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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 NWCRG J. Detchart 3 Internet-Draft E. Lochin 4 Intended status: Experimental J. Lacan 5 Expires: January 7, 2016 ISAE 6 V. Roca 7 INRIA 8 July 6, 2015 10 Tetrys, an On-the-Fly Network Coding protocol 11 draft-detchart-nwcrg-tetrys-02 13 Abstract 15 This document describes Tetrys, an On-The-Fly Network Coding (NC) 16 protocol that can be used to transport delay and loss sensitive data 17 over a lossy network. Tetrys can recover from erasures within a RTT- 18 independent delay, thanks to the transmission of coded packets. It 19 can be used for both unicast, multicast and anycast communications. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on January 7, 2016. 38 Copyright Notice 40 Copyright (c) 2015 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 57 2. Definitions, Notations and Abbreviations . . . . . . . . . . 3 58 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4 59 3.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 61 4. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 6 62 4.1. Common Header Format . . . . . . . . . . . . . . . . . . 6 63 4.1.1. Header Extensions . . . . . . . . . . . . . . . . . . 8 64 4.2. Source Packet Format . . . . . . . . . . . . . . . . . . 9 65 4.3. Coded Packet Format . . . . . . . . . . . . . . . . . . . 10 66 4.4. Acknowledgement Packet Format . . . . . . . . . . . . . . 11 67 5. The Coding Coefficient Generator Identifiers . . . . . . . . 12 68 5.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 12 69 5.2. Table of Identifiers . . . . . . . . . . . . . . . . . . 12 70 6. Tetrys Basic Functions . . . . . . . . . . . . . . . . . . . 12 71 6.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 12 72 6.1.1. Encoding Vector Formats . . . . . . . . . . . . . . . 13 73 6.2. The Elastic Encoding Window . . . . . . . . . . . . . . . 16 74 6.3. Recoding . . . . . . . . . . . . . . . . . . . . . . . . 17 75 6.3.1. Principle . . . . . . . . . . . . . . . . . . . . . . 17 76 6.3.2. Generating a coded symbol at an intermediate node . . 17 77 6.4. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 17 78 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 79 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 80 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 81 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 82 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 83 11.1. Normative References . . . . . . . . . . . . . . . . . . 18 84 11.2. Informative References . . . . . . . . . . . . . . . . . 18 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 87 1. Introduction 89 This document describes Tetrys, a novel network coding protocol. 90 Network codes were introduced in the early 2000s [AHL-00] to address 91 the limitations of transmission over the Internet (delay, capacity 92 and packet loss). While the use of network codes is fairly recent in 93 the Internet community, the use of application layer erasure codes in 94 the IETF has already been standardized in the RMT [RMT] and the 95 FECFRAME [FECFRAME] working groups. The protocol presented here can 96 be seen as a network coding extension to standards solutions. The 97 current proposal can be considered as a combination of network 98 erasure coding and feedback mechanisms [Tetrys]. 100 The main innovation of the Tetrys protocol is in the generation of 101 coded packets from an elastic encoding window periodically updated 102 with the receiver's feedbacks. This update is done in such a way 103 that any source packets coming from an input flow is included in the 104 encoding window as long as it is not acknowledged or the encoding 105 window did not reach a size limit. This mechanism allows for losses 106 on both the forward and return paths and in particular is resilient 107 to acknowledgement losses. 109 With Tetrys, a coded packet is a linear combination over a finite 110 field of the data source packets belonging to the coding window. The 111 choice of the finite field of the coefficients is a trade-off between 112 the best performance (with non-binary coefficients) and the system 113 constraints (binary codes in an energy constrained environment) and 114 is driven by the application. 116 Thanks to the elastic encoding window, the coded packets are built 117 on-the-fly, by using an algorithm or a function to choose the 118 coefficients. The redundancy ratio can be dynamically adjusted, and 119 the coefficients can be generated in different ways along a 120 transmission. Compared to FEC block codes, this allows to reduce the 121 bandwidth use and the decoding delay. 