RFC6330

Internet Engineering Task Force (IETF) M. Luby Request for Comments: 6330 Qualcomm Incorporated Category: Standards Track A. Shokrollahi ISSN: 2070-1721 EPFL

                                                           M. Watson
                                                        Netflix Inc.
                                                      T. Stockhammer
                                                      Nomor Research
                                                           L. Minder
                                               Qualcomm Incorporated
                                                         August 2011

  RaptorQ Forward Error Correction Scheme for Object Delivery

Abstract

This document describes a Fully-Specified Forward Error Correction (FEC) scheme, corresponding to FEC Encoding ID 6, for the RaptorQ FEC 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 RFC 5053. RaptorQ is also a fountain code, i.e., as many encoding symbols as needed can be generated on the fly by the encoder from the source symbols of a source block of data. The decoder is able to recover the source block from almost any set of encoding symbols of sufficient cardinality -- in most cases, a set of cardinality equal to the number of source symbols is sufficient; in rare cases, a set of cardinality slightly more than the number of source symbols is required.

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 is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6330.

Copyright Notice

Copyright (c) 2011 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 Simplified BSD License.

   5.3.3.  First Encoding Step: Intermediate Symbol Generation  . 21

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 RFC 5052 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 RFC 5053 RFC5053. The RaptorQ code provides superior reliability, better coding efficiency, and support for larger source block sizes than the Raptor code of RFC 5053 RFC5053. These improvements simplify the usage of the RaptorQ code in an object delivery Content Delivery Protocol compared to RFC 5053 RFC 5053 RFC5053. A detailed mathematical design and analysis of the RaptorQ code together with extensive simulation results are provided in [RaptorCodes].

The RaptorQ FEC scheme is a Fully-Specified FEC scheme corresponding to FEC Encoding ID 6.

RaptorQ is a fountain code, i.e., as many encoding symbols as needed can be generated on the fly by the encoder from the source symbols of a block. The decoder is able to recover the source block from almost any set of encoding symbols of cardinality only slightly larger than the number of source symbols.

The code described in this document is a systematic code; that is, the original unmodified source symbols, as well as a number of repair symbols, can be sent from sender to receiver. For more background on the use of Forward Error Correction codes in reliable multicast, see RFC3453.

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.

Formats and Codes

Introduction

The octet order of all fields is network byte order, i.e., big- endian.

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

o Source Block Number (SBN): 8-bit unsigned integer. A non-negative

  integer identifier for the source block that the encoding symbols
  within the packet relate to.

o Encoding Symbol ID (ESI): 24-bit unsigned integer. A non-negative

  integer identifier for the encoding symbols within the packet.

The interpretation of the Source Block Number and Encoding Symbol Identifier is defined in Section 4.

FEC Object Transmission Information

Mandatory

The value of the FEC Encoding ID MUST be 6, as assigned by IANA (see Section 7).

Common

The Common FEC Object Transmission Information elements used by this FEC scheme are:

o Transfer Length (F): 40-bit unsigned integer. A non-negative

  integer that is at most 946270874880.  This is the transfer length
  of the object in units of octets.

o Symbol Size (T): 16-bit unsigned integer. A positive integer that

  is less than 2^^16.  This is the size of a symbol in units of
  octets.

The encoded Common FEC Object Transmission Information (OTI) format is 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 RaptorQ FEC Scheme

  NOTE: The limit of 946270874880 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 56403,
  and the limitation on the number of source blocks to 2^^8.

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): 8-bit unsigned integer.

o The number of sub-blocks (N): 16-bit unsigned integer.

o A symbol alignment parameter (Al): 8-bit unsigned integer.

These parameters are all positive integers. The encoded Scheme- specific Object Transmission Information is a 4-octet field consisting of the parameters Z, N, and Al 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 described in Section 4.4.1.2.

Procedures

Introduction

For any undefined symbols or functions used in this section, in particular the functions "ceil" and "floor", refer to Section 5.1.

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 F: the transfer length of the object, in octets

o Al: the symbol alignment parameter

o T: the symbol size in octets, 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

The RaptorQ encoder scheme for object delivery additionally requires:

- the object to be encoded, which is F octets long

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.

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 octets

o WS: the maximum size block that is decodable in working memory, in

  octets

o P': the maximum payload size in octets, which is assumed to be a

  multiple of Al

o Al: the symbol alignment parameter, in octets

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 56403.

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' <= 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 4.4.1.2 ensure that the sub-symbol sizes are a multiple of the symbol alignment parameter, Al. This is useful because the sum operations used for encoding and decoding are generally performed several octets at a

time, for example, at least 4 octets 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 octets.

The recommended setting for the input parameter Al is 4.

The parameter WS 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 WS depends on the implementation, but it is possible to implement decoding using working memory only slightly larger than WS.

Object Delivery

Source Block Construction

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 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 octets each. Each source symbol is identified by a unique 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 an 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 4.4.1.2 below.

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 octets

o Al: a symbol alignment parameter, in octets

o T: the symbol size, in octets, 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 4.3.

The function Partition[I,J] derives parameters for partitioning a block of size I into J approximately equal-sized blocks. More specifically, it partitions I into JL blocks of length IL and JS blocks of length IS. The output of Partition[I, J] is the sequence (IL, IS, JL, JS), where IL = ceil(I/J), IS = floor(I/J), JL = I - IS

• J, and JS = J - JL.

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 octets, i.e., KL source symbols of T octets each, and the remaining ZS source blocks each having KS*T octets, i.e., KS source symbols of T octets 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 octets.

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 octets and the remaining NS sub-blocks each consisting of K contiguous sub- symbols of size of TS*Al octets. The symbol alignment parameter Al ensures that sub-symbols are always a multiple of Al octets.

Finally, the mth symbol of a source block consists of the concatenation of the mth sub-symbol from each of the N sub-blocks. Note that this implies that when N > 1, a symbol is NOT a contiguous portion of the object.

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. Each encoding packet contains encoding symbols generated from the one source block identified by the SBN carried in the encoding packet. 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 contains only source symbols (source packet) or contains only 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 octets added for FEC encoding purposes, then these octets need not be included in the packet. Otherwise, each packet MUST contain only whole symbols.

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.

Example Receiver Recovery Strategies

A receiver can use the received encoding symbols for each source block of an object to recover the source symbols for that source block independently of all other source blocks.

If there is one sub-block per source block, i.e., N = 1, then the portion of the data in the original object in its original order associated with a source block consists of the concatenation of the source symbols of a source block in consecutive ESI order.

If there are multiple sub-blocks per source block, i.e., if N > 1, then the portion of the data in the original object in its original order associated with a source block consists of the concatenation of the sub-blocks associated with the source block, where sub-symbols within each sub-block are in consecutive ESI order. In this case, there are different receiver source block recovery strategies worth considering depending on the available amount of Random Access Memory (RAM) at the receiver, as outlined below.

One strategy is to recover the source symbols of a source block using the decoding procedures applied to the received symbols for the source block to recover the source symbols as described in Section 5, and then to reorder the sub-symbols of the source symbols so that all consecutive sub-symbols of the first sub-block are first, followed by all consecutive sub-symbols of the second sub-block, etc., followed by all consecutive sub-symbols of the Nth sub-block. This strategy is especially applicable if the receiver has enough RAM to decode an entire source block.

Another strategy is to separately recover the sub-blocks of a source block. For example, a receiver may demultiplex and store sub-symbols associated with each sub-block separately as packets containing encoding symbols arrive, and then use the stored sub-symbols received for a sub-block to recover that sub-block using the decoding procedures described in Section 5. This strategy is especially applicable if the receiver has enough RAM to decode only one sub- block at a time.

