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Versions: (draft-roca-fecframe-ldpc) 00 01 02 03 04 RFC 6816

FecFrame                                                         V. Roca
Internet-Draft                                                     INRIA
Intended status: Standards Track                               M. Cunche
Expires: April 12, 2013                                  INSA-Lyon/INRIA
                                                                J. Lacan
                                                 ISAE, Univ. of Toulouse
                                                         October 9, 2012


Simple LDPC-Staircase Forward Error Correction (FEC) Scheme for FECFRAME
                      draft-ietf-fecframe-ldpc-04

Abstract

   This document describes a fully-specified simple FEC scheme for LDPC-
   Staircase codes that can be used to protect media streams along the
   lines defined by the FECFRAME framework.  These codes have many
   interesting properties: they are systematic codes, they perform close
   to ideal codes in many use-cases and they also feature very high
   encoding and decoding throughputs.  LDPC-Staircase codes are
   therefore a good solution to protect a single high bitrate source
   flow, or to protect globally several mid-rate flows within a single
   FECFRAME instance.  They are also a good solution whenever the
   processing load of a software encoder or decoder must be kept to a
   minimum.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 12, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions Notations and Abbreviations  . . . . . . . . . . .  4
     3.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Notations  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Common Procedures Related to the ADU Block and Source
       Block Creation . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Restrictions . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  ADU Block Creation . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Source Block Creation  . . . . . . . . . . . . . . . . . .  9
   5.  LDPC-Staircase FEC Scheme for Arbitrary ADU Flows  . . . . . . 11
     5.1.  Formats and Codes  . . . . . . . . . . . . . . . . . . . . 11
       5.1.1.  FEC Framework Configuration Information  . . . . . . . 11
       5.1.2.  Explicit Source FEC Payload ID . . . . . . . . . . . . 13
       5.1.3.  Repair FEC Payload ID  . . . . . . . . . . . . . . . . 14
     5.2.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.3.  FEC Code Specification . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     6.1.  Attacks Against the Data Flow  . . . . . . . . . . . . . . 16
       6.1.1.  Access to Confidential Content . . . . . . . . . . . . 16
       6.1.2.  Content Corruption . . . . . . . . . . . . . . . . . . 16
     6.2.  Attacks Against the FEC Parameters . . . . . . . . . . . . 16
     6.3.  When Several Source Flows are to be Protected Together . . 17
     6.4.  Baseline Secure FEC Framework Operation  . . . . . . . . . 17
   7.  Operations and Management Considerations . . . . . . . . . . . 17
     7.1.  Operational Recommendations  . . . . . . . . . . . . . . . 18
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     10.2. Informative References . . . . . . . . . . . . . . . . . . 20







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1.  Introduction

   The use of Forward Error Correction (FEC) codes is a classic solution
   to improve the reliability of unicast, multicast and broadcast
   Content Delivery Protocols (CDP) and applications [RFC3453].  The
   [RFC6363] document describes a generic framework to use FEC schemes
   with media delivery applications, and for instance with real-time
   streaming media applications based on the RTP real-time protocol.
   Similarly the [RFC5052] document describes a generic framework to use
   FEC schemes with objects (e.g., files) delivery applications based on
   the Asynchronous Layered Coding (ALC) [RFC5775] and NACK-Oriented
   Reliable Multicast (NORM) [RFC5740] reliable multicast transport
   protocols.

   More specifically, the [RFC5053] (Raptor) and [RFC5170] (LDPC-
   Staircase and LDPC-Triangle) FEC schemes introduce erasure codes
   based on sparse parity check matrices for object delivery protocols
   like ALC and NORM.  Similarly, the [RFC5510] document introduces
   Reed-Solomon codes based on Vandermonde matrices for the same object
   delivery protocols.  All these codes are systematic codes, meaning
   that the k source symbols are part of the n encoding symbols.
   Additionally, the Reed-Solomon FEC codes belong to the class of
   Maximum Distance Separable (MDS) codes that are optimal in terms of
   erasure recovery capabilities.  It means that a receiver can recover
   the k source symbols from any set of exactly k encoding symbols out
   of n.  This is not the case with either Raptor or LDPC-Staircase
   codes, and these codes require a certain number of encoding symbols
   in excess to k.  However, this number is small in practice when an
   appropriate decoding scheme is used at the receiver [Cunche08].
   Another key difference is the high encoding/decoding complexity of
   Reed-Solomon codecs compared to Raptor or LDPC-Staircase codes.  A
   difference of one or more orders of magnitude or more in terms of
   encoding/decoding speed exists between the Reed-Solomon and LDPC-
   Staircase software codecs [Cunche08][CunchePHD10].  Finally, Raptor
   and LDPC-Staircase codes are large block FEC codes, in the sense of
   [RFC3453], since they can efficiently deal with a large number of
   source symbols.

