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RFC 5445 - Basic Forward Error Correction (FEC) Schemes


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Network Working Group                                          M. Watson
Request for Comments: 5445                              Digital Fountain
Obsoletes: 3452, 3695                                         March 2009
Category: Standards Track

              Basic Forward Error Correction (FEC) Schemes

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document provides Forward Error Correction (FEC) Scheme
   specifications according to the Reliable Multicast Transport (RMT)
   FEC building block for the Compact No-Code FEC Scheme, the Small
   Block, Large Block, and Expandable FEC Scheme, the Small Block
   Systematic FEC Scheme, and the Compact FEC Scheme.  This document
   obsoletes RFC 3695 and assumes responsibility for the FEC Schemes
   defined in RFC 3452.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Compact No-Code FEC Scheme . . . . . . . . . . . . . . . . . .  4
     3.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Formats and Codes  . . . . . . . . . . . . . . . . . . . .  4
       3.2.1.  FEC Payload ID(s)  . . . . . . . . . . . . . . . . . .  4
       3.2.2.  FEC Object Transmission Information  . . . . . . . . .  5
     3.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  FEC Code Specification . . . . . . . . . . . . . . . . . .  7
       3.4.1.  Source Block Logistics . . . . . . . . . . . . . . . .  7
       3.4.2.  Sending and Receiving a Source Block . . . . . . . . .  8
   4.  Small Block, Large Block, and Expandable FEC Scheme  . . . . .  9
     4.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Formats and Codes  . . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  FEC Payload ID(s)  . . . . . . . . . . . . . . . . . .  9
       4.2.2.  FEC Object Transmission Information  . . . . . . . . . 10
     4.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.4.  FEC Code Specification . . . . . . . . . . . . . . . . . . 12
   5.  Small Block Systematic FEC Scheme  . . . . . . . . . . . . . . 12
     5.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Formats and Codes  . . . . . . . . . . . . . . . . . . . . 12
       5.2.1.  FEC Payload ID(s)  . . . . . . . . . . . . . . . . . . 12
       5.2.2.  FEC Object Transmission Information  . . . . . . . . . 13
     5.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.4.  FEC Code Specification . . . . . . . . . . . . . . . . . . 15
   6.  Compact FEC Scheme . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Formats and Codes  . . . . . . . . . . . . . . . . . . . . 15
       6.2.1.  FEC Payload ID(s)  . . . . . . . . . . . . . . . . . . 15
       6.2.2.  FEC Object Transmission Information  . . . . . . . . . 15
     6.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.4.  FEC Code Specification . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Changes from Schemes Defined in RFC 3452 and RFC 3695  . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18

1.  Introduction

   The document specifies the following FEC Schemes according to the
   specification requirements of the FEC building block [RFC5052]:

   o  Compact No-Code FEC Scheme

   o  Small Block, Large Block, and Expandable FEC Scheme

   o  Small Block Systematic FEC Scheme

   o  Compact FEC Scheme

   This document inherits the context, language, declarations and
   restrictions of the FEC building block [RFC5052].  This document also
   uses the terminology of the companion document [RFC3453], which
   describes the use of FEC codes within the context of reliable IP
   multicast transport and provides an introduction to some commonly
   used FEC codes.

   Building blocks are defined in [RFC3048].  This document follows the
   general guidelines provided in [RFC3269].

   [RFC3452] and [RFC3695] contain previous versions of the FEC Schemes
   defined in this specification.  These RFCs were published in the
   "Experimental" category.  It was the stated intent of the RMT working
   group to re-submit these specifications as an IETF Proposed Standard
   in due course.  This document obsoletes [RFC3695].  [RFC3452] has
   already been obsoleted by [RFC5052], and this document assumes
   responsibility for aspects of [RFC3452] that were not included in
   [RFC5052].

   This Proposed Standard specification is thus based on and backwards
   compatible with the FEC Schemes defined in [RFC3452] and [RFC3695],
   updated according to accumulated experience and growing protocol
   maturity since their original publication.  Said experience applies
   both to this specification itself and to congestion control
   strategies related to the use of this specification.

   The differences between the FEC Scheme specifications in [RFC3452]
   and [RFC3695] and this document are listed in Section 10.

