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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Lars-Erik Jonsson, Ericsson 3 INTERNET-DRAFT Ghyslain Pelletier, Ericsson 4 Expires: March 2004 5 September 25, 2003 7 RObust Header Compression (ROHC): 8 A Compression Profile for IP 9 11 Status of this memo 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that other 18 groups may also distribute working documents as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or cite them other than as "work in progress". 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/lid-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html 31 This document is a submission of the IETF ROHC WG. Comments should be 32 directed to the ROHC WG mailing list, rohc@ietf.org. 34 Abstract 36 The original RObust Header Compression (ROHC) RFC, RFC 3095, defines 37 a framework for header compression, along with compression protocols 38 (profiles) for IP/UDP/RTP, IP/ESP, IP/UDP, and also a profile for 39 uncompressed packet streams. However, no profile was defined for 40 compression of IP only, which has been identified as a missing piece 41 in RFC 3095. This document defines a ROHC compression profile for IP, 42 similar to the IP/UDP profile defined by RFC 3095, but simplified to 43 exclude UDP, and enhanced to compress IP header chains of arbitrary 44 length. 46 Table of Contents 48 1. Introduction..................................................2 49 2. Terminology...................................................3 50 3. ROHC IP Compression (Profile 0x0004)..........................3 51 3.1. Static chain termination...............................3 52 3.2. Handling multiple levels of IP headers.................3 53 3.3. Constant IP-ID.........................................4 54 3.4. Additional mode transition logic.......................6 55 3.5. Initialization.........................................7 56 3.6. Packet types...........................................8 57 3.7. The CONTEXT_MEMORY feedback option.....................9 58 4. Security Considerations.......................................9 59 5. IANA Considerations...........................................9 60 6. Intellectual Property Right Claim Considerations.............10 61 7. Acknowledgements.............................................10 62 8. References...................................................10 63 9. Authors' Addresses...........................................11 64 Appendix A. Detailed procedures for canceling mode transitions..12 65 A.1. Transition from Optimistic to Reliable mode................12 66 A.2. Transition from Unidirectional to Reliable mode............13 67 A.3. Transition from Reliable to Optimistic mode................13 68 A.4. Transition back to Unidirectional mode.....................14 70 1. Introduction 72 The original RObust Header Compression (ROHC) RFC [RFC-3095] defines 73 a framework for header compression, along with compression protocols 74 (profiles) for IP/UDP/RTP, IP/ESP, IP/UDP, and also a profile for 75 uncompressed packet streams. The profile for uncompressed data was 76 defined to provide means to encapsulate all traffic over a link 77 within ROHC packets. Through this profile, the lower layers do not 78 have to provide multiplexing for different packet types, but instead 79 ROHC can handle any packet stream, even if compression profiles for 80 all kinds of packet streams have yet not been defined or implemented 81 over the link. 83 Although the profile without compression is simple and can tunnel 84 arbitrary packets, it has of course a major weakness in that it does 85 not compress the headers at all. When considering that normally all 86 packets are expected to be IP [RFC-791, RFC-2460] packets, and that 87 the IP header often represent a major part of the total header, a 88 useful alternative to no compression would for most packets be 89 compression of the IP header only. Unfortunately, such a profile was 90 not defined in [RFC-3095], and this has thus been identified as an 91 important missing piece in the ROHC toolbox. 93 This document addresses this missing compression support and defines 94 a ROHC compression profile for IP [RFC-791, RFC-2460] only, similar 95 to the IP/UDP profile defined by [RFC-3095], but simplified to 96 exclude UDP. Due to the similarities with the IP/UDP profile, the IP 97 compression profile is described based on the IP/UDP profile, mainly 98 covering differences. The most important differences are a different 99 way of terminating the static header chain, and the capability to 100 compress IP header chains of arbitrary length. 