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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Stewart 3 Internet-Draft M. Ramalho 4 Expires: February 20, 2004 Cisco Systems, Inc. 5 Q. Xie 6 Motorola, Inc. 7 M. Tuexen 8 Univ. of Applied Sciences Muenster 9 P. Conrad 10 Temple University 11 August 22, 2003 13 SCTP Partial Reliability Extension 14 draft-ietf-tsvwg-prsctp-01.txt 16 Status of this Memo 18 This document is an Internet-Draft and is in full conformance with 19 all provisions of Section 10 of RFC2026. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that other 23 groups may also distribute working documents as Internet-Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at http:// 31 www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on February 20, 2004. 38 Copyright Notice 40 Copyright (C) The Internet Society (2003). All Rights Reserved. 42 Abstract 44 This memo describes an extension to the Stream Control Transmission 45 Protocol (SCTP) RFC2960 [5] that allows an SCTP endpoint to signal to 46 its peer that it should move the cumulative ack point forward. When 47 both sides of an SCTP association support this extension, it can be 48 used by an SCTP implementation to provide partially reliable data 49 transmission service to an upper layer protocol. This memo describes 50 (1) the protocol extensions, which consist of a new parameter for 51 INIT and INIT ACK, and a new FORWARD TSN chunk type (2) one example 52 partially reliable service that can be provided to the upper layer 53 via this mechanism. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 1.1 Overview of Protocol Extensions . . . . . . . . . . . . . . 3 59 1.2 Overview of New Services Provided to the Upper Layer . . . . 3 60 1.3 Benefits of PR-SCTP . . . . . . . . . . . . . . . . . . . . 4 61 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 6 62 3. Protocol Changes to support PR-SCTP . . . . . . . . . . . . 7 63 3.1 Forward-TSN-Supported Parameter For INIT and INIT ACK . . . 7 64 3.2 Forward Cumulative TSN Chunk Definition (FORWARD TSN) . . . 7 65 3.3 Negotiation of Forward-TSN-Supported parameter . . . . . . . 8 66 3.3.1 Sending Forward-TSN-Supported param in INIT . . . . . . . . 8 67 3.3.2 Receipt of Forward-TSN-Supported param in INIT or 68 INIT-ACK . . . . . . . . . . . . . . . . . . . . . . . . . . 9 69 3.3.3 Receipt of Op. Error for Forward-TSN-Supported Param . . . . 9 70 3.4 Definition of "abandoned" in the context of PR-SCTP . . . . 10 71 3.5 Sender Side Implementation of PR-SCTP . . . . . . . . . . . 10 72 3.6 Receiver Side Implementation of PR-SCTP . . . . . . . . . . 13 73 4. Services provided by PR-SCTP to the upper layer . . . . . . 16 74 4.1 PR-SCTP Service Definition for "timed reliability" . . . . . 16 75 4.2 PR-SCTP Association Establishment . . . . . . . . . . . . . 18 76 4.3 Guidelines for defining other PR-SCTP Services . . . . . . . 19 77 4.4 Usage Notes . . . . . . . . . . . . . . . . . . . . . . . . 20 78 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21 79 6. Security Considerations . . . . . . . . . . . . . . . . . . 22 80 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 23 81 References . . . . . . . . . . . . . . . . . . . . . . . . . 24 82 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 24 83 Intellectual Property and Copyright Statements . . . . . . . 26 85 1. Introduction 87 This memo describes an extension to the Stream Control Transmission 88 Protocol (SCTP) RFC2960 [5] that allows an SCTP sender to signal to 89 its peer that it should no longer expect to receive one or more DATA 90 chunks. 92 1.1 Overview of Protocol Extensions 94 The protocol extension described in this document consists of two new 95 elements: 97 1. a single new parameter in the INIT/INIT-ACK exchange that 98 indicates whether the endpoint supports the extension 100 2. a single new chunk type, FORWARD TSN, that indicates that the 101 receiver should move its cumulative ack point forward (possibly 102 skipping past one or more Data chunks that may not yet have been 103 received and/or acknowledged.) 105 1.2 Overview of New Services Provided to the Upper Layer 107 When this extension is supported by both sides of an SCTP 108 association, it can be used to provide partially reliable transport 109 service over an SCTP association. We define partially reliable 110 transport service as a service that allows the user to specify, on a 111 per message basis, the rules governing how persistent the transport 112 service should be in attempting to send the message to the receiver. 114 One example of partially reliable service is specified in this 115 document, namely a "timed reliability" service. This service allows 116 the service user to indicate a limit on the duration of time that the 117 sender should try to transmit/retransmit the message (this is a 118 natural extension of the "lifetime" parameter already in the base 119 protocol). 