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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                                                  M. Watson
Internet-Draft                                             Netflix, Inc.
Intended status: Standards Track                                A. Begen
Expires: December 11, 2011                                         Cisco
                                                                 V. Roca
                                                                   INRIA
                                                            June 9, 2011


                Forward Error Correction (FEC) Framework
                    draft-ietf-fecframe-framework-15

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 FEC 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 FEC 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 11, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   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 and Abbreviations  . . . . . . . . . . . . . . . .  6
   3.  Architecture Overview  . . . . . . . . . . . . . . . . . . . .  9
   4.  Procedural Overview  . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.2.  Sender Operation . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  Receiver Operation . . . . . . . . . . . . . . . . . . . . 17
   5.  Protocol Specification . . . . . . . . . . . . . . . . . . . . 21
     5.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . . 21
     5.2.  Structure of the Source Block  . . . . . . . . . . . . . . 21
     5.3.  Packet Format for FEC Source Packets . . . . . . . . . . . 21
       5.3.1.  Generic Explicit Source FEC Payload ID . . . . . . . . 23
     5.4.  Packet Format for FEC Repair Packets . . . . . . . . . . . 23
       5.4.1.  Packet Format for FEC Repair Packets over RTP  . . . . 24
     5.5.  FEC Framework Configuration Information  . . . . . . . . . 24
     5.6.  FEC Scheme Requirements  . . . . . . . . . . . . . . . . . 26
   6.  Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   7.  Transport Protocols  . . . . . . . . . . . . . . . . . . . . . 30
   8.  Congestion Control . . . . . . . . . . . . . . . . . . . . . . 31
     8.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  Normative Requirements . . . . . . . . . . . . . . . . . . 32
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
     9.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . . 33
     9.2.  Attacks Against the Data Flows . . . . . . . . . . . . . . 34
       9.2.1.  Access to Confidential Content . . . . . . . . . . . . 34
       9.2.2.  Content Corruption . . . . . . . . . . . . . . . . . . 35
     9.3.  Attacks Against the FEC Parameters . . . . . . . . . . . . 36
     9.4.  When Several Source Flows are to be Protected Together . . 37
     9.5.  Baseline Secure FEC Framework Operation  . . . . . . . . . 37
   10. Operations and Management Considerations . . . . . . . . . . . 39
     10.1. What are the Key Aspects to Consider?  . . . . . . . . . . 39
     10.2. Operational and Management Recommendations . . . . . . . . 40
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 44
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 45
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     13.1. Normative references . . . . . . . . . . . . . . . . . . . 46
     13.2. Informative references . . . . . . . . . . . . . . . . . . 46
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48











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1.  Introduction

   Many applications have a requirement to transport a continuous stream
   of packetized 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 (FEC) 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 and, the term "Forward
   Error Correction" is more widely used.

   This document defines a framework for the definition of CDPs which
   provide for FEC protection for 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 CDP, but rather defines only those aspects that are
   expected to be common to all CDPs 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 CDP specification which makes use of
   this framework will address these signaling requirements.  However,
   this document does specify the information which is required by the
   FEC Framework at the sender and receiver, e.g., 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 fulfill a
   number of requirements defined in this document.  These requirements
   are different from those defined in [RFC5052] for FEC schemes for



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   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 in this document.

   Since the RTP protocol is (often) used over UDP, this framework can
   be applied to RTP flows as well.  FEC repair packets may be sent
   directly over UDP or RTP.  The latter approach has the advantage that
   RTP instrumentation, based on RTP Control Protocol (RTCP), can be
   used for the repair flow.  Additionally, the post-repair RTCP
   extended reports [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 and Abbreviations

   Application Data Unit (ADU): 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
   port, destination IP address, destination port, transport protocol}).

   AL-FEC: Application-layer Forward Error Correction.

   Application Protocol: Control protocol used to establish and control
   the source flow being protected, e.g., RTSP.

   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 FEC
   capabilities.

   FEC Code: An algorithm for encoding data such that the encoded data
   flow is resilient to data loss.  Note that in general FEC codes may
   also be used to make a data flow resilient to corruption, but that is
   not considered in this document.

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

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

   FEC Payload ID: Information which identifies the contents of a packet
   with respect to the FEC scheme.

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

   FEC Scheme: A specification which defines the additional protocol
   aspects required to use a particular FEC code with the FEC Framework.

   FEC Source Packet: At a sender (respectively, at a receiver) a
   payload submitted to (respectively, received from) the transport
   protocol containing an ADU along with an optional Explicit Source FEC
   Payload ID.

   Protection Amount: The relative increase in data sent due to the use



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   of FEC.

   Repair Flow: The packet flow carrying FEC data.

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

   Source Flow: The packet flow to which FEC protection is to be
   applied.  A source flow consists of ADUs.

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

   Source Protocol: A protocol used for the source flow being protected,
   e.g., RTP.

   Transport Protocol: The protocol used for transport of the source and
   repair flows, e.g., UDP and DCCP.

   The following definitions are aligned with [RFC5052]:

   Code Rate: 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.

   Encoding Symbol: Unit of data generated by the encoding process.
   With systematic codes, source symbols are part of the encoding
   symbols.

   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.

   Repair Symbol: Encoding symbol that is not a source symbol.

   Source Block: Group of ADUs which are to be FEC protected as a single
   block.

   Source Symbol: Unit of data used during the encoding process.

   Systematic Code: FEC code in which the source symbols are part of the
   encoding symbols.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

















































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3.  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.  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 by the
   standard 5-tuple {Source IP address, source port, destination IP
   address, destination port, transport protocol}.  In the case that RTP
   is used for the repair flows, the source and repair data can be
   multiplexed using RTP onto a single UDP flow and needs to be
   consequently 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 functional 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 cases 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(s)).  The FEC Framework Configuration
   Information also includes information fields which are specific to
   the FEC scheme.  This information is analogous 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 has to be defined by any CDP 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 (ADUs)) 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.  We do require 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
   addresses/ports) 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 can also be used as a transport for
   repair packet flows.  In this case an FEC scheme has to define an RTP
   payload format for the repair data.

   The architecture outlined above is illustrated in the Figure 1.  In
   this architecture, two (optional) 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 one considers 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 Layer    |     | RTP Processing |      | |
      | (Optional)   |     +-- -- -- |- -- -+      |
    | |        +-- -- -- -- -- -- -- |--+          | |
      |        |  RTP (De)multiplexing  |          |
    | +-- -- -- --- -- -- -- -- -- -- -- -- -- -- -+ |
                             |
    | +--------------------------------------------+ |
      |          Transport Layer (e.g., UDP)       |
    | +--------------------------------------------+ |
                             |
    | +--------------------------------------------+ |
      |                     IP                     |
    | +--------------------------------------------+ |

    | Content Delivery Protocol                      |
    + - - - - - - - - - - - - - - -  - - - - - - - - +

                   Figure 1: FