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Versions: 00 01                                                         
FecFrame                                                         V. Roca
Internet-Draft                                                     INRIA
Intended status: Experimental                                  M. Cunche
Expires: April 25, 2011                                            NICTA
                                                                J. Lacan
                                                          ISAE/LAAS-CNRS
                                                        October 22, 2010


Simple LDPC-Staircase Forward Error Correction (FEC) Scheme for FECFRAME
                      draft-roca-fecframe-ldpc-01

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 25, 2011.

Copyright Notice

   Copyright (c) 2010 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 . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Source Block Creation  . . . . . . . . . . . . . . . . . .  8
   5.  LDPC-Staircase FEC Scheme for Arbitrary ADU Flows  . . . . . . 10
     5.1.  Formats and Codes  . . . . . . . . . . . . . . . . . . . . 10
       5.1.1.  FEC Framework Configuration Information  . . . . . . . 10
       5.1.2.  Explicit Source FEC Payload ID . . . . . . . . . . . . 12
       5.1.3.  Repair FEC Payload ID  . . . . . . . . . . . . . . . . 13
     5.2.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.3.  FEC Code Specification . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     6.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Attacks Against the Data Flow  . . . . . . . . . . . . . . 15
       6.2.1.  Access to Confidential Contents  . . . . . . . . . . . 15
       6.2.2.  Content Corruption . . . . . . . . . . . . . . . . . . 15
     6.3.  Attacks Against the FEC Parameters . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18








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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
   [FECFRAME-FRAMEWORK] 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 with objects (e.g., files) delivery
   applications based on the ALC [RFC5775] and 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 [SPSC08].
   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 [SPSC08][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 to protect a
   single high bitrate source flow as in [LCN10], or to protect globally
   several mid-rate source flows within a single FECFRAME instance.
   They are also a good solution whenever processing requirements at a
   software encoder or decoder must be kept to a minimum, independently
   of the ADU flow(s) bitrate.

   This documents inherits from [RFC5170] the specifications of the core
   LDPC-Staircase codes.  Therefore this document specifies only the



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   information specific to the FECFRAME context and refers to [RFC5170]
   for the core specifications of the codes.  To that purpose, the
   present document introduces:
   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 ADU flows.

   Finally, a publicly available reference implementation of these codes
   is available and distributed under a GNU/LGPL (Lesser General Public
   License) [LDPC-codec].


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 Reed-Solomon 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.

   Some of them are FECFRAME framework specific and are in line with



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   [FECFRAME-FRAMEWORK]:
   Application Data Unit (ADU):  a unit of data coming from (sender) or
      given to (receiver) the media delivery application.  Depending on
      the use-case, an ADU can use an RTP encapsulation.  In this
      specification, there is always one source symbol per ADU.
   (Source) ADU Flow:  a flow of ADUs from a media delivery application
      and to which FEC protection is applied.  Depending on the use-
      case, several ADU flows can 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 F[], L[], and 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:  the FEC scheme specific
      information that enables the synchronization of the FECFRAME
      sender and receiver instances.
   FEC Source Packet:  a data packet submitted to (sender) or received
      from (receiver) the transport protocol.  It contains an ADU along
      with its optional Explicit Source FEC Payload ID.
   FEC Repair Packet:  a repair packet submitted to (sender) or received
      from (receiver) the transport protocol.  It contains a repair
      symbol along with its Repair FEC Payload ID.

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























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   +----------------------+
   |     Application      |
   +----------------------+
              |
    ADU flow  | (1) Application Data Unit (ADU)
              v
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |                |
   |                      |------------------------- >|  FEC Scheme    |
   |(2) Construct an ADU  | (4) Source Symbols for    |                |
   |    block             |     this Source Block     |(5) Perform FEC |
   |(3) Construct ADU Info|                           |    Encoding    |
   |(7) Construct FEC Src |< -------------------------|                |
   |    Packets and FEC   |(6) Ex src FEC Payload Ids,|                |
   |    Repair Packets    |    Repair FEC Payload Ids,|                |
   +----------------------+    Repair Symbols         +----------------+
       |             |
       |(8) FEC Src  |(8') FEC Repair
       |    packets  |     packets
       v             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.
   N1     denotes the target number of "1s" per column in the left side
          of the parity check matrix.
   N1m3   denotes the value N1 - 3.
   a^^b   denotes a raised to the power b.

