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rfc:rfc6816

Internet Engineering Task Force (IETF) V. Roca Request for Comments: 6816 INRIA Category: Standards Track M. Cunche ISSN: 2070-1721 INSA-Lyon/INRIA

                                                              J. Lacan
                                               ISAE, Univ. of Toulouse
                                                         December 2012
          Simple Low-Density Parity Check (LDPC) Staircase
         Forward Error Correction (FEC) Scheme for FECFRAME

Abstract

 This document describes a fully specified simple Forward Error
 Correction (FEC) scheme for Low-Density Parity Check (LDPC) Staircase
 codes that can be used to protect media streams along the lines
 defined by FECFRAME.  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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6816.

Roca, et al. Standards Track [Page 1] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Roca, et al. Standards Track [Page 2] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

Table of Contents

 1. Introduction ....................................................4
 2. Terminology .....................................................5
 3. Definitions Notations and Abbreviations .........................5
    3.1. Definitions ................................................5
    3.2. Notations ..................................................7
    3.3. Abbreviations ..............................................8
 4. Common Procedures Related to the ADU Block and Source
    Block Creation ..................................................8
    4.1. Restrictions ...............................................9
    4.2. ADU Block Creation .........................................9
    4.3. Source Block Creation .....................................11
 5. LDPC-Staircase FEC Scheme for Arbitrary ADU Flows ..............13
    5.1. Formats and Codes .........................................13
         5.1.1. FEC Framework Configuration Information ............13
         5.1.2. Explicit Source FEC Payload ID .....................14
         5.1.3. Repair FEC Payload ID ..............................16
    5.2. Procedures ................................................17
    5.3. FEC Code Specification ....................................17
 6. Security Considerations ........................................17
    6.1. Attacks against the Data Flow .............................17
         6.1.1. Access to Confidential Content .....................17
         6.1.2. Content Corruption .................................18
    6.2. Attacks against the FEC Parameters ........................18
    6.3. When Several Source Flows Are to Be Protected Together ....19
    6.4. Baseline Secure FEC Framework Operation ...................19
 7. Operations and Management Considerations .......................19
    7.1. Operational Recommendations ...............................19
 8. IANA Considerations ............................................21
 9. Acknowledgments ................................................21
 10. References ....................................................21
    10.1. Normative References .....................................21
    10.2. Informative References ...................................22

Roca, et al. Standards Track [Page 3] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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 (CDPs) and applications [RFC3453].
 "Forward Error Correction (FEC) Framework" [RFC6363] 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, "Forward Error Correction
 (FEC) Building Block" [RFC5052] describes a generic framework to use
 FEC schemes with objects (e.g., files) delivery applications based on
 either the Asynchronous Layered Coding (ALC) [RFC5775] or the NACK-
 Oriented Reliable Multicast (NORM) [RFC5740] 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, "Reed-Solomon Forward Error Correction
 (FEC) Schemes" [RFC5510] 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 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 document inherits from [RFC5170], Section 6 "Full Specification
 of the LDPC-Staircase Scheme", the specifications of the core LDPC-
 Staircase codes, and from Section 5.7 "Pseudo-Random Number
 Generator", the specifications of the PRNG used by these codes.
 Therefore, this document specifies only the information specific to

Roca, et al. Standards Track [Page 4] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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 7 that
    specifies a simple way of using LDPC-Staircase codes in order to
    protect arbitrary Application Data Unit (ADU) flows.
 Therefore Sections 4 and 5 (except Section 5.7, see above) of
 [RFC5170], that define [RFC5052] specific Formats and Procedures, are
 not considered and are replaced by FECFRAME specific Formats and
 Procedures.
 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.  Those in the
 list below 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.

Roca, et al. Standards Track [Page 5] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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.
 The following are FECFRAME 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 FECFRAME.
 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 that
    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):

Roca, et al. Standards Track [Page 6] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 +----------------------+
 |     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.  Those in the list below
 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.

Roca, et al. Standards Track [Page 7] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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.
 The following are FECFRAME 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    Application Data Unit
 ESI    Encoding Symbol ID
 FEC    Forward Error (or Erasure) Correction
 FFCI   FEC Framework Configuration Information
 FSSI   FEC Scheme-Specific Information
 LDPC   Low-Density Parity Check
 MDS    Maximum Distance Separable
 PRNG   Pseudo-Random Number Generator
 SDP    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 related source block creation, for the FEC scheme
 considered.

Roca, et al. Standards Track [Page 8] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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

 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 the use of the 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 be
 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) and 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

Roca, et al. Standards Track [Page 9] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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:
    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.

Roca, et al. Standards Track [Page 10] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

4.3. Source Block Creation

 In its most general form, FECFRAME 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 be 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 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.  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 FECFRAME.
 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.

Roca, et al. Standards Track [Page 11] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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.
                      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.

Roca, et al. Standards Track [Page 12] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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.

5.1.1.1. Mandatory Information

 o  FEC Encoding ID: the value assigned to this fully specified FEC
    scheme MUST be 7, 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].

Roca, et al. Standards Track [Page 13] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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:
 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.