RaptorQ FEC Code Specification

Background

For the purpose of the RaptorQ FEC code specification in this section, the following definitions, symbols, and abbreviations apply. A basic understanding of linear algebra, matrix operations, and finite fields is assumed in this section. In particular, matrix multiplication and matrix inversion operations over a mixture of the

finite fields GF[2] and GF[256] are used. A basic familiarity with sparse linear equations, and efficient implementations of algorithms that take advantage of sparse linear equations, is also quite beneficial to an implementer of this specification.

Definitions

o Source block: a block of K source symbols that 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 octets, of a symbol is known

  as the symbol size.  The symbol size is always a positive 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 based on pre-coding
  relationships.  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 for the purposes of the encoding process.

o LT symbols: a process similar to that described in [LTCodes] is

  used to generate part of the contribution to each generated
  encoding symbol from the portion of the intermediate symbols
  designated as LT symbols.

o PI symbols: a process even simpler than that described in

  [LTCodes] is used to generate the other part of the contribution
  to each generated encoding symbol from the portion of the
  intermediate symbols designated as PI symbols.  In the decoding
  algorithm suggested in Section 5.4, the PI symbols are inactivated
  at the start, i.e., are placed into the matrix U at the beginning
  of the first phase of the decoding algorithm.  Because the symbols
  corresponding to the columns of U are sometimes called the
  "inactivated" symbols, and since the PI symbols are inactivated at
  the beginning, they are considered "permanently inactivated".

o HDPC symbols: there is a small subset of the intermediate symbols

  that are HDPC symbols.  Each HDPC symbol has a pre-coding
  relationship with a large fraction of the other intermediate
  symbols.  HDPC means "High Density Parity Check".

o LDPC symbols: there is a moderate-sized subset of the intermediate

  symbols that are LDPC symbols.  Each LDPC symbol has a pre-coding
  relationship with a small fraction of the other intermediate
  symbols.  LDPC means "Low Density Parity Check".

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 (ESI): information that uniquely identifies

  each encoding symbol associated with a source block for sending
  and receiving purposes.

o Internal Symbol ID (ISI): information that uniquely identifies

  each symbol associated with an extended source block for encoding
  and decoding purposes.

o Arithmetic operations on octets and symbols and matrices: the

  operations that are used to produce encoding symbols from source
  symbols and vice versa.  See Section 5.7.

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 that is greater than or equal

    to x, where x is a real value.

floor(x) denotes the largest integer that 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 value of the values x and y, and in

    general the maximum value of all the argument values.

i % j denotes i modulo j.

i + j denotes the sum of i and j. If i and j are octets or symbols,

    this designates the arithmetic on octets or symbols,
    respectively, as defined in Section 5.7.  If i and j are
    integers, then it denotes the usual integer addition.

i * j denotes the product of i and j. If i and j are octets, this

    designates the arithmetic on octets, as defined in Section 5.7.
    If i is an octet and j is a symbol, this denotes the
    multiplication of a symbol by an octet, as also defined in
    Section 5.7.  Finally, if i and j are integers, i * j denotes
    the usual product of integers.

a ^^ b denotes the operation a raised to the power b. If a is an

    octet and b is a non-negative integer, this is understood to
    mean a*a*...*a (b terms), with '*' being the octet product as
    defined in Section 5.7.

u ^ v denotes, for equal-length bit strings u and v, the bitwise

    exclusive-or of u and v.

Transpose[A] denotes the transposed matrix of matrix A. In this

    specification, all matrices have entries that are octets.

A^^-1 denotes the inverse matrix of matrix A. In this

    specification, all the matrices have octets as entries, so it is
    understood that the operations of the matrix entries are to be
    done as stated in Section 5.7 and A^^-1 is the matrix inverse of
    A with respect to octet arithmetic.

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 56403.

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 32-bit unsigned 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.

Rand[y, i, m] denotes a pseudo-random number generator.

Deg[v] denotes a degree generator.

Enc[K', C ,(d, a, b, d1, a1, b1)] denotes an encoding symbol

    generator.

Tuple[K', X] denotes a tuple generator function.

T denotes the symbol size in octets.

J(K') denotes the systematic index associated with K'.

G denotes any generator matrix.

I_S denotes the S x S identity matrix.

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, referred to as the symbol size T. The atomic operations performed on symbols for both encoding and decoding are the arithmetic operations defined in Section 5.7.

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.

Requirements for a RaptorQ-compliant decoder are provided in Section 5.8. A number of decoding algorithms are possible to achieve these requirements. An efficient decoding algorithm to achieve these requirements 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 that must be available to both sender and receiver. These numbers are

provided in Section 5.5. Encoding and decoding operations for RaptorQ use operations on octets. Section 5.7 describes how 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 = 56403 source symbols. Thus, the RaptorQ code supports source blocks with between 1 and 56403 source symbols.

Systematic RaptorQ Encoder

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 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

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. Also, it ensures 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 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), as well as determining the proper padding of the extended source block with padding symbols (used for encoding and decoding), is the internal responsibility of the RaptorQ encoder/decoder.

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 octets where the value of each octet is zero. K' MUST be selected as the smallest value of K' from the table of Section 5.6 that 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 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 non- negative 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 are 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].

First Encoding Step: Intermediate Symbol Generation

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:

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 Tuple[] generator as described in
   Section 5.3.5.4.

2. A number 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.

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]

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')

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)

  *  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

  *  b = (i+1) % P

  *  D[i] = D[i] + C[W+a] + C[W+b]

Recall that the addition of symbols is to be carried out as specified in Section 5.7.

Note that the LDPC relations as defined in the algorithm above are linear, so there exists an S x B matrix G_LDPC,1 and an S x P matrix G_LDPC,2 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

(The matrix G_LDPC,1 is defined by the first loop in the above algorithm, and G_LDPC,2 can be deduced from the second loop.)

The second set of relations among the intermediate symbols C[0], ..., C[L-1] are the HDPC relations and they are defined as follows:

Let

o alpha denote the octet represented by integer 2 as defined in

  Section 5.7.

o MT denote an H x (K' + S) matrix of octets, where for j=0, ...,

  K'+S-2, the entry MT[i,j] is the octet represented by the integer
  1 if i= Rand[j+1,6,H] or i = (Rand[j+1,6,H] + Rand[j+1,7,H-1] + 1)
  % H, and MT[i,j] is the zero element for all other values of i,
  and for j=K'+S-1, MT[i,j] = alpha^^i for i=0, ..., H-1.

o GAMMA denote a (K'+S) x (K'+S) matrix of octets, where

     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]] + MT * GAMMA *
  Transpose[C[0], ..., C[K'+S-1]] = 0,

where '*' represents standard matrix multiplication utilizing the octet multiplication to define the multiplication between a matrix of octets and a matrix of symbols (in particular, the column vector of symbols), and '+' denotes addition over octet vectors.

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 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 of octets 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 as defined in

  Section 5.3.3.3.

o G_HDPC be the H x (K'+S) matrix such that

     G_HDPC * Transpose(C[0], ..., C[K'+S-1]) = Transpose(C[K'+S],
     ..., C[L-1]),

     i.e., G_HDPC = MT*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 matrix such that

     G_ENC * Transpose[(C[0], ..., C[L-1])] =
     Transpose[(C'[0],C'[1], ...,C'[K'-1])],

     i.e., G_ENC[i,j] = 1 if and only if C[j] is included in the
     symbols that are summed 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 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.

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].

Generators

Random Number 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 the arrays 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

Degree Generator

The degree generator Deg[v] is defined as follows, where v is a non- negative integer that is 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). Recall that W is derived from K' as described in Section 5.3.3.3.

             +---------+---------+---------+---------+
             | 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

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.

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 a positive integer denoting an encoding symbol LT degree

  *  a is a positive integer between 1 and W-1 inclusive

  *  b is a non-negative integer between 0 and W-1 inclusive

  *  d1 is a positive integer that has value either 2 or 3 denoting
     an encoding symbol PI degree

  *  a1 is a positive integer between 1 and P1-1 inclusive

  *  b1 is a non-negative integer between 0 and P1-1 inclusive

The encoding symbol generator produces a single encoding symbol as output (referred to as result), 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

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 ISI

Let

o L be determined from K' as described in Section 5.3.3.3

o J = J(K') be the systematic index associated with K', as defined

  in Table 2 in Section 5.6

The output of the tuple generator is a tuple, (d, a, b, d1, a1, b1), determined as follows:

o A = 53591 + J*997

o if (A % 2 == 0) { A = A + 1 }

o B = 10267*(J+1)

o y = (B + X*A) % 2^^32

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[X, 3, 2] } else { d1 = 2 }

o a1 = 1 + Rand[X, 4, P1-1]

o b1 = Rand[X, 5, P1]

Example FEC Decoder

General

This section describes an efficient decoding algorithm for the RaptorQ code introduced in this specification. Note that each received encoding symbol is a known linear combination of the intermediate symbols. So, each received encoding symbol provides a

linear equation among the intermediate symbols, which, together with the known linear pre-coding relationships amongst the intermediate symbols, gives a system of linear equations. Thus, any algorithm for solving systems of linear 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.

Decoding an Extended Source Block

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 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 sum is equal to the encoding symbol are 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 that 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 that must be summed 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 x L identity matrix. The decoding schedule consists of the sequence of row operations and row and column reorderings during the Gaussian elimination process, and it 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 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 an octet 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 x 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.

First Phase

In 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 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 consists of the first L-P columns of A, and U consists of the last P columns corresponding to the PI symbols. 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 nonzero 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.

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 zeros submatrix above V have disappeared, and A consists of I, the all zeros submatrix below I, and U. The phase ends unsuccessfully in decoding failure if at some step before V disappears there is no nonzero 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 nonzeros in V.

  *  If r != 2, then choose a row with exactly r nonzeros 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 and there is a row with exactly 2 ones in V, then
     choose any row with exactly 2 ones in V that is part of a
     maximum size component in the graph described above that is
     defined by V.

  *  If r = 2 and there is no row with exactly 2 ones in V, then
     choose any row with exactly 2 nonzeros in 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 nonzeros in the chosen row appears in the first column of V and so that the remaining r-1 nonzeros 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 nonzero 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 that are not eventually used in the decoding process, and in particular this avoids 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.

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 and i columns. 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 either to 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 x 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.

Third Phase

After the second phase, the only portion of A that needs to be zeroed out to finish converting A into the L x 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.

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 the matrix I_u.

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.

After this phase, A is the L x L identity matrix and a complete decoding schedule has been successfully formed. Then, the corresponding decoding consisting of summing 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 summed to recover the missing source symbols.

Random Numbers

The four arrays V0, V1, V2, and V3 used in Section 5.3.5.1 are provided below. There are 256 entries in each of the four arrays. The indexing into each array starts at 0, and the entries are 32-bit unsigned integers.

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, 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

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, 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

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,
  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

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, 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