   Integer fields specified in this document are all encoded in network
   byte order.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Compact No-Code FEC Scheme

3.1.  Introduction

   The Compact No-code FEC Scheme is a Fully-Specified FEC Scheme.  The
   scheme requires no FEC coding and is specified primarily to allow
   simple interoperability testing between different implementations of
   protocol instantiations that use the FEC building block.

3.2.  Formats and Codes

3.2.1.  FEC Payload ID(s)

   The FEC Payload ID for the Compact No-Code FEC Scheme is composed of
   a Source Block Number and an Encoding Symbol ID as shown in Figure 1.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Source Block Number       |      Encoding Symbol ID       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: FEC Payload ID Format for Compact No-Code FEC Scheme

   The Source Block Number (SBN) is a 16-bit unsigned integer that is
   used to identify from which source block of the object the encoding
   symbol in the payload of the packet is generated.  There are two
   possible modes: in the unique SBN mode, each source block within the
   object has a unique Source Block Number associated with it, and in
   the non-unique SBN mode, the same Source Block Number may be used for
   more than one source block within the object.  Which mode is being
   used for an object is outside the scope of this document and MUST be
   communicated, either explicitly or implicitly, out-of-band to
   receivers.

   If the unique SBN mode is used, then successive Source Block Numbers
   are associated with consecutive source blocks of the object starting
   with Source Block Number 0 for the first source block of the object.
   In this case, there are at most 2^^16 source blocks in the object.

   If the non-unique SBN mode is used, then the mapping from source
   blocks to Source Block Numbers MUST be communicated out-of-band to
   receivers, and how this is done is outside the scope of this
   document.  This mapping could be implicit, for example, determined by
   the transmission order of the source blocks.  In non-unique SBN mode,
   packets for two different source blocks mapped to the same Source
   Block Number SHOULD NOT be sent within an interval of time that is
   shorter than the transport time of a source block.  The transport
   time of a source block includes the amount of time needed to process
   the source block at the sender transport layer, the network transit
   time for packets, and the amount of time needed to process the source
   block at the receiver transport.  This allows the receiver to clearly
   decide which packets belong to which source block.

   The Encoding Symbol ID is a 16-bit unsigned integer that identifies
   which specific encoding symbol generated from the source block is
   carried in the packet payload.  The exact details of the
   correspondence between Encoding Symbol IDs and the encoding symbols
   in the packet payload are specified in Section 3.4.

3.2.2.  FEC Object Transmission Information

3.2.2.1.  Mandatory

   The mandatory FEC Object Transmission Information element for the
   Compact No-Code FEC Scheme is:

   o  FEC Encoding ID: zero (0)

3.2.2.2.  Common

   The Common FEC Object Transmission Information elements and their
   value ranges for the Compact No-Code FEC Scheme are:

   Transfer-Length:  a non-negative integer, less than 2^^48, indicating
      the length of the object in octets.

   Encoding-Symbol-Length:  a non-negative integer, less than 2^^16,
      indicating the length of each encoding symbol in octets.

   Maximum-Source-Block-Length:  a non-negative integer, less than
      2^^32, indicating the maximum number of source symbols in a source
      block.

   The encoded Common FEC Object Transmission Information is defined 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                          |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Encoding Symbol Length     | Max. Source Block Length (MSB)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Max. Source Block Length (LSB)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 2: Encoded Common FEC Object Transmission Information (OTI)
                      for Compact No-Code FEC Scheme

   The Transfer Length, Encoding Symbol Length, and Maximum Source Block
   Length are encoded as unsigned integers, of length 48 bits, 16 bits,
   and 32 bits, respectively.

   All Encoding Symbols of a transport object MUST have length equal to
   the length specified in the Encoding Symbol Length element, with the
   optional exception of the last source symbol of the last source block
   (so that redundant padding is not mandatory in this last symbol).
   This last source symbol MUST be logically padded out with zeroes when
   another Encoding Symbol is computed based on this source symbol to
   ensure the same interpretation of this Encoding Symbol value by the
   sender and receiver.  However, this padding does not actually need to
   be sent with the data of the last source symbol.

   The "Reserved" field in the Encoded FEC Object Transmission
   Information MUST be set to zero by senders and its value MUST be
   ignored by receivers.