121 In addition to this example, we will also show that defining the 122 semantics of a particular partially reliable service involves two 123 elements, namely: 125 1. how the service user indicates the level of reliability required 126 for a particular message, and 128 2. how the sender side implementation uses that reliability level to 129 determine when to give up on further retransmissions of that 130 message. 132 Note that other than the fact that the FORWARD-TSN chunk is required, 133 neither of these two elements impacts the "on-the-wire" protocol; 134 only the API, and the sender side implementation is affected by the 135 way in which the service is defined to the upper layer. Therefore, 136 in principle, it is feasible to implement many varieties of partially 137 reliable services in a particular SCTP implementation without 138 changing the on-the-wire protocol. Also, the SCTP receiver does not 139 necessarily need to know which semantics of partially reliable 140 service are being used by the sender, since the receiver's only role 141 is to correctly interpret FORWARD TSN chunks, thereby skipping past 142 messages that the sender has decided to no longer transmit (or 143 retransmit). 145 Nevertheless, it is recommended that a limited number of standard 146 definitions of partially reliable services be standardized by the 147 IETF so that that the designers of IETF application layer protocols 148 can match the requirements of their upper layer protocols to standard 149 service definitions provided by a particular SCTP implementation. 150 One such definition, "timed reliability" is included in this 151 document. Given the extensions proposed in this document, other 152 definitions may be standardized as the need arises without further 153 changes to the on-the-wire protocol. 155 1.3 Benefits of PR-SCTP 157 Hereafter, we use the notation "PR-SCTP" to refer to the SCTP 158 protocol extended as defined in this document. 160 The following are some of the advantages for integrating partially 161 reliable data service into SCTP, i.e., benefits of PR-SCTP: 163 1. Some application layer protocols may benefit from being able to 164 use a single SCTP association to carry both reliable content, -- 165 such as text pages, billing and accounting information, setup 166 signaling -- and unreliable content, e.g. state that is highly 167 sensitive to timeliness, where generating a new packet is more 168 advantageous than transmitting an old one [1]. 170 2. Partially reliable data traffic carried by PR-SCTP will enjoy the 171 same communication failure detection and protection capabilities 172 as the normal reliable SCTP data traffic does. This includes the 173 ability to: - quickly detect a failed destination address; - 174 fail-over to an alternate destination address, and; - be notified 175 if the data receiver becomes unreachable. 177 3. In addition to providing unordered unreliable data transfer as 178 UDP does, PR-SCTP can provide ordered unreliable data transfer 179 service. 181 4. PR-SCTP employs the same congestion control and congestion 182 avoidance for all data traffic, whether reliable or partially 183 reliable - this is very desirable since SCTP enforces 184 TCP-friendliness (unlike UDP.) 186 5. Because of the chunk bundling function of SCTP, reliable and 187 unreliable messages can be multiplexed over a single PR-SCTP 188 association. Therefore, the number of IP datagrams (and hence 189 the network overhead) can be reduced versus having to send these 190 different types of data using separate protocols. Additionally, 191 this multiplexing allows for port savings versus using different 192 ports for reliable and unreliable connections. 194 2. Conventions 196 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, 197 SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when 198 they appear in this document, are to be interpreted as described in 199 RFC2119 [3]. 201 Comparisons and arithmetic on TSNs are governed by the rules in 202 Section 1.6 of RFC2960 [5]. 204 3. Protocol Changes to support PR-SCTP 206 3.1 Forward-TSN-Supported Parameter For INIT and INIT ACK 208 The following new OPTIONAL parameter is added to the INIT and INIT 209 ACK chunks. 211 Parameter Name Status Type Value 212 ------------------------------------------------------------- 213 Forward-TSN-Supported OPTIONAL 0xC000 215 At the initialization of the association, the sender of the INIT or 216 INIT ACK chunk shall include this OPTIONAL parameter to inform its 217 peer that it is able to support the Forward TSN chunk. The format of 218 this parameter is defined as follows: 220 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 1 221 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 222 | Parameter Type = 0xC000 | Parameter Length = 4 | 223 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 225 Type: 16 bit u_int 227 0xC000, indicating Forward-TSN-Supported parameter 229 Length: 16 bit u_int 231 Indicate the size of the parameter i.e., 4. 233 3.2 Forward Cumulative TSN Chunk Definition (FORWARD TSN) 235 The following new chunk type is defined: 237 Chunk Type Chunk Name 238 ------------------------------------------------------ 239 0xC0 Forward Cumulative TSN (FORWARD TSN) 241 This chunk shall be used by the data sender to inform the data 242 receiver to adjust its cumulative received TSN point forward because 243 some missing TSNs are associated with data chunks that SHOULD NOT be 244 transmitted or retransmitted by the sender. 246 Forward Cumulative TSN chunk has the following format: 248 0 1 2 3 249 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 1 250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 251 | Type = 0xC0 | Flags = 0x00 | Length = Variable | 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 253 | New Cumulative TSN | 254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 255 | Stream-1 | Stream Sequence-1 | 256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 257 \ / 258 / \ 259 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 260 | Stream-N | Stream Sequence-N | 261 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 263 Chunk Flags: 265 Set to all zeros on transmit and ignored on receipt. 267 New Cumulative TSN: 32 bit u_int 269 This indicates the new cumulative TSN to the data receiver. Upon 270 the reception of this value, the data receiver MUST consider 271 any missing TSNs earlier than or equal to this value as received 272 and stop reporting them as gaps in any subsequent SACKs. 274 Stream-N: 16 bit u_int 276 This field holds a stream number that was skipped by this 277 FWD-TSN. 279 Stream Sequence-N: 16 bit u_int 281 This field holds the sequence number associated with the stream 282 that was skipped. The stream sequence field holds the largest stream 283 sequence number in this stream being skipped. The receiver of 284 the FWD-TSN's can use the Stream-N and Stream Sequence-N fields 285 to enable delivery of any stranded TSN's that remain on the stream 286 re-ordering queues. This field MUST NOT report TSN's cooresponding 287 to DATA chunk that are marked as unordered. For ordered DATA 288 chunks this field MUST be filled in. 290 3.3 Negotiation of Forward-TSN-Supported parameter 292 3.3.1 Sending Forward-TSN-Supported param in INIT 294 If an SCTP endpoint supports the FORWARD TSN chunk, then any time it 295 sends an INIT during association establishment, it SHOULD include the 296 Forward-TSN-supported parameter in the INIT chunk to indicate this 297 fact to its peer. 299 3.3.2 Receipt of Forward-TSN-Supported param in INIT or INIT-ACK 301 When a receiver of an INIT detects a Forward-TSN-Supported parameter, 302 and does not support the Forward-TSN chunk type, the receiver SHOULD 303 treat this parameter as an invalid or unrecognized parameter and 304 respond to the data sender with an unrecognized parameter in the 305 INIT-ACK, following the rules defined in Section 3.3.3 of RFC2960 306 [5]. 308 When a receiver of an INIT-ACK detects a Forward-TSN-Supported 309 parameter, and does not support the Forward-TSN chunk type, the 310 receiver SHOULD treat this parameter as an invalid or unrecognized 311 parameter and respond to the data sender with an unrecognized 312 parameter error, following the rules defined in Section 3.3.3 of 313 RFC2960 [4]. This error SHOULD be reported in an ERROR chunk bundled 314 with the COOKIE-ECHO. 316 When a receiver of an INIT detects a Forward-TSN-Supported parameter, 317 and does support the Forward-TSN chunk type, the receiver SHOULD 318 respond with a Forward-TSN-supported parameter in the INIT-ACK chunk. 320 When an endpoint that supports the FORWARD TSN chunk receives an INIT 321 that does not contain the Forward-TSN-Supported Parameter, that 322 endpoint: 324 o MAY include the Forward-TSN-Supported parameter in the INIT-ACK, 326 o SHOULD record the fact that the peer does not support the FORWARD 327 TSN chunk, 329 o MUST NOT send a FORWARD TSN chunk at any time during the 330 associations life, 332 o SHOULD inform the upper layer, if the upper layer has requested 333 such notification. 335 3.3.3 Receipt of Op. Error for Forward-TSN-Supported Param 337 When an SCTP endpoint that desires to use the FORWARD TSN chunk 338 feature for partially reliable data transfer receives an operational 339 error from the remote endpoint (either bundled with the COOKIE or as 340 a unrecognized parameter in the INIT-ACK), indicating that the remote 341 endpoint does not recognize the Forward-TSN-Supported parameter, the 342 local endpoint SHOULD inform its upper layer of the remote endpoint's 343 inability to support partially reliable data transfer. 