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') |
             +-------+-------+-------+-------+-------+
             | 10    | 254   | 7     | 10    | 17    |
             +-------+-------+-------+-------+-------+
             | 12    | 630   | 7     | 10    | 19    |
             +-------+-------+-------+-------+-------+
             | 18    | 682   | 11    | 10    | 29    |
             +-------+-------+-------+-------+-------+
             | 20    | 293   | 11    | 10    | 31    |
             +-------+-------+-------+-------+-------+
             | 26    | 80    | 11    | 10    | 37    |
             +-------+-------+-------+-------+-------+
             | 30    | 566   | 11    | 10    | 41    |
             +-------+-------+-------+-------+-------+
             | 32    | 860   | 11    | 10    | 43    |
             +-------+-------+-------+-------+-------+
             | 36    | 267   | 11    | 10    | 47    |
             +-------+-------+-------+-------+-------+
             | 42    | 822   | 11    | 10    | 53    |
             +-------+-------+-------+-------+-------+
             | 46    | 506   | 13    | 10    | 59    |
             +-------+-------+-------+-------+-------+
             | 48    | 589   | 13    | 10    | 61    |
             +-------+-------+-------+-------+-------+
             | 49    | 87    | 13    | 10    | 61    |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 55    | 520   | 13    | 10    | 67    |
             +-------+-------+-------+-------+-------+
             | 60    | 159   | 13    | 10    | 71    |
             +-------+-------+-------+-------+-------+
             | 62    | 235   | 13    | 10    | 73    |
             +-------+-------+-------+-------+-------+
             | 69    | 157   | 13    | 10    | 79    |
             +-------+-------+-------+-------+-------+
             | 75    | 502   | 17    | 10    | 89    |
             +-------+-------+-------+-------+-------+
             | 84    | 334   | 17    | 10    | 97    |
             +-------+-------+-------+-------+-------+
             | 88    | 583   | 17    | 10    | 101   |
             +-------+-------+-------+-------+-------+
             | 91    | 66    | 17    | 10    | 103   |
             +-------+-------+-------+-------+-------+
             | 95    | 352   | 17    | 10    | 107   |
             +-------+-------+-------+-------+-------+
             | 97    | 365   | 17    | 10    | 109   |
             +-------+-------+-------+-------+-------+
             | 101   | 562   | 17    | 10    | 113   |
             +-------+-------+-------+-------+-------+
             | 114   | 5     | 19    | 10    | 127   |
             +-------+-------+-------+-------+-------+
             | 119   | 603   | 19    | 10    | 131   |
             +-------+-------+-------+-------+-------+
             | 125   | 721   | 19    | 10    | 137   |
             +-------+-------+-------+-------+-------+
             | 127   | 28    | 19    | 10    | 139   |
             +-------+-------+-------+-------+-------+
             | 138   | 660   | 19    | 10    | 149   |
             +-------+-------+-------+-------+-------+
             | 140   | 829   | 19    | 10    | 151   |
             +-------+-------+-------+-------+-------+
             | 149   | 900   | 23    | 10    | 163   |
             +-------+-------+-------+-------+-------+
             | 153   | 930   | 23    | 10    | 167   |
             +-------+-------+-------+-------+-------+
             | 160   | 814   | 23    | 10    | 173   |
             +-------+-------+-------+-------+-------+
             | 166   | 661   | 23    | 10    | 179   |
             +-------+-------+-------+-------+-------+
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             | 179   | 780   | 23    | 10    | 191   |
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             | 187   | 777   | 23    | 10    | 199   |
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             | 217   | 764   | 29    | 10    | 233   |
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             | 225   | 843   | 29    | 10    | 241   |
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             | 236   | 646   | 29    | 10    | 251   |
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             | 242   | 557   | 29    | 10    | 257   |
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             | 257   | 265   | 29    | 10    | 271   |
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             | 263   | 505   | 29    | 10    | 277   |
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             | 269   | 722   | 29    | 10    | 283   |
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             | 280   | 263   | 29    | 10    | 293   |
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             | 295   | 999   | 29    | 10    | 307   |
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             | 301   | 874   | 29    | 10    | 313   |
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             | 305   | 160   | 29    | 10    | 317   |
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             | 362   | 932   | 31    | 10    | 373   |
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             | 486   | 912   | 37    | 10    | 499   |
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             | 491   | 617   | 37    | 10    | 503   |
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             | 563   | 667   | 41    | 10    | 577   |
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             | 588   | 152   | 41    | 10    | 601   |
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             | 594   | 526   | 41    | 10    | 607   |
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             | 600   | 268   | 41    | 10    | 613   |
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             | 619   | 45    | 41    | 10    | 631   |
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             | 640   | 527   | 43    | 10    | 653   |
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             | 648   | 558   | 43    | 10    | 661   |
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             | 675   | 5     | 47    | 10    | 691   |
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             | 693   | 996   | 47    | 10    | 709   |
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             | 718   | 513   | 47    | 10    | 733   |
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             | 728   | 865   | 47    | 10    | 743   |
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             | 736   | 870   | 47    | 10    | 751   |
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             | 747   | 239   | 47    | 10    | 761   |
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             | 835   | 321   | 53    | 10    | 853   |
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             | 845   | 287   | 53    | 10    | 863   |
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             | 870   | 251   | 53    | 10    | 887   |
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             | 891   | 30    | 53    | 10    | 907   |
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             | 903   | 621   | 53    | 10    | 919   |
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             | 913   | 555   | 53    | 10    | 929   |
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             | 926   | 127   | 53    | 10    | 941   |
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             | 938   | 400   | 53    | 10    | 953   |
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             | 963   | 916   | 59    | 10    | 983   |
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             | 977   | 935   | 59    | 10    | 997   |
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             | 989   | 691   | 59    | 10    | 1009  |
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             | 1020  | 282   | 59    | 10    | 1039  |
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             | 1032  | 824   | 59    | 10    | 1051  |
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             | 1050  | 536   | 59    | 11    | 1069  |
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             | 1074  | 596   | 59    | 11    | 1093  |
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             | 1099  | 947   | 59    | 11    | 1117  |
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             | 1111  | 162   | 59    | 11    | 1129  |
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             | 1152  | 1000  | 61    | 11    | 1171  |
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             | 1169  | 251   | 61    | 11    | 1187  |
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             | 1183  | 673   | 61    | 11    | 1201  |
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             | 1205  | 559   | 61    | 11    | 1223  |
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             | 1220  | 923   | 61    | 11    | 1237  |
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             | 1236  | 81    | 67    | 11    | 1259  |
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             | 1255  | 478   | 67    | 11    | 1277  |
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             | 1269  | 198   | 67    | 11    | 1291  |
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             | 1285  | 137   | 67    | 11    | 1307  |
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             | 1306  | 75    | 67    | 11    | 1327  |
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             | 1347  | 29    | 67    | 11    | 1367  |
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             | 1361  | 231   | 67    | 11    | 1381  |
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             | 1389  | 532   | 67    | 11    | 1409  |
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             | 1404  | 58    | 67    | 11    | 1423  |
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             | 1420  | 60    | 67    | 11    | 1439  |
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             | 1436  | 964   | 71    | 11    | 1459  |
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             | 1461  | 624   | 71    | 11    | 1483  |
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             | 1477  | 502   | 71    | 11    | 1499  |
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             | 1502  | 636   | 71    | 11    | 1523  |
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             | 1522  | 986   | 71    | 11    | 1543  |
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             | 1539  | 950   | 71    | 11    | 1559  |
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             | 1561  | 735   | 73    | 11    | 1583  |
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             | 1579  | 866   | 73    | 11    | 1601  |