   The present document focuses on LDPC-Staircase codes, that belong to
   the well-known class of "Low Density Parity Check" codes.  Because of
   their key features, these codes are a good solution in many
   situations, as detailed in Section 7.

   This documents inherits from [RFC5170] the specifications of the core
   LDPC-Staircase codes.  Therefore this document specifies only the
   information specific to the FECFRAME context and refers to [RFC5170]
   for the core specifications of the codes.  To that purpose, the
   present document introduces:



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   o  the Fully-Specified FEC Scheme with FEC Encoding ID XXX that
      specifies a simple way of using LDPC-Staircase codes in order to
      protect arbitrary Application Data Unit (ADU) flows.

   Finally, publicly available reference implementations of these codes
   are available [LDPC-codec] [LDPC-codec-OpenFEC].

2.  Terminology

   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 RFC 2119 [RFC2119].

3.  Definitions Notations and Abbreviations

3.1.  Definitions

   This document uses the following terms and definitions.  Some of them
   are FEC scheme specific and are in line with [RFC5052]:

   Source symbol:  unit of data used during the encoding process.  In
      this specification, there is always one source symbol per ADU.

   Encoding symbol:  unit of data generated by the encoding process.
      With systematic codes, source symbols are part of the encoding
      symbols.

   Repair symbol:  encoding symbol that is not a source symbol.

   Code rate:  the k/n ratio, i.e., the ratio between the number of
      source symbols and the number of encoding symbols.  By definition,
      the code rate is such that: 0 < code rate <= 1.  A code rate close
      to 1 indicates that a small number of repair symbols have been
      produced during the encoding process.

   Systematic code:  FEC code in which the source symbols are part of
      the encoding symbols.  The LDPC-Staircase codes introduced in this
      document are systematic.

   Source block:  a block of k source symbols that are considered
      together for the encoding.

   Packet Erasure Channel:  a communication path where packets are
      either dropped (e.g., by a congested router, or because the number
      of transmission errors exceeds the correction capabilities of the
      physical layer codes) or received.  When a packet is received, it
      is assumed that this packet is not corrupted.




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   Some of them are FECFRAME framework specific and are in line with
   [RFC6363]:

   Application Data Unit (ADU):  the unit of source data provided as
      payload to the transport layer.  Depending on the use-case, an ADU
      may use an RTP encapsulation.

   (Source) ADU Flow:  a sequence of ADUs associated with a transport-
      layer flow identifier (such as the standard 5-tuple {Source IP
      address, source port, destination IP address, destination port,
      transport protocol}).  Depending on the use-case, several ADU
      flows may be protected together by the FECFRAME framework.

   ADU Block:  a set of ADUs that are considered together by the
      FECFRAME instance for the purpose of the FEC scheme.  Along with
      the flow ID (F[]), length (L[]), and padding (Pad[]) fields, they
      form the set of source symbols over which FEC encoding will be
      performed.

   ADU Information (ADUI):  a unit of data constituted by the ADU and
      the associated Flow ID, Length and Padding fields (Section 4.3).
      This is the unit of data that is used as source symbol.

   FEC Framework Configuration Information (FFCI):  information which
      controls the operation of the FEC Framework.  The FFCI enables the
      synchronization of the FECFRAME sender and receiver instances.

   FEC Source Packet:  at a sender (respectively, at a receiver) a
      payload submitted to (respectively, received from) the transport
      protocol containing an ADU along with an optional Explicit Source
      FEC Payload ID.

   FEC Repair Packet:  at a sender (respectively, at a receiver) a
      payload submitted to (respectively, received from) the transport
      protocol containing one repair symbol along with a Repair FEC
      Payload ID and possibly an RTP header.

   The above terminology is illustrated in Figure 1 (sender's point of
   view):












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   +----------------------+
   |     Application      |
   +----------------------+
              |
              | (1) Application Data Units (ADUs)
              |
              v
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |                |
   |                      |-------------------------->|   FEC Scheme   |
   |(2) Construct source  |(3) Source Block           |                |
   |    blocks            |                           |(4) FEC Encoding|
   |(6) Construct FEC     |<--------------------------|                |
   |    source and repair |                           |                |
   |    packets           |(5) Explicit Source FEC    |                |
   +----------------------+    Payload IDs            +----------------+
              |                Repair FEC Payload IDs
              |                Repair symbols
              |
              |(7) FEC source and repair packets
              v
   +----------------------+
   |   Transport Layer    |
   |     (e.g., UDP)      |
   +----------------------+

           Figure 1: Terminology used in this document (sender).