102 2. Terminology 104 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 105 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 106 document are to be interpreted as described in RFC 2119 [RFC-2119]. 108 ROHC UDP 110 "ROHC UDP" in this document refers to the IP/UDP profile 111 (Profile 0x0002) as defined in [RFC-3095]. 113 3. ROHC IP Compression (Profile 0x0004) 115 In general, there are no major difference between the ROHC UDP 116 profile and the IP profile (ROHC IP) defined in this document, since 117 the removal of UDP has no impact on the compression mechanisms. As 118 for ROHC UDP, the compressor generates a 16-bit sequence number which 119 increases by one for each packet compressed in the packet stream, 120 simply called SN below. The most important difference between this 121 profile and ROHC UDP is about static chain termination and handling 122 of multiple IP headers. Unless stated explicitly below, mechanisms 123 and formats are as for ROHC UDP. 125 3.1. Static chain termination 127 One difference for IP-only compression, compared to IP/UDP 128 compression, is related to the termination of the static chain in IR 129 headers. For the UDP profile, the chain always ends with a UDP header 130 part, which per definition provides the boundaries for the chain. The 131 UDP header is also the last header in the uncompressed packet (except 132 for potential application header). For the IP-only profile, there is 133 no single last header that per profile definition terminates the 134 chain. Instead, the static chain is terminated if the "Next Header / 135 Protocol" field of a static IP header part indicates anything but IP 136 (IPinIP or IPv6). 138 3.2. Handling multiple levels of IP headers 140 The ROHC IR and IR-DYN packets defined in [RFC-3095] are used to 141 communicate static and/or dynamic parts of a context. For each of the 142 compression profiles defined in [RFC-3095], there is a single last 143 header in the header chain that clearly marks the termination of the 144 static chain. The length of the dynamic chain is then inferred from 145 the static chain in the IR header itself, or from the static chain in 146 the context for the IR-DYN header. The length of both static and 147 dynamic chains may thus be of arbitrary length and may, in theory, 148 initialize a context with an arbitrary number of IP levels. 150 However, the general compressed header formats defined in [RFC-3095, 151 section 5.7.] specifies that at most two levels of IP headers (the 152 'Inner' and the 'Outer' level of IP headers) may be included in a 153 compressed header. Specifically, the format defined for Extension 3 154 [RFC-3095, section 5.7.5.] can only carry the 'Outer' IP header. In 155 addition, while list compression may be used to compress other types 156 of headers, it cannot be used to compress additional IP headers as IP 157 headers may not be part of an extension header chain in compressed 158 headers [ROHC, section 5.8.]. 160 For the compression profiles defined in [RFC-3095], the consequence 161 is that at most two levels of IP headers can be compressed. In other 162 words, the presence of additional IP headers at best partially 163 disables header compression and compression will not go beyond the IR 164 state (as only IR and IR-DYNs can be used in such case). 166 For the compression of IP headers only, the additional IP headers 167 would however not have to cause header compression to be disabled 168 because there is no single packet type that ends the compressed 169 chain. The excess IP headers could simply be left uncompressed by 170 implicitly terminating the static and dynamic chains after at most 171 two levels of IP headers. 173 The IP-only profile defined in this document takes one step further 174 and supports compression of an arbitrary number of IP levels. This is 175 achieved by adding a dynamic chain to the general format of 176 compressed headers, to include the header part of each IP level in 177 excess of the first two. 179 As explained above, the static chain within IR packets can be of 180 arbitrary length, and the chain is terminated by the presence of a 181 non-IP header (not IPinIP nor IPv6). The dynamic chain is structured 182 analogously. 184 For compressed headers, the information related to the initial two IP 185 headers is carried as for the IP/UDP profile, and a chain of dynamic 186 header information is added to the end of the compressed header for 187 each and every additional IP header. This additional data structure 188 is thus exactly the same as the one used in IR and IR-DYN packets. 189 The length of the chain is inferred from the chain of static 190 parameters in the context. While a dynamic chain carries dynamically 191 changing parameters using an uncompressed representation, this 192 ensures that flows with arbitrary levels of IP headers will not 193 impair compression efficiency. 195 3.3. Constant IP-ID 197 Most IPv4 stacks assigns IP-ID according to the value of a counter 198 increasing by one for each outgoing packet. ROHC UDP compresses the 199 IP-ID field using offset IP-ID encoding based on the UDP SN [RFC- 200 3095]. For stacks generating IP-ID values using a pseudo-random 201 number generator, the field is not compressed and is sent as-is in 202 its entirety as additional octets after the compressed header. 204 Cases have also been found where an IPv4 stack uses a constant value 205 for the IP Identifier. When the IP-ID field is constant, it cannot be 206 compressed using offset IP-ID encoding and the field must be sent in 207 its entirety. This overhead can be avoided with the addition of a 208 flag within the dynamic part of the chain used to initialize the IPv4 209 header, as follow: 211 Dynamic part: 213 +---+---+---+---+---+---+---+---+ 214 | Type of Service | 215 +---+---+---+---+---+---+---+---+ 216 | Time to Live | 217 +---+---+---+---+---+---+---+---+ 218 / Identification / 2 octets 219 +---+---+---+---+---+---+---+---+ 220 | DF|RND|NBO|SID| 0 | 221 +---+---+---+---+---+---+---+---+ 222 / Generic extension header list / variable length 223 +---+---+---+---+---+---+---+---+ 225 SID: Static IP Identifier. 227 For IR and IR-DYN packets, the logic is the same as for ROHC UDP 228 with the addition that field(SID) must be kept in the context. 230 For compressed headers other than IR and IR-DYN: 232 If value(RND) = 0 and context(SID) = 0, hdr(IP-ID) is 233 compressed using Offset IP-ID encoding (see [RFC-3095 section 234 4.5.5]) using p = 0 and default-slope(IP-ID offset) = 0. 236 If value(RND) = 0 and context(SID) = 1, hdr(IP-ID) is constant 237 and compressed away; hdr(IP-ID) is the value of context(IP-ID). 239 If value(RND) = 1, IP-ID is the uncompressed hdr(IP-ID). IP-ID 240 is then passed as additional octets at the end of the 241 compressed header, after any extensions. 243 Note: Only IR and IR-DYN packets can update context(SID). 245 Note: All other fields are the same as for ROHC UDP [RFC-3095]. 247 3.4. Additional mode transition logic 249 The profiles defined in [ROHC] operate using different modes of 250 compression. A mode transition can be requested once a packet has 251 reached the decompressor by sending feedback indicating the desired 252 mode. As per the specifications found in [ROHC], the compressor is 253 compelled to honor such request. 255 For the IP profile defined in this document, the Mode parameter for 256 the value mode = 0 (packet types UOR-2, IR and IR-DYN) is redefined 257 to allow the compressor to decline a mode transition requested by the 258 decompressor: 260 Mode: Compression mode. 0 = (C)ancel Mode Transition 262 Upon receiving the Mode parameter set to '0', the decompressor MUST 263 stay in its current mode of operation and SHOULD refrain from sending 264 further mode transition requests for the declined mode for a certain 265 amount of time. 267 More specifically, with reference to the parameters C_TRANS, C_MODE, 268 D_TRANS and D_MODE defined in [ROHC, section 5.6.1.], the following 269 modifications apply when the compressor cancels a mode transition: 271 Parameters for the compressor side: 273 - C_MODE: 274 This value must not be changed when sending mode information 275 within packets when the mode parameter set to '0' (as a 276 response to a mode transition request from the decompressor). 278 - C_TRANS: 279 C_TRANS is (P)ending when receiving a mode transition request 280 from the decompressor. C_TRANS is set to (D)one when the 281 compressor receives an ACK for a UOR-2, IR-DYN, or IR packet 282 sent with the mode parameter set to the mode in use at the time 283 when the mode transition request was initiated. 285 Parameters for the decompressor side: 287 - D_MODE: 288 D_MODE MUST remain unchanged when receiving a UOR-2, an IR-DYN, 289 or an IR packet sent with the mode parameter set to '0'. 291 - D_TRANS: 292 D_TRANS is (P)ending when a UOR-2, IR-DYN, or IR packet sent 293 with the mode parameter set to '0' is received. It is set to 294 (D)one when a packet of type 1 or 0 corresponding to the 295 unchanged mode is received. 