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             | 1616  | 83    | 73    | 11    | 1637  |
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             | 1649  | 14    | 73    | 11    | 1669  |
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             | 1673  | 522   | 79    | 11    | 1699  |
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             | 1698  | 226   | 79    | 11    | 1723  |
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             | 1716  | 282   | 79    | 11    | 1741  |
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             | 1734  | 88    | 79    | 11    | 1759  |
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             | 1759  | 636   | 79    | 11    | 1783  |
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             | 1777  | 860   | 79    | 11    | 1801  |
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             | 1800  | 324   | 79    | 11    | 1823  |
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             | 1824  | 424   | 79    | 11    | 1847  |
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             | 1844  | 999   | 79    | 11    | 1867  |
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             | 1863  | 682   | 83    | 11    | 1889  |
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             | 1887  | 814   | 83    | 11    | 1913  |
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             | 1906  | 979   | 83    | 11    | 1931  |
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             | 1926  | 538   | 83    | 11    | 1951  |
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             | 1954  | 278   | 83    | 11    | 1979  |
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             | 1979  | 580   | 83    | 11    | 2003  |
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             | 2005  | 773   | 83    | 11    | 2029  |
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             | 2040  | 911   | 89    | 11    | 2069  |
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             | 2070  | 506   | 89    | 11    | 2099  |
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             | 2103  | 628   | 89    | 11    | 2131  |
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             | 2125  | 282   | 89    | 11    | 2153  |
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             | 2152  | 309   | 89    | 11    | 2179  |
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             | 2195  | 858   | 89    | 11    | 2221  |
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             | 2217  | 442   | 89    | 11    | 2243  |
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             | 2247  | 654   | 89    | 11    | 2273  |
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             | 2278  | 82    | 97    | 11    | 2311  |
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             | 2315  | 428   | 97    | 11    | 2347  |
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             | 2339  | 442   | 97    | 11    | 2371  |
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             | 2367  | 283   | 97    | 11    | 2399  |
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             | 2392  | 538   | 97    | 11    | 2423  |
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             | 2416  | 189   | 97    | 11    | 2447  |
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             | 2447  | 438   | 97    | 11    | 2477  |
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             | 2473  | 912   | 97    | 11    | 2503  |
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             | 2502  | 1     | 97    | 11    | 2531  |
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             | 2528  | 167   | 97    | 11    | 2557  |
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             | 2565  | 272   | 97    | 11    | 2593  |
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             | 2640  | 927   | 101   | 11    | 2671  |
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             | 2668  | 386   | 101   | 11    | 2699  |
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             | 2701  | 653   | 101   | 11    | 2731  |
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             | 2737  | 669   | 101   | 11    | 2767  |
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             | 2772  | 431   | 101   | 11    | 2801  |
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             | 2802  | 793   | 103   | 11    | 2833  |
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             | 2831  | 588   | 103   | 11    | 2861  |
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             | 2875  | 777   | 107   | 11    | 2909  |
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             | 2906  | 939   | 107   | 11    | 2939  |
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             | 2938  | 864   | 107   | 11    | 2971  |
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             | 2979  | 627   | 107   | 11    | 3011  |
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             | 3015  | 265   | 109   | 11    | 3049  |
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             | 3056  | 976   | 109   | 11    | 3089  |
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             | 3101  | 988   | 113   | 11    | 3137  |
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             | 3151  | 507   | 113   | 11    | 3187  |
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             | 3186  | 640   | 113   | 11    | 3221  |
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             | 3265  | 667   | 113   | 11    | 3299  |
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             | 3299  | 24    | 127   | 11    | 3347  |
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             | 3344  | 877   | 127   | 11    | 3391  |
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             | 3423  | 720   | 127   | 11    | 3469  |
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             | 3466  | 93    | 127   | 11    | 3511  |
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             | 3502  | 919   | 127   | 11    | 3547  |
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             | 3539  | 635   | 127   | 11    | 3583  |
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             | 3579  | 174   | 127   | 11    | 3623  |
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             | 3616  | 647   | 127   | 11    | 3659  |
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             | 3658  | 820   | 127   | 11    | 3701  |
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             | 3697  | 56    | 127   | 11    | 3739  |
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             | 3883  | 546   | 127   | 11    | 3923  |
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             | 3924  | 954   | 131   | 11    | 3967  |
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             | 4015  | 927   | 131   | 11    | 4057  |
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             | 4165  | 583   | 137   | 11    | 4211  |
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             | 4207  | 782   | 137   | 11    | 4253  |
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             | 4252  | 37    | 137   | 11    | 4297  |
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             | 4318  | 758   | 137   | 11    | 4363  |
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             | 4365  | 777   | 137   | 11    | 4409  |
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             | 4468  | 476   | 139   | 11    | 4513  |
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             | 4513  | 113   | 149   | 11    | 4567  |
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             | 4567  | 313   | 149   | 11    | 4621  |
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             | 4626  | 102   | 149   | 11    | 4679  |
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             | 4731  | 332   | 149   | 11    | 4783  |
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             | 4780  | 786   | 149   | 11    | 4831  |
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             | 4838  | 99    | 149   | 11    | 4889  |
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             | 4954  | 794   | 149   | 11    | 5003  |
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             | 5008  | 37    | 151   | 11    | 5059  |
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             | 5063  | 471   | 151   | 11    | 5113  |
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             | 5116  | 94    | 157   | 11    | 5171  |
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             | 5172  | 873   | 157   | 11    | 5227  |
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             | 5225  | 918   | 157   | 11    | 5279  |
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             | 5279  | 945   | 157   | 11    | 5333  |
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             | 5334  | 211   | 157   | 11    | 5387  |
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             | 5391  | 341   | 157   | 11    | 5443  |
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             | 5449  | 11    | 163   | 11    | 5507  |
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             | 5506  | 578   | 163   | 11    | 5563  |
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             | 5566  | 494   | 163   | 11    | 5623  |
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             | 5637  | 694   | 163   | 11    | 5693  |
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             | 5694  | 252   | 163   | 11    | 5749  |
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             | 5763  | 451   | 167   | 11    | 5821  |
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             | 5823  | 83    | 167   | 11    | 5881  |
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             | 5896  | 689   | 167   | 11    | 5953  |
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             | 5975  | 488   | 173   | 11    | 6037  |
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             | 6039  | 214   | 173   | 11    | 6101  |
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             | 6102  | 17    | 173   | 11    | 6163  |
             +-------+-------+-------+-------+-------+
             | 6169  | 469   | 173   | 11    | 6229  |
             +-------+-------+-------+-------+-------+
             | 6233  | 263   | 179   | 11    | 6299  |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 6296  | 309   | 179   | 11    | 6361  |
             +-------+-------+-------+-------+-------+
             | 6363  | 984   | 179   | 11    | 6427  |
             +-------+-------+-------+-------+-------+
             | 6427  | 123   | 179   | 11    | 6491  |
             +-------+-------+-------+-------+-------+
             | 6518  | 360   | 179   | 11    | 6581  |
             +-------+-------+-------+-------+-------+
             | 6589  | 863   | 181   | 11    | 6653  |
             +-------+-------+-------+-------+-------+
             | 6655  | 122   | 181   | 11    | 6719  |
             +-------+-------+-------+-------+-------+
             | 6730  | 522   | 191   | 11    | 6803  |
             +-------+-------+-------+-------+-------+
             | 6799  | 539   | 191   | 11    | 6871  |
             +-------+-------+-------+-------+-------+
             | 6878  | 181   | 191   | 11    | 6949  |
             +-------+-------+-------+-------+-------+
             | 6956  | 64    | 191   | 11    | 7027  |
             +-------+-------+-------+-------+-------+
             | 7033  | 387   | 191   | 11    | 7103  |
             +-------+-------+-------+-------+-------+
             | 7108  | 967   | 191   | 11    | 7177  |
             +-------+-------+-------+-------+-------+
             | 7185  | 843   | 191   | 11    | 7253  |
             +-------+-------+-------+-------+-------+
             | 7281  | 999   | 193   | 11    | 7351  |
             +-------+-------+-------+-------+-------+
             | 7360  | 76    | 197   | 11    | 7433  |
             +-------+-------+-------+-------+-------+
             | 7445  | 142   | 197   | 11    | 7517  |
             +-------+-------+-------+-------+-------+
             | 7520  | 599   | 197   | 11    | 7591  |
             +-------+-------+-------+-------+-------+
             | 7596  | 576   | 199   | 11    | 7669  |
             +-------+-------+-------+-------+-------+
             | 7675  | 176   | 211   | 11    | 7759  |
             +-------+-------+-------+-------+-------+
             | 7770  | 392   | 211   | 11    | 7853  |
             +-------+-------+-------+-------+-------+
             | 7855  | 332   | 211   | 11    | 7937  |
             +-------+-------+-------+-------+-------+
             | 7935  | 291   | 211   | 11    | 8017  |
             +-------+-------+-------+-------+-------+
             | 8030  | 913   | 211   | 11    | 8111  |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 8111  | 608   | 211   | 11    | 8191  |
             +-------+-------+-------+-------+-------+
             | 8194  | 212   | 211   | 11    | 8273  |
             +-------+-------+-------+-------+-------+
             | 8290  | 696   | 211   | 11    | 8369  |
             +-------+-------+-------+-------+-------+
             | 8377  | 931   | 223   | 11    | 8467  |
             +-------+-------+-------+-------+-------+
             | 8474  | 326   | 223   | 11    | 8563  |
             +-------+-------+-------+-------+-------+
             | 8559  | 228   | 223   | 11    | 8647  |
             +-------+-------+-------+-------+-------+
             | 8654  | 706   | 223   | 11    | 8741  |
             +-------+-------+-------+-------+-------+
             | 8744  | 144   | 223   | 11    | 8831  |
             +-------+-------+-------+-------+-------+
             | 8837  | 83    | 223   | 11    | 8923  |
             +-------+-------+-------+-------+-------+
             | 8928  | 743   | 223   | 11    | 9013  |
             +-------+-------+-------+-------+-------+
             | 9019  | 187   | 223   | 11    | 9103  |
             +-------+-------+-------+-------+-------+
             | 9111  | 654   | 227   | 11    | 9199  |
             +-------+-------+-------+-------+-------+
             | 9206  | 359   | 227   | 11    | 9293  |
             +-------+-------+-------+-------+-------+
             | 9303  | 493   | 229   | 11    | 9391  |
             +-------+-------+-------+-------+-------+
             | 9400  | 369   | 233   | 11    | 9491  |
             +-------+-------+-------+-------+-------+
             | 9497  | 981   | 233   | 11    | 9587  |
             +-------+-------+-------+-------+-------+
             | 9601  | 276   | 239   | 11    | 9697  |
             +-------+-------+-------+-------+-------+
             | 9708  | 647   | 239   | 11    | 9803  |
             +-------+-------+-------+-------+-------+
             | 9813  | 389   | 239   | 11    | 9907  |
             +-------+-------+-------+-------+-------+
             | 9916  | 80    | 239   | 11    | 10009 |
             +-------+-------+-------+-------+-------+
             | 10017 | 396   | 241   | 11    | 10111 |
             +-------+-------+-------+-------+-------+
             | 10120 | 580   | 251   | 11    | 10223 |
             +-------+-------+-------+-------+-------+
             | 10241 | 873   | 251   | 11    | 10343 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 10351 | 15    | 251   | 11    | 10453 |
             +-------+-------+-------+-------+-------+
             | 10458 | 976   | 251   | 11    | 10559 |
             +-------+-------+-------+-------+-------+
             | 10567 | 584   | 251   | 11    | 10667 |
             +-------+-------+-------+-------+-------+
             | 10676 | 267   | 257   | 11    | 10781 |
             +-------+-------+-------+-------+-------+
             | 10787 | 876   | 257   | 11    | 10891 |
             +-------+-------+-------+-------+-------+
             | 10899 | 642   | 257   | 12    | 11003 |
             +-------+-------+-------+-------+-------+
             | 11015 | 794   | 257   | 12    | 11119 |
             +-------+-------+-------+-------+-------+
             | 11130 | 78    | 263   | 12    | 11239 |
             +-------+-------+-------+-------+-------+
             | 11245 | 736   | 263   | 12    | 11353 |
             +-------+-------+-------+-------+-------+
             | 11358 | 882   | 269   | 12    | 11471 |
             +-------+-------+-------+-------+-------+
             | 11475 | 251   | 269   | 12    | 11587 |
             +-------+-------+-------+-------+-------+
             | 11590 | 434   | 269   | 12    | 11701 |
             +-------+-------+-------+-------+-------+
             | 11711 | 204   | 269   | 12    | 11821 |
             +-------+-------+-------+-------+-------+
             | 11829 | 256   | 271   | 12    | 11941 |
             +-------+-------+-------+-------+-------+
             | 11956 | 106   | 277   | 12    | 12073 |
             +-------+-------+-------+-------+-------+
             | 12087 | 375   | 277   | 12    | 12203 |
             +-------+-------+-------+-------+-------+
             | 12208 | 148   | 277   | 12    | 12323 |
             +-------+-------+-------+-------+-------+
             | 12333 | 496   | 281   | 12    | 12451 |
             +-------+-------+-------+-------+-------+
             | 12460 | 88    | 281   | 12    | 12577 |
             +-------+-------+-------+-------+-------+
             | 12593 | 826   | 293   | 12    | 12721 |
             +-------+-------+-------+-------+-------+
             | 12726 | 71    | 293   | 12    | 12853 |
             +-------+-------+-------+-------+-------+
             | 12857 | 925   | 293   | 12    | 12983 |
             +-------+-------+-------+-------+-------+
             | 13002 | 760   | 293   | 12    | 13127 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 13143 | 130   | 293   | 12    | 13267 |
             +-------+-------+-------+-------+-------+
             | 13284 | 641   | 307   | 12    | 13421 |
             +-------+-------+-------+-------+-------+
             | 13417 | 400   | 307   | 12    | 13553 |
             +-------+-------+-------+-------+-------+
             | 13558 | 480   | 307   | 12    | 13693 |
             +-------+-------+-------+-------+-------+
             | 13695 | 76    | 307   | 12    | 13829 |
             +-------+-------+-------+-------+-------+
             | 13833 | 665   | 307   | 12    | 13967 |
             +-------+-------+-------+-------+-------+
             | 13974 | 910   | 307   | 12    | 14107 |
             +-------+-------+-------+-------+-------+
             | 14115 | 467   | 311   | 12    | 14251 |
             +-------+-------+-------+-------+-------+
             | 14272 | 964   | 311   | 12    | 14407 |
             +-------+-------+-------+-------+-------+
             | 14415 | 625   | 313   | 12    | 14551 |
             +-------+-------+-------+-------+-------+
             | 14560 | 362   | 317   | 12    | 14699 |
             +-------+-------+-------+-------+-------+
             | 14713 | 759   | 317   | 12    | 14851 |
             +-------+-------+-------+-------+-------+
             | 14862 | 728   | 331   | 12    | 15013 |
             +-------+-------+-------+-------+-------+
             | 15011 | 343   | 331   | 12    | 15161 |
             +-------+-------+-------+-------+-------+
             | 15170 | 113   | 331   | 12    | 15319 |
             +-------+-------+-------+-------+-------+
             | 15325 | 137   | 331   | 12    | 15473 |
             +-------+-------+-------+-------+-------+
             | 15496 | 308   | 331   | 12    | 15643 |
             +-------+-------+-------+-------+-------+
             | 15651 | 800   | 337   | 12    | 15803 |
             +-------+-------+-------+-------+-------+
             | 15808 | 177   | 337   | 12    | 15959 |
             +-------+-------+-------+-------+-------+
             | 15977 | 961   | 337   | 12    | 16127 |
             +-------+-------+-------+-------+-------+
             | 16161 | 958   | 347   | 12    | 16319 |
             +-------+-------+-------+-------+-------+
             | 16336 | 72    | 347   | 12    | 16493 |
             +-------+-------+-------+-------+-------+
             | 16505 | 732   | 347   | 12    | 16661 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 16674 | 145   | 349   | 12    | 16831 |
             +-------+-------+-------+-------+-------+
             | 16851 | 577   | 353   | 12    | 17011 |
             +-------+-------+-------+-------+-------+
             | 17024 | 305   | 353   | 12    | 17183 |
             +-------+-------+-------+-------+-------+
             | 17195 | 50    | 359   | 12    | 17359 |
             +-------+-------+-------+-------+-------+
             | 17376 | 351   | 359   | 12    | 17539 |
             +-------+-------+-------+-------+-------+
             | 17559 | 175   | 367   | 12    | 17729 |
             +-------+-------+-------+-------+-------+
             | 17742 | 727   | 367   | 12    | 17911 |
             +-------+-------+-------+-------+-------+
             | 17929 | 902   | 367   | 12    | 18097 |
             +-------+-------+-------+-------+-------+
             | 18116 | 409   | 373   | 12    | 18289 |
             +-------+-------+-------+-------+-------+
             | 18309 | 776   | 373   | 12    | 18481 |
             +-------+-------+-------+-------+-------+
             | 18503 | 586   | 379   | 12    | 18679 |
             +-------+-------+-------+-------+-------+
             | 18694 | 451   | 379   | 12    | 18869 |
             +-------+-------+-------+-------+-------+
             | 18909 | 287   | 383   | 12    | 19087 |