      Note: this FEC Scheme was first defined in [RFC3695], which did
      not require that the Encoding Symbol Length should be the same for
      every source block.  This document introduces a general
      requirement that the Encoding Symbol Length be the same across
      source blocks.  Since no protocols were defined that support
      variation in the Encoding Symbol Length between source blocks,
      this can be done without introducing backwards compatibility
      issues.

3.2.2.3.  Scheme-Specific

   No Scheme-Specific FEC Object Transmission Information elements are
   defined by this FEC Scheme.

3.3.  Procedures

   The algorithm defined in Section 9.1. of [RFC5052] MUST be used to
   partition the file into source blocks.

3.4.  FEC Code Specification

   The Compact No-Code FEC Scheme does not require FEC encoding or
   decoding.  Instead, each encoding symbol consists of consecutive
   bytes of a source block of the object.

   The following two subsections describe the details of how the Compact
   No-Code FEC Scheme operates for each source block of an object.

3.4.1.  Source Block Logistics

   Let X > 0 be the length of a source block in bytes.  Let L > 0 be the
   length of the encoding symbol contained in the payload of each
   packet.  The value of X and L are part of the FEC Object Transmission
   Information, and how this information is communicated to a receiver
   is outside the scope of this document.

   For a given source block X bytes in length with Source Block Number
   I, let N = X/L rounded up to the nearest integer.  The encoding
   symbol carried in the payload of a packet consists of a consecutive
   portion of the source block.  The source block is logically
   partitioned into N encoding symbols, each L bytes in length, and the
   corresponding Encoding Symbol IDs range from 0 through N-1 starting
   at the beginning of the source block and proceeding to the end.
   Thus, the encoding symbol with Encoding Symbol ID Y consists of bytes
   L*Y through L*(Y+1)-1 of the source block, where the bytes of the
   source block are numbered from 0 through X-1.  If X/L is not integral
   then the last encoding symbol with Encoding Symbol ID = N-1 consists
   of bytes L*(N-1) through the last byte X-1 of the source block, and
   the remaining L*N - X bytes of the encoding symbol can by padded out
   with zeroes.

   As an example, suppose that the source block length X = 20,400 and
   encoding symbol length L = 1,000.  The encoding symbol with Encoding
   Symbol ID = 10 contains bytes 10,000 through 10,999 of the source
   block, and the encoding symbol with Encoding Symbol ID = 20 contains
   bytes 20,000 through the last byte 20,399 of the source block and the
   remaining 600 bytes of the encoding symbol can be padded with zeroes.

   There are no restrictions beyond the rules stated above on how a
   sender generates encoding symbols to send from a source block.
   However, it is recommended that an implementor refer to the companion
   document [RFC3452] for general advice.

   In the next subsection, a procedure is recommended for sending and
   receiving source blocks.

3.4.2.  Sending and Receiving a Source Block

   The following carousel procedure is RECOMMENDED for a sender to
   generate packets containing FEC Payload IDs and corresponding
   encoding symbols for a source block with Source Block Number I.  Set
   the length in bytes of an encoding symbol to a fixed value L, which
   is reasonable for a packet payload (e.g., ensure that the total
   packet size does not exceed the MTU) and that is smaller than the
   source block length X, e.g., L = 1,000 for X >= 1,000.  Initialize Y
   to a value randomly chosen in the interval [0..N-1].  Repeat the
   following for each packet of the source block to be sent.

   o  If Y <= N-1, then generate the encoding symbol Y.

   o  Within the FEC Payload ID, set the Source Block Length to X, set
      the Source Block Number = I, set the Encoding Symbol ID = Y, place
      the FEC Payload ID and the encoding symbol into the packet to
      send.

   o  In preparation for the generation of the next packet: if Y < N-1
      then increment Y by one else if Y = N-1 then reset Y to zero.

