6LoWPAN Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track October 23, 2010
Expires: April 26, 2011
6LoWPAN Generic Compression of Headers and Header-like Payloads
draft-bormann-6lowpan-ghc-01
Abstract
This short I-D provides a complete design for a simple 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. Integrating 6LoWPAN-GHC into 6LoWPAN-HC . . . . . . . . . . . 11
4.1. Compressing extension headers . . . . . . . . . . . . . . 11
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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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 a header or payload within a L2 packet
(= fragment), variables "sa" and "na" are initialized as zero.
The code bytes are defined as follows:
+----------+---------------------------------------------+----------+
| code | Action | Argument |
| byte | | |
+----------+---------------------------------------------+----------+
| 0kkkkkkk | Append k = 0b0kkkkkkk bytes of data in the | The k |
| | bytecode argument (k < 96) | bytes of |
| | | data |
| | | |
| 0110iiii | Append all bytes (possibly filling an | |
| | incomplete byte with zero bits) from | |
| | Context i | |
| | | |
| 0111iiii | Append 8 bytes from Context i; i.e., the | |
| | context value truncated/extended to 8 | |
| | bytes, and then append 0000 00FF FE00 | |
| | (i.e., 14 bytes total) | |
| | | |
| 1000nnnn | Append 0b0000nnnn+2 bytes of zeroes | |
| | | |
| 1001nnnn | reserved | |
| | | |
| 101nssss | sa += 0b0ssss000, na += 0b0000n000 | |
| | | |
| 11nnnkkk | 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 | |
+----------+---------------------------------------------+----------+
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 some relatively 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 DHCP examples.)
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: 82
Compressed:
04 9b 00 6b de 82
Was 8 bytes; compressed to 6 bytes, compression factor 1.33
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
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 5c 00 00 00 3a
copy: 09 9b 01 7a 5f 00 f0 01 00 88
3 nulls: 81
copy: 04 20 02 0d b8
7 nulls: 85
ref(52): ff fe 00 -> ref 101nssss 0 6/11nnnkkk 1 1: a6 c9
copy: 08 fa ce 04 0e 00 14 09 ff
2 nulls: 80
copy: 01 01
7 nulls: 85
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:
09 9b 01 7a 5f 00 f0 01 00 88 81 04 20 02 0d b8
85 a6 c9 08 fa ce 04 0e 00 14 09 ff 80 01 01 85
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 53 bytes, compression factor 1.74
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: 82
ref(32): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
-> ref 101nssss 1 2/11nnnkkk 6 0: b2 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 b2 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 3: An ND neighbor solicitation
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Figure 4 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
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: 81
ref(48): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23
-> ref 101nssss 1 4/11nnnkkk 6 0: b4 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 b4 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 4: An ND neighbor advertisement
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Figure 5 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
Pseudoheader:
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
00 00 00 18 00 00 00 3a
copy: 04 85 00 90 65
ref(33): 00 00 00 00 01 -> ref 101nssss 0 3/11nnnkkk 3 4: a3 dc
copy: 02 02 ac
ref(42): de 48 00 00 00 00 01
-> ref 101nssss 0 4/11nnnkkk 5 3: a4 eb
6 nulls: 84
Compressed:
04 85 00 90 65 a3 dc 02 02 ac a4 eb 84
Was 24 bytes; compressed to 13 bytes, compression factor 1.85
Figure 5
Figure 6 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.
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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
Pseudoheader:
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
00 00 00 60 00 00 00 3a
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 -> ref 101nssss 0 9/11nnnkkk 3 7: a9 df
ref(40): 00 20 02 0d b8 00 00 00 00 00 00 00
-> ref 101nssss 1 3/11nnnkkk 2 4: b3 d4
ref(120): ff fe 00 11 22
-> ref 101nssss 0 14/11nnnkkk 3 3: ae 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 df b3 d4 ae db
Was 96 bytes; compressed to 58 bytes, compression factor 1.66
Figure 6: An ND router advertisement
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4. Integrating 6LoWPAN-GHC into 6LoWPAN-HC
6LoWPAN-GHC is intended to plug in as an NHC format for 6LoWPAN-HC
[I-D.ietf-6lowpan-hc]. This section shows how this can be done
(without supplying the detailed normative text yet, although it could
be implemented from this page).
GHC is by definition generic and can be applied to different kinds of
packets. All the examples given above are for ICMPv6 packets; it is
trivial to define an NHC format for ICMPv6 based on GHC.
In addition it may be useful to include an NHC format for UDP, as
many headerlike payloads (e.g., DHCPv6) are carried in UDP.
[I-D.ietf-6lowpan-hc] already defines an NHC format for UDP
(11110CPP). What remains to be done is to define an analogous NHC
byte formatted, e.g. as shown in Figure 7, and simply reference the
existing specification, indicating that for 0b11010cpp 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 7: A possible NHC byte for UDP GHC
To stay in the same general numbering space, we propose 0b11011111 as
the NHC byte for IPCMPv6 GHC.
4.1. Compressing extension headers
If the compression of specific extension headers is considered
desirable, this can be added in a similar way, e.g. as in Figure 8
(however, probably only EID 0 to 3 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. Instead, the reserved bytecode 0b10010000
would be assigned as a stop marker.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 0 | 1 | 1 | EID |NH |
+---+---+---+---+---+---+---+---+
Figure 8: A possible NHC byte for extension header GHC
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5. IANA considerations
In the IANA registry for the LOWPAN_NHC header type, IANA would need
to add the assigments in Figure 9.
10110IIN: Extension header GHC*) [RFCthis]
11010CPP: UDP GHC [RFCthis]
11011111: ICMPv6 GHC [RFCthis]
Figure 9: IANA assignments for the NHC byte
*) if the functionality of Section 4.1 is made part of this document.
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6. 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.
The examples given are based on pcap files that Colin O'Flynn and
Owen Kirby provided.
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7. Security Considerations
(To be worked out. Probably mostly about the need to avoid buffer
overflows and out-of-area references during decompression.)
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8. References
8.1. 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.
8.2. 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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