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. 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 April 26, 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 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. Bormann Expires April 26, 2011 [Page 1] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 2] Internet-Draft 6lowpan-ghc October 2010 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". Bormann Expires April 26, 2011 [Page 3] Internet-Draft 6lowpan-ghc October 2010 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. Bormann Expires April 26, 2011 [Page 4] Internet-Draft 6lowpan-ghc October 2010 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. Bormann Expires April 26, 2011 [Page 5] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 6] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 7] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 8] Internet-Draft 6lowpan-ghc October 2010 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. Bormann Expires April 26, 2011 [Page 9] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 10] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 11] Internet-Draft 6lowpan-ghc October 2010 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. Bormann Expires April 26, 2011 [Page 12] Internet-Draft 6lowpan-ghc October 2010 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. Bormann Expires April 26, 2011 [Page 13] Internet-Draft 6lowpan-ghc October 2010 7. Security Considerations (To be worked out. Probably mostly about the need to avoid buffer overflows and out-of-area references during decompression.) Bormann Expires April 26, 2011 [Page 14] Internet-Draft 6lowpan-ghc October 2010 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. Bormann Expires April 26, 2011 [Page 15] Internet-Draft 6lowpan-ghc October 2010 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 Bormann Expires April 26, 2011 [Page 16]