             +-------+-------+-------+-------+-------+
             | 19126 | 246   | 389   | 12    | 19309 |
             +-------+-------+-------+-------+-------+
             | 19325 | 222   | 389   | 12    | 19507 |
             +-------+-------+-------+-------+-------+
             | 19539 | 563   | 397   | 12    | 19727 |
             +-------+-------+-------+-------+-------+
             | 19740 | 839   | 397   | 12    | 19927 |
             +-------+-------+-------+-------+-------+
             | 19939 | 897   | 401   | 12    | 20129 |
             +-------+-------+-------+-------+-------+
             | 20152 | 409   | 401   | 12    | 20341 |
             +-------+-------+-------+-------+-------+
             | 20355 | 618   | 409   | 12    | 20551 |
             +-------+-------+-------+-------+-------+
             | 20564 | 439   | 409   | 12    | 20759 |
             +-------+-------+-------+-------+-------+
             | 20778 | 95    | 419   | 13    | 20983 |
             +-------+-------+-------+-------+-------+
             | 20988 | 448   | 419   | 13    | 21191 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 21199 | 133   | 419   | 13    | 21401 |
             +-------+-------+-------+-------+-------+
             | 21412 | 938   | 419   | 13    | 21613 |
             +-------+-------+-------+-------+-------+
             | 21629 | 423   | 431   | 13    | 21841 |
             +-------+-------+-------+-------+-------+
             | 21852 | 90    | 431   | 13    | 22063 |
             +-------+-------+-------+-------+-------+
             | 22073 | 640   | 431   | 13    | 22283 |
             +-------+-------+-------+-------+-------+
             | 22301 | 922   | 433   | 13    | 22511 |
             +-------+-------+-------+-------+-------+
             | 22536 | 250   | 439   | 13    | 22751 |
             +-------+-------+-------+-------+-------+
             | 22779 | 367   | 439   | 13    | 22993 |
             +-------+-------+-------+-------+-------+
             | 23010 | 447   | 443   | 13    | 23227 |
             +-------+-------+-------+-------+-------+
             | 23252 | 559   | 449   | 13    | 23473 |
             +-------+-------+-------+-------+-------+
             | 23491 | 121   | 457   | 13    | 23719 |
             +-------+-------+-------+-------+-------+
             | 23730 | 623   | 457   | 13    | 23957 |
             +-------+-------+-------+-------+-------+
             | 23971 | 450   | 457   | 13    | 24197 |
             +-------+-------+-------+-------+-------+
             | 24215 | 253   | 461   | 13    | 24443 |
             +-------+-------+-------+-------+-------+
             | 24476 | 106   | 467   | 13    | 24709 |
             +-------+-------+-------+-------+-------+
             | 24721 | 863   | 467   | 13    | 24953 |
             +-------+-------+-------+-------+-------+
             | 24976 | 148   | 479   | 13    | 25219 |
             +-------+-------+-------+-------+-------+
             | 25230 | 427   | 479   | 13    | 25471 |
             +-------+-------+-------+-------+-------+
             | 25493 | 138   | 479   | 13    | 25733 |
             +-------+-------+-------+-------+-------+
             | 25756 | 794   | 487   | 13    | 26003 |
             +-------+-------+-------+-------+-------+
             | 26022 | 247   | 487   | 13    | 26267 |
             +-------+-------+-------+-------+-------+
             | 26291 | 562   | 491   | 13    | 26539 |
             +-------+-------+-------+-------+-------+
             | 26566 | 53    | 499   | 13    | 26821 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 26838 | 135   | 499   | 13    | 27091 |
             +-------+-------+-------+-------+-------+
             | 27111 | 21    | 503   | 13    | 27367 |
             +-------+-------+-------+-------+-------+
             | 27392 | 201   | 509   | 13    | 27653 |
             +-------+-------+-------+-------+-------+
             | 27682 | 169   | 521   | 13    | 27953 |
             +-------+-------+-------+-------+-------+
             | 27959 | 70    | 521   | 13    | 28229 |
             +-------+-------+-------+-------+-------+
             | 28248 | 386   | 521   | 13    | 28517 |
             +-------+-------+-------+-------+-------+
             | 28548 | 226   | 523   | 13    | 28817 |
             +-------+-------+-------+-------+-------+
             | 28845 | 3     | 541   | 13    | 29131 |
             +-------+-------+-------+-------+-------+
             | 29138 | 769   | 541   | 13    | 29423 |
             +-------+-------+-------+-------+-------+
             | 29434 | 590   | 541   | 13    | 29717 |
             +-------+-------+-------+-------+-------+
             | 29731 | 672   | 541   | 13    | 30013 |
             +-------+-------+-------+-------+-------+
             | 30037 | 713   | 547   | 13    | 30323 |
             +-------+-------+-------+-------+-------+
             | 30346 | 967   | 547   | 13    | 30631 |
             +-------+-------+-------+-------+-------+
             | 30654 | 368   | 557   | 14    | 30949 |
             +-------+-------+-------+-------+-------+
             | 30974 | 348   | 557   | 14    | 31267 |
             +-------+-------+-------+-------+-------+
             | 31285 | 119   | 563   | 14    | 31583 |
             +-------+-------+-------+-------+-------+
             | 31605 | 503   | 569   | 14    | 31907 |
             +-------+-------+-------+-------+-------+
             | 31948 | 181   | 571   | 14    | 32251 |
             +-------+-------+-------+-------+-------+
             | 32272 | 394   | 577   | 14    | 32579 |
             +-------+-------+-------+-------+-------+
             | 32601 | 189   | 587   | 14    | 32917 |
             +-------+-------+-------+-------+-------+
             | 32932 | 210   | 587   | 14    | 33247 |
             +-------+-------+-------+-------+-------+
             | 33282 | 62    | 593   | 14    | 33601 |
             +-------+-------+-------+-------+-------+
             | 33623 | 273   | 593   | 14    | 33941 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 33961 | 554   | 599   | 14    | 34283 |
             +-------+-------+-------+-------+-------+
             | 34302 | 936   | 607   | 14    | 34631 |
             +-------+-------+-------+-------+-------+
             | 34654 | 483   | 607   | 14    | 34981 |
             +-------+-------+-------+-------+-------+
             | 35031 | 397   | 613   | 14    | 35363 |
             +-------+-------+-------+-------+-------+
             | 35395 | 241   | 619   | 14    | 35731 |
             +-------+-------+-------+-------+-------+
             | 35750 | 500   | 631   | 14    | 36097 |
             +-------+-------+-------+-------+-------+
             | 36112 | 12    | 631   | 14    | 36457 |
             +-------+-------+-------+-------+-------+
             | 36479 | 958   | 641   | 14    | 36833 |
             +-------+-------+-------+-------+-------+
             | 36849 | 524   | 641   | 14    | 37201 |
             +-------+-------+-------+-------+-------+
             | 37227 | 8     | 643   | 14    | 37579 |
             +-------+-------+-------+-------+-------+
             | 37606 | 100   | 653   | 14    | 37967 |
             +-------+-------+-------+-------+-------+
             | 37992 | 339   | 653   | 14    | 38351 |
             +-------+-------+-------+-------+-------+
             | 38385 | 804   | 659   | 14    | 38749 |
             +-------+-------+-------+-------+-------+
             | 38787 | 510   | 673   | 14    | 39163 |
             +-------+-------+-------+-------+-------+
             | 39176 | 18    | 673   | 14    | 39551 |
             +-------+-------+-------+-------+-------+
             | 39576 | 412   | 677   | 14    | 39953 |
             +-------+-------+-------+-------+-------+
             | 39980 | 394   | 683   | 14    | 40361 |
             +-------+-------+-------+-------+-------+
             | 40398 | 830   | 691   | 15    | 40787 |
             +-------+-------+-------+-------+-------+
             | 40816 | 535   | 701   | 15    | 41213 |
             +-------+-------+-------+-------+-------+
             | 41226 | 199   | 701   | 15    | 41621 |
             +-------+-------+-------+-------+-------+
             | 41641 | 27    | 709   | 15    | 42043 |
             +-------+-------+-------+-------+-------+
             | 42067 | 298   | 709   | 15    | 42467 |
             +-------+-------+-------+-------+-------+
             | 42490 | 368   | 719   | 15    | 42899 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 42916 | 755   | 727   | 15    | 43331 |
             +-------+-------+-------+-------+-------+
             | 43388 | 379   | 727   | 15    | 43801 |
             +-------+-------+-------+-------+-------+
             | 43840 | 73    | 733   | 15    | 44257 |
             +-------+-------+-------+-------+-------+
             | 44279 | 387   | 739   | 15    | 44701 |
             +-------+-------+-------+-------+-------+
             | 44729 | 457   | 751   | 15    | 45161 |
             +-------+-------+-------+-------+-------+
             | 45183 | 761   | 751   | 15    | 45613 |
             +-------+-------+-------+-------+-------+
             | 45638 | 855   | 757   | 15    | 46073 |
             +-------+-------+-------+-------+-------+
             | 46104 | 370   | 769   | 15    | 46549 |
             +-------+-------+-------+-------+-------+
             | 46574 | 261   | 769   | 15    | 47017 |
             +-------+-------+-------+-------+-------+
             | 47047 | 299   | 787   | 15    | 47507 |
             +-------+-------+-------+-------+-------+
             | 47523 | 920   | 787   | 15    | 47981 |
             +-------+-------+-------+-------+-------+
             | 48007 | 269   | 787   | 15    | 48463 |
             +-------+-------+-------+-------+-------+
             | 48489 | 862   | 797   | 15    | 48953 |
             +-------+-------+-------+-------+-------+
             | 48976 | 349   | 809   | 15    | 49451 |
             +-------+-------+-------+-------+-------+
             | 49470 | 103   | 809   | 15    | 49943 |
             +-------+-------+-------+-------+-------+
             | 49978 | 115   | 821   | 15    | 50461 |
             +-------+-------+-------+-------+-------+
             | 50511 | 93    | 821   | 16    | 50993 |
             +-------+-------+-------+-------+-------+
             | 51017 | 982   | 827   | 16    | 51503 |
             +-------+-------+-------+-------+-------+
             | 51530 | 432   | 839   | 16    | 52027 |
             +-------+-------+-------+-------+-------+
             | 52062 | 340   | 853   | 16    | 52571 |
             +-------+-------+-------+-------+-------+
             | 52586 | 173   | 853   | 16    | 53093 |
             +-------+-------+-------+-------+-------+
             | 53114 | 421   | 857   | 16    | 53623 |
             +-------+-------+-------+-------+-------+
             | 53650 | 330   | 863   | 16    | 54163 |
             +-------+-------+-------+-------+-------+

