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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) ** Obsolete normative reference: RFC 3775 (Obsoleted by RFC 6275) -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Hui, Ed. 3 Internet-Draft Arch Rock Corporation 4 Updates: 4944 (if approved) P. Thubert 5 Intended status: Standards Track Cisco 6 Expires: January 27, 2011 July 26, 2010 8 Compression Format for IPv6 Datagrams in 6LoWPAN Networks 9 draft-ietf-6lowpan-hc-08 11 Abstract 13 This document specifies an IPv6 header compression format for IPv6 14 packet delivery in 6LoWPAN networks. The compression format relies 15 on shared context to allow compression of arbitrary prefixes. How 16 the information is maintained in that shared context is out of scope. 17 This document specifies compression of multicast addresses and a 18 framework for compressing next headers. UDP header compression is 19 specified within this framework. 21 Status of this Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on January 27, 2011. 38 Copyright Notice 40 Copyright (c) 2010 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 57 2. Specific Updates to RFC 4944 . . . . . . . . . . . . . . . . . 4 58 3. IPv6 Header Compression . . . . . . . . . . . . . . . . . . . 5 59 3.1. LOWPAN_IPHC Encoding Format . . . . . . . . . . . . . . . 6 60 3.1.1. Base Format . . . . . . . . . . . . . . . . . . . . . 6 61 3.1.2. Context Identifier Extension . . . . . . . . . . . . . 8 62 3.2. IPv6 Header Encoding . . . . . . . . . . . . . . . . . . . 9 63 3.2.1. Traffic Class and Flow Label Compression . . . . . . . 9 64 3.2.2. Mapping Link-Layer Addresses to Interface IDs . . . . 10 65 3.2.3. Stateless Multicast Addresses Compression . . . . . . 11 66 3.2.4. Stateful Multicast Addresses Compression . . . . . . . 12 67 4. IPv6 Next Header Compression . . . . . . . . . . . . . . . . . 13 68 4.1. LOWPAN_NHC Format . . . . . . . . . . . . . . . . . . . . 13 69 4.2. IPv6 Extension Header Compression . . . . . . . . . . . . 14 70 4.3. UDP Header Compression . . . . . . . . . . . . . . . . . . 15 71 4.3.1. Compressing UDP ports . . . . . . . . . . . . . . . . 16 72 4.3.2. Compressing UDP checksum . . . . . . . . . . . . . . . 16 73 4.3.3. UDP LOWPAN_NHC Format . . . . . . . . . . . . . . . . 17 74 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 75 6. Security Considerations . . . . . . . . . . . . . . . . . . . 18 76 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 77 8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 78 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 79 9.1. Normative References . . . . . . . . . . . . . . . . . . . 20 80 9.2. Informative References . . . . . . . . . . . . . . . . . . 20 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 83 1. Introduction 85 The [IEEE 802.15.4] standard specifies an MTU of 128 bytes, yielding 86 about 80 octets of actual MAC payload with security enabled, on a 87 wireless link with a link throughput of 250 kbps or less. The 88 6LoWPAN adaptation format [RFC4944] was specified to carry IPv6 89 datagrams over such constrained links, taking into account limited 90 bandwidth, memory, or energy resources that are expected in 91 applications such as wireless sensor networks. [RFC4944] defines a 92 Mesh Addressing header to support sub-IP forwarding, a Fragmentation 93 header to support the IPv6 minimum MTU requirement [RFC2460], and 94 stateless header compression for IPv6 datagrams (LOWPAN_HC1 and 95 LOWPAN_HC2) to reduce the relatively large IPv6 and UDP headers down 96 to (in the best case) several bytes. 98 LOWPAN_HC1 and LOWPAN_HC2 are insufficient for most practical uses of 99 6LoWPAN networks. LOWPAN_HC1 is most effective for link-local 100 unicast communication, where IPv6 addresses carry the link-local 101 prefix and an Interface Identifier (IID) directly derived from IEEE 102 802.15.4 addresses. In this case, both addresses may be completely 103 elided. However, though link-local addresses are commonly used for 104 local protocol interactions such as IPv6 ND [RFC4861], DHCPv6 105 [RFC3315] or routing protocols, they are usually not used for 106 application-layer data traffic, so the actual value of this 107 compression mechanism is limited. 109 Routable addresses must be used when communicating with devices 110 external to the LoWPAN or in a route-over configuration where IP 111 forwarding occurs within the LoWPAN. For routable addresses, 112 LOWPAN_HC1 requires both IPv6 source and destination addresses to 113 carry the prefix in-line. In cases where the Mesh Addressing header 114 is not used, the IID of a routable address must be carried in-line. 115 However, LOWPAN_HC1 requires 64-bits for the IID when carried in-line 116 and cannot be shortened even when it is derived from the IEEE 117 802.15.4 16-bit short address. When the destination is an IPv6 118 multicast address, LOWPAN_HC1 requires the full 128-bit address to be 119 carried in-line. 121 As a result, this document defines an encoding format, LOWPAN_IPHC, 122 for effective compression of Unique Local, Global, and multicast IPv6 123 Addresses based on shared state within contexts. In addition, this 124 document also introduces a number of additional improvements over the 125 header compression format defined in [RFC4944]. 