3.2.  Notations

   This document uses the following notations: Some of them are FEC
   scheme specific:

   k      denotes the number of source symbols in a source block.

   max_k  denotes the maximum number of source symbols for any source
          block.

   n      denotes the number of encoding symbols generated for a source
          block.

   E      denotes the encoding symbol length in bytes.

   CR     denotes the "code rate", i.e., the k/n ratio.







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   N1     denotes the target number of "1s" per column in the left side
          of the parity check matrix.

   N1m3   denotes the value N1 - 3.

   G      G denotes the number of encoding symbols per group, i.e., the
          number of symbols sent in the same packet.

   a^^b   denotes a raised to the power b.

   Some of them are FECFRAME framework specific:

   B      denotes the number of ADUs per ADU block.

   max_B  denotes the maximum number of ADUs for any ADU block.

3.3.  Abbreviations

   This document uses the following abbreviations:

   ADU    stands for Application Data Unit.

   ESI    stands for Encoding Symbol ID.

   FEC    stands for Forward Error (or Erasure) Correction code.

   FFCI   stands for FEC Framework Configuration Information.

   FSSI   stands for FEC Scheme Specific Information.

   LDPC   stands for Low Density Parity Check.

   MDS    stands for Maximum Distance Separable code.

   SDP    stands for Session Description Protocol.

4.  Common Procedures Related to the ADU Block and Source Block Creation

   This section introduces the procedures that are used during the ADU
   block and the related source block creation, for the FEC scheme
   considered.

4.1.  Restrictions

   This specification has the following restrictions:

   o  there MUST be exactly one source symbol per ADUI, and therefore
      per ADU;



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   o  there MUST be exactly one repair symbol per FEC Repair Packet;

   o  there MUST be exactly one source block per ADU block;

   o  the use of the LDPC-Staircase scheme is such that there MUST be
      exactly one encoding symbol per group, i.e., G MUST be equal to 1
      [RFC5170];

4.2.  ADU Block Creation

   Two kinds of limitations exist that impact the ADU block creation:

   o  at the FEC Scheme level: the FEC Scheme and the FEC codec have
      limitations that define a maximum source block size;

   o  at the FECFRAME instance level: the target use-case can have real-
      time constraints that can/will define a maximum ADU block size;

   Note that terminology "maximum source block size" and "maximum ADU
   block size" depends on the point of view that is adopted (FEC Scheme
   versus FECFRAME instance).  However, in this document, both refer to
   the same value since Section 4.1 requires there is exactly one source
   symbol per ADU.  We now detail each of these aspects.

   The maximum source block size in symbols, max_k, depends on several
   parameters: the code rate (CR), the Encoding Symbol ID (ESI) field
   length in the Explicit Source/Repair FEC Payload ID (16 bits), as
   well as possible internal codec limitations.  More specifically,
   max_k cannot be larger than the following values, derived from the
   ESI field size limitation, for a given code rate:

      max1_k = 2^^(16 - ceil(Log2(1/CR)))

   Some common max1_k values are:

   o  CR == 1 (no repair symbol): max1_k = 2^^16 = 65536 symbols

   o  1/2 <= CR < 1: max1_k = 2^^15 = 32,768 symbols

   o  1/4 <= CR < 1/2: max1_k = 2^^14 = 16,384 symbols

   Additionally, a codec can impose other limitations on the maximum
   source block size, for instance, because of a limited working memory
   size.  This decision MUST be clarified at implementation time, when
   the target use-case is known.  This results in a max2_k limitation.

   Then, max_k is given by:




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      max_k = min(max1_k, max2_k)

   Note that this calculation is only required at the encoder (sender),
   since the actual k parameter (k <= max_k) is communicated to the
   decoder (receiver) through the Explicit Source/Repair FEC Payload ID.

   The source ADU flows can have real-time constraints.  When there are
   multiple flows, with different real-time constraints, let us consider
   the most stringent constraints (see [RFC6363], section 10.2, item 6
   for recommendations when several flows are globally protected).  In
   that case the maximum number of ADUs of an ADU block must not exceed
   a certain threshold since it directly impacts the decoding delay.
   The larger the ADU block size, the longer a decoder may have to wait
   until it has received a sufficient number of encoding symbols for
   decoding to succeed, and therefore the larger the decoding delay.
   When the target use-case is known, these real-time constraints result
   in an upper bound to the ADU block size, max_rt.

   For instance, if the use-case specifies a maximum decoding latency,
   l, and if each source ADU covers a duration d of a continuous media
   (we assume here the simple case of a constant bit rate ADU flow),
   then the ADU block size must not exceed:

      max_rt = floor(l / d)

   After encoding, this block will produce a set of at most n = max_rt /
   CR encoding symbols.  These n encoding symbols will have to be sent
   at a rate of n / l packets per second.  For instance, with d = 10 ms,
   l = 1 s, max_rt = 100 ADUs.

   If we take into account all these constraints, we find:

      max_B = min(max_k, max_rt)

   This max_B parameter is an upper bound to the number of ADUs that can
   constitute an ADU block.

4.3.  Source Block Creation

   In its most general form the FECFRAME framework and the LDPC-
   Staircase FEC scheme are meant to protect a set of independent flows.
   Since the flows have no relationship to one another, the ADU size of
   each flow can potentially vary significantly.  Even in the special
   case of a single flow, the ADU sizes can largely vary (e.g., the
   various frames of a "Group of Pictures (GOP) of an H.264 flow).  This
   diversity must be addressed since the LDPC-Staircase FEC scheme
   requires a constant encoding symbol size (E parameter) per source
   block.  Since this specification requires that there is only one



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   source symbol per ADU, E must be large enough to contain all the ADUs
   of an ADU block along with their prepended 3 bytes (see below).

   In situations where E is determined per source block (default,
   specified by the FFCI/FSSI with S = 0, Section 5.1.1.2), E is equal
   to the size of the largest ADU of this source block plus three (for
   the prepended 3 bytes, see below).  In this case, upon receiving the
   first FEC Repair Packet for this source block, since this packet MUST
   contain a single repair symbol (Section 5.1.3), a receiver determines
   the E parameter used for this source block.

   In situations where E is fixed (specified by the FFCI/FSSI with S =
   1, Section 5.1.1.2), then E must be greater or equal to the size of
   the largest ADU of this source block plus three (for the prepended 3
   bytes, see below).  If this is not the case, an error is returned.
   How to handle this error is use-case specific (e.g., a larger E
   parameter may be communicated to the receivers in an updated FFCI
   message, using an appropriate mechanism) and is not considered by
   this specification.

   The ADU block is always encoded as a single source block.  There are
   a total of B <= max_B ADUs in this ADU block.  For the ADU i, with 0
   <= i <= B-1, 3 bytes are prepended (Figure 2):

   o  The first byte, F[i] (Flow ID), contains the integer identifier
      associated to the source ADU flow to which this ADU belongs to.
      It is assumed that a single byte is sufficient, or said
      differently, that no more than 256 flows will be protected by a
      single instance of the FECFRAME framework.

   o  The following two bytes, L[i] (Length), contain the length of this
      ADU, in network byte order (i.e., big endian).  This length is for
      the ADU itself and does not include the F[i], L[i], or Pad[i]
      fields.

   Then zero padding is added to ADU i (if needed) in field Pad[i], for
   alignment purposes up to a size of exactly E bytes.  The data unit
   resulting from the ADU i and the F[i], L[i] and Pad[i] fields, is
   called ADU Information (or ADUI).  Each ADUI contributes to exactly
   one source symbol of the source block.











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                        Encoding Symbol Length (E)
   < -------------------------------------------------------------- >
   +----+----+-----------------------+------------------------------+
   |F[0]|L[0]|        ADU[0]         |            Pad[0]            |
   +----+----+----------+------------+------------------------------+
   |F[1]|L[1]| ADU[1]   |                         Pad[1]            |
   +----+----+----------+-------------------------------------------+
   |F[2]|L[2]|                    ADU[2]                            |
   +----+----+------+-----------------------------------------------+
   |F[3]|L[3]|ADU[3]|                             Pad[3]            |
   +----+----+------+-----------------------------------------------+
   \_______________________________  _______________________________/
                                   \/
                          simple FEC encoding

   +----------------------------------------------------------------+
   |                            Repair 4                            |
   +----------------------------------------------------------------+
   .                                                                .
   .                                                                .
   +----------------------------------------------------------------+
   |                            Repair 7                            |
   +----------------------------------------------------------------+

    Figure 2: Source block creation, for code rate 1/2 (equal number of
         source and repair symbols, 4 in this example), and S = 0.

   Note that neither the initial 3 bytes nor the optional padding are
   sent over the network.  However, they are considered during FEC
   encoding.  It means that a receiver who lost a certain FEC source
   packet (e.g., the UDP datagram containing this FEC source packet)
   will be able to recover the ADUI if FEC decoding succeeds.  Thanks to
   the initial 3 bytes, this receiver will get rid of the padding (if
   any) and identify the corresponding ADU flow.

5.  LDPC-Staircase FEC Scheme for Arbitrary ADU Flows

5.1.  Formats and Codes

5.1.1.  FEC Framework Configuration Information

   The FEC Framework Configuration Information (or FFCI) includes
   information that MUST be communicated between the sender and
   receiver(s).  More specifically, it enables the synchronization of
   the FECFRAME sender and receiver instances.  It includes both
   mandatory elements and scheme-specific elements, as detailed below.





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5.1.1.1.  Mandatory Information

   o  FEC Encoding ID: the value assigned to this fully-specified FEC
      scheme MUST be XXX, as assigned by IANA (Section 8).

   When SDP is used to communicate the FFCI, this FEC Encoding ID is
   carried in the 'encoding-id' parameter.

5.1.1.2.  FEC Scheme-Specific Information

   The FEC Scheme Specific Information (FSSI) includes elements that are
   specific to the present FEC scheme.  More precisely:

   o  PRNG seed (seed): a non-negative 32 bit integer used as the seed
      of the Pseudo Random Number Generator, as defined in [RFC5170].

   o  Encoding symbol length (E): a non-negative integer that indicates
      either the length of each encoding symbol in bytes (strict mode,
      i.e., if S = 1), or the maximum length of any encoding symbol
      (i.e., if S = 0).

   o  Strict (S) flag: when set to 1 this flag indicates that the E
      parameter is the actual encoding symbol length value for each
      block of the session (unless otherwise notified by an updated FFCI
      if this possibility is considered by the use-case or CDP).  When
      set to 0 this flag indicates that the E parameter is the maximum
      encoding symbol length value for each block of the session (unless
      otherwise notified by an updated FFCI if this possibility is
      considered by the use-case or CDP).

   o  N1 minus 3 (n1m3): an integer between 0 (default) and 7,
      inclusive.  The number of "1s" per column in the left side of the
      parity check matrix, N1, is then equal to N1m3 + 3, as specified
      in [RFC5170].

   These elements are required both by the sender (LDPC-Staircase
   encoder) and the receiver(s) (LDPC-Staircase decoder).

   When SDP is used to communicate the FFCI, this FEC scheme-specific
   information is carried in the 'fssi' parameter in textual
   representation as specified in [RFC6364].  For instance:

   fssi=seed:1234,E:1400,S:0,n1m3:0

   If another mechanism requires the FSSI to be carried as an opaque
   octet string (for instance after a Base64 encoding), the encoding
   format consists of the following 7 octets:




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   o  PRNG seed (seed): 32 bit field.

   o  Encoding symbol length (E): 16 bit field.

   o  Strict (S) flag: 1 bit field.

   o  Reserved: a 4 bit field that MUST be set to zero.

   o  N1m3 parameter (n1m3): 3 bit field.


    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      PRNG seed (seed)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Encoding Symbol Length (E)  |S| resvd | n1m3|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: FSSI encoding format.

5.1.2.  Explicit Source FEC Payload ID

   A FEC source packet MUST contain an Explicit Source FEC Payload ID
   that is appended to the end of the packet as illustrated in Figure 4.

   +--------------------------------+
   |           IP Header            |
   +--------------------------------+
   |        Transport Header        |
   +--------------------------------+
   |              ADU               |
   +--------------------------------+
   | Explicit Source FEC Payload ID |
   +--------------------------------+

    Figure 4: Structure of a FEC Source Packet with the Explicit Source
                              FEC Payload ID.

   More precisely, the Explicit Source FEC Payload ID is composed of the
   following fields (Figure 5):

   o  Source Block Number (SBN) (16 bit field): this field identifies
      the source block to which this FEC source packet belongs.

   o  Encoding Symbol ID (ESI) (16 bit field): this field identifies the
      source symbol contained in this FEC source packet.  This value is
      such that 0 <= ESI <= k - 1 for source symbols.



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   o  Source Block Length (k) (16 bit field): this field provides the
      number of source symbols for this source block, i.e., the k
      parameter.


    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 (SBN)   |   Encoding Symbol ID (ESI)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Source Block Length (k)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: Source FEC Payload ID encoding format.

5.1.3.  Repair FEC Payload ID

   A FEC repair packet MUST contain a Repair FEC Payload ID that is
   prepended to the repair symbol(s) as illustrated in Figure 6.  There
   MUST be a single repair symbol per FEC repair packet.

   +--------------------------------+
   |           IP Header            |
   +--------------------------------+
   |        Transport Header        |
   +--------------------------------+
   |      Repair FEC Payload ID     |
   +--------------------------------+
   |         Repair Symbol          |
   +--------------------------------+

      Figure 6: Structure of a FEC Repair Packet with the Repair FEC
                                Payload ID.

   More precisely, the Repair FEC Payload ID is composed of the
   following fields (Figure 7):

   o  Source Block Number (SBN) (16 bit field): this field identifies
      the source block to which the FEC repair packet belongs.

   o  Encoding Symbol ID (ESI) (16 bit field): this field identifies the
      repair symbol contained in this FEC repair packet.  This value is
      such that k <= ESI <= n - 1 for repair symbols.

   o  Source Block Length (k) (16 bit field): this field provides the
      number of source symbols for this source block, i.e., the k
      parameter.




