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Begen 3 Internet-Draft Cisco 4 Intended status: Standards Track July 12, 2010 5 Expires: January 13, 2011 7 Session Description Protocol (SDP) Elements for FEC Framework 8 draft-ietf-fecframe-sdp-elements-07 10 Abstract 12 This document specifies the use of Session Description Protocol (SDP) 13 to describe the parameters required to signal the Forward Error 14 Correction (FEC) Framework Configuration Information between the 15 sender(s) and receiver(s). This document also provides examples that 16 show the semantics for grouping multiple source and repair flows 17 together for the applications that simultaneously use multiple 18 instances of the FEC Framework. 20 Status of this Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on January 13, 2011. 37 Copyright Notice 39 Copyright (c) 2010 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 This document may contain material from IETF Documents or IETF 53 Contributions published or made publicly available before November 54 10, 2008. The person(s) controlling the copyright in some of this 55 material may not have granted the IETF Trust the right to allow 56 modifications of such material outside the IETF Standards Process. 57 Without obtaining an adequate license from the person(s) controlling 58 the copyright in such materials, this document may not be modified 59 outside the IETF Standards Process, and derivative works of it may 60 not be created outside the IETF Standards Process, except to format 61 it for publication as an RFC or to translate it into languages other 62 than English. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4 68 3. Forward Error Correction (FEC) and FEC Framework . . . . . . . 4 69 3.1. Forward Error Correction (FEC) . . . . . . . . . . . . . . 4 70 3.2. FEC Framework . . . . . . . . . . . . . . . . . . . . . . 5 71 3.3. FEC Framework Configuration Information . . . . . . . . . 5 72 4. SDP Elements . . . . . . . . . . . . . . . . . . . . . . . . . 6 73 4.1. Transport Protocol Identifiers . . . . . . . . . . . . . . 6 74 4.2. Media Stream Grouping . . . . . . . . . . . . . . . . . . 7 75 4.3. Source IP Addresses . . . . . . . . . . . . . . . . . . . 7 76 4.4. Source Flows . . . . . . . . . . . . . . . . . . . . . . . 7 77 4.5. Repair Flows . . . . . . . . . . . . . . . . . . . . . . . 8 78 4.6. Repair Window . . . . . . . . . . . . . . . . . . . . . . 9 79 4.7. Bandwidth Specification . . . . . . . . . . . . . . . . . 10 80 5. Scenarios and Examples . . . . . . . . . . . . . . . . . . . . 11 81 5.1. Declarative Considerations . . . . . . . . . . . . . . . . 11 82 5.2. Offer/Answer Model Considerations . . . . . . . . . . . . 11 83 6. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12 84 6.1. One Source Flow, One Repair Flow and One FEC Scheme . . . 12 85 6.2. Two Source Flows, One Repair Flow and One FEC Scheme . . . 13 86 6.3. Two Source Flows, Two Repair Flows and Two FEC Schemes . . 14 87 6.4. One Source Flow, Two Repair Flows and Two FEC Schemes . . 15 88 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 89 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 90 8.1. Registration of Transport Protocols . . . . . . . . . . . 16 91 8.2. Registration of SDP Attributes . . . . . . . . . . . . . . 17 92 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 93 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 94 10.1. Normative References . . . . . . . . . . . . . . . . . . . 18 95 10.2. Informative References . . . . . . . . . . . . . . . . . . 18 97 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 99 1. Introduction 101 The Forward Error Correction (FEC) Framework, described in 102 [I-D.ietf-fecframe-framework], outlines a general framework for using 103 FEC-based error recovery in packet flows carrying media content. 104 While a continuous signaling between the sender(s) and receiver(s) is 105 not required for a Content Delivery Protocol (CDP) that uses the FEC 106 Framework, a set of parameters pertaining to the FEC Framework has to 107 be initially communicated between the sender(s) and receiver(s). A 108 signaling protocol (such as the one described in 109 [I-D.ietf-fecframe-config-signaling]) is required to enable such 110 communication and the parameters need to be appropriately encoded so 111 that they can be carried by the signaling protocol. 