345 The local endpoint may then choose to either: 347 1) end the initiation process (in cases where the initiation 348 process has already ended the endpoint may need to send an ABORT), 349 in consideration of the peer's inability to supply the requested 350 features for the new association, or 352 2) continue the initiation process (in cases where the initiation 353 process has already completed the endpoint MUST just mark the 354 association as not supporting partial reliability), but with the 355 understanding that partially reliable data transmission is not 356 supported. In this case, the endpoint receiving the operational 357 error SHOULD note that the FORWARD TSN chunk is not supported, and 358 MUST NOT transmit a FORWARD TSN chunk at any time during the life 359 of the association. 361 3.4 Definition of "abandoned" in the context of PR-SCTP 363 At some point, a sending PR-SCTP implementation MAY determine that a 364 particular data chunk SHOULD NOT be transmitted or retransmitted 365 further, in accordance with the rules governing some particular 366 PR-SCTP service definition (such as the definition of "timed 367 reliability" in Section 4.1.) For purposes of this document, we 368 define the term "abandoned" to refer to any data chunk about which 369 the SCTP sender has made this determination. 371 Each PR-SCTP service defines the rules for determining when a TSN is 372 "abandoned", and accordingly, the rules that govern how, whether, and 373 when to "abandon" a TSN may vary from one service definition to 374 another. However, the rules governing the actions taken when a TSN 375 is "abandoned" do NOT vary between service definitions; these rules 376 are included in Section 3.5. 378 3.5 Sender Side Implementation of PR-SCTP 380 The sender side implementation of PR-SCTP is identical to that of the 381 base SCTP protocol, except for: 383 o actions a sending side PR-SCTP implementation must take when a TSN 384 is "abandoned" (as per the rules of whatever PR-SCTP service 385 definition is in effect) 387 o special actions that a PR-SCTP implementation must take upon 388 receipt of SACK 390 o rules governing generation of FORWARD TSN chunks. 392 In detail, these exceptions are as follows: 394 A1) The sender maintains an "Advanced.Peer.Ack.Point" for each peer 395 to track a theoretical cumulative TSN point of the peer (Note, 396 this is a _new_ protocol variable and its value is NOT necessarily 397 the same as the SCTP "Cumulative TSN Ack Point" as defined in 398 Section 1.4 of RFC2960 [5]) and discussed throughout that 399 document. 401 A2) From time to time, as governed by the rules of a particular 402 PR-SCTP service definition (see Section 4), the SCTP data sender 403 may make a determination that a particular data chunk that has 404 already been assigned a TSN SHOULD be "abandoned". 406 When a data chunk is "abandoned", the sender MUST treat the data 407 chunk as being finally acked and no longer outstanding. 409 The sender MUST NOT credit an "abandoned" data chunk to the 410 partial_bytes_acked as defined in Section 7.2.2 of RFC2960 [5], 411 and MUST NOT advance the cwnd based on this "abandoned" data 412 chunk. 414 A3) When a TSN is "abandoned", if it is part of a fragmented message, 415 all other TSN's within that fragmented message MUST be abandoned 416 at the same time. 418 A4) Whenever the data sender receives a SACK from the data receiver, 419 it MUST first process the SACK using the normal procedures as 420 defined in Section 6.2.1 of RFC2960 [5]. 422 The data sender MUST then perform the following additional steps : 424 C1) Let SackCumAck be the Cumulative TSN ACK carried in the 425 received SACK. 427 If (Advanced.Peer.Ack.Point < SackCumAck), then update 428 Advanced.Peer.Ack.Point to be equal to SackCumAck. 430 C2) Try to further advance the "Advanced.Peer.Ack.Point" locally, 431 that is, to move "Advanced.Peer.Ack.Point" up as long as the 432 chunk next in the out-queue space is marked as "abandoned" as 433 shown in the following example: 435 Assuming that a SACK arrived with the Cumulative TSN ACK = 436 102 and the Advanced.Peer.Ack.Point is updated to this 437 value: 439 out-queue at the end of ==> out-queue after Adv.Ack.Point 440 normal SACK processing local advancement 442 ... ... 443 Adv.Ack.Pt-> 102 acked 102 acked 444 103 abandoned 103 abandoned 445 104 abandoned Adv.Ack.P-> 104 abandoned 446 105 105 447 106 acked 106 acked 448 ... ... 450 In this example, the data sender successfully advanced the 451 "Advanced.Peer.Ack.Point" from 102 to 104 locally. 453 C3) If, after step C1 and C2, the "Advanced.Peer.Ack.Point" is 454 greater than the Cumulative TSN ACK carried in the received 455 SACK, the data sender MUST send the data receiver a FORWARD TSN 456 chunk containing the latest value of the 457 "Advanced.Peer.Ack.Point". 