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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 FEC Framework U. Kozat 3 Internet-Draft DoCoMo USA Labs 4 Intended status: Informational A. Begen 5 Expires: December 22, 2012 Cisco 6 June 20, 2012 8 Pseudo Content Delivery Protocol (CDP) for Protecting Multiple Source 9 Flows in FEC Framework 10 draft-ietf-fecframe-pseudo-cdp-04 12 Abstract 14 This document provides a pseudo Content Delivery Protocol (CDP) to 15 protect multiple source flows with one or more repair flows based on 16 the FEC Framework and the Session Description Protocol (SDP) elements 17 defined for the framework. The purpose of the document is not to 18 provide a full-pledged protocol, but to show how the defined 19 framework and SDP elements can be combined together to design a CDP. 21 Status of this Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on December 22, 2012. 38 Copyright Notice 40 Copyright (c) 2012 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 This document may contain material from IETF Documents or IETF 54 Contributions published or made publicly available before November 55 10, 2008. The person(s) controlling the copyright in some of this 56 material may not have granted the IETF Trust the right to allow 57 modifications of such material outside the IETF Standards Process. 58 Without obtaining an adequate license from the person(s) controlling 59 the copyright in such materials, this document may not be modified 60 outside the IETF Standards Process, and derivative works of it may 61 not be created outside the IETF Standards Process, except to format 62 it for publication as an RFC or to translate it into languages other 63 than English. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 68 2. Definitions/Abbreviations . . . . . . . . . . . . . . . . . . 3 69 3. Construction of a Repair Flow from Multiple Source Flows . . . 3 70 3.1. Example: Two Source Flows Protected by a Single Repair 71 Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 4. Reconstruction of Source Flows from Repair Flow(s) . . . . . . 9 73 4.1. Example: Multiple Source Flows Protected by a Single 74 Repair Flow . . . . . . . . . . . . . . . . . . . . . . . 9 75 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 76 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 77 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 78 8. Normative References . . . . . . . . . . . . . . . . . . . . . 10 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 81 1. Introduction 83 The Forward Error Correction (FEC) Framework (described in [RFC6363]) 84 and SDP Elements for FEC Framework (described in [RFC6364]) together 85 define mechanisms sufficient enough to build an actual Content 86 Delivery Protocol (CDP) with FEC protection. Methods to convey FEC 87 Framework Configuration Information (described in 88 [I-D.ietf-fecframe-config-signaling]) on the other hand provides the 89 signaling protocols that may be used as part of CDP to communicate 90 FEC Scheme-Specific Information from FEC sender to a single as well 91 as multiple FEC receivers. This document aims at providing a 92 guideline on how the mechanisms defined in [RFC6363] and [RFC6364] 93 can be sufficiently used to design a CDP over a non-trivial scenario, 94 namely protection of multiple source flows with one or more repair 95 flows. 97 In particular, we provide clarifications and descriptions on how: 99 o source and repair flows may be uniquely identified, 101 o source blocks may be generated from one or more source flows, 103 o repair flows may be paired with the source flows, 105 o the receiver explicitly and implicitly identifies individual 106 flows, 108 o source blocks are regenerated at the receiver and the missing 109 source symbols in a source block are recovered. 111 2. Definitions/Abbreviations 113 This document uses all the definitions and abbreviations from Section 114 2 of [RFC6363]. 116 3. Construction of a Repair Flow from Multiple Source Flows 118 At the sender side, CDP constructs the source blocks (SB) by 119 multiplexing transport payloads from multiple flows (See Figure 1 and 120 Figure 2). According to the FEC Framework, each source block is FEC- 121 protected separately. Each source block is given to the specific FEC 122 encoder used within the CDP as input and as the outputs Explicit 123 Source FEC Payload ID, Repair FEC Payload ID, and Repair Payloads 124 corresponding to that source block are generated. Note that Explicit 125 Source FEC payload ID is optional and if CDP has implicit means of 126 constructing the source block at the sender/receiver (e.g., by using 127 any existing sequence numbers in the payload), the Explicit Source 128 FEC payload ID might not be output. 130 +------------+ 131 s_1 --------> | | 132 . Source | Source | +--------+ +--------+ +--------+ 133 . Flows | Block |==> ..|SB_(j+1)| | SB_j | |SB_(j-1)| .. 