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<rfc number="5053" category="std" >
	<front>
		<title abbrev="Raptor FEC Scheme">Raptor Forward Error Correction Scheme for Object Delivery</title>


	<author initials="M." surname="Luby" fullname="Michael Luby">
			<organization>Digital Fountain</organization>
			<address>
				<postal>
					<street>39141 Civic Center Drive</street>
					<street>Suite 300</street>
					<city>Fremont</city>
					<region>CA</region>
					<code>94538</code>
					<country>U.S.A.</country>
				</postal>
				<email>luby@digitalfountain.com</email>
			</address>
		</author>
		<author initials="A." surname="Shokrollahi" fullname="Amin Shokrollahi">
			<organization>EPFL</organization>
			<address>
				<postal>
					<street>Laboratory of Algorithmic Mathematics</street>
					<street>IC-IIF-ALGO</street>
					<street>PSE-A</street>
					<city>Lausanne</city>
					<code>1015</code>
					<country>Switzerland</country>
				</postal>
				<email>amin.shokrollahi@epfl.ch</email>
			</address>
		</author>
		<author initials="M." surname="Watson" fullname="Mark Watson">
			<organization>Digital Fountain</organization>
			<address>
				<postal>
					<street>39141 Civic Center Drive</street>
					<street>Suite 300</street>
					<city>Fremont</city>
					<region>CA</region>
					<code>94538</code>
					<country>U.S.A.</country>
				</postal>
				<email>mark@digitalfountain.com</email>
			</address>
		</author>
		<author initials="T." surname="Stockhammer" fullname="Thomas Stockhammer">
			<organization>Nomor Research</organization>
			<address>
				<postal>
				<street>Nomor Research</street>
					<city>Bergen</city>
					<code>83346</code>
					<country>Germany</country>
				</postal>
				<email>stockhammer@nomor.de</email>
			</address>
		</author>
		   <date month="September" year="2007"/>
		<area>Transport</area>
		<workgroup>Reliable Multicast Transport</workgroup>

<!-- [rfced] Please insert any keywords (beyond those that appear in
the title) for use on http://www.rfc-editor.org/rfcsearch.html. -->

<keyword>example</keyword>

		<abstract>
			<t>This document describes a Fully-Specified
			  Forward Error Correction (FEC) scheme, corresponding to FEC Encoding ID 1, for the Raptor forward error correction code and its application to reliable
delivery of data objects.</t>
			<t>Raptor 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
source block of data.  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.</t>
			<t>The Raptor code described here is a systematic code, meaning that all the source symbols are among the encoding symbols that can be generated.</t>
		</abstract>
	</front>
	<middle>
		<section title="Introduction">
			<t>This document specifies an FEC Scheme for the Raptor forward error correction code for object delivery applications. The concept of an FEC Scheme is defined  in <xref target="RFC5052"/> and this document follows the format prescribed there and uses the terminology of that document. Raptor Codes were introduced in <xref target="Raptor"/>. For an overview, see, for example, <xref target="CCNC"/>. 
			</t>
			<t>The Raptor FEC Scheme is a Fully-Specified FEC Scheme corresponding to FEC Encoding ID 1.</t>
			<t>Raptor 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.</t>
			<t>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 background on the use of Forward Error Correction codes in reliable multicast, see <xref target="RFC3453"/>.</t>
			<t>The code described here is identical to that described in <xref target="MBMS"/>.
			</t>
		</section>
		<section title="Requirements Notation">
			<t>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 <xref target="RFC2119"/>.</t>
		</section>
		<section title="Formats and Codes">
			<section title="FEC Payload IDs">
				<t>The FEC Payload ID MUST be a 4 octet field defined as follows:
<figure title="FEC Payload ID format" anchor="fecpayload">
<artwork>
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Source Block Number       |      Encoding Symbol ID       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
					</figure>
				</t>
				<t>
					<list>
						<t>Source Block Number (SBN), (16 bits): An integer identifier for the source block that the encoding symbols within the packet relate to.</t>
						<t>Encoding Symbol ID (ESI), (16 bits): An integer identifier for the encoding symbols within the packet.</t>
					</list>
				</t>
				<t>The interpretation of the Source Block Number and Encoding Symbol Identifier is defined in <xref target="codespecification"/>.
				</t>
			</section>
			<section title="FEC Object Transmission
					Information (OTI)">
				<section title="Mandatory">
					<t>The value of the FEC Encoding ID MUST be 1 (one), as assigned by IANA (see <xref target="IANA"/>).</t>
				</section>
				<section title="Common">
					<t>The Common FEC Object Transmission Information elements used by this FEC Scheme are:
<list>
							<t>- Transfer Length (F)</t>
							<t>- Encoding Symbol Length (T)</t>
						</list>
					</t>
					<t>The Transfer Length is a non-negative integer less than 2^^45. The Encoding Symbol Length is a non-negative integer less than 2^^16.</t>
					<t>The encoded Common FEC Object Transmission Information format is shown in <xref target="CommonFOTIfigure"/>.
					<figure title="Encoded Common FEC OTI for Raptor FEC Scheme" anchor="CommonFOTIfigure">
<artwork>
    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                          |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Encoding Symbol Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
</t>
<t><list>
	<t>NOTE 1: The limit of 2^^45 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 2^^13, and the limitation on the number of source blocks to 2^^16. However, the Transfer Length is encoded as a 48-bit field for simplicity.</t>
</list></t>
				</section>
				<section title="Scheme-Specific">
					<t>The following parameters are carried in the Scheme-Specific FEC Object Transmission Information element for this FEC Scheme:
<list>
							<t>- The number of source blocks (Z)</t>
							<t>- The number of sub-blocks (N)</t>
							<t>- A symbol alignment parameter (Al)</t>
						</list>
					</t>
					<t>These parameters are all non-negative integers. The encoded Scheme-specific Object
					 Transmission Information is a
					 4-octet field consisting of
					 the parameters Z (2 octets),
					 N (1 octet),
					 and Al (1 octet) as shown in <xref target="OTIfigure"/>.
<figure title="Encoded Scheme-Specific FEC Object Transmission Information" anchor="OTIfigure">
							<artwork>
     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      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
</artwork>
						</figure>
					</t>
					<t>The encoded FEC Object Transmission Information is a 14-octet field consisting
						of the concatenation of the encoded Common FEC Object Transmission Information
						and the encoded Scheme-Specific FEC Object Transmission Information.</t>
						<t>These three parameters define the source block partitioning as described in <xref target="partitioning"/>.
						</t>
				</section>
			</section>
		</section>
		<section title="Procedures">
			<section title="Content Delivery Protocol Requirements">
				<t>This section describes the information exchange between the Raptor FEC Scheme and any Content Delivery Protocol (CDP) that makes use of the Raptor FEC Scheme for object delivery.</t>
				<t>The Raptor encoder and decoder for object delivery require the following information from the CDP:
<list>
						<t>-	The transfer length of the object, F, in bytes</t>
						<t>-	A symbol alignment parameter, Al</t>
						<t>-	The symbol size, T, in bytes, which MUST be a multiple of Al</t>
						<t>-	The number of source blocks, Z</t>
						<t>-	The number of sub-blocks in each source block, N</t>
					</list>
				</t>
				<t>The Raptor encoder for object delivery additionally requires:
<list>
						<t>-	the object to be encoded, F bytes</t>
					</list>
				</t>
				<t>The Raptor encoder supplies the CDP with the following information for each packet to be sent:
<list>
						<t>- Source Block Number (SBN)</t>
						<t>- Encoding Symbol ID (ESI)</t>
						<t>- Encoding symbol(s)</t>
					</list>
				</t>
				<t>The CDP MUST communicate this information to the receiver.</t>
			</section>
			<section title="Example Parameter Derivation Algorithm" anchor="recommend">
				<t>This section provides
				recommendations for the derivation of
				the three transport parameters, T, Z,
				and N. &nbsp;This recommendation is based on the following input parameters:
<list style="hanging" hangIndent="5">
						<t hangText="- F">the transfer length of the object, in bytes</t>
						<t hangText="- W">a target on the sub-block size, in bytes</t>
						<t hangText="- P">the maximum packet payload size, in bytes, which is assumed to be a multiple of Al</t>
						<t hangText="- Al">the symbol alignment parameter, in bytes</t>
						<t hangText="- Kmax">the maximum number of source symbols per source block.
						<list style="empty">
								<t>Note: <xref target="symbols"/> defines Kmax to be 8192.</t>
							</list>
						</t>
						<t hangText="- Kmin">a minimum target on the number of symbols per source block</t>
						<t hangText="- Gmax">a maximum target number of symbols per packet</t>
					</list>
				</t>
				<t>Based on the above inputs, the transport parameters T, Z, and N are calculated as follows:</t>
				<t> Let
<list>
						<t>G = min{ceil(P*Kmin/F), P/Al, Gmax}</t>
						<t>T = floor(P/(Al*G))*Al</t>
						<t>Kt = ceil(F/T)</t>
						<t>Z = ceil(Kt/Kmax)</t>
						<t>N = min{ceil(ceil(Kt/Z)*T/W), T/Al}</t>
					</list>
				</t>
				<t>The value G represents the maximum number of symbols to be transported in a single packet. The value Kt is the total
				   number of symbols required to represent the source data of the object. The values of G and N derived above should be
				    considered as lower bounds. It may be advantageous to increase these values, for example, to the nearest power of two.
				    In particular, the above algorithm does not guarantee that the symbol size, T, divides the maximum packet size,
				    P, and so it may not be possible to use the packets of size exactly P. &nbsp;If, instead, G is chosen to be a value
				    that divides P/Al, then the symbol size, T, will be a divisor of P and packets of size P can be used.</t>
				<t>The algorithm above and that defined in <xref target="partitioning"/> 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.</t>
				<t>Recommended settings for the input parameters, Al, Kmin, and Gmax are as follows: Al = 4, Kmin = 1024, Gmax = 10.</t>
				<t>The parameter W can be used to generate encoded data that can be decoded efficiently with limited working memory at the decoder. Note that the actual maximum decoder memory requirement for a given value of W depends on the implementation, but it is possible to implement decoding using working memory only slightly larger than W.</t>
			</section>
		</section>
		<section title="Raptor FEC Code Specification" anchor="codespecification">