   Some of them are FECFRAME framework specific:







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   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.
   LDPC   stands for Low Density Parity Check.
   RS     stands for Reed-Solomon.
   MDS    stands for Maximum Distance Separable code.


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;
   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

   Several aspects must be considered, that impact the ADU block
   creation:
   o  the maximum source block size (max_k parameter);
   o  the potential real-time constraints, that impact the maximum ADU
      block size, since the larger the block size, the larger the
      decoding delay;
   We now detail each of these aspects.

   The maximum source block length 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:



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      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 MAY impose other limitations on the maximum
   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:
      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 usually have real-time constraints.  It means
   that the maximum number of ADUs of an ADU block must not exceed a
   certain threshold since it directly impacts the decoding delay.  It
   is the role of the developer, who knows the flow real-time features,
   to define an appropriate upper bound to the ADU block size, max_rt.

   If we take into account these constraints, we find: max_B =
   min(max_k, max_rt).  Then max_B gives 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 RS FEC scheme requires a
   constant encoding symbol size (E parameter) per source block.  Since
   this specification requires that there is only one 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 FCCI/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.



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   In situations where E is fixed (specified by the FCCI/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, FID[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 FID[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 to 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

   FEC Encoding ID:  the value assigned to this fully-specified FEC
      scheme MUST be XXX, as assigned by IANA (Section 7).
   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:
   PRNG seed (seed):  a non-negative 32 bit integer used as the seed of
      the Pseudo Random Number Generator, as defined in [RFC5170].
   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).
   Strict (S) flag:  when set to 1 this flag indicates that the E
      parameter is valid for the whole session, unless otherwise
      notified.  When set to 0 this flag indicates that the E parameter
      is only the maximum length of each encoding symbol, for the whole
      session, unless otherwise notified.
   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 [SDP_ELEMENTS].  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:
   o  PRNG seed (seed): 32 bit field.
   o  Encoding symbol length (E): 16 bit field.
   o  Strict (S) flag: 1 bit field.
   o  N1m3 parameter (n1m3): 7 bit field.










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



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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):
   Source Block Number (SBN) (16 bit field):  this field identifies the
      source block to which the FEC repair packet belongs.
   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.
   Source Block Length (k) (16 bit field):  this field provides the
      number of source symbols for this source block, i.e., the k
      parameter.
   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:





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   o  The source block creation procedures are specified in Section 4.3.
   o  The SBN value is incremented for each new source block, starting
      at 0 for the first block of the ADU flow.  Wrapping to zero will
      happen for long sessions, after value 2^^16 - 1.
   o  The ESI of encoding symbols is managed sequentially, starting at 0
      for the first symbol.  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 procedures are 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".


6.  Security Considerations

6.1.  Problem Statement

   A content delivery system is potentially subject to many attacks.
   Some of them target the network (e.g., to compromise the routing
   infrastructure, by compromising the congestion control component),
   others target the Content Delivery Protocol (CDP) (e.g., to
   compromise its normal behavior), and finally some attacks target the
   content itself.  Since this document focuses on various FEC schemes,
   this section only discusses the additional threats that their use
   within the FECFRAME framework can create to an arbitrary CDP.

   More specifically, these attacks may have several goals:
   o  those that are meant to give access to a confidential content
      (e.g., in case of a non-free content),
   o  those that try to corrupt the ADU Flows being transmitted (e.g.,
      to prevent a receiver from using it),
   o  and those that try to compromise the receiver's behavior (e.g., by
      making the decoding of an object computationally expensive).
   These attacks can be launched either against the data flow itself
   (e.g., by sending forged FEC Source/Repair Packets) or against the
   FEC parameters that are sent either in-band (e.g., in the Repair FEC
   Payload ID) or out-of-band (e.g., in a session description).




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6.2.  Attacks Against the Data Flow

   First of all, let us consider the attacks against the data flow.

6.2.1.  Access to Confidential Contents

   Access control to the ADU Flow being transmitted is typically
   provided by means of encryption.  This encryption can be done within
   the content provider itself, by the application (for instance by
   using the Secure Real-time Transport Protocol (SRTP) [RFC3711]), or
   at the Network Layer, on a packet per packet basis when IPSec/ESP is
   used [RFC4303].  If confidentiality is a concern, it is RECOMMENDED
   that one of these solutions be used.  Even if we mention these
   attacks here, they are not related nor facilitated by the use of FEC.