459 C4) For each "abandoned" TSN the sender of the FORWARD TSN MUST 460 determine if the chunk has a valid stream and sequence number 461 (i.e., it was ordered). If the chunk has a valid stream and 462 sequence number the sender MUST include the stream and sequence 463 number in the FORWARD TSN. This information will enable the 464 receiver to easily find any stranded TSN's waiting on stream 465 reorder queues. Each stream SHOULD only be reported once; this 466 means that if multiple abandoned messages occur in the same 467 stream then only the highest abandoned stream sequence number 468 is reported. If the total size of the FORWARD TSN does NOT fit 469 in a single MTU then the sender of the FORWARD TSN SHOULD lower 470 the Advanced.Peer.Ack.Point to the last TSN that will fit in a 471 single MTU. 473 C5) If a FORWARD TSN is sent, the sender MUST assure that at least 474 one T3-rtx timer is running. 476 A5) Any time the T3-rtx timer expires, on any destination, the sender 477 SHOULD try to advance the "Advanced.Peer.Ack.Point" by following 478 the procedures outlined in C1 - C5. 480 The following additional rules govern the generation of FORWARD TSN 481 chunks: 483 F1) An endpoint MUST NOT use the FORWARD TSN for any purposes other 484 than circumstances described in this document. 486 F2) The data sender SHOULD always attempt to bundle an outgoing 487 FORWARD TSN with outbound DATA chunks for efficiency. 489 A sender MAY even choose to delay the sending of the FORWARD TSN 490 in the hope of bundling it with an outbound DATA chunk. 492 IMPLEMENTATION NOTE: An implementation may wish to limit the 493 number of duplicate FORWARD TSN chunks it sends by either only 494 sending a duplicate FORWARD TSN every other SACK or waiting a full 495 RTT before sending a duplicate FORWARD TSN. 497 IMPLEMENTATION NOTE: An implementation may allow the maximum delay 498 for generating a FORWARD TSN to be configured either statically or 499 dynamically in order to meet the specific timing requirements of 500 the protocol being carried, but see the next rule: 502 F3) Any delay applied to the sending of FORWARD TSN chunk SHOULD NOT 503 exceed 200ms and MUST NOT exceed 500ms. In other words an 504 implementation MAY lower this value below 500ms but MUST NOT raise 505 it above 500ms. 507 NOTE: Delaying the sending of FORWARD TSN chunks may cause delays 508 in the receiver's ability to deliver other data being held at the 509 receiver for re-ordering. 511 F4) The detection criterion for out-of-order SACKs MUST remain the 512 same as stated in RFC2960, that is, a SACK is only considered 513 out-of-order if the Cumulative TSN ACK carried in the SACK is 514 earlier than that of the previous received SACK (i.e., the 515 comparison MUST NOT be made against "Advanced.Peer.Ack.Point"). 517 F5) If the decision to "abandon" a chunk is made, no matter how such 518 a decision is made, the appropriate congestion adjustment MUST be 519 made as specified in RFC2960 if the chunk would have been marked 520 for retransmission later (e.g. either by T3-Timeout or by Fast 521 Retransmit). 523 3.6 Receiver Side Implementation of PR-SCTP 525 The receiver side implementation of PR-SCTP at an SCTP endpoint A is 526 capable of supporting any PR-SCTP service definition used by the 527 sender at endpoint B, even if that service definition is not 528 supported by the sending side functionality of host A. All that is 529 necessary is that the receiving side correctly handle the 530 Forward-TSN-Supported parameter as specified in Section 3.3, and 531 correctly handle the receipt of FORWARD TSN chunks as specified 532 below. 534 DATA chunk arrival at a PR-SCTP receiver proceeds exactly as for DATA 535 chunk arrival at a base protocol SCTP receiver---that is, the 536 receiver MUST perform the same TSN handling including duplicate 537 detection, gap detection, SACK generation, cumulative TSN 538 advancement, etc. as defined in RFC2960 [5]---with the following 539 exceptions and additions. 541 When a FORWARD TSN chunk arrives, the data receiver MUST first update 542 its cumulative TSN point to the value carried in the FORWARD TSN 543 chunk, and then MUST further advance its cumulative TSN point locally 544 if possible, as shown by the following example: 546 Assuming that the new cumulative TSN carried in the arrived 547 FORWARD TSN is 103: 549 in-queue before processing in-queue after processing the 550 the FORWARD TSN ==> the FORWARD TSN and further 551 advancement 553 cum.TSN.Pt-> 102 received 102 -- 554 103 missing 103 -- 555 104 received 104 -- 556 105 received cum.TSN.Pt-> 105 received 557 106 missing 106 missing 558 107 received 107 received 559 ... ... 561 In this example, the receiver's cumulative TSN point is first 562 updated to 103 and then further advanced to 105. 564 After the above processing, the data receiver MUST stop reporting any 565 missing TSNs earlier than or equal to the new cumulative TSN point. 