297 The resulting mode transition procedure is described below: 299 Compressor Decompressor 300 ---------------------------------------------- 301 C_MODE = X | | D_MODE = X 302 | Mode Request(Y) +-<-<-<-| D_TRANS = I 303 | +-<-<-<-<-<-<-<-+ | 304 C_TRANS = P |-<-<-<-+ | 305 C_MODE = X | | 306 |->->->-+ IR/IR-DYN/UOR-2(SN,C) | 307 | +->->->->->->->-+ | 308 |->-.. +->->->-| D_TRANS = P 309 |->-.. | D_MODE = X 310 | ACK(SN,X) +-<-<-<-| 311 | +-<-<-<-<-<-<-<-+ | 312 C_TRANS = D |-<-<-<-+ | 313 | | 314 |->->->-+ X-0, X-1* | 315 | +->->->->->->->-+ | 316 | +->->->-| D_TRANS = D 317 | | 319 where X: mode in use before the mode transition was initiated 320 Y: mode requested by the decompressor 321 C: (C)ancel mode transition 323 3.5. Initialization 325 The static context for ROHC IP compression can be initialized in 326 either of two ways: 328 1) By using an IR packet as in ROHC UDP, where the profile is 329 0x0004, and the static chain ends with the static part of an 330 IP header, where the Next Header/Protocol field has any value but 331 IPinIP (4) or IPv6 (41) [PROTOCOL]. At the compressor, SN is 332 initialized to a random value when the first IR packet is sent. 334 2) By reusing an existing context. This is done with an IR-DYN 335 packet, identifying profile 0x0004, where the dynamic chain 336 corresponds to the prefix of the existing static chain, ending 337 with an IP header where the Next Header/Protocol field has any 338 value but IPinIP (4) or IPv6 (41) [PROTOCOL]. At the compressor, 339 SN is initialized to a random value when the first IR-DYN packet 340 is sent. 342 For ROHC IP, the dynamic part of an IR or IR-DYN packet is similar to 343 the one for ROHC UDP, with a two-octet field containing the SN 344 present at the end of the dynamic chain in IR and IR-DYN packets. It 345 should be noted that the static and dynamic chains have an arbitrary 346 length, and the SN is added only once, at the end of the dynamic 347 chain in IR and IR-DYN packets. 349 3.6. Packet types 351 Except for one new feedback option (see section 3.7), the only packet 352 format that differs from ROHC UDP is the general format for 353 compressed packets, which has no UDP checksum in the end. Instead, it 354 ends with a list of dynamic header portions, one for each IP header 355 above the initial two (if any, as indicated by the presence of 356 corresponding header portions in the static chain). 358 The general format for a compressed header is thus as follows: 360 0 1 2 3 4 5 6 7 361 --- --- --- --- --- --- --- --- 362 : Add-CID octet : | 363 +---+---+---+---+---+---+---+---+ | 364 | first octet of base header | | 365 +---+---+---+---+---+---+---+---+ | 366 : : | 367 / 0, 1, or 2 octets of CID / | 368 : : | 369 +---+---+---+---+---+---+---+---+ | 370 / remainder of base header / | 371 +---+---+---+---+---+---+---+---+ | 372 : : | 373 / Extension / | 374 : : | 375 --- --- --- --- --- --- --- --- | 376 : : | 377 + IP-ID of outer IPv4 header + 378 : : (see section 5.7 of [RFC-3095]) 379 --- --- --- --- --- --- --- --- 380 / AH data for outer list / | 381 --- --- --- --- --- --- --- --- | 382 : : | 383 + GRE checksum + | 384 : : | 385 --- --- --- --- --- --- --- --- | 386 : : | 387 + IP-ID of inner IPv4 header + | 388 : : | 389 --- --- --- --- --- --- --- --- | 390 / AH data for inner list / | 391 --- --- --- --- --- --- --- --- | 392 : : | 393 + GRE checksum + | 394 : : | 395 --- --- --- --- --- --- --- --- 396 : List of : 397 / Dynamic chains / variable, given by static chain 398 : for additional IP headers : (includes no SN) 399 --- --- --- --- --- --- --- --- 401 Note that the list of dynamic chains for the additional IP headers in 402 compressed packets do not have a sequence number at the end of the 403 chain, as SN is present within compressed base headers. 405 3.7. The CONTEXT_MEMORY feedback option 407 The CONTEXT_MEMORY option informs the compressor that the 408 decompressor does not have sufficient memory resources to handle the 409 context of the packet stream, as the stream is currently compressed. 411 0 1 2 3 4 5 6 7 412 +---+---+---+---+---+---+---+---+ 413 | Opt Type = 9 | Opt Len = 0 | 414 +---+---+---+---+---+---+---+---+ 416 When receiving a CONTEXT_MEMORY option, the compressor should take 417 actions to compress the packet stream in a way that requires less 418 decompressor memory resources, or stop compressing the packet stream. 420 4. Security Considerations 422 The security considerations of [RFC-3095] apply equally to this 423 document, without exceptions or additions. 425 5. IANA Considerations 427 ROHC profile identifier 0x0004 has been reserved by the IANA for the 428 profile defined in this document. 