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Versions: (draft-watson-fecframe-framework) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 6363

FEC Framework Working Group                                    M. Watson
Internet-Draft                                            Qualcomm, Inc.
Intended status: Standards Track                            June 1, 2010
Expires: December 3, 2010


                Forward Error Correction (FEC) Framework
                    draft-ietf-fecframe-framework-08

Abstract

   This document describes a framework for using forward error
   correction (FEC) codes with applications in public and private IP
   networks to provide protection against packet loss.  The framework
   supports applying Forward Error Correction to arbitrary packet flows
   over unreliable transport and is primarily intended for real-time, or
   streaming, media.  This framework can be used to define Content
   Delivery Protocols that provide Forward Error Correction for
   streaming media delivery or other packet flows.  Content Delivery
   Protocols defined using this framework can support any FEC Scheme
   (and associated FEC codes) which is compliant with various
   requirements defined in this document.  Thus, Content Delivery
   Protocols can be defined which are not specific to a particular FEC
   Scheme and FEC Schemes can be defined which are not specific to a
   particular Content Delivery Protocol.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 3, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.






























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions/Abbreviations  . . . . . . . . . . . . . . . . . .  6
   3.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  9
   4.  Architecture Overview  . . . . . . . . . . . . . . . . . . . . 10
   5.  Procedural overview  . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.2.  Sender Operation . . . . . . . . . . . . . . . . . . . . . 16
     5.3.  Receiver Operation . . . . . . . . . . . . . . . . . . . . 18
   6.  Protocol Specification . . . . . . . . . . . . . . . . . . . . 22
     6.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . . 22
     6.2.  Structure of the source block  . . . . . . . . . . . . . . 22
     6.3.  Packet format for FEC Source packets . . . . . . . . . . . 22
       6.3.1.  Generic Explicit Source FEC Payload Id . . . . . . . . 24
     6.4.  Packet Format for FEC Repair packets . . . . . . . . . . . 24
       6.4.1.  Packet Format for FEC Repair packets over RTP  . . . . 24
     6.5.  FEC Framework Configuration Information  . . . . . . . . . 25
     6.6.  FEC Scheme requirements  . . . . . . . . . . . . . . . . . 27
   7.  Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   8.  Transport Protocols  . . . . . . . . . . . . . . . . . . . . . 31
   9.  Congestion Control . . . . . . . . . . . . . . . . . . . . . . 32
     9.1.  Normative requirements . . . . . . . . . . . . . . . . . . 33
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 36
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 37
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     13.1. Normative references . . . . . . . . . . . . . . . . . . . 38
     13.2. Informative references . . . . . . . . . . . . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40





















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1.  Introduction

   Many applications have a requirement to transport a continuous stream
   of packetised data from a source (sender) to one or more destinations
   (receivers) over networks which do not provide guaranteed packet
   delivery.  Primary examples are real-time, or streaming, media
   applications such as broadcast, multicast or on-demand audio, video
   or multimedia.

   Forward Error Correction is a well-known technique for improving
   reliability of packet transmission over networks which do not provide
   guaranteed packet delivery, especially in multicast and broadcast
   applications.  The FEC Building Block defined in [RFC5052] provides a
   framework for definition of Content Delivery Protocols (CDPs) for
   object delivery (including, primarily, file delivery) which make use
   of separately defined FEC Schemes.  Any CDP defined according to the
   requirements of the FEC Building Block can then easily be used with
   any FEC Scheme which is also defined according to the requirements of
   the FEC Building Block.  (Note that the term "Forward Erasure
   Correction" is sometimes used, 'erasures' being a type of error in
   which data is lost and this loss can be detected, rather than being
   received in corrupted form - the focus of this document is strictly
   on erasures, however the term Forward Error Correction is more widely
   used).

   This document defines a framework for the definition of CDPs which
   provide for FEC protection of arbitrary packet flows over unreliable
   transports such as UDP.  As such, this document complements the FEC
   Building Block of [RFC5052], by providing for the case of arbitrary
   packet flows over unreliable transport, the same kind of framework as
   that document provides for object delivery.  This document does not
   define a complete Content Delivery Protocol, but rather defines only
   those aspects that are expected to be common to all Content Delivery
   Protocols based on this framework.

   This framework does not define how the flows to be protected are
   determined, nor how the details of the protected flows and the FEC
   streams which protect them are communicated from sender to receiver.
   It is expected that any complete Content Delivery Protocol
   specification which makes use of this framework will address these
   signalling requirements.  However, this document does specify the
   information which is required by the FEC Framework at the sender and
   receiver - for example details of the flows to be FEC protected, the
   flow(s) that will carry the FEC protection data and an opaque
   container for FEC-Scheme-specific information.

   FEC Schemes designed for use with this framework must fulfil a number
   of requirements defined in this document.  Note that these



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   requirements are different from those defined in [RFC5052] for FEC
   Schemes for object delivery.  However there is a great deal of
   commonality and FEC Schemes defined for object delivery may be easily
   adapted for use with the framework defined here.

   Since the RTP protocol layer is used over UDP, this framework can be
   applied to RTP flows as well.  FEC repair packets may be sent
   directly over UDP or over RTP.  The latter approach has the advantage
   that RTP instrumentation, based on RTCP, can be used for the repair
   flow.  Additionally, the post-repair RTCP extended report [RFC5725]
   may be used to obtain information about the loss rate after FEC
   recovery.

   The use of RTP for repair flows is defined for each FEC Scheme by
   defining an RTP Payload Format for that particular FEC Scheme
   (possibly in the same document).



































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2.  Definitions/Abbreviations

   'FEC'  Forward Error Correction.

   'AL-FEC'  Application Layer Forward Error Correction

   'FEC Framework'  A protocol framework for definition of Content
      Delivery Protocols using FEC, such as the framework defined in
      this document.

   'Source data flow'  The packet flow or flows to which FEC protection
      is to be applied.  A source data flow consists of ADUs.

   '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
      and repair data flows - e.g.  UDP, DCCP.

   'Application Data Unit'  The unit of source data provided as payload
      to the transport layer

   'ADU Flow'  A sequence of ADUs associated with a transport layer flow
      identifier (such as the standard 5-tuple { Source IP Address,
      Source Transport Port, Destination IP Address, Destination
      Transport Port, Transport Protocol } in the case of UDP)

   'Application protocol'  Control protocol used to establish and
      control the source data flow being protected - e.g.  RTSP.