113 One format to encode the parameters is the Session Description 114 Protocol (SDP) [RFC4566]. SDP provides a simple text-based format 115 for announcements and invitations to describe multimedia sessions. 116 These SDP announcements and invitations include sufficient 117 information for the sender(s) and receiver(s) to participate in the 118 multimedia sessions. SDP also provides a framework for capability 119 negotiation, which can be used to negotiate all or a subset of the 120 parameters pertaining to the individual sessions. 122 The purpose of this document is to introduce the SDP elements that 123 are used by the CDPs using the FEC Framework that choose SDP 124 [RFC4566] as their session description protocol. 126 2. Requirements Notation 128 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 129 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 130 document are to be interpreted as described in [RFC2119]. 132 3. Forward Error Correction (FEC) and FEC Framework 134 This section gives a brief overview of FEC and the FEC Framework. 136 3.1. Forward Error Correction (FEC) 138 Any application that needs a reliable transmission over an unreliable 139 packet network has to cope with packet losses. FEC is an effective 140 approach that provides reliable transmission particularly in 141 multicast and broadcast applications where the feedback from the 142 receiver(s) is either not available or quite limited. 144 In a nutshell, FEC groups source packets into blocks and applies 145 protection to generate a desired number of repair packets. These 146 repair packets can be sent on demand or independently of any receiver 147 feedback. The choice depends on the FEC scheme or the Content 148 Delivery Protocol used by the application, the packet loss 149 characteristics of the underlying network, the transport scheme 150 (e.g., unicast, multicast and broadcast) and the application. At the 151 receiver side, lost packets can be recovered by erasure decoding 152 provided that a sufficient number of source and repair packets have 153 been received. 155 3.2. FEC Framework 157 The FEC Framework [I-D.ietf-fecframe-framework] outlines a general 158 framework for using FEC codes in multimedia applications that stream 159 audio, video or other types of multimedia content. It defines the 160 common components and aspects of Content Delivery Protocols (CDP). 161 The FEC Framework also defines the requirements for the FEC schemes 162 that need to be used within a CDP. However, the details of the FEC 163 schemes are not specified within the FEC Framework. For example, the 164 FEC Framework defines what configuration information has to be known 165 at the sender and receiver(s) at minimum, but the FEC Framework 166 neither specifies how the FEC repair packets are generated and used 167 to recover missing source packets, nor dictates how the configuration 168 information is communicated between the sender and receiver(s). 169 These are rather specified by the individual FEC schemes or CDPs. 171 3.3. FEC Framework Configuration Information 173 The FEC Framework [I-D.ietf-fecframe-framework] defines a minimum set 174 of information that has to be communicated between the sender and 175 receiver(s) for a proper operation of an FEC scheme. This 176 information is called the FEC Framework Configuration Information. 177 This information includes unique identifiers for the source and 178 repair flows that carry the source and repair packets, respectively. 179 It also specifies how the sender applies protection to the source 180 flow(s) and how the repair flow(s) can be used to recover lost data. 182 Multiple instances of the FEC Framework can simultaneously exist at 183 the sender and the receiver(s) for different source flows, for the 184 same source flow, or for various combinations of the source flows. 185 Each instance of the FEC Framework provides the following FEC 186 Framework Configuration Information: 188 1. Identification of the repair flows. 190 2. For each source flow protected by the repair flow(s): 192 a. Definition of the source flow. 194 b. An integer identifier for this flow definition (i.e., tuple). 195 This identifier MUST be unique amongst all source flows that are 196 protected by the same FEC repair flow. The identifiers SHOULD be 197 allocated starting from zero and increasing by one for each flow. 198 A source flow identifier need not be carried in source packets 199 since source packets are directly associated with a flow by virtue 200 of their packet headers. 202 3. The FEC Encoding ID, identifying the FEC scheme. 204 4. The length of the Explicit Source FEC Payload ID (in bytes). 206 5. Zero or more FEC-Scheme-Specific Information (FSSI) elements, each 207 consisting of a name and a value where the valid element names and 208 value ranges are defined by the FEC scheme. 210 FSSI includes the information that is specific to the FEC scheme used 211 by the CDP. FSSI is used to communicate the information that cannot 212 be adequately represented otherwise and is essential for proper FEC 213 encoding and decoding operations. The motivation behind separating 214 the FSSI required only by the sender (which is carried in Sender-Side 215 FEC-Scheme-Specific Information (SS-FSSI) container) from the rest of 216 the FSSI is to provide the receiver or the third party entities a 217 means of controlling the FEC operations at the sender. Any FSSI 218 other than the one solely required by the sender MUST be communicated 219 via the FSSI container. 221 The variable-length SS-FSSI and FSSI containers transmit the 222 information in textual representation and contain zero or more 223 distinct elements, whose descriptions are provided by the fully- 224 specified FEC schemes. 226 4. SDP Elements 228 This section defines the SDP elements that MUST be used to describe 229 the FEC Framework Configuration Information in multimedia sessions by 230 the CDPs that choose SDP [RFC4566] as their session description 231 protocol. Example SDP descriptions can be found in Section 6. 233 4.1. Transport Protocol Identifiers 235 This specification defines a class of new transport protocol 236 identifiers for SDP media descriptions. For all existing identifiers 237 (listed in the table for the 'proto' field in the Session 238 Description Protocol (SDP) Parameters registry), this specification 239 defines the identifier 'FEC/'. This identifier MAY be used as 240 the transport protocol identifier in the media descriptions for the 241 source data to indicate that the FEC Source Packet format defined in 242 Section 5.3 of [I-D.ietf-fecframe-framework] is used, where the 243 original transport payload field is formatted according to . 244 However, if the FEC scheme does not use the Explicit Source FEC 245 Payload ID as described in Section 4.1 of 246 [I-D.ietf-fecframe-framework], then the original transport protocol 247 identifier MUST be used to support backward compatibility with the 248 receivers that do not support FEC at all. 250 This specification also defines another transport protocol 251 identifier, 'UDP/FEC', to indicate the FEC Repair Packet format 252 defined in Section 5.4 of [I-D.ietf-fecframe-framework]. 254 4.2. Media Stream Grouping 256 In FEC Framework, the 'group' attribute and the FEC grouping 257 semantics defined in [RFC5888] and [I-D.ietf-mmusic-rfc4756bis], 258 respectively are used to associate source and repair flows together. 260 4.3. Source IP Addresses 262 The 'source-filter' attribute of SDP ("a=source-filter") as defined 263 in [RFC4570] is used to express the source addresses or fully 264 qualified domain names in the FEC Framework. 266 4.4. Source Flows 268 The FEC Framework allows that multiple source flows MAY be grouped 269 and protected together by a single or multiple FEC Framework 270 instances. For this reason, as described in Section 3.3, individual 271 source flows MUST be identified with unique identifiers. For this 272 purpose, we introduce the attribute 'fec-source-flow'. 274 The syntax for the new attribute in ABNF [RFC5234] is as follows: 276 fec-source-flow-line = "a=fec-source-flow:" source-id 277 [";" SP tag-length] CRLF 279 source-id = "id=" src-id 280 src-id = 1*DIGIT 282 tag-length = "tag-len=" tlen 283 tlen = *DIGIT 285 The REQUIRED parameter 'id' is used to identify the source flow. 286 Parameter 'id' MUST be an integer. 288 The OPTIONAL 'tag-len' parameter is used to specify the length of the 289 Explicit Source FEC Payload ID field (in bytes). If no value is 290 specified for the 'tag-len' parameter, it indicates a value of zero. 291 However, in the case that an Explicit Source FEC Payload ID is used, 292 the 'tag-len' parameter MUST exist and indicate its length. 