Reed-Solomon Forward Error Correction (FEC) Schemes for FECFRAME
draft-roca-fecframe-rs-02

Abstract

This document describes two fully-specified FEC schemes for Reed-Solomon codes that can be used to protect media streams along the lines defined by the FECFRAME framework. Reed-Solomon codes belong to the class of Maximum Distance Separable (MDS) codes which means they offer optimal protection against packet erasures. They are also systematic codes, which means that the source symbols are part of the encoding symbols. The price to pay is a limit on the maximum source block size, on the maximum number of encoding symbols, and a computational complexity higher than that of sparse parity check based FEC codes. However, this complexity remains compatible with software codecs.

The first scheme is for Reed-Solomon codes over GF(2^^m), with m in {2..16}, a simple FEC encoding and arbitrary packet flows. The second scheme is similar to the first scheme, with the exception that it is for a single sequenced flow.

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Table of Contents

1.  Introduction
2.  Terminology
3.  Definitions Notations and Abbreviations
    3.1.  Definitions
    3.2.  Notations
    3.3.  Abbreviations
4.  Common Procedures Related to the ADU Block and Source Block Creation
    4.1.  Problem Statement
    4.2.  Source Block Creation
5.  Reed-Solomon FEC Encoding Scheme over GF(2^^m) for Arbitrary ADU Flows
    5.1.  Formats and Codes
        5.1.1.  FEC Framework Configuration Information
        5.1.2.  Explicit Source FEC Payload ID
        5.1.3.  Repair FEC Payload ID
    5.2.  Procedures
    5.3.  FEC Code Specification
6.  Reed-Solomon FEC Encoding Scheme over GF(2^^m) for a Single Sequenced ADU Flow
7.  Security Considerations
    7.1.  Problem Statement
    7.2.  Attacks Against the Data Flow
        7.2.1.  Access to Confidential Contents
        7.2.2.  Content Corruption
    7.3.  Attacks Against the FEC Parameters
8.  IANA Considerations
9.  Acknowledgments
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Authors' Addresses

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] (Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and J. Crowcroft, “The Use of Forward Error Correction (FEC) in Reliable Multicast,” December 2002.). The [FECFRAME‑FRAMEWORK] (Watson, M., “Forward Error Correction (FEC) Framework,” March 2010.) 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] (Watson, M., Luby, M., and L. Vicisano, “Forward Error Correction (FEC) Building Block,” August 2007.) document describes a generic framework to use FEC schemes with with objects (e.g., files) delivery applications based on the ALC [RMT‑PI‑ALC] (Luby, M., Watson, M., and L. Vicisano, “Asynchronous Layered Coding (ALC) Protocol Instantiation,” November 2009.) and NORM [RFC5740] (Adamson, B., Bormann, C., Handley, M., and J. Macker, “Negative-acknowledgment (NACK)-Oriented Reliable Multicast (NORM) Protocol,” November 2009.) reliable multicast transport protocols.

More specifically, the [RFC5053] (Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer, “Raptor Forward Error Correction Scheme,” June 2007.) and [RFC5170] (Roca, V., Neumann, C., and D. Furodet, “Low Density Parity Check (LDPC) Forward Error Correction,” June 2008.) FEC schemes introduce erasure codes based on sparse parity check matrices for object delivery protocols like ALC and NORM. These codes are efficient in terms of processing but not optimal in terms of erasure recovery capabilities when dealing with "small" objects.

The Reed-Solomon FEC codes described in this document belong to the class of Maximum Distance Separable (MDS) codes that are optimal in terms of erasure recovery capability. It means that a receiver can recover the k source symbols from any set of exactly k encoding symbols. These codes are also systematic codes, which means that the k source symbols are part of the encoding symbols. However they are limited in terms of maximum source block size and number of encoding symbols. Since the real-time constraints of media delivery applications usually limit the maximum source block size, this is not considered to be a major issue in the context of the FEC Framework for many (but not necessarily all) use-cases. Additionally, if the encoding/decoding complexity is higher with Reed-Solomon codes than it is with [RFC5053] (Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer, “Raptor Forward Error Correction Scheme,” June 2007.) or [RFC5170] (Roca, V., Neumann, C., and D. Furodet, “Low Density Parity Check (LDPC) Forward Error Correction,” June 2008.) codes, it remains reasonable for most use-cases, even in case of a software codec.

