks is
   required.

   One way to know that a neighbor does implement GHC is receiving a
   packet from that neighbor with GHC in it ("implicit capability
   detection").  However, there needs to be a way to bootstrap this, as
   nobody would ever start sending packets with GHC otherwise.

   To minimize the impact on [RFC6775], we define a Neighbor Discovery
   (ND) option called the 6LoWPAN Capability Indication Option (6CIO),
   as illustrated in Figure 5.  (For the fields marked by an underscore
   in Figure 5, see Section 3.4.)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |   Length = 1  |_____________________________|G|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |_______________________________________________________________|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 5: 6LoWPAN Capability Indication Option (6CIO)

   The G bit indicates whether the node sending the option is GHC
   capable.

   Once a node receives either an explicit or implicit indication of GHC
   capability from another node, it may send GHC-compressed packets to
   that node.  Where that capability has not been recently confirmed,
   similar to the way Packetization Layer Path MTU Discovery (PLPMTUD)
   [RFC4821] finds out about changes in the network, a node SHOULD make
   use of Neighbor Unreachability Detection (NUD) failures to switch
   back to basic 6LoWPAN header compression [RFC6282].

3.4.  Using the 6CIO

   The 6CIO will typically only be sent in 6LoWPAN-ND Router
   Solicitation (RS) packets (which cannot themselves be GHC compressed
   unless the host desires to limit itself to talking to GHC-capable
   routers).  The resulting 6LoWPAN-ND Router Advertisement (RA) can
   then already make use of GHC and thus indicate GHC capability
   implicitly, which in turn allows both nodes to use GHC in the
   6LoWPAN-ND Neighbor Solicitation / Neighbor Advertisement (NS/NA)
   exchange.

   The 6CIO can also be used for future options that need to be
   negotiated between 6LoWPAN peers; an IANA registry is used to assign
   the flags.  Bits marked by underscores in Figure 5 are unassigned and
   available for future assignment.  They MUST be sent as zero and MUST
   be ignored on reception until assigned by IANA.  Length values larger
   than 1 MUST be accepted by implementations in order to enable future
   extensions; the additional bits in the option are then deemed
   unassigned in the same way.  For the purposes of the IANA registry,
   the bits are numbered in most-significant-bit-first order from the
   16th bit of the option onward: the 16th bit is flag number 0, the
   31st bit (the G bit) is flag number 15, up to the 63rd bit for flag
   number 47.  (Additional bits may also be used by a follow-on version
   of this document if some bit combinations that have been left
   unassigned here are then used in an upward-compatible manner.)

   Flag numbers 0 to 7 are reserved for experimental use.  They MUST NOT
   be used for actual deployments.

   Where the use of this option by other specifications or for
   experimental use is envisioned, the following items have to be kept
   in mind:

   o  The option can be used in any ND packet.

   o  Specific bits are set in the option to indicate that a capability
      is present in the sender.  (There may be other ways to infer this
      information, as is the case in this specification.)  Bit
      combinations may be used as desired.  The absence of the
      capability _indication_ is signaled by setting these bits to zero;
      this does not necessarily mean that the capability is absent.

   o  The intention is not to modify the semantics of the specific ND
      packet carrying the option but to provide the general capability
      indication described above.

   o  Specifications have to be designed such that receivers that do not
      receive or do not process such a capability indication can still
      interoperate (presumably without exploiting the indicated
      capability).

   o  The option is meant to be used sparsely, i.e., once a sender has
      reason to believe the capability indication has been received,
      there is no longer a need to continue sending it.

4.  IANA Considerations

   IANA has added the assignments listed in Figure 6 in the "LOWPAN_NHC
   Header Type" registry (under "IPv6 Low Power Personal Area Network
   Parameters").

           10110EEN: Extension header GHC              [RFC7400]
           11010CPP: UDP GHC                           [RFC7400]
           11011111: ICMPv6 GHC                        [RFC7400]

                Figure 6: IANA Assignments for the NHC Byte

   IANA has allocated ND option number 36 for the "6LoWPAN Capability
   Indication Option (6CIO)" ND option format in the "IPv6 Neighbor
   Discovery Option Formats" registry [RFC4861].