430 { NOTE TO IANA - TO BE REMOVED BEFORE PUBLICATION } 432 A ROHC profile identifier must be reserved by the IANA for the 433 profile defined in this document. Profile number 0x0004 has 434 previously been saved for this purpose, and should thus be used. 435 As for previous ROHC profiles, profile numbers 0xnn04 must also be 436 reserved for future variants of this profile. A suggested 437 registration in the "RObust Header Compression (ROHC) Profile 438 Identifiers" name space would then be: 440 OLD: 0xnn04 To be Assigned by IANA 442 NEW: 0x0004 ROHC IP [RFCXXXX (this)] 443 0xnn04 Reserved 445 { END OF NOTE } 447 6. Intellectual Property Right Claim Considerations 449 The IETF has been notified of intellectual property rights claimed in 450 regard to some or all of the specification contained in this 451 document. For more information consult the online list of claimed 452 rights. 454 The IETF takes no position regarding the validity or scope of any 455 intellectual property or other rights that might be claimed to 456 pertain to the implementation or use of the technology described in 457 this document or the extent to which any license under such rights 458 might or might not be available; neither does it represent that it 459 has made any effort to identify any such rights. Information on the 460 IETF's procedures with respect to rights in standards-track and 461 standards-related documentation can be found in BCP-11. Copies of 462 claims of rights made available for publication and any assurances of 463 licenses to be made available, or the result of an attempt made to 464 obtain a general license or permission for the use of such 465 proprietary rights by implementors or users of this specification can 466 be obtained from the IETF Secretariat. 468 The IETF invites any interested party to bring to its attention any 469 copyrights, patents or patent applications, or other proprietary 470 rights which may cover technology that may be required to practice 471 this standard. Please address the information to the IETF Executive 472 Director. 474 7. Acknowledgements 476 The authors would like to thank Carsten Bormann, Fredrik Lindstrom, 477 Kristofer Sandlund and Mark West for valuable input and review. 479 8. References 481 [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate 482 Requirement Levels", RFC 2119, March 1997. 484 [RFC-3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, 485 H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., 486 Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, 487 K., Wiebke, T., Yoshimura, T. and H. Zheng, "Robust 488 Header Compression (ROHC)", RFC 3095, July 2001. 490 [RFC-791] Postel, J., "Internet Protocol", RFC 791, September 1981. 492 [RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 493 (IPv6) Specification", RFC 2460, December 1998. 494 [PROTOCOL] "Assigned Internet Protocol Numbers", IANA registry at: 495 http://www.iana.org/assignments/protocol-numbers 497 9. Authors' Addresses 499 Lars-Erik Jonsson 500 Ericsson AB 501 Box 920 502 SE-971 28 Lulea, Sweden 504 Phone: +46 920 20 21 07 505 Fax: +46 920 20 20 99 506 Email: lars-erik.jonsson@ericsson.com 508 Ghyslain Pelletier 509 Box 920 510 Ericsson AB 511 SE-971 28 Lulea, Sweden 513 Phone: +46 920 20 24 32 514 Fax: +46 920 20 20 99 515 Email: ghyslain.pelletier@ericsson.com 517 Appendix A. Detailed procedures for canceling mode transitions 519 The profiles defined in [ROHC] operate using different modes of 520 compression: Unidirectional (U-Mode), Bi-directional Optimistic (O- 521 Mode) and Bi-directional Reliable (R-Mode). Compression always starts 522 in the U-Mode, and mode transitions can only be initiated by the 523 decompressor [ROHC, section 5.6.]. A mode transition can be requested 524 once a packet has reached the decompressor by sending feedback 525 indicating the desired mode. 527 With reference to the parameters C_TRANS, C_MODE, D_TRANS and D_MODE 528 defined in [ROHC, section 5.6.1.], the following sub-sections 529 describe the resulting procedures when a compressor declines a mode 530 transition request from the decompressor as described in section 3.4. 532 A.1. Transition from Optimistic to Reliable mode 534 When the decompressor initiates a mode transition from Optimistic to 535 Reliable mode, the cancellation of the transition procedure is 536 described as follows: 538 Compressor Decompressor 539 ---------------------------------------------- 540 | | 541 | ACK(R)/NACK(R) +-<-<-<-| D_TRANS = I 542 | +-<-<-<-<-<-<-<-+ | 543 C_TRANS = P |-<-<-<-+ | 544 C_MODE = O | | 545 |->->->-+ IR/IR-DYN/UOR-2(SN,C) | 546 | +->->->->->->->-+ | 547 |->-.. +->->->-| D_TRANS = P 548 |->-.. | D_MODE = O 549 | ACK(SN,O) +-<-<-<-| 550 | +-<-<-<-<-<-<-<-+ | 551 C_TRANS = D |-<-<-<-+ | 552 | | 553 |->->->-+ UO-0, UO-1* | 554 | +->->->->->->->-+ | 555 | +->->->-| D_TRANS = D 557 The compressor must not send packet types 1 or 0 when C_TRANS is P, 558 i.e. not until it has received an ACK for a UOR-2, IR-DYN, or IR 559 packet sent with the mode transition parameter set to C. When the 560 decompressor receives a UOR-2, IR-DYN, or IR packet sent with the 561 mode transition parameter set to C, it must keep the value D_MODE to 562 O and it sets D_TRANS to P. When the decompressor receives packet 563 types 0 or 1, after having ACKed a UOR-2, IR-DYN, or IR packet, it 564 sets D_TRANS to D. 566 A.2. Transition from Unidirectional to Reliable mode 568 The cancellation of a transition from Unidirectional to Reliable mode 569 follows the same procedure as defined in section 4.2 above. 571 A.3. Transition from Reliable to Optimistic mode 573 When the decompressor initiates a mode transition from Reliable to 574 Optimistic mode, the cancellation of the transition procedure is 575 described as follows: 577 Compressor Decompressor 578 ---------------------------------------------- 579 | | 580 | ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I 581 | +-<-<-<-<-<-<-<-+ | 582 C_TRANS = P |-<-<-<-+ | 583 C_MODE = R | | 584 |->->->-+ IR/IR-DYN/UOR-2(SN,C) | 585 | +->->->->->->->-+ | 586 |->-.. +->->->-| D_MODE = R 587 |->-.. | 588 | ACK(SN,R) +-<-<-<-| 589 | +-<-<-<-<-<-<-<-+ | 590 C_TRANS = D |-<-<-<-+ | 591 | | 592 |->->->-+ R-0, R-1* | 593 | +->->->->->->->-+ | 594 | +->->->-| D_TRANS = D 595 | | 597 The compressor must not send packet types 1 or 0 when C_TRANS is P, 598 i.e. not until it has received an ACK for a UOR-2, IR-DYN, or IR 599 packet sent with the mode transition parameter set to C. When the 600 decompressor receives a UOR-2, IR-DYN, or IR packet sent with the 601 mode transition parameter set to C, it must keep the value D_MODE to 602 R. When the decompressor receives packet types 0 or 1, after having 603 ACKed a UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D. 605 A.4. Transition back to Unidirectional mode 607 When the decompressor initiates a mode transition from Reliable or 608 Optimistic mode back to Unidirectional mode, the cancellation of the 609 transition procedure is described as follows: 611 Compressor Decompressor 612 ---------------------------------------------- 613 | | 614 | ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I 615 | +-<-<-<-<-<-<-<-+ | 616 C_TRANS = P |-<-<-<-+ | 617 C_MODE = O/R| | 618 |->->->-+ IR/IR-DYN/UOR-2(SN,C) | 619 | +->->->->->->->-+ | 620 |->-.. +->->->-| 621 |->-.. | 622 | ACK(SN,O/R) +-<-<-<-| 623 | +-<-<-<-<-<-<-<-+ | 624 C_TRANS = D |-<-<-<-+ | 625 | R-0, R-1* or | 626 |->->->-+ UO-0, UO-1* | 627 | +->->->->->->->-+ | 628 | +->->->-| D_TRANS = D 629 D_MODE = O/R 631 When the decompressor receives a UOR-2, IR-DYN, or IR packet sent 632 with the mode transition parameter set to C, it must keep the value 633 D_MODE to the bi-directional mode already in use (either O- or R- 634 mode). After ACKing the first UOR-2(C), IR-DYN(C), or IR(C), the 635 decompressor MUST continue to send feedback with the Mode parameter 636 set to the bi-directional mode in use (either O- or R-mode) until it 637 receives packet types 0 or 1. When the decompressor receives packet 638 types 0 or 1, after having ACKed a UOR-2, IR-DYN, or IR packet, it 639 sets D_TRANS to D. 641 Full Copyright Statement 643 Copyright (C) The Internet Society (2003). All Rights Reserved. 645 This document and translations of it may be copied and furnished to 646 others, and derivative works that comment on or otherwise explain it 647 or assist in its implementation may be prepared, copied, published 648 and distributed, in whole or in part, without restriction of any 649 kind, provided that the above copyright notice and this paragraph are 650 included on all such copies and derivative works. However, this 651 document itself may not be modified in any way, such as by removing 652 the copyright notice or references to the Internet Society or other 653 Internet organizations, except as needed for the purpose of 654 developing Internet standards in which case the procedures for 655 copyrights defined in the Internet Standards process must be 656 followed, or as required to translate it into languages other than 657 English. 659 The limited permissions granted above are perpetual and will not be 660 revoked by the Internet Society or its successors or assigns. 662 This document and the information contained herein is provided on an 663 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 664 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 665 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 666 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 667 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 669 This Internet-Draft expires March 25, 2004.