             +-------+-------+-------+-------+-------+
             | 54188 | 624   | 877   | 16    | 54713 |
             +-------+-------+-------+-------+-------+
             | 54735 | 233   | 877   | 16    | 55259 |
             +-------+-------+-------+-------+-------+
             | 55289 | 362   | 883   | 16    | 55817 |
             +-------+-------+-------+-------+-------+
             | 55843 | 963   | 907   | 16    | 56393 |
             +-------+-------+-------+-------+-------+
             | 56403 | 471   | 907   | 16    | 56951 |
             +-------+-------+-------+-------+-------+

         Table 2: Systematic Indices and Other Parameters

Operating with Octets, Symbols, and Matrices

General

The remainder of this section describes the arithmetic operations that are used to generate encoding symbols from source symbols and to generate source symbols from encoding symbols. Mathematically, octets can be thought of as elements of a finite field, i.e., the finite field GF(256) with 256 elements, and thus the addition and multiplication operations and identity elements and inverses over both operations are defined. Matrix operations and symbol operations are defined based on the arithmetic operations on octets. This allows a full implementation of these arithmetic operations without having to understand the underlying mathematics of finite fields.

Arithmetic Operations on Octets

Octets are mapped to non-negative integers in the range 0 through 255 in the usual way: A single octet of data from a symbol, B[7],B[6],B[5],B[4],B[3],B[2],B[1],B[0], where B[7] is the highest order bit and B[0] is the lowest order bit, is mapped to the integer i=B[7]*128+B[6]*64+B[5]*32+B[4]*16+B[3]*8+B[2]*4+B[1]*2+B[0].

The addition of two octets u and v is defined as the exclusive-or operation, i.e.,

  u + v = u ^ v.

Subtraction is defined in the same way, so we also have

  u - v = u ^ v.

The zero element (additive identity) is the octet represented by the integer 0. The additive inverse of u is simply u, i.e.,

  u + u = 0.

The multiplication of two octets is defined with the help of two tables OCT_EXP and OCT_LOG, which are given in Section 5.7.3 and Section 5.7.4, respectively. The table OCT_LOG maps octets (other than the zero element) to non-negative integers, and OCT_EXP maps non-negative integers to octets. For two octets u and v, we define

  u * v =

     0, if either u or v are 0,

     OCT_EXP[OCT_LOG[u] + OCT_LOG[v]] otherwise.

Note that the '+' on the right-hand side of the above is the usual integer addition, since its arguments are ordinary integers.

The division u / v of two octets u and v, and where v != 0, is defined as follows:

  u / v =

     0, if u == 0,

     OCT_EXP[OCT_LOG[u] - OCT_LOG[v] + 255] otherwise.

The one element (multiplicative identity) is the octet represented by the integer 1. For an octet u that is not the zero element, i.e., the multiplicative inverse of u is

  OCT_EXP[255 - OCT_LOG[u]].

The octet denoted by alpha is the octet with the integer representation 2. If i is a non-negative integer 0 <= i < 256, we have

  alpha^^i = OCT_EXP[i].

The Table OCT_EXP

The table OCT_EXP contains 510 octets. The indexing starts at 0 and ranges to 509, and the entries are the octets with the following positive integer representation:

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, 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

The Table OCT_LOG

The table OCT_LOG contains 255 non-negative integers. The table is indexed by octets interpreted as integers. The octet corresponding to the zero element, which is represented by the integer 0, is excluded as an index, and thus indexing starts at 1 and ranges up to 255, and the entries are the following:

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

Operations on Symbols

Operations on symbols have the same semantics as operations on vectors of octets of length T in this specification. Thus, if U and V are two symbols formed by the octets u[0], ..., u[T-1] and v[0], ..., v[T-1], respectively, the sum of symbols U + V is defined to be the component-wise sum of octets, i.e., equal to the symbol D formed by the octets d[0], ..., d[T-1], such that

  d[i] = u[i] + v[i], 0 <= i < T.

Furthermore, if beta is an octet, the product beta*U is defined to be the symbol D obtained by multiplying each octet of U by beta, i.e.,

  d[i] = beta*u[i], 0 <= i < T.

Operations on Matrices

All matrices in this specification have entries that are octets, and thus matrix operations and definitions are defined in terms of the underlying octet arithmetic, e.g., operations on a matrix, matrix rank, and matrix inversion.

Requirements for a Compliant Decoder

If a RaptorQ-compliant decoder 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 SHOULD recover the entire source block.

A RaptorQ-compliant decoder SHALL have the following recovery properties for source blocks with K' source symbols for all values of K' in Table 2 of Section 5.6.

1. If the decoder receives K' encoding symbols generated according

   to the encoder specification in Section 5.3 with corresponding
   ESIs chosen independently and uniformly at random from the range
   of possible ESIs, then on average the decoder will fail to
   recover the entire source block at most 1 out of 100 times.

2. If the decoder receives K'+1 encoding symbols generated according

   to the encoder specification in Section 5.3 with corresponding
   ESIs chosen independently and uniformly at random from the range
   of possible ESIs, then on average the decoder will fail to
   recover the entire source block at most 1 out of 10,000 times.

3. If the decoder receives K'+2 encoding symbols generated according

   to the encoder specification in Section 5.3 with corresponding
   ESIs chosen independently and uniformly at random from the range
   of possible ESIs, then on average the decoder will fail to
   recover the entire source block at most 1 out of 1,000,000 times.

Note that the Example FEC Decoder specified in Section 5.4 fulfills both requirements, i.e.,

1. it can reconstruct a source block as long as it receives a

   mathematically sufficient set of encoding symbols generated
   according to the encoder specification in Section 5.3, and

2. it fulfills the mandatory recovery properties from above.

Security Considerations

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. 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-256 hash [FIPS.180-3.2008] of an object may be appended before transmission, and the SHA-256 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.

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. IANA has assigned the value 6 under the ietf:rmt:fec:encoding registry to "RaptorQ Code" as the Fully-Specified FEC Encoding ID value associated with this specification.

Acknowledgements

Thanks are due to Ranganathan (Ranga) Krishnan. Ranga Krishnan has been very supportive in finding and resolving implementation details and in finding the systematic indices. In addition, Habeeb Mohiuddin Mohammed and Antonios Pitarokoilis, both from the Munich University of Technology (TUM), and Alan Shinsato have done two independent implementations of the RaptorQ encoder/decoder that have helped to clarify and to resolve issues with this specification.

References

Normative References

[FIPS.180-3.2008]

          National Institute of Standards and Technology, "Secure
          Hash Standard", FIPS PUB 180-3, October 2008.

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.

RFC5052 Watson, M., Luby, M., and L. Vicisano, "Forward Error

          Correction (FEC) Building Block", RFC 5052, August 2007.

Informative References

[LTCodes] Luby, M., "LT codes", Annual IEEE Symposium on Foundations

          of Computer Science, pp. 271-280, November 2002.

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.

RFC5053 Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer,

          "Raptor Forward Error Correction Scheme for Object
          Delivery", RFC 5053, October 2007.

[RaptorCodes]

          Shokrollahi, A. and M. Luby, "Raptor Codes", Foundations
          and Trends in Communications and Information Theory: Vol.
          6: No. 3-4, pp. 213-322, 2011.

Authors' Addresses

Michael Luby Qualcomm Incorporated 3165 Kifer Road Santa Clara, CA 95051 U.S.A.

EMail: [email protected]

Amin Shokrollahi EPFL Laboratoire d'algorithmique Station 14 Batiment BC Lausanne 1015 Switzerland

EMail: [email protected]

Mark Watson Netflix Inc. 100 Winchester Circle Los Gatos, CA 95032 U.S.A.

EMail: [email protected]

Thomas Stockhammer Nomor Research Brecherspitzstrasse 8 Munich 81541 Germany

EMail: [email protected]

Lorenz Minder Qualcomm Incorporated 3165 Kifer Road Santa Clara, CA 95051 U.S.A.

EMail: [email protected]