127 LOWPAN_IPHC allows for compression of some commonly-used IPv6 Hop 128 Limit values. If the LoWPAN is a mesh-under stub, a Hop Limit of 1 129 for inbound and a default value such as 64 for outbound are usually 130 enough for application layer data traffic. Additionally, a hop-limit 131 value of 255 is often used to verify that a communication occurs over 132 a single-hop. This specification enables compression of the IPv6 Hop 133 Limit field in those common cases, whereas LOWPAN_HC1 does not. 135 This document also defines LOWPAN_NHC, an encoding format for 136 arbitrary next headers. LOWPAN_IPHC indicates whether the following 137 header is encoded using LOWPAN_NHC. If so, the bits immediately 138 following the compressed IPv6 header start the LOWPAN_NHC encoding. 139 In contrast, LOWPAN_HC1 could be extended to support compression of 140 next headers using LOWPAN_HC2, but only for UDP, TCP, and ICMPv6. 141 Furthermore, the LOWPAN_HC2 octet sits between the LOWPAN_HC1 octet 142 and uncompressed IPv6 header fields. This specification moves the 143 next header encoding bits to follow all IPv6-related bits, allowing 144 for a properly layered structure and direct support for IPv6 145 extension headers. 147 Using LOWPAN_NHC, this document defines a compression mechanism for 148 UDP. While [RFC4944] defines a compression mechanism for UDP, that 149 mechanism does not enable checksum compression when rendered possible 150 by additional upper layer mechanisms such as upper layer Message 151 Integrity Check (MIC). This specification adds the capability to 152 elide the UDP checksum over the LoWPAN, which enables saving of a 153 further two octets. 155 Also using LOWPAN_NHC, this document defines encoding formats for 156 IPv6-in-IPv6 encapsulation as well as IPv6 Extension Headers. With 157 LOWPAN_HC1 and LOWPAN_HC2, chains of next headers cannot be encoded 158 efficiently. 160 1.1. Requirements Language 162 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 163 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 164 document are to be interpreted as described in RFC 2119 [RFC2119]. 166 2. Specific Updates to RFC 4944 168 This document specifies a header compression format that is intended 169 to replace that defined in Section 10 of [RFC4944]. Implementation 170 of Section 10 of [RFC4944] is now NOT RECOMMENDED. New 171 implementations MAY implement compression according to Section 10 of 172 [RFC4944], but SHOULD NOT send packets compressed according to 173 Section 10 of [RFC4944]. 175 Section 5.3 of [RFC4944] also defines how to fragment compressed IPv6 176 datagrams that do not fit within a single link frame. Section 5.3 of 177 [RFC4944] defines the fragment header's datagram_size and 178 datagram_offset values as the size and offset of the IPv6 datagram 179 before compression. As a result, all fragment payload outside the 180 first fragment must carry their respective portions of the IPv6 181 datagram before compression. This document does not change that 182 requirement. When using the fragmentation mechanism described in 183 Section 5.3 of [RFC4944], any header that cannot fit within the first 184 fragment MUST NOT be compressed. 186 The header compression format defined in this document preempts the 187 ESC dispatch value defined in Section 5.1 of [RFC4944]. Instead, the 188 value of 01 000000 is reserved as a replacement value for ESC, to be 189 finally assigned with the first assignment of extension bytes. 191 3. IPv6 Header Compression 193 In this section, we define the LOWPAN_IPHC encoding format for 194 compressing the IPv6 header. To enable effective compression 195 LOWPAN_IPHC relies on information pertaining to the entire 6LoWPAN 196 network. LOWPAN_IPHC assumes the following will be the common case 197 for 6LoWPAN communication: Version is 6; Traffic Class and Flow Label 198 are both zero; Payload Length can be inferred from lower layers from 199 either the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header; 200 Hop Limit will be set to a well-known value by the source; addresses 201 assigned to 6LoWPAN interfaces will be formed using the link-local 202 prefix or a small set of routable prefixes assigned to the entire 203 6LoWPAN network; addresses assigned to 6LoWPAN interfaces are formed 204 with an IID derived directly from either the 64-bit extended or 16- 205 bit short IEEE 802.15.4 addresses. 207 +-------------------------------------+---------------------------- 208 | Dispatch + LOWPAN_IPHC (2-3 octets) | In-line IPv6 Header Fields 209 +-------------------------------------+---------------------------- 211 Figure 1: LOWPAN_IPHC Header 213 The LOWPAN_IPHC encoding utilizes 13 bits, 5 of which are taken from 214 the rightmost bit of the dispatch type. The encoding may be extended 215 by another octet to support additional contexts. Any information 216 from the uncompressed IPv6 header fields carried in-line follow the 217 LOWPAN_IPHC encoding, as shown in Figure 1. In the best case, the 218 LOWPAN_IPHC can compress the IPv6 header down to two octets (the 219 dispatch octet and the LOWPAN_IPHC encoding) with link-local 220 communication. 