567 Note, if the "New Cumulative TSN" value carried in the arrived 568 FORWARD TSN chunk is found to be behind or at the current cumulative 569 TSN point, the data receiver MUST treat this FORWARD TSN as 570 out-of-date and MUST NOT update its Cumulative TSN. The receiver 571 SHOULD send a SACK to its peer (the sender of the FORWARD TSN) since 572 such a duplicate may indicate the previous SACK was lost in the 573 network. 575 Any time a FORWARD TSN chunk arrives, for the purposes of sending a 576 SACK, the receiver MUST follow the same rules as if a DATA chunk had 577 been received (i.e., follow the delayed sack rules specified in 578 RFC2960 [5] section 6.2). 580 Whenever a DATA chunk arrives with the 'U' bit set to '0' (indicating 581 ordered delivery) and is out of order, the receiver must hold the 582 chunk for reordering. Since it is possible with PR-SCTP that a DATA 583 chunk being waited upon will not be retransmitted, special actions 584 will need to be taken upon the arrival of a FORWARD TSN. 586 In particular, during processing of a FORWARD TSN, the receiver MUST 587 use the stream sequence information to examine all of the listed 588 stream reordering queues, and immediately make available for delivery 589 stream sequence numbers earlier than or equal to the stream sequence 590 number listed inside the FORWARD TSN. 592 After receiving and processing a FORWARD TSN, the data receiver MUST 593 take cautions in updating its re-assembly queue. The receiver MUST 594 remove any partially reassembled message which is still missing one 595 or more TSNs earlier than or equal to the new cumulative TSN point. 596 In the event that the receiver has invoked the partial delivery API a 597 notification SHOULD also be generated to inform the upper layer API 598 that the message being partially delivered will NOT be completed. 600 4. Services provided by PR-SCTP to the upper layer 602 As described in Section 1.2, it is feasible to implement a variety of 603 partially reliable transport services using the new protocol 604 mechanisms introduced in Section 3; introducing these new services 605 requires making changes only at the sending side API, and the sending 606 side protocol implementation. Thus, there may be a temptation to 607 standardize only the protocol, and leave the service definition as 608 "implementation specific" or leave it to be defined in 609 "informational" documents. 611 However, for those who may wish to write IETF standards for upper 612 layer protocols implemented over PR-SCTP, it is important to be able 613 to refer to a standard definition of services provided. Therefore, 614 this section provides an example definitions of one such service, 615 while also providing guidelines for the definition of additional 616 services as required. Each such service may be proposed as a 617 separate new standard. 619 Section 4 is organized as follows: 621 Section 4.1 provides the definition of one specific PR-SCTP 622 service: timed reliability. 624 Section 4.2 describes how a particular PR-SCTP service definition 625 is requested by the upper layer during association establishment, 626 and how the upper layer is notified if that request cannot be 627 satisfied. 629 Section 4.3 then provides guidelines for the specification of 630 PR-SCTP services other then the one defined in this memo. 632 Finally, Section 4.4 describes some additional usage notes that 633 upper layer protocol designers and implementors may find helpful. 635 4.1 PR-SCTP Service Definition for "timed reliability" 637 The "timed reliability" service is a natural extension of the 638 "lifetime" concept already present in the base SCTP protocol. 640 When this service is requested for an SCTP association, it changes 641 the meaning of the lifetime parameter specified in the SEND primitive 642 (see Section 10.1, part (E) of RFC2960 [5]; note that the parameter 643 is spelled "life time" in that document.) 645 In the base SCTP protocol, the lifetime parameter is used to avoid 646 sending stale data. When a lifetime value is indicated for a 647 particular message, SCTP cancels the sending of this message, and 648 notifies the ULP if the first transmission of the data does not take 649 place (because of rwnd or cwnd limitations, or for any other reason) 650 before the lifetime expires. However, in the base protocol, if SCTP 651 has sent the first transmission before the lifetime expires, then the 652 message MUST be sent as a normal reliable message. During episodes 653 of congestion this is particularly unfortunate, as retransmission 654 wastes bandwidth that could have been used for other (non-lifetime 655 expired) messages. 657 When the "timed reliability" service is invoked, this latter 658 restriction is removed. Specifically, when the "timed reliability" 659 service is in effect, the following rules govern all messages that 660 are sent with a lifetime parameter: 662 TR1) If the lifetime parameter of a message is SCTP_LIFETIME_RELIABLE 663 (or unspecified) that message is treated as a normal reliable SCTP 664 message, just as in the base SCTP protocol. 