Many applications dealing with reliable content transmission or content storage already rely on packet-based Reed-Solomon erasure recovery codes. In particular, many of them use the Reed-Solomon codec of Luigi Rizzo [RS‑codec] (Rizzo, L., “Reed-Solomon FEC codec (revised version of July 2nd, 1998), available at http://info.iet.unipi.it/~luigi/vdm98/vdm980702.tgz and mirrored at http://planete-bcast.inrialpes.fr/,” July 1998.) [Rizzo97] (Rizzo, L., “Effective Erasure Codes for Reliable Computer Communication Protocols,” April 1997.). The goal of the present document is to specify Reed-Solomon schemes that are compatible with this codec.

More specifically, the [RFC5510] (Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, “Reed-Solomon Forward Error Correction (FEC) Schemes,” April 2009.) document introduced such Reed-Solomon codes and several associated FEC schemes that are compatible with the [RFC5052] (Watson, M., Luby, M., and L. Vicisano, “Forward Error Correction (FEC) Building Block,” August 2007.) framework. The present document inherits from [RFC5510] (Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, “Reed-Solomon Forward Error Correction (FEC) Schemes,” April 2009.) the specification of the core Reed-Solomon codes based on Vandermonde matrices, and specifies FEC schemes that are compatible with the FECFRAME framework [FECFRAME‑FRAMEWORK] (Watson, M., “Forward Error Correction (FEC) Framework,” March 2010.). Therefore this document specifies only the information specific to the FECFRAME context and refers to [RFC5510] (Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, “Reed-Solomon Forward Error Correction (FEC) Schemes,” April 2009.) for the core specifications of the codes.

To that purpose, the present document introduces:

• the Fully-Specified FEC Scheme with FEC Encoding ID XXX that specifies the use of Reed-Solomon codes over GF(2^^m), with m in {2..16}, with a simple FEC encoding for arbitrary packet flows;
• the Fully-Specified FEC Scheme with FEC Encoding ID XXX is similar to Scheme XXX except that it is for a single sequenced flow;

With the simple FEC encoding, a set of Application Data Units (or ADUs) coming from one or several media delivery applications (e.g., a set of RTP packets), are grouped in a ADU block and FEC encoded as a whole. With Reed-Solomon codes over GF(2^^8), there is a strict limit over the number ADUs that can be protected together, since the number of encoded symbols, n, must be inferior or equal to 255.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” .).

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] (Watson, M., Luby, M., and L. Vicisano, “Forward Error Correction (FEC) Building Block,” August 2007.):

Source symbol:

unit of data used during the encoding process.

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 [FECFRAME‑FRAMEWORK] (Watson, M., “Forward Error Correction (FEC) Framework,” March 2010.):

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.

(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 4.2 (Source Block Creation)). This is the unit of data that is used to define source symbols.

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 Explicit Repair FEC Payload ID.

The above terminology is illustrated in Figure 1 (Terminology used in this document (sender).) (sender point of view):

+----------------------+
|     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 )      |
+----------------------+

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_r

denotes the number of repair symbols generated for a source block.

n

denotes the number of encoding symbols generated for a source block. Therefore: n = k + n_r.

E

denotes the encoding symbol length in bytes.

S

denotes the encoding symbol length in units of m-bit elements. When m = 8, then S and E are equal.

GF(q)

denotes a finite field (also known as Galois Field) with q elements. We assume that q = 2^^m in this document.

m

defines the length of the elements in the finite field, in bits. In this document, m belongs to {2..16}.

q

defines the number of elements in the finite field. We have: q = 2^^m in this specification.

CR

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

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.

NumSPA

(Symbols Per ADU) denotes the size of an ADUI in units of encoding symbols This size is constant for a given ADU block but may vary for different ADU blocks. We have: 1 ≤ NumSPA ≤ 16.

tot_k

denotes the total number of source symbols in an ADU block, tot_k = B * NumSPA.

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 Correction code.

FFCI

stands for FEC Framework Configuration Information.

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(s) creation, for the various FEC schemes considered.

4.1.  Problem Statement

Several aspects must be considered, that impact the source block creation:

• the distribution of ADU sizes for the ADU flow(s) protected by the FECFRAME instance;
• the maximum source block size (k parameter) and number of encoding symbols (n parameter), that are constrained by the finite field size (m parameter);
• 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.

In its most general form the FECFRAME framework and the RS FEC schemes are meant to protect a set of independent flows. Since the flows have no relationship to one another, the ADU size of each flow will potentially vary significantly. Even in the special case of a single flow, the ADU sizes may largely vary (e.g., the various frames of a "Group of Pictures (GOP) of an H.264 flow can have different sizes). This diversity must be addressed by the source block creation procedure since the RS FEC scheme requires a constant encoding symbol size (E parameter).