   The following procedure is RECOMMENDED for a receiver to recover the
   source block based on receiving packets for the source block from a
   sender that is using the carousel procedure described above.  The
   receiver can determine from which source block a received packet was
   generated by the Source Block Number carried in the FEC Payload ID.
   Upon receipt of the first FEC Payload ID for a source block, the
   receiver uses the Source Block Length and Encoding Symbol Length
   received out-of-band as part of the FEC Object Transmission
   Information to determine the length X in bytes of the source block
   and length L in bytes of each encoding symbol.  The receiver
   allocates space for the X bytes that the source block requires.  The
   receiver also computes the length of the encoding symbol in the
   payload of the packet by subtracting the packet header length from
   the total length of the received packet.  The receiver checks that
   this symbol length is equal to L, except in the case that this is the
   last symbol of the source block in which case the symbol length in
   the packet may be less than L.  After calculating N = X/L rounded up
   to the nearest integer, the receiver allocates a boolean array
   RECEIVED[0..N-1] with all N entries initialized to false to track
   received encoding symbols.  The receiver keeps receiving packets for
   the source block as long as there is at least one entry in RECEIVED
   still set to false or until the application decides to give up on
   this source block and move on to other source blocks.  For each

   received packet for the source block (including the first packet),
   the steps to be taken to help recover the source block are as
   follows.  Let Y be the value of the Encoding Symbol ID within the FEC
   Payload ID of the packet.  If Y <= N-1, then the receiver copies the
   encoding symbol into the appropriate place within the space reserved
   for the source block and sets RECEIVED[Y] = true.  If all N entries
   of RECEIVED are true, then the receiver has recovered the entire
   source block.

4.  Small Block, Large Block, and Expandable FEC Scheme

4.1.  Introduction

   This section defines an Under-Specified FEC Scheme for Small Block
   FEC codes, Large Block FEC codes, and Expandable FEC codes as
   described in [RFC3453].

4.2.  Formats and Codes

4.2.1.  FEC Payload ID(s)

   The FEC Payload ID is composed of a Source Block Number and an
   Encoding Symbol ID structured 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Source Block Number                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Encoding Symbol ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 3: FEC Payload ID Format for Small Block, Large Block, and
                           Expandable FEC Codes

   The Source Block Number is a 32-bit unsigned integer that identifies
   from which source block of the object the encoding symbol(s) in the
   payload are generated.  These blocks are numbered consecutively from
   0 to N-1, where N is the number of source blocks in the object.

   The Encoding Symbol ID is a 32-bit unsigned integer that identifies
   which specific encoding symbol(s) generated from the source block are
   carried in the packet payload.  The exact details of the
   correspondence between Encoding Symbol IDs and the encoding symbol(s)
   in the packet payload are dependent on the particular FEC Scheme
   instance used as identified by the FEC Encoding ID and by the FEC
   Instance ID, and these details may be proprietary.

4.2.2.  FEC Object Transmission Information

4.2.2.1.  Mandatory

   The mandatory FEC Object Transmission Information element for the
   Small Block, Large Block, and Expandable FEC Scheme are:

   o  FEC Encoding ID: 128

4.2.2.2.  Common

   The Common FEC Object Transmission Information elements and their
   value ranges for the Small Block, Large Block, and Expandable FEC
   Scheme are:

   FEC Instance ID:  a non-negative integer less than 2^^16.

   Transfer-Length:  a non-negative integer less than 2^^48, indicating
      the length of the object in octets.

   Encoding-Symbol-Length:  a non-negative integer less than 2^^16,
      indicating the length of each encoding symbol in octets.

   Maximum-Source-Block-Length:  a non-negative integer less than 2^^32,
      indicating the maximum number of source symbols in a source block.

   The encoded Common FEC Object Transmission Information is defined in
   Figure 4.

       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                          |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |         FEC Instance ID       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Encoding Symbol Length     | Max. Source Block Length (MSB)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Max. Source Block Length (LSB)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 4: Encoded Common FEC OTI for Small Block, Large Block, and
                           Expandable FEC Scheme

   The Transfer Length (48 bits), FEC Instance ID (16 bits), Encoding
   Symbol Length (16 bits), and Maximum Source Block Length (32 bits)
   are encoded as unsigned integers.