   IANA has created a subregistry for "6LoWPAN capability Bits" under
   the "Internet Control Message Protocol version 6 (ICMPv6) Parameters"
   registry.  The bits are assigned by giving their numbers as small,
   non-negative integers as defined in Section 3.4, in the range 0-47.
   The policy is "IETF Review" or "IESG Approval" [RFC5226].  The
   initial content of the registry is as shown in Figure 7:

            0-7: Reserved for Experimental Use         [RFC7400]
           8-14: Unassigned
             15: GHC capable bit (G bit)               [RFC7400]
          16-47: Unassigned

        Figure 7: IANA Assignments for the 6LoWPAN Capability Bits

5.  Security Considerations

   The security considerations of [RFC4944] and [RFC6282] apply.  As
   usual in protocols with packet parsing/construction, care must be
   taken in implementations to avoid buffer overflows and, in particular
   (with respect to the backreferencing), out-of-area references during
   decompression.

   One additional consideration is that an attacker may send a forged
   packet that makes a second node believe a third victim node is GHC
   capable.  If it is not, this may prevent packets sent by the second
   node from reaching the third node (at least until robustness features
   such as those discussed in Section 3.3 kick in).

   No mitigation is proposed (or known) for this attack, except that a
   victim node that does implement GHC is not vulnerable.  However, with
   unsecured ND, a number of attacks with similar outcomes are already
   possible, so there is little incentive to make use of this additional
   attack.  With secured ND, the 6CIO is also secured; nodes relying on
   secured ND therefore should use the 6CIO bidirectionally (and limit
   the implicit capability detection to secured ND packets carrying GHC)
   instead of basing their neighbor capability assumptions on receiving
   any kind of unprotected packet.

   As with any LZ77 scheme, decompression bombs (compressed packets
   crafted to expand so much that the decompressor is overloaded) are a
   problem.  An attacker cannot send a GHC decompressor into a tight
   loop for too long, because the MTU will be reached quickly.  Some
   amplification of an attack from inside the compressed link is
   possible, though.  Using a constrained node in a constrained network
   as a DoS attack source is usually not very useful, though, except
   maybe against other nodes in that constrained network.  The worst
   case for an attack to the outside is a not-so-constrained device
   using a (typically not-so-constrained) edge router to attack by
   forwarding out of its Ethernet interface.  The worst-case
   amplification of GHC is 17, so an MTU-size packet can be generated
   from a 6LoWPAN packet of 76 bytes.  The 6LoWPAN network is still
   constrained, so the amplification at the edge router turns an entire
   250 kbit/s 802.15.4 network (assuming a theoretical upper bound of
   225 kbit/s throughput to a single-hop adjacent node) into a
   3.8 Mbit/s attacker.

   The amplification may be more important inside the 6LoWPAN, if there
   is a way to obtain reflection (otherwise, the packet is likely to
   simply stay compressed on the way and do little damage), e.g., by
   pinging a node using a decompression bomb, somehow keeping that node
   from re-compressing the ping response (which would probably require
   something more complex than simple runs of zeroes, so the worst-case
   amplification is likely closer to 9).  Or, if there are nodes that do
   not support GHC, those can be attacked via a router that is then
   forced to decompress.

   All these attacks are mitigated by some form of network access
   control.

   In a 6LoWPAN stack, sensitive information will normally be protected
   by transport- or application-layer (or even IP-layer) security, which
   are all above the adaptation layer, leaving no sensitive information
   to compress at the GHC level.  However, a 6LoWPAN deployment that
   entirely depends on Media Access Control (MAC) layer security may be
   vulnerable to attacks that exploit redundancy information disclosed
   by compression to recover information about secret values.  The
   attacker would need to be in radio range to observe the compressed
   packets.  Since compression is stateless, the attacker would need to
   entice the party sending the secret value to also send some value
   controlled (or at least usefully varying and knowable) by the
   attacker in (what becomes the first adaptation-layer fragment of) the
   same packet.  This attack is fully mitigated by not exposing secret
   values to the adaptation layer or by not using GHC in deployments
   where this is done.

6.  References

6.1.  Normative References

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007, <http://www.rfc-editor.org/info/rfc4861>.

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

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

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

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

6.2.  Informative References

   [ICMPv6-ND]
              O'Flynn, C., "ICMPv6/ND Compression for 6LoWPAN Networks",
              Work in Progress, draft-oflynn-6lowpan-icmphc-00,
              July 2010.

   [LZ77]     Ziv, J. and A. Lempel, "A Universal Algorithm for
              Sequential Data Compression", IEEE Transactions on
              Information Theory, Vol. 23, No. 3, pp. 337-343, May 1977.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996,
              <http://www.rfc-editor.org/info/rfc1951>.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, July 2001,
              <http://www.rfc-editor.org/info/rfc3095>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              March 2010, <http://www.rfc-editor.org/info/rfc5795>.

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012,
              <http://www.rfc-editor.org/info/rfc6550>.

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

   [Roadmap-6LoWPAN]
              Bormann, C., "6LoWPAN Roadmap and Implementation Guide",
              Work in Progress, draft-bormann-6lo-6lowpan-roadmap-00,
              October 2013.

Appendix A.  Examples

   This section demonstrates some relatively realistic examples derived
   from actual packet captures taken at previous interops.

   For the Routing Protocol for Low-Power and Lossy Networks (RPL)
   [RFC6550], Figure 8 shows a Destination-Oriented Directed Acyclic
   Graph (DODAG) Information Solicitation (DIS), a quite short RPL
   message that obviously cannot be improved much.

   IP header:
    60 00 00 00 00 08 3a ff fe 80 00 00 00 00 00 00
    02 1c da ff fe 00 20 24 ff 02 00 00 00 00 00 00
    00 00 00 00 00 00 00 1a
   Payload:
    9b 00 6b de 00 00 00 00
   Dictionary:
    fe 80 00 00 00 00 00 00 02 1c da ff fe 00 20 24
    ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 04 9b 00 6b de
   4 nulls: 82
   Compressed:
    04 9b 00 6b de 82
   Was 8 bytes; compressed to 6 bytes, compression factor 1.33

                      Figure 8: A Simple RPL Example

   Figure 9 shows a RPL DODAG Information Object, a longer RPL control
   message that is improved a bit more.  Note that the compressed output
   exposes an inefficiency in the simple-minded compressor used to
   generate it; this does not devalue the example, since constrained
   nodes are quite likely to make use of simple-minded compressors.

   IP header:
    60 00 00 00 00 5c 3a ff fe 80 00 00 00 00 00 00
    02 1c da ff fe 00 30 23 ff 02 00 00 00 00 00 00
    00 00 00 00 00 00 00 1a
   Payload:
    9b 01 7a 5f 00 f0 01 00 88 00 00 00 20 02 0d b8
    00 00 00 00 00 00 00 ff fe 00 fa ce 04 0e 00 14
    09 ff 00 00 01 00 00 00 00 00 00 00 08 1e 80 20
    ff ff ff ff ff ff ff ff 00 00 00 00 20 02 0d b8
    00 00 00 00 00 00 00 ff fe 00 fa ce 03 0e 40 00
    ff ff ff ff 20 02 0d b8 00 00 00 00
   Dictionary:
    fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
    ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 06 9b 01 7a 5f 00 f0
   ref(9): 01 00 -> ref 11nnnkkk 0 7: c7
   copy: 01 88
   3 nulls: 81
   copy: 04 20 02 0d b8
   7 nulls: 85
   ref(60): ff fe 00 -> ref 101nssss 0 7/11nnnkkk 1 1: a7 c9
   copy: 08 fa ce 04 0e 00 14 09 ff
   ref(39): 00 00 01 00 00 -> ref 101nssss 0 4/11nnnkkk 3 2: a4 da
   5 nulls: 83
   copy: 06 08 1e 80 20 ff ff
   ref(2): ff ff -> ref 11nnnkkk 0 0: c0
   ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0
   4 nulls: 82
   ref(48): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 fa ce
    -> ref 101nssss 1 4/11nnnkkk 6 0: b4 f0
   copy: 03 03 0e 40
   ref(9): 00 ff -> ref 11nnnkkk 0 7: c7
   ref(28): ff ff ff -> ref 101nssss 0 3/11nnnkkk 1 1: a3 c9
   ref(24): 20 02 0d b8 00 00 00 00
    -> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0
   Compressed:
    06 9b 01 7a 5f 00 f0 c7 01 88 81 04 20 02 0d b8
    85 a7 c9 08 fa ce 04 0e 00 14 09 ff a4 da 83 06
    08 1e 80 20 ff ff c0 d0 82 b4 f0 03 03 0e 40 c7
    a3 c9 a2 f0
   Was 92 bytes; compressed to 52 bytes, compression factor 1.77

                      Figure 9: A Longer RPL Example

   Similarly, Figure 10 shows a RPL Destination Advertisement Object
   (DAO) message.  One of the embedded addresses is copied right out of
   the pseudo-header; the other one is effectively converted from global
   to local by providing the prefix FE80 literally, inserting a number
   of nulls, and copying (some of) the Interface Identifier part again
   out of the pseudo-header.  Note that a simple implementation would
   probably emit fewer nulls and copy the entire Interface Identifier;
   there are multiple ways to encode this 50-byte payload into 27 bytes.

   IP header:
    60 00 00 00 00 32 3a ff 20 02 0d b8 00 00 00 00
    00 00 00 ff fe 00 33 44 20 02 0d b8 00 00 00 00
    00 00 00 ff fe 00 11 22
   Payload:
    9b 02 58 7d 01 80 00 f1 05 12 00 80 20 02 0d b8
    00 00 00 00 00 00 00 ff fe 00 33 44 06 14 00 80
    f1 00 fe 80 00 00 00 00 00 00 00 00 00 ff fe 00
    11 22
   Dictionary:
    20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44
    20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80
   ref(60): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44
    -> ref 101nssss 1 5/11nnnkkk 6 4: b5 f4
   copy: 08 06 14 00 80 f1 00 fe 80
   9 nulls: 87
   ref(66): ff fe 00 11 22 -> ref 101nssss 0 7/11nnnkkk 3 5: a7 dd
   Compressed:
    0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 b5 f4 08
    06 14 00 80 f1 00 fe 80 87 a7 dd
   Was 50 bytes; compressed to 27 bytes, compression factor 1.85

                       Figure 10: A RPL DAO Message

   Figure 11 shows the effect of compressing a simple ND neighbor
   solicitation.

   IP header:
    60 00 00 00 00 30 3a ff 20 02 0d b8 00 00 00 00
    00 00 00 ff fe 00 3b d3 fe 80 00 00 00 00 00 00
    02 1c da ff fe 00 30 23
   Payload:
    87 00 a7 68 00 00 00 00 fe 80 00 00 00 00 00 00
    02 1c da ff fe 00 30 23 01 01 3b d3 00 00 00 00
    1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24
   Dictionary:
    20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3
    fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 04 87 00 a7 68
   4 nulls: 82
   ref(40): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
    -> ref 101nssss 1 3/11nnnkkk 6 0: b3 f0
   copy: 04 01 01 3b d3
   4 nulls: 82
   copy: 02 1f 02
   5 nulls: 83
   copy: 02 06 00
   ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db
   copy: 02 20 24
   Compressed:
    04 87 00 a7 68 82 b3 f0 04 01 01 3b d3 82 02 1f
    02 83 02 06 00 a2 db 02 20 24
   Was 48 bytes; compressed to 26 bytes, compression factor 1.85

                  Figure 11: An ND Neighbor Solicitation

   Figure 12 shows the compression of an ND neighbor advertisement.

   IP header:
    60 00 00 00 00 30 3a fe fe 80 00 00 00 00 00 00
    02 1c da ff fe 00 30 23 20 02 0d b8 00 00 00 00
    00 00 00 ff fe 00 3b d3
   Payload:
    88 00 26 6c c0 00 00 00 fe 80 00 00 00 00 00 00
    02 1c da ff fe 00 30 23 02 01 fa ce 00 00 00 00
    1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24
   Dictionary:
    fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
    20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 05 88 00 26 6c c0
   3 nulls: 81
   ref(56): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
    -> ref 101nssss 1 5/11nnnkkk 6 0: b5 f0
   copy: 04 02 01 fa ce
   4 nulls: 82
   copy: 02 1f 02
   5 nulls: 83
   copy: 02 06 00
   ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db
   copy: 02 20 24
   Compressed:
    05 88 00 26 6c c0 81 b5 f0 04 02 01 fa ce 82 02
    1f 02 83 02 06 00 a2 db 02 20 24
   Was 48 bytes; compressed to 27 bytes, compression factor 1.78

                  Figure 12: An ND Neighbor Advertisement

   Figure 13 shows the compression of an ND router solicitation.  Note
   that the relatively good compression is not caused by the many zero
   bytes in the link-layer address of this particular capture (which are
   unlikely to occur in practice): 7 of these 8 bytes are copied from
   the pseudo-header (the 8th byte cannot be copied, as the universal/
   local bit needs to be inverted).

   IP header:
    60 00 00 00 00 18 3a ff fe 80 00 00 00 00 00 00
    ae de 48 00 00 00 00 01 ff 02 00 00 00 00 00 00
    00 00 00 00 00 00 00 02
   Payload:
    85 00 90 65 00 00 00 00 01 02 ac de 48 00 00 00
    00 01 00 00 00 00 00 00
   Dictionary:
    fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01
    ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 02
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 04 85 00 90 65
   ref(11): 00 00 00 00 01 -> ref 11nnnkkk 3 6: de
   copy: 02 02 ac
   ref(50): de 48 00 00 00 00 01
    -> ref 101nssss 0 5/11nnnkkk 5 3: a5 eb
   6 nulls: 84
   Compressed:
    04 85 00 90 65 de 02 02 ac a5 eb 84
   Was 24 bytes; compressed to 12 bytes, compression factor 2.00

                   Figure 13: An ND Router Solicitation

   Figure 14 shows the compression of an ND router advertisement.  The
   indefinite lifetime is compressed to four bytes by backreferencing;
   this could be improved (at the cost of minor additional decompressor
   complexity) by including some simple runlength mechanism.

   IP header:
    60 00 00 00 00 60 3a ff fe 80 00 00 00 00 00 00
    10 34 00 ff fe 00 11 22 fe 80 00 00 00 00 00 00
    ae de 48 00 00 00 00 01
   Payload:
    86 00 55 c9 40 00 0f a0 1c 5a 38 17 00 00 07 d0
    01 01 11 22 00 00 00 00 03 04 40 40 ff ff ff ff
    ff ff ff ff 00 00 00 00 20 02 0d b8 00 00 00 00
    00 00 00 00 00 00 00 00 20 02 40 10 00 00 03 e8
    20 02 0d b8 00 00 00 00 21 03 00 01 00 00 00 00
    20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22
   Dictionary:
    fe 80 00 00 00 00 00 00 10 34 00 ff fe 00 11 22
    fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   copy: 0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17
   2 nulls: 80
   copy: 06 07 d0 01 01 11 22
   4 nulls: 82
   copy: 06 03 04 40 40 ff ff
   ref(2): ff ff -> ref 11nnnkkk 0 0: c0
   ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0
   4 nulls: 82
   copy: 04 20 02 0d b8
   12 nulls: 8a
   copy: 04 20 02 40 10
   ref(38): 00 00 03 -> ref 101nssss 0 4/11nnnkkk 1 3: a4 cb
   copy: 01 e8
   ref(24): 20 02 0d b8 00 00 00 00
    -> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0
   copy: 02 21 03
   ref(84): 00 01 00 00 00 00
    -> ref 101nssss 0 9/11nnnkkk 4 6: a9 e6
   ref(40): 20 02 0d b8 00 00 00 00 00 00 00
    -> ref 101nssss 1 3/11nnnkkk 1 5: b3 cd
   ref(128): ff fe 00 11 22
    -> ref 101nssss 0 15/11nnnkkk 3 3: af db
   Compressed:
    0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17 80 06 07
    d0 01 01 11 22 82 06 03 04 40 40 ff ff c0 d0 82
    04 20 02 0d b8 8a 04 20 02 40 10 a4 cb 01 e8 a2
    f0 02 21 03 a9 e6 b3 cd af db
   Was 96 bytes; compressed to 58 bytes, compression factor 1.66

                   Figure 14: An ND Router Advertisement

   Figure 15 shows the compression of a DTLS application data packet
   with a net payload of 13 bytes of cleartext and 8 bytes of
   authenticator (note that the IP header is not relevant for this
   example and has been set to 0).  This makes good use of the static
   dictionary and is quite effective crunching out the redundancy in the
   TLS_PSK_WITH_AES_128_CCM_8 header, leading to a net reduction by 15
   bytes.

   IP header:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00
   Payload:
    17 fe fd 00 01 00 00 00 00 00 01 00 1d 00 01 00
    00 00 00 00 01 09 b2 0e 82 c1 6e b6 96 c5 1f 36
    8d 17 61 e2 b5 d4 22 d4 ed 2b
   Dictionary:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   ref(13): 17 fe fd 00 01 00 00 00 00 00 01 00
    -> ref 101nssss 1 0/11nnnkkk 2 1: b0 d1
   copy: 01 1d
   ref(10): 00 01 00 00 00 00 00 01 -> ref 11nnnkkk 6 2: f2
   copy: 15 09 b2 0e 82 c1 6e b6 96 c5 1f 36 8d 17 61 e2
   copy: b5 d4 22 d4 ed 2b
   Compressed:
    b0 d1 01 1d f2 15 09 b2 0e 82 c1 6e b6 96 c5 1f
    36 8d 17 61 e2 b5 d4 22 d4 ed 2b
   Was 42 bytes; compressed to 27 bytes, compression factor 1.56

                 Figure 15: A DTLS Application Data Packet

   Figure 16 shows that the compression is slightly worse in a
   subsequent packet (containing 6 bytes of cleartext and 8 bytes of
   authenticator, yielding a net compression of 13 bytes).  The total
   overhead does stay at a quite acceptable 8 bytes.

   IP header:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00
   Payload:
    17 fe fd 00 01 00 00 00 00 00 05 00 16 00 01 00
    00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4
    cb 35 b9
   Dictionary:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   ref(13): 17 fe fd 00 01 00 00 00 00 00
    -> ref 101nssss 1 0/11nnnkkk 0 3: b0 c3
   copy: 03 05 00 16
   ref(10): 00 01 00 00 00 00 00 05 -> ref 11nnnkkk 6 2: f2
   copy: 0e ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9
   Compressed:
    b0 c3 03 05 00 16 f2 0e ae a0 15 56 67 92 4d ff
    8a 24 e4 cb 35 b9
   Was 35 bytes; compressed to 22 bytes, compression factor 1.59

              Figure 16: Another DTLS Application Data Packet

   Figure 17 shows the compression of a DTLS handshake message, here a
   client hello.  There is little that can be compressed about the 32
   bytes of randomness.  Still, the net reduction is by 14 bytes.

   IP header:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00
   Payload:
    16 fe fd 00 00 00 00 00 00 00 00 00 36 01 00 00
    2a 00 00 00 00 00 00 00 2a fe fd 51 52 ed 79 a4
    20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe c6 89 2f
    32 26 9a 16 4e 31 7e 9f 20 92 92 00 00 00 02 c0
    a8 01 00
   Dictionary:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00
   ref(16): 16 fe fd -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd
   9 nulls: 87
   copy: 01 36
   ref(16): 01 00 00 -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd
   copy: 01 2a
   7 nulls: 85
   copy: 23 2a fe fd 51 52 ed 79 a4 20 c9 62 56 11 47 c9
   copy: 39 ee 6c c0 a4 fe c6 89 2f 32 26 9a 16 4e 31 7e
   copy: 9f 20 92 92
   3 nulls: 81
   copy: 05 02 c0 a8 01 00
   Compressed:
    a1 cd 87 01 36 a1 cd 01 2a 85 23 2a fe fd 51 52
    ed 79 a4 20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe
    c6 89 2f 32 26 9a 16 4e 31 7e 9f 20 92 92 81 05
    02 c0 a8 01 00
   Was 67 bytes; compressed to 53 bytes, compression factor 1.26

             Figure 17: A DTLS Handshake Packet (Client Hello)

Acknowledgements

   Colin O'Flynn has repeatedly insisted that some form of compression
   for ICMPv6 and ND packets might be beneficial.  He actually wrote his
   own document, [ICMPv6-ND], which compresses better, but that document
   only addresses basic ICMPv6/ND and needs a much longer specification
   (around 17 pages of detailed specification, as compared to the single
   page of core specification here).  This motivated the author to try
   something simple, yet general.  Special thanks go to Colin for
   indicating that he indeed considers his document superseded by
   this one.

   The examples given are based on packet capture files that Colin
   O'Flynn, Owen Kirby, Olaf Bergmann, and others provided.

   Using these files as a corpus, the static dictionary was developed,
   and the bit allocations validated, based on research by Sebastian
   Dominik.

   Erik Nordmark provided input that helped shape the 6CIO.  Thomas
   Bjorklund proposed simplifying the predefined dictionary.

   Yoshihiro Ohba insisted on clarifying the notation used for the
   definition of the bytecodes and their bitfields.  Ulrich Herberg
   provided some additional review and suggested expanding the
   introductory material, and with Hannes Tschofenig and Brian Haberman
   he helped come up with the IANA policy for the "6LoWPAN capability
   bits" assignments in the 6CIO.

   The IESG reviewers Richard Barnes and Stephen Farrell contributed
   topics to the Security Considerations section; they and Barry Leiba,
   as well as GEN-ART reviewer Vijay K. Gurbani, also provided editorial
   improvements.

Author's Address

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

   Phone: +49-421-218-63921
   EMail: cabo@tzi.org