   'FEC Code'  An algorithm for encoding data such that the encoded data
      flow is resiliant to data loss (Note: in general FEC Codes may
      also be used to make a data flow resiliant to corruption, but that
      is not considered here).

   '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.

   'Protection amount'  The relative increase in data sent due to the
      use of FEC.






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   'FEC Framework Configuration Information'  Information which controls
      the operation of the FEC Framework.

   'FEC Source Packet'  At a sender (resp. receiver) a payload submitted
      to (resp. received from) the Transport protocol containing an ADU
      along with an optional Source FEC Payload ID.

   'FEC Repair Packet'  At a sender (resp. receiver) a payload submitted
      to (resp. received from) the Transport protocol containing one or
      more repair symbols along with a Repair FEC Payload ID and
      possibly an RTP header.

   'FEC Payload ID'  Information which identifies the contents of a
      packet with respect to the FEC Scheme.

   'Source FEC Payload ID'  An FEC Payload ID specifically for use with
      source packets.

   'Repair FEC Payload ID'  An FEC Payload ID specifically for use with
      repair packets.

   'Content Delivery Protocol (CDP)'  A complete application protocol
      specification which, through the use of the framework defined in
      this document, is able to make use of FEC Schemes to provide
      Forward Error Correction capabilities

   The following definitions are aligned with [RFC5052]

   'Source symbol'  unit of data used during the encoding process.

   'Encoding symbol'  unit of data generated by the encoding process.
      With systematic codes, source symbols are part of the encoding
      symbols.

   'Repair symbol'  encoding symbol that is not a source symbol.

   'Code rate'  the k/n ratio, i.e., the ratio between the number of
      source symbols and the number of encoding symbols.  By definition,
      the code rate is such that: 0 < code rate <= 1.  A code rate close
      to 1 indicates that a small number of repair symbols have been
      produced during the encoding process.

   'Systematic code'  FEC code in which the source symbols are part of
      the encoding symbols.  The Reed-Solomon codes introduced in this
      document are systematic.






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   'Source Block'  group of ADUs which are to be FEC protected as a
      single block.

   '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.











































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3.  Requirements notation

   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 [RFC2119].














































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4.  Architecture Overview

   The FEC Framework is described in terms of an additional layer
   between the transport layer (e.g.  UDP or DCCP) and protocols running
   over this transport layer.  Examples of such protocols are RTP, RTCP,
   etc.  As such, the data path interface between the FEC Framework and
   both underlying and overlying layers can be thought of as being the
   same as the standard interface to the transport layer - i.e. the data
   exchanged consists of datagram payloads each associated with a single
   ADU flow identified (in the case of UDP) by the standard 5-tuple {
   Source IP Address, Source Transport Port, Destination IP Address,
   Destination Transport Port, Transport Protocol }.  In the case that
   RTP is used for the repair flows, the source and repair data may be
   multiplexed using RTP onto a single UDP flow and must consequently be
   demultiplexed at the receiver.  There are various ways in which this
   multiplexing can be done, for example as described in [RFC4588].

   It is important to understand that the main purpose of the FEC
   Framework architecture is to allocate fuctional responsibilities to
   separately documented components in such a way that specific
   instances of the components can be combined in different ways to
   describe different protocols.

   The FEC Framework makes use of an FEC Scheme, in a similar sense to
   that defined in [RFC5052] and uses the terminology of that document.
   The FEC Scheme defines the FEC encoding and decoding and defines the
   protocol fields and procedures used to identify packet payload data
   in the context of the FEC Scheme.  The interface between the FEC
   Framework and an FEC Scheme, which is described in this document, is
   a logical one, which exists for specification purposes only.  At an
   encoder, the FEC Framework passes ADUs to the FEC Scheme for FEC
   encoding.  The FEC Scheme returns repair symbols with their
   associated Repair FEC Payload IDs, and in some case Source FEC
   Payload IDs, depending on the FEC Scheme.  At a decoder, the FEC
   Framework passes transport packet payloads (source and repair) to the
   FEC Scheme and the FEC Scheme returns additional recovered source
   packet payloads.

   This document defines certain FEC Framework Configuration Information
   which MUST be available to both sender and receiver(s).  For example,
   this information includes the specification of the ADU flows which
   are to be FEC protected, specification of the ADU flow(s) which will
   carry the FEC protection (repair) data and the relationship(s)
   between these source and repair flows (i.e. which source flow(s) are
   protected by each repair flow.  The FEC Framework Configuration
   Information also includes information fields which are specific to
   the FEC Scheme.  This information is analagous to the FEC Object
   Transmission Information defined in [RFC5052].



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   The FEC Framework does not define how the FEC Framework Configuration
   Information for the stream is communicated from sender to receiver.
   This must be defined by any Content Delivery Protocol specification
   as described in the following sections.

   In this architecture we assume that the interface to the transport
   layer supports the concepts of data units (referred to here as
   Application Data Units) to be transported and identification of ADU
   flows on which those data units are transported.  Since this is an
   interface internal to the architecture, we do not specify this
   interface explicitly, except to say that ADU flows which are distinct
   from the transport layer point of view (for example, distinct UDP
   flows as identified by the UDP source/destination ports/addresses)
   are also distinct on the interface between the transport layer and
   the FEC Framework.

   As noted above, RTP flows are a specific example of ADU flows which
   might be protected by the FEC Framework.  From the FEC Framework
   point of view, RTP source flows are ADU flows like any other, with
   the RTP header included within the ADU.

   Depending on the FEC Scheme, RTP may also be used as a transport for
   repair packet flows.  In this case an FEC Scheme must define an RTP
   Payload Format for the repair data.

   The architecture outlined above is illustrated in the Figure 1.  In
   this architecture, two RTP instances are shown, for the source and
   repair data respectively.  This is because the use of RTP for the
   source data is separate from and independent of the use of RTP for
   the repair data.  The appearance of two RTP instances is more natural
   when you consider that in many FEC codes, the repair payload contains
   repair data calculated across the RTP headers of the source packets.
   Thus a repair packet carried over RTP starts with an RTP header of
   its own which is followed (after the Repair Payload ID) by repair
   data containing bytes which protect the source RTP headers (as well
   as repair data for the source RTP payloads).















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   +--------------------------------------------+
   |                 Application                |
   +--------------------------------------------+
                          |
                          |
                          |
 + - - - - - - - - - - - - - - - - - - - - - - - -+
 | +--------------------------------------------+ |
   |            Application Layer               |
 | +--------------------------------------------+ |
                        |                  |
 | + -- -- -- -- -- -- -- -- -- -- --+     |      |
   |  RTP (optional)                 |     |
 | |                                 |     |-Configuration/Coordination
   +- -- -- -- -- -- -- -- -- -- -- -+     |
 |                    |                    |      |
                      | ADU flows          |
 |                    |                    v      |
   +--------------------------------------------+     +----------------+
 | |      FEC Framework (this document)         |<--->|   FEC Scheme   |
   +--------------------------------------------+     +----------------+
 |                |               |               |
           Source |        Repair |
 |                |               |               |
   +-- -- -- -- --|-- --+ -- -- -- -- -- + -- --+
 | |  RTP         |     | RTP processing |      |<--- Optional
   |              |     +-- -- -- |- -- -+      |       - dependent on
 | |        +-- -- -- -- -- -- -- |--+          | |       FEC Scheme
   |        |  RTP (de)multiplexing  |          |
 | +-- -- -- --- -- -- -- -- -- -- -- -- -- -- -+ |
                        |
 | +--------------------------------------------+ |
   |          Transport Layer (e.g. UDP)        |
 | +--------------------------------------------+ |
                        |
 | +--------------------------------------------+ |
   |                  IP                        |
 | +--------------------------------------------+ |
  Content Delivery Protocol
 + - - - - - - - - - - - - - - -  - - - - - - - - +

                   Figure 1: FEC Framework Architecture

   The contents of the transport payload for repair packets is fully
   defined by the FEC Scheme.  For a specific FEC Scheme, a means MAY be
   defined for repair data to be carried over RTP, in which case the
   repair packet payload format starts with the RTP header.  This
   corresponds to defining an RTP Payload Format for the specific FEC



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   Scheme.  Guidelines for writers of RTP Payload Formats are provided
   in [RFC2736].

   The use of RTP for repair packets is independent of the protocols
   used for source packets: if RTP is used for source packets then
   repair packets may or may not use RTP and vice versa (although it is
   unlikely that there are useful scenarios where non-RTP source flows
   are protected by RTP repair flows).  FEC Schemes are expected to
   recover entire transport payloads for recovered source packets in all
   cases.  For example if RTP is used for source flows, the FEC Scheme
   is expected to recover the entire UDP payload, including the RTP
   header.







































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5.  Procedural overview

5.1.  General

   The mechanism defined in this document does not place any
   restrictions on the Application Data Units which can be protected
   together, except that the Application Data Unit is carried over a
   supported transport protocol (See Section 8).  The data may be from
   multiple Source Data Flows that are protected jointly.  The FEC
   framework handles the Source Data Flows as a sequence of 'source
   blocks' each consisting of a set of Application Data Units, possibly
   from multiple Source Data Flows which are to be protected together.
   For example, each source block may be constructed from those
   Application Data Units related to a particular segment in time of the
   flow.

   At the sender, the FEC Framework passes the payloads for a given
   block to the FEC Scheme for FEC encoding.  The FEC Scheme performs
   the FEC encoding operation and returns the following information:

   o  optionally, FEC Payload IDs for each of the source payloads
      (encoded according to an FEC-Scheme-specific format)

   o  one or more FEC repair packet payloads

   o  FEC Payload IDs for each of the repair packet payloads (encoded
      according to an FEC-Scheme-specific format)

   The FEC framework then performs two operations: Firstly, it appends
   the FEC payload IDs, if provided, to each of the Application Data
   Units, and sends the resulting packets, known as 'FEC source
   packets', to the receiver and secondly it places the provided 'FEC
   repair packet payloads' and corresponding 'FEC Repair Payload IDs'
   appropriately to construct 'FEC repair packets' and send them to the
   receiver.  Note that FEC repair packets MAY be sent to a different
   multicast group or groups from the source packets.

   This document does not define how the sender determines which
   Application Data Units are included in which source blocks or the
   sending order and timing of FEC source and FEC repair packets.  A
   specific Content Delivery Protocol MAY define this mapping or it MAY
   be left as implementation dependent at the sender.  However, a CDP
   specification MUST define how a receiver determines a mimimum length
   of time that it should wait to receive FEC repair packets for any
   given source block.  FEC Schemes MAY define limitations on this
   mapping, such as maximum size of source blocks, but SHOULD NOT
   attempt to define specific mappings.  The sequence of operations at
   the sender is described in more detail in Section 5.2.



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   At the receiver, original Application Data Units are recovered by the
   FEC Framework directly from any FEC Source Packets received simply by
   removing the Source FEC Payload ID, if present.  The receiver also
   passes the contents of the received Application Data Units, plus
   their FEC Payload IDs to the FEC Scheme for possible decoding.

   If any Application Data Units related to a given source block have
   been lost, then the FEC Scheme may perform FEC decoding to recover
   the missing Application Data Units (assuming sufficient FEC Source
   and FEC Repair packets related to that source block have been
   received).

   Note that the receiver may need to buffer received source packets to
   allow time for the FEC Repair packets to arrive and FEC decoding to
   be performed before some or all of the received or recovered packets
   are passed to the application.  If such a buffer is not provided,
   then the application must be able to deal with the severe re-ordering
   of packets that may occur.  However, such buffering is Content
   Delivery Protocol and/or implementation-specific and is not specified
   here.  The receiver operation is described in more detail in
   Section 5.3

   The FEC Source packets MUST contain information which identifies the
   source block and the position within the source block (in terms
   specific to the FEC Scheme) occupied by the Application Data Unit.
   This information is known as the 'Source FEC Payload ID'.  The FEC
   Scheme is responsible for defining and interpreting this information.
   This information MAY be encoded into a specific field within the FEC
   Source packet format defined in this specification, called the
   Explicit Source FEC Payload ID field.  The exact contents and format
   of the Explicit Source FEC Payload ID field are defined by the FEC
   Scheme.  Alternatively, the FEC Scheme MAY define how the Source FEC
   Payload ID is derived from other fields within the source packets.
   This document defines the way that the Explicit Source FEC Payload ID
   field is appended to source packets to form FEC Source packets.