294 4.5. Repair Flows 296 A repair flow MUST contain only repair packets formatted as described 297 in [I-D.ietf-fecframe-framework] for a single FEC Framework instance, 298 i.e., packets belonging to source flows or other repair flows from a 299 different FEC Framework instance cannot be sent within this flow. We 300 introduce the attribute 'fec-repair-flow' to describe the repair 301 flows. 303 The syntax for the new attribute in ABNF is as follows: 305 fec-repair-flow-line = "a=fec-repair-flow:" SP fec-encoding-id 306 [";" SP flow-preference] 307 [";" SP sender-side-scheme-specific] 308 [";" SP scheme-specific] CRLF 310 fec-encoding-id = "encoding-id=" enc-id 311 enc-id = 1*DIGIT ; FEC Encoding ID 313 flow-preference = "preference-lvl=" preference-level-of-the-flow 314 preference-level-of-the-flow = *DIGIT 316 sender-side-scheme-specific = "ss-fssi=" sender-info 317 sender-info = *CHAR 319 scheme-specific = "fssi=" scheme-info 320 scheme-info = *CHAR 322 The REQUIRED parameter 'encoding-id' is used to identify the FEC 323 scheme used to generate this repair flow. These identifiers MUST be 324 registered with IANA by the FEC schemes that use the FEC Framework. 326 The OPTIONAL parameter 'preference-lvl' is used to indicate the 327 preferred order of using the repair flows. The exact usage of the 328 parameter 'preference-lvl' and the pertaining rules MAY be defined by 329 the FEC scheme or the CDP. If no value is specified for the 330 parameter 'preference-lvl', it means that the receiver(s) MAY receive 331 and use the repair flows in any order. However, if a preference 332 level is assigned to the repair flow(s), the receivers are encouraged 333 to follow the specified order in receiving and using the repair 334 flow(s). 336 The OPTIONAL parameters 'ss-fssi' and 'fssi' are containers to convey 337 the FEC-Scheme-Specific Information (FSSI) that includes the 338 information that is specific to the FEC scheme used by the CDP and is 339 necessary for proper FEC encoding and decoding operations. The FSSI 340 required only by the sender (called Sender-Side FSSI) MUST be 341 communicated in the container specified by the parameter 'ss-fssi'. 342 Any other FSSI MUST be communicated in the container specified by the 343 parameter 'fssi'. In both containers, FSSI is transmitted in the 344 form of textual representation and MAY contain multiple distinct 345 elements. If the FEC scheme does not require any specific 346 information, the 'ss-fssi' and 'fssi' parameters MAY be null and 347 ignored. 349 4.6. Repair Window 351 Repair window is the time that spans an FEC block, which consists of 352 the source block and the corresponding repair packets. 354 At the sender side, the FEC encoder processes a block of source 355 packets and generates a number of repair packets. Then both the 356 source and repair packets are transmitted within a certain duration 357 not larger than the value of the repair window. The value of the 358 repair window impacts the maximum number of source packets that can 359 be included in an FEC block. 361 At the receiver side, the FEC decoder should wait at least for the 362 duration of the repair window after getting the first packet in an 363 FEC block to allow all the repair packets to arrive (The waiting time 364 can be adjusted if there are mising packets at the beginning of the 365 FEC block). The FEC decoder can start decoding the already received 366 packets sooner, however, it SHOULD NOT register an FEC decoding 367 failure until it waits at least for the repair-window duration. 369 This document specifies a new attribute to describe the size of the 370 repair window in milliseconds and microseconds. 372 The syntax for the attribute in ABNF is as follows: 374 repair-window-line = "a=repair-window:" window-size 375 [unit] CRLF 377 window-size = 1*DIGIT 379 unit = ms / us 381 is the unit of time the repair window size is specified with. 382 Two units are defined here: 'ms', which stands for milliseconds and 383 'us', which stands for microseconds. 385 The 'a=repair-window' attribute is a media-level attribute since each 386 repair flow MAY have a different repair window size. 388 Specifying the repair window size in an absolute time value does not 389 necessarily correspond to an integer number of packets or exactly 390 match with the clock rate used in RTP (in case of RTP transport) 391 causing mismatches among subsequent repair windows. However, in 392 practice, this mismatch does not break anything in the FEC decoding 393 process. 395 4.7. Bandwidth Specification 397 The bandwidth specification as defined in [RFC4566] denotes the 398 proposed bandwidth to be used by the session or media. The 399 specification of bandwidth is OPTIONAL. 401 In the context of the FEC Framework, the bandwidth specification can 402 be used to express the bandwidth of the repair flows or the bandwidth 403 of the session. If included in the SDP, it SHALL adhere to the 404 following rules: 406 The session-level bandwidth for an FEC Framework instance or the 407 media-level bandwidth for the individual repair flows MAY be 408 specified. In this case, it is RECOMMENDED to use the Transport 409 Independent Application Specific (TIAS) bandwidth modifier [RFC3890] 410 and the 'a=maxprate' attribute unless the Application Specific (AS) 411 bandwidth modifier [RFC4566] is used. The use of AS bandwidth 412 modifier is NOT RECOMMENDED since TIAS allows the calculation of the 413 bitrate according to the IP version and transport protocol, whereas 414 AS does not. Thus, in TIAS-based bitrate calculations, the packet 415 size SHALL include all headers and payload, excluding the IP and UDP 416 headers. In AS-based bitrate calculations, the packet size SHALL 417 include all headers and payload, plus the IP and UDP headers. 419 For the ABNF syntax information of the TIAS and AS, refer to 420 [RFC3890] and [RFC4566], respectively. 422 5. Scenarios and Examples 424 This section discusses the considerations for Session Announcement 425 and Offer/Answer Models. 427 5.1. Declarative Considerations 429 In multicast-based applications, the FEC Framework Configuration 430 Information pertaining to all FEC protection options available at the 431 sender MAY be advertised to the receivers as a part of a session 432 announcement. This way, the sender can let the receivers know all 433 available options for FEC protection. Based on their needs, the 434 receivers MAY choose protection provided by one or more FEC Framework 435 instances and subscribe to the respective multicast session(s) to 436 receive the repair flow(s). Unless explicitly required by the CDP, 437 the receivers SHOULD NOT send an answer back to the sender specifying 438 their choices since this can easily overwhelm the sender particularly 439 in large-scale multicast applications. 441 5.2. Offer/Answer Model Considerations 443 In unicast-based applications, a sender and receiver MAY adopt the 444 Offer/Answer Model [RFC3264] to set the FEC Framework Configuration 445 Information. In this case, the sender offers the options available 446 to this particular receiver and the receiver answers back to the 447 sender with its choice(s). 449 Receivers supporting the SDP Capability Negotiation Framework 450 [I-D.ietf-mmusic-sdp-capability-negotiation] MAY also use this 451 framework to negotiate all or a subset of the FEC Framework 452 parameters. 454 The backward compatibility in Offer/Answer Model is handled as 455 specified in [I-D.ietf-mmusic-rfc4756bis]. 457 6. SDP Examples 459 This section provides SDP examples that can be used by the FEC 460 Framework. 462 [RFC5888] defines the media stream identification attribute ('mid') 463 as a token in ABNF. In contrast, the identifiers for the source 464 flows MUST be integers and SHOULD be allocated starting from zero and 465 increasing by one for each flow. To avoid any ambiguity, using the 466 same values for identifying the media streams and source flows is NOT 467 RECOMMENDED, even when 'mid' values are integers. 469 In the examples below, random FEC Encoding IDs will be used for 470 illustrative purposes. Artificial content for the SS-FSSI and FSSI 471 will also be provided. 473 6.1. One Source Flow, One Repair Flow and One FEC Scheme 475 SOURCE FLOWS | INSTANCE #1 476 S1: Source Flow |--------| R1: Repair Flow 477 | 479 Figure 1: Scenario #1 481 In this example, we have one source video flow (mid:S1) and one FEC 482 repair flow (mid:R1). We form one FEC group with the "a=group:FEC-FR 483 S1 R1" line. The source and repair flows are sent to the same port 484 on different multicast groups. The repair window is set to 150 ms. 486 v=0 487 o=ali 1122334455 1122334466 IN IP4 fec.example.com 488 s=FEC Framework Examples 489 t=0 0 490 a=group:FEC-FR S1 R1 491 m=video 30000 RTP/AVP 100 492 c=IN IP4 233.252.0.1/127 493 a=rtpmap:100 MP2T/90000 494 a=fec-source-flow: id=0 495 a=mid:S1 496 m=application 30000 udp/fec 497 c=IN IP4 233.252.0.2/127 498 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 499 a=repair-window:150 500 a=mid:R1 502 6.2. Two Source Flows, One Repair Flow and One FEC Scheme 504 SOURCE FLOWS 505 S2: Source Flow | | INSTANCE #1 506 |---------| R2: Repair Flow 507 S3: Source Flow | 509 Figure 2: Scenario #2 511 In this example, we have two source video flows (mid:S2 and mid:S3) 512 and one FEC repair flow (mid:R2), protecting both source flows. We 513 form one FEC group with the "a=group:FEC-FR S2 S3 R2" line. The 514 source and repair flows are sent to the same port on different 515 multicast groups. The repair window is set to 150500 us. 517 v=0 518 o=ali 1122334455 1122334466 IN IP4 fec.example.com 519 s=FEC Framework Examples 520 t=0 0 521 a=group:FEC-FR S2 S3 R2 522 m=video 30000 RTP/AVP 100 523 c=IN IP4 233.252.0.1/127 524 a=rtpmap:100 MP2T/90000 525 a=fec-source-flow: id=0 526 a=mid:S2 527 m=video 30000 RTP/AVP 101 528 c=IN IP4 233.252.0.2/127 529 a=rtpmap:101 MP2T/90000 530 a=fec-source-flow: id=1 531 a=mid:S3 532 m=application 30000 udp/fec 533 c=IN IP4 233.252.0.3/127 534 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 535 a=repair-window:150500us 536 a=mid:R2 538 6.3. Two Source Flows, Two Repair Flows and Two FEC Schemes 540 SOURCE FLOWS | INSTANCE #1 541 S4: Source Flow |--------| R3: Repair Flow 543 S5: Source Flow |--------| INSTANCE #2 544 | R4: Repair Flow 546 Figure 3: Scenario #3 548 In this example, we have two source video flows (mid:S4 and mid:S5) 549 and two FEC repair flows (mid:R3 and mid:R4). The source flows 550 mid:S4 and mid:S5 are protected by the repair flows mid:R3 and 551 mid:R4, respectively. We form two FEC groups with the "a=group: 552 FEC-FR S4 R3" and "a=group:FEC-FR S5 R4" lines. The source and 553 repair flows are sent to the same port on different multicast groups. 554 The repair window is set to 200 ms and 400 ms for the first and 555 second FEC group, respectively. 557 v=0 558 o=ali 1122334455 1122334466 IN IP4 fec.example.com 559 s=FEC Framework Examples 560 t=0 0 561 a=group:FEC-FR S4 R3 562 a=group:FEC-FR S5 R4 563 m=video 30000 RTP/AVP 100 564 c=IN IP4 233.252.0.1/127 565 a=rtpmap:100 MP2T/90000 566 a=fec-source-flow: id=0 567 a=mid:S4 568 m=video 30000 RTP/AVP 101 569 c=IN IP4 233.252.0.2/127 570 a=rtpmap:101 MP2T/90000 571 a=fec-source-flow: id=1 572 a=mid:S5 573 m=application 30000 udp/fec 574 c=IN IP4 233.252.0.3/127 575 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 576 a=repair-window:200ms 577 a=mid:R3 578 m=application 30000 udp/fec 579 c=IN IP4 233.252.0.4/127 580 a=fec-repair-flow: encoding-id=0; ss-fssi=n:14,k:10 581 a=repair-window:400ms 582 a=mid:R4 584 6.4. One Source Flow, Two Repair Flows and Two FEC Schemes 586 SOURCE FLOWS | INSTANCE #1 587 S6: Source Flow |--------| R5: Repair Flow 588 | 589 |--------| INSTANCE #2 590 | R6: Repair Flow 592 Figure 4: Scenario #4 594 In this example, we have one source video flow (mid:S6) and two FEC 595 repair flows (mid:R5 and mid:R6) with different preference levels. 596 The source flow mid:S6 is protected by both of the repair flows. We 597 form two FEC groups with the "a=group:FEC-FR S6 R5" and "a=group: 598 FEC-FR S6 R6" lines. The source and repair flows are sent to the 599 same port on different multicast groups. The repair window is set to 600 200 ms for both FEC groups. 602 v=0 603 o=ali 1122334455 1122334466 IN IP4 fec.example.com 604 s=FEC Framework Examples 605 t=0 0 606 a=group:FEC-FR S6 R5 607 a=group:FEC-FR S6 R6 608 m=video 30000 RTP/AVP 100 609 c=IN IP4 233.252.0.1/127 610 a=rtpmap:100 MP2T/90000 611 a=fec-source-flow: id=0 612 a=mid:S6 613 m=application 30000 udp/fec 614 c=IN IP4 233.252.0.3/127 615 a=fec-repair-flow: encoding-id=0; preference-lvl=0; ss-fssi=n:7,k:5 616 a=repair-window:200ms 617 a=mid:R5 618 m=application 30000 udp/fec 619 c=IN IP4 233.252.0.4/127 620 a=fec-repair-flow: encoding-id=1; preference-lvl=1; ss-fssi=t:3 621 a=repair-window:200ms 622 a=mid:R6 624 7. Security Considerations 626 There is a weak threat if the SDP is modified in a way that it shows 627 incorrect association and/or grouping of the source and repair flows. 628 Such attacks may result in failure of FEC protection and/or 629 mishandling of other media streams. It is RECOMMENDED that the 630 receiver SHOULD do integrity check on SDP and follow the security 631 considerations of SDP [RFC4566] to only trust SDP from trusted 632 sources. For other general security considerations related to SDP, 633 refer to [RFC4566]. For the security considerations related to the 634 use of source address filters in SDP, refer to [RFC4570]. 636 The security considerations for the FEC Framework also apply. Refer 637 to [I-D.ietf-fecframe-framework] for details. 639 8. IANA Considerations 641 Note to the RFC Editor: In the following, please replace "XXXX" with 642 the number of this document prior to publication as an RFC. 644 8.1. Registration of Transport Protocols 646 This specification updates the Session Description Protocol (SDP) 647 Parameters registry as defined in Section 8.2.2 of [RFC4566]. 649 Specifically, it adds the following values to the table for the 650 'proto' field. 652 Type SDP Name Reference 653 ------ ---------- ----------- 654 proto UDP/FEC [RFCXXXX] 656 8.2. Registration of SDP Attributes 658 This document registers new attribute names in SDP. 660 SDP Attribute ("att-field"): 661 Attribute name: fec-source-flow 662 Long form: Pointer to FEC Source Flow 663 Type of name: att-field 664 Type of attribute: Media level 665 Subject to charset: No 666 Purpose: Provide parameters for an FEC source flow 667 Reference: [RFCXXXX] 668 Values: See [RFCXXXX] 670 SDP Attribute ("att-field"): 671 Attribute name: fec-repair-flow 672 Long form: Pointer to FEC Repair Flow 673 Type of name: att-field 674 Type of attribute: Media level 675 Subject to charset: No 676 Purpose: Provide parameters for an FEC repair flow 677 Reference: [RFCXXXX] 678 Values: See [RFCXXXX] 680 SDP Attribute ("att-field"): 681 Attribute name: repair-window 682 Long form: Pointer to FEC Repair Window 683 Type of name: att-field 684 Type of attribute: Media level 685 Subject to charset: No 686 Purpose: Indicate the size of the repair window 687 Reference: [RFCXXXX] 688 Values: See [RFCXXXX] 690 9. Acknowledgments 692 The author would like to thank the FEC Framework Design Team for 693 their inputs, suggestions and contributions. 695 10. References 697 10.1. Normative References 699 [I-D.ietf-fecframe-framework] 700 Watson, M., "Forward Error Correction (FEC) Framework", 701 draft-ietf-fecframe-framework-09 (work in progress), 702 July 2010. 704 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 705 Requirement Levels", BCP 14, RFC 2119, March 1997. 707 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 708 Description Protocol", RFC 4566, July 2006. 710 [RFC4570] Quinn, B. and R. Finlayson, "Session Description Protocol 711 (SDP) Source Filters", RFC 4570, July 2006. 713 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 714 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 716 [I-D.ietf-mmusic-rfc4756bis] 717 Begen, A., "Forward Error Correction Grouping Semantics in 718 Session Description Protocol", 719 draft-ietf-mmusic-rfc4756bis-10 (work in progress), 720 June 2010. 722 [RFC3890] Westerlund, M., "A Transport Independent Bandwidth 723 Modifier for the Session Description Protocol (SDP)", 724 RFC 3890, September 2004. 726 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 727 Specifications: ABNF", STD 68, RFC 5234, January 2008. 729 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 730 with Session Description Protocol (SDP)", RFC 3264, 731 June 2002. 733 10.2. Informative References 735 [I-D.ietf-fecframe-config-signaling] 736 Asati, R., "Methods to convey FEC Framework Configuration 737 Information", draft-ietf-fecframe-config-signaling-03 738 (work in progress), June 2010. 740 [I-D.ietf-mmusic-sdp-capability-negotiation] 741 Andreasen, F., "SDP Capability Negotiation", 742 draft-ietf-mmusic-sdp-capability-negotiation-13 (work in 743 progress), March 2010. 745 Author's Address 747 Ali Begen 748 Cisco 749 181 Bay Street 750 Toronto, ON M5J 2T3 751 Canada 753 Email: abegen@cisco.com