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   o  Number of Encoding Symbols (n) (16 bit field): this field provides
      the number of encoding symbols for this source block, i.e., the n
      parameter.


    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 (SBN)   |   Encoding Symbol ID (ESI)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Source Block Length (k)    |  Number Encoding Symbols (n)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 7: Repair FEC Payload ID encoding format.

5.2.  Procedures

   The following procedures apply:

   o  The source block creation MUST follow the procedures specified in
      Section 4.3.

   o  The SBN value MUST start with value 0 for the first block of the
      ADU flow and MUST be incremented by 1 for each new source block.
      Wrapping to zero will happen for long sessions, after value 2^^16
      - 1.

   o  The ESI of encoding symbols MUST start with value 0 for the first
      symbol and MUST be managed sequentially.  The first k values (0 <=
      ESI <= k - 1) identify source symbols whereas the last n-k values
      (k <= ESI <= n - 1) identify repair symbols.

   o  The FEC repair packet creation MUST follow the procedures
      specified in Section 5.1.3.

5.3.  FEC Code Specification

   The present document inherits from [RFC5170] the specification of the
   core LDPC-Staircase codes for a packet erasure transmission channel.

   Because of the requirement to have exactly one encoding symbol per
   group, i.e., because G MUST be equal to 1 (Section 4.1), several
   parts of [RFC5170] are useless.  In particular, this is the case of
   Section 5.6.  "Identifying the G Symbols of an Encoding Symbol
   Group".






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6.  Security Considerations

   The FEC Framework document [RFC6363] provides a comprehensive
   analysis of security considerations applicable to FEC schemes.
   Therefore the present section follows the security considerations
   section of [RFC6363] and only discusses topics that are specific to
   the use of LDPC-Staircase codes.

6.1.  Attacks Against the Data Flow

6.1.1.  Access to Confidential Content

   The LDPC-Staircase FEC Scheme specified in this document does not
   change the recommendations of [RFC6363].  To summarize, if
   confidentiality is a concern, it is RECOMMENDED that one of the
   solutions mentioned in [RFC6363] is used, with special considerations
   to the way this solution is applied (e.g., is encryption applied
   before or after FEC protection, within the end-system or in a
   middlebox), to the operational constraints (e.g., performing FEC
   decoding in a protected environment may be complicated or even
   impossible) and to the threat model.

6.1.2.  Content Corruption

   The LDPC-Staircase FEC Scheme specified in this document does not
   change the recommendations of [RFC6363].  To summarize, it is
   RECOMMENDED that one of the solutions mentioned in [RFC6363] is used
   on both the FEC Source and Repair Packets.

6.2.  Attacks Against the FEC Parameters

   The FEC Scheme specified in this document defines parameters that can
   be the basis of several attacks.  More specifically, the following
   parameters of the FFCI may be modified by an attacker
   (Section 5.1.1.2):

   o  FEC Encoding ID: changing this parameter leads the receiver to
      consider a different FEC Scheme, which enables an attacker to
      create a Denial of Service (DoS).

   o  Encoding symbol length (E): setting this E parameter to a value
      smaller than the valid one enables an attacker to create a DoS
      since the repair symbols and certain source symbols will be larger
      than E, which is an incoherency for the receiver.  Setting this E
      parameter to a value larger than the valid one has similar impacts
      when S=1 since the received repair symbol size will be smaller
      than expected.  On the opposite it will not lead to any
      incoherency when S=0 since the actual symbol length value for the



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      block is determined by the size of any received repair symbol, as
      long as this value is smaller than E. However setting this E
      parameter to a larger value may have impacts on receivers that
      pre-allocate memory space in advance to store incoming symbols.

   o  Strict (S) flag: flipping this S flag from 0 to 1 (i.e., E is now
      considered as a strict value) enables an attacker to mislead the
      receiver if the actual symbol size varies over different source
      blocks.  Flipping this S flag from 1 to 0 has no major
      consequences unless the receiver requires to have a fixed E value
      (e.g., because the receiver pre-allocates memory space).

   o  N1 minus 3 (n1m3): changing this parameter leads the receiver to
      consider a different code, which enables an attacker to create a
      DoS.

   It is therefore RECOMMENDED that security measures are taken to
   guarantee the FFCI integrity, as specified in [RFC6363].  How to
   achieve this depends on the way the FFCI is communicated from the
   sender to the receiver, which is not specified in this document.

   Similarly, attacks are possible against the Explicit Source FEC
   Payload ID and Repair FEC Payload ID: by modifying the Source Block
   Number (SBN), or the Encoding Symbol ID (ESI), or the Source Block
   Length (k), or the Number Encoding Symbols (n), an attacker can
   easily corrupt the block identified by the SBN.  Other consequences,
   that are use-case and/or CDP dependant, may also happen.  It is
   therefore RECOMMENDED that security measures are taken to guarantee
   the FEC Source and Repair Packets as stated in [RFC6363].