222 When routing over multiple IP hops, LOWPAN_IPHC can compress the IPv6 223 header down to 7 octets (1-octet dispatch, 1-octet LOWPAN_IPHC, 224 1-octet Hop Limit, 2-octet Source Address, and 2-octet Destination 225 Address). The Hop Limit may not be compressed because it needs to 226 decremented at each hop and may take any value. Stateful address 227 compression must be applied to the source and destination IPv6 228 addresses because they do not statelessly match the source and 229 destination link layer addresses on intermediate hops. 231 3.1. LOWPAN_IPHC Encoding Format 233 This section specifies the format of the LOWPAN_IPHC encoding that 234 describes how an IPv6 header is compressed. The encoding can be 2 235 octets long for the base encoding or 3 octets long when an additional 236 context encoding is present. The IPv6 header fields that are not 237 fully elided are placed immediately after the LOWPAN_IPHC, either in 238 a compressed form if the field is partially elided, or literally. 240 3.1.1. Base Format 242 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 243 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 244 | 0 | 1 | 1 | TF |NH | HLIM |CID|SAC| SAM | M |DAC| DAM | 245 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 247 Figure 2: LOWPAN_IPHC base Encoding 249 TF: Traffic Class, Flow Label: 250 00: ECN + DSCP + 4-bit Pad + Flow Label (4 bytes) 251 01: ECN + 2-bit Pad + Flow Label (3 bytes), DSCP is elided 252 10: ECN + DSCP (1 byte), Flow Label is elided 253 11: Traffic Class and Flow Label are elided. 255 NH: Next Header: 256 0: Full 8 bits for Next Header are carried in-line. 257 1: The Next Header field is compressed and the next header is 258 encoded using LOWPAN_NHC, which is discussed in Section 4. 260 HLIM: Hop Limit: 261 00: The Hop Limit field is carried in-line. 262 01: The Hop Limit field is compressed and the hop limit is 1. 263 10: The Hop Limit field is compressed and the hop limit is 64. 264 11: The Hop Limit field is compressed and the hop limit is 255. 266 CID: Context Identifier Extension: 267 0: No additional 8-bit Context Identifier Extension is used. If 268 context-based compression is specified in either SAC or DAC, 269 context 0 is used. 270 1: An additional 8-bit Context Identifier Extension field 271 immediately follows the DAM field. 273 SAC: Source Address Compression 274 0: Source address compression uses stateless compression. 275 1: Source address compression uses stateful, context-based 276 compression. 278 SAM: Source Address Mode: 279 If SAC=0: 280 00: 128 bits. The full address is carried in-line. 281 01: 64 bits. The first 64-bits of the address are elided. 282 The value of those bits is the link-local prefix padded with 283 zeros. The remaining 64 bits are carried in-line. 284 10: 16 bits. The first 112 bits of the address are elided. 285 The value of those bits is the link-local prefix padded with 286 zeros. The remaining 16 bits are carried in-line. 287 11: 0 bits. The address is fully elided. The first 64 bits 288 of the address are the link-local prefix padded with zeros. 289 The remaining 64 bits are computed from the encapsulating 290 header. 291 If SAC=1: 292 00: The UNSPECIFIED address, :: 293 01: 64 bits. The address is derived using context information 294 and the 64 bits carried in-line. 295 10: 16 bits. The address is derived using context information 296 and the 16 bits carried in-line. 297 11: 0 bits. The address is fully elided. The prefix is 298 derived using context information. Any of the remaining 64 299 bits not covered by the context information are computed 300 from the encapsulating header. 302 M: Multicast Compression 303 0: Destination address is not a multicast address. 304 1: Destination address is a multicast address. 306 DAC: Destination Address Compression 307 0: Destination address compression uses stateless compression. 308 1: Destination address compression uses stateful, context-based 309 compression. 311 DAM: Destination Address Mode: 312 If M=0 and DAC=0 This case matches SAC=0 but for the destination 313 address: 314 00: 128 bits. The full address is carried in-line. 315 01: 64 bits. The first 64-bits of the address are elided. 316 The value of those bits is the link-local prefix padded with 317 zeros. The remaining 64 bits are carried in-line. 318 10: 16 bits. The first 112 bits of the address are elided. 319 The value of those bits is the link-local prefix padded with 320 zeros. The remaining 16 bits are carried in-line. 321 11: 0 bits. The address is fully elided. The first 64 bits 322 of the address are the link-local prefix padded with zeros. 323 The remaining 64 bits are computed from the encapsulating 324 header. 325 If M=0 and DAC=1: 326 00: Reserved. 