Roca, et al. Standards Track [Page 14] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

                +--------------------------------+
                |           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.
 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

Roca, et al. Standards Track [Page 15] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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 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.
 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

Roca, et al. Standards Track [Page 16] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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
 (see Section 1).
 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 not of use.  In particular, this is the case
 of Section 5.6, "Identifying the G Symbols of an Encoding Symbol
 Group".

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] be used, with special considerations

Roca, et al. Standards Track [Page 17] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 to the way this solution is applied (e.g., Is encryption applied
 before or after FEC protection?  Is it 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] be 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.  Contrarily, it will not lead to any incoherency
    when S=0 since the actual symbol length value for the 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.

Roca, et al. Standards Track [Page 18] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 Therefore, it is RECOMMENDED that security measures be 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 dependent, may also happen.  It is
 therefore RECOMMENDED that security measures be 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
 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, let us consider a source block of size
 k=1024 symbols, CR=2/3 (i.e., 512 repair symbols are added), N1=7,
 G=1, a transmission scheme where all the symbols are sent in a random
 order, and a hybrid ITerative/Maximum Likelihood (IT/ML) decoder (see
 below).  An ideal MDS code with code rate 2/3 can recover from
 erasures up to a 33.33% channel loss rate.  With LDPC-Staircase
 codes, the average overhead amounts to 0.237% (i.e., receiving 2.43
 symbols in addition to k, which corresponds to a 33.18% channel loss

Roca, et al. Standards Track [Page 19] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 rate, enables a successful decoding with a probability 0.5), and an
 overhead of 1.46% (i.e., receiving 15 symbols in addition to k, which
 corresponds to a 32.36% channel loss rate) is sufficient to reduce
 the probability that decoding fails down 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 hundred (or
 more) source symbols.
 LDPC-Staircase codes are also a good solution whenever the processing
 load at a software encoder or decoder must be kept to a minimum.
 This is true when the decoder uses an IT decoding algorithm, an ML
 algorithm (we use a Gaussian Elimination as the ML algorithm) when
 carefully implemented, or a mixture of both techniques, which is the
 recommended solution [Cunche08][CunchePHD10][LDPC-codec-OpenFEC].
 Let us consider the same conditions as above (k=1024 source symbols,
 CR=2/3, N1=7, G=1), with encoding symbols of size 1024 bytes.  With
 an Intel Xeon 5120/1.86 GHz workstation running Linux/64 bits, the
 average decoding speed is between 1.78 Gbps (overhead of 2 symbols in
 addition to k, corresponding to a very bad channel with a 33.20% loss
 rate, close to the theoretical decoding limit, where ML decoding is
 required) and 3.91 Gbps (corresponding to a good channel with a 5%
 loss rate only, where IT decoding is sufficient).  Under the same
 conditions, on a Samsung Galaxy SII smartphone (GT-I9100P model,
 featuring an ARM Cortex-A9/1.2 GHz processor and running Android
 2.3.4), the decoding speed is between 397 Mbps (bad channel with a
 33.20% loss rate, close to the theoretical decoding limit) and 813
 Mbps (good channel with a 5% loss rate only).
 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 a
 source block of size k=256 symbols, CR=2/3 (i.e., 128 repair symbols
 are added), 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 in addition to k) is sufficient to reduce the
 decoding failure probability to 5.9*10^^-5.
 The processing load also decreases with the source block size.  For
 instance, under these conditions (k=256 source symbols, CR=2/3, N1=7,
 and G=1), with encoding symbols of size 1024 bytes, on a Samsung
 Galaxy SII smartphone, the decoding speed is between 518 Mbps (bad
 channel) and 863 Mbps (good channel with a 5% loss rate only).

Roca, et al. Standards Track [Page 20] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

 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  7 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", BCP 14, RFC 2119, March 1997.
 [RFC5170]    Roca, V., Neumann, C., and D. Furodet, "Low Density
              Parity Check (LDPC) Staircase and Triangle Forward Error
              Correction (FEC) Schemes", RFC 5170, June 2008.
 [RFC6363]    Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", RFC 6363, October 2011.
 [RFC6364]    Begen, A., "Session Description Protocol Elements for
              the Forward Error Correction (FEC) Framework", RFC 6364,
              October 2011.

Roca, et al. Standards Track [Page 21] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

10.2. Informative References

 [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 the
              erasure channel : variation around LDPC codes", PhD
              dissertation (in French) (http://
              tel.archives-ouvertes.fr/tel-00451336/en/), June 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/>.
 [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.
 [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.
 [RFC5053]    Luby, M., Shokrollahi, A., Watson, M., and T.
              Stockhammer, "Raptor Forward Error Correction Scheme for
              Object Delivery", RFC 5053, October 2007.
 [RFC5510]    Lacan, J., Roca, V., Peltotalo, J., and S.  Peltotalo,
              "Reed-Solomon Forward Error Correction (FEC) Schemes",
              RFC 5510, April 2009.

Roca, et al. Standards Track [Page 22] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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

Roca, et al. Standards Track [Page 23] RFC 6816 Simple LDPC-Staircase FEC Scheme December 2012

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/
 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/

Roca, et al. Standards Track [Page 24]

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