6.2.2.  Content Corruption

   Protection against corruptions (e.g., after sending forged FEC
   Source/Repair Packets) is achieved by means of a content integrity
   verification/sender authentication scheme.  This service is usually
   provided at the packet level.  In this case, after removing all
   forged packets, the ADU Flow may be sometimes recovered.  Several
   techniques can provide this source authentication/content integrity
   service:
   o  at the application level, the Secure Real-time Transport Protocol
      (SRTP) [RFC3711] provides several solutions to authenticate the
      source and check the integrity of RTP and RTCP messages, among
      other services.  For instance, associated to the Timed Efficient
      Stream Loss-Tolerant Authentication (TESLA) [RFC4383], SRTP is an
      attractive solution that is robust to losses, provides a true
      authentication/integrity service, and does not create any
      prohibitive processing load or transmission overhead.  Yet,
      checking a packet requires a small delay (a second or more) after
      its reception with TESLA.  Other building blocks can be used
      within SRTP to provide authentication/content integrity services.
   o  at the Network Layer, IPSec/ESP offers (among other services) an
      integrity verification mechanism that can be used to provide
      authentication/content integrity services.

   It is up to the developer and the person in charge of deployment, who
   know the security requirements and features of the target application
   area, to define which solution is the most appropriate.  Nonetheless
   it is RECOMMENDED that at least one of these techniques be used.

6.3.  Attacks Against the FEC Parameters

   Let us now consider attacks against the FEC parameters included in
   the FFCI that are usually sent out-of-band (e.g., in a session



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   description).  Attacks on these FEC parameters can prevent the
   decoding of the associated object.  For instance modifying the PRNG
   seed or N1m3 fields will lead a receiver to consider a different
   parity check matrix, i.e., a different code.  Modifying the E
   parameter will lead a receiver to consider bad Repair Symbols for a
   received FEC Repair Packet.

   It is therefore RECOMMENDED that security measures be taken to
   guarantee the FFCI integrity.  When the FFCI is sent out-of-band in a
   session description, this latter SHOULD be protected, for instance by
   digitally signing it.

   Attacks are also possible against some FEC parameters included in the
   Explicit Source FEC Payload ID and Repair FEC Payload ID.  For
   instance modifying the Source Block Number of a FEC Source of Repair
   Packet will lead a receiver to assign this packet to a wrong block.

   It is therefore RECOMMENDED that security measures be taken to
   guarantee the Explicit Source FEC Payload ID and Repair FEC Payload
   ID integrity.  To that purpose, one of the packet-level source
   authentication/content integrity techniques of Section 6.2.2 can be
   used.


7.  IANA Considerations

   The FEC Encoding ID value is subject to IANA registration.

   TBD


8.  Acknowledgments

   TBD


9.  References

9.1.  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.

   [FECFRAME-FRAMEWORK]



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              Watson, M., "Forward Error Correction (FEC) Framework",
              draft-ietf-fecframe-framework-10 (Work in Progress),
              September 2010.

   [SDP_ELEMENTS]
              Begen, A., "SDP Elements for FEC Framework",
              draft-ietf-fecframe-sdp-elements-10 (Work in Progress),
              October 2010.

9.2.  Informative References

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

   [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.

   [RFC5053]  Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer,
              "Raptor Forward Error Correction Scheme", 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.

   [SPSC08]   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 the
              erasure channel : variation around LDPC codes", PhD
              dissertation (in
              French) (http://tel.archives-ouvertes.fr/tel-
              00451336/en/), June 2010.

   [LCN10]    Matsuzono, K., Detchart, J., Cunche, M., Roca, V., and H.



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              Asaeda, "Performance Analysis of a High-Performance Real-
              Time Application with Several AL-FEC Schemes",  35th
              Annual IEEE Conference on Local Computer Networks 2010
              (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/>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              February 2006.


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
   NICTA
   Australia

   Email: mathieu.cunche@nicta.com.au
   URI:   http://mathieu.cunche.free.fr/








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   Jerome Lacan
   ISAE/LAAS-CNRS
   1, place Emile Blouin
   Toulouse  31056
   France

   Email: jerome.lacan@isae.fr
   URI:   http://dmi.ensica.fr/auteur.php3?id_auteur=5











































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