327 01: 64 bits. The address is derived using context information 328 and the 64 bits carried in-line. 329 10: 16 bits. The address is derived using context information 330 and the 16 bits carried in-line. 331 11: 0 bits. The address is fully elided. The prefix is 332 derived using context information. Any of the remaining 64 333 bits not covered by the context information are computed 334 from the encapsulating header. 335 If M=1 and DAC=0: 336 00: 128 bits. The full address is carried in-line. 337 01: 48 bits. The address takes the form FFXX::00XX:XXXX:XXXX. 338 10: 32 bits. The address takes the form FFXX::00XX:XXXX. 339 11: 8 bits. The address takes the form FF02::00XX. 340 If M=1 and DAC=1: 341 00: 48 bits. This format is designed to match Unicast-Prefix- 342 based IPv6 Multicast Addresses as defined in [RFC3306] and 343 [RFC3956]. The multicast address takes the form FFXX:XXLL: 344 PPPP:PPPP:PPPP:PPPP:XXXX:XXXX. where the X are the nibbles 345 that are carried in-line, in the order in which they appear 346 in this format. P denotes nibbles used to encode the prefix 347 itself. L denotes nibbles used to encode the prefix length. 348 The prefix information P and L is taken from the specified 349 context. 350 01: reserved 351 10: reserved 352 11: reserved 354 3.1.2. Context Identifier Extension 356 This specification expects that a conceptual context is shared 357 between the node that compresses a packet and the node(s) that need 358 to expand it. How the contexts are shared and maintained is out of 359 scope. What information is contained within a context information is 360 out of scope. Actions in response to unknown and/or invalid contexts 361 are out of scope. The specification enables a node to use up to 16 362 contexts. The context used to encode the source address does not 363 have to be the same as the context used to encode the destination 364 address. 366 If the CID field is set to '1' in the LOWPAN_IPHC encoding, then an 367 additional octet extends the LOWPAN_IPHC encoding following the DAM 368 bits but before the IPv6 header fields that are carried in-line. The 369 additional octet identifies the pair of contexts to be used when the 370 IPv6 source and/or destination address is compressed. The context 371 identifier is 4 bits for each address, supporting up to 16 contexts. 372 Context 0 is the default context. The encoding is shown in Figure 3. 374 0 1 2 3 4 5 6 7 375 +---+---+---+---+---+---+---+---+ 376 | SCI | DCI | 377 +---+---+---+---+---+---+---+---+ 379 Figure 3: LOWPAN_IPHC Encoding 381 SCI: Source Context Identifier Identifies the prefix that is used 382 when the IPv6 source address is statefully compressed. 383 DCI: Destination Context Identifier Identifies the prefix that is 384 used when the IPv6 destination address is statefully compressed. 386 3.2. IPv6 Header Encoding 388 Fields carried in-line (in part or in whole) appear in the same order 389 as they do in the IPv6 header format [RFC2460]. The Version field is 390 always elided. Unicast IPv6 addresses may be compressed to 64 or 16 391 bits or completely elided. Multicast IPv6 addresses may be 392 compressed to 8, 32, or 48 bits. The IPv6 Payload Length field MUST 393 always be elided and inferred from lower layers using the 6LoWPAN 394 Fragmentation header or the IEEE 802.15.4 header. 396 3.2.1. Traffic Class and Flow Label Compression 398 The Traffic Class field in the IPv6 header comprises 6 bits of 399 diffserv extension [RFC2474] and 2 bits of Explicit Congestion 400 Notification (ECN) [RFC3168]. If the ECN information is carried by 401 the Lower Layers in a compatible fashion then it can be elided from 402 the 6LoWPAN header. Otherwise, it has to be transported in one of 403 the following encodings. 405 The TF field in the LOWPAN_IPHC encoding indicates whether the 406 Traffic Class and Flow Label are carried in-line in the compressed 407 IPv6 header. When Flow Label is included while the Traffic Class is 408 compressed, an additional 4 bits are included to maintain byte- 409 alignment. Two of the 4 bits contain the ECN bits from the Traffic 410 Class field. 412 To ensure that the ECN bits appear in the same location for all 413 encodings that include them, the Traffic Class field is rotated right 414 by 2 bits in the compressed IPv6 header. The encodings are shown 415 below: 417 1 2 3 418 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 1 419 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 420 |ECN| DSCP | rsv | Flow Label | 421 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 423 TF = 00: Traffic Class and Flow Label carried in-line. 425 1 2 426 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 427 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 428 |ECN|rsv| Flow Label | 429 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 431 TF = 01: Flow Label carried in-line. 433 0 1 2 3 4 5 6 7 434 +-+-+-+-+-+-+-+-+ 435 |ECN| DSCP | 436 +-+-+-+-+-+-+-+-+ 438 TF = 10: Traffic Class carried in-line. 440 3.2.2. Mapping Link-Layer Addresses to Interface IDs 442 LOWPAN_IPHC elides the IIDs of source or destination addresses when 443 SAM = 3 or DAM = 3, respectively. In this mode, the IID is derived 444 from the encapsulating header. When the encapsulating header carries 445 IPv6 addresses, the corresponding bits map directly. 