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6LoWPAN Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                        October 18, 2010
Expires: April 21, 2011


    6LoWPAN Generic Compression of Headers and Header-like Payloads
                      draft-bormann-6lowpan-ghc-00

Abstract

   This short I-D provides a basic design for a an addition to 6lowpan
   Header Compression that enables the compression of generic headers
   and header-like payloads.

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
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   This Internet-Draft will expire on April 21, 2011.

Copyright Notice

   Copyright (c) 2010 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.





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Table of Contents

   1.  The Header Compression Coupling Problem  . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  6lowpan-ghc  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12







































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1.  The Header Compression Coupling Problem

   [I-D.ietf-6lowpan-hc] defines a scheme for header compression in
   6LoWPAN [RFC4944] packets.  As with most header compression schemes,
   a new specification is needed for every new kind of header that needs
   to be compressed.  In addition, [I-D.ietf-6lowpan-hc] does not define
   an extensibility scheme like the ROHC profiles defined in ROHC
   [RFC3095] [RFC5795].  This leads to the difficult situation that
   [I-D.ietf-6lowpan-hc] tends 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.  At this rate, [I-D.ietf-6lowpan-hc] will never get completed
   (fortunately, by now it has passed WGLC, but the underlying problem
   remains unsolved).

   The purpose of the present contribution is to plug into
   [I-D.ietf-6lowpan-hc] as is, using its NHC (next header compression)
   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.
   The objective is to arrive at something that can be defined on a
   single page and implemented in a couple of lines of code, as opposed
   to a general data compression scheme such as that defined in
   [RFC1951].

1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14 [RFC2119]
   and indicate requirement levels for compliant CoAP implementations.

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

















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2.  6lowpan-ghc

   The format of a 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, some simply modify some decompression variables.

   At the start of decompressing an L2 packet (= fragment), variable s
   is initialized as zero.

   The code bytes are defined as follows:

   +----------+--------------------------------------------+-----------+
   | code     | Action                                     | Argument  |
   | byte     |                                            |           |
   +----------+--------------------------------------------+-----------+
   | 0kkkkkkk | Copy k+1 bytes of actual data (k < 96)     | The k+1   |
   |          |                                            | bytes of  |
   |          |                                            | data      |
   |          |                                            |           |
   | 011sssss | s = (sssss * 8)                            |           |
   |          |                                            |           |
   | 10000nnn | reserved                                   |           |
   |          |                                            |           |
   | 10001kkk | Insert 8 bytes copied from previous bytes, |           |
   |          | at k + s bytes distance; s += 8            |           |
   |          |                                            |           |
   | 1001nnnn | Insert n+2 bytes of zeroes                 |           |
   |          |                                            |           |
   | 1010iiii | Insert all bytes (possibly filling an      |           |
   |          | incomplete byte with zero bits) from       |           |
   |          | Context i                                  |           |
   |          |                                            |           |
   | 1011iiii | Insert 8 bytes from Context i; i.e., the   |           |
   |          | context value truncated/extended to 8      |           |
   |          | bytes, and then insert 0000 00FF FE00      |           |
   |          |                                            |           |
   | 11nnnkkk | Insert n+2 bytes from previous bytes, k +  |           |
   |          | s bytes distance; s = 0                    |           |
   +----------+--------------------------------------------+-----------+

   For the purposes of the backreferences, the expansion buffer is
   initialized with the pseudo-header as defined in [RFC2460], at the
   end of which the target buffer begins.  These pseudo-header bytes are
   therefore available for backreferencing, but not copied into the
   final result.




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3.  Examples

   This section demonstrates a couple of realistic examples derived from
   actual PCAP dumps taken at previous interops.  Unfortunately, for
   these dumps, no context information was available, so the relatively
   powerful effect of context-based compression is not shown.  (TBD: Add
   a couple more general ICMP, ND, DHCP and RPL examples that show how
   nifty all this is.)

   Figure 1 shows a quite short RPL control 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
   Pseudoheader:
   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
   00 00 00 08 00 00 00 3a
   copy: 04 9b 00 6b de
   4 nulls: 92
   Compressed:
   04 9b 00 6b de 92
   Was 8 bytes; compressed to 6 bytes, 75 %

                      Figure 1: A simple RPL example

   Figure 2 shows a longer RPL control message that is improved a bit
   more (but would likely benefit additionally from a context
   reference).  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.















