FecFrame V. Roca
Internet-Draft M. Cunche
Intended status: Experimental INRIA
Expires: April 9, 2011 J. Lacan
A. Bouabdallah
ISAE/LAAS-CNRS
K. Matsuzono
Keio University
October 6, 2010
Simple Reed-Solomon Forward Error Correction (FEC) Schemes for FECFRAME
draft-roca-fecframe-simple-rs-00
Abstract
This document describes a fully-specified simple FEC scheme for Reed-
Solomon codes over GF(2^^m), with 2 <= m <= 16, that can be used to
protect arbitrary 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 LDPC codes for instance.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 9, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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. Simple Reed-Solomon FEC Encoding Scheme over GF(2^^m) 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 . . . . . . . . . . . . 11
5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 12
5.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 14
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 . . . . . . . . . . . . 16
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. 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] and [RFC5170] 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] or [RFC5170] 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] [Rizzo97]. The goal of the present
document is to specify a simple Reed-Solomon scheme that is
compatible with this codec.
More specifically, the [RFC5510] document introduced such Reed-
Solomon codes and several associated FEC schemes that are compatible
with the [RFC5052] framework. The present document inherits from
[RFC5510] the specification of the core Reed-Solomon codes based on
Vandermonde matrices, and specifies a simple FEC scheme that is
compatible with the FECFRAME framework [FECFRAME-FRAMEWORK].
Therefore this document specifies only the information specific to
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the FECFRAME context and refers to [RFC5510] 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 of Reed-Solomon codes over
GF(2^^m), with 2 <= m <= 16, with a simple FEC encoding for
arbitrary packet flows;
For instance, with this scheme, a set of Application Data Units (or
ADUs) coming from one or several (resp. one) media delivery
applications (e.g., a set of RTP packets), are grouped in an ADU
block and FEC encoded as a whole. With Reed-Solomon codes over
GF(2^^8), there is a strict limit over the number of ADUs that can be
protected together, since the number of encoded symbols, n, must be
inferior or equal to 255. This constraint is relaxed when using a
higher finite field size (m > 8), at the price of an increased
computational complexity.
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.
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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]:
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 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 (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.
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.
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a^^b denotes a raised to the power b.
Some of them are FECFRAME framework specific:
B denotes the number of ADUs per ADU block.
max_B denotes the maximum number of ADUs for any ADU block.
3.3. Abbreviations
This document uses the following abbreviations:
ADU stands for Application Data Unit.
ESI stands for Encoding Symbol ID.
FEC stands for Forward Error 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. Restrictions
This specification has the following restrictions:
o there MUST be exactly one source symbol 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;
4.2. ADU Block Creation
Several aspects must be considered, that impact the ADU block
creation:
o the maximum source block size (k parameter) and number of encoding
symbols (n parameter), that are constrained by the finite field
size (m 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 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
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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. Since there is one source symbol
per ADU, max_k is also an upper bound to the maximum number of ADUs
per ADU block.
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 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
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.
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
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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.
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 with the simple encoding scheme, for
code rate 1/2 (equal number of source and repair symbols, 4 in this
example), S = 0.
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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. Simple 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 2 <= m <= 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
o 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:
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 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.
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o m parameter (m): an integer that defines the length of the
elements in the finite field, in bits. We have: 2 <= m <= 16.
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 in textual
representation as specified in [SDP_ELEMENTS]. For instance:
fssi = E:1400,S:0,m:8
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 3 octets:
o Encoding symbol length (E): 16 bit field.
o Strict (S) flag: 1 bit field.
o m parameter (m): 7 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length (E) |S| m |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
Source Block Number, the Encoding Symbol ID, and the Source Block
Length. The length of the first two fields depends on the m
parameter (transmitted separately in the FFCI, Section 5.1.1.2):
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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 4.3). 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. If 16 bits are too much when m <= 8, it is needed when
8 < m <= 16. Therefore we provide a single common format
regardless of m.
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 (24 bits) | Enc. Symb. ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length (k) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Source FEC Payload ID encoding format for m = 8 (default).
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 Nb (16 bits) | Enc. Symbol ID (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length (k) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Source FEC Payload ID encoding format for m = 16.
The format of the Source FEC Payload ID for m = 8 and m = 16 are
illustrated in Figure 5 and Figure 6 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. There
MUST be a single repair symbol per FEC repair packet.
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+--------------------------------+
| IP Header |
+--------------------------------+
| Transport Header |
+--------------------------------+
| Repair FEC Payload ID |
+--------------------------------+
| Repair Symbol |
+--------------------------------+
Figure 7: Structure of a repair packet with the Repair FEC Payload
ID.
More precisely, the Repair FEC Payload ID is composed of the Source
Block Number, the Encoding Symbol ID, and the Source Block Length.
The length of the first two fields depends on the m parameter
(transmitted separately in the FFCI, Section 5.1.1.2):
Source Block Number (SBN) (32-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
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. If 16 bits are too much when m <= 8, it is needed when
8 < m <= 16. Therefore we provide a single common format
regardless of m.
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 (24 bits) | Enc. Symb. ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length (k) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Repair FEC Payload ID encoding format for m = 8 (default).
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 Nb (16 bits) | Enc. Symbol ID (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length (k) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Repair FEC Payload ID encoding format for m = 16.
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The format of the Repair FEC Payload ID for m = 8 and m = 16 are
illustrated in Figure 8 and Figure 9 respectively.
5.2. Procedures
The following procedures apply:
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^^(32-m) - 1.
o 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 (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 [RFC5510] the specification of the
core Reed-Solomon codes based on Vandermonde matrices for a packet
transmission channel.
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
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Payload ID) or out-of-band (e.g., in a session description).
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 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.
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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
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 6.2.2 can be
used.
7. IANA Considerations
The FEC Encoding ID value is subject to IANA registration.
TBD
8. Acknowledgments
The authors want to thank Hitoshi Asaeda for his valuable comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119.
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052, August 2007.
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[RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo,
"Reed-Solomon Forward Error Correction (FEC) Schemes",
RFC 5510, April 2009.
[FECFRAME-FRAMEWORK]
Watson, M., "Forward Error Correction (FEC) Framework",
Work in Progress, July 2010.
[SDP_ELEMENTS]
Begen, A., "SDP Elements for FEC Framework", Work
in Progress, April 2010.
9.2. Informative References
[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", ACM SIGCOMM Computer
Communication Review Vol.27, No.2, pp.24-36, April 1997.
[RFC5170] Roca, V., Neumann, C., and D. Furodet, "Low Density Parity
Check (LDPC) Forward Error Correction", RFC 5170,
June 2008.
[RFC5053] Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer,
"Raptor Forward Error Correction Scheme", RFC 5053,
June 2007.
[RFC5775] Luby, M., Watson, M., and L. Vicisano, "Asynchronous
Layered Coding (ALC) Protocol Instantiation", RFC 5775,
April 2010.
[RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-acknowledgment (NACK)-Oriented Reliable
Multicast (NORM) Protocol", RFC 5740, November 2009.
[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.
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[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
INRIA
655, av. de l'Europe
Inovallee; Montbonnot
ST ISMIER cedex 38334
France
Email: mathieu.cunche@inria.fr
URI: http://planete.inrialpes.fr/people/cunche/
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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Amine Bouabdallah
ISAE/LAAS-CNRS
1, place Emile Blouin
Toulouse 31056
France
Email: Amine.Bouabdallah@isae.fr
URI: http://dmi.ensica.fr/
Kazuhisa Matsuzono
Keio University
Graduate School of Media and Governance
5322 Endo
Fujisawa, Kanagawa 252-8520
Japan
Email: kazuhisa@sfc.wide.ad.jp
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