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Versions: 00 01 02                                                      
FECFRAME Working Group                                         M. Watson
Internet-Draft                                          Digital Fountain
Intended status: Informational                          December 3, 2007
Expires: June 5, 2008


                         FECFRAME requirements
                       draft-ietf-fecframe-req-02

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Copyright Notice

   Copyright (C) The IETF Trust (2007).














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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 [RFC5052]

   The term "Forward Error Correction" refers here to application/



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   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 [RFC5052]
   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
      [RFC5052] (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
      [RFC5052].  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

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052, August 2007.















































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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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Full Copyright Statement

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   Administrative Support Activity (IASA).





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