			<section title="Definitions, Symbols, and Abbreviations">
				<section title="Definitions">
					<t>For the purposes of this specification, the following terms and definitions apply.
<list>
							<t>Source block: a block of K source symbols that are considered together for Raptor encoding purposes.</t>
							<t>Source symbol: the smallest unit of data used during the encoding process. All source symbols within a source block have the same size.</t>
							<t>Encoding symbol: a symbol that is included in a data packet. The encoding symbols consist of the source symbols and the repair symbols. Repair symbols generated from a source block have the same size as the source symbols of that source block.</t>
							<t>Systematic code: a code in which all the source symbols may be included as part of the encoding symbols sent for a source block.</t>
							<t>Repair symbol:  the encoding symbols sent for a source block that are not the source symbols. The repair symbols are generated based on the source symbols.</t>
							<t>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 included in data packets.</t>
							<t>Symbol: a unit of data. The size, in bytes, of a symbol is known as the symbol size.</t>
							<t>Encoding symbol group: a group of encoding symbols that are sent together, i.e., within the same packet whose relationship to the source symbols can be derived from a single Encoding Symbol ID.</t>
							<t>Encoding Symbol ID: information that defines the relationship between the symbols of an encoding symbol group and the source symbols.</t>
							<t>Encoding packet: data packets that contain encoding symbols</t>
							<t>Sub-block: a source block is sometimes broken into sub-blocks, each of which is sufficiently small to be decoded in working memory. For a source block consisting of K source symbols, each sub-block consists of K sub-symbols, each symbol of the source block being composed of one sub-symbol from each sub-block.</t>
							<t>Sub-symbol: part of a symbol. Each source symbol is composed of as many sub-symbols as there are sub-blocks in the source block.</t>
							<t>Source packet: data packets that contain source symbols.</t>
							<t>Repair packet: data packets that contain repair symbols.</t>
						</list>
					</t>
				</section>
				<section title="Symbols" anchor="symbols">
					<t>
						<list style="hanging" hangIndent="5">
				<t hangText="i, j, x, h, a, b, d, v, m">represent positive integers.</t>
				<t hangText="ceil(x)">denotes the smallest positive integer that is greater than or equal to x.</t>
				<t hangText="choose(i,j)">denotes the number of ways j objects can be chosen from among i objects without repetition.</t>
				<t hangText="floor(x)">denotes the largest positive integer that is less than or equal to x.</t>
				<t hangText="i % j">denotes i modulo j.</t>
				<t hangText="X ^ Y">denotes, for equal-length bit strings X and Y, the bitwise exclusive-or of X and Y.</t>
				<t hangText="Al">denotes a symbol alignment parameter. Symbol and sub-symbol sizes are restricted to be multiples of Al.</t>
				<t hangText="A">denotes a matrix over GF(2).</t>
				<t hangText="Transpose[A]">denotes the transposed matrix of matrix A.</t>
				<t hangText="A^^-1">denotes the inverse matrix of matrix A.</t>
				<t hangText="K">denotes the number of symbols in a single source block.</t>
				<t hangText="Kmax">denotes the maximum number of source symbols that can be in a single source block.  Set to 8192.</t>
				<t hangText="L">denotes the number of pre-coding symbols for a single source block.</t>
				<t hangText="S">denotes the number of LDPC symbols for a single source block.</t>
				<t hangText="H">denotes the number of Half symbols for a single source block.</t>
				<t hangText="C">denotes an array of intermediate symbols, C[0], C[1], C[2],..., C[L-1].</t>
				<t hangText="C'">denotes an array of
				source symbols, C&apos;[0],
				C&apos;[1], C&apos;[2],...,
				C&apos;[K-1].</t>

				<t hangText="X">a non-negative integer value</t>
				<t hangText="V0, V1">two arrays of 4-byte integers, V0[0], V0[1],..., V0[255] and V1[0], V1[1],..., V1[255]</t>
				<t hangText="Rand[X, i, m]">a pseudo-random number generator</t>
				<t hangText="Deg[v]">a degree generator</t>
				<t hangText="LTEnc[K, C ,(d, a, b)]">a LT encoding symbol generator</t>
				<t hangText="Trip[K, X]">a triple generator function</t>
				<t hangText="G">the number of symbols within an encoding symbol group</t>
				<t hangText="GF(n)">The Galois field with n elements.</t>
				<t hangText="N">the number of sub-blocks within a source block</t>
				<t hangText="T">the symbol size in bytes.  If the source block is partitioned into sub-blocks, then T = T&apos;*N.</t>
				<t hangText="T'">the sub-symbol size, in bytes.  If the source block is not partitioned into sub-blocks, then T&apos; is not relevant.  </t>
				<t hangText="F">the transfer length of an object, in bytes</t>
				<t hangText="I">the sub-block size in bytes</t>
				<t hangText="P">for object delivery, the payload size of each packet, in bytes, that is used in the recommended derivation of the object delivery transport parameters.</t>
				<t hangText="Q">Q = 65521, i.e., Q is the largest prime smaller than 2^^16</t>
				<t hangText="Z">the number of source blocks, for object delivery</t>
				<t hangText="J(K)">the systematic index associated with K</t>
				<t hangText="I_S">denotes the SxS identity matrix.</t>
				<t hangText="0_SxH">denotes the SxH zero matrix.</t>
				<t hangText="a ^^ b">a raised to the power b</t>
						</list>
					</t>
				</section>
				<section title="Abbreviations">
					<t>
For the purposes of the present document, the following abbreviations apply:
<list style="hanging" hangIndent="10">
							<t hangText="ESI">Encoding Symbol ID</t>
							<t hangText="LDPC">Low Density Parity Check</t>
							<t hangText="LT">Luby Transform</t>
							<t hangText="SBN">Source Block Number</t>
							<t hangText="SBL">Source Block Length (in units of symbols)</t>
						</list>
					</t>
				</section>
			</section>
			<section title="Overview">
				<t>The principal component of the systematic Raptor code is the basic encoder described in <xref target="encoder"/>. First, it is described how to derive values for a set of intermediate symbols from the original source symbols such that knowledge of the intermediate symbols is sufficient to reconstruct the source symbols. Secondly, the encoder produces repair symbols, which are each the exclusive OR of a number of the intermediate symbols. The encoding symbols are the combination of the source and repair symbols.  The repair symbols are produced in such a way that the intermediate symbols, and therefore also the source symbols, can be recovered from any sufficiently large set of encoding symbols.</t>
				<t>This document specifies the systematic Raptor code encoder. A number of possible decoding algorithms are possible. An efficient decoding algorithm is provided in <xref target="decoder"/>.</t>
				<t>The construction of the intermediate and repair symbols is based in part on a pseudo-random number generator described in <xref target="randomgenerator"/>. This generator is based on a fixed set of 512 random numbers that MUST be available to both sender and receiver. These are provided in <xref target="random"/>.</t>
				<t>Finally, the construction of the intermediate symbols from the source symbols is governed by a &apos;systematic index&apos;, values of which are provided in <xref target="systematic"/> for source block sizes from 4 source symbols to Kmax  = 8192 source symbols.</t>
			</section>
			<section title="Object Delivery">
				<section title="Source Block Construction">
					<section title="General">
						<t>In order to apply the Raptor encoder to a source object, the object may be broken into Z &gt;= 1 blocks, known as source blocks. The Raptor 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.</t>
						<t>Each source block with K source symbols is divided into N &gt;= 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&apos;. </t>
						<t>Note that the value of K is not necessarily the same for each source block of an object and the value of T&apos; 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 <xref target="partitioning"/> below.</t>
					</section>
					<section title="Source Block and Sub-Block Partitioning" anchor="partitioning">
						<t>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:
<list style="hanging" hangIndent="5">
					<t hangText="- F">      the transfer length of the object, in bytes</t>
					<t hangText="- Al">	a symbol alignment parameter, in bytes</t>
					<t hangText="- T">	the symbol size, in bytes, which MUST be a multiple of Al</t>
					<t hangText="- Z">	the number of source blocks</t>
					<t hangText="- N">	the number of sub-blocks in each source block</t>
							</list>
						</t>
						<t>These parameters MUST be set so that ceil(ceil(F/T)/Z) &lt;= Kmax.
						  Recommendations for derivation of these parameters are provided in <xref target="recommend"/>.
						</t>
						<t>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.</t>
						<t>The source object MUST be partitioned into source blocks and sub-blocks as follows:</t>
						<t>Let
<list>
								<t>Kt = ceil(F/T)</t>
								<t>(KL, KS, ZL, ZS) = Partition[Kt, Z]</t>
								<t>(TL, TS, NL, NS) = Partition[T/Al, N]</t>
							</list>
						</t>
						<t>Then, the object MUST be partitioned into Z = ZL + ZS contiguous source blocks, the first ZL source blocks each having length KL*T bytes, and the remaining ZS source blocks each having KS*T bytes.</t>
						<t>If Kt*T &gt; F, then for encoding purposes, the last symbol MUST be padded at the end with Kt*T - F zero bytes.</t>
						<t>Next, each source block 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 and the remaining NS sub-blocks each consisting of K contiguous sub-symbols of size of TS*Al. The symbol alignment parameter Al ensures that sub-symbols are always a multiple of Al bytes.</t>
						<t>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 &gt; 1, then a symbol is NOT a contiguous portion of the object.</t>
					</section>
				</section>
				<section title="Encoding Packet Construction" anchor="encodingpackets">
					
						<t>Each encoding packet contains the following information:
<list>
								<t>-	Source Block Number (SBN)</t>
								<t>-	Encoding Symbol ID (ESI)</t>
								<t>-	encoding symbol(s)</t>
							</list>
						</t>
						<t>Each source block is encoded independently of the others. Source blocks are numbered consecutively from zero.</t>
						<t>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 symbols in the source block. Encoding Symbol IDs from K onwards identify repair symbols.</t>
					
						<t>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.</t>
						<t>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.</t>
						<t>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. </t>
						<t>Note that it is not necessary for the receiver to know the total number of repair packets.</t>
						<t>Associated with each symbol is a triple of integers (d, a, b).</t>
						<t>The G repair symbol triples (d[0], a[0], b[0]),..., (d[G-1], a[G-1], b[G-1]) for the repair symbols placed into a repair packet with ESI X are computed using the Triple generator defined in <xref target="triplegenerator"/> as follows:
<list>
								<t>For each i = 0, ..., G-1, (d[i], a[i], b[i]) = Trip[K,X+i]</t>
							</list>
						</t>
						<t>The G repair symbols to be placed in repair packet with ESI X are calculated based on the repair symbol triples, as described in <xref target="encoder"/>, using the intermediate symbols C and the LT encoder LTEnc[K, C, (d[i], a[i], b[i])].</t>
					
				</section>
			</section>
			<section title="Systematic Raptor Encoder" anchor="encoder">
				<section title="Encoding Overview">
					<t>The systematic Raptor encoder is used to generate repair symbols from a source block that consists of K source symbols. </t>
					<t>Symbols are the fundamental data units of the encoding and decoding process. For each source block (sub-block), all symbols (sub-symbols) are the same size.  The atomic operation performed on symbols (sub-symbols) for both encoding and decoding is the exclusive-or operation.</t>
					<t>Let C&apos;[0],..., C&apos;[K-1] denote the K source symbols.</t>
					<t>Let C[0],..., C[L-1] denote L intermediate symbols.</t>
					<t>The first step of encoding is to generate a number, L &gt; K, of intermediate symbols from the K source symbols. In this step, K source symbol triples (d[0], a[0], b[0]), ..., (d[K-1], a[K-1], b[K-1]) are generated using the Trip[] generator as described in <xref target="sourcetriples"/>. The K source symbol triples are associated with the K source symbols and are then used to determine the L intermediate symbols C[0],..., C[L-1] from the source symbols using an inverse encoding process. This process can be realized by a Raptor decoding process.</t>
					<t>Certain &quot;pre-coding relationships&quot; MUST hold within the L intermediate symbols. <xref target="precoding"/> describes these relationships and how the intermediate symbols are generated from the source symbols.</t>
					<t>
Once the intermediate symbols have been generated, repair symbols are produced and one or more repair symbols are placed as a group into a single data packet. Each repair symbol group is associated with an Encoding Symbol ID (ESI) and a number, G, of repair symbols.  The ESI is used to generate a triple of three integers, (d, a,  b) for each repair symbol, again using the Trip[] generator as described in <xref target="triplegenerator"/>.  Then, each (d,a,b)-triple is used to generate the corresponding repair symbol from the intermediate symbols using the LTEnc[K, C[0],..., C[L-1], (d,a,b)] generator described in <xref target="ltenc"/>.</t>
				</section>
				<section title="First Encoding Step: Intermediate Symbol Generation">
					<section title="General">
						<t>The first encoding step is a pre-coding step to generate the L intermediate symbols C[0], ..., C[L-1] from the source symbols C&apos;[0], ..., C&apos;[K-1]. The intermediate symbols are uniquely defined by two sets of constraints:
<list>
								<t>1. The intermediate symbols are related to the source symbols by a set of source symbol triples. The generation of the source symbol triples is defined in <xref target="sourcetriples"/> using the Trip[] generator described in <xref target="triplegenerator"/>.</t>
								<t>2. A set of pre-coding relationships hold within the intermediate symbols themselves. These are defined in <xref target="precoding"/>.
								</t>
							</list>
						</t>
						<t>The generation of the L intermediate symbols is then defined in <xref target="intermediatesymbols"/>
						</t>
					</section>
					<section title="Source Symbol Triples" anchor="sourcetriples">
						<t>Each of the K source symbols is associated with a triple (d[i], a[i], b[i]) for 0 &lt;= i &lt; K. &nbsp;The source symbol triples are determined using the Triple generator defined in <xref target="triplegenerator"/> as:
<list>
								<t>For each i, 0 &lt;= i &lt; K
<list>
										<t>(d[i], a[i], b[i]) = Trip[K, i]</t>
									</list>
								</t>
							</list>
						</t>
					</section>
					<section title="Pre-Coding Relationships" anchor="precoding">
						<t>The pre-coding relationships amongst the L intermediate symbols are defined by expressing the last L-K intermediate symbols in terms of the first K intermediate symbols.</t>
						<t>The last L-K intermediate symbols C[K],...,C[L-1] consist of S LDPC symbols and H Half symbols The values of S and H are determined from K as described below. Then L = K+S+H.</t>
						<t>Let 
<list>
								<t>X    be the smallest positive integer such that X*(X-1) &gt;= 2*K.</t>
								<t>S    be the smallest prime integer such that S &gt;= ceil(0.01*K) + X</t>
								<t>H    be the smallest integer such that choose(H,ceil(H/2)) &gt;= K + S</t>
								<t>H&apos; = ceil(H/2)</t>
								<t>L = K+S+H</t>
								<t>C[0],...,C[K-1] denote the first K intermediate symbols </t>
								<t>C[K],...,C[K+S-1] denote the S LDPC symbols, initialised to zero</t>
								<t>C[K+S],...,C[L-1] denote the H Half symbols, initialised to zero</t>
							</list>
						</t>
						<t>The S LDPC symbols are defined to be the values of  C[K],...,C[K+S-1] at the end of the following process:
<list>
								<t>For i = 0,...,K-1 do
<list>
										<t>a = 1 + (floor(i/S) % (S-1))</t>
										<t>b = i % S</t>
										<t>C[K + b] = C[K + b] ^ C[i]</t>
										<t>b = (b + a) % S</t>
										<t>C[K + b] = C[K + b] ^ C[i]</t>
										<t>b = (b + a) % S</t>
										<t>C[K + b] = C[K + b] ^ C[i]</t>
									</list>
								</t>
							</list>
						</t>
						<t>The H Half symbols are defined as follows:</t>
						<t>Let
<list>
								<t>g[i] = i ^ (floor(i/2)) for all positive integers i

	<list>
										<t>Note: g[i] is the Gray sequence, in which each element differs from the previous one in a single bit position</t>
									</list>
								</t>
								<t>m[k] denote the subsequence of g[.] whose elements have exactly k non-zero bits in their binary representation.</t>
								<t>m[j,k] denote the jth element of the sequence m[k], where j=0, 1, 2, ...</t>
							</list>
						</t>
						<t>Then, the Half symbols are defined as the values of C[K+S],...,C[L-1] after the following process:
<list>
								<t>For h = 0,...,H-1 do
<list>
										<t>For j = 0,...,K+S-1 do
<list>
												<t>If bit h of m[j,H&apos;] is equal to 1 then C[h+K+S] = C[h+K+S] ^ C[j].</t>
											</list>
										</t>
									</list>
								</t>
							</list>
						</t>
					</section>
					<section title="Intermediate Symbols" anchor="intermediatesymbols">
						<section title="Definition" anchor="intermediatesymbolsdefinition">
							<t>Given the K source symbols C&apos;[0], C&apos;[1],..., C&apos;[K-1] the L intermediate symbols C[0], C[1],..., C[L-1] are the uniquely defined symbol values that satisfy the following conditions:  
<list>
									<t>1. The K source symbols C&apos;[0], C&apos;[1],..., C&apos;[K-1] satisfy the K constraints
									<list>
										<t>C&apos;[i] = LTEnc[K, (C[0],..., C[L-1]), (d[i], a[i], b[i])], for all i, 0 &lt;= i &lt; K.</t>
									</list> 
</t>
									<t>2. The L intermediate symbols C[0], C[1],..., C[L-1] satisfy the pre-coding relationships defined in <xref target="precoding"/>.
									</t>
								</list>
							</t>
						</section>
						<section
						   title="Example
							  Method for Calculation of Intermediate Symbols" anchor="intermediateexample">
							<t>This subsection describes a possible method for calculation of the L intermediate symbols C[0], C[1],..., C[L-1] satisfying the constraints in <xref target="intermediatesymbolsdefinition"/>.
							</t>
							<t>The 'generator matrix' for a code that generates N output symbols from K input symbols is an NxK
							matrix over GF(2), where each row corresponds to one of the output symbols and each column to one of
							the input symbols and where the ith output symbol is equal to the sum of those input symbols whose
							column contains a non-zero entry in row i.</t>
							<t>Then, the L intermediate symbols can be calculated as follows:</t>
							<t>Let
<list>
									<t>C denote the column vector of the L intermediate symbols, C[0], C[1],..., C[L-1].</t>
									<t>D denote the column vector consisting of S+H zero symbols followed by the K source symbols C&apos;[0], C&apos;[1], ..., C&apos;[K-1]</t>
								</list>
							</t>
							<t>Then the above constraints define an LxL matrix over GF(2), A, such that:
<list>
									<t>A*C = D</t>
								</list>
							</t>
							<t>The matrix A can be constructed as follows:</t>
							<t>Let:
<list>
									<t>G_LDPC be the S x K generator matrix of the LDPC symbols. So,

<list>
											<t>G_LDPC * Transpose[(C[0],...., C[K-1])] = Transpose[(C[K], ..., C[K+S-1])]</t>
										</list>
									</t>
									<t>G_Half be the H x (K+S) generator matrix of the Half symbols, So,

<list>
											<t>G_Half * Transpose[(C[0], ..., C[S+K-1])] = Transpose[(C[K+S], ..., C[K+S+H-1])]</t>
										</list>
									</t>
									<t>I_S be the S x S identity matrix</t>
									<t>I_H be the H x H identity matrix</t>
									<t>0_SxH be the S x H zero matrix</t>
									<t>G_LT be the KxL generator matrix of the encoding symbols generated by the LT Encoder. 
So,

<list>
											<t>G_LT * Transpose[(C[0], ..., C[L-1])] = Transpose[(C&apos;[0],C&apos;[1],...,C&apos;[K-1])]</t>
											<t>i.e., G_LT(i,j) = 1 if and only if C[j] is included in the symbols that are XORed to produce LTEnc[K, (C[0], ..., C[L-1]), (d[i], a[i], b[i])].</t>
										</list>
									</t>
								</list>
							</t>
							<t>Then:
<list>
									<t>The first S rows of A are equal to G_LDPC | I_S | 0_SxH.</t>
									<t>The next H rows of A are equal to G_Half | I_H. </t>
									<t>The remaining K rows of A are equal to G_LT.</t>
								</list>
							</t>
							<t>The matrix A is depicted in <xref target="MatrixA"/> below:
<figure title="The matrix A" anchor="MatrixA">
									<artwork>
              K               S       H
  +-----------------------+-------+-------+
  |                       |       |       |
S |        G_LDPC         |  I_S  | 0_SxH |
  |                       |       |       |
  +-----------------------+-------+-------+
  |                               |       |
H |        G_Half                 |  I_H  |
  |                               |       |
  +-------------------------------+-------+
  |                                       |
  |                                       |
K |                 G_LT                  |
  |                                       |
  |                                       |
  +---------------------------------------+	
	</artwork>
								</figure>
							</t>
							<t>The intermediate symbols can then be calculated as:
<list>
									<t>C = (A^^-1)*D</t>
								</list>
							</t>
							<t>The source symbol triples are generated such that for any K matrix, A has full rank and is therefore invertible. This calculation can be realized by applying a Raptor decoding process to the K source symbols C&apos;[0], C&apos;[1],..., C&apos;[K-1] to produce the L intermediate symbols C[0], C[1],..., C[L-1].</t>
							<t>To efficiently generate the intermediate symbols from the source symbols, it is recommended that an efficient decoder implementation such as that described in <xref target="decoder"/> be used.  The source symbol triples are designed to facilitate efficient decoding of the source symbols using that algorithm.</t>
						</section>
					</section>
				</section>
				<section title="Second Encoding Step: LT Encoding">
					<t>In the second encoding step, the repair symbol with ESI X is generated by applying the generator LTEnc[K, (C[0], C[1],..., C[L-1]), (d, a, b)] defined in <xref target="ltenc"/> to the L intermediate symbols C[0], C[1],..., C[L-1] using the triple (d, a, b)=Trip[K,X] generated according to  <xref target="encodingpackets"/>
					</t>
				</section>
				<section title="Generators" anchor="generators">
					<section title="Random Generator" anchor="randomgenerator">
						<t>The random number generator Rand[X, i, m] is defined as follows, where X is a non-negative integer, i is a non-negative integer, and m is a positive integer and the value produced is an integer between 0 and m-1.   Let V0 and V1 be arrays of 256 entries each, where each entry is a 4-byte unsigned integer. These arrays are provided in <xref target="random"/>.</t>
						<t>Then,
	<list>
								<t>Rand[X, i, m] = (V0[(X + i) % 256] ^ V1[(floor(X/256)+ i) % 256]) % m</t>
							</list>
						</t>
					</section>
					<section title="Degree Generator" anchor="degreegenerator">
						<t>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.</t>
						<t>
							<list>
								<t>In <xref target="degreedistribution"/>, find the index j such that f[j-1] &lt;= v &lt; f[j]</t>
								<t>Then, Deg[v] = d[j]</t>
							</list>
						</t>
						<texttable title="Defines the degree distribution for encoding symbols" anchor="degreedistribution">
							<ttcol>Index j</ttcol>
							<ttcol>f[j]</ttcol>
							<ttcol>d[j]</ttcol>
							<c>0</c>
							<c>0</c>
							<c>--</c>
							<c>1</c>
							<c>	10241</c>
							<c>1</c>
							<c>2</c>
							<c>491582</c>
							<c>2</c>
							<c>3</c>
							<c>712794</c>
							<c>3</c>
							<c>4</c>
							<c>831695</c>
							<c>4</c>
							<c>5</c>
							<c>948446</c>
							<c>10</c>
							<c>6</c>
							<c>1032189</c>
							<c>11</c>
							<c>7</c>
							<c>1048576</c>
							<c>40</c>
						</texttable>
					</section>
					<section title="LT Encoding Symbol Generator" anchor="ltenc">
						<t>The encoding symbol generator LTEnc[K, (C[0], C[1],..., C[L-1]), (d, a, b)] takes the following inputs:
<list>
								<t>K is the number of source symbols (or sub-symbols) for the source block (sub-block). Let L be derived from K as described in <xref target="precoding"/>, and let L&apos; be the smallest prime integer greater than or equal to L.</t>
								<t>(C[0], C[1],..., C[L-1]) is the array of L intermediate symbols (sub-symbols) generated as described in <xref target="intermediatesymbols"/>.
								</t>
								<t>(d, a, b) is a source triple determined using the Triple generator defined in <xref target="triplegenerator"/>, whereby
<list>
										<t>d is an integer denoting an encoding symbol degree</t>
										<t>a is an integer between 1 and L&apos;-1 inclusive</t>
										<t>b is an integer between 0 and L&apos;-1 inclusive</t>
									</list>
								</t>
							</list>
						</t>
						<t>The encoding symbol generator produces a single encoding symbol as output, according to the following algorithm:
<list>
								<t>While (b &gt;= L) do b = (b + a) % L&apos;</t>
								<t>Let result = C[b].</t>
								<t>For j = 1,...,min(d-1,L-1) do
									<list>
										<t>b = (b + a) % L&apos;</t>
										<t>While (b &gt;= L) do b = (b + a) % L&apos;</t>
										<t>result = result ^ C[b]</t>
									</list>
								</t>
								<t>Return result</t>
							</list>
						</t>
					</section>
					<section title="Triple Generator" anchor="triplegenerator">
						<t>The triple generator Trip[K,X] takes the following inputs:
<list>
								<t>K - The number of source symbols</t>
								<t>X - An encoding symbol ID</t>
							</list>
						</t>
						<t>Let
<list>
								<t>L be determined from K as described in <xref target="precoding"/>
								</t>
								<t>L&apos; be the smallest prime that is greater than or equal to L</t>
								<t>Q = 65521, the largest prime smaller than 2^^16.</t>
								<t>J(K) be the systematic index associated with K, as defined in <xref target="systematic"/>.
								</t>
							</list>
						</t>
						<t>The output of the triple generator is a triple, (d, a, b) determined as follows:
<list>
								<t>A = (53591 + J(K)*997) % Q</t>
								<t>B = 10267*(J(K)+1) % Q</t>
								<t>Y = (B + X*A) % Q</t>
								<t>v = Rand[Y, 0, 2^^20]</t>
								<t>d = Deg[v]</t>
								<t>a = 1 + Rand[Y, 1, L&apos;-1]</t>
								<t>b = Rand[Y, 2, L&apos;]</t>
							</list>
						</t>
					</section>
				</section>
			</section>
			<section title="Example FEC Decoder" anchor="decoder">
				<section title="General">
					<t>This section describes an efficient decoding algorithm for the Raptor codes described 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 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.</t>
				</section>
				<section title="Decoding a Source Block">
					<section title="General" anchor="decodegeneral">
						<t>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.</t>
						<t>From the algorithms described in <xref target="encoder"/>, the Raptor decoder can calculate the total
						number L = K+S+H of pre-coding symbols and determine how they were generated from the source block to be decoded.
						  In this description, it is assumed that the received encoding symbols for the source block to be decoded are
						  passed to the decoder. Note that, as described in <xref target="encodingpackets"/>, the last source symbol of a source
						  packet may have included padding bytes added for FEC encoding purposes. These padding bytes may not be actually included
						  in the packet sent and so must be reinserted at the received before passing the symbol to the decoder.</t>
						   <t>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 also passed to the decoder. 
						  In the case of source symbols, the source symbol triples described in <xref target="sourcetriples"/> indicate
						  the number and set of intermediate symbols that sum to give each source symbol. </t>
						<t>Let N &gt;= K be the number of received encoding symbols for a source block and let M = S+H+N. &nbsp;The following M by L bit matrix A can be derived from the information passed to the decoder for the source block to be decoded.  Let C be the column vector of the L intermediate symbols, and let D be the column vector of M symbols with values known to the receiver, where the first S+H of the M symbols are zero-valued symbols that correspond to LDPC and Half symbols (these are check symbols for the LDPC and Half symbols, and not the LDPC and Half symbols themselves), and the remaining N of the M symbols are the received encoding symbols for the source block.  Then, A is the bit matrix that satisfies A*C = D, where here * denotes matrix multiplication over GF[2].  In particular, A[i,j] = 1 if  the intermediate symbol corresponding to index j is exclusive-ORed into the LDPC, Half, or encoding symbol corresponding to index i in the encoding, or if index i corresponds to a LDPC or Half symbol and index j corresponds to the same LDPC or Half symbol.  For all other i and j, A[i,j] = 0.</t>
						<t>Decoding a source block is equivalent to decoding C from known A and D. &nbsp;It is clear that C can be decoded if and only if the rank of A over GF[2] is L. &nbsp;Once C has been decoded, missing source symbols can be obtained by using the source symbol triples to determine the number and set of intermediate symbols that MUST be exclusive-ORed to obtain each missing source symbol.</t>
						<t>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 reorderings during the Gaussian elimination process, and only depends on A and not on D. &nbsp;The decoding of C from D can take place concurrently with the forming of the decoding schedule, or the decoding can take place afterwards based on the decoding schedule.</t>
						<t>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.
<list style="hanging">
								<t
 hangText="-">Each time row i of A is exclusive-ORed into row i&apos;
 in the decoding schedule, then in the decoding process, symbol D[d[i]] is exclusive-ORed into symbol D[d[i&apos;]].</t>
								<t hangText="-">Each time row i is exchanged with row i&apos; in the decoding schedule, then in the decoding process, the value of d[i] is exchanged with the value of d[i&apos;]. </t>
								<t hangText="-">Each time column j is exchanged with column j&apos; in the decoding schedule, then in the decoding process, the value of c[j] is exchanged with the value of c[j&apos;].</t>
							</list>
						</t>
						<t>From this correspondence, it is clear that the total number of exclusive-ORs of symbols in the decoding of the 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]].</t>
						<t>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.</t>
					</section>
					<section title="First Phase">
						<t>The first phase of the Gaussian elimination, the matrix A, is conceptually partitioned into submatrices.   The submatrix sizes are parameterized by non-negative integers i and u, which are initialized to 0.  The submatrices of A are:
<list>
								<t>(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.</t>
								<t>(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.</t>
								<t>(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.</t>
								<t>(4)	The submatrix U defined by the intersection of all the rows and the last u columns.</t>
								<t>(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.</t>
							</list>
						</t>
						<t>
							<xref target="matrixA"/> illustrates the submatrices of A. &nbsp;At the beginning of the first phase, V = A. &nbsp;In each step, a row of A is chosen.

<figure title="Submatrices of A in the first phase" anchor="matrixA">
								<artwork>
+-----------+-----------------+---------+
|           |                 |         |
|     I     |    All Zeros    |         |
|           |                 |         | 	
+-----------+-----------------+    U    |
|           |                 |         |
|           |                 |         |
| All Zeros |       V         |         |
|           |                 |         |
|           |                 |         |
+-----------+-----------------+---------+
	</artwork>
							</figure>
						</t>
						<t>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 ones in V 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.</t>
						<t>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. &nbsp;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.  
						Whenever there are non-zero rows in V, then the next step starts by choosing a row of A as follows:
<list style="symbols">
								<t>Let r be the minimum integer such that at least one row of A has exactly r ones in V.
<list style="symbols">
										<t>If r != 2, then choose a row with exactly r ones in V with minimum original degree among all such rows.</t>
										<t>If r = 2, then choose any row with exactly 2 ones in V that is part of a maximum size
											component in the graph defined by V.</t>
									</list>
								</t>
							</list>
						</t>
						<t>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. &nbsp;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. &nbsp;Then, the chosen row is exclusive-ORed into all the other rows of A below the chosen row that have a one in the first column of V. &nbsp;Finally, i is incremented by 1 and u is incremented by r-1, which completes the step.</t>
					</section>
					<section title="Second Phase">
						<t>The submatrix U is further partitioned into the first i rows, U_upper, and the remaining M - i rows, U_lower.  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.</t>
					</section>
					<section title="Third Phase">
						<t>After the second phase, the only portion of A that 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.  To zero out U_upper efficiently, the following precomputation matrix U&apos; is computed based on I_u in the third phase and then U&apos; is used in the fourth phase to zero out U_upper.   The u rows of Iu are partitioned into ceil(u/8) groups of 8 rows each.  Then, for each group of 8 rows, all non-zero combinations of the 8 rows are computed, resulting in 2^^8 - 1 = 255 rows (this can be done with 2^^8-8-1 = 247 exclusive-ors of rows per group, since the combinations of Hamming weight one that appear in I_u do not need to be recomputed).  Thus, the resulting precomputation matrix U&apos; has ceil(u/8)*255 rows and u columns.  Note that U&apos; is not formally a part of matrix A, but will be used in the fourth phase to zero out U_upper.</t>
					</section>
					<section title="Fourth Phase">
						<t>For each of the first i rows of A, for each group of 8 columns in the U_upper submatrix of this row, if the set of 8 column entries in U_upper are not all zero, then the row of the precomputation matrix U&apos; that matches the pattern in the 8 columns is exclusive-ORed into the row, thus zeroing out those 8 columns in the row at the cost of exclusive-ORing one row of U&apos; into the row.  </t>
						<t>After this phase, A is the L by L identity matrix and a complete decoding schedule has been successfully formed.  Then, as explained in <xref target="decodegeneral"/>, the corresponding decoding consisting of exclusive-ORing known encoding symbols can be executed to recover the intermediate symbols based on the decoding schedule.
The triples associated with all source symbols are computed according to <xref target="sourcetriples"/>. The triples for received source symbols are used in the decoding. The triples for missing source symbols are used to determine which intermediate symbols need to be exclusive-ORed to recover the missing source symbols.</t>
					</section>
				</section>
			</section>
			<section title="Random Numbers" anchor="random">
				<t>The two tables V0  and V1 described in <xref target="randomgenerator"/> are given below. Each entry is a 32-bit integer in decimal representation. </t>
				<section title="The Table V0">
					<t>  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, 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</t>
				</section>
				<section title="The Table V1">
					<t> 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, 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</t>
				</section>
			</section>
			<section title="Systematic Indices J(K)" anchor="systematic">
				<t>For each value of K, the systematic index J(K) is designed to have the property that the set of source symbol triples (d[0], a[0], b[0]), ..., (d[L-1], a[L-1], b[L-1]) are such that the L intermediate symbols are uniquely defined, i.e., the matrix A in <xref target="intermediateexample"/> has full rank and is therefore invertible.</t>
				<t>The following is the list of the systematic indices for values of K between 4 and 8192 inclusive.</t>
				<t>18, 14, 61, 46, 14, 22, 20, 40, 48, 1, 29, 40, 43, 46, 18, 8, 20, 2, 61, 26, 13, 29, 36, 19, 58, 5, 58, 0, 54, 56, 24, 14, 5, 67, 39, 31, 25, 29, 24, 19, 14, 56, 49, 49, 63, 30, 4, 39, 2, 1, 20, 19, 61, 4, 54, 70, 25, 52, 9, 26, 55, 69, 27, 68, 75, 19, 64, 57, 45, 3, 37, 31, 100, 41, 25, 41, 53, 23, 9, 31, 26, 30, 30, 46, 90, 50, 13, 90, 77, 61, 31, 54, 54, 3, 21, 66, 21, 11, 23, 11, 29, 21, 7, 1, 27, 4, 34, 17, 85, 69, 17, 75, 93, 57, 0, 53, 71, 88, 119, 88, 90, 22, 0, 58, 41, 22, 96, 26, 79, 118, 19, 3, 81, 72, 50, 0, 32, 79, 28, 25, 12, 25, 29, 3, 37, 30, 30, 41, 84, 32, 31, 61, 32, 61, 7, 56, 54, 39, 33, 66, 29, 3, 14, 75, 75, 78, 84, 75, 84, 25, 54, 25, 25, 107, 78, 27, 73, 0, 49, 96, 53, 50, 21, 10, 73, 58, 65, 27, 3, 27, 18, 54, 45, 69, 29, 3, 65, 31, 71, 76, 56, 54, 76, 54, 13, 5, 18, 142, 17, 3, 37, 114, 41, 25, 56, 0, 23, 3, 41, 22, 22, 31, 18, 48, 31, 58, 37, 75, 88, 3, 56, 1, 95, 19, 73, 52, 52, 4, 75, 26, 1, 25, 10, 1, 70, 31, 31, 12, 10, 54, 46, 11, 74, 84, 74, 8, 58, 23, 74, 8, 36, 11, 16, 94, 76, 14, 57, 65, 8, 22, 10, 36, 36, 96, 62, 103, 6, 75, 103, 58, 10, 15, 41, 75, 125, 58, 15, 10, 34, 29, 34, 4, 16, 29, 18, 18, 28, 71, 28, 43, 77, 18, 41, 41, 41, 62, 29, 96, 15, 106, 43, 15, 3, 43, 61, 3, 18, 103, 77, 29, 103, 19, 58, 84, 58, 1, 146, 32, 3, 70, 52, 54, 29, 70, 69, 124, 62, 1, 26, 38, 26, 3, 16, 26, 5, 51, 120, 41, 16, 1, 43, 34, 34, 29, 37, 56, 29, 96, 86, 54, 25, 84, 50, 34, 34, 93, 84, 96, 29, 29, 50, 50, 6, 1, 105, 78, 15, 37, 19, 50, 71, 36, 6, 54, 8, 28, 54, 75, 75, 16, 75, 131, 5, 25, 16, 69, 17, 69, 6, 96, 53, 96, 41, 119, 6, 6, 88, 50, 88, 52, 37, 0, 124, 73, 73, 7, 14, 36, 69, 79, 6, 114, 40, 79, 17, 77, 24, 44, 37, 69, 27, 37, 29, 33, 37, 50, 31, 69, 29, 101, 7, 61, 45, 17, 73, 37, 34, 18, 94, 22, 22, 63, 3, 25, 25, 17, 3, 90, 34, 34, 41, 34, 41, 54, 41, 54, 41, 41, 41, 163, 143, 96, 18, 32, 39, 86, 104, 11, 17, 17, 11, 86, 104, 78, 70, 52, 78, 17, 73, 91, 62, 7, 128, 50, 124, 18, 101, 46, 10, 75, 104, 73, 58, 132, 34, 13, 4, 95, 88, 33, 76, 74, 54, 62, 113, 114, 103, 32, 103, 69, 54, 53, 3, 11, 72, 31, 53, 102, 37, 53, 11, 81, 41, 10, 164, 10, 41, 31, 36, 113, 82, 3, 125, 62, 16, 4, 41, 41, 4, 128, 49, 138, 128, 74, 103, 0, 6, 101, 41, 142, 171, 39, 105, 121, 81, 62, 41, 81, 37, 3, 81, 69, 62, 3, 69, 70, 21, 29, 4, 91, 87, 37, 79, 36, 21, 71, 37, 41, 75, 128, 128, 15, 25, 3, 108, 73, 91, 62, 114, 62, 62, 36, 36, 15, 58, 114, 61, 114, 58, 105, 114, 41, 61, 176, 145, 46, 37, 30, 220, 77, 138, 15, 1, 128, 53, 50, 50, 58, 8, 91, 114, 105, 63, 91, 37, 37, 13, 169, 51, 102, 6, 102, 23, 105, 23, 58, 6, 29, 29, 19, 82, 29, 13, 36, 27, 29, 61, 12, 18, 127, 127, 12, 44, 102, 18, 4, 15, 206, 53, 127, 53, 17, 69, 69, 69, 29, 29, 109, 25, 102, 25, 53, 62, 99, 62, 62, 29, 62, 62, 45, 91, 125, 29, 29, 29, 4, 117, 72, 4, 30, 71, 71, 95, 79, 179, 71, 30, 53, 32, 32, 49, 25, 91, 25, 26, 26, 103, 123, 26, 41, 162, 78, 52, 103, 25, 6, 142, 94, 45, 45, 94, 127, 94, 94, 94, 47, 209, 138, 39, 39, 19, 154, 73, 67, 91, 27, 91, 84, 4, 84, 91, 12, 14, 165, 142, 54, 69, 192, 157, 185, 8, 95, 25, 62, 103, 103, 95, 71, 97, 62, 128, 0, 29, 51, 16, 94, 16, 16, 51, 0, 29, 85, 10, 105, 16, 29, 29, 13, 29, 4, 4, 132, 23, 95, 25, 54, 41, 29, 50, 70, 58, 142, 72, 70, 15, 72, 54, 29, 22, 145, 29, 127, 29, 85, 58, 101, 34, 165, 91, 46, 46, 25, 185, 25, 77, 128, 46, 128, 46, 188, 114, 46, 25, 45, 45, 114, 145, 114, 15, 102, 142, 8, 73, 31, 139, 157, 13, 79, 13, 114, 150, 8, 90, 91, 123, 69, 82, 132, 8, 18, 10, 102, 103, 114, 103, 8, 103, 13, 115, 55, 62, 3, 8, 154, 114, 99, 19, 8, 31, 73, 19, 99, 10, 6, 121, 32, 13, 32, 119, 32, 29, 145, 30, 13, 13, 114, 145, 32, 1, 123, 39, 29, 31, 69, 31, 140, 72, 72, 25, 25, 123, 25, 123, 8, 4, 85, 8, 25, 39, 25, 39, 85, 138, 25, 138, 25, 33, 102, 70, 25, 25, 31, 25, 25, 192, 69, 69, 114, 145, 120, 120, 8, 33, 98, 15, 212, 155, 8, 101, 8, 8, 98, 68, 155, 102, 132, 120, 30, 25, 123, 123, 101, 25, 123, 32, 24, 94, 145, 32, 24, 94, 118, 145, 101, 53, 53, 25, 128, 173, 142, 81, 81, 69, 33, 33, 125, 4, 1, 17, 27, 4, 17, 102, 27, 13, 25, 128, 71, 13, 39, 53, 13, 53, 47, 39, 23, 128, 53, 39, 47, 39, 135, 158, 136, 36, 36, 27, 157, 47, 76, 213, 47, 156, 25, 25, 53, 25, 53, 25, 86, 27, 159, 25, 62, 79, 39, 79, 25, 145, 49, 25, 143, 13, 114, 150, 130, 94, 102, 39, 4, 39, 61, 77, 228, 22, 25, 47, 119, 205, 122, 119, 205, 119, 22, 119, 258, 143, 22, 81, 179, 22, 22, 143, 25, 65, 53, 168, 36, 79, 175, 37, 79, 70, 79, 103, 70, 25, 175, 4, 96, 96, 49, 128, 138, 96, 22, 62, 47, 95, 105, 95, 62, 95, 62, 142, 103, 69, 103, 30, 103, 34, 173, 127, 70, 127, 132, 18, 85, 22, 71, 18, 206, 206, 18, 128, 145, 70, 193, 188, 8, 125, 114, 70, 128, 114, 145, 102, 25, 12, 108, 102, 94, 10, 102, 1, 102, 124, 22, 22, 118, 132, 22, 116, 75, 41, 63, 41, 189, 208, 55, 85, 69, 8, 71, 53, 71, 69, 102, 165, 41, 99, 69, 33, 33, 29, 156, 102, 13, 251, 102, 25, 13, 109, 102, 164, 102, 164, 102, 25, 29, 228, 29, 259, 179, 222, 95, 94, 30, 30, 30, 142, 55, 142, 72, 55, 102, 128, 17, 69, 164, 165, 3, 164, 36, 165, 27, 27, 45, 21, 21, 237, 113, 83, 231, 106, 13, 154, 13, 154, 128, 154, 148, 258, 25, 154, 128, 3, 27, 10, 145, 145, 21, 146, 25, 1, 185, 121, 0, 1, 95, 55, 95, 95, 30, 0, 27, 95, 0, 95, 8, 222, 27, 121, 30, 95, 121, 0, 98, 94, 131, 55, 95, 95, 30, 98, 30, 0, 91, 145, 66, 179, 66, 58, 175, 29, 0, 31, 173, 146, 160, 39, 53, 28, 123, 199, 123, 175, 146, 156, 54, 54, 149, 25, 70, 178, 128, 25, 70, 70, 94, 224, 54, 4, 54, 54, 25, 228, 160, 206, 165, 143, 206, 108, 220, 234, 160, 13, 169, 103, 103, 103, 91, 213, 222, 91, 103, 91, 103, 31, 30, 123, 13, 62, 103, 50, 106, 42, 13, 145, 114, 220, 65, 8, 8, 175, 11, 104, 94, 118, 132, 27, 118, 193, 27, 128, 127, 127, 183, 33, 30, 29, 103, 128, 61, 234, 165, 41, 29, 193, 33, 207, 41, 165, 165, 55, 81, 157, 157, 8, 81, 11, 27, 8, 8, 98, 96, 142, 145, 41, 179, 112, 62, 180, 206, 206, 165, 39, 241, 45, 151, 26, 197, 102, 192, 125, 128, 67, 128, 69, 128, 197, 33, 125, 102, 13, 103, 25, 30, 12, 30, 12, 30, 25, 77, 12, 25, 180, 27, 10, 69, 235, 228, 343, 118, 69, 41, 8, 69, 175, 25, 69, 25, 125, 41, 25, 41, 8, 155, 146, 155, 146, 155, 206, 168, 128, 157, 27, 273, 211, 211, 168, 11, 173, 154, 77, 173, 77, 102, 102, 102, 8, 85, 95, 102, 157, 28, 122, 234, 122, 157, 235, 222, 241, 10, 91, 179, 25, 13, 25, 41, 25, 206, 41, 6, 41, 158, 206, 206, 33, 296, 296, 33, 228, 69, 8, 114, 148, 33, 29, 66, 27, 27, 30, 233, 54, 173, 108, 106, 108, 108, 53, 103, 33, 33, 33, 176, 27, 27, 205, 164, 105, 237, 41, 27, 72, 165, 29, 29, 259, 132, 132, 132, 364, 71, 71, 27, 94, 160, 127, 51, 234, 55, 27, 95, 94, 165, 55, 55, 41, 0, 41, 128, 4, 123, 173, 6, 164, 157, 121, 121, 154, 86, 164, 164, 25, 93, 164, 25, 164, 210, 284, 62, 93, 30, 25, 25, 30, 30, 260, 130, 25, 125, 57, 53, 166, 166, 166, 185, 166, 158, 94, 113, 215, 159, 62, 99, 21, 172, 99, 184, 62, 259, 4, 21, 21, 77, 62, 173, 41, 146, 6, 41, 128, 121, 41, 11, 121, 103, 159, 164, 175, 206, 91, 103, 164, 72, 25, 129, 72, 206, 129, 33, 103, 102, 102, 29, 13, 11, 251, 234, 135, 31, 8, 123, 65, 91, 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2132, 1176, 1176, 516, 2610, 2230, 2230, 64, 1501, 537, 1501, 173, 2230, 2988, 1501, 2694, 2694, 537, 537, 173, 173, 1501, 537, 64, 173, 173, 64, 2230, 537, 2230, 537, 2230, 2230, 2069, 3142, 1645, 689, 1165, 1165, 1963, 514, 488, 1963, 1145, 235, 1145, 1078, 1145, 231, 2405, 552, 21, 57, 57, 57, 1297, 1455, 1988, 2310, 1885, 2854, 2014, 734, 1705, 734, 2854, 734, 677, 1988, 1660, 734, 677, 734, 677, 677, 734, 2854, 1355, 677, 1397, 2947, 2386, 1698, 128, 1698, 3028, 2386, 2437, 2947, 2386, 2643, 2386, 2804, 1188, 335, 746, 1187, 1187, 861, 2519, 1917, 2842, 1917, 675, 1308, 234, 1917, 314, 314, 2339, 2339, 2592, 2576, 902, 916, 2339, 916, 2339, 916, 2339, 916, 1089, 1089, 2644, 1221, 1221, 2446, 308, 308, 2225, 2225, 3192, 2225, 555, 1592, 1592, 555, 893, 555, 550, 770, 3622, 2291, 2291, 3419, 465, 250, 2842, 2291, 2291, 2291, 935, 160, 1271, 308, 325, 935, 1799, 1799, 1891, 2227, 1799, 1598, 112, 1415, 1840, 2014, 1822, 2014, 677, 1822, 1415, 1415, 1822, 2014, 2386, 2159, 1822, 1415, 1822, 179, 1976, 1033, 179, 1840, 2014, 1415, 1970, 1970, 1501, 563, 563, 563, 462, 563, 1970, 1158, 563, 563, 1541, 1238, 383, 235, 1158, 383, 1278, 383, 1898, 2938, 21, 2938, 1313, 2201, 2059, 423, 2059, 1313, 872, 1313, 2044, 89, 173, 3327, 1660, 2044, 1623, 173, 1114, 1114, 1592, 1868, 1651, 1811, 383, 3469, 1811, 1651, 869, 383, 383, 1651, 1651, 3223, 2166, 3469, 767, 383, 1811, 767, 2323, 3355, 1457, 3341, 2640, 2976, 2323, 3341, 2323, 2640, 103, 103, 1161, 1080, 2429, 370, 2018, 2854, 2429, 2166, 2429, 2094, 2207, 871, 1963, 1963, 2023, 2023, 2336, 663, 2893, 1580, 691, 663, 705, 2046, 2599, 409, 2295, 1118, 2494, 1118, 1950, 549, 2494, 2453, 2046, 2494, 2453, 2046, 2453, 2046, 409, 1118, 4952, 2291, 2225, 1894, 1423, 2498, 567, 4129, 1475, 1501, 795, 463, 2084, 828, 828, 232, 828, 232, 232, 1818, 1818, 666, 463, 232, 220, 220, 2162, 2162, 833, 4336, 913, 35, 913, 21, 2927, 886, 3037, 383, 886, 876, 1747, 383, 916, 916, 916, 2927, 916, 1747, 837, 1894, 717, 423, 481, 1894, 1059, 2262, 3206, 4700, 1059, 3304, 2262, 871, 1831, 871, 3304, 1059, 1158, 1934, 1158, 756, 1511, 41, 978, 1934, 2603, 720, 41, 756, 41, 325, 2611, 1158, 173, 1123, 1934, 1934, 1511, 2045, 2045, 2045, 1423, 3206, 3691, 2512, 3206, 2512, 2000, 1811, 2504, 2504, 2611, 2437, 2437, 2437, 1455, 893, 150, 2665, 1966, 605, 398, 2331, 1177, 516, 1962, 4241, 94, 1252, 760, 1292, 1962, 1373, 2000, 1990, 3684, 42, 1868, 3779, 1811, 1811, 2041, 3010, 5436, 1780, 2041, 1868, 1811, 1780, 1811, 1868, 1811, 2041, 1868, 1811, 5627, 4274, 1811, 1868, 4602, 1811, 1811, 1474, 2665, 235, 1474, 2665</t>
			</section>
		</section>
		<section title="Security Considerations">
			<t>Data delivery can be subject to denial-of-service attacks by
   attackers that 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 <xref target="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 <xref target="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.</t>
			<t>   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.</t>
		</section>
		<section title="IANA Considerations" anchor="IANA">
			<t>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 <xref target="RFC5052"/>.  This document assigns the
   Fully-Specified FEC Encoding ID 1 under the ietf:rmt:fec:encoding
   name-space to "Raptor Code".
<!-- [rfced] Should it be "Raptor Code" or "Raptor"?  The IANA site
lists the following:

   1  Raptor        [RFC-ietf-rmt-bb-fec-raptor-object-09.txt]  -->
</t>
		</section>
		
		<section title="Acknowledgements">
			<t>Numerous editorial improvements and clarifications were made to this specification during the review process within 3GPP. Thanks are due to the members of 3GPP Technical Specification Group SA, Working Group 4, for these.</t>
		</section>
	</middle>
	<back>
		<references title="Normative References">
		&rfc2119;
		&rfc5052;
		&rfc4082;
		</references>


		<references title="Informative References">

		<reference anchor="SHA1">
		<front>
		<title>Secure Hash Standard</title>
		<date day="17" month="April" year="2005"/>
		</front>
		<seriesInfo name="Federal Information Processing Standards Publication
         (FIPS PUB)" value="180-1"/>
         <format type="HTML" target="http://www.itl.nist.gov/fipspubs/fip180-1.htm"/>
		</reference>

		<reference anchor="Raptor">
		<front><title>Raptor Codes</title>
		<author initials="A." surname="Shokrollahi" fullname="Amin Shokrollahi"></author>
		<date month="June" year="2006"/>
		</front>
		<seriesInfo name="IEEE Transactions on Information Theory" value="no. 6"/>
		</reference>

<reference anchor="MBMS">
				<front>
					<title>Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs</title>
					<author>
						<organization>3GPP</organization>
					</author>
					<date day="08" month="June" year="2005"/>
				</front>
				<seriesInfo name="3GPP TS" value="26.346 6.1.0"/>
				<format type="HTML" target="http://www.3gpp.org/ftp/Specs/html-info/26346.htm"/>
			</reference>
			<reference anchor="CCNC">
			<front>
			<title>Raptor Codes for Reliable Download Delivery in Wireless Broadcast Systems</title>
			<author initials="M." surname="Luby" fullname="Michael Luby"></author>
			<author initials="M." surname="Watson" fullname="Mark Watson"></author>
			<author initials="T." surname="Gasiba" fullname="Tiago Gasiba"></author>
			<author initials="T." surname="Stockhammer" fullname="Thomas Stockhammer"></author>
			<author initials="W." surname="Xu"></author>
			<date month="Jan" year="2006"/>
			</front>
			<seriesInfo name="CCNC 2006, Las Vegas, NV" value=""/>
			</reference>

			&rfc3453;

		</references>
	</back>
<vspace blankLines="100"/>
</rfc>
