6Lo Working GroupInternet Engineering Task Force (IETF) C. BormannInternet-DraftRequest for Comments: 7400 Universitaet Bremen TZIIntended status:Category: Standards TrackSeptember 19,November 2014Expires: March 23, 2015 6LoWPANISSN: 2070-1721 6LoWPAN-GHC: Generic Header Compressionof Headers and Header-like Payloads (GHC) draft-ietf-6lo-ghc-05for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) AbstractThis short specification providesRFC 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 additionto 6LoWPAN Header Compressionthat enables the compression of generic headers and header-like payloads, without a need to define a new header compression scheme for eachnewsuch new header or header-like payload. Status of This Memo ThisInternet-Draftissubmitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documentsan Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF).Note that other groups may also distribute working documents as Internet-Drafts. The listIt represents the consensus ofcurrent Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents validthe IETF community. It has received public review and has been approved fora maximumpublication 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 ofsix monthsthis document, any errata, and how to provide feedback on it may beupdated, replaced, or obsoleted by other documentsobtained atany 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 March 23, 2015.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 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. Table of Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 2....................................................2 1.1. The Header Compression Coupling Problem. . . . . . . . . 2....................2 1.2. Compression Approach. . . . . . . . . . . . . . . . . . 3.......................................3 1.3. Terminology. . . . . . . . . . . . . . . . . . . . . . . 3................................................3 1.4. Notation. . . . . . . . . . . . . . . . . . . . . . . . 4...................................................4 2. 6LoWPAN-GHC. . . . . . . . . . . . . . . . . . . . . . . . . 5.....................................................4 3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC. . . . . . . . . . . 6.........................6 3.1. CompressingpayloadsPayloads (UDP and ICMPv6). . . . . . . . . . 6......................6 3.2. Compressingextension headers . . . . . . . . . . . . . . 6Extension Headers ..............................6 3.3. Indicating GHCcapability . . . . . . . . . . . . . . . . 7Capability ..................................7 3.4. Using the 6CIOOption . . . . . . . . . . . . . . . . . . 8.............................................8 4. IANAconsiderations . . . . . . . . . . . . . . . . . . . . . 9Considerations .............................................9 5. Securityconsiderations . . . . . . . . . . . . . . . . . . . 10Considerations ........................................10 6.Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 7.References. . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1......................................................11 6.1. Normative References. . . . . . . . . . . . . . . . . . 13 7.2.......................................11 6.2. Informative References. . . . . . . . . . . . . . . . . 13....................................12 Appendix A. Examples. . . . . . . . . . . . . . . . . . . . . . 14..............................................14 Acknowledgements ..................................................24 Author's Address. . . . . . . . . . . . . . . . . . . . . . . . 24..................................................24 1. Introduction 1.1. The Header Compression Coupling Problem6LoWPAN-HC[RFC6282] defines a scheme for header compression in 6LoWPAN [RFC4944]packets.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 isneedednecessary for every new kind of header that needs to be compressed. In addition, [RFC6282] does not define an extensibility scheme like theROHCRobust Header Compression (ROHC) profiles defined in ROHC [RFC3095] [RFC5795]. This leads to the difficult situationthatin 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.WhileAlthough [RFC6282] was finallygot completed,completed and published, the underlying problem remains unsolved. The purpose of the present contribution is to plug into [RFC6282] as is, using itsNHC (next header compression)Next Header Compression (NHC) concept. We add a slightly less efficient, but vastly moregeneralgeneral, 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,(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 indetail,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 itsnow customarynow-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 notationis: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 sameletterletter, 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 inmost-significant-bit firstmost-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 zerobit,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 sameorder,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 withinaan 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 |STOPstop code (end of compresseddata,data; see | | | | Section 3.2) | | | | | | | 101nssss | Set up extended arguments for a | | | | backreference: sa += 0b0ssss000,na +=| | | | na += 0b0000n000 | | | | | | | 11nnnkkk | Backreference: n = na+0b00000nnn+2;s =| | | | 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 forgeneric header compressionGeneric Header Compression Note that the following bit combinations are reserved at this time:011xxxxx, ando 011xxxxx o 1001nnnn (where 0b0000nnnn >0).