666 TR2) If the lifetime parameter is not SCTP_LIFETIME_RELIABLE, then 667 the SCTP sender MUST treat the message just as if it were a normal 668 reliable SCTP message as long as the lifetime has not yet expired. 670 TR3) Before assigning a TSN to any message, the SCTP sender MUST 671 evaluate the lifetime of that message. If it is expired, the SCTP 672 sender MUST NOT assign a TSN to that message, but instead, SHOULD 673 issue a notification to the upper layer and abandon the message. 675 TR4) Before transmitting or retransmitting a message for which a TSN 676 is already assigned, the SCTP sender MUST evaluate the lifetime of 677 the message. If the lifetime of the message is expired, the SCTP 678 sender MUST "abandon" the message, as per the rules specified in 679 Section 3.5. 681 TR5) The sending SCTP MAY evaluate the lifetime of messages at 682 anytime. Expired messages that have not been assigned a TSN MAY be 683 handled as per rule TR3. Expired messages that HAVE been assigned 684 a TSN MAY be handled as per rule TR4. 686 TR6) The sending application MUST NOT change the lifetime parameter 687 once the message is passed to the sending SCTP. 689 Implementation Note: Rules TR1 through TR4 are designed in such as 690 way to avoid requiring the implementer to maintain a separate timer 691 for each message; instead, the lifetime need only be evaluated at 692 points in the life of the message where actions are already being 693 taken, such as TSN assignment, transmission, or expiration of a 694 retransmission timeout. Rule TR5 is intended to give the SCTP 695 implementor flexibility to evaluate lifetime at any other convenient 696 opportunity, WITHOUT requiring that lifetime be evaluated immediately 697 at the point in time where it expires. 699 4.2 PR-SCTP Association Establishment 701 An upper layer protocol (ULP) that uses PR-SCTP may need to know 702 whether PR-SCTP can be supported on a given association. Therefore, 703 the ULP needs to have some indication of whether the FORWARD-TSN 704 chunk is supported by its peer. 706 Section 10.1 of RFC2960 [5] describes abstract primitives for the 707 ULP-to-SCTP interface, while noting that "individual implementations 708 must define their own exact format, and may provide combinations or 709 subsets of the basic functions in single calls." 711 In this section, we describe one additional return value that may be 712 added to the ASSOCIATE primitive to allow an SCTP service user to 713 indicate whether the FORWARD-TSN chunk is supported by its peer. 715 RFC2960 indicates that the associate primitive "allows the upper 716 layer to initiate an association to a specific peer endpoint". It is 717 structured as follows: 719 Format: ASSOCIATE(local SCTP instance name, destination transport addr, 720 outbound stream count) 721 -> association id [,destination transport addr list] 722 [,outbound stream count] 724 This extension adds one new OPTIONAL return value, such that the new 725 primitive reads as follows: 727 Format: ASSOCIATE(local SCTP instance name, destination transport addr, 728 outbound stream count ) 729 -> association id [,destination transport addr list] 730 [,outbound stream count] [,forward tsn supported] 732 NOTE: As per RFC2960, if the ASSOCIATE primitive is implemented as a 733 non-blocking call, the new OPTIONAL return value shall be passed with 734 the association parameters using the COMMUNICATION UP notification. 736 The new OPTIONAL parameter "forward tsn supported" is a boolean flag: 738 (0) false [default] indicates that FORWARD TSN is not supported by 739 the peer. 741 (1) true indicates that FORWARD TSN is supported by the peer. 743 4.3 Guidelines for defining other PR-SCTP Services 745 Other PR-SCTP services may be defined and implemented as dictated by 746 the needs of upper layer protocols. If such upper layer protocols 747 are to be standardized and require some particular PR-SCTP service 748 other than the one defined in this document (i.e., "timed 749 reliability") then those additional PR-SCTP services should also be 750 specified and standardized. 752 It is suggested that any such additional service definitions be 753 modeled after the contents of Section 4.1 . In particular, the 754 service definition should provide: 756 1. A description of how the service user specifies any parameters 757 that need to be associated with a particular message (and/or any 758 other communication that takes place between the application and 759 the SCTP transport sender) that provides the SCTP transport 760 sender with the information needed to determine when to give up 761 on transmission of a particular message. 763 Preferably this description should reference the primitives in 764 the abstract API provided in Section 10 of RFC2960 [5], 765 indicating any: 767 * changes to the interpretation of the existing parameters of 768 existing primitives, 770 * additional parameters to be added to existing primitives 771 (these should be OPTIONAL, and default values should be 772 indicated), 774 * additional primitives that may be needed. 776 2. A description of the rules used by the sender side implementation 777 to determine when to give up on messages that have not yet been 778 assigned a TSN. This description should also indicate what 779 protocol events trigger the evaluation, and what actions to take 780 (e.g. notifications.) 782 3. A description of the rules used by the sender side implementation 783 to determine when to give up on the transmission or 784 retransmission of messages that have already been assigned a TSN, 785 and may have been transmitted and possibly retransmitted zero or 786 more times. 788 Items (2) and (3) in the list above should also indicate what 789 protocol events trigger the evaluation, and what actions to take if 790 the determination is made that the sender should give up on 791 transmitting the message (e.g. notifications to the ULP.) 793 Note that in any PR-SCTP service, the following rule MUST be 794 specified to avoid a protocol deadlock: 796 (G1) When the sender side implementation gives up on transmitting a 797 message that has been assigned a TSN (i.e., when that message is 798 "abandoned", as defined in Section 3.4) the sender side MUST mark 799 that TSN as eligible for forward TSN, and the rules in Section 3.4 800 regarding the sending of FORWARD TSN chunks MUST be followed. 802 Finally, a PR-SCTP service definition should specify a "canonical 803 service name" to uniquely identify the service, and distinguish it 804 from other PR-SCTP services. This name can then be used in upper 805 layer protocol standards, to indicate which PR-SCTP service 806 definition is required by that upper layer protocol. It can also be 807 used in the documentation of APIs of PR-SCTP implementations to 808 indicate how an upper layer indicates which definition of PR-SCTP 809 service should apply. The canonical service name for the PR-SCTP 810 service defined in Section 4.1 is "timed reliability". 812 4.4 Usage Notes 814 Detecting missing data in a PR-SCTP stream is useful for some 815 applications (e.g. Fiber channel or SCSI over IP). With PR-SCTP this 816 becomes possible - the upper layer simply needs to examine the stream 817 sequence number of the arrived user messages of that stream to detect 818 any missing data. Note, this detection only works when all the 819 messages on that stream are sent in order, i.e., the "U" bit is not 820 set. 822 5. Acknowledgments 824 The authors would like to thank Brian Bidulock, Scott Bradner, Jon 825 Berger, Armando L. Caro Jr., John Loughney, Ivan Arias Rodriguez, Ian 826 Rytina, Chip Sharp, and others for their comments. 828 6. Security Considerations 830 This document does not introduce any new security concerns to SCTP 831 other than the ones already documented in RFC2960 [5]. In particular 832 this document shares the same security issues as unordered data 833 within RFC2960 [5]. An application using the PR-SCTP extension should 834 not use transport layer security. Further details can be found in 835 RFC3436 [4]. 837 7. IANA Considerations 839 One new chunk type is added to SCTP ('0xC0') by this document. 841 One new parameter type code is defined by this document to be added 842 to SCTP ('0xC000'). 844 References 846 [1] Clark, D. and D. Tennenhouse, "Architectural Considerations for 847 a New Generation of Protocols", SIGCOMM 1990 pp. 200-208, 848 September 1990. 850 [2] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 851 9, RFC 2026, October 1996. 853 [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement 854 Levels", BCP 14, RFC 2119, March 1997. 856 [4] Jungmaier, A., Rescorla, E. and M. Tuexen, "TLS over SCTP", RFC 857 3436, December 2002. 859 [5] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, 860 H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson, 861 "Stream Control Transmission Protocol", RFC 2960, October 2000. 863 Authors' Addresses 865 Randall R. Stewart 866 Cisco Systems, Inc. 867 8725 West Higgins Road 868 Suite 300 869 Chicago, IL 60631 870 USA 872 Phone: +1-815-477-2127 873 EMail: rrs@cisco.com 875 Michael A. Ramalho 876 Cisco Systems, Inc. 877 1802 Rue de la Porte 878 Wall Township, NJ 07719-3784 879 USA 881 Phone: +1.732.449.5762 882 EMail: mramalho@cisco.com 883 Qiaobing Xie 884 Motorola, Inc. 885 1501 W. Shure Drive, #2309 886 Arlington Heights, IL 60004 887 USA 889 Phone: +1-847-632-3028 890 EMail: qxie1@email.mot.com 892 Michael Tuexen 893 Univ. of Applied Sciences Muenster 894 Stegerwaldstr. 39 895 48565 Steinfurt 896 Germany 898 EMail: tuexen@fh-muenster.de 900 Phillip T. Conrad 901 Temple University 902 CIS Department 903 Room 303, Computer Building (038-24) 904 1805 N. 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