6.3.  When Several Source Flows are to be Protected Together

   The LDPC-Staircase FEC Scheme specified in this document does not
   change the recommendations of [RFC6363].

6.4.  Baseline Secure FEC Framework Operation

   The LDPC-Staircase FEC Scheme specified in this document does not
   change the recommendations of [RFC6363] concerning the use of the
   IPsec/ESP security protocol as a mandatory to implement (but not
   mandatory to use) security scheme.  This is well suited to situations
   where the only insecure domain is the one over which the FEC
   Framework operates.

7.  Operations and Management Considerations

   The FEC Framework document [RFC6363] provides a comprehensive
   analysis of operations and management considerations applicable to



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   FEC schemes.  Therefore the present section only discusses topics
   that are specific to the use of LDPC-Staircase codes as specified in
   this document.

7.1.  Operational Recommendations

   LDPC-Staircase codes have excellent erasure recovery capabilities
   with large source blocks, close to ideal MDS codes.  For instance,
   independently of FECFRAME, with source block size k=1024 symbols,
   CR=2/3, N1=7, G=1, a hybrid ITerative/Maximum Likelihood (IT/ML)
   decoding approach (see below) and when all symbols are sent in a
   random order, the average overhead amounts to 0.237% (i.e., receiving
   2.43 symbols in addition to k enables a successful decoding with a
   probability 0.5) and an overhead of 1.46% (i.e., receiving 15 symbols
   in addition to k) is sufficient to reduce the decoding failure
   probability to 8.2*10^^-5.  This is why these codes are a good
   solution to protect a single high bitrate source flow as in
   [Matsuzono10], or to protect globally several mid-rate source flows
   within a single FECFRAME instance: in both cases the source block
   size can be assumed to be equal to a few hundreds (or more) source
   symbols.

   LDPC-Staircase codes are also a good solution whenever processing
   requirements at a software encoder or decoder must be kept to a
   minimum.  This is true when the decoder uses an IT decoding
   algorithm, or an ML algorithm (we use a Gaussian Elimination as the
   ML algorithm) when this latter is carefully implemented, or a mixture
   of both techniques which is the recommended solution
   [Cunche08][CunchePHD10][LDPC-codec-OpenFEC].  For instance an average
   decoding speed between 1.78 Gbps (overhead of 2 symbols in addition
   to k, corresponding to a very bad channel, close to the theoretical
   decoding limit, where ML decoding is required) and 3.41 Gbps
   (corresponding to a medium quality channel where IT decoding is
   sufficient) is easily achieved with a source block size composed of
   k=1024 source symbols, a code rate CR=2/3 (i.e., 512 repair symbols),
   1024 byte long symbols, G=1, and N1=7, on an Intel Xeon 5120/1.86GHz
   workstation running Linux/64 bits.  Under the same conditions, on a
   Samsung Galaxy SII (GT-I9100P model, featuring an ARM Cortex-A9/1.2
   GHz processor and running Android 2.3.4), decoding speed is between
   278 Mbps (overhead of 2 symbols and ML decoding) and 626 Mbps (IT
   decoding).

   As the source block size decreases, the erasure recovery capabilities
   of LDPC codes in general also decrease.  In the case of LDPC-
   Staircase codes, in order to limit this phenomenon, it is recommended
   to use a value of the N1 parameter at least equal to 7 (e.g.,
   experiments carried out in [Matsuzono10] use N1=7 if k=170 symbols,
   and N1=5 otherwise).  For instance, independently of FECFRAME, with



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   source block size k=256 symbols, CR=2/3, N1=7, and G=1, the average
   overhead amounts to 0.706% (i.e., receiving 1.8 symbols in addition
   to k enables a successful decoding with a probability 0.5), and an
   overhead of 5.86% (i.e., receiving 15 symbols ina addition to k) is
   sufficient to reduce the decoding failure probability to 5.9*10^^-5.

   With very small source blocks (e.g., a few tens of symbols), using
   for instance Reed-Solomon codes [SIMPLE_RS] or 2D parity check codes
   may be more appropriate.

   The way the FEC Repair Packets are transmitted is of high importance.
   A good strategy, that works well for any kind of channel loss model,
   consists in sending FEC Repair Packets in random order (rather than
   in sequence) while FEC Source Packets are sent first and in sequence.
   Sending all packets in a random order is another possibility, but it
   requires that all repair symbols for a source block be produced
   first, which adds some extra delay at a sender.