134 s_n --------> | Generation | +--------+ +--------+ +--------+ 135 +------------+ 137 Figure 1: Source Block generation for an FEC scheme 139 Figure 2 shows the structure of a source block. A CDP must clearly 140 specify which payload corresponds to which source flow and the length 141 of each payload. 143 <------------------ Source Block (SB) -------------------> 145 +-------...-----+-------...-----+- -+-------...-----+ 146 | Payload_1 | Payload_2 | . . . | Payload_n | 147 +-------...-----+-------...-----+- -+-------...-----+ 148 \______ _______|______ _______| |______ _______| 149 \/ \/ \/ 150 FID_1,Len_1 FID_2,Len_2 FID_n,Len_n 152 Figure 2: Structure of a Source Block 154 Flow ID (FID) value provides a unique short-hand identifier for the 155 source flows. FID is specified and associated with the possibly 156 wildcarded tuple of {source IP address, source port, destination IP 157 address, destination port, transport protocol} in the SDP 158 description. When wildcarded, certain fields in the tuple are not 159 needed for distinguishing the source flows. The tuple is carried in 160 the IP and transport headers of the source packets. Since FID is 161 utilized by the CDP and FEC scheme to distinguish between the source 162 packets, the tuple must have a one-to-one mapping to a valid FID. 163 This point will be clearer in the specific example given later in 164 this section. The length of FID must be a priori fixed and known to 165 both the receiver and sender. Alternatively, it might be specified 166 in the FEC-Scheme-Specific Information field in the SDP element 167 [RFC6364]. 169 The payload length (Len) information is needed to figure out how many 170 bits, bytes, or symbols (depending on the FEC scheme) from a 171 particular source flow are included in the source block. If the 172 payload is not an integer multiple of the specified symbol length, 173 the remaining portion is padded with zeros (See Figure 3 and 174 Figure 4). 176 +------+ 177 +--------+ +--------+ +--------+ | | -------> r_1 178 .. |SB_(j+1)| | SB_j | |SB_(j-1)| .. ==> | FEC | Repair . 179 +--------+ +--------+ +--------+ |Scheme| Flows . 180 | | -------> r_k 181 +------+ 183 Figure 3: Repair flow generation by an FEC scheme 185 <------------------ Source Block (SB) -------------------> 186 | | | | | | 187 +-------...-----+-------...-----+- -+-------...-----+ | 188 | Payload_1 | Payload_2 | . . . | Payload_n |0| 189 +-------...-----+-------...-----+- -+-------...-----+ | 190 | | | | | | 191 | Symbol_1 | Symbol_2 | Symbol_3 | . . . | Symbol_m | 192 |<-------->|<-------->|<-------->| |<-------->| 194 +------+ 195 Symbol_1,..,Symbol_m => | FEC | => Symbol_u,..,Symbol_1 196 | Enc. | 197 +------+ 199 Figure 4: Repair flow payload generation 201 FEC schemes typically expect a source block of certain size, say m 202 symbols. Therefore, the FEC encoder divides each source block into m 203 symbols (with some padding if the source block is shorter than the 204 expected m symbols) and generates u repair symbols which are 205 functions of the m symbols in the original source block. The repair 206 symbols are grouped by the FEC scheme into repair payloads with each 207 repair payload assigned a Repair FEC Payload ID in order to associate 208 each repair payload with a particular source block at the receiver. 209 If the payloads in a given source block have sequence numbers that 210 can uniquely specify their location in the source block, an Explicit 211 Source FEC Payload ID may not be generated for these payloads. 212 Otherwise, Explicit Source FEC Payload IDs are generated for each 213 payload and indicate the order the payloads appear in the source 214 block. 216 Note that FID and length information are not actually transmitted 217 with the source payloads since both information can be gathered by 218 other means as it will be clear in the next sections. 220 3.1. Example: Two Source Flows Protected by a Single Repair Flow 222 In this section, we present an example of source flow and repair flow 223 generation by the CDP. We have two source flows with flow IDs of 0 224 and 1 to be protected by a single repair flow (See Figure 5). The 225 first source flow is multicast to 233.252.0.1 and the second source 226 flow is multicast to 233.252.0.2. Both flows use the port number 227 30000. The SDP description below states that the source flow defined 228 by the tuple {*,*,233.252.0.1,30000} is identified with FID=0 and the 229 source flow defined by the tuple {*,*,233.252.0.2,30000} is 230 identified with FID=1. The SDP description also states that the 231 repair flow is to be received at the multicast address of 233.252.0.3 232 and at port 30000. 234 SOURCE FLOWS 235 S1: Source Flow | | INSTANCE #1 236 |---------| R3: Repair Flow 237 S2: Source Flow | 239 Figure 5: Example: Two source flows and one repair flow 241 v=0 242 o=ali 1122334455 1122334466 IN IP4 fec.example.com 243 s=FEC Framework Examples 244 t=0 0 245 a=group:FEC-FR S1 S2 R3 246 m=video 30000 RTP/AVP 100 247 c=IN IP4 233.252.0.1/127 248 a=rtpmap:100 MP2T/90000 249 a=fec-source-flow: id=0 250 a=mid:S1 251 m=video 30000 RTP/AVP 101 252 c=IN IP4 233.252.0.2/127 253 a=rtpmap:101 MP2T/90000 254 a=fec-source-flow: id=1 255 a=mid:S2 256 m=application 30000 UDP/FEC 257 c=IN IP4 233.252.0.3/127 258 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 259 a=repair-window:150ms 260 a=mid:R3 262 Figure 6 shows the first and the second source blocks (SB_1 and SB_2) 263 generated from these two source flows. In this example, SB_1 is of 264 length 10000 bytes. Suppose that the FEC scheme uses a symbol length 265 of 512 bytes. Then SB_1 can be divided into 20 symbols after padding 266 the source block for 240 bytes. Assume that the FEC scheme is 267 rate-2/3 erasure code, hence, it generates 10 repair symbols from 20 268 original symbols for SB_1. On the other hand, SB_2 is 7000-byte long 269 and can be divided into 14 symbols after padding 168 bytes. Using 270 the same encoder, suppose that 7 repair symbols are generated for 271 SB_2. 273 <-------- Source Block 1 --------> 274 +------------+-------------------+ 275 | $1 $2 $3 $4| #1 #2 #3 #4 #5 #6 | 0..00 276 +------------+-------------------+ 277 \__________________ __________________/ 278 \/ 279 @1 @2 @3 @4 @5 @6 @7 @8 @9 @10 281 <---- Source Block 2 ----> 282 +----------------+-------+ 283 | $5 $6 $7 $8 $9 | #7 #8 |0..00 284 +----------------+-------+ 285 \______________ _____________/ 286 \/ 287 @11 @12 @13 @14 @15 @16 @17 289 $: 1000-byte payload from source flow 1 290 #: 1000-byte payload from source flow 2 291 @: Repair symbol 293 Figure 6: Source block with two source flows 295 The information on the unit of payload length, FEC scheme, symbol 296 size, and coding rates can be specified in the FEC Scheme-Specific 297 Information (FSSI) field of the SDP element. If the values of the 298 payload lengths from each source flow and the order of appearance of 299 source flows in every source block are fixed during the session, 300 these values may be also provided in the FSSI field. To carry FSSI 301 information to the FEC receivers, one may use the signaling methods 302 described in [I-D.ietf-fecframe-config-signaling]. In our example, 303 we will consider the case where the ordering is fixed and known both 304 at the sender and the receiver, but the payload lengths will be 305 variable from one source block to another. We assume that the 306 payload of a source flow with an FID smaller than another flow's FID 307 precedes other payloads in a source block. 309 The FEC scheme gets the source blocks as input and generates the 310 parity blocks for each source block to protect the whole source 311 block. In the example, the repair payloads for SB_1 consist of 512- 312 byte symbols, denoted by @1 to @10. Similarly @11 to @17 constitute 313 the repair payloads for SB_2. The FEC scheme outputs the repair 314 payloads along with the Repair FEC Payload IDs. In our example, 315 Repair FEC Payload ID provides information on the source block 316 sequence number and the order the repair symbols are generated. For 317 instance @3 is the third FEC repair symbol for SB_1 and the three 318 tuple {@3,SB_1,3} can uniquely deliver this information. In our 319 example, the FEC scheme also provides Explicit Source FEC Payload IDs 320 that carry information to indicate which source symbols correspond to 321 which source block sequence number and the relative position in the 322 source block. For instance the two tuple {SB_2,2} can be attached to 323 $6 as the Explicit Source FEC Payload ID to indicate that $6 is 324 protected together with packets belonging to SB_2, and $6 is the 325 second payload in SB_2. 327 The source packets are generated from the source symbols by 328 concatenating consecutive symbols in one packet. There should not be 329 any fragmentation of a source symbol, e.g., symbols #7 and #8 can be 330 concatenated in one transport payload of 2000-bytes (The 331 implementation should make sure that the size of the resulting source 332 packet - payload plus the overhead - is not larger than the path 333 MTU), but one portion of symbol #7 should not be put in one source 334 packet and the remaining portion in another source packet. The 335 simplest implementation is to place each source symbol in a different 336 source packet as shown in Figure 7. 338 +------------------------------------+ 339 | IP header {233.252.0.1} | 340 +------------------------------------+ 341 | Transport header {30000} | 342 +------------------------------------+ 343 | Original Transport Payload {$6} | 344 +------------------------------------+ 345 | Source FEC Payload ID {SB_2,2} | 346 +------------------------------------+ 348 Figure 7: Example of a source packet 350 The repair packets are generated from the repair symbols belonging to 351 the same source block by grouping consecutive symbols in one packet. 352 There should not be any fragmentation of a repair symbol, e.g., 353 symbols @4, @5, and @6 can be concatenated in one transport payload 354 of 1536-bytes, but @6 should not be divided into smaller sub-symbols 355 and spread over multiple repair packets. The Repair FEC Payload ID 356 must carry sufficient information for the decoding process and in our 357 example indicating source block sequence number, length of each 358 source payload, and the order that the first parity block in a repair 359 packet is generated are sufficient. The exact header format of 360 Repair FEC Payload ID may be specified in the FSSI field of the SDP 361 element. In Figure 8 for instance, the repair symbols @4, @5, and @6 362 are concatenated together. The Payload ID {SB_1,4,4,6} states that 363 the repair symbols protect SB_1, the first repair symbol in the 364 payload is generated as the 4th symbol and the source block consists 365 of two source flows carrying 4 and 6 packets from each. 367 +------------------------------------+ 368 | IP header {233.252.0.3} | 369 +------------------------------------+ 370 | Transport header {30000} | 371 +------------------------------------+ 372 | Repair FEC Payload ID {SB_1,4,4,6} | 373 +------------------------------------+ 374 | Repair Symbols {@4,@5,@6} | 375 +------------------------------------+ 377 Figure 8: Example of a repair packet 379 4. Reconstruction of Source Flows from Repair Flow(s) 381 4.1. Example: Multiple Source Flows Protected by a Single Repair Flow 383 At the receiver, source flows 1 and 2 are received at 384 {233.252.0.1,30000} and {233.252.0.2,30000}, while the repair flow is 385 received at {233.252.0.3,30000}. The CDP can map these tuples to the 386 flow IDs using the SDP elements. Accordingly, the payloads received 387 at {233.252.0.1,30000} and {233.252.0.2,30000} are mapped to flow IDs 388 0 and 1, respectively. 390 The CDP passes the flow IDs and received payloads along with the 391 Explicit Source FEC Payload ID to the FEC scheme defined in the SDP 392 description. The CDP also passes the received repair packet payloads 393 and Repair FEC Payload ID to the FEC scheme. The FEC scheme can 394 construct the original source block with missing packets by using the 395 information given in the FEC Payload IDs. The FEC Repair Payload ID 396 provides the information that SB_1 has packets from two flows with 4 397 packets from the first one and 6 packets from the second one. Flow 398 IDs state that the packets from source flow 0 precedes the packets 399 from source flow 1. Explicit Source FEC Payload IDs on the other 400 hand provide the information about which source payload appears in 401 what order. Therefore, the FEC scheme can depict an source block 402 with exact locations of the missing packets. Figure 9 depicts the 403 case for SB_1. Since the original source block with missing packets 404 can be constructed at the decoder and the FEC scheme knows the coding 405 rate (e.g., it might be carried in the FSSI field in the SDP 406 description), a proper decoding operation can start as soon as the 407 repair symbols are provided to the FEC scheme. 409 <-------- Source Block 1 --------> 410 +------------+-------------------+ 411 | $1 $2 X X | #1 X #3 #4 #5 #6 | 412 +------------+-------------------+ 414 O: Symbols received from the source flow 1 for SB_1 415 #: Symbols received from the source flow 2 for SB_1 416 X: Lost source symbols 418 Figure 9: Source block regeneration 420 When the FEC scheme can recover any missing symbol while more repair 421 symbols are arriving, it provides the recovered blocks along with the 422 source flow IDs of the recovered blocks as outputs to the CDP. The 423 receiver knows how long to wait to repair the remaining missing 424 packets (e.g., specified by the 'repair-window' attribute in the SDP 425 description). After the associated timer expires, the CDP hands over 426 whatever could be recovered from the source flow to the application 427 layer and continues with processing the next source block. 429 5. Security Considerations 431 For the general security considerations related to the FEC Framework, 432 refer to [RFC6363]. There are no additional security considerations 433 that apply to this document. 435 6. IANA Considerations 437 There are no IANA related issues considered in this document. 439 7. Acknowledgments 441 The authors would like to thank the FEC Framework Design Team for 442 their inputs, suggestions and contributions. 444 8. Normative References 446 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 447 Correction (FEC) Framework", RFC 6363, October 2011. 449 [RFC6364] Begen, A., "Session Description Protocol Elements for the 450 Forward Error Correction (FEC) Framework", RFC 6364, 451 October 2011. 453 [I-D.ietf-fecframe-config-signaling] 454 Asati, R., "Methods to convey FEC Framework Configuration 455 Information", draft-ietf-fecframe-config-signaling-09 456 (work in progress), June 2012. 458 Authors' Addresses 460 Ulas C. Kozat 461 DoCoMo USA Labs 462 3240 Hillview Avenue 463 Palo Alto, CA 94304-1201 464 USA 466 Phone: +1 650 496 4739 467 Email: kozat@docomolabs-usa.com 469 Ali Begen 470 Cisco 471 181 Bay Street 472 Toronto, ON M5J 2T3 473 Canada 475 Email: abegen@cisco.com