123 1.1. Requirements Notation 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in [RFC2119]. 129 2. Definitions, Notations and Abbreviations 131 The terminology used in this document is presented below. It is 132 aligned with the FECFRAME terminology as well as with recent 133 activities in the Network Coding Research Group. 135 Source symbol: a symbol that has to be transmitted between the 136 ingress and egress of the network. 138 Coded symbol: a linear combination over a finite field of a set of 139 source symbols. 141 Source symbol ID: a sequence number to identify the source 142 symbols. 144 Coded symbol ID: a sequence number to identify the coded symbols. 146 Encoding coefficients: elements of the finite field characterizing 147 the linear combination used to generate a coded symbol. 149 Encoding vector: set of the encoding coefficients and input source 150 symbol IDs. 152 Source packet: a source packet contains a source symbol with its 153 associated IDs. 155 Coded packet: a coded packet contains a coded symbol, the coded 156 symbol's ID and encoding vector. 158 Input symbol: a symbol at the input of the Tetrys Encoding 159 Building Block. 161 Output symbol: a symbol generated by the Tetrys Encoding Building 162 Block. For a non systematic mode, all output symbols are coded 163 symbols. For a systematic mode, output symbols can be the input 164 symbols and a number of coded symbols that are linear combinations 165 of the input symbols. 167 Feedback packet: a feedback packet is a packet containing 168 information about the decoded or received source symbols. It can 169 also bring additional information about the Packet Error Rate or 170 the number of various packets in the receiver decoding window. 172 Elastic Encoding Window: an encoder-side buffer that stores all 173 the non-acknowledged source packets of the input flow that are 174 involved in the coding process. 176 Coding Coefficient Generator Identifier: a unique identifier that 177 define a function or an algorithm allowing to generate the 178 encoding vector. 180 Code rate: Define the rate between the number of input symbols and 181 the number of output symbols. 183 3. Architecture 185 -- Editor's note: The architecture used in this document should be 186 aligned with the future NC Architecture document [NWCRG-ARCH]. -- 188 3.1. Use Cases 190 Tetrys is well suited, but not limited to the use case where there is 191 a single flow originated by a single source, with intra stream coding 192 that takes place at a single encoding node. Note that the input 193 stream can be a multiplex of several upper layer streams. 195 Transmission can be over a single path or multiple paths. In 196 addition, the flow can be sent in unicast, multicast, or anycast 197 mode. 199 3.2. Overview 201 +----------+ +----------+ 202 | | | | 203 | App | | App | 204 | | | | 205 +----------+ +----------+ 206 | ^ 207 | source source | 208 | symbols symbols | 209 | | 210 v | 211 +----------+ +----------+ +----------+ 212 | | output packets | | output packets | | 213 | Tetrys |----------------->| Tetrys |----------------->| Tetrys | 214 | Encoder | feedback packets | Recoder | feedback packets | Decoder | 215 | |<-----------------| |<-----------------| | 216 +----------+ +----------+ +----------+ 218 Figure 1: Tetrys Architecture 220 The Tetrys protocol features several key functionalities: 222 o On-the-fly encoding; 224 o Recoding; 226 o Decoding; 228 o Signaling, to carry in particular the symbol identifiers in the 229 encoding window and the associated coding coefficients when 230 meaningful, in a manner that was previously used in FEC; 232 o Feedback management; 234 o Elastic window management; 236 o Channel estimation; 238 o Dynamic adjustment of the code rate and flow control; 240 o Congestion control management (if appropriate); 242 -- Editor's note: must be discussed -- 244 o Tetrys packet header creation and processing; 246 o -- Editor's note: something else? -- 248 These functionalities are provided by several building blocks: 250 o The Tetrys Building Block: this BB is used during encoding, 251 recoding and decoding processes. It must be noted that Tetrys 252 does not mandate a specific building block. Instead any building 253 block compatible with the elastic encoding window feature of 254 Tetrys can be used. 256 o The Window Management Building Block: this building block is in 257 charge of managing the encoding encoding window at a Tetrys 258 sender. 260 -- Editor's note: Is it worth moving it in a dedicated BB? To 261 be discussed -- 263 o Other ? 265 In order to enable future components and services to be added 266 dynamically, Tetrys adds a header extension mechanism, compatible 267 with that of LCT, NORM, FECFRAME [REFS]. 