The finite field size parameter, m, defines the number of non zero elements in this field which is equal to: q - 1 = 2^^m - 1. This q - 1 value is also the theoretical maximum number of encoding symbols that can be produced for a source block. For instance, when m = 8 (default) there is a maximum of 2^^8 - 1 = 255 encoding symbols. So: k < n ≤ 255. Given the target FEC code rate (e.g., provided by the end-user or upper application when starting the FECFRAME framework, and taking into account the (known or estimated) packet loss rate), the sender calculates:

max_k = floor((2^^m - 1) * CR)

This max_k value leaves enough room for the sender to produce the desired number of repair symbols.

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_B.

4.2.  Source Block Creation

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 (Source block creation with the simple encoding scheme, for code rate 1/2 (equal number of source and repair symbols, 7 in this example).)):

• 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.
• 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.

Let us now assume that the largest ADU of this source block, with the prepended 3 bytes, must be segmented into NumSPA source symbols, each of size E. By construction, we must have: 1 ≤ NumSPA ≤ 16. Since E is fixed for the whole duration of the FECFRAME session, this also means that the maximum ADUI size is 16 * E bytes.

Then zero padding is added to ADU i (if needed) in field Pad[i], for alignment purposes up to a size of exactly NumSPA * E bytes. The data unit resulting from the ADU and the F[], L[] and Pad[] fields, is called ADU Information (or ADUI). Because of padding, each ADUI contributes to exactly NumSPA source symbols to the source block. However some of the NumSPA source symbols may be totally filled with zero bytes. As a special case, the encoding symbol size (E) can be chosen to be equal to the largest ADUI, in which case NumSPA = 1.

Thanks to the padding, a source symbol never straddles several ADUIs. As a direct consequence, a source symbol will never straddle several FEC source packets that are the payloads transmitted (and possibly erased) over the physical network.

  Enc Symbol Len (E)    Enc Symbol Len (E)    Enc Symbol Len (E)
< ------------------ >< ------------------ >< ------------------ >
+----+----+-----------------------+------------------------------+
|F[0]|L[0]|          R[0]         |            Pad[1]            |
+----+----+----------+------------+------------------------------+
|F[1]|L[1]|   R[1]   |                         Pad[2]            |
+----+----+----------+--------------------------------------+----+
|F[2]|L[2]|                      R[2]                       |P[2]|
+----+----+----------+--------------------------------------+----+
|F[3]|L[3]| R[3] |                             Pad[3]            |
+----+----+------+-----------------------------------------------+
\_______________________________  _______________________________/
                                \/
                       simple FEC encoding

+--------------------+
|      Repair 7      |
+--------------------+
.                    .
.                    .
+--------------------+
|      Repair 13     |
+--------------------+

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.  Reed-Solomon FEC Encoding Scheme over GF(2^^m) for Arbitrary ADU Flows

This Fully-Specified FEC Scheme specifies the use of Reed-Solomon codes over GF(2^^m), with m in {2..16}, with a simple FEC encoding for arbitrary packet 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

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

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:

• Encoding symbol length (E) (16 bit field): a non-negative integer that indicates the length of each encoding symbol in bytes.
• m parameter (8 bit field): an integer that defines the length of the elements in the finite field, in bits. In this scheme, m belongs to {2..16}.

The encoding format consists of the following 3 octet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Encoding Symbol Length (E)  |       m       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

These elements are required both by the sender (RS encoder) and the receiver(s) (RS decoder). When SDP is used to communicate the FFCI, this FEC scheme-specific information is carried in the 'fssi' parameter as an opaque octet string, using a Base64 encoding, as specified in [SDP_ELEMENTS] (Begen, A., “SDP Elements for FEC Framework,” August 2009.).

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 (Structure of a FEC source packet with the Explicit Source FEC Payload ID.).

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

More precisely, the Explicit Source FEC Payload ID is composed of the NumSPA, the Source Block Number and the Encoding Symbol ID. The length of these two fields depends on the m parameter (transmitted separately in the FFCI, Section 5.1.1.2 (FEC Scheme-Specific Information)):

Number of Source Symbols per ADU (NumSPA) (4 bit field):

this field provides the number of source symbols per ADU for this source block, i.e., the NumSPA parameter. Though this field might be recovered upon receiving the first FEC repair packet for this block, i.e., just on time before doing FEC decoding, it is transmitted in the Explicit Source FEC Payload ID too in order to simplify source block preparation at the receiver while receiving FEC source packets.

Source Block Number (SBN) (32-m bit field):

this field identifies the source block to which this FEC source packet belongs.