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 forbackreferencing,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 16bytesBytes ofstatic dictionaryStatic Dictionary (inhex)Hex) 3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC 6LoWPAN-GHC plugs in as an NHC format for 6LoWPAN-HC [RFC6282]. 3.1. CompressingpayloadsPayloads (UDP and ICMPv6) By definition, GHC isby definitiongeneric 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). Inadditionaddition, it is useful to include an NHC format for UDP, as manyheaderlikeheader-like payloads (e.g., DHCPv6,DTLS)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 for0b11010cpp11010CPP 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: NHCbyteByte for UDP GHC(to be allocated by IANA)(11010CPP) To stay in the same general numbering space, we use0b1101111111011111 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: NHCbyteByte for ICMPv6 GHC(to be allocated by IANA)(11011111) 3.2. Compressingextension headersExtension Headers Compression of specific extension headers is added in a similar way (Figure 4) (however, probably onlyEIDExtension Header ID (EID) 0 to 3 [RFC6282] need to be assigned). As there is no easy way to extract thelengthLength field from theGHC- encodedGHC-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 thelengthLength field (in the same way NHC already elides thenext headerNext Header field in certain cases) and reconstruct it only on decompression. To serve as a terminator for the extension header, thereservedbytecode 0b10010000 has been assigned as a stopmarker.code. Note that the stopmarkercode 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: NHCbyteByte forextension headerExtension Header GHC 3.3. Indicating GHCcapabilityCapability The 6LoWPAN baseline includes just [RFC4944], [RFC6282], and [RFC6775] (see[I-D.bormann-6lo-6lowpan-roadmap]).[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 nobodyeverwould ever start sending packets with GHC otherwise. To minimize the impact on [RFC6775], we definean NDa 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 oranimplicit 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 wayPLPMTUDPacketization Layer Path MTU Discovery (PLPMTUD) [RFC4821] finds out about changes in the network, a node SHOULD make use ofNUD (neighbor unreachability detection)Neighbor Unreachability Detection (NUD) failures to switch back to basic 6LoWPAN header compression [RFC6282]. 3.4. Using the 6CIOOptionThe 6CIOoptionwill typically only beeversent in 6LoWPAN-NDRSRouter Solicitation (RS) packets (which cannotitselfthemselves be GHC compressed unless the host desires to limit itself to talking toGHC capableGHC-capable routers). The resulting 6LoWPAN-NDRARouter 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-NDNS/NANeighbor 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 anupward compatibleupward-compatible manner.) Flag numbers 0 to 7 are reserved forexperiments.experimental use. They MUST NOT be used for actual deployments. Where the use of this option by other specifications orby experimentsfor 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 theoption,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.i.e., once a sender has reason to believe the capability indication has been received, there is no longerisa need to continue sending it. 4. IANAconsiderations [This section to be removed/replaced by the RFC Editor.] In theConsiderations IANAregistry forhas added the assignments listed in Figure 6 in the "LOWPAN_NHC Header Type"(in theregistry (under "IPv6 Low Power Personal Area NetworkParameters"), IANA is requested to add the assignments in Figure 6. 10110IIN:Parameters"). 10110EEN: Extension header GHC[RFCthis][RFC7400] 11010CPP: UDP GHC[RFCthis][RFC7400] 11011111: ICMPv6 GHC[RFCthis][RFC7400] Figure 6: IANAassignmentsAssignments for the NHCbyteByte IANAis requested to allocate anhas allocated ND option number 36 for the "6LoWPAN Capability Indication Option (6CIO)" ND option format in theRegistry"IPv6 Neighbor Discovery Option Formats" registry [RFC4861]. IANAis requested to createhas created a subregistry for "6LoWPAN capabilitybits" withinBits" under the "Internet Control Message Protocol version 6 (ICMPv6)Parameters".Parameters" registry. The bits are assigned by giving their numbers assmallsmall, non-negative integers as defined insectionSection 3.4,preferablyin the range0..47.0-47. The policy is "IETF Review" or "IESG Approval" [RFC5226]. The initial content of the registry is as shown in Figure 7:0..7: reserved0-7: Reserved forexperiments [RFCthis] 8..14: unassignedExperimental Use [RFC7400] 8-14: Unassigned 15: GHC capable bit (G bit)[RFCthis] 16..47: unassigned[RFC7400] 16-47: Unassigned Figure 7: IANAassignmentsAssignments for the 6LoWPANcapability bitsCapability Bits 5. SecurityconsiderationsConsiderations 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 overflowsandand, in particular (with respect to theback-referencing)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 isGHC-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 3.8Mbit/ sMbit/s attacker. The amplification may be more important inside the 6LoWPAN, if there is a way to obtain reflection(otherwise(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 bytransporttransport- orapplicationapplication-layer (or evenIP) layerIP-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 onMACMedia 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 firstadaptation layeradaptation-layer fragment of) the same packet. This attack is fully mitigated by not exposing secret values to the adaptationlayer,layer or by not using GHC in deployments where this is done. 6.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 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 of core spec here). This motivated the author to try something simple, yet general. Special thanks go to Colin for indicating that he indeed considers his draft superseded by the present one. The examples given are based on pcap files that Colin O'Flynn, Owen Kirby, Olaf Bergmann and others provided. Using these pcap 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 shaping the 6CIO option. 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 option. The IESG reviewers Richard Barnes and Stephen Farrell have contributed issues to the security considerations section; they and Barry Leiba, as well as GEN-ART reviewer Vijay K. Gurbani also have provided editorial improvements. 7.References7.1.6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March1997.1997, <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, September2007.2007, <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, September2007.2007, <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, May2008.2008, <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, September2011.2011, <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, November2012. 7.2.2012, <http://www.rfc-editor.org/info/rfc6775>. 6.2. Informative References[I-D.bormann-6lo-6lowpan-roadmap] Bormann, C., "6LoWPAN Roadmap and Implementation Guide", draft-bormann-6lo-6lowpan-roadmap-00 (work in progress), October 2013. [I-D.oflynn-6lowpan-icmphc][ICMPv6-ND] O'Flynn, C., "ICMPv6/ND Compression for 6LoWPAN Networks",draft-oflynn-6lowpan-icmphc-00 (workWork inprogress),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, May1996.1996, <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, July2001.2001, <http://www.rfc-editor.org/info/rfc3095>. [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March2007.2007, <http://www.rfc-editor.org/info/rfc4821>. [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust Header Compression (ROHC) Framework", RFC 5795, March2010.2010, <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, May2014.2014, <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 actualPCAP dumpspacket captures taken at previous interops. For the Routing Protocol for Low-Power and Lossy Networks (RPL) [RFC6550], Figure 8 showsan RPL DODAGa Destination-Oriented Directed Acyclic Graph (DODAG) InformationSolicitation,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: AsimpleSimple RPLexampleExample Figure 9 showsana 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 theexampleexample, 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: AlongerLonger RPLexampleExample Similarly, Figure 10 showsana RPLDAODestination Advertisement Object (DAO) message. One of the embedded addresses is copied right out of thepseudo-header,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) theIIDInterface Identifier part again out of the pseudo-header. Note that a simple implementation would probably emit fewer nulls and copy the entireIID;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:AnA RPL DAOmessageMessage 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 NDneighbor solicitationNeighbor 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 NDneighbor advertisementNeighbor 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 becopiedcopied, 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 NDrouter solicitationRouter 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 NDrouter advertisementRouter Advertisement Figure 15 shows the compression of a DTLS application data packet with a net payload of 13 bytes ofcleartext,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 staticdictionary,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 DTLSapplication data packetApplication 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 DTLSapplication data packetApplication 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 DTLShandshakeHandshake 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(client hello)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: +49-421-218-63921Email:EMail: cabo@tzi.org