269 4. Packet Format 271 4.1. Common Header Format 273 All types of Tetrys packets share the same common header format (see 274 Figure 2). 276 0 1 2 3 277 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 1 278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 279 | V | C |S| Reserved | HDR_LEN | Packet Type | 280 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 281 | Congestion Control Information (CCI, length = 32*C bits) | 282 | ... | 283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 284 | Transport Session Identifier (TSI, length = 32*S bits) | 285 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 286 | Header Extensions (if applicable) | 287 | ... | 288 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 290 Figure 2: Common Header Format 292 -- Editor's note: this format inherits from the LCT header format 293 (RFC 5651) with slight modifications. -- 295 o Tetrys version number (V): 4 bits. Indicates the Tetrys version 296 number. The Tetrys version number for this specification is 1. 298 o Congestion control flag (C): 2 bits. C=0 indicates the Congestion 299 Control Information (CCI) field is 0 bits in length. C=1 300 indicates the CCI field is 32 bits in length. C=2 indicates the 301 CCI field is 64 bits in length. C=3 indicates the CCI field is 96 302 bits in length. 304 -- Editor's note: version number and congestion control to be 305 discussed -- 307 o Transport Session Identifier flag (S): 1 bit. This is the number 308 of full 32-bit words in the TSI field. The TSI field is 32*S bits 309 in length, i.e., the length is either 0 bits or 32 bits. 311 o Reserved (Resv): 9 bits. These bits are reserved. In this 312 version of the specification, they MUST be set to zero by senders 313 and MUST be ignored by receivers. 315 o Header length (HDR_LEN): 8 bits. Total length of the Tetrys 316 header in units of 32-bit words. The length of the Tetrys header 317 MUST be a multiple of 32 bits. This field can be used to directly 318 access the portion of the packet beyond the Tetrys header, i.e., 319 to the first other header if it exists, or to the packet payload 320 if it exists and there is no other header, or to the end of the 321 packet if there are no other headers or packet payload. 323 o Packet Type: 8 bits. Type of packet. 325 o Congestion Control Information (CCI): 0, 32, 64, or 96 bits Used 326 to carry congestion control information. For example, the 327 congestion control information could include layer numbers, 328 logical channel numbers, and sequence numbers. This field is 329 opaque for the purpose of this specification. This field MUST be 330 0 bits (absent) if C=0. This field MUST be 32 bits if C=1. This 331 field MUST be 64 bits if C=2. This field MUST be 96 bits if C=3. 333 o Transport Session Identifier (TSI): 0 or 32 bits. The TSI 334 uniquely identifies a session among all sessions from a particular 335 sender. The TSI is scoped by the IP address of the sender, and 336 thus the IP address of the sender and the TSI together uniquely 337 identify the session. Although a TSI in conjunction with the IP 338 address of the sender always uniquely identifies a session, 339 whether or not the TSI is included in the Tetrys header depends on 340 what is used as the TSI value. If the underlying transport is 341 UDP, then the 16-bit UDP source port number MAY serve as the TSI 342 for the session. If the TSI value appears multiple times in a 343 packet, then all occurrences MUST be the same value. If there is 344 no underlying TSI provided by the network, transport or any other 345 layer, then the TSI MUST be included in the Tetrys header. 347 4.1.1. Header Extensions 349 Header Extensions are used in Tetrys to accommodate optional header 350 fields that are not always used or have variable size. The presence 351 of Header Extensions can be inferred by the Tetrys header length 352 (HDR_LEN). If HDR_LEN is larger than the length of the standard 353 header, then the remaining header space is taken by Header Extension 354 fields. 356 If present, Header Extensions MUST be processed to ensure that they 357 are recognized before performing any congestion control procedure or 358 otherwise accepting a packet. The default action for unrecognized 359 Header Extensions is to ignore them. This allows the future 360 introduction of backward-compatible enhancements to Tetrys without 361 changing the Tetrys version number. Non-backward-compatible Header 362 Extensions CANNOT be introduced without changing the Tetrys version 363 number. 365 There are two formats for Header Extension fields, as depicted in 366 Figure 3. The first format is used for variable-length extensions, 367 with Header Extension Type (HET) values between 0 and 127. The 368 second format is used for fixed-length (one 32-bit word) extensions, 369 using HET values from 128 to 255. 371 0 1 2 3 372 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 1 373 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 374 | HET (<=127) | HEL | | 375 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 376 . . 377 . Header Extension Content (HEC) . 378 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 380 0 1 2 3 381 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 1 382 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 383 | HET (>=128) | Header Extension Content (HEC) | 384 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 386 Figure 3: Header Extension Format 388 o Header Extension Type (HET): 8 bits The type of the Header 389 Extension. This document defines a number of possible types. 390 Additional types may be defined in future versions of this 391 specification. HET values from 0 to 127 are used for variable- 392 length Header Extensions. HET values from 128 to 255 are used for 393 fixed-length 32-bit Header Extensions. 395 o Header Extension Length (HEL): 8 bits The length of the whole 396 Header Extension field, expressed in multiples of 32-bit words. 397 This field MUST be present for variable-length extensions (HETs 398 between 0 and 127) and MUST NOT be present for fixed-length 399 extensions (HETs between 128 and 255). 401 o Header Extension Content (HEC): variable length The content of the 402 Header Extension. The format of this sub-field depends on the 403 Header Extension Type. For fixed-length Header Extensions, the 404 HEC is 24 bits. For variable-length Header Extensions, the HEC 405 field has variable size, as specified by the HEL field. Note that 406 the length of each Header Extension field MUST be a multiple of 32 407 bits. Also note that the total size of the Tetrys header, 408 including all Header Extensions and all optional header fields, 409 cannot exceed 255 32-bit words. 411 4.2. Source Packet Format 413 A source packet is the encapsulation of a source symbol, a source 414 symbol ID and a Common Packet Header. The source symbols can have 415 variable sizes. 417 0 1 2 3 418 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 419 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 420 | | 421 / Common Packet Header / 422 | | 423 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 424 | Source Symbol ID | 425 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 426 | | 427 / Payload / 428 | | 429 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 431 Figure 4: Source Packet Format 433 Common Packet Header: a common packet header where Packet Type=0. 435 Source Symbol ID: the sequence number to identify a source symbol. 437 Payload: the payload (source symbol) 439 4.3. Coded Packet Format 441 A coded packet is the encapsulation of a coded symbol, a coded symbol 442 ID, the associated encoding vector and the Common Packet Header. As 443 the source symbols CAN have variable sizes, each source symbol size 444 need to be encoded, and the result must be stored in the coded packet 445 as the Encoded Payload Size (16 bits): as it is an optional field, 446 the encoding vector MUST signal the use of variable source symbol 447 sizes with the field V (see Section 6.1.1.2). 449 0 1 2 3 450 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 1 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 | | 453 / Common Packet Header / 454 | | 455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 456 | Coded Symbol ID | 457 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 458 | | 459 / Encoding Vector / 460 | | 461 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 462 | Encoded Payload Size | | 463 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 464 | | 465 / Payload / 466 | | 467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 469 Figure 5: Coded Packet Format 471 Common Packet Header: a common packet header where Packet Type=1. 473 Coded Symbol ID: the sequence number to identify a coded symbol. 475 Encoding Vector: an encoding vector to define the linear combination 476 used (coefficients, and source symbols). 478 Encoded Payload Size: the coded payload size used if the source 479 symbols have variable size (optional, Section 6.1.1.2)). 481 Payload: the coded symbol. 483 4.4. Acknowledgement Packet Format 485 A Tetrys Decoding Building Block or Tetrys Recoding Building Block 486 MAY send back to another building block some Acknowledgement packets. 487 They contain information about what it is received and/or decoded, 488 and other information such as a packet loss rate or the size of the 489 decoding buffers. The acknowledgement packets are OPTIONAL hence 490 they could be omitted or lost in transmission without impacting the 491 basic protocol performance. 493 0 1 2 3 494 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 1 495 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 496 | | 497 / Common Packet Header / 498 | | 499 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 500 | Nb of missing source symbols | 501 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 502 | Nb of not already used coded symbols | 503 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 504 | First Source Symbol ID | 505 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 506 | SACK size | | 507 +-+-+-+-+-+-+-+-+ + 508 | | 509 / SACK Vector / 510 | | 511 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 513 Figure 6: Acknowledgement Packet Format 515 Common Packet Header: a common packet header where Packet Type=2. 517 Nb missing source symbols: the number of missing source symbols in 518 the receiver. 520 Nb of not already used coded symbols: the number of not already used 521 coded symbols in the receiver that have not already been used for 522 decoding. Meaning the number of linear combinations containing at 523 least 2 unknown source symbols. 525 First Source Symbol ID: ID of the first source symbol to acknowledge. 527 SACK size: the size of the SACK vector in 32-bit words. For 528 instance, with value 2, the SACK vector is 64 bits long. 530 SACK vector: bit vector indicating the acknowledged symbols following 531 the first source symbol ID. The "First Source Symbol" is not 532 included in this bit vector. A bit equal to 1 at position i means 533 that the source symbol of ID equal to "First Source Symbol ID" + i + 534 1 is acknowledged by this acknowledgment packet. 536 5. The Coding Coefficient Generator Identifiers 538 5.1. Definition 540 The Coding Coefficient Generator Identifier defines a function or an 541 algorithm to build the coding coefficients used to generate the coded 542 symbols. They MUST be known by all the Building Blocks. 544 5.2. Table of Identifiers 546 0000: GF256 Vandermonde based coefficients. Each coefficient is 547 build as alpha^( (source_symbol_id*coded_symbol_id) % 255). 549 0001: GF16 Vandermonde based coefficients. Each coefficient is build 550 as alpha^( (source_symbol_id*coded-symbol_id) % 15). 552 0010: SRLC. 554 Others: To be discussed. 556 6. Tetrys Basic Functions 558 6.1. Encoding 560 At the beginning of a transmission, a Tetrys Encoding Building Block 561 MUST choose an initial code rate (added redundancy) as it doesn't 562 know the packet loss rate of the channel. In steady state, the 563 Tetrys Encoding Building Block generates coded symbols when it 564 receives some information from the decoding or recoding blocks. 566 When a Tetrys Encoding Building Block needs to generate a coded 567 symbol, it considers the set of source symbols stored in the Elastic 568 Encoding Window. These source symbols are the set of source symbols 569 which are not yet acknowledged by the receiver. 571 A Tetrys Encoding Building Block SHOULD set a limit of the Elastic 572 Encoding Window size. This allows to reduce the complexity by 573 considering less source symbols. It also provides a coping mechanism 574 if all the acknowledgment packets are lost. 576 At the generation of a coded symbol, the Tetrys Encoding Building 577 Block generates an encoding vector containing the IDs of the source 578 symbols stored in the Elastic Encoding Window. For each source 579 symbol, a finite field coefficient is determined using a Coding 580 Coefficient Generator. This generator CAN take as input the source 581 symbol ID and the coded symbol ID and CAN determine a coefficient in 582 a deterministic way. A classical example of such deterministic 583 function is a generator matrix where the rows are indexed by the 584 source symbol IDs and the columns by the coded symbol IDs. For 585 example, the entries of this matrix can be built from a Vandermonde 586 structure, like Reed-Solomon codes, or from a sparse binary matrix, 587 like Low-Density Generator Matrix codes. Finally, the coded symbol 588 is the sum of the source symbols multiplied by their corresponding 589 coefficients. 591 6.1.1. Encoding Vector Formats 593 The encoding vectors are sent in each coded symbols. They CAN 594 contain the source symbol IDs and/or the coefficients. 596 To avoid the overhead of transmitting all the source symbol IDs, the 597 following algorithm is used to compress them. 599 6.1.1.1. Transmitting the source symbol IDs 601 The source symbol IDs are organized as a sorted list of 32-bit 602 integers. Instead of sending the full list, a differential transform 603 to reduce the number of bits needed to represent an ID is used. 605 6.1.1.1.1. Compressing the Source symbol IDs 607 Assume the symbol IDs used in the combination are: 608 [1..3],[5..6],[8..10]. 