4.2.2.3.  Scheme-Specific

   The Scheme-Specific FEC Object Transmission Information field for the
   Small Block, Large Block, and Expandable FEC Scheme provides for the
   possibility of Instance-specific FEC Object Transmission Information.
   The format of the Scheme-Specific FEC Object Transmission Information
   for this FEC Scheme is defined in Figure 5.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Length    |           Instance-specific FEC OTI           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |               Instance-specific FEC OTI contd.                |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 5: Encoded Scheme-Specific FEC OTI for Small Block, Large
                     Block, and Expandable FEC Scheme

   The Scheme-Specific FEC Object Transmission Information field
   contains the following sub-fields:

   Length (1 octet):  an unsigned integer that specifies the length of
      the Scheme-Specific FEC OTI in four-octet words (including this
      length field), except that the value zero indicates that no
      Instance-specific FEC OTI Information is provided.  When the
      Length is zero, three padding bytes containing value zero SHALL
      follow the Length field to maintain 4-octet alignment.

   Instance-specific FEC OTI Information:   the contents of this field
      are FEC Scheme Instance-specific.

   Note that in the case of a Content Delivery protocol that supports
   external signaling of the total FEC Object Transmission Information
   length, then the Scheme-Specific FEC OTI field defined here is
   optional.  Otherwise, this field MUST be included.

4.3.  Procedures

   The algorithm defined in Section 9.1. of [RFC5052] MUST be used to
   partition the file into source blocks.

4.4.  FEC Code Specification

   The FEC code specification and the correspondence of Encoding Symbols
   IDs to encoding symbols are defined by specific instances of this
   scheme and so are out of scope of this document.

5.  Small Block Systematic FEC Scheme

5.1.  Introduction

   This section defines an Under-Specified FEC Scheme for Small Block
   Systematic FEC codes as described in [RFC3453].  For Small Block
   Systematic FEC codes, each source block is of length at most 65535
   source symbols.

   Although these codes can generally be accommodated by the FEC
   Encoding ID described in Section 4, a specific FEC Encoding ID is
   defined for Small Block Systematic FEC codes to allow more
   flexibility and to retain header compactness.  The small source block
   length and small expansion factor that often characterize systematic
   codes may require the data source to frequently change the source
   block length.  To allow the dynamic variation of the source block
   length and to communicate it to the receivers with low overhead, the
   block length is included in the FEC Payload ID.

5.2.  Formats and Codes

5.2.1.  FEC Payload ID(s)

   The FEC Payload ID is composed of the Source Block Number, Source
   Block Length, and the Encoding Symbol ID structured as shown in
   Figure 6.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Source Block Number                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Source Block Length      |       Encoding Symbol ID      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: FEC Payload ID Format for Small Block Systematic FEC Scheme

   The Source Block Number is a 32-bit unsigned integer that identifies
   from which source block of the object the encoding symbol(s) in the
   payload are generated.  These blocks are numbered consecutively from
   0 to N-1, where N is the number of source blocks in the object.

   The Source Block Length is a 16-bit unsigned integer that specifies
   the length in units of source symbols of the source block identified
   by the Source Block Number.

   The Encoding Symbol ID is a 16-bit unsigned integer that identifies
   which specific encoding symbol(s) generated from the source block are
   carried in the packet payload.  Each encoding symbol is either an
   original source symbol or a redundant symbol generated by the
   encoder.  The exact details of the correspondence between Encoding
   Symbol IDs and the encoding symbol(s) in the packet payload are
   dependent on the particular FEC Scheme instance used as identified by
   the FEC Instance ID, and these details may be proprietary.

5.2.2.  FEC Object Transmission Information

5.2.2.1.  Mandatory

   The mandatory FEC Object Transmission Information element for the
   Small Block Systematic FEC Scheme is:

   o  FEC Encoding ID: 129

5.2.2.2.  Common

   The Common FEC Object Transmission Information elements and their
   value ranges for the Small Block Systematic FEC Scheme are:

   FEC Instance ID:  a non-negative integer less than 2^^16.

   Transfer-Length:  a non-negative integer less than 2^^48, indicating
      the length of the object in octets.

   Encoding-Symbol-Length:  a non-negative integer less than 2^^16,
      indicating the length of each encoding symbol in octets.

   Maximum-Source-Block-Length:  a non-negative integer less than 2^^16,
      indicating the maximum number of source symbols in a source block.

   Max-Number-of-Encoding-Symbols:  a non-negative integer less than
      2^^16, indicating the maximum number of encoding symbols per block
      (i.e., source plus repair symbols in the case of a systematic
      code).

   The encoded Common FEC Object Transmission Information is defined in
   Figure 7.

       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                          |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |         FEC Instance ID       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Encoding Symbol Length     |  Maximum Source Block Length  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Max. Num. of Encoding Symbols |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 7: FEC OTI Format for Small Block Systematic FEC Scheme

   The Transfer Length (48 bits), FEC Instance ID (16 bits), Encoding
   Symbol Length (16 bits), Maximum Source Block Length (16 bits), and
   Maximum Number of Encoding Symbols (16 bits) are encoded as unsigned
   integers.

   All Encoding Symbols of a transport object MUST have length equal to
   the length specified in the Encoding Symbol Length field, with the
   optional exception of the last source symbol of the last source block
   (so that redundant padding is not mandatory in this last symbol).
   This last source symbol MUST be logically padded out with zeroes when
   another Encoding Symbol is computed based on this source symbol to
   ensure the same interpretation of this Encoding Symbol value by the
   sender and receiver.  However, this padding need not be actually sent
   with the data of the last source symbol.

      Note: this FEC Scheme was first defined in [RFC3452], which did
      not require that the Encoding Symbol Length should be the same for
      every source block.  However, no protocols have been defined that
      support variation in the Encoding Symbol Length between source
      blocks, and thus introduction of a general requirement that the
      Encoding Symbol Length be the same across source blocks (as
      defined here) should not cause backwards compatibility issues and
      will aid interoperability.

5.2.2.3.  Scheme-Specific

   The Scheme-Specific FEC Object Transmission Information format
   defined in Section 4.2.2.3 SHALL be used.

5.3.  Procedures

   The algorithm defined in Section 9.1. of [RFC5052] MAY be used to
   partition the file into source blocks.  Otherwise, the FEC Scheme
   instance MUST specify the algorithm that is used.

5.4.  FEC Code Specification

   The FEC code specification and the correspondence of Encoding Symbols
   IDs to encoding symbols are defined by specific instances of this
   scheme and so are out of scope of this document.

6.  Compact FEC Scheme

6.1.  Introduction

   The Compact FEC Scheme is an Under-Specified FEC Scheme.  This FEC
   Scheme is similar in spirit to the Compact No-Code FEC Scheme, except
   that a non-trivial FEC encoding (that is Under-Specified) may be used
   to generate encoding symbol(s) placed in the payload of each packet
   and a corresponding FEC decoder may be used to produce the source
   block from received packets.

6.2.  Formats and Codes

6.2.1.  FEC Payload ID(s)

   The FEC Payload ID format defined in Section 3.2.1 SHALL be used.

6.2.2.  FEC Object Transmission Information

6.2.2.1.  Mandatory

   The mandatory FEC Object Transmission Information element for the
   Compact No-Code FEC Scheme is:

   o  FEC Encoding ID: 130

6.2.2.2.  Common

   The Common FEC Object Transmission Information elements and their
   encoding are the same as defined for the Small Block, Large Block,
   and Expandable FEC Scheme in Figure 4.

6.2.2.3.  Scheme-Specific

   The Scheme-Specific FEC Object Transmission Information format
   defined in Section 4.2.2.3 SHALL be used.

6.3.  Procedures

   The algorithm defined in Section 9.1. of [RFC5052] MUST be used to
   partition the file into source blocks.

6.4.  FEC Code Specification

   The FEC code specification and the correspondence of Encoding Symbols
   IDs to encoding symbols are defined by specific instances of this
   scheme and so are out of scope of this document.

7.  Security Considerations

   This specification does not introduce any further security
   considerations beyond those described in [RFC5052].

8.  Acknowledgements

   This document is substantially based on [RFC3695] by Michael Luby and
   Lorenzo Vicisano and [RFC3452] by Michael Luby, Lorenzo Vicisano, Jim
   Gemmell, Luigi Rizzo, Mark Handley, and Jon Crowcroft.

9.  IANA Considerations

   FEC Encoding IDs 0 and 130 were first defined and registered in the
   ietf:rmt:fec:encoding namespace by [RFC3695].  This document updates
   and obsoletes the definitions from that specification.  References to
   that specification should be replaced with references to this
   document.

   FEC Encoding IDs 128 and 129 were first defined and registered in the
   ietf:rmt:fec:encoding namespace by [RFC3452].  This document updates
   and obsoletes the definitions from that specification.  References to
   that specification should be replaced with references to this
   document.

   Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA
   registration.  For general guidelines on IANA considerations as they
   apply to this document, see [RFC5052].

   This document assigns the Fully-Specified FEC Encoding ID 0 under the
   ietf:rmt:fec:encoding name-space (which was previously assigned by
   [RFC3695], which is obsoleted by this document) to "Compact No-Code"
   as specified in Section 3 above.

   This document assigns the Under-Specified FEC Encoding ID 128 under
   the ietf:rmt:fec:encoding name-space (which was previously assigned
   by [RFC3452]) to "Small Block, Large Block, and Please note that we
   have added a comma between large block and expandable throughout this
   document (RFC Editor style is to include a comme before the last item
   of a series).  If you do not object, we will ask IANA to include this
   comma in their registry for consistency. --> Expandable FEC Codes" as
   specified in Section 4 above.

   This document assigns the Under-Specified FEC Encoding ID 129 under
   the ietf:rmt:fec:encoding name-space (which was previously assigned
   by [RFC3452]) to "Small Block Systematic FEC Codes" as specified in
   Section 5 above.

   This document assigns the Under-Specified FEC Encoding ID 130 under
   the ietf:rmt:fec:encoding name-space (which was previously assigned
   by [RFC3695], which is obsoleted by this document) to "Compact FEC"
   as specified in Section 6 above.

   As FEC Encoding IDs 128, 129, and 130 are Under-Specified, "FEC
   Instance ID" sub-name-spaces must be established, in accordance to
   [RFC5052].  Hence, this document also assumes responsibility for the
   "FEC Instance ID" registries named.

      ietf:rmt:fec:encoding:instance:128, scoped by ietf:rmt:fec:
      encoding = 128

      ietf:rmt:fec:encoding:instance:129, scoped by ietf:rmt:fec:
      encoding = 129

      ietf:rmt:fec:encoding:instance:130, scoped by ietf:rmt:fec:
      encoding = 130

   The values that can be assigned within these namespaces are non-
   negative numeric indices.  Assignment requests are granted on a
   "First Come First Served" basis.  [RFC5052] specifies additional
   criteria that MUST be met for the assignment within the generic ietf:
   rmt:fec:encoding:instance name-space.  These criteria also apply to
   ietf:rmt:fec:encoding:instance:128, ietf:rmt:fec:encoding:instance:
   129, and ietf:rmt:fec:encoding:instance:130.

10.  Changes from Schemes Defined in RFC 3452 and RFC 3695

   This section describes the changes between the Experimental versions
   of these FEC Scheme specifications contained in RFC 3452 [RFC3452]
   and RFC 3695 [RFC3695] and those defined in this specification:

   o  Scheme definitions have been updated to meet the requirements of
      [RFC5052].

   o  Complete encoding formats for the FEC Object Transmission
      Information for each scheme are defined here, instead of within
      content delivery protocol specifications, since the exact format
      depends on the FEC Scheme.

   o  The previous specifications for the Compact No-Code and Small
      Block Systematic FEC Schemes did not require that all encoding
      symbols of the object should have the same length.  This
      requirement is introduced in this specification.  Since no
      protocols have been defined that support variation of the encoding
      symbol length within an object this should not cause backwards
      compatibility issues.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052, August 2007.

11.2.  Informative References

   [RFC3452]  Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
              M., and J. Crowcroft, "Forward Error Correction (FEC)
              Building Block", RFC 3452, December 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.

   [RFC3269]  Kermode, R. and L. Vicisano, "Author Guidelines for
              Reliable Multicast Transport (RMT) Building Blocks and
              Protocol Instantiation documents", RFC 3269, April 2002.

   [RFC3048]  Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
              Floyd, S., and M. Luby, "Reliable Multicast Transport
              Building Blocks for One-to-Many Bulk-Data Transfer",
              RFC 3048, January 2001.

   [RFC3695]  Luby, M. and L. Vicisano, "Compact Forward Error
              Correction (FEC) Schemes", RFC 3695, February 2004.

Author's Address

   Mark Watson
   Digital Fountain
   39141 Civic Center Drive
   Suite 300
   Fremont, CA  94538
   USA

   EMail: mark@digitalfountain.com

 

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