One document matched: draft-bormann-6lowpan-ghc-00.txt
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.
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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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