447 The remainder of this section defines the mapping from IEEE 802.15.4 448 link-layer addresses to IIDs for both short and extended IEEE 449 802.15.4 addresses. IID bits not covered by the context information 450 MAY be elided if they match the link-layer address mapping and MUST 451 NOT be elided if they do not. 453 An extended IEEE 802.15.4 address takes the form of an IEEE EUI-64 454 address. Generating an IID from an extended address is identical to 455 that defined in Appendix A of [RFC4291]. The only change needed to 456 transform an IEEE EUI-64 identifier to an interface identifier is to 457 invert the universal/local bit. 459 A short IEEE 802.15.4 address is 16 bits in length. Short addresses 460 are mapped into the restricted space of IEEE EUI-64 addresses by 461 setting the middle 16 bits to 0xfffe, the bottom 16 bits to the short 462 address, and all other bits to zero. As a result, an IID generated 463 from a short address has the form: 465 0000:00ff:fe00:XXXX 467 where XXXX carries the short address. The universal/local bit is 468 zero to indicate local scope. 470 This mapping for non-EUI-64 identifiers differs from that presented 471 in Appendix A of [RFC4291] for a couple reasons. Using the 472 restricted space ensures no overlap with IIDs generated from 473 unrestricted IEEE EUI-64 addresses. Also, including 0xfffe in the 474 middle of the IID helps avoid overlap with other locally managed 475 IIDs. 477 3.2.3. Stateless Multicast Addresses Compression 479 LOWPAN_IPHC supports stateless compression of multicast address when 480 M = 1 and DAC = 0. An IPv6 multicast address may be compressed down 481 to 48, 32, or 8 bits using stateless compression. The format 482 supports compression of the Solicited-Node Multicast Address (FF02:: 483 1:FFXX:XXXX) as well as any IPv6 multicast address where the upper 484 bits of the multicast group identifier are zero. The 8-bit 485 compressed form only carries the least-significant bits of the 486 multicast group identifier. The 48 and 32-bit compressed forms carry 487 the multicast scope and flags in-line, in addition to the least- 488 significant bits of the multicast group identifier. 490 1 2 3 491 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 1 492 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 493 | Flags | Scope | Group Identifier | 494 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 495 | Group Identifier | 496 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 498 DAM = 01. 48-bit Compressed Multicast Address (FFfs::00gg:gggg:gggg) 500 1 2 3 501 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 1 502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 503 | Flags | Scope | Group Identifier | 504 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 506 DAM = 10. 32-bit Compressed Multicast Address (FFfs::00gg:gggg). 508 0 1 2 3 4 5 6 7 509 +-+-+-+-+-+-+-+-+ 510 | Group ID | 511 +-+-+-+-+-+-+-+-+ 513 DAM = 11. 8-bit Compressed Multicast Address (FF02::gg). 515 3.2.4. Stateful Multicast Addresses Compression 517 LOWPAN_IPHC supports stateful compression of multicast addresses when 518 M = 1 and DAC = 1. This document currently defines DAM = 00: 519 context-based compression of Unicast-Prefix-based IPv6 Multicast 520 Addresses [RFC3306][RFC3956]. In particular, the Prefix Length and 521 Network Prefix can be taken from a context. As a result, LOWPAN_IPHC 522 can compress a Unicast-Prefix-based IPv6 Multicast Address down to 6 523 octets by only carrying the 4-bit Flags, 4-bit Scope, 8-bit RIID, and 524 32-bit Group Identifier in-line. 526 1 2 3 527 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 1 528 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 529 | Flags | Scope | Rsvd / RIID | Group Identifier | 530 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 531 | Group Identifier | 532 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 534 DAM = 01. Unicast-Prefix-based IPv6 Multicast Address Compression 536 Note that the Reserved field MUST carry the reserved bits from the 537 multicast address format as described in [RFC3306]. When a 538 Rendezvous Point is encoded in the multicast address as described in 539 [RFC3956], the Reserved field carries the RIID bits in-line. 541 4. IPv6 Next Header Compression 543 LOWPAN_IPHC elides the IPv6 Next Header field when the NH bit is set 544 to 1. It also indicates the use of 6LoWPAN next header compression, 545 LOWPAN_NHC. The value of IPv6 Next Header is recovered from the 546 first bits in the LOWPAN_NHC encoding. The following bits are 547 specific to the IPv6 Next Header value. Figure 4 shows the structure 548 of an IPv6 datagram compressed using LOWPAN_IPHC and LOWPAN_NHC. 550 +-------------+-------------+-------------+-----------------+-------- 551 | LOWPAN_IPHC | In-line | LOWPAN_NHC | In-line Next | Payload 552 | Encoding | IP Fields | Encoding | Header Fields | 553 +-------------+-------------+-------------+-----------------+-------- 555 Figure 4: Typical LOWPAN_IPHC/LOWPAN_NHC Header Configuration 557 4.1. LOWPAN_NHC Format 559 Compression formats for different next headers are identified by a 560 variable-length bit-pattern immediately following the LOWPAN_IPHC 561 compressed header. When defining a next header compression format, 562 the number of bits used SHOULD be determined by the perceived 563 frequency of using that format. However, the number of bits and any 564 remaining encoding bits SHOULD respect octet alignment. The 565 following bits are specific to the next header compression format. 