Encoding Symbol ID (ESI) (m bit field):

this field identifies the first source symbol associated to this FEC source packet in the source block (remember there can be several source symbols per ADUI, Section 4.2 (Source Block Creation)). This value belongs to interval {0..k - 1} inclusive for source symbols.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumSPA|   Source Block Number (28-8=20 bits)  | Enc. Symb. ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumSPA|Source Block Nb (12 b.)|   Enc. Symbol ID (16 bits)    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the Source FEC Payload ID for m = 8 and m = 16 are illustrated in Figure 5 (Source FEC Payload ID encoding format for m = 8 (default).) and Figure 6 (Source FEC Payload ID encoding format for m = 16.) respectively.

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 7 (Structure of a repair packet with the Repair FEC Payload ID.). There can be several repair symbols per repair packet.

+--------------------------------+
|           IP Header            |
+--------------------------------+
|        Transport Header        |
+--------------------------------+
|      Repair FEC Payload ID     |
+--------------------------------+
|        Repair Symbol(s)        |
+--------------------------------+

More precisely, the Repair FEC Payload ID is composed of the NumSPA, the Source Block Number, the Encoding Symbol ID, and the Source Block Length. The length of some of these fields depends on the m parameter (transmitted separately in the FFCI, Section 5.1.1.2 (FEC Scheme-Specific Information)):

Number of Source Symbols per ADU (NumSPA) (4 bit field):

this field provides the number of source symbols per ADU for this source block, i.e., the NumSPA parameter.

Source Block Number (SBN) (28-m bit field):

this field identifies the source block to which the FEC repair packet belongs.

Encoding Symbol ID (ESI) (m bit field)

this field identifies the first repair symbol contained in this FEC repair packet (remember there can be several repair symbols per FEC repair packet). This value belongs to interval {k..n - k - 1} inclusive 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. If 16 bits are too much when m ≤ 8, it is needed when 8 < m ≤ 16.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumSPA|   Source Block Number (28-8=20 bits)  | Enc. Symb. ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Source Block Length (k)    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumSPA|Source Block Nb (12 b.)|   Enc. Symbol ID (16 bits)    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Source Block Length (k)    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the Repair FEC Payload ID for m = 8 and m = 16 are illustrated in Figure 8 (Repair FEC Payload ID encoding format for m = 8 (default).) and Figure 9 (Repair FEC Payload ID encoding format for m = 16.) respectively.

The actual number of repair symbols contained in a FEC repair packet can be determined by the receiver by comparing the FEC repair packet length, PL, and the encoding symbol length, E, (transmitted in the FFCI). More precisely, this number is given by SN = PL / E. When multiple repair symbols are sent in the same FEC repair packet, the FEC Payload ID refers to the first repair symbol. The other repair symbols can be deduced from the ESI of the first repair symbol by incrementing sequentially this ESI.

5.2.  Procedures

The following procedures apply:

• The source block creation procedures are specified in Section 4.2 (Source Block Creation).
• 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^^(32-m) - 1.
• The ESI of source symbols is managed sequentially, starting at 0 for the first symbol. There are a maximum of 2^^m encoding symbols per block. The first k values (from 0 to k - 1 inclusive) identify source symbols, whereas the last n-k values (from k to n - k - 1 inclusive) identify repair symbols.
• The FEC repair packet creation procedures are specified in Section 5.1.3 (Repair FEC Payload ID).

5.3.  FEC Code Specification

The present document inherits from [RFC5510] (Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, “Reed-Solomon Forward Error Correction (FEC) Schemes,” April 2009.) the specification of the core Reed-Solomon codes based on Vandermonde matrices for a packet transmission channel.

6.  Reed-Solomon FEC Encoding Scheme over GF(2^^m) for a Single Sequenced ADU Flow

TBD

7.  Security Considerations

7.1.  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:

• those that are meant to give access to a confidential content (e.g., in case of a non-free content),
• those that try to corrupt the ADU Flows being transmitted (e.g., to prevent a receiver from using it),
• 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).

7.2.  Attacks Against the Data Flow

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

7.2.1.  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] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.)), or at the Network Layer, on a packet per packet basis when IPSec/ESP is used [RFC4303] (Kent, S., “IP Encapsulating Security Payload (ESP),” December 2005.). 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.

7.2.2.  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:

• at the application level, the Secure Real-time Transport Protocol (SRTP) [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) 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] (Baugher, M. and E. Carrara, “The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Secure Real-time Transport Protocol (SRTP),” February 2006.), 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.
• 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 deployer, 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.

7.3.  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 description). Attacks on these FEC parameters can prevent the decoding of the associated object. For instance modifying the m field (when applicable) will lead a receiver to consider 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 7.2.2 (Content Corruption) can be used.

8.  IANA Considerations

Values of FEC Encoding IDs are subject to IANA registration.

TBD

9.  Acknowledgments

The authors want to thank Hitoshi Asaeda for his valuable comments.

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses