Reliable Multicast Transport M. Luby Internet-Draft Qualcomm, Inc. Intended status: Standards Track A. Shokrollahi Expires: August 1, 2010 EPFL M. Watson Qualcomm, Inc. T. Stockhammer Nomor Research January 28, 2010 RaptorQ Forward Error Correction Scheme for Object Delivery draft-ietf-rmt-bb-fec-raptorq-00 Abstract This document describes a Fully-Specified FEC scheme, corresponding to FEC Encoding ID 6 (to be confirmed (tbc)), for the RaptorQ forward error correction code and its application to reliable delivery of data objects. RaptorQ codes are a new family of codes that provide superior flexibility, support for larger source block sizes and better coding efficiency than Raptor codes in RFC5053. RaptorQ is also a fountain code, i.e., as many encoding symbols as needed can be generated by the encoder on-the-fly from the source symbols of a source block of data. The decoder is able to recover the source block from any set of encoding symbols for most cases equal to the number of source symbols and in rare cases with slightly more than the number of source symbols. The RaptorQ code described here is a systematic code, meaning that all the source symbols are among the encoding symbols that can be generated. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference Luby, et al. Expires August 1, 2010 [Page 1]

Internet-Draft RaptorG FEC Scheme January 2010 material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 1, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. Luby, et al. Expires August 1, 2010 [Page 2]

Internet-Draft RaptorG FEC Scheme January 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements notation . . . . . . . . . . . . . . . . . . . . . 3 3. Formats and Codes . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. FEC Payload IDs . . . . . . . . . . . . . . . . . . . . . . 4 3.2. FEC Object Transmission Information . . . . . . . . . . . . 4 3.2.1. Mandatory . . . . . . . . . . . . . . . . . . . . . . . 4 3.2.2. Common . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2.3. Scheme-Specific . . . . . . . . . . . . . . . . . . . . 5 4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Content Delivery Protocol Requirements . . . . . . . . . . 7 4.2. Example Parameter Derivation Algorithm . . . . . . . . . . 7 4.3. Object Delivery . . . . . . . . . . . . . . . . . . . . . . 9 4.3.1. Source block construction . . . . . . . . . . . . . . . 9 4.3.2. Encoding packet construction . . . . . . . . . . . . 11 5. RaptorG FEC Code Specification . . . . . . . . . . . . . . . 12 5.1. Definitions, Symbols and Abbreviations . . . . . . . . . 12 5.1.1. Definitions . . . . . . . . . . . . . . . . . . . . . 12 5.1.2. Symbols . . . . . . . . . . . . . . . . . . . . . . . 13 5.1.3. Abbreviations . . . . . . . . . . . . . . . . . . . . 15 5.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15 5.3. Systematic RaptorG encoder . . . . . . . . . . . . . . . 17 5.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 17 5.3.2. Encoding overview . . . . . . . . . . . . . . . . . . 18 5.3.3. First encoding step: Intermediate Symbol Generation . 19 5.3.4. Second encoding step: Encoding . . . . . . . . . . . 27 5.3.5. Generators . . . . . . . . . . . . . . . . . . . . . 27 5.4. Example FEC decoder . . . . . . . . . . . . . . . . . . . 30 5.4.1. General . . . . . . . . . . . . . . . . . . . . . . . 30 5.4.2. Decoding an extended source block . . . . . . . . . . 32 5.5. Random Numbers . . . . . . . . . . . . . . . . . . . . . 36 5.5.1. The table V0 . . . . . . . . . . . . . . . . . . . . 36 5.5.2. The table V1 . . . . . . . . . . . . . . . . . . . . 37 5.5.3. The table V2 . . . . . . . . . . . . . . . . . . . . 38 5.5.4. The table V3 . . . . . . . . . . . . . . . . . . . . 39 5.6. Systematic indices and other parameters . . . . . . . . . 40 5.7. Arithmetic in GF(256) . . . . . . . . . . . . . . . . . . 44 5.7.1. Introduction . . . . . . . . . . . . . . . . . . . . 44 5.7.2. The table GF256_EXP . . . . . . . . . . . . . . . . . 44 5.7.3. The table GF256_LOG . . . . . . . . . . . . . . . . . 45 6. Security Considerations . . . . . . . . . . . . . . . . . . . 48 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 51 9.1. Normative references . . . . . . . . . . . . . . . . . . 51 9.2. Informative references . . . . . . . . . . . . . . . . . 51 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52 Luby, et al. Expires August 1, 2010 [Page 3]

Internet-Draft RaptorG FEC Scheme January 2010 1. Introduction This document specifies an FEC Scheme for the RaptorQ forward error correction code for object delivery applications. The concept of an FEC Scheme is defined in RFC5052 [RFC5052] and this document follows the format prescribed there and uses the terminology of that document. The RaptorQ code described herein is a next generation of the Raptor code described in RFC5053 [RFC5053]. The RaptorQ code provides superior reliability, better coding efficiency, and support for larger source block sizes than the Raptor code of RFC5053 [RFC5053]. These improvements simplify the usage of the RaptorQ code in an object delivery Content Delivery Protocol compared to RFC5053 [RFC5053]. The RaptorQ FEC Scheme is a Fully-Specified FEC Scheme corresponding to FEC Encoding ID 6 (tbc). Editor's Note: The finalized FEC encoding ID is still to be defined, but '6 (tbc)' is used as temporary value in this Internet Draft expecting sequential use of FEC encoding IDs in the IANA registration process. RaptorQ is a fountain code, i.e., as many encoding symbols as needed can be generated by the encoder on-the-fly from the source symbols of a block. The decoder is able to recover the source block from any set of encoding symbols only slightly more in number than the number of source symbols. The code described in this document is a systematic code, that is, the original source symbols can be sent unmodified from sender to receiver, as well as a number of repair symbols. For more backgound on the use of Forward Error Correction codes in reliable multicast, see [RFC3453]. Luby, et al. Expires August 1, 2010 [Page 4]

Internet-Draft RaptorG FEC Scheme January 2010 2. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Luby, et al. Expires August 1, 2010 [Page 5]

Internet-Draft RaptorG FEC Scheme January 2010 3. Formats and Codes 3.1. FEC Payload IDs The FEC Payload ID MUST be a 4 octet field defined as follows: 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SBN | Encoding Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: FEC Payload ID format Source Block Number (SBN), (8 bits): An integer identifier for the source block that the encoding symbols within the packet relate to. Encoding Symbol ID (ESI), (24 bits): An integer identifier for the encoding symbols within the packet. The interpretation of the Source Block Number and Encoding Symbol Identifier is defined in Section 5. 3.2. FEC Object Transmission Information 3.2.1. Mandatory The value of the FEC Encoding ID MUST be 6, as assigned by IANA (see Section 7). 3.2.2. Common The Common FEC Object Transmission Information elements used by this FEC Scheme are: o Transfer Length (F) o Symbol Size (T) The Transfer Length is a non-negative integer that is at most 946287651840, which can be represented by 40 bits. The Symbol Size is a non-negative integer less than 2^^16. The Transfer Length is a field of 40 bits in its definition, and the Symbol Size field is 16 bits, and both length units are bytes. The encoded Common FEC Object Transmission Information format is Luby, et al. Expires August 1, 2010 [Page 6]

Internet-Draft RaptorG FEC Scheme January 2010 shown in Figure 2. 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transfer Length (F) | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Reserved | Symbol Size (T) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Encoded Common FEC OTI for RaptorG FEC Scheme NOTE 1: The limit of 946287651840 on the transfer length is a consequence of the limitation on the symbol size to 2^^16-1, the limitation on the number of symbols in a source block to 56404 and the limitation on the number of source blocks to 2^^8. 3.2.3. Scheme-Specific The following parameters are carried in the Scheme-Specific FEC Object Transmission Information element for this FEC Scheme: o The number of source blocks (Z) o The number of sub-blocks (N) o A symbol alignment parameter (Al) These parameters are all non-negative integers. The encoded Scheme- specific Object Transmission Information is a 4-octet field consisting of the parameters Z (12 bits), N (12 bits) and Al (8 bits) as shown in Figure 3. 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Z | N | Al | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Encoded Scheme-specific FEC Object Transmission Information The encoded FEC Object Transmission Information is a 12-octet field consisting of the concatenation of the encoded Common FEC Object Transmission Information and the encoded Scheme-specific FEC Object Transmission Information. These three parameters define the source block partitioning as Luby, et al. Expires August 1, 2010 [Page 7]

Internet-Draft RaptorG FEC Scheme January 2010 described in Section 4.3.1.2 Luby, et al. Expires August 1, 2010 [Page 8]

Internet-Draft RaptorG FEC Scheme January 2010 4. Procedures 4.1. Content Delivery Protocol Requirements This section describes the information exchange between the RaptorQ FEC Scheme and any Content Delivery Protocol (CDP) that makes use of the RaptorQ FEC Scheme for object delivery. The RaptorQ encoder scheme and RaptorQ decoder scheme for object delivery require the following information from the CDP: o The transfer length of the object, F, in bytes o A symbol alignment parameter, Al o The symbol size, T, in bytes, which MUST be a multiple of Al o The number of source blocks, Z o The number of sub-blocks in each source block, N The RaptorQ encoder scheme for object delivery additionally requires: - the object to be encoded, F bytes The RaptorQ encoder scheme supplies the CDP with the following information for each packet to be sent: o Source Block Number (SBN) o Encoding Symbol ID (ESI) o Encoding symbol(s) The CDP MUST communicate this information to the receiver. 4.2. Example Parameter Derivation Algorithm This section provides recommendations for the derivation of the three transport parameters, T, Z and N. This recommendation is based on the following input parameters: o F the transfer length of the object, in bytes o WS the maximum size block that is decodable in working memory, in bytes Luby, et al. Expires August 1, 2010 [Page 9]

Internet-Draft RaptorG FEC Scheme January 2010 o P' the maximum payload size in bytes, which is assumed to be a multiple of Al o Al the symbol alignment parameter, in bytes o SS a parameter where the desired lower bound on the sub-symbol size is SS*Al o K'_max the maximum number of source symbols per source block. Note: Section 5.1.2 defines K'_max to be 56404 Based on the above inputs, the transport parameters T, Z and N are calculated as follows: Let, o T = P' o Kt = ceil(F/T) o N_max = floor(T/(SS*Al)) o for all n=1, ..., N_max * KL(n) is the maximum K' value in Table 2 in Section 5.6 such that K' <= floor (WS/(Al*(ceil(T/(Al*n))))) o Z = ceil(Kt/KL(N_max)) o N is the minimum n=1, ..., N_max such that ceil (Kt/Z) <= KL(n) It is RECOMMENDED that each packet contains exactly one symbol. However, receivers SHALL support the reception of packets that contain multiple symbols. The value Kt is the total number of symbols required to represent the source data of the object. The algorithm above and that defined in Section 4.3.1.2 ensure that the sub-symbol sizes are a multiple of the symbol alignment parameter, Al. This is useful because the XOR operations used for encoding and decoding are generally performed several bytes at a time, for example at least 4 bytes at a time on a 32 bit processor. Thus the encoding and decoding can be performed faster if the sub- symbol sizes are a multiple of this number of bytes. Luby, et al. Expires August 1, 2010 [Page 10]

Internet-Draft RaptorG FEC Scheme January 2010 The recommended settings for the input parameter Al is 4. The parameter WS can be used to generate encoded data which can be decoded efficiently with limited working memory at the decoder. Note that the actual maximum decoder memory requirement for a given value of WS depends on the implementation, but it is possible to implement decoding using working memory only slightly larger than WS. 4.3. Object Delivery 4.3.1. Source block construction 4.3.1.1. General In order to apply the RaptorQ encoder to a source object, the object may be broken into Z >= 1 blocks, known as source blocks. The RaptorQ encoder is applied independently to each source block. Each source block is identified by a unique integer Source Block Number (SBN), where the first source block has SBN zero, the second has SBN one, etc. Each source block is divided into a number, K, of source symbols of size T bytes each. Each source symbol is identified by a unique integer Encoding Symbol Identifier (ESI), where the first source symbol of a source block has ESI zero, the second has ESI one, etc. Each source block with K source symbols is divided into N >= 1 sub- blocks, which are small enough to be decoded in the working memory. Each sub-block is divided into K sub-symbols of size T'. Note that the value of K is not necessarily the same for each source block of a object and the value of T' may not necessarily be the same for each sub-block of a source block. However, the symbol size T is the same for all source blocks of an object and the number of symbols, K is the same for every sub-block of a source block. Exact partitioning of the object into source blocks and sub-blocks is described in Section 4.3.1.2 below. 4.3.1.2. Source block and sub-block partitioning The construction of source blocks and sub-blocks is determined based on five input parameters, F, Al, T, Z and N and a function Partition[]. The five input parameters are defined as follows: o F the transfer length of the object, in bytes o Al a symbol alignment parameter, in bytes Luby, et al. Expires August 1, 2010 [Page 11]

Internet-Draft RaptorG FEC Scheme January 2010 o T the symbol size, in bytes, which MUST be a multiple of Al o Z the number of source blocks o N the number of sub-blocks in each source block These parameters MUST be set so that ceil(ceil(F/T)/Z) <= K'_max. Recommendations for derivation of these parameters are provided in Section 4.2. The function Partition[] takes a pair of integers (I, J) as input and derives four integers (IL, IS, JL, JS) as output. Specifically, the value of Partition[I, J] is a sequence of four integers (IL, IS, JL, JS), where IL = ceil(I/J), IS = floor(I/J), JL = I - IS * J and JS = J - JL. Partition[] derives parameters for partitioning a block of size I into J approximately equal sized blocks. Specifically, JL blocks of length IL and JS blocks of length IS. The source object MUST be partitioned into source blocks and sub- blocks as follows: Let, o Kt = ceil(F/T) o (KL, KS, ZL, ZS) = Partition[Kt, Z] o (TL, TS, NL, NS) = Partition[T/Al, N] Then, the object MUST be partitioned into Z = ZL + ZS contiguous source blocks, the first ZL source blocks each having KL*T bytes, i.e. KL source symbols of T bytes each, and the remaining ZS source blocks each having KS*T bytes, i.e. KS source symbols of T bytes each. If Kt*T > F then for encoding purposes, the last symbol of the last source block MUST be padded at the end with Kt*T-F zero bytes. Next, each source block with K source symbols MUST be divided into N = NL + NS contiguous sub-blocks, the first NL sub-blocks each consisting of K contiguous sub-symbols of size of TL*Al bytes and the remaining NS sub-blocks each consisting of K contiguous sub-symbols of size of TS*Al bytes. The symbol alignment parameter Al ensures that sub-symbols are always a multiple of Al bytes. Finally, the m-th symbol of a source block consists of the concatenation of the m-th sub-symbol from each of the N sub-blocks. Note that this implies that when N > 1 then a symbol is NOT a Luby, et al. Expires August 1, 2010 [Page 12]

Internet-Draft RaptorG FEC Scheme January 2010 contiguous portion of the object. 4.3.2. Encoding packet construction Each encoding packet contains the following information: o Source Block Number (SBN) o Encoding Symbol ID (ESI) o encoding symbol(s) Each source block is encoded independently of the others. Source blocks are numbered consecutively from zero. Encoding Symbol ID values from 0 to K-1 identify the source symbols of a source block in sequential order, where K is the number of source symbols in the source block. Encoding Symbol IDs K onwards identify repair symbols generated from the source symbols using the RaptorQ encoder. Each encoding packet either consists entirely of source symbols (source packet) or entirely of repair symbols (repair packet). A packet may contain any number of symbols from the same source block. In the case that the last source symbol in a source packet includes padding bytes added for FEC encoding purposes then these bytes need not be included in the packet. Otherwise, only whole symbols MUST be included. The Encoding Symbol ID, X, carried in each source packet is the Encoding Symbol ID of the first source symbol carried in that packet. The subsequent source symbols in the packet have Encoding Symbol IDs, X+1 to X+G-1, in sequential order, where G is the number of symbols in the packet. Similarly, the Encoding Symbol ID, X, placed into a repair packet is the Encoding Symbol ID of the first repair symbol in the repair packet and the subsequent repair symbols in the packet have Encoding Symbol IDs X+1 to X+G-1 in sequential order, where G is the number of symbols in the packet. Note that it is not necessary for the receiver to know the total number of repair packets. Luby, et al. Expires August 1, 2010 [Page 13]

Internet-Draft RaptorG FEC Scheme January 2010 5. RaptorG FEC Code Specification 5.1. Definitions, Symbols and Abbreviations For the purpose of the RaptorQ FEC code specification in this section, the following definitions, symbols and abbreviations apply. 5.1.1. Definitions o Source block: a block of K source symbols which are considered together for RaptorQ encoding and decoding purposes. o Extended Source Block: a block of K' source symbols, where K' >= K constructed from a source block and zero or more padding symbols. o Symbol: a unit of data. The size, in bytes, of a symbol is known as the symbol size. The symbol size is always an integer. o Source symbol: the smallest unit of data used during the encoding process. All source symbols within a source block have the same size. o Padding symbol: a symbol with all zero bits that is added to the source block to form the extended source block. o Encoding symbol: a symbol that can be sent as part of the encoding of a source block. The encoding symbols of a source block consist of the source symbols of the source block and the repair symbols generated from the source block. Repair symbols generated from a source block have the same size as the source symbols of that source block. o Repair symbol: the encoding symbols of a source block that are not source symbols. The repair symbols are generated based on the source symbols of a source block. o Intermediate symbols: symbols generated from the source symbols using an inverse encoding process. The repair symbols are then generated directly from the intermediate symbols. The encoding symbols do not include the intermediate symbols, i.e., intermediate symbols are not sent as part of the encoding of a source block. The intermediate symbols are partitioned into LT symbols and PI symbols. o LT symbols: The subset of the intermediate symbols that can be LT neighbors of an encoding symbol. Luby, et al. Expires August 1, 2010 [Page 14]

Internet-Draft RaptorG FEC Scheme January 2010 o PI symbols: The subset of the intermediate symbols that can be PI neighbors of an encoding symbol. o Systematic code: a code in which all source symbols are included as part of the encoding symbols of a source block. The RaptorQ code as described herein is a systematic code. o Encoding Symbol ID: information that uniquely identifies each encoding symbol associated with a source block for sending and receiving purposes. o Internal Symbol ID: information that uniquely identifies each symbol associated with an extended source block for encoding and decoding purposes. 5.1.2. Symbols i, j, u, v, h, d, a, b, d1, a1, b1, v, m, x, y represent values or variables of one type or another, depending on the context. X denotes a non-negative integer value that is either an ISI value or an ESI value, depending on the context. ceil(x) denotes the smallest integer which is greater than or equal to x, where x is a real value. floor(x) denotes the largest positive integer which is less than or equal to x, where x is a real value. min(x,y) denotes the minimum value of the values x and y, and in general the minimum value of all the argument values. max(x,y) denotes the maximum vaue of the values x and y, and in general the maximum value of all the argument values. i % j denotes i modulo j. u ^ v denotes, for equal-length bit strings u and v, the bitwise exclusive-or of u and v. A denotes a matrix A. Transpose[A] denotes the transposed matrix of matrix A. A^^-1 denotes the inverse matrix of matrix A. Luby, et al. Expires August 1, 2010 [Page 15]

Internet-Draft RaptorG FEC Scheme January 2010 K denotes the number of symbols in a single source block. K' denotes the number of source plus padding symbols in an extended source block. For the majority of this specification, the padding symbols are considered to be additional source symbols. K'_max denotes the maximum number of source symbols that can be in a single source block. Set to 56404. L denotes the number of intermediate symbols for a single extended source block. S denotes the number of LDPC symbols for a single extended source block. These are LT symbols. For each value of K' shown in Table 2 in Section 5.6, the corresponding value of S is a prime number. H denotes the number of HDPC symbols for a single extended source block. These are PI symbols. B denotes the number of intermediate symbols that are LT symbols excluding the LDPC symbols. W denotes the number of intermediate symbols that are LT symbols. For each value of K' in Table 2 shown in Section 5.6, the corresponding value of W is a prime number. P denotes the number of intermediate symbols that are PI symbols. These contain all HDPC symbols. P1 denotes the smallest prime number greater than or equal to P. U denotes the number of non-HDPC intermediate symbols that are PI symbols. C denotes an array of intermediate symbols, C[0], C[1], C[2],..., C[L-1]. C' denotes an array of the symbols of the extended source block, where C'[0], C'[1], C'[2],..., C'[K-1] are the source symbols of the source block and C'[K], C'[K+1],..., C'[K'-1] are padding symbols. V0, V1, V2, V3 denote four arrays of 4-byte integers, V0[0], V0[1],..., V0[255] ; V1[0], V1[1],..., V1[255]; V2[0], V2[1],..., V2[255]; and V3[0], V3[1],..., V3[255] as shown in Section 5.5. Luby, et al. Expires August 1, 2010 [Page 16]

Internet-Draft RaptorG FEC Scheme January 2010 Rand[y, i, m] a pseudo-random number generator Deg[v] a degree generator Enc[K', C ,(d, a, b, d1, a1, b1)] an encoding symbol generator Tuple[K', X] a tuple generator function GF(n) denotes the Galois field with n elements. T denotes the symbol size in bytes. Q Q = 4294967291, i.e., Q is the largest prime smaller than 2^^32. J(K') denotes the systematic index associated with K'. G denotes any generator matrix. I_S denotes the SxS identity matrix. a ^^ b denotes the operation a raised to the power b. 5.1.3. Abbreviations ESI Encoding Symbol ID GF Galois Field HDPC High Density Parity Check ISI Internal Symbol ID LDPC Low Density Parity Check LT Luby Transform PI Permanently Interactive SBN Source Block Number SBL Source Block Length (in units of symbols) 5.2. Overview This section defines the systematic RaptorQ FEC code. Symbols are the fundamental data units of the encoding and decoding process. For each source block all symbols are the same size, Luby, et al. Expires August 1, 2010 [Page 17]

Internet-Draft RaptorG FEC Scheme January 2010 referred to as the symbol size T. The atomic operations performed on symbols for both encoding and decoding are the exclusive-or operation between symbols and an operation of the elements of the finite field GF(256) upon symbols. The basic encoder is described in Section 5.3. The encoder first derives a block of intermediate symbols from the source symbols of a source block. This intermediate block has the property that both source and repair symbols can be generated from it using the same process. The encoder produces repair symbols from the intermediate block using an efficient process, where each such repair symbol is the exclusive OR of a small number of intermediate symbols from the block. Source symbols can also be reproduced from the intermediate block using the same process. The encoding symbols are the combination of the source and repair symbols. An example of a decoder is described in Section 5.4. The process for producing source and repair symbols from the intermediate block is designed so that the intermediate block can be recovered from any sufficiently large set of encoding symbols, independent of the mix of source and repair symbols in the set. Once the intermediate block is recovered, missing source symbols of the source block can be recovered using the encoding process. If a RaptorQ compliant decoding algorithm receives a mathematically sufficient set of encoding symbols generated according to the encoder specification in Section 5.3 for reconstruction of a source block then such a decoder SHALL recover the entire source block. A number of decoding algorithms are possible to achieve this optimal behavior. An efficient decoding algorithm to achieve this is provided in Section 5.4. The construction of the intermediate and repair symbols is based in part on a pseudo-random number generator described in Section 5.3. This generator is based on a fixed set of 1024 random numbers which must be available to both sender and receiver. These numbers are provided in Section 5.5. Encoding and decoding operations for RaptorQ use operations in the field GF(256). Section 5.7 provides a recommended way to perform these operations. Finally, the construction of the intermediate symbols from the source symbols is governed by "systematic indices", values of which are provided in Section 5.6 for specific extended source block sizes between 6 and K'_max = 56404 source symbols. Thus, the RaptorQ code supports source blocks with between 1 and 56404 source symbols. Luby, et al. Expires August 1, 2010 [Page 18]

Internet-Draft RaptorG FEC Scheme January 2010 5.3. Systematic RaptorG encoder 5.3.1. Introduction For a given source block of K source symbols, for encoding and decoding purposes the source block is augmented with K'-K additional padding symbols, where K' is the smallest value that is at least K in the systematic index Table 2 of Section 5.6. The reason for padding out a source block to a multiple of K' is to enable faster encoding and decoding, and to minimize the amount of table information that needs to be stored in the encoder and decoder. For purposes of transmitting and receiving data, the value of K is used to determine the number of source symbols in a source block, and thus K needs to be known at the sender and the receiver. In this case the sender and receiver can compute K' from K and the K'-K padding symbols can be automatically added to the source block without any additional communication. The encoding symbol ID (ESI) is used by a sender and receiver to identify the encoding symbols of a source block, where the encoding symbols of a source block consist of the source symbols and the repair symbols associated with the source block. For a source block with K source symbols, the ESIs for the source symbols are 0,1,2,...,K-1 and the ESIs for the repair symbols are K, K+1, K+2,... . Using the ESI for identifying encoding symbols in transport ensures that the ESI values continue consecutively between the source and repair symbols. For purposes of encoding and decoding data, the value of K' derived from K is used as the number of source symbols of the extended source block upon which encoding and decoding operations are performed, where the K' source symbols consist of the original K source symbols and an additional K'-K padding symbols. The internal symbol ID (ISI) is used by the encoder and decoder to identify the symbols associated with the extended source block, i.e., for generating encoding symbols and for decoding. For a source block with K original source symbols, the ISIs for the original source symbols are 0,1,2,...,K-1, the ISIs for the K'-K padding symbols are K, K+1, K+2,..., K'-1, and the ISIs for the repair symbols are K', K'+1, K'+2,... . Using the ISI for encoding and decoding allows the padding symbols of the extended source block to be treated the same way as other source symbols of the extended source block, and that a given prefix of repair symbols are generated in a consistent way for a given number K' of source symbols in the extended source block independent of K. The relationship between the ESIs and the ISIs is simple: the ESIs and the ISIs for the original K source symbols are the same, the K'-K padding symbols have an ISI but do not have a corresponding ESI (since they are symbols that are neither sent nor received), and a Luby, et al. Expires August 1, 2010 [Page 19]

Internet-Draft RaptorG FEC Scheme January 2010 repair symbol ISI is simply the repair symbol ESI plus K'-K. The translation between ESIs used to identify encoding symbols sent and received and the corresponding ISIs used for encoding and decoding, and the proper padding of the extended source block with padding symbols used for encoding and decoding, is the responsibility of the padding function in the RaptorQ encoder/decoder. 5.3.2. Encoding overview The systematic RaptorQ encoder is used to generate any number of repair symbols from a source block that consists of K source symbols placed into an extended source block C'. Figure 4 shows the encoding overview. The first step of encoding is to construct an extended source block by adding zero or more padding symbols such that the total number of symbols, K', is one of the values listed in Section 5.6. Each padding symbol consists of T bytes where the value of each byte is zero. K' MUST be selected as the smallest value of K' from the table of Section 5.6 which is greater than or equal to K. -----------------------------------------------------------+ | | | +-----------+ +--------------+ +-------------+ | C' | | | C' | Intermediate | C | | | ----+--->| Padding |--->| Symbol |--->| Encoding |--+--> K | | | K' | Generation | L | | | | +-----------+ +--------------+ +-------------+ | | | (d,a,b, ^ | | | d1,a1,b1)| | | | +------------+ | | | K' | Tuple | | | +----------------------------->| | | | | Generation | | | +------------+ | | ^ | +-------------------------------------------------+--------+ | ISI X Figure 4: Encoding Overview Let C'[0],..., C'[K-1] denote the K source symbols. Let C'[K], ..., C'[K'-1] denote the K'-K padding symbols, which are Luby, et al. Expires August 1, 2010 [Page 20]

Internet-Draft RaptorG FEC Scheme January 2010 all set to zero bits. Then, C'[0],..., C'[K'-1] are the symbols of the extended source block upon which encoding and decoding are performed. In the remainder of this description these padding symbols will be considered as additional source symbols and referred to as such. However, these padding symbols are not part of the encoding symbols, i.e., they are not sent as part of the encoding. At a receiver, the value of K' can be computed based on K, then the receiver can insert K'-K padding symbols at the end of a source block of K' source symbols and recover the remaining K source symbols of the source block from received encoding symbols. The second step of encoding is to generate a number, L > K', of intermediate symbols from the K' source symbols. In this step, K' source tuples (d[0], a[0], b[0], d1[0], a1[0], b1[0]), ..., (d[K'-1], a[K'-1], b[K'-1], d1[K'-1], a1[K'-1], b1[K'-1]) are generated using the Tuple[] generator as described in Section 5.3.5.4. The K' source tuples and the ISIs associated with the K' source symbols are used to determine L intermediate symbols C[0],..., C[L-1] from the source symbols using an inverse encoding process. This process can be realized by a RaptorQ decoding process. Certain "pre-coding relationships" must hold within the L intermediate symbols. Section 5.3.3.3 describes these relationships. Section 5.3.3.4 describes how the intermediate symbols are generated from the source symbols. Once the intermediate symbols have been generated, repair symbols can be produced. For a repair symbol with ISI X>K', the tuple of integers, (d, a, b, d1, a1, b1) can be generated, using the Tuple[] generator as described in Section 5.3.5.4. Then, the (d, a, b, d1, a1, b1)-tuple and the ISI X is used to generate the corresponding repair symbol from the intermediate symbols using the Enc[] generator described in Section 5.3.5.3. The corresponding ESI for this repair symbol is then X-(K'-K). Note that source symbols of the extended source block can also be generated using the same process, i.e., for any X < K', the symbol generated using this process has the same value as C'(X). 5.3.3. First encoding step: Intermediate Symbol Generation 5.3.3.1. General This encoding step is a pre-coding step to generate the L intermediate symbols C[0], ..., C[L-1] from the source symbols C'[0], ..., C'[K'-1], , where L > K is defined in Section 5.3.3.3. The intermediate symbols are uniquely defined by two sets of constraints: Luby, et al. Expires August 1, 2010 [Page 21]

Internet-Draft RaptorG FEC Scheme January 2010 1. The intermediate symbols are related to the source symbols by a set of source symbol tuples and by the ISIs of the source symbols. The generation of the source symbol tuples is defined in Section 5.3.3.2 using the the Tuple[] generator as described in Section 5.3.5.4. 2. A set of pre-coding relationships hold within the intermediate symbols themselves. These are defined in Section 5.3.3.3 The generation of the L intermediate symbols is then defined in Section 5.3.3.4 5.3.3.2. Source symbol tuples Each of the K' source symbols is associated with a source symbol tuple (d[X], a[X], b[X], d1[X], a1[X], b1[X]) for 0 <= X < K'. The source symbol tuples are determined using the Tuple generator defined in Section 5.3.5.4 as: For each X, 0 <= X < K' (d[X], a[X], b[X], d1[X], a1[X], b1[X]) = Tuple[K, X] 5.3.3.3. Pre-coding relationships The pre-coding relationships amongst the L intermediate symbols are defined by requiring that a set of S+H linear combinations of the intermediate symbols evaluate to zero. There are S LDPC and H HDPC symbols, and thus L = K'+S+H. Another partition of the L intermediate symbols is into two sets, one set of W LT symbols and another set of P PI symbols, and thus it is also the case that L = W+P. The P PI symbols are treated differently than the W LT symbols in the encoding process. The P PI symbols consist of the H HDPC symbols together with a set of U = P-H of the other K' intermediate symbols. The W LT symbols consist of the S LDPC symbols together with W-S of the other K' intermediate symbols. The values of these parameters are determined from K' as described below where H(K'), S(K'), and W(K') are derived from Table 2 in Section 5.6. Let o S = S(K') o H = H(K') o W = W(K') Luby, et al. Expires August 1, 2010 [Page 22]

Internet-Draft RaptorG FEC Scheme January 2010 o L = K' + S + H o P = L - W o P1 denote the smallest prime number greater than or equal to P o U =P - H o B = W - S o C[0], ..., C[B-1] denote the intermediate symbols that are LT symbols but not LDPC symbols. o C[B], ..., C[B+S-1] denote the S LDPC symbols that are also LT symbols. o C[W], ..., C[W+U-1] denote the intermediate symbols that are PI symbols but not HDPC symbols. o C[L-H], ..., C[L-1] denote the H HDPC symbols that are also PI symbols. The first set of pre-coding relations, called LDPC relations, is described below and requires that at the end of this process the set of symbols D[0] , ..., D[S-1] are all zero: o Initialize the symbols D[0] = C[B], ... , D[S-1] = C[B+S-1]. o For i = 0, ..., B-1 do * a = 1 + (floor(i/S) % (S-1)) * b = i % S * D[b] = D[b] ^ C[i] * b = (b + a) % S * D[b] = D[b] ^ C[i] * b = (b + a) % S * D[b] = D[b] ^ C[i] o For i = 0, ..., S-1 do * a = i % P Luby, et al. Expires August 1, 2010 [Page 23]

Internet-Draft RaptorG FEC Scheme January 2010 * b = (i+1) % P * D[i] = D[i] ^ C[W+a] ^ C[W+b] The second set of relations, called HDPC relations, is obtained by considering each intermediate symbol as a sequence of elements from the finite field GF(256). We represent elements of GF(256) in the usual way as polynomials in one variable, x, with coefficients from the finite field GF(2) modulo an irreducible polynomial f(x). A single byte of data from a symbol, b7,b6,b5,b4,b3,b2,b1,b0, where b7 is the highest order bit and b0 is the lowest order bit, corresponds to the finite field element b7 x^^7 + b6 x^^6 + b5 x^^5 + b4 x^^4 + b3 x^^3 + b2 x^^2 + b1 x + b0 mod f(x) The irreducible polynomial f(x) is defined to be: f(x) = x^^8 + x^^4 + x^^3 + x^^2 + 1. We then define the operation of elements of GF(256) on symbols as follows: Let o beta denote an element of GF(256). o C denote a symbol of length T bytes. o c[0], ..., c[T-1] denote the bytes of C. o gamma[0], ..., gamma[T-1] denote the elements of GF(256) corresponding to c[0], ..., c[T-1] respectively. Then we define delta[i] = beta*gamma[i] for i=0, ..., T-1 where '*' represents the usual multiplication operation in GF(256). A multiplication table for GF(256) and a recommended way to perform calculations in GF(256) is provided in Section 5.7. Then the operation of beta on C is defined as follows: beta*C = d[0], ..., d[T-1] where d[i] is the byte value corresponding to delta[i] for i=0,..., T-1. Luby, et al. Expires August 1, 2010 [Page 24]

Internet-Draft RaptorG FEC Scheme January 2010 The set of HDPC relations among the intermediate symbols C[0], ..., C[K'+S+H-1] is defined as follows: Let o alpha denote a generator element of GF(256), specifically the element represented by the polynomial x mod f(x). o T denote an H x (K' +S ) matrix with elements from GF(256), where for j=0,...,K'+S-2 the entry T[i,j] is the identity element if i= Rand[j,6,H] or i = (Rand[j,6,H] + Rand[j,7,H-1] + 1) % H and T[i,j] is the zero element for all other values of i, and for j=K'+S-1, T[i,j] = alpha^^i for i=0,...,H-1. o GAMMA denote a (K'+S ) x (K'+S ) matrix with elements from GF(256), where o GAMMA[i,j] = alpha ^^ (i-j) for i <= j 0 otherwise Then the relationship between the first K'+S intermediate symbols C[0], ..., C[K'+S-1] and the H HDPC symbols C[K'+S], ..., C[K'+S+H-1] is given by: Transpose[C[K'+S], ..., C[K'+S+H-1]] + T* GAMMA * Transpose[C[0], ..., C[K'+S-1]] = 0 where '*' represents standard matrix multiplication utilizing the above defined operation to define the multiplication between a matrix over GF(256) and a matrix of symbols (in particular the column vector of symbols). The H HDPC relations may be conveniently described using the following algorithm, where u is a working register containing a single symbol. These relations require that the values of the symbols D[S], ..., D[S+H-1] are zero at the end of the following process. o Initialize the symbols D[S] = C[K'+S], ..., D[S+H-1] = C[K'+S+H-1] o u = C[0] Luby, et al. Expires August 1, 2010 [Page 25]

Internet-Draft RaptorG FEC Scheme January 2010 o For j = 1, ..., K'+S-1 do * pos1 = Rand[j,6,H] * pos2 = (pos1 + Rand[j,7,H-1] + 1) % H * D[S+pos1] = D[S+pos1] ^ u * D[S+pos2] = D[S+pos2] ^ u * u = (alpha*u) ^ C[j] o For i = 0, ..., H-1 * D[S+i] = D[S+i] ^ u * u = (alpha*u) 5.3.3.4. Intermediate symbols 5.3.3.4.1. Definition Given the K' source symbols C'[0], C'[1],..., C'[K'-1] the L intermediate symbols C[0], C[1],..., C[L-1] are the uniquely defined symbol values that satisfy the following conditions: 1. The K' source symbols C'[0], C'[1],..., C'[K'-1] satisfy the K' constraints C'[X] = Enc[K', (C[0],..., C[L-1]), (d[X], a[X], b[X], d1[X], a1[X], b1[X])], for all X, 0 <= X < K', where (d[X], a[X], b[X], d1[X], a1[X], b1[X])) = Tuple[K',X], Tuple[] is defined in Section 5.3.5.4 and Enc[] is described in Section 5.3.5.3. 2. The L intermediate symbols C[0], C[1],..., C[L-1] satisfy the pre-coding relationships defined in Section 5.3.3.3 5.3.3.4.2. Example method for calculation of intermediate symbols This section describes a possible method for calculation of the L intermediate symbols C[0], C[1],..., C[L-1] satisfying the constraints in Section 5.3.3.4.1 The generator matrix G for a code which generates n output symbols from k input symbols is an n x k matrix over GF(256), where each row corresponds to one of the output symbols and each column to one of Luby, et al. Expires August 1, 2010 [Page 26]

Internet-Draft RaptorG FEC Scheme January 2010 the input symbols and where the i-th output symbol is equal to the sum of the product of each input symbol with the corresponding entry in row i. Then, the L intermediate symbols can be calculated as follows: Let o C denote the column vector of the L intermediate symbols, C[0], C[1],..., C[L-1]. o D denote the column vector consisting of S+H zero symbols followed by the K' source symbols C'[0], C'[1], ..., C'[K'-1]. Then the above constraints define an L x L matrix A over GF(256) such that: A*C = D The matrix A can be constructed as follows: Let: o G_LDPC,1 and G_LDPC,2 be S x B and S x P matrices such that G_LDPC,1 * Transpose[(C[0],...., C[B-1])] + G_LDPC,2 * Transpose(C[W], ..., C[W+P-1]) + Transpose[(C[B], ..., C[B+S-1])] = 0 and "+" is the component-wise XOR of the vectors involved. o G_HDPC be the H x (K'+S) generator matrix of the HDPC symbols, So, G_HDPC * Transpose(C[0], ..., C[K'+S-1]) = Transpose(C[K'+S], ..., C[L-1]), i.e. G_HDPC = H*GAMMA o I_S be the S x S identity matrix o I_H be the H x H identity matrix o G_ENC be the K'x L generator matrix of the encoding symbols generated by the Encoder. So, G_ENC * Transpose[(C[0], ..., C[L-1])] = Transpose[(C'[0],C'[1],...,C'[K'-1])], Luby, et al. Expires August 1, 2010 [Page 27]

Internet-Draft RaptorG FEC Scheme January 2010 i.e. G_ENC[i,j] = 1 if and only if C[j] is included in the symbols which are XORed to produce Enc[K', (C[0], ..., C[L-1]), (d[i], a[i], b[i], d1[i], a1[i], b1[i])] and G_ENC[i,j] = 0 otherwise. Then: o The first S rows of A are equal to G_LDPC,1 | I_S | G_LDPC,2. o The next H rows of A are equal to G_HDPC | I_H. o The remaining K' rows of A are equal to G_ENC. The matrix A is depicted in Figure (Figure 5) below: B S U H +-----------------------+-------+------------------+ | | | | S | G_LDPC,1 | I_S | G_LDPC,2 | | | | | +-----------------------+-------+----------+-------+ | | | H | G_HDPC | I_H | | | | +------------------------------------------+-------+ | | | | K' | G_ENC | | | | | +--------------------------------------------------+ Figure 5: The matrix A The intermediate symbols can then be calculated as: C = (A^^-1)*D The source tuples are generated such that for any K' matrix A has full rank and is therefore invertible. This calculation can be realized by applying a RaptorQ decoding process to the K' source symbols C'[0], C'[1],..., C'[K'-1] to produce the L intermediate symbols C[0], C[1],..., C[L-1]. To efficiently generate the intermediate symbols from the source symbols, it is recommended that an efficient decoder implementation such as that described in Section 5.4 be used. Luby, et al. Expires August 1, 2010 [Page 28]

Internet-Draft RaptorG FEC Scheme January 2010 5.3.4. Second encoding step: Encoding In the second encoding step, the repair symbol with ISI X (X >= K') is generated by applying the generator Enc[K', (C[0], C[1],..., C[L-1]), (d, a, b, d1, a1, b1)] defined in Section 5.3.5.3 to the L intermediate symbols C[0], C[1],..., C[L-1] using the tuple (d, a, b, d1, a1, b1)=Tuple[K',X]. 5.3.5. Generators 5.3.5.1. Random Generator The random number generator Rand[y, i, m] is defined as follows, where y is a non-negative integer, i is a non-negative integer less than 256, and m is a positive integer and the value produced is an integer between 0 and m-1. Let V0, V1, V2 and V3 be arrays of 256 entries each, where each entry is a 4-byte unsigned integer. These arrays are provided in Section 5.5. Let o x0 = (y + i) mod 2^^8 o x1 = (floor(y / 2^^8) + i) mod 2^^8 o x2 = (floor(y / 2^^16) + i) mod 2^^8 o x3 = (floor(y / 2^^24) + i) mod 2^^8 Then, Rand[y, i, m] = (V0[x0] ^ V1[x1] ^ V2[x2] ^ V3[x3]) % m 5.3.5.2. Degree Generator The degree generator Deg[v] is defined as follows, where v is an integer that is at least 0 and less than 2^^20 = 1048576. Given v, find index d in Table 1 such that f[d-1] <= v < f[d], and set Deg[v] = min(d, W-2). Luby, et al. Expires August 1, 2010 [Page 29]

Internet-Draft RaptorG FEC Scheme January 2010 +---------+---------+---------+---------+ | Index d | f[d] | Index d | f[d] | +---------+---------+---------+---------+ | 0 | 0 | 1 | 5243 | +---------+---------+---------+---------+ | 2 | 529531 | 3 | 704294 | +---------+---------+---------+---------+ | 4 | 791675 | 5 | 844104 | +---------+---------+---------+---------+ | 6 | 879057 | 7 | 904023 | +---------+---------+---------+---------+ | 8 | 922747 | 9 | 937311 | +---------+---------+---------+---------+ | 10 | 948962 | 11 | 958494 | +---------+---------+---------+---------+ | 12 | 966438 | 13 | 973160 | +---------+---------+---------+---------+ | 14 | 978921 | 15 | 983914 | +---------+---------+---------+---------+ | 16 | 988283 | 17 | 992138 | +---------+---------+---------+---------+ | 18 | 995565 | 19 | 998631 | +---------+---------+---------+---------+ | 20 | 1001391 | 21 | 1003887 | +---------+---------+---------+---------+ | 22 | 1006157 | 23 | 1008229 | +---------+---------+---------+---------+ | 24 | 1010129 | 25 | 1011876 | +---------+---------+---------+---------+ | 26 | 1013490 | 27 | 1014983 | +---------+---------+---------+---------+ | 28 | 1016370 | 29 | 1017662 | +---------+---------+---------+---------+ | 30 | 1048576 | | | +---------+---------+---------+---------+ Table 1: Defines the degree distribution for encoding symbols 5.3.5.3. Encoding Symbol Generator The encoding symbol generator Enc[K', (C[0], C[1],..., C[L-1]), (d, a, b, d1, a1, b1)] takes the following inputs: o K' is the number of source symbols for the extended source block. Let L, W, B, S, P and P1 be derived from K' as described in Section 5.3.3.3. Luby, et al. Expires August 1, 2010 [Page 30]

Internet-Draft RaptorG FEC Scheme January 2010 o (C[0], C[1],..., C[L-1]) is the array of L intermediate symbols (sub-symbols) generated as described in Section 5.3.3.4 o (d, a, b, d1, a1, b1) is a source tuple determined from ISI X using the Tuple generator defined in Section 5.3.5.4, whereby * d is an integer denoting an encoding symbol LT degree * a is an integer between 1 and W-1 inclusive * b is an integer between 0 and W-1 inclusive * d1 is an integer between 2 and 3 inclusive denoting an encoding symbol PI degree * a1 is an integer between 1 and P1-1 inclusive * b1 is an integer between 0 and P1-1 inclusive The encoding symbol generator produces a single encoding symbol as output, according to the following algorithm: o result = C[b]. o For j = 1, ..., d-1 do * b = (b + a) % W * result = result ^ C[b] o while (b1 >= P) do b1 = (b1+a1) % P1 o result = result ^ C[W+b1] o For j = 1, ..., d1-1 do * b1 = (b1 + a1) % P1 * while (b1 >= P) do b1 = (b1+a1) % P1 * result = result ^ C[W+b1] o Return result Luby, et al. Expires August 1, 2010 [Page 31]

Internet-Draft RaptorG FEC Scheme January 2010 5.3.5.4. Tuple generator The tuple generator Tuple[K',X] takes the following inputs: o K' - The number of source symbols in the extended source block o X - An Intermediate symbol ID (ISI) Let o L be determined from K' as described in Section 5.3.3.3 o Q = 4294967291, the largest prime smaller than 2^^32. o J=J(K') be the systematic index associated with K', as defined inTable 2 inSection 5.6 The output of the source symbol tuple generator is a tuple, (d, a, b, d1, a1, b1) determined as follows: o A = 1 + (53591 + J*997) % Q o B = 10267*(J+1) % Q o y = (B + X*A) % Q o v = Rand[y, 0, 2^^20] o d = Deg[v] o a = 1 + Rand[y, 1, W-1] o b = Rand[y, 2, W] o if (d<4) { d1 = 2 + Rand[y, 3, 2] } else { d1 = 2 } o a1 = 1 + Rand[y, 4, P1-1] o b1 = Rand[y, 5, P1] 5.4. Example FEC decoder 5.4.1. General This section describes an efficient decoding algorithm for the RaptorQ code introduced in this specification. Note that each received encoding symbol can be considered as the value of an equation amongst the intermediate symbols. From these simultaneous Luby, et al. Expires August 1, 2010 [Page 32]

Internet-Draft RaptorG FEC Scheme January 2010 equations, and the known pre-coding relationships amongst the intermediate symbols, any algorithm for solving simultaneous equations can successfully decode the intermediate symbols and hence the source symbols. However, the algorithm chosen has a major effect on the computational efficiency of the decoding. 5.4.2. Decoding an extended source block 5.4.2.1. General It is assumed that the decoder knows the structure of the source block it is to decode, including the symbol size, T, and the number K of symbols in the source block and the number K' of source symbols in the extended source block. From the algorithms described in Sections Section 5.3, the RaptorQ decoder can calculate the total number L = K'+S+H of intermediate symbols and determine how they were generated from the extended source block to be decoded. In this description it is assumed that the received encoding symbols for the extended source block to be decoded are passed to the decoder. Furthermore, for each such encoding symbol it is assumed that the number and set of intermediate symbols whose exclusive-or is equal to the encoding symbol is passed to the decoder. In the case of source symbols, including padding symbols, the source symbol tuples described in Section 5.3.3.2 indicate the number and set of intermediate symbols which sum to give each source symbol. Let N >= K' be the number of received encoding symbols to be used for decoding, including padding symbols for an extended source block and let M = S+H+N. Then with the notation of Section 5.3.3.4.2 we have A*C=D. Decoding an extended source block is equivalent to decoding C from known A and D. It is clear that C can be decoded if and only if the rank of A is L. Once C has been decoded, missing source symbols can be obtained by using the source symbol tuples to determine the number and set of intermediate symbols which must be exclusive-ORed to obtain each missing source symbol. The first step in decoding C is to form a decoding schedule. In this step A is converted, using Gaussian elimination (using row operations and row and column reorderings) and after discarding M - L rows, into the L by L identity matrix. The decoding schedule consists of the sequence of row operations and row and column re-orderings during the Gaussian elimination process, and only depends on A and not on D. The decoding of C from D can take place concurrently with the forming of the decoding schedule, or the decoding can take place afterwards Luby, et al. Expires August 1, 2010 [Page 33]

Internet-Draft RaptorG FEC Scheme January 2010 based on the decoding schedule. The correspondence between the decoding schedule and the decoding of C is as follows. Let c[0] = 0, c[1] = 1...,c[L-1] = L-1 and d[0] = 0, d[1] = 1...,d[M-1] = M-1 initially. o Each time a multiple, beta, of row i of A is added to row i' in the decoding schedule then in the decoding process the symbol beta*D[d[i]] is added to symbol D[d[i']] . o Each time a row i of A is multiplied by a field element beta, then in the decoding process the symbol D[d[i]] is also multiplied by beta. o Each time row i is exchanged with row i' in the decoding schedule then in the decoding process the value of d[i] is exchanged with the value of d[i']. o Each time column j is exchanged with column j' in the decoding schedule then in the decoding process the value of c[j] is exchanged with the value of c[j']. From this correspondence it is clear that the total number of operations on symbols in the decoding of the extended source block is the number of row operations (not exchanges) in the Gaussian elimination. Since A is the L by L identity matrix after the Gaussian elimination and after discarding the last M - L rows, it is clear at the end of successful decoding that the L symbols D[d[0]], D[d[1]],..., D[d[L-1]] are the values of the L symbols C[c[0]], C[c[1]],..., C[c[L-1]]. The order in which Gaussian elimination is performed to form the decoding schedule has no bearing on whether or not the decoding is successful. However, the speed of the decoding depends heavily on the order in which Gaussian elimination is performed. (Furthermore, maintaining a sparse representation of A is crucial, although this is not described here). The remainder of this section describes an order in which Gaussian elimination could be performed that is relatively efficient. 5.4.2.2. First Phase The first phase of the Gaussian elimination the matrix A is conceptually partitioned into submatrices and additionally, a matrix X is created. This matrix has as many rows and columns as A, and it will be a lower triangular matrix throughout the first phase. At the beginning of this phase, the matrix A is copied into the matrix X. The submatrix sizes are parameterized by non-negative integers i and Luby, et al. Expires August 1, 2010 [Page 34]

Internet-Draft RaptorG FEC Scheme January 2010 u which are initialized to 0 and P, the number of PI symbols, respectively. The submatrices of A are: 1. The submatrix I defined by the intersection of the first i rows and first i columns. This is the identity matrix at the end of each step in the phase. 2. The submatrix defined by the intersection of the first i rows and all but the first i columns and last u columns. All entries of this submatrix are zero. 3. The submatrix defined by the intersection of the first i columns and all but the first i rows. All entries of this submatrix are zero. 4. The submatrix U defined by the intersection of all the rows and the last u columns. 5. The submatrix V formed by the intersection of all but the first i columns and the last u columns and all but the first i rows. Figure 6 illustrates the submatrices of A. At the beginning of the first phase V = A. In each step, a row of A is chosen. +-----------+-----------------+---------+ | | | | | I | All Zeros | | | | | | +-----------+-----------------+ U | | | | | | | | | | All Zeros | V | | | | | | | | | | +-----------+-----------------+---------+ Figure 6: Submatrices of A in the first phase The following graph defined by the structure of V is used in determining which row of A is chosen. The columns that intersect V are the nodes in the graph, and the rows that have exactly 2 non-zero entries in V and are not HDPC rows are the edges of the graph that connect the two columns (nodes) in the positions of the two ones. A component in this graph is a maximal set of nodes (columns) and edges (rows) such that there is a path between each pair of nodes/edges in the graph. The size of a component is the number of nodes (columns) in the component. Luby, et al. Expires August 1, 2010 [Page 35]

Internet-Draft RaptorG FEC Scheme January 2010 There are at most L steps in the first phase. The phase ends successfully when i + u = L, i.e., when V and the all zeroes submatrix above V have disappeared and A consists of I, the all zeroes submatrix below I, and U. The phase ends unsuccessfully in decoding failure if at some step before V disappears there is no non- zero row in V to choose in that step. In each step, a row of A is chosen as follows: o If all entries of V are zero then no row is chosen and decoding fails. o Let r be the minimum integer such that at least one row of A has exactly r ones in V. * If r != 2 then choose a row with exactly r ones in V with minimum original degree among all such rows, except that HDPC rows should not be chosen until all non-HDPC rows have been processed. * If r = 2 then choose any row with exactly 2 ones in V that is part of a maximum size component in the graph described above which is defined by V. After the row is chosen in this step the first row of A that intersects V is exchanged with the chosen row so that the chosen row is the first row that intersects V. The columns of A among those that intersect V are reordered so that one of the r ones in the chosen row appears in the first column of V and so that the remaining r-1 ones appear in the last columns of V. The same row and column operations are also performed on the matrix X. Then, an appropriate multiple of the chosen row is added to all the other rows of A below the chosen row that have a non-zero entry in the first column of V. Specifically, if a row below the chosen row has entry beta in the first column of V, and the chosen row has entry alpha in the first column of V, then beta/alpha multiplied by the chosen row is added to this row to leave a zero value in the first column of V. Finally, i is incremented by 1 and u is incremented by r-1, which completes the step. Note that efficiency can be improved if the row operations identified above are not actually performed until the affected row is itself chosen during the decoding process. This avoids processing of row operations for rows which are not eventually used in the decoding process and in particular avoid those rows for which beta!=1 until they are actually required. Furthermore, the row operations required for the HDPC rows may be performed for all such rows in one process, by using the algorithm described in Section 5.3.3.3. Luby, et al. Expires August 1, 2010 [Page 36]

Internet-Draft RaptorG FEC Scheme January 2010 5.4.2.3. Second Phase At this point, all the entries of X outside the first i rows and i columns are discarded, so that X has lower triangular form. The last i rows and columns of X are discarded, so that X now has i rows i columns. The submatrix U is further partitioned into the first i rows, U_upper, and the remaining M - i rows, Ulower. Gaussian elimination is performed in the second phase on U_lower to either determine that its rank is less than u (decoding failure) or to convert it into a matrix where the first u rows is the identity matrix (success of the second phase). Call this u by u identity matrix I_u. The M - L rows of A that intersect U_lower - I_u are discarded. After this phase A has L rows and L columns. 5.4.2.4. Third Phase After the second phase the only portion of A which needs to be zeroed out to finish converting A into the L by L identity matrix is U_upper. The number of rows i of the submatrix U_upper is generally much larger than the number of columns u of U_upper. Moreover, at this time, the matrix U_upper is typically dense, i.e., the number of nonzero entries of this matrix is large. To reduce this matrix to a sparse form, the sequence of operations performed to obtain the matrix U_lower needs to be inverted. To this end, the matrix X is multiplied with the submatrix of A consisting of the first i rows of A. After this operation the submatrix of A consisting of the intersection of the first i rows and columns equals to X, whereas the matrix U_upper is transformed to a sparse form. 5.4.2.5. Fourth Phase For each of the first i rows of U_upper do the following: if the row has a nonzero entry at position j, and if the value of that nonzero entry is b, then add to this row b times row j of I_u. After this step, the submatrix of A consisting of the intersection of the first i rows and columns is equal to X, the submatrix U_upper consists of zeros, the submatrix consisting of the intersection of the last u rows and the first i columns consists of zeros, and the submatrix consisting of the last u rows and columns is is the matrix I_u. 5.4.2.6. Fifth Phase For j from 1 to i perform the following operations: 1. if A[j,j] is not one, then divide row j of A by A[j,j]. 2. For l from 1 to j-1, if A[j,l] is nonzero, then add A[j,l] multiplied with row l of A to row j of A. Luby, et al. Expires August 1, 2010 [Page 37]

Internet-Draft RaptorG FEC Scheme January 2010 After this phase A is the L by L identity matrix and a complete decoding schedule has been successfully formed. Then, the corresponding decoding consisting of exclusive-ORing known encoding symbols can be executed to recover the intermediate symbols based on the decoding schedule. The tuples associated with all source symbols are computed according to Section 5.3.3.2. The tuples for received source symbols are used in the decoding. The tuples for missing source symbols are used to determine which intermediate symbols need to be exclusive-ORed to recover the missing source symbols. 5.5. Random Numbers The four tables V0, V1, V2 and V3 described in Section 5.3.5.1 are given below. Each entry is a 32-bit integer in decimal representation. 5.5.1. The table V0 251291136, 3952231631, 3370958628, 4070167936, 123631495, 3351110283, 3218676425, 2011642291, 774603218, 2402805061, 1004366930, 1843948209, 428891132, 3746331984, 1591258008, 3067016507, 1433388735, 504005498, 2032657933, 3419319784, 2805686246, 3102436986, 3808671154, 2501582075, 3978944421, 246043949, 4016898363, 649743608, 1974987508, 2651273766, 2357956801, 689605112, 715807172, 2722736134, 191939188, 3535520147, 3277019569, 1470435941, 3763101702, 3232409631, 122701163, 3920852693, 782246947, 372121310, 2995604341, 2045698575, 2332962102, 4005368743, 218596347, 3415381967, 4207612806, 861117671, 3676575285, 2581671944, 3312220480, 681232419, 307306866, 4112503940, 1158111502, 709227802, 2724140433, 4201101115, 4215970289, 4048876515, 3031661061, 1909085522, 510985033, 1361682810, 129243379, 3142379587, 2569842483, 3033268270, 1658118006, 932109358, 1982290045, 2983082771, 3007670818, 3448104768, 683749698, 778296777, 1399125101, 1939403708, 1692176003, 3868299200, 1422476658, 593093658, 1878973865, 2526292949, 1591602827, 3986158854, 3964389521, 2695031039, 1942050155, 424618399, 1347204291, 2669179716, 2434425874, 2540801947, 1384069776, 4123580443, 1523670218, 2708475297, 1046771089, 2229796016, 1255426612, 4213663089, 1521339547, 3041843489, 420130494, 10677091, 515623176, 3457502702, 2115821274, 2720124766, 3242576090, 854310108, 425973987, 325832382, 1796851292, 2462744411, 1976681690, 1408671665, 1228817808, 3917210003, 263976645, 2593736473, 2471651269, 4291353919, 650792940, 1191583883, 3046561335, 2466530435, 2545983082, 969168436, 2019348792, 2268075521, 1169345068, 3250240009, 3963499681, 2560755113, 911182396, 760842409, 3569308693, 2687243553, 381854665, 2613828404, 2761078866, 1456668111, 883760091, 3294951678, 1604598575, 1985308198, 1014570543, 2724959607, 3062518035, 3115293053, 138853680, 4160398285, 3322241130, 2068983570, 2247491078, Luby, et al. Expires August 1, 2010 [Page 38]

Internet-Draft RaptorG FEC Scheme January 2010 3669524410, 1575146607, 828029864, 3732001371, 3422026452, 3370954177, 4006626915, 543812220, 1243116171, 3928372514, 2791443445, 4081325272, 2280435605, 885616073, 616452097, 3188863436, 2780382310, 2340014831, 1208439576, 258356309, 3837963200, 2075009450, 3214181212, 3303882142, 880813252, 1355575717, 207231484, 2420803184, 358923368, 1617557768, 3272161958, 1771154147, 2842106362, 1751209208, 1421030790, 658316681, 194065839, 3241510581, 38625260, 301875395, 4176141739, 297312930, 2137802113, 1502984205, 3669376622, 3728477036, 234652930, 2213589897, 2734638932, 1129721478, 3187422815, 2859178611, 3284308411, 3819792700, 3557526733, 451874476, 1740576081, 3592838701, 1709429513, 3702918379, 3533351328, 1641660745, 179350258, 2380520112, 3936163904, 3685256204, 3156252216, 1854258901, 2861641019, 3176611298, 834787554, 331353807, 517858103, 3010168884, 4012642001, 2217188075, 3756943137, 3077882590, 2054995199, 3081443129, 3895398812, 1141097543, 2376261053, 2626898255, 2554703076, 401233789, 1460049922, 678083952, 1064990737, 940909784, 1673396780, 528881783, 1712547446, 3629685652, 1358307511 5.5.2. The table V1 807385413, 2043073223, 3336749796, 1302105833, 2278607931, 541015020, 1684564270, 372709334, 3508252125, 1768346005, 1270451292, 2603029534, 2049387273, 3891424859, 2152948345, 4114760273, 915180310, 3754787998, 700503826, 2131559305, 1308908630, 224437350, 4065424007, 3638665944, 1679385496, 3431345226, 1779595665, 3068494238, 1424062773, 1033448464, 4050396853, 3302235057, 420600373, 2868446243, 311689386, 259047959, 4057180909, 1575367248, 4151214153, 110249784, 3006865921, 4293710613, 3501256572, 998007483, 499288295, 1205710710, 2997199489, 640417429, 3044194711, 486690751, 2686640734, 2394526209, 2521660077, 49993987, 3843885867, 4201106668, 415906198, 19296841, 2402488407, 2137119134, 1744097284, 579965637, 2037662632, 852173610, 2681403713, 1047144830, 2982173936, 910285038, 4187576520, 2589870048, 989448887, 3292758024, 506322719, 176010738, 1865471968, 2619324712, 564829442, 1996870325, 339697593, 4071072948, 3618966336, 2111320126, 1093955153, 957978696, 892010560, 1854601078, 1873407527, 2498544695, 2694156259, 1927339682, 1650555729, 183933047, 3061444337, 2067387204, 228962564, 3904109414, 1595995433, 1780701372, 2463145963, 307281463, 3237929991, 3852995239, 2398693510, 3754138664, 522074127, 146352474, 4104915256, 3029415884, 3545667983, 332038910, 976628269, 3123492423, 3041418372, 2258059298, 2139377204, 3243642973, 3226247917, 3674004636, 2698992189, 3453843574, 1963216666, 3509855005, 2358481858, 747331248, 1957348676, 1097574450, 2435697214, 3870972145, 1888833893, 2914085525, 4161315584, 1273113343, 3269644828, 3681293816, 412536684, 1156034077, 3823026442, 1066971017, 3598330293, 1979273937, 2079029895, 1195045909, 1071986421, 2712821515, 3377754595, 2184151095, 750918864, 2585729879, 4249895712, Luby, et al. Expires August 1, 2010 [Page 39]

Internet-Draft RaptorG FEC Scheme January 2010 1832579367, 1192240192, 946734366, 31230688, 3174399083, 3549375728, 1642430184, 1904857554, 861877404, 3277825584, 4267074718, 3122860549, 666423581, 644189126, 226475395, 307789415, 1196105631, 3191691839, 782852669, 1608507813, 1847685900, 4069766876, 3931548641, 2526471011, 766865139, 2115084288, 4259411376, 3323683436, 568512177, 3736601419, 1800276898, 4012458395, 1823982, 27980198, 2023839966, 869505096, 431161506, 1024804023, 1853869307, 3393537983, 1500703614, 3019471560, 1351086955, 3096933631, 3034634988, 2544598006, 1230942551, 3362230798, 159984793, 491590373, 3993872886, 3681855622, 903593547, 3535062472, 1799803217, 772984149, 895863112, 1899036275, 4187322100, 101856048, 234650315, 3183125617, 3190039692, 525584357, 1286834489, 455810374, 1869181575, 922673938, 3877430102, 3422391938, 1414347295, 1971054608, 3061798054, 830555096, 2822905141, 167033190, 1079139428, 4210126723, 3593797804, 429192890, 372093950, 1779187770, 3312189287, 204349348, 452421568, 2800540462, 3733109044, 1235082423, 1765319556, 3174729780, 3762994475, 3171962488, 442160826, 198349622, 45942637, 1324086311, 2901868599, 678860040, 3812229107, 19936821, 1119590141, 3640121682, 3545931032, 2102949142, 2828208598, 3603378023, 4135048896 5.5.3. The table V2 1629829892, 282540176, 2794583710, 496504798, 2990494426, 3070701851, 2575963183, 4094823972, 2775723650, 4079480416, 176028725, 2246241423, 3732217647, 2196843075, 1306949278, 4170992780, 4039345809, 3209664269, 3387499533, 293063229, 3660290503, 2648440860, 2531406539, 3537879412, 773374739, 4184691853, 1804207821, 3347126643, 3479377103, 3970515774, 1891731298, 2368003842, 3537588307, 2969158410, 4230745262, 831906319, 2935838131, 264029468, 120852739, 3200326460, 355445271, 2296305141, 1566296040, 1760127056, 20073893, 3427103620, 2866979760, 2359075957, 2025314291, 1725696734, 3346087406, 2690756527, 99815156, 4248519977, 2253762642, 3274144518, 598024568, 3299672435, 556579346, 4121041856, 2896948975, 3620123492, 918453629, 3249461198, 2231414958, 3803272287, 3657597946, 2588911389, 242262274, 1725007475, 2026427718, 46776484, 2873281403, 2919275846, 3177933051, 1918859160, 2517854537, 1857818511, 3234262050, 479353687, 200201308, 2801945841, 1621715769, 483977159, 423502325, 3689396064, 1850168397, 3359959416, 3459831930, 841488699, 3570506095, 930267420, 1564520841, 2505122797, 593824107, 1116572080, 819179184, 3139123629, 1414339336, 1076360795, 512403845, 177759256, 1701060666, 2239736419, 515179302, 2935012727, 3821357612, 1376520851, 2700745271, 966853647, 1041862223, 715860553, 171592961, 1607044257, 1227236688, 3647136358, 1417559141, 4087067551, 2241705880, 4194136288, 1439041934, 20464430, 119668151, 2021257232, 2551262694, 1381539058, 4082839035, 498179069, 311508499, 3580908637, 2889149671, 142719814, 1232184754, 3356662582, 2973775623, 1469897084, 1728205304, 1415793613, 50111003, 3133413359, 4074115275, 2710540611, 2700083070, 2457757663, 2612845330, Luby, et al. Expires August 1, 2010 [Page 40]

Internet-Draft RaptorG FEC Scheme January 2010 3775943755, 2469309260, 2560142753, 3020996369, 1691667711, 4219602776, 1687672168, 1017921622, 2307642321, 368711460, 3282925988, 213208029, 4150757489, 3443211944, 2846101972, 4106826684, 4272438675, 2199416468, 3710621281, 497564971, 285138276, 765042313, 916220877, 3402623607, 2768784621, 1722849097, 3386397442, 487920061, 3569027007, 3424544196, 217781973, 2356938519, 3252429414, 145109750, 2692588106, 2454747135, 1299493354, 4120241887, 2088917094, 932304329, 1442609203, 952586974, 3509186750, 753369054, 854421006, 1954046388, 2708927882, 4047539230, 3048925996, 1667505809, 805166441, 1182069088, 4265546268, 4215029527, 3374748959, 373532666, 2454243090, 2371530493, 3651087521, 2619878153, 1651809518, 1553646893, 1227452842, 703887512, 3696674163, 2552507603, 2635912901, 895130484, 3287782244, 3098973502, 990078774, 3780326506, 2290845203, 41729428, 1949580860, 2283959805, 1036946170, 1694887523, 4880696, 466000198, 2765355283, 3318686998, 1266458025, 3919578154, 3545413527, 2627009988, 3744680394, 1696890173, 3250684705, 4142417708, 915739411, 3308488877, 1289361460, 2942552331, 1169105979, 3342228712, 698560958, 1356041230, 2401944293, 107705232, 3701895363, 903928723, 3646581385, 844950914, 1944371367, 3863894844, 2946773319, 1972431613, 1706989237, 29917467, 3497665928 5.5.4. The table V3 1191369816, 744902811, 2539772235, 3213192037, 3286061266, 1200571165, 2463281260, 754888894, 714651270, 1968220972, 3628497775, 1277626456, 1493398934, 364289757, 2055487592, 3913468088, 2930259465, 902504567, 3967050355, 2056499403, 692132390, 186386657, 832834706, 859795816, 1283120926, 2253183716, 3003475205, 1755803552, 2239315142, 4271056352, 2184848469, 769228092, 1249230754, 1193269205, 2660094102, 642979613, 1687087994, 2726106182, 446402913, 4122186606, 3771347282, 37667136, 192775425, 3578702187, 1952659096, 3989584400, 3069013882, 2900516158, 4045316336, 3057163251, 1702104819, 4116613420, 3575472384, 2674023117, 1409126723, 3215095429, 1430726429, 2544497368, 1029565676, 1855801827, 4262184627, 1854326881, 2906728593, 3277836557, 2787697002, 2787333385, 3105430738, 2477073192, 748038573, 1088396515, 1611204853, 201964005, 3745818380, 3654683549, 3816120877, 3915783622, 2563198722, 1181149055, 33158084, 3723047845, 3790270906, 3832415204, 2959617497, 372900708, 1286738499, 1932439099, 3677748309, 2454711182, 2757856469, 2134027055, 2780052465, 3190347618, 3758510138, 3626329451, 1120743107, 1623585693, 1389834102, 2719230375, 3038609003, 462617590, 260254189, 3706349764, 2556762744, 2874272296, 2502399286, 4216263978, 2683431180, 2168560535, 3561507175, 668095726, 680412330, 3726693946, 4180630637, 3335170953, 942140968, 2711851085, 2059233412, 4265696278, 3204373534, 232855056, 881788313, 2258252172, 2043595984, 3758795150, 3615341325, 2138837681, 1351208537, 2923692473, 3402482785, Luby, et al. Expires August 1, 2010 [Page 41]

Internet-Draft RaptorG FEC Scheme January 2010 2105383425, 2346772751, 499245323, 3417846006, 2366116814, 2543090583, 1828551634, 3148696244, 3853884867, 1364737681, 2200687771, 2689775688, 232720625, 4071657318, 2671968983, 3531415031, 1212852141, 867923311, 3740109711, 1923146533, 3237071777, 3100729255, 3247856816, 906742566, 4047640575, 4007211572, 3495700105, 1171285262, 2835682655, 1634301229, 3115169925, 2289874706, 2252450179, 944880097, 371933491, 1649074501, 2208617414, 2524305981, 2496569844, 2667037160, 1257550794, 3399219045, 3194894295, 1643249887, 342911473, 891025733, 3146861835, 3789181526, 938847812, 1854580183, 2112653794, 2960702988, 1238603378, 2205280635, 1666784014, 2520274614, 3355493726, 2310872278, 3153920489, 2745882591, 1200203158, 3033612415, 2311650167, 1048129133, 4206710184, 4209176741, 2640950279, 2096382177, 4116899089, 3631017851, 4104488173, 1857650503, 3801102932, 445806934, 3055654640, 897898279, 3234007399, 1325494930, 2982247189, 1619020475, 2720040856, 885096170, 3485255499, 2983202469, 3891011124, 546522756, 1524439205, 2644317889, 2170076800, 2969618716, 961183518, 1081831074, 1037015347, 3289016286, 2331748669, 620887395, 303042654, 3990027945, 1562756376, 3413341792, 2059647769, 2823844432, 674595301, 2457639984, 4076754716, 2447737904, 1583323324, 625627134, 3076006391, 345777990, 1684954145, 879227329, 3436182180, 1522273219, 3802543817, 1456017040, 1897819847, 2970081129, 1382576028, 3820044861, 1044428167, 612252599, 3340478395, 2150613904, 3397625662, 3573635640, 3432275192 5.6. Systematic indices and other parameters Table 2 below specifies the supported values of K'. The table also specifies for each supported value of K' the systematic index J(K'), the number H(K') of HDPC symbols, the number S(K') of LDPC symbols, and the number W(K') of LT symbols. For each value of K', the corresponding values of S(K') and W(K') are prime numbers. The systematic index J(K') is designed to have the property that the set of source symbol tuples (d[0], a[0], b[0], d1[0], a1[0], b1[0]), ..., (d[K'-1], a[K'-1], b[K'-1], d1[K'-1], a1[K'-1], b1[K'-1]) are such that the L intermediate symbols are uniquely defined, i.e., the matrix A in Figure 6 has full rank and is therefore invertible. +-------+-------+-------+-------+-------+ | K' | J(K') | S(K') | H(K') | W(K') | +-------+-------+-------+-------+-------+ | 6 | 3 | 5 | 10 | 11 | +-------+-------+-------+-------+-------+ | 12 | 57 | 7 | 10 | 19 | +-------+-------+-------+-------+-------+ | 18 | 27 | 11 | 10 | 29 | Luby, et al. Expires August 1, 2010 [Page 42]

Internet-Draft RaptorG FEC Scheme January 2010 | 26 | 96 | 11 | 10 | 37 | +-------+-------+-------+-------+-------+ | 32 | 959 | 11 | 10 | 43 | +-------+-------+-------+-------+-------+ | 36 | 564 | 11 | 10 | 47 | +-------+-------+-------+-------+-------+ | 42 | 39 | 11 | 10 | 53 | +-------+-------+-------+-------+-------+ | 48 | 10 | 13 | 10 | 61 | +-------+-------+-------+-------+-------+ | 55 | 531 | 13 | 10 | 67 | +-------+-------+-------+-------+-------+ | 62 | 55 | 13 | 10 | 73 | +-------+-------+-------+-------+-------+ | 69 | 235 | 13 | 10 | 79 | +-------+-------+-------+-------+-------+ | 75 | 234 | 17 | 10 | 89 | +-------+-------+-------+-------+-------+ | 88 | 113 | 17 | 10 | 101 | +-------+-------+-------+-------+-------+ | 101 | 8 | 17 | 10 | 113 | +-------+-------+-------+-------+-------+ | 114 | 8 | 19 | 10 | 127 | +-------+-------+-------+-------+-------+ | 127 | 184 | 19 | 10 | 139 | +-------+-------+-------+-------+-------+ | 140 | 7 | 19 | 10 | 151 | +-------+-------+-------+-------+-------+ | 160 | 39 | 23 | 10 | 173 | +-------+-------+-------+-------+-------+ | 185 | 751 | 23 | 10 | 197 | +-------+-------+-------+-------+-------+ | 213 | 1 | 23 | 10 | 223 | +-------+-------+-------+-------+-------+ | 242 | 10 | 29 | 10 | 257 | +-------+-------+-------+-------+-------+ | 267 | 195 | 29 | 10 | 281 | +-------+-------+-------+-------+-------+ | 295 | 572 | 29 | 10 | 307 | +-------+-------+-------+-------+-------+ | 324 | 447 | 31 | 10 | 337 | +-------+-------+-------+-------+-------+ | 362 | 751 | 31 | 10 | 373 | +-------+-------+-------+-------+-------+ | 403 | 234 | 37 | 10 | 419 | +-------+-------+-------+-------+-------+ | 443 | 974 | 37 | 10 | 457 | +-------+-------+-------+-------+-------+ Luby, et al. Expires August 1, 2010 [Page 43]

Internet-Draft RaptorG FEC Scheme January 2010 +-------+-------+-------+-------+-------+ | 497 | 115 | 37 | 10 | 509 | +-------+-------+-------+-------+-------+ | 555 | 17 | 41 | 10 | 569 | +-------+-------+-------+-------+-------+ | 619 | 75 | 41 | 10 | 631 | +-------+-------+-------+-------+-------+ | 685 | 476 | 47 | 10 | 701 | +-------+-------+-------+-------+-------+ | 759 | 112 | 47 | 10 | 773 | +-------+-------+-------+-------+-------+ | 839 | 454 | 53 | 10 | 857 | +-------+-------+-------+-------+-------+ | 932 | 424 | 53 | 10 | 947 | +-------+-------+-------+-------+-------+ | 1032 | 34 | 59 | 10 | 1051 | +-------+-------+-------+-------+-------+ | 1144 | 600 | 61 | 11 | 1163 | +-------+-------+-------+-------+-------+ | 1281 | 75 | 67 | 11 | 1303 | +-------+-------+-------+-------+-------+ | 1420 | 726 | 67 | 11 | 1439 | +-------+-------+-------+-------+-------+ | 1575 | 39 | 73 | 11 | 1597 | +-------+-------+-------+-------+-------+ | 1734 | 83 | 79 | 11 | 1759 | +-------+-------+-------+-------+-------+ | 1906 | 394 | 83 | 11 | 1931 | +-------+-------+-------+-------+-------+ | 2103 | 75 | 89 | 11 | 2131 | +-------+-------+-------+-------+-------+ | 2315 | 772 | 97 | 11 | 2347 | +-------+-------+-------+-------+-------+ | 2550 | 726 | 97 | 11 | 2579 | +-------+-------+-------+-------+-------+ | 2812 | 683 | 103 | 11 | 2843 | +-------+-------+-------+-------+-------+ | 3101 | 512 | 113 | 11 | 3137 | +-------+-------+-------+-------+-------+ | 3411 | 650 | 127 | 11 | 3457 | +-------+-------+-------+-------+-------+ | 3751 | 838 | 127 | 11 | 3793 | +-------+-------+-------+-------+-------+ | 4086 | 547 | 131 | 11 | 4127 | +-------+-------+-------+-------+-------+ | 4436 | 305 | 139 | 11 | 4481 | +-------+-------+-------+-------+-------+ | 4780 | 3 | 149 | 11 | 4831 | Luby, et al. Expires August 1, 2010 [Page 44]

Internet-Draft RaptorG FEC Scheme January 2010 | 5134 | 518 | 157 | 11 | 5189 | +-------+-------+-------+-------+-------+ | 5512 | 229 | 163 | 11 | 5569 | +-------+-------+-------+-------+-------+ | 6070 | 980 | 173 | 11 | 6131 | +-------+-------+-------+-------+-------+ | 6688 | 596 | 191 | 11 | 6761 | +-------+-------+-------+-------+-------+ | 7360 | 960 | 197 | 11 | 7433 | +-------+-------+-------+-------+-------+ | 8087 | 85 | 211 | 11 | 8167 | +-------+-------+-------+-------+-------+ | 8886 | 479 | 223 | 11 | 8971 | +-------+-------+-------+-------+-------+ | 9793 | 200 | 239 | 11 | 9887 | +-------+-------+-------+-------+-------+ | 10779 | 290 | 257 | 11 | 10883 | +-------+-------+-------+-------+-------+ | 11864 | 543 | 277 | 12 | 11981 | +-------+-------+-------+-------+-------+ | 13046 | 893 | 293 | 12 | 13171 | +-------+-------+-------+-------+-------+ | 14355 | 527 | 311 | 12 | 14489 | +-------+-------+-------+-------+-------+ | 15786 | 601 | 337 | 12 | 15937 | +-------+-------+-------+-------+-------+ | 17376 | 479 | 359 | 12 | 17539 | +-------+-------+-------+-------+-------+ | 19126 | 518 | 389 | 12 | 19309 | +-------+-------+-------+-------+-------+ | 21044 | 933 | 419 | 13 | 21247 | +-------+-------+-------+-------+-------+ | 23177 | 85 | 449 | 13 | 23399 | +-------+-------+-------+-------+-------+ | 25491 | 710 | 479 | 13 | 25733 | +-------+-------+-------+-------+-------+ | 28035 | 11 | 521 | 13 | 28319 | +-------+-------+-------+-------+-------+ | 30898 | 738 | 557 | 14 | 31219 | +-------+-------+-------+-------+-------+ | 33988 | 602 | 599 | 14 | 34351 | +-------+-------+-------+-------+-------+ | 37372 | 545 | 647 | 14 | 37783 | +-------+-------+-------+-------+-------+ | 41127 | 11 | 701 | 15 | 41593 | +-------+-------+-------+-------+-------+ | 45245 | 639 | 757 | 15 | 45767 | +-------+-------+-------+-------+-------+ Luby, et al. Expires August 1, 2010 [Page 45]

Internet-Draft RaptorG FEC Scheme January 2010 +-------+-------+-------+-------+-------+ | 49791 | 249 | 821 | 15 | 50377 | +-------+-------+-------+-------+-------+ | 54768 | 300 | 877 | 16 | 55411 | +-------+-------+-------+-------+-------+ | 56404 | 733 | 907 | 16 | 57077 | +-------+-------+-------+-------+-------+ Table 2: Systematic indices and other parameters 5.7. Arithmetic in GF(256) 5.7.1. Introduction Elements of GF(256) are represented by bytes. In this section, we opt to represent them by integers in the range 0 through 255. For ease of exposition, operations in GF(256) are facilitated by two tables: GF256_EXP, and GF256_LOG. GF256_EXP has 512 entries, whereas GF256_LOG has 256 entries. For an integer i between 0 and 511, GF256_EXP[i] is the binary value of the polynomial x^^i modulo x^^8 + x^^4 + x^^3 + x^^2 + 1, whereas for i between 1 and 255 the value of GF256_LOG[i] is the integer j such that the binary value of x^^j modulo x^^8 + x^^4 + x^^3 + x^^2 + 1 is i. In this representation we have i + j = i ^ j, and i * j = 0, if either i or j are 0, GF256_EXP[GF256_LOG[i] + GF256_LOG[j]] otherwise 5.7.2. The table GF256_EXP 1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38, 76, 152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192, 157, 39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159, 35, 70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111, 222, 161, 95, 190, 97, 194, 153, 47, 94, 188, 101, 202, 137, 15, 30, 60, 120, 240, 253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223, 163, 91, 182, 113, 226, 217, 175, 67, 134, 17, 34, 68, 136, 13, 26, 52, 104, 208, 189, 103, 206, 129, 31, 62, 124, 248, 237, 199, 147, 59, 118, 236, 197, 151, 51, 102, 204, 133, 23, 46, 92, 184, 109, 218, 169, 79, 158, 33, 66, 132, 21, 42, 84, 168, 77, 154, 41, 82, 164, 85, 170, 73, 146, 57, 114, 228, 213, 183, 115, 230, 209, 191, 99, 198, 145, 63, 126, 252, 229, 215, 179, 123, 246, 241, 255, 227, 219, 171, 75, 150, 49, 98, 196, 149, 55, 110, 220, 165, 87, 174, 65, 130, 25, Luby, et al. Expires August 1, 2010 [Page 46]

Internet-Draft RaptorG FEC Scheme January 2010 50, 100, 200, 141, 7, 14, 28, 56, 112, 224, 221, 167, 83, 166, 81, 162, 89, 178, 121, 242, 249, 239, 195, 155, 43, 86, 172, 69, 138, 9, 18, 36, 72, 144, 61, 122, 244, 245, 247, 243, 251, 235, 203, 139, 11, 22, 44, 88, 176, 125, 250, 233, 207, 131, 27, 54, 108, 216, 173, 71, 142, 1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38, 76, 152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192, 157, 39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159, 35, 70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111, 222, 161, 95, 190, 97, 194, 153, 47, 94, 188, 101, 202, 137, 15, 30, 60, 120, 240, 253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223, 163, 91, 182, 113, 226, 217, 175, 67, 134, 17, 34, 68, 136, 13, 26, 52, 104, 208, 189, 103, 206, 129, 31, 62, 124, 248, 237, 199, 147, 59, 118, 236, 197, 151, 51, 102, 204, 133, 23, 46, 92, 184, 109, 218, 169, 79, 158, 33, 66, 132, 21, 42, 84, 168, 77, 154, 41, 82, 164, 85, 170, 73, 146, 57, 114, 228, 213, 183, 115, 230, 209, 191, 99, 198, 145, 63, 126, 252, 229, 215, 179, 123, 246, 241, 255, 227, 219, 171, 75, 150, 49, 98, 196, 149, 55, 110, 220, 165, 87, 174, 65, 130, 25, 50, 100, 200, 141, 7, 14, 28, 56, 112, 224, 221, 167, 83, 166, 81, 162, 89, 178, 121, 242, 249, 239, 195, 155, 43, 86, 172, 69, 138, 9, 18, 36, 72, 144, 61, 122, 244, 245, 247, 243, 251, 235, 203, 139, 11, 22, 44, 88, 176, 125, 250, 233, 207, 131, 27, 54, 108, 216, 173, 71, 142, 1, 2 5.7.3. The table GF256_LOG 255, 0, 1, 25, 2, 50, 26, 198, 3, 223, 51, 238, 27, 104, 199, 75, 4, 100, 224, 14, 52, 141, 239, 129, 28, 193, 105, 248, 200, 8, 76, 113, 5, 138, 101, 47, 225, 36, 15, 33, 53, 147, 142, 218, 240, 18, 130, 69, 29, 181, 194, 125, 106, 39, 249, 185, 201, 154, 9, 120, 77, 228, 114, 166, 6, 191, 139, 98, 102, 221, 48, 253, 226, 152, 37, 179, 16, 145, 34, 136, 54, 208, 148, 206, 143, 150, 219, 189, 241, 210, 19, 92, 131, 56, 70, 64, 30, 66, 182, 163, 195, 72, 126, 110, 107, 58, 40, 84, 250, 133, 186, 61, 202, 94, 155, 159, 10, 21, 121, 43, 78, 212, 229, 172, 115, 243, 167, 87, 7, 112, 192, 247, 140, 128, 99, 13, 103, 74, 222, 237, 49, 197, 254, 24, 227, 165, 153, 119, 38, 184, 180, 124, 17, 68, 146, 217, 35, 32, 137, 46, 55, 63, 209, 91, 149, 188, 207, 205, 144, 135, 151, 178, 220, 252, 190, 97, 242, 86, 211, 171, 20, 42, 93, 158, 132, 60, 57, 83, 71, 109, 65, 162, 31, 45, 67, 216, 183, 123, 164, 118, 196, 23, 73, 236, 127, 12, 111, 246, 108, 161, 59, 82, 41, 157, 85, 170, 251, 96, 134, 177, 187, 204, 62, 90, 203, 89, 95, 176, 156, 169, 160, 81, 11, 245, 22, 235, 122, 117, 44, 215, 79, 174, 213, 233, 230, 231, 173, 232, 116, 214, 244, 234, 168, 80, 88, 175 Luby, et al. Expires August 1, 2010 [Page 47]

Internet-Draft RaptorG FEC Scheme January 2010 6. Security Considerations Data delivery can be subject to denial-of-service attacks by attackers which send corrupted packets that are accepted as legitimate by receivers. This is particularly a concern for multicast delivery because a corrupted packet may be injected into the session close to the root of the multicast tree, in which case the corrupted packet will arrive at many receivers. This is particularly a concern when the code described in this document is used because the use of even one corrupted packet containing encoding data may result in the decoding of an object that is completely corrupted and unusable. It is thus RECOMMENDED that source authentication and integrity checking are applied to decoded objects before delivering objects to an application. For example, a SHA-1 hash [SHA1] of an object may be appended before transmission, and the SHA-1 hash is computed and checked after the object is decoded but before it is delivered to an application. Source authentication SHOULD be provided, for example by including a digital signature verifiable by the receiver computed on top of the hash value. It is also RECOMMENDED that a packet authentication protocol such as TESLA [RFC4082] be used to detect and discard corrupted packets upon arrival. This method may also be used to provide source authentication. Furthermore, it is RECOMMENDED that Reverse Path Forwarding checks be enabled in all network routers and switches along the path from the sender to receivers to limit the possibility of a bad agent successfully injecting a corrupted packet into the multicast tree data path. Another security concern is that some FEC information may be obtained by receivers out-of-band in a session description, and if the session description is forged or corrupted then the receivers will not use the correct protocol for decoding content from received packets. To avoid these problems, it is RECOMMENDED that measures be taken to prevent receivers from accepting incorrect session descriptions, e.g., by using source authentication to ensure that receivers only accept legitimate session descriptions from authorized senders. Luby, et al. Expires August 1, 2010 [Page 48]

Internet-Draft RaptorG FEC Scheme January 2010 7. IANA Considerations Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA registration. For general guidelines on IANA considerations as they apply to this document, see [RFC5052]. This document assigns the Fully-Specified FEC Encoding ID 6 (tbc) under the ietf:rmt:fec: encoding name-space to "RaptorQ Code". Luby, et al. Expires August 1, 2010 [Page 49]

Internet-Draft RaptorG FEC Scheme January 2010 8. Acknowledgements Thanks are due to Lorenz Minder and Ranganathan (Ranga) Krishnan. Lorenz Minder did the original implementation of RaptorQ, supervised by Amin Shokrollahi. Ranga Krishnan has been very supportive in finding and resolving implementation details and in finding the systematic indices. Luby, et al. Expires August 1, 2010 [Page 50]

Internet-Draft RaptorG FEC Scheme January 2010 9. References 9.1. Normative references [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005. [SHA1] "Secure Hash Standard", Federal Information Processing Standards Publication (FIPS PUB) 180-1, April 2005. [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error Correction (FEC) Building Block", RFC 5052, August 2007. [RFC5053] Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer, "Raptor Forward Error Correction Scheme for Object Delivery", RFC 5053, October 2007. 9.2. Informative references [RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and J. Crowcroft, "The Use of Forward Error Correction (FEC) in Reliable Multicast", RFC 3453, December 2002. Luby, et al. Expires August 1, 2010 [Page 51]

```
Internet-Draft RaptorG FEC Scheme January 2010
Authors' Addresses
Michael Luby
Qualcomm, Inc.
3165 Kifer Road
Santa Clara, 95051 94538
U.S.A.
Email: luby@qualcomm.com
Amin Shokrollahi
EPFL
Laboratoire d'algorithmique
EPFL
Station 14
Batiment BC
Lausanne 1015
Switzerland
Email: amin.shokrollahi@epfl.ch
Mark Watson
Qualcomm, Inc.
3165 Kifer Road
Santa Clara, CA 95051
U.S.A.
Email: watson@qualcomm.com
Thomas Stockhammer
Nomor Research
Brecherspitzstrasse 8
Munich 81541
Germany
Email: stockhammer@nomor.de
Luby, et al. Expires August 1, 2010 [Page 52]
```