FecFrame V. Roca
Internet-Draft INRIA
Intended status: Standards Track M. Cunche
Expires: March 17, 2012 NICTA
J. Lacan
A. Bouabdallah
ISAE/LAAS-CNRS
K. Matsuzono
Keio University
September 14, 2011
Simple Reed-Solomon Forward Error Correction (FEC) Scheme for FECFRAME
draft-ietf-fecframe-simple-rs-01
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
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. ADU Block Creation . . . . . . . . . . . . . . . . . . . . 8
4.3. Source Block Creation . . . . . . . . . . . . . . . . . . 9
5. Simple Reed-Solomon FEC Scheme over GF(2^^m) for Arbitrary
ADU Flows . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 11
5.1.1. FEC Framework Configuration Information . . . . . . . 11
5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . . 12
5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 13
5.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3. FEC Code Specification . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 15
6.1.1. Access to Confidential Content . . . . . . . . . . . . 15
6.1.2. Content Corruption . . . . . . . . . . . . . . . . . . 16
6.2. Attacks Against the FEC Parameters . . . . . . . . . . . . 16
6.3. When Several Source Flows are to be Protected Together . . 17
6.4. Baseline Secure FEC Framework Operation . . . . . . . . . 17
7. Operations and Management Considerations . . . . . . . . . . . 17
7.1. Operational Recommendations: Finite Field Size (m) . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 19
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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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 [RFC6363]
document describes a generic framework to use FEC schemes with media
delivery applications, and for instance with real-time streaming
media applications based on the RTP real-time protocol. Similarly
the [RFC5052] document describes a generic framework to use FEC
schemes with 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 [RFC6363]. Therefore this
document specifies only the information specific to the FECFRAME
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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.
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.
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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
[RFC6363]:
Application Data Unit (ADU): a unit of data coming from (sender) or
given to (receiver) the media delivery application. Depending on
the use-case, an ADU can use an RTP encapsulation. In this
specification, there is always one source symbol per ADU.
(Source) ADU Flow: a flow of ADUs from a media delivery application
and to which FEC protection is applied. Depending on the use-
case, several ADU flows can be protected together by the FECFRAME
framework.
ADU Block: a set of ADUs that are considered together by the
FECFRAME instance for the purpose of the FEC scheme. Along with
the F[], L[], and Pad[] fields, they form the set of source
symbols over which FEC encoding will be performed.
ADU Information (ADUI): a unit of data constituted by the ADU and
the associated Flow ID, Length and Padding fields (Section 4.3).
This is the unit of data that is used as source symbol.
FEC Framework Configuration Information: the FEC scheme specific
information that enables the synchronization of the FECFRAME
sender and receiver instances.
FEC Source Packet: a data packet submitted to (sender) or received
from (receiver) the transport protocol. It contains an ADU along
with its optional Explicit Source FEC Payload ID.
FEC Repair Packet: a repair packet submitted to (sender) or received
from (receiver) the transport protocol. It contains a repair
symbol along with its Repair FEC Payload ID.
The above terminology is illustrated in Figure 1 (sender's point of
view):
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+----------------------+
| Application |
+----------------------+
|
ADU flow | (1) Application Data Unit (ADU)
v
+----------------------+ +----------------+
| FEC Framework | | |
| |------------------------- >| FEC Scheme |
|(2) Construct an ADU | (4) Source Symbols for | |
| block | this Source Block |(5) Perform FEC |
|(3) Construct ADU Info| | Encoding |
|(7) Construct FEC Src |< -------------------------| |
| Packets and FEC |(6) Ex src FEC Payload Ids,| |
| Repair Packets | Repair FEC Payload Ids,| |
+----------------------+ Repair Symbols +----------------+
| |
|(8) FEC Src |(8') FEC Repair
| packets | packets
v v
+----------------------+
| Transport Layer |
| (e.g., UDP ) |
+----------------------+
Figure 1: Terminology used in this document (sender).
3.2. Notations
This document uses the following notations: Some of them are FEC
scheme specific:
k denotes the number of source symbols in a source block.
max_k denotes the maximum number of source symbols for any source
block.
n denotes the number of encoding symbols generated for a source
block.
E denotes the encoding symbol length in bytes.
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 is such that 2 <= m <= 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 (or Erasure) Correction code.
FFCI stands for FEC Framework Configuration Information.
FSSI stands for FEC Scheme Specific Information.
MDS stands for Maximum Distance Separable code.
4. Common Procedures Related to the ADU Block and Source Block Creation
This section introduces the procedures that are used during the ADU
block and the related source block creation, for the FEC scheme
considered.
4.1. Restrictions
This specification has the following restrictions:
o there MUST be exactly one source symbol per ADUI, and therefore
per ADU;
o there MUST be exactly one repair symbol per FEC Repair Packet;
o there MUST be exactly one source block per ADU block;
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:
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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. 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 Reed-Solomon
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 Reed-Solomon 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 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
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message, using an appropriate mechanism) and is not considered by
this specification.
The ADU block is always encoded as a single source block. There are
a total of B <= max_B ADUs in this ADU block. For the ADU i, with 0
<= i <= B-1, 3 bytes are prepended (Figure 2):
o The first byte, FID[i] (Flow ID), contains the integer identifier
associated to the source ADU flow to which this ADU belongs to.
It is assumed that a single byte is sufficient, or said
differently, that no more than 256 flows will be protected by a
single instance of the FECFRAME framework.
o The following two bytes, L[i] (Length), contain the length of this
ADU, in network byte order (i.e., big endian). This length is for
the ADU itself and does not include the FID[i], L[i], or Pad[i]
fields.
Then zero padding is added to ADU i (if needed) in field Pad[i], for
alignment purposes up to a size of exactly E bytes. The data unit
resulting from the ADU i and the F[i], L[i] and Pad[i] fields, is
called ADU Information (or ADUI). Each ADUI contributes to exactly
one source symbol to the source block.
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.
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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 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
FEC Encoding ID: the value assigned to this fully-specified FEC
scheme MUST be XXX, as assigned by IANA (Section 8).
When SDP is used to communicate the FFCI, this FEC Encoding ID is
carried in the 'encoding-id' parameter.
5.1.1.2. FEC Scheme-Specific Information
The FEC Scheme Specific Information (FSSI) includes elements that are
specific to the present FEC scheme. More precisely:
Encoding symbol length (E): a non-negative integer that indicates
either the length of each encoding symbol in bytes (strict mode,
i.e., if S = 1), or the maximum length of any encoding symbol
(i.e., if S = 0).
Strict (S) flag: when set to 1 this flag indicates that the E
parameter is 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).
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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 (Reed-Solomon encoder)
and the receiver(s) (Reed-Solomon 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
source symbol contained in this FEC source packet. This value is
such that 0 <= ESI <= k - 1 for source symbols.
Source Block Length (k) (16 bit field): this field provides the
number of source symbols for this source block, i.e., the k
parameter. 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 FEC 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 encoding symbols is managed sequentially, starting at 0
for the first symbol. The first k values (0 <= ESI <= k - 1)
identify source symbols, whereas the last n-k values (k <= ESI <=
n - 1) identify repair symbols.
o The FEC repair packet creation procedures are specified in
Section 5.1.3.
5.3. FEC Code Specification
The present document inherits from [RFC5510] the specification of the
core Reed-Solomon codes based on Vandermonde matrices for a packet
transmission channel.
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 Reed-Solomon codes.
6.1. Attacks Against the Data Flow
6.1.1. Access to Confidential Content
The Reed-Solomon FEC Scheme specified in this document does not
change the recommendations of [RFC6363]. To summarize, if
confidentiality is a concern, it is RECOMMENDED that one of the
solutions mentioned in [RFC6363] is used, with special considerations
to the way this solution is applied (e.g., before versus after FEC
protection, and within the end-system versus 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.
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6.1.2. Content Corruption
The Reed-Solomon FEC Scheme specified in this document does not
change the recommendations of [RFC6363]. To summarize, it is
RECOMMENDED that one of the solutions mentioned in [RFC6363] is used
on both the FEC Source and Repair Packets.
6.2. Attacks Against the FEC Parameters
The FEC Scheme specified in this document defines parameters that can
be the basis of several attacks. More specifically, the following
parameters of the FFCI may be modified by an attacker
(Section 5.1.1.2):
o FEC Encoding ID: changing this parameter leads the receiver to
consider a different FEC Scheme, which enables an attacker to
create a Denial of Service (DoS).
o Encoding symbol length (E): setting this E parameter to a value
smaller than the valid one enables an attacker to create a DoS
since the repair symbols and certain source symbols will be larger
than E, which is an incoherency for the receiver. Setting this E
parameter to a value larger than the valid one has similar impacts
when S=1 since the received repair symbol size will be smaller
than expected. On the opposite it will not lead to any
incoherency when S=0 since the actual symbol length value for the
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 m parameter: changing this parameter triggers a DoS since the
receiver and sender will consider different codes, and the
receiver will interpret the Explicit Source FEC Payload ID and
Repair FEC Payload ID differently. Additionally, by increasing
this m parameter to a larger value (typically m=16 rather than 8,
when both values are possible in the target use-case) will create
additional processing load at a receiver if decoding is attempted.
It is therefore RECOMMENDED that security measures are taken to
guarantee the FFCI integrity, as specified in [RFC6363]. How to
achieve this depends on the way the FFCI is communicated from the
sender to the receiver, which is not specified in this document.
Similarly, attacks are possible against the Explicit Source FEC
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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), an attacker can easily corrupt the block identified by
the SBN. Other consequences, that are use-case and/or CDP dependant,
may also happen. It is therefore RECOMMENDED that security measures
are taken to guarantee the FEC Source and Repair Packets as stated in
[RFC6363].
6.3. When Several Source Flows are to be Protected Together
The Reed-Solomon FEC Scheme specified in this document does not
change the recommendations of [RFC6363].
6.4. Baseline Secure FEC Framework Operation
The Reed-Solomon 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 Reed-Solomon codes as specified in
this document.
7.1. Operational Recommendations: Finite Field Size (m)
The present document requires that m, the length of the elements in
the finite field, in bits, is such that 2 <= m <= 16. However all
possibilities are not equally interesting from a practical point of
view. It is expected that m=8, the default value, will be mostly
used since it offers the possibility to have small to medium sized
source blocks and/or a significant number of repair symbols (i.e., k
< n <= 255). Additionally, elements in the finite field are 8 bits
long which makes read/write memory operations aligned on bytes during
encoding and decoding.
An alternative when it is known that only very small source blocks
will be used is m=4 (i.e., k < n <= 15). Elements in the finite
field are 4 bits long, so if two elements are accessed at a time,
read/write memory operations are aligned on bytes during encoding and
decoding.
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An alternative when very large source blocks are needed is m=16
(i.e., k < n <= 65535). However this choice has significant impacts
on the processing load. For instance using pre-calculated tables to
speedup operations over the finite field (as done with smaller finite
fields) may require a prohibitive amount of memory to be used on
embedded platforms. Alternative lightweight solutions (e.g.,
[RFC5170]) MAY be preferred in situations where the processing load
is an issue [Matsuzono10].
Since several values for the m parameter are possible, the use-case
SHOULD define which value(s) need(s) to be supported. In situations
where this is not specified, the default m=8 value SHOULD be
supported and used.
8. IANA Considerations
Values of FEC Encoding IDs are subject to IANA registration.
[RFC6363] defines general guidelines on IANA considerations. In
particular it defines a registry called FEC Framework (FECFRAME) FEC
Encoding IDs whose values are granted on an IETF Consensus basis.
This document registers one value in the FEC Framework (FECFRAME) FEC
Encoding IDs registry as follows:
o XXX refers to the Simple Reed-Solomon [RFC5510] FEC Scheme over
GF(2^^m) for Arbitrary Packet Flows.
9. Acknowledgments
The authors want to thank Hitoshi Asaeda for his valuable comments.
10. References
10.1. 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.
[RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo,
"Reed-Solomon Forward Error Correction (FEC) Schemes",
RFC 5510, April 2009.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
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Correction (FEC) Framework", RFC 6363, September 2011.
[SDP_ELEMENTS]
Begen, A., "SDP Elements for FEC Framework",
draft-ietf-fecframe-sdp-elements-11 (Work in Progress),
October 2010.
10.2. 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,
mirrored at http://planete-bcast.inrialpes.fr/ and
http://openfec.org/", 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.
[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.
[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.
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Authors' Addresses
Vincent Roca
INRIA
655, av. de l'Europe
Inovallee; Montbonnot
ST ISMIER cedex 38334
France
Email: vincent.roca@inria.fr
URI: http://planete.inrialpes.fr/people/roca/
Mathieu Cunche
NICTA
Australia
Email: mathieu.cunche@nicta.com.au
URI: http://mathieu.cunche.free.fr/
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
Amine Bouabdallah
ISAE/LAAS-CNRS
1, place Emile Blouin
Toulouse 31056
France
Email: Amine.Bouabdallah@isae.fr
URI: http://dmi.ensica.fr/
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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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