   The FEC Repair packets MUST contain information which identifies the
   source block and the relationship between the contained repair
   payloads and the original source block.  This is known as the 'Repair
   FEC Payload ID'.  This information MUST be encoded into a specific
   field, the Repair FEC Payload ID field, the contents and format of
   which are defined by the FEC Scheme.

   The FEC Scheme MAY use different FEC Payload ID field formats for FEC
   Source packets and FEC Repair packets.






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5.2.  Sender Operation

   It is assumed that the sender has constructed or received original
   data packets for the session.  These may be RTP, RTCP, MIKEY or
   indeed any other type of packet.  The following operations,
   illustrated in Figure 2, for the case of UDP repair flows and
   Figure 3 for the case of RTP repair flows, describe a possible way to
   generate compliant FEC Source packet and FEC repair packet streams:

      1.  Application Data Units are provided by the application.

      2.  A source block is constructed as specified in Section 6.2.

      3.  The source block is passed to the FEC Scheme for FEC encoding.
      The Source FEC Payload ID information of each Source packet is
      determined by the FEC Scheme.  If required by the FEC Scheme the
      Source FEC Payload ID is encoded into the Explicit Source FEC
      Payload ID field.

      4.  The FEC Scheme performs FEC Encoding, generating repair packet
      payloads from a source block and a Repair FEC Payload ID field for
      each repair payload.

      5.  The Explicit Source FEC Payload IDs (if used), Repair FEC
      Payload IDs and repair packet payloads are provided back from the
      FEC Scheme to the FEC Framework.

      6.  The FEC Framework constructs FEC Source packets according to
      Section 6.3 and FEC Repair packets according to Section 6.4 using
      the FEC Payload IDs and repair packet payloads provided by the FEC
      Scheme.

      7.  The FEC Source and FEC Repair packets are sent using normal
      transport layer procedures.  The port(s) and multicast group(s) to
      be used for FEC Repair packets are defined in the FEC Framework
      Configuration Information.  The FEC Source packets are sent using
      the same ADU flow identification information as would have been
      used for the original source packets if the FEC Framework were not
      present (for example, in the UDP case, the UDP source and
      destination addresses and ports on the IP datagram carrying the
      Source Packet will be the same whether or not the FEC Framework is
      applied).









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 +----------------------+
 |     Application      |
 +----------------------+
            |
            | (1) Application Data Units
            |
            v
 +----------------------+                           +------------------+
 |    FEC Framework     |                           |                  |
 |                      |-------------------------->|   FEC Scheme     |
 |(2) Construct source  |   (3) Source Block        |                  |
 |    blocks            |                           | (4) FEC Encoding |
 |(6) Construct FEC src |<--------------------------|                  |
 |    packets and FEC   |                           |                  |
 |    repair packets    |(5) Ex src FEC Payload Ids,|                  |
 +----------------------+    Repair FEC Payload Ids,+------------------+
            |                Repair symbols
            |
            | (7) FEC Source packets and FEC repair packets
            v
 +----------------------+
 |   Transport Layer    |
 |     (e.g. UDP )      |
 +----------------------+

                        Figure 2: Sender operation

























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 +----------------------+
 |     Application      |
 +----------------------+
            |
            | (1) Application Data Units
            v
 +----------------------+                           +------------------+
 |    FEC Framework     |                           |                  |
 |                      |-------------------------->|   FEC Scheme     |
 |(2) Construct source  |    (3) Source Block       |                  |
 |    blocks            |                           | (4) FEC Encoding |
 |(6) Construct FEC src |<--------------------------|                  |
 |    packets and FEC   |                           |                  |
 |    repair payloads   |(5) Ex src FEC Payload Ids,|                  |
 +----------------------+    Repair FEC Payload Ids,+------------------+
     |             |         Repair symbols
     |(7) Source   |
     |             |(7') Repair RTP payloads
     |   + -- -- -- -- -+
     |   |     RTP      |
     |   +-- -- -- -- --+
     v             v
 +----------------------+
 |   Transport Layer    |
 |     (e.g. UDP )      |
 +----------------------+

             Figure 3: Sender operation with RTP repair flows

5.3.  Receiver Operation

   The following describes a possible receiver algorithm, illustrated in
   Figure 4 and Figure 5 for the case of RTP repair flows, when
   receiving an FEC source or repair packet:

      1.  FEC Source Packets and FEC Repair packets are received and
      passed to the FEC Framework.  The type of packet (Source or
      Repair) and the Source Data Flow to which it belongs (in the case
      of source packets) is indicated by the ADU flow information which
      identifies the flow at the transport layer (for example source and
      destination ports and addresses in the case of UDP).

      1a.  In the special case that RTP is used for repair packets and
      source and repair packets are multiplexed onto the same UDP flow,
      then RTP demultiplexing is required to demultiplex source and
      repair flows.  However, RTP processing is applied only to the
      repair packets at this stage: source packets continue to be
      handled as UDP payloads (i.e. including their RTP headers).



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      2.  The FEC Framework extracts the Explicit Source FEC Payload ID
      field (if present) from FEC Source Packets and the Repair FEC
      Payload ID from FEC Repair Packets.

      3.  The Explicit Source FEC Payload IDs (if present), Repair FEC
      Payload IDs, FEC Source payloads and FEC Repair payloads are
      passed to the FEC Scheme.

      4.  The FEC Scheme uses the received FEC Payload IDs (and derived
      FEC Source Payload IDs in the case that the Explicit Source FEC
      Payload ID field is not used) to group source and repair packets
      into source blocks.  If at least one source packet is missing from
      a source block, and at least one repair packet has been received
      for the same source block then FEC decoding may be performed in
      order to recover missing source payloads.  The FEC Scheme
      determines whether source packets have been lost and whether
      enough data for decoding of any or all of the missing source
      payloads in the source block has been received.

      5.  The FEC Scheme returns the Application Data Units to the FEC
      Framework in the form of source blocks containing received and
      decoded Application Data Units and indications of any Application
      Data Units which were missing and could not be decoded.

      6.  The FEC Framework passes the received and recovered
      Application Data Units to the application.

   Note that the description above defines functionality
   responsibilities but does not imply a specific set of timing
   relationships.  For example, ADUs may eb provided to the application
   as soon as they are received or recovered (and hence potentially out-
   of-order) or they may be buffered are delivered to the application
   in-order.


















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+----------------------+
|     Application      |
+----------------------+
           ^
           | (6) Application Data Units
           |
+----------------------+                            +------------------+
|   FEC Framework      |                            |                  |
|                      |<---------------------------|   FEC Scheme     |
|(2)Extract FEC Payload| (5) Application Data Units |                  |
|   IDs and pass IDs & |                            | (4) FEC Decoding |
|   Payloads to FEC    |--------------------------->|                  |
|   Scheme             | (3) Ex src FEC Payload IDs,|                  |
+----------------------+     FEC Repair Payload IDs,+------------------+
           ^                 FEC Source Payloads,
           |                 FEC Repair Payloads
           |
           | (1) FEC Source packets and FEC repair packets
           |
+----------------------+
|   Transport Layer    |
|     (e.g. UDP )      |
+----------------------+

                       Figure 4: Receiver Operation


























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+----------------------+
|     Application      |
+----------------------+
           ^
           | (6) Application Data Units
           |
+----------------------+                            +------------------+
|   FEC Framework      |                            |                  |
|                      |<---------------------------|   FEC Scheme     |
|(2)Extract FEC Payload| (5) Application Data Units |                  |
|   IDs and pass IDs & |                            | (4) FEC Decoding |
|   Payloads to FEC    |--------------------------->|                  |
|   Scheme             | (3) Ex src FEC Payload IDs,|                  |
+----------------------+     FEC Repair Payload IDs,+------------------+
    ^             ^          FEC Source Payloads,
    |             |          FEC Repair Payloads
    |Source pkts  |
    |             |(1a) FEC repair payloads
+-- |- -- -- -- -- -- -+
|RTP| | RTP processing |
|   | +-- -- -- --|-- -+
| +-- -- -- -- -- |--+ |
| | RTP demux        | |
+-- -- -- -- -- -- -- -+
           |  (1) FEC Source packets and FEC repair packets
+----------------------+
|   Transport Layer    |
|     (e.g. UDP )      |
+----------------------+

                       Figure 5: Receiver Operation

   Note that the above procedure may result in a situation in which not
   all ADUs are recovered.

   Source packets which are correctly received and those which are
   reconstructed MAY be delivered to the application out of order and in
   a different order from the order of arrival at the receiver.
   Alternatively, buffering and packet re-ordering MAY be applied to re-
   order received and reconstructed source packets into the order they
   were placed into the source block, if that is necessary according to
   the application.









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6.  Protocol Specification

6.1.  General

   This section specifies the protocol elements for the FEC Framework.
   Three components of the protocol are defined in this document and are
   described in the following sections:

      1.  Construction of a source block from Application Data Units.
      The FEC code will be applied to this source block to produce the
      repair payloads.

      2.  A format for packets containing source data.

      3.  A format for packets containing repair data.

   The operation of the FEC Framework is governed by certain FEC
   Framework Configuation Information.  This configuration information
   is also defined in this section.  A complete protocol specification
   that uses this framework MUST specify the means to determine and
   communicate this information between sender and receiver.

6.2.  Structure of the source block

   The FEC Framework and FEC Scheme exchange Application Data Units in
   the form of source blocks.  A source block is generated by the FEC
   Framework from an ordered sequence of Application Data Units.  The
   allocation of Application Data Units to blocks is dependent on the
   application.  Note that some Application Data Units may not be
   included in any block.  Each Source Block provided to the FEC scheme
   consists of an ordered sequence of Application Data Units where the
   following information is provided for each ADU:

   o  A description of the Source Data Flow with which the Application
      Data Unit is associated (See 6.5)

   o  The Application Data Unit itself

   o  The length of the Application Data Unit

6.3.  Packet format for FEC Source packets

   The packet format for FEC Source packets MUST be used to transport
   the payload of an original source packet.  As depicted in Figure 6,
   it consists of the original packet, optionally followed by the
   Explicit Source FEC Payload ID field.  The FEC Scheme determines
   whether the Explicit Source FEC Payload ID field is required.  This
   determination is specific to each ADU flow.



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   +------------------------------------+
   |             IP header              |
   +------------------------------------+
   |          Transport header          |
   +------------------------------------+
   |        Application Data Unit       |
   +------------------------------------+
   |   Explicit Source FEC Payload ID   |
   +------------------------------------+

    Figure 6: Structure of the FEC packet format for FEC Source packets

   The FEC Source packets MUST be sent using the same ADU flow as would
   have been used for the original source packets if the FEC Framework
   were not present.  The transport payload of the FEC Source packet
   MUST consist of the Application Data Unit followed by the Explicit
   Source FEC Payload ID field, if required.

   The Explicit Source FEC Payload ID field contains information
   required to associate the source packet with a source block and for
   the operation of the FEC algorithm and is defined by the FEC Scheme.
   The format of the Source FEC Payload ID field is defined by the FEC
   Scheme.  Note that in the case that the FEC Scheme or CDP defines a
   means to derive the Source FEC Payload ID from other information in
   the packet (for example the a sequence number of some kind used by
   the application protocol), then the Source FEC Payload ID field is
   not included in the packet.  In this case the original source packet
   and FEC Source Packet are identical.

   Since the addition of the Explicit Source FEC Payload ID increases
   the packet length, then in applications where avoidance of IP packet
   fragmentation is a goal, Content Delivery Protocols SHOULD consider
   the Explicit Source FEC Payload ID size when determining the size of
   Application Data Units that will be delivered using the FEC
   Framework.

   Note: The Explicit Source FEC Payload ID is placed at the end of the
   packet so that in the case that Robust Header Compression [RFC3095]
   or other header compression mechanisms are used and in the case that
   a ROHC profile is defined for the protocol carried within the
   transport payload (for example RTP), then ROHC will still be applied
   for the FEC Source packets.  Applications that may be used with this
   Framework should consider that FEC Schemes may add this Explicit
   Source FEC Payload ID and thereby increase the packet size.

   In many applications, support for Forward Error Correction is added
   to a pre-existing protocol and in this case use of the Explicit
   Source FEC Payload ID may break backwards compatibility, since source



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   packets are modified.

6.3.1.  Generic Explicit Source FEC Payload Id

   In order to apply FEC protection using multiple FEC Schemes to a
   single source flow all schemes must use the same Explicit Source FEC
   Payload Id format.  In order to enable this, it is RECOMMENDED that
   FEC Schemes support the Generic Explicit Source FEC Payload Id format
   described below.

   The Generic Explicit Source FEC Payload Id has length 2 bytes and
   consists of an unsigned packet sequence number in network byte order.
   The allocation of sequence numbers to packets is independent of any
   FEC Scheme and of the Source Block contruction, except that the use
   of this sequence number places a constraint on source block
   construction source packets within a given source block MUST have
   consecutive sequence numbers (where consecutive includes wrap-around
   from 65535 to 0).  Sequence numbers SHOULD NOT be reused until all
   values in the sequence number space have been used.

6.4.  Packet Format for FEC Repair packets

   The packet format for FEC Repair packets is shown in Figure 7.  The
   transport payload consists of a Repair FEC Payload ID field followed
   by repair data generated in the FEC encoding process.

   +------------------------------------+
   |             IP header              |
   +------------------------------------+
   |          Transport header          |
   +------------------------------------+
   |       Repair FEC Payload ID        |
   +------------------------------------+
   |          Repair Symbols            |
   +------------------------------------+

                Figure 7: Packet format for repair packets

   The Repair FEC Payload ID field contains information required for the
   operation of the FEC algorithm at the receiver.  This information is
   defined by the FEC Scheme.  The format of the Repair FEC Payload ID
   field is defined by the FEC Scheme.

6.4.1.  Packet Format for FEC Repair packets over RTP

   For FEC Schemes which specify the use of RTP for repair packets, the
   packet format for repair packets includes an RTP header as shown in
   Figure 8.



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   +------------------------------------+
   |             IP header              |
   +------------------------------------+
   |      Transport header (UDP)        |
   +------------------------------------+
   |             RTP Header             |
   +------------------------------------+
   |       Repair FEC Payload ID        |
   +------------------------------------+
   |          Repair Symbols            |
   +------------------------------------+

                Figure 8: Packet format for repair packets

6.5.  FEC Framework Configuration Information

   The FEC Framework Configuration Information is information that the
   FEC Framework needs in order to apply FEC protection to the ADU
   flows.  A complete Content Delivery Protocol specification that uses
   the framework specified here MUST include details of how this
   information is derived and communicated between sender and receiver.

   The FEC Framework Configuration Information includes identification
   of the set of Source Data Flows.  For example, in the case of UDP,
   each Source Data Flow is uniquely identified by a tuple { Source IP
   Address, Destination IP Address, Source UDP port, Destination UDP
   port }.  Note that in some applications some of these fields may be
   wildcarded, so that the flow is identified by a subset of the fields
   and in particular in many applications the limited tuple {
   Destination IP Address, Destination UDP port } is sufficient.

   A single instance of the FEC Framework provides FEC protection for
   packets of the specified set of Source Data Flows, by means of one or
   more packet flows consisting of repair packets.  The FEC Framework
   Configuation Information includes, for each instance of the FEC
   Framework:

      1.  Identification of the packet flow(s) carrying FEC Repair
      packets, known as the FEC repair flow(s).

      2.  For each Source Data Flow protected by the FEC repair flow(s):

         a.  Defintion of the Source Data Flow carrying source packets
         (for example, by means of a tuple as describe above for UDP).

         b.  An integer identifier for this flow definition (i.e.
         tuple).  This identifier MUST be unique amongst all Source Data
         Flows which are protected by the same FEC repair flow.



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      3.  The FEC Encoding ID, identifying the FEC Scheme

      4.  The length of the Explicit Source FEC Payload Id, in bytes

      5.  Zero or more FEC-Scheme-specific information elements, each
      consisting of a name and a value where the valid element names and
      value ranges are defined by the FEC Scheme

   Multiple instances of the FEC Framework, with separate and
   independent FEC Framework Configuration Information, may be present
   at a sender or receiver.  A single instance of the FEC Framework
   protects packets of the Source Data Flows identified in (2) above
   i.e. all packets sent on those flows MUST be FEC Source packets as
   defined in Section 6.3.  A single Source Data Flow may be protected
   by multiple instances of the FEC Framework.

   The integer flow identifier identified in 2(b) is a "shorthand" to
   identify source flows between the FEC Framework and the FEC Scheme.
   The reason for defining this as an integer, and including it in the
   FEC Framework Configuration Information is so that the FEC Scheme at
   the sender and receiver may use it to identify the source flow with
   which a recovered packet is associated.  The integer flow identifier
   may therefore take the place of the complete flow description (e.g.
   UDP 4-tuple).

   Whether and how this flow identifier is used is defined by the FEC
   Scheme.  Since source packets are directly associated with a flow by
   virtue of their packet headers, this identifier need not be carried
   in source packets.  Since repair packets may provide protection for
   multiple source flows, repair packets would either not carry the
   identifier at all or may carry multiple identifiers.  However, in any
   case, the flow identifier associated with a particular source packet
   may be recovered from the repair packets as part of an FEC decoding
   operation.  Integer flow identifiers SHOULD be allocated starting
   from zero and increasing by one for each flow.

   A single FEC repair flow provides repair packets for a single
   instance of the FEC Framework.  Other packets MUST NOT be sent within
   this flow i.e. all packets in the FEC repair flow MUST be FEC repair
   packets as defined in Section 6.4 and MUST relate to the same FEC
   Framework instance.

   In the case that RTP is used for repair packets, the identification
   of the repair packet flow MAY also include the RTP Payload Type to be
   used for repair packets.

   FEC Scheme-specific information elements MAY be encoded into a text
   string for transport within Content Delivery Protocols as according



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   to the following ABNF [RFC5234]:

          scheme-specific-info  = [ element *( ',' element ) ]
          element               = name ':' value
          name                  = token
          token                 = 1*<any CHAR except CTLs or separators>
          value                 = *<any CHAR except CTLs or separators>
          separators            = "(" | ")" | "<" | ">" | "@"
                                | "," | ";" | ":" | "\" | <">
                                | "/" | "[" | "]" | "?" | "="
                                | "{" | "}" | SP | HT

6.6.  FEC Scheme requirements

   In order to be used with this framework, an FEC Scheme MUST be
   capable of processing data arranged into blocks of Application Data
   Units (source blocks).

   A specification for a new FEC scheme MUST include the following
   things:

   1.  The FEC Encoding ID value that uniquely identifies the FEC
       scheme.  This value MUST be registered with IANA as described in
       Section 11.