8.  IANA Considerations

   This document registers one value in the FEC Framework (FECFRAME) FEC
   Encoding IDs registry [RFC6363] as follows:

   o  XXX refers to the Simple LDPC-Staircase FEC Scheme for Arbitrary
      Packet Flows, as defined in Section 5 of this document.

9.  Acknowledgments

   The authors want to thank K. Matsuzono, J. Detchart and H. Asaeda for
   their contributions in evaluating the use of LDPC-Staircase codes in
   the context of FECFRAME [Matsuzono10].

10.  References

10.1.  Normative References

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

   [RFC5170]             Roca, V., Neumann, C., and D. Furodet, "Low
                         Density Parity Check (LDPC) Forward Error
                         Correction", RFC 5170, June 2008.

   [RFC6363]             Watson, M., Begen, A., and V. Roca, "Forward
                         Error Correction (FEC) Framework", RFC 6363,
                         September 2011.

   [RFC6364]             Begen, A., "Session Description Protocol



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                         Elements for the Forward Error Correction (FEC)
                         Framework", RFC 6364, October 2011.

10.2.  Informative References

   [RFC3453]             Luby, M., Vicisano, L., Gemmell, J., Rizzo, L.,
                         Handley, M., and J. Crowcroft, "The Use of
                         Forward Error Correction (FEC) in Reliable
                         Multicast", RFC 3453, December 2002.

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

   [RFC5510]             Lacan, J., Roca, V., Peltotalo, J., and S.
                         Peltotalo, "Reed-Solomon Forward Error
                         Correction (FEC) Schemes", RFC 5510,
                         April 2009.

   [SIMPLE_RS]           Roca, V., Cunche, M., Lacan, J., Bouabdallah,
                         A., and K. Matsuzono, "Simple Reed-Solomon
                         Forward Error Correction (FEC) Scheme for
                         FECFRAME",
                         draft-ietf-fecframe-simple-rs-04 (Work in
                         Progress), October 2012.

   [RFC5053]             Luby, M., Shokrollahi, A., Watson, M., and T.
                         Stockhammer, "Raptor Forward Error Correction
                         Scheme for Object Delivery", RFC 5053,
                         June 2007.

   [RFC5740]             Adamson, B., Bormann, C., Handley, M., and J.
                         Macker, "NACK-Oriented Reliable Multicast
                         (NORM) Transport Protocol", RFC 5740,
                         November 2009.

   [RFC5775]             Luby, M., Watson, M., and L. Vicisano,
                         "Asynchronous Layered Coding (ALC) Protocol
                         Instantiation", RFC 5775, April 2010.

   [Cunche08]            Cunche, M. and V. Roca, "Optimizing the Error
                         Recovery Capabilities of LDPC-Staircase Codes
                         Featuring a Gaussian Elimination Decoding
                         Scheme",  10th IEEE International Workshop on
                         Signal Processing for Space Communications
                         (SPSC'08), October 2008.

   [CunchePHD10]         Cunche, M., "High performances AL-FEC codes for



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                         the erasure channel : variation around LDPC
                         codes", PhD dissertation (in French) (http://
                         tel.archives-ouvertes.fr/tel-00451336/en/),
                         June 2010.

   [Matsuzono10]         Matsuzono, K., Detchart, J., Cunche, M., Roca,
                         V., and H. Asaeda, "Performance Analysis of a
                         High-Performance Real-Time Application with
                         Several AL-FEC Schemes", 35th Annual IEEE
                         Conference on Local Computer Networks (LCN
                         2010), October 2010.

   [LDPC-codec]          Cunche, M., Roca, V., Neumann, C., and J.
                         Laboure, "LDPC-Staircase/LDPC-Triangle Codec
                         Reference Implementation", INRIA Rhone-Alpes
                         and STMicroelectronics,
                         <http://planete-bcast.inrialpes.fr/>.

   [LDPC-codec-OpenFEC]  "The OpenFEC project", <http://openfec.org/>.

Authors' Addresses

   Vincent Roca
   INRIA
   655, av. de l'Europe
   Inovallee; Montbonnot
   ST ISMIER cedex  38334
   France

   EMail: vincent.roca@inria.fr
   URI:   http://planete.inrialpes.fr/people/roca/


   Mathieu Cunche
   INSA-Lyon/INRIA
   Laboratoire CITI
   6 av. des Arts
   Villeurbanne cedex  69621
   France

   EMail: mathieu.cunche@inria.fr
   URI:   http://mathieu.cunche.free.fr/









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   Jerome Lacan
   ISAE, Univ. of Toulouse
   10 av. Edouard Belin; BP 54032
   Toulouse cedex 4  31055
   France

   EMail: jerome.lacan@isae.fr
   URI:   http://personnel.isae.fr/jerome-lacan/











































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