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   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 0a
   Pseudoheader:
   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
   00 00 00 5d 00 00 00 3a
   copy: 09 9b 01 7a 5f 00 f0 01 00 88
   3 nulls: 91
   copy: 04 20 02 0d b8
   7 nulls: 95
   ref(52): ff fe 00 -> ref 011sssss 6/11nnnkkk 1 4: 66 cc
   copy: 08 fa ce 04 0e 00 14 09 ff
   2 nulls: 90
   copy: 01 01
   7 nulls: 95
   copy: 06 08 1e 80 20 ff ff
   ref(2): ff ff -> ref 11nnnkkk 0 2: c2
   ref(4): ff ff ff ff -> ref 11nnnkkk 2 4: d4
   4 nulls: 92
   ref(48): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 fa ce
   -> ref 011sssss 6/10001kkk 0/11nnnkkk 6 0: 66 88 f0
   copy: 03 03 0e 40
   ref(9): 00 ff -> ref 011sssss 1/11nnnkkk 0 1: 61 c1
   ref(28): ff ff ff -> ref 011sssss 3/11nnnkkk 1 4: 63 cc
   ref(24): 20 02 0d b8 00 00 00 00
   -> ref 011sssss 3/11nnnkkk 6 0: 63 f0
   copy: 01 0a
   Compressed:
   09 9b 01 7a 5f 00 f0 01 00 88 91 04 20 02 0d b8
   95 66 cc 08 fa ce 04 0e 00 14 09 ff 90 01 01 95
   06 08 1e 80 20 ff ff c2 d4 92 66 88 f0 03 03 0e
   40 61 c1 63 cc 63 f0 01 0a
   Was 93 bytes; compressed to 57 bytes, 61 %

                      Figure 2: A longer RPL example







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   Figure 3 shows an the effect of compressing a simple ND neighbor
   solicitation (again, no context-based compression).

   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
   Pseudoheader:
   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
   00 00 00 30 00 00 00 3a
   copy: 04 87 00 a7 68
   4 nulls: 92
   ref(32): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
   -> ref 011sssss 4/10001kkk 0/11nnnkkk 6 0: 64 88 f0
   copy: 04 01 01 3b d3
   4 nulls: 92
   copy: 02 1f 02
   5 nulls: 93
   copy: 02 06 00
   ref(24): 1c da ff fe 00 -> ref 011sssss 3/11nnnkkk 3 0: 63 d8
   copy: 02 20 24
   Compressed:
   04 87 00 a7 68 92 64 88 f0 04 01 01 3b d3 92 02
   1f 02 93 02 06 00 63 d8 02 20 24
   Was 48 bytes; compressed to 27 bytes, 56 %

                   Figure 3: An ND neighbor solicitation



















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   Figure 4 shows the compression of an ND neighbor advertisement.  No
   RS or RA examples were available at this time; note that particularly
   RA messages might improve (even if they often will not be able to
   make use of context-based compression).

   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
   Pseudoheader:
   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
   00 00 00 30 00 00 00 3a
   copy: 05 88 00 26 6c c0
   3 nulls: 91
   ref(48): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
   -> ref 011sssss 6/10001kkk 0/11nnnkkk 6 0: 66 88 f0
   copy: 04 02 01 fa ce
   4 nulls: 92
   copy: 02 1f 02
   5 nulls: 93
   copy: 02 06 00
   ref(24): 1c da ff fe 00
   -> ref 011sssss 3/11nnnkkk 3 0: 63 d8
   copy: 02 20 24
   Compressed:
   05 88 00 26 6c c0 91 66 88 f0 04 02 01 fa ce 92
   02 1f 02 93 02 06 00 63 d8 02 20 24
   Was 48 bytes; compressed to 28 bytes, 58 %

                  Figure 4: An ND neighbor advertisement
















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4.  Acknowledgements

   Colin O'Flynn has repeatedly insisted that some form of compression
   for ICMPv6 and ND packets might be beneficial.  He actually has his
   own draft, [I-D.oflynn-6lowpan-icmphc], which compresses better, but
   addresses basic ICMPv6/ND only and needs a much longer spec (around
   17 pages of detailed spec, as compared to the single page here).
   This motivated the author to try something simple, yet general.











































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5.  Security Considerations

   (To be worked out.)
















































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6.  References

6.1.  Normative References

   [I-D.ietf-6lowpan-hc]
              Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams in 6LoWPAN Networks", draft-ietf-6lowpan-hc-13
              (work in progress), September 2010.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

6.2.  Informative References

   [I-D.oflynn-6lowpan-icmphc]
              O'Flynn, C., "ICMPv6/ND Compression for 6LoWPAN Networks",
              draft-oflynn-6lowpan-icmphc-00 (work in progress),
              July 2010.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [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.

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              March 2010.












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Author's Address

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

   Phone: +49-421-218-63921
   Fax:   +49-421-218-7000
   Email: cabo@tzi.org








































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