FECFRAME Working Group Watson
Internet-Draft Digital Fountain
Intended status: Informational October 20, 2006
Expires: April 23, 2007
FECFRAME requirements
draft-ietf-fecframe-req-01
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Copyright (C) The Internet Society (2006).
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Abstract
This document defines requirements for a "FEC Framework" to be
defined by the IETF FECFRAME working group. The object of this group
is primarily to develop specifications for using forward error
correction (FEC) codes with applications in the Internet to provide
protection against packet loss.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Essential requirements . . . . . . . . . . . . . . . . . . . . 8
5. Non-essential requirements . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
This document defines requirements for a "FEC Framework" to be
defined by the IETF FECFRAME working group. The purpose of this
working group is defined in the working group charter as follows:
"The object of this group is to develop specifications for using
forward error correction (FEC) codes with applications in the
Internet to provide protection against packet loss. The group
will develop a protocol framework for application of FEC codes to
arbitrary packet flows over unreliable transport protocols over
both IP multicast and unicast."
This document defines requirements for this protocol framework. Both
'essential' ('SHALL') and 'non-essential' ('SHOULD') requirements are
considered.
A 'protocol framework' is a partial specification of a protocol,
along with a formal description of the missing aspects which are
required to form a full protocol specification - i.e. a protocol
framework is a protocol with 'holes' and a detailed description of
the 'shape' of those holes. Protocol frameworks provide for maximum
commonality between different complete protocols which provide
similar functions and therefore simplify implementation and
understanding of a set of alternative protocols which perform similar
functions. In this case, support for different complete protocols is
valuable for two reasons. Firstly because there exist many different
forward error correction codes, with different properties in terms of
error correction capability, computational complexity, flexibility
and intellectual property rights. Secondly, there are many
applications which could benefit from the use of forward error
correction. The FEC framework therefore replaces the "full mesh" of
application/FEC code combinations with a single general approach
which specifies how any FEC code meeting the FEC code requirments
defined in the framework can be used with any application meeting the
application requirements defined in the framework.
The FEC protocol framework must therefore define as much as possible
of a protocol for providing forward error correction for arbitrary
packet flows over unreliable transport, without defining a particular
FEC code or assuming a particular application. Furthermore, the
protocol framework specification will define a clear interface
between the specified parts and the unspecified, FEC-code-specific
and application-specific, parts. For this purpose, the building
block techniques applied in the Reliable Multicast (RMT) working
group will be re-used, specifically the FEC Building Block
[I-D.ietf-rmt-fec-bb-revised]
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The term "Forward Error Correction" refers here to application/
transport layer techniques for recovering lost packets of data. More
accurately, the term "Forward Erasure Correction" should be used. In
many contexts the term "Application Layer FEC (AL-FEC)" is also used,
although the mechanisms considered here could be considered as either
application or transport layer (the important point being that they
are end-to-end).
Generally, an "FEC Code" is defined in terms of the operations
required to construct encoded data from source data (at an encoder)
and to reconstruct source data from encoded data (at a decoder). In
order to apply an FEC Code to arbitrary packet flows, additional
elements are required such as protocol elements to identify encoded
data within packets, pre-processing of source data (e.g. segmentation
and/or addition of FEC-related indications into the source data).
Therefore, in order to adapt an FEC Code for use in the context of
the FEC Framework, additional FEC-code specific specification is
required. Following the approach of the FEC Building Block, this
specification is known as an "FEC Scheme". The FEC Framework will
define the requirements that FEC Schemes must meet for use with the
framework.
Generally, it is required to add forward error correction to existing
applications, for example media streaming applications. In this
case, the application protocols must be extended to support this.
The FEC framework will describe the requirements that application
protocols must meet in order to be used with the FEC framework.
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2. Terminology
'FEC' Forward Erasure Correction.
'AL-FEC' Application Layer Forward Erasure Correction
'FEC Framework' The protocol framework which is to be defined by
FECFRAME and for which this document provides requirements.
'Source data flow' The packet flow or flows to which FEC protection
is to be applied.
'Repair data flow' The packet flow or flows carrying forward error
correction data
'Source protocol' A protocol used for the source data flow being
protected - e.g. RTP.
'Transport protocol' The protocol used for transport of the source
data flow being protected - e.g. UDP, DCCP.
'Control protocol' Application layer protocols used to establish and
modify the source data flow being protected - e.g. RTSP.
'FEC Code' An algorithm for encoding data such that the encoded dats
flow is resiliant to data loss or corruption.
'FEC Scheme' A specification which defines the additional protocol
aspects required to use a particular FEC code with the FEC
framework, or (in the context of RMT), with the RMT FEC Building
Block.
'Source Block' the group of source data packets which are to be FEC
protected as a single block
'Protection amount' The relative increase in data sent due to the
use of FEC.
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3. Motivation
One approach to the problem addressed in this document would be to
arrange the source packet flows into a sequence of 'objects' and then
apply FEC protection using the mechanisms defined by the RMT working
group for object transport. This section describes the motivation
for following a separate approach, although one that draws heavily on
the RMT work.
FEC Schemes defined according to the RMT FEC Building Block
[I-D.ietf-rmt-fec-bb-revised] envisage objects with a finite size.
Mapping arbitrary flows to this environment one would need to
consider the flows as a sequence of such objects (also known as
Source Blocks). For each object, the RMT FEC Schemes expect FEC
Object Transmission Information to be communicated with the object.
In many cases some or all of this information will be the same for
every block. Thus there is some advantage in explicitly introducing
the concept of a flow (or bundle of flows) for which some or all of
the FEC Object Transmission Information can be the same for every
source block. As well as reducing overhead, it is advantageous to be
able to inform the receiver that these parameters won't change during
the lifetime of the flow or flows.
A second issue is that FEC Schemes in RMT generally also include
recommendations for parameter settings, which are based on single-
object delivery. Recommendations for protection of packet flows may
be different from these for a variety of reasons. There is a need,
therefore, for FEC-Scheme specific specification material which is
specific to the case of arbitrary packet flows and different from the
recommendations for single-object delivery. One of the key aspects
of the FEC Framework contemplated here is that it provides a context
for such material, in the form of an explicit description of the
requirements that FEC Schemes must meet in order to be used with this
framework.
A third issue is the question of how source data from a packet flow
or flows is formatted into data blocks that an 'object-based' FEC
Scheme could process. RMT FEC Schemes expect an object which is just
a sequence of bytes. We therefore would need to build such an object
out of a sequence of potentially variable-length source packets.
There are several ways this could be done and different FEC Schemes
may require different approaches. Again, the framework contemplated
here provides a context for the definition of these mechanisms
through the concept of FEC Schemes which are adapted for use with
this framework. The RMT work then envisages that both source packets
and repair packets consist of symbols which are extracted from or
generated from (respectively) this source block. In the case of FEC
protection of arbitrary packet flows it is desirable to support cases
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where the source packets are transmitted unchanged, thereby providing
backwards compatibility. This is not compatible with in the RMT
approach.
As a result of the considerations above, this document describes
requirements for an FEC Framework for arbitrary packet flows which is
independent of the RMT FEC Building Block, although we draw heavily
on the concepts developed there. FEC Schemes defined for use with
this FEC Framework are distinct from FEC Schemes defined for object
delivery in the context of the RMT FEC Building Block. However, it
is expected that in many cases the task of generalising an RMT FEC
Scheme into one which can be used with both the RMT protocols and
this FEC Framework will be a simple one.
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4. Essential requirements
Req-10: The FEC Framework shall support a wide range of FEC codes,
using the abstractions of the FEC Building Block defined in RMT
[I-D.ietf-rmt-fec-bb-revised] (including short and long block FEC
codes, systematic and non-systematic codes). Specifically, the
FEC Framework shall define the requirements that FEC code
specifications shall meet in order to be used with the framework,
re-using, as far as possible, the FEC code specification approach
and requirements from the FEC Building Block and specifying any
further requirements that must be met for the FEC Framework.
Req-20: The FEC Framework shall support a wide range of application
protocols, using the abstractions of the FEC Building Block
[I-D.ietf-rmt-fec-bb-revised]. Specifically, the FEC Framework
shall define the requirements that application protocol
specifications shall meet in order to be used with the framework,
re-using, as far as possible, the Content Delivery Protocol
specification approach and requirements from the FEC Building
Block and specifying any further requirements that must be met for
the FEC Framework.
Req-30: The FEC Framework shall support variable source block sizes,
including real-time variation of source block size between blocks
of a given source data flow.
Req-35: The FEC Framework shall support variable protection amounts,
including dynamic variation of protection amount between blocks
within a given source data flow.
Req-40: The FEC Framework shall be independent of the source
protocols (provided that source protocol uses one of the supported
transport protocols).
Req-50: The FEC Framework shall place minimal requirements on the
application protocols.
Req-60: The FEC Framework shall support variable source data flow
rates.
Req-70: The FEC Framework shall support variable source data flow
packet sizes.
Req-80: The FEC Framework shall provide support of combined
protection of multiple source data flows.
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Req-90: The FEC Framework shall provide support of multiple
transport protocols for the source data protocols (UDP, DCCP,
others ?).
Req-100: The FEC Framework shall provide support for definition of
backwards-compatible FEC protocols (i.e. where the source packets
are not modified in any way).
Req-110: The FEC Framework shall provide support for different
source data protocols (RTP, MIKEY, others ?).
Req-120: The FEC Framework shall shall address the security issues,
if any, associated with the use of FEC.
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5. Non-essential requirements
The FEC Framework should be constructed such that the FEC
streaming protocol defined by 3GPP in TS26.346 is a valid protocol
according to the FEC Framework.
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6. Security Considerations
This document defines requirements for the work of the FECFRAME
working group and includes a requirement that the security
implications of the use of FEC, if any, should be addressed in that
work.
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7. References
[I-D.ietf-rmt-fec-bb-revised]
Watson, M., "Forward Error Correction (FEC) Building
Block", draft-ietf-rmt-fec-bb-revised-04 (work in
progress), September 2006.
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Author's Address
Mark Watson
Digital Fountain
39141 Civic Center Dr.
Suite 300
Fremont, CA 94538
US
Email: mark@digitalfountain.com
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