   2.  The type, semantics and encoding format of the Repair FEC Payload
       ID.

   3.  The name, type, semantics and text value encoding rules for zero
       or more FEC Scheme-specific FEC Framework Configuration
       Information elements.  Names must conform to the
       "name"__production and values encodings to the "value" __
       production defined in Section 6.5

   4.  A full specification of the FEC code.

       This specification MUST precisely define the valid FEC-Scheme-
       Specific FEC Framework Configuration Information values, the
       valid FEC Payload ID values and the valid packet payload sizes
       (where packet payload refers to the space within a packet
       dedicated to carrying encoding symbol bytes).

       Furthermore, given a source block as defined in Section 6.2,
       valid values of the FEC-Scheme-Specific FEC Framework
       Configuration Information, a valid Repair FEC Payload ID value
       and a valid packet payload size, the specification MUST uniquely
       define the values of the encoding symbol bytes to be included in
       the repair packet payload of a packet with the given Repair FEC



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       Payload ID value.

       A common and simple way to specify the FEC code to the required
       level of detail is to provide a precise specification of an
       encoding algorithm which, given a source block, valid values of
       the FEC-Scheme-Specific FEC Framework Configuration Information,
       a valid Repair FEC Payload ID value and a valid packet payload
       size as input produces the exact value of the encoding symbol
       bytes as output.

   5.  A description of practical encoding and decoding algorithms.

       This description need not be to the same level of detail as for
       the encoding above, however it must be sufficient to demonstrate
       that encoding and decoding of the code is both possible and
       practical.

   FEC scheme specifications MAY additionally define the following:

   1.  Type, semantics and encoding format of an Explicit Source FEC
       Payload ID.

   Whenever an FEC scheme specification defines an 'encoding format' for
   an element, this must be defined in terms of a sequence of bytes
   which can be embedded within a protocol.  The length of the encoding
   format MUST either be fixed or it must be possible to derive the
   length from examining the encoded bytes themselves.  For example, the
   initial bytes may include some kind of length indication.

   FEC scheme specifications SHOULD use the terminology defined in this
   document and SHOULD follow the following format:

   1. Introduction  <describe the use-cases addressed by this FEC
      scheme>


   2. Formats and Codes

      2.1 Source FEC Payload ID(s)  <Either, define the type and format
         of the Explicit Source FEC Payload ID, or define how Source FEC
         Payload ID information is derived from source packets>


      2.2 Repair FEC Payload Id  <Define the type and format of the
         Repair FEC Payload ID>






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      2.3 FEC Framework Configuration Information  <Define the names,
         types and text value encoding formats of the FEC Scheme-
         specific FEC Framework configuration information elements>

   3. Procedures  <describe any procedures which are specific to this
      FEC scheme, in particular derivation and interpretation of the
      fields in the FEC Payload ID and FEC Scheme-specific FEC Framework
      configuration information.>

   4. FEC code specification  <provide a complete specification of the
      FEC Code>

   Specifications MAY include additional sections, for example,
   examples.

   Each FEC scheme MUST be specified independently of all other FEC
   schemes; for example, in a separate specification or a completely
   independent section of larger specification (except, of course, a
   specification of one FEC Scheme may include portions of another by
   reference).

   Where an RTP Payload Format is defined for repair data for a specific
   FEC Scheme, the RTP Payload Format and the FEC Scheme MAY be
   specified within the same document.



























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7.  Feedback

   Many applications require some kind of feedback on transport
   performance: how much data arrived at the receiver, at what rate,
   when etc.  When FEC is added to such applications, feedback
   mechanisms may also need to be enhanced to report on the performance
   of the FEC (for example how much lost data was recovered by the FEC).

   When used to provide instrumentation for engineering purposes, it is
   important to remember that FEC is generally applied to relatively
   small blocks of data (in time) and so feedback information averaged
   over longer periods of time than the FEC block size will likely not
   provide sufficient information for engineering purposes.  For example
   see [RFC5725].

   Applications which used feedback for congestion control purposes MUST
   calculate such feedback on the basis of packets received before FEC
   recovery is applied.  If this requirement conflicts with other uses
   of the feedback information then the application MUST be enhanced to
   support both information calculated pre- and post- FEC recovery.
   This is to ensure that congestion control mechanisms operate
   correctly based on congestion indications received from the network,
   rather than on post-FEC recovery information which would give an
   inaccurate picture of congestion conditions.

   New applications which require such feedback SHOULD use RTP/RTCP
   [RFC3550].
























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8.  Transport Protocols

   The following transport protocols are supported:

   o  User Datagram Protocol (UDP)

   o  Datagram Congestion Control Protocol (DCCP)












































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9.  Congestion Control

   This section starts with a informative section on the motivation of
   the normative requirements for congestion control, which are spelled
   out in Section 9.1.

      Informative Note: The enforcement of Congestion Control (CC)
      principles has gained a lot of momentum in the IETF over the
      recent years.  While the need of CC over the open Internet is
      unquestioned, and the goal of TCP friendliness is generally agreed
      for most (but not all) applications, the subject of congestion
      detection and measurement in heterogenous networks can hardly be
      considered as solved.  Most congestion control algorithms detect
      and measure congestion by taking (primarily or exclusively) the
      packet loss rate into account.  This appears to be inappropriate
      in environments where a large percentage of the packet losses are
      the result link-layer errors and independent of the network load.
      Note that such environments exist in the "open Internet", as well
      as in "closed" IP based networks.  An example for the former would
      be the use of IP/UDP/RTP based streaming from an Internet-
      connected streaming server to a device attached to the Internet
      using cellular technology.

      The authors of this draft are primarily interested in applications
      where the application reliability requirements and end-to-end
      reliability of the network differ, such that it warrants higher
      layer protection of the packet stream - for example due to the
      presence of unreliable links in the end-to-end path - and where
      real-time, scalability or other constraints prohibit the use of
      higher layer (transport or application) feedback.  A typical
      example for such applications is multicast and broadcast streaming
      or multimedia transmission over heterogenous networks.  In other
      cases, application reliability requirements may be so high that
      the required end-to-end reliability is difficult to achieve even
      over wired networks.  Furthermore the end-to-end network
      reliability may not be known in advance.

      This FEC framework is not proposed, nor intended, as a QoS
      enhancement tool to combat losses resulting from highly congested
      networks.  It should not be used for such purposes.

      In order to prevent such mis-use, one approach would be to leave
      standardisation to bodies most concerned with the problem
      described above.  However, the IETF defines base standards used by
      several bodies, including DVB, 3GPP, 3GPP2, all of which appear to
      share the environment and the problem described.





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      Another approach would be to write a clear applicability statement
      - for example restricting use of the framework to networks with
      wireless links.  However, there may be applications where the use
      of FEC may be justified to combat congestion-induced packet losses
      - particularly in lightly loaded networks, where congestion is the
      result of relatively rare random peaks in instantaneous traffic
      load - thereby intentionally violating congestion control
      principles.  One possible example for such an application could be
      a no-matter-what, brute-force FEC protection of traffic generated
      as an emergency signal.

      We propose a third approach, which is to require at a minimum that
      the use of this framework with any given application, in any given
      environment, does not cause congestion issues which the
      application alone would not itself cause i.e. the use of this
      framework must not make things worse.

      Taking above considerations into account, the normative text of
      this section implements a small set of constraints for the FEC,
      which are mandatory for all senders compliant with this FEC
      framework.  Further restrictions may be imposed for certain
      Content Delivery Protocols.  In this it follows the spirit of the
      congestion control section of RTP and its Audio-Visual Profile
      (RFC3550/STD64 and RFC3551/STD65).

      One of the constraints effectively limits the bandwidth for the
      FEC protected packet stream to be no more than roughly twice as
      high as the original, non-FEC protected packet stream.  This
      disallows the (static or dynamic) use of excessively strong FEC to
      combat high packet loss rates, which may otherwise be chosen by
      naively implemented dynamic FEC-strength selection mechanisms.  We
      acknowledge that there may be a few exotic applications, e.g.  IP
      traffic from space-based senders, or senders in certain hardened
      military devices, which would warrant a higher FEC strength.
      However, in this specification we give preference to the overall
      stability and network friendliness of the average application, and
      for those a factor of 2 appears to be appropriate.

      A second constraint requires that the FEC protected packet stream
      be in compliance with the congestion control in use for the
      application and network in question.

9.1.  Normative requirements

   The bandwidth of FEC Repair packet flows MUST NOT exceed the
   bandwidth of the source packet flows being protected.  In addition,
   whenever the source packet flow bandwidth is adapted due to the
   operation of congestion control mechanisms, the FEC repair packet



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   flow bandwidth MUST be similarly adapted.


















































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10.  Security Considerations

   The application of FEC protection to a stream does not provide any
   kind of security protection.

   If security services are required for the stream, then they MUST
   either be applied to the original source data before FEC protection
   is applied, or to both the source and repair data, after FEC
   protection has been applied.

   If integrity protection is applied to source packets before FEC
   protection is applied, and no further integrity protection is applied
   to repair packets, then a denial of service attack is possible if an
   attacker is in a position to inject fake repair transport payloads.
   If received by a receiver, such fake repair transport payloads could
   cause incorrect FEC decoding resulting in incorrect Application Data
   Units being passed up to the application protocol.  A similar attack
   may be possible if an attacker is in a position to inject fack FEC
   Framework Configuration Information or fake FEC Payload IDs.  Such
   incorrect decoded Application Data Units would then be detected by
   the source integrity protection and discarded, resulting in partial
   or complete denial of service.  Therefore, in such environments,
   integrity protection MUST also be applied to the FEC repair transport
   payloads, FEC Framework Configuration Information and FEC Payload
   IDs, for example using IPsec to integrity protect all packets.
   Receivers MUST also verify the integrity of source symbols before
   including the source symbols into the source block for FEC purposes.

   It is possible that multiple streams with different confidentiality
   requirements (for example, the streams may be visible to different
   sets of users) can be FEC protected by a single repair stream.  This
   scenario is not recommended, since resources will be used to
   distribute and decode data which cannot then be decrypted by at least
   some receivers.  However, in this scenario, confidentiality
   protection MUST be applied before FEC encoding of the streams,
   otherwise repair transport payload may be used by a receiver to
   decode unencrypted versions of source streams which they do not have
   permissionions to view.













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11.  IANA Considerations

   FEC Schemes for use with this framework may be identified in
   protocols using FEC Encoding IDs.  Values of FEC Encoding IDs are
   subject to IANA registration.  They are in the registry named "FEC
   Framework (FECFRAME) FEC Encoding IDs" located at time of publication
   at <tbd>.

   The values that can be assigned within the FEC Framework (FECFRAME)
   FEC Encoding ID registry are numeric indexes in the range [0, 255],
   boundaries included.  Assignment requests are granted on a "IETF
   Consensus" basis as defined in[RFC5226] .  Section 6.6 defines
   explicit requirements that documents defining new FEC Encoding IDs
   should meet.





































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12.  Acknowledgments

   This document is based in part on [I-D.watson-tsvwg-fec-sf] and so
   thanks are due to the additional authors of that document, Mike Luby,
   Magnus Westerlund and Stephan Wenger.  That document was in turn
   based on the FEC streaming protocol defined by 3GPP in [MBMSTS] and
   thus thanks are also due to the participants in 3GPP TSG SA working
   group 4.  Further thanks are due to the members of the FECFRAME
   working group for their comments and review.










































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13.  References

13.1.  Normative references

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, July 2001.

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

13.2.  Informative references

   [I-D.watson-tsvwg-fec-sf]
              Watson, M., "Forward Error Correction (FEC) Streaming
              Framework", draft-watson-tsvwg-fec-sf-00 (work in
              progress), July 2005.

   [RFC5725]  Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
              Report Block Type for RTP Control Protocol (RTCP) Extended
              Reports (XRs)", RFC 5725, February 2010.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

   [RFC2736]  Handley, M. and C. Perkins, "Guidelines for Writers of RTP
              Payload Format Specifications", BCP 36, RFC 2736,
              December 1999.

   [MBMSTS]   3GPP, "Multimedia Broadcast/Multicast Service (MBMS);



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              Protocols and codecs", 3GPP TS 26.346, April 2005.


















































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Author's Address

   Mark Watson
   Qualcomm, Inc.
   3165 Kifer Road
   Santa Clara, CA  95051
   U.S.A.

   Email: watson@qualcomm.com










































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