566 This document defines a compression format for IPv6 Extension and UDP 567 headers. 569 +----------------+--------------------------- 570 | var-len NHC ID | compressed next header... 571 +----------------+--------------------------- 573 Figure 5: LOWPAN_NHC Encoding 575 4.2. IPv6 Extension Header Compression 577 A necessary property of encoding headers using LOWPAN_NHC is that the 578 immediately preceding header must either be encoded using LOWPAN_IPHC 579 or LOWPAN_NHC. In other words, all headers encoded using the 6LoWPAN 580 encoding format defined in this document must be contiguous. As a 581 result, this document defines a set of LOWPAN_NHC encodings for 582 selected IPv6 Extension Headers such that the UDP Header Compression 583 defined in Section 4.3 may be used in the presence of those extension 584 headers. 586 The LOWPAN_NHC encodings for IPv6 Extension Headers are composed of a 587 single LOWPAN_NHC octet followed by the IPv6 Extension Header. The 588 format of the LOWPAN_NHC octet is shown in Figure 6. The first 7 589 bits serve as an identifier for the IPv6 Extension Header immediately 590 following the LOWPAN_NHC octet. The remaining bit indicates whether 591 or not the following header utilizes LOWPAN_NHC encoding. 593 0 1 2 3 4 5 6 7 594 +---+---+---+---+---+---+---+---+ 595 | 1 | 1 | 1 | 0 | EID |NH | 596 +---+---+---+---+---+---+---+---+ 598 Figure 6: IPv6 Extension Header Encoding 600 EID: IPv6 Extension Header ID: 601 0: IPv6 Hop-by-Hop Options Header[RFC2460] 602 1: IPv6 Routing Header[RFC2460] 603 2: IPv6 Fragment Header[RFC2460] 604 3: IPv6 Destination Options Header[RFC2460] 605 4: IPv6 Mobility Header [RFC3775] 606 5: Reserved 607 6: Reserved 608 7: IPv6 Header 610 NH: Next Header: 611 0: Full 8 bits for Next Header are carried in-line. 612 1: The Next Header field is elided and the next header is encoded 613 using LOWPAN_NHC, which is discussed in Section 4. 615 For the most part, the IPv6 Extension Header is carried verbatim in 616 the bytes immediately following the LOWPAN_NHC octet, with two 617 important exceptions: Length Field and Next Header Field. 619 The Next Header Field contained in IPv6 Extension Headers is elided 620 when the NH bit is set in the LOWPAN_NHC encoding octet. Note that 621 doing so allows LOWPAN_NHC to utilize no more overhead than the non- 622 encoded IPv6 Extension Header. 624 The Length Field contained in IPv6 Extension Headers indicate the 625 length of the IPv6 Extension Header in octets, not including the 626 LOWPAN_NHC byte. Note that this changes the Length Field definition 627 in [RFC2460] from indicating the header size in 8-octet units, not 628 including the first 8 octets. Changing the Length Field to be in 629 units of octets removes wasteful internal fragmentation. However, 630 specifying units in octets also means that LOWPAN_NHC MUST NOT be 631 used to encode IPv6 Extension Headers that exceed 255 octets. 633 IPv6 Hop-by-Hop and Destination Options Headers may use Pad1 and PadN 634 to pad out the header for octet-alignment purposes. When using 635 LOWPAN_NHC, Pad1 and PadN options that appear at the end of the 636 options header MAY be elided. When converting from the LOWPAN_NHC 637 encoding back to the standard IPv6 encoding, Pad1 and PadN options 638 MUST be used to pad out the containing header to a multiple of 8 639 octets in length. Note that Pad1 and PadN options that appear in 640 locations other than the end MUST be carried in-line as they are used 641 to align subsequent options. 643 When the identified next header is an IPv6 Header (EID=7), the NH bit 644 of the LOWPAN_NHC encoding is unused and SHOULD be set to zero. The 645 following bytes MUST be encoded using LOWPAN_IPHC as defined in 646 Section 3. 648 4.3. UDP Header Compression 650 This document defines a compression format for UDP headers using 651 LOWPAN_NHC. The UDP compression format is shown in Figure 7. Bits 0 652 through 4 represent the NHC ID and '11110' indicates the specific UDP 653 header compression encoding defined in this section. 655 4.3.1. Compressing UDP ports 657 This specification introduces a range of well-known ports (0xF0Bx) 658 that can be compressed to 4 bits. Considering that this represents 659 only 16 contiguous ports, it can be expected that many incompatible 660 applications will use the same port numbers for their own end-to-end 661 needs. 663 The overloading of the 0xF0Bx ports increases the risk of getting the 664 wrong type of payload and misinterpreting the content compared to 665 ports that are reserved at IANA. As a result, it is recommended that 666 the use of those ports be associated with a mechanism such as a 667 Transport Layer Security (TLS) Message Integrity Check (MIC) that 668 validates that the content is expected and checked for integrity. 670 4.3.2. Compressing UDP checksum 672 The UDP checksum operation is mandatory with IPv6 [RFC2460] for all 673 packets. For that reason [RFC4944] disallows the compression of the 674 UDP checksum. 676 With this specification, a compressor in the source transport 677 endpoint MAY elide the UDP checksum if it is authorized by the Upper 678 Layer. The compressor SHOULD NOT set the C bit unless it has 679 received such authorization. The Upper Layer SHOULD only provide the 680 authorization in the following cases: 682 Tunneling: In this case, 6LoWPAN is deployed as a wireless pseudo- 683 fieldbus by tunneling existing field protocols over UDP. If the 684 tunneled PDU possesses its own addressing, security and integrity 685 check, the tunneling mechanism MAY authorize to elide the UDP 686 checksum in order to save on the encapsulation overhead. 687 Upper Layer Message Integrity Check: In this case, there is some 688 other form of integrity check in the UDP payload that covers at 689 least the same information as the UDP checksum (pseudo-header, 690 data) and has at least the same strength. 692 A forwarding node MAY imply authorization from an incoming packet if 693 the C bit is set. A forwarding node that cannot unambiguously derive 694 such authorization SHOULD NOT elide the UDP checksum when performing 695 6LoWPAN compression. The forwarding node that expands a 6LoWPAN 696 packet with the C bit on MUST compute the UDP checksum on behalf of 697 the source node and place that checksum in the restored UDP header as 698 specified in the incumbent standards [RFC0768], [RFC2460]. 700 If a 6LoWPAN termination is also the transport endpoint and it 701 receives a compressed packet with the C bit set, then it is entitled 702 to ignore the UDP checksum process completely. If the C bit is not 703 set, the packet might have been forwarded by an edge router, so this 704 is not an indication that the MIC is not present. If the terminating 705 node knows that the message integrity will be validated by the upper 706 layer by some state associated to the Service Access Point, it is 707 entitled to ignore the checksum operation as if the C bit was set. 709 4.3.3. UDP LOWPAN_NHC Format 711 0 1 2 3 4 5 6 7 712 +---+---+---+---+---+---+---+---+ 713 | 1 | 1 | 1 | 1 | 0 | C | P | 714 +---+---+---+---+---+---+---+---+ 716 Figure 7: UDP Header Encoding 718 C: Checksum: 719 0: All 16 bits of Checksum are carried in-line. 720 1: All 16 bits of Checksum are elided. The Checksum is recovered 721 by recomputing it on the 6LoWPAN termination point. 723 P: Ports: 724 00: All 16 bits for both Source Port and Destination Port are 725 carried in-line. 726 01: All 16 bits for Source Port are carried in-line. First 8 727 bits of Destination Port is 0xF0 and elided. The remaining 8 728 bits of Destination Port are carried in-line. 729 10: First 8 bits of Source Port are 0xF0 and elided. The 730 remaining 8 bits of Source Port are carried in-line. All 16 731 bits for Destination Port are carried in-line. 732 11: First 12 bits of both Source Port and Destination Port are 733 0xF0B and elided. The remaining 4 bits for each are carried 734 in-line. 736 Fields carried in-line (in part or in whole) appear in the same order 737 as they do in the UDP header format [RFC0768]. The UDP Length field 738 MUST always be elided and is inferred from lower layers using the 739 6LoWPAN Fragmentation header or the IEEE 802.15.4 header. 741 5. IANA Considerations 743 This document defines a new IPv6 header compression format for 744 6LoWPAN networks. The document allocates the following 32 Dispatch 745 type field values for LOWPAN_IPHC: 747 01 100000 748 through 749 01 111111 751 This assignment preempts the assignment of 01 111111 for ESC 752 [RFC4944], which is possible as no extension bytes have been 753 allocated yet that would enable the use of ESC. Instead, the value: 755 01 000000 757 is reserved as a replacement value for ESC, to be finally assigned 758 with the first assignment of extension bytes. 760 6. Security Considerations 762 The definition of LOWPAN_IPHC permits the compression of header 763 information on communication that could take place in its absence, 764 albeit in a less efficient form. It recognizes that a IEEE 802.15.4 765 PAN may have associated with it a number of prefixes through shared 766 context. How the shared context is assigned and managed is beyond 767 the scope of this document. 769 The overloading of the 0xF0Bx ports increases the risk of getting the 770 wrong type of payload and misinterpreting the content compared to 771 ports that reserved at IANA. It is thus recommended that the use of 772 those ports be associated with a mechanism such as a Transport Layer 773 Security (TLS) Message Integrity Check (MIC) that validates that the 774 content is expected and checked for integrity. 776 7. Acknowledgements 778 Thanks to Julien Abeille, Robert Assimiti, Dominique Barthel, Carsten 779 Bormann, Robert Cragie, Stephen Dawson-Haggerty, Mathilde Durvy, Erik 780 Nordmark, Christos Polyzois, Shoichi Sakane, Zach Shelby, Tony 781 Viscardi, and Jay Werb for useful design consideration and 782 implementation feedback. 784 8. Changes 786 Draft 08: 787 - Clarified that the lower bits of an IPv6 address may be derived 788 from an IPv6 header, not just an 802.15.4 header. Change text 789 from "derived from link-layer header" to "derived from 790 encapsulating header". 792 Draft 07: 793 - Added section on mapping link-layer addresses to IIDs. 794 - Added text on restricting compressed headers to first fragment 795 when using fragment headers defined in Section 5.3 of [RFC4944]. 796 - Minor editorial edits. 798 Draft 06: 799 - Reworked introduction. 800 - Added section on updates to [RFC4944]. 801 - Fixed description of number of bits used for IPHC encoding. 802 - Specify M=0 only for non-multicast addresses and M=1 only for 803 multicast addresses. 804 - Move 128-bit multicast encoding to DAC=0. 805 - Redefined ESC dispatch value to 01 000000. 806 - Many detailed edits. 808 Draft 05: 809 - Added LOWPAN_NHC encodings for IPv6 Extension Headers. 810 - Specify use of context 0 when CID is 0. 811 - Indicate that first 64-bits is link-local prefix padded with 812 zeros when link-local prefix is elided. 813 - Made prefix-based multicast encoding format more explicit for 814 clarity. 815 - Changed wording around stateful compression to allow for using 816 the in-line bits as an additional index to identify the compressed 817 address. 818 - Removed support for compressing unspecified address. 819 - Full 128-bit addr in-line only in stateless encoding. 821 Draft 04: 822 - Fixed typos leftover from the changes in 03. 823 - Gave more details on UDP checksum compression. 824 - Clarify that the context information is out of scope. 825 - Added security concern on 0xF0Bx port overloading. 827 Draft 03: 828 - Decoupled meaning of SAM bits from the destination address. 829 - Have separate bit to indicate multicast address compression. 830 - More extensive support for multicast address compression, 831 including Unicast-Prefix-based Multicast Addresses. 833 Draft 02: 834 - Updated wording with compression mode to clarify that a 835 compression mode does not enforce what kind of destination address 836 is being used. Specifically changed Destination Dependent Field 837 to Compression Mode. 839 - Specify that the configuration and management of contexts is out 840 of scope of this document. 842 Draft 01: 843 - HC back to 1 byte by default by stealing a few bits from the 844 dispatch field. 845 - Added better support for multicast address compression. 846 - Fixed alignment for UDP port compression. 847 - Better support for Traffic Class and Flow Label compression. 848 - Pascal joined as an author. 850 9. References 852 9.1. Normative References 854 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 855 August 1980. 857 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 858 Requirement Levels", BCP 14, RFC 2119, March 1997. 860 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 861 (IPv6) Specification", RFC 2460, December 1998. 863 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 864 "Definition of the Differentiated Services Field (DS 865 Field) in the IPv4 and IPv6 Headers", RFC 2474, 866 December 1998. 868 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 869 of Explicit Congestion Notification (ECN) to IP", 870 RFC 3168, September 2001. 872 [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support 873 in IPv6", RFC 3775, June 2004. 875 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 876 Architecture", RFC 4291, February 2006. 878 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 879 "Transmission of IPv6 Packets over IEEE 802.15.4 880 Networks", RFC 4944, September 2007. 882 9.2. Informative References 884 [IEEE 802.15.4] 885 IEEE Computer Society, "IEEE Std. 802.15.4-2006", 886 October 2006. 888 [RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 889 Multicast Addresses", RFC 3306, August 2002. 891 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 892 and M. Carney, "Dynamic Host Configuration Protocol for 893 IPv6 (DHCPv6)", RFC 3315, July 2003. 895 [RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous 896 Point (RP) Address in an IPv6 Multicast Address", 897 RFC 3956, November 2004. 899 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 900 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 901 September 2007. 903 Authors' Addresses 905 Jonathan W. Hui (editor) 906 Arch Rock Corporation 907 501 2nd St. Ste. 410 908 San Francisco, California 94107 909 USA 911 Phone: +415 692 0828 912 Email: jhui@archrock.com 914 Pascal Thubert 915 Cisco Systems 916 Village d'Entreprises Green Side 917 400, Avenue de Roumanille 918 Batiment T3 919 Biot - Sophia Antipolis 06410 920 FRANCE 922 Phone: +33 4 97 23 26 34 923 Email: pthubert@cisco.com