610 1. Keep the first element in the packet as the first_source_id: 1. 612 2. Apply a differential transform to the others elements 613 ([3,5,6,8,10]) which removes the element i-1 to the element i, 614 starting with the first_source_id as i0, and get the list L => 615 [2,2,1,2,2] 617 3. Compute b, the number of bits needed to store all the elements, 618 which is ceil(log2(max(L))): here, 2 bits. 620 4. Write b in the corresponding field, and write all the b * [(2 * 621 NB blocks) - 1] elements in a bit vector, here: 10 10 01 10 10. 623 6.1.1.1.2. Decompressing the Source symbol IDs 625 When a Tetrys Decoding Building Block wants to reverse the 626 operations, this algorithm is used: 628 1. Rebuild the list of the transmitted elements by reading the bit 629 vector and b: [10 10 01 10 10] => [2,2,1,2,2] 631 2. Apply the reverse transform by adding successively the elements, 632 starting with first_source_id: [1,1+2,(1+2)+2,(1+2+2)+1,...] => 633 [1,3,5,6,8,10] 635 3. Rebuild the blocks using the list and first_source_id: 636 [1..3],[5..6],[8..10]. 638 6.1.1.2. Encoding Vector Format 640 The encoding vector CAN be used to store the source symbol IDs 641 included in the associated coded symbol, the coefficients used in the 642 combination, or both. It CAN be used to send only the number of 643 source symbols included in the coded symbol. 645 If the source IDs are stored, the nb of blocks MUST be different from 646 0. 648 The encoding vector format uses a 4-bit Coding Coefficient Generator 649 Identifier to identity the algorithm to generate the coefficients, 650 and contains a set of blocks for the source symbol IDs used in the 651 combination. In this format, the number of blocks is stored as a 652 8-bit unsigned integer. To reduce the overhead, a compressed way to 653 store the symbol IDs is used: the IDs are not stored as themselves, 654 but stored as the difference between the previous. 656 0 1 2 3 657 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 1 658 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 659 | EV_LEN | CCGI |I|C|V| | NB_BLOCKS | NB_COEFS | 660 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 661 | FIRST_SOURCE_ID | 662 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 663 | b_id | | 664 +-+-+-+-+-+-+-+ id_bit_vector +-+-+-+-+-+-+-+ 665 | | Padding | 666 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 667 | b_coef | | 668 +-+-+-+-+-+-+-+ coef_bit_vector +-+-+-+-+-+-+-+ 669 | | Padding | 670 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 672 Figure 7: Encoding Vector Format 674 o Encoding Vector Length (EV_LEN): size in units of 32-bit words. 676 o Coding Coefficient Generator Identifier (CCGI): 4-bit ID to 677 identify the algorithm or the function used to generate the 678 coefficients (see Section 5). As a CCGI is included in each 679 encoded vector, it can dynamically change between the generation 680 of 2 coded symbols. 682 o Store the IDs flag (I): 1 bit to know if an encoding vector 683 contains the list of the IDs used. MUST be 1 if the Encoding 684 Vector stores the source symbol IDs. 686 o Store the coefficients flag (C): 1 bit to know if an encoding 687 vector contains information about the coefficients used. 689 o Having source symbols with variable size flag (V): set V to 1 if 690 the combination which refers the encoding vector is a combination 691 of source symbols with variable sizes. In this case, the coded 692 packets MUST have the 'Encoded Payload Size' field. 694 o Number of blocks used to store the source symbol IDs (NB_BLOCKS): 695 the number of blocks used to store all the source symbol IDs. 697 o Number of coefficients (NB_COEFS): The number of the coefficients 698 used to generate the associated coded symbol. 700 o The first source Identifier (FIRST_SOURCE_ID): the first source 701 symbol ID used in the combination. 703 o Number of bits for each edge block (b_id): the number of bits 704 needed to store the edge (see Section 6.1.1.1). 706 o The compressed edge blocks (id_bit_vector): equal to b_id * 707 (NB_BLOCKS * 2 - 1). 709 o Number of bits needed to store each coefficient (b_coef): the 710 number of bits used to store the coefficients. 712 o The coefficients (coef_bit_vector): The coefficients stored (as a 713 vector of b_coef * NB_COEFS). 715 o Padding: padding to have an Encoding Vector size multiple of 716 32-bit (for the id and coefficient part). 718 6.2. The Elastic Encoding Window 720 When an input source symbol is passed to a Tetrys Encoding Building 721 Block, it is added to the Elastic Encoding Window. This window MUST 722 have a limit set by the encoding building Block (depending of the use 723 case: unicast, multicast, file transfer, real-time transfer, ...). 724 If the Elastic Encoding Window reached its limit, the window slides 725 over the symbols: the first (oldest) symbols are removed. Then, a 726 packet containing this symbol can be sent onto the network. As an 727 element of the coding window, this symbol is included in the next 728 linear combinations created to generate the coded symbols. 730 As explained below, the receiver or the recoder sends periodic 731 feedback indicating the received or decoded source symbols. In the 732 case of a unicast transmission, when the sender receives the 733 information that a source symbol was received and/or decoded by the 734 receiver, it removes this symbol from the coding window. 736 In a multicast transmission: 738 o If the acknowledgement packets are not enabled, the coding window 739 grows up to a limit. When the limit is reached, the oldest 740 symbols are removed from the coding window. 742 o If the acknowledgement packets are enabled, a source symbol is 743 removed from the coding window when all the receivers have 744 received or decoded it or when the coding window reaches its 745 limit. 747 6.3. Recoding 749 6.3.1. Principle 751 A Tetrys Recoding Block maintains a list of the ID of the source 752 symbols included in the Elastic Coding Window of the sender. It also 753 stores a set of received source and coded symbols able to regenerate 754 the set or a subset of the symbols of the Elastic Coding Window. In 755 other words, if R1, ..., Rt represent t received symbols and S1, ..., 756 Sk represent the set or a subset of the source symbols of the Elastic 757 Coding Window, there exists a t*k-matrix M such that (R1, ..., Rt).M 758 = (S1, ..., Sk). 760 6.3.2. Generating a coded symbol at an intermediate node 762 At the generation of a coded symbol, the Tetrys Recoding Building 763 Block generates an encoding vector containing the IDs of the source 764 symbols stored in the Elastic Encoding Window or in the subset of the 765 Elastic Encoding Window that it is able to regenerate. The Tetrys 766 Recoding Building Block then generates a new coded symbol ID 767 different from the received coded symbol IDs. From this coded symbol 768 ID and the source symbol IDs of (S1, ..., Sk), a vector of 769 coefficients is determined using a Coding Coefficient Generator. Let 770 (a1, ...,ak) denote the obtained coefficients. To compute the linear 771 combination (s1, ..., Sk).transpose(a1, ..., ak) the Tetrys Recoding 772 Building block computes the vector v = (a1, ...,ak).transpose(M) and 773 then computes the coded symbol R = (R1, ..., Rt).transpose(v). It 774 can be verified that the new coded symbol is obtained without any 775 decoding operation. 777 6.4. Decoding 779 A classical matrix inversion is sufficient to recover the source 780 symbols. 782 7. Security Considerations 784 N/A 786 8. Privacy Considerations 788 N/A 790 9. IANA Considerations 792 N/A 794 10. Acknowledgments 796 N/A 798 11. References 800 11.1. Normative References 802 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 803 Requirement Levels", BCP 14, RFC 2119, March 1997. 805 11.2. Informative References 807 [AHL-00] Ahlswede, R., Ning Cai, , Li, S., and R. Yeung, "Network 808 information flow", IEEE Transactions on Information Theory 809 vol.46, no.4, pp.1204,1216, July 2000. 811 [FECFRAME] 812 Watson, M., Begen, A., and V. Roca, "Forward Error 813 Correction (FEC) Framework", Request for Comments 6363, 814 October 2011. 816 [NWCRG-ARCH] 817 NWCRG, , "Network Coding Architecture", TBD TBD. 819 [RMT] Vicisano, L., Gemmel, J., Rizzo, L., Handley, M., and J. 820 Crowcroft, "Forward Error Correction (FEC) Building 821 Block", Request for Comments 3452, December 2002. 823 [Tetrys] Lacan, J. and E. Lochin, "Rethinking reliability for long- 824 delay networks", International Workshop on Satellite and 825 Space Communications 2008 (IWSSC08), October 2008. 827 Authors' Addresses 829 Jonathan Detchart 830 ISAE 831 10, avenue Edouard-Belin 832 BP 54032 833 Toulouse CEDEX 4 31055 834 France 836 Email: jonathan.detchart@isae.fr 837 Emmanuel Lochin 838 ISAE 839 10, avenue Edouard-Belin 840 BP 54032 841 Toulouse CEDEX 4 31055 842 France 844 Email: emmanuel.lochin@isae.fr 846 Jerome Lacan 847 ISAE 848 10, avenue Edouard-Belin 849 BP 54032 850 Toulouse CEDEX 4 31055 851 France 853 Email: jerome.lacan@isae.fr 855 Vincent Roca 856 INRIA 857 655, av. de l'Europe 858 Inovallee; Montbonnot 859 ST ISMIER cedex 38334 860 France 862 Email: vincent.roca@inria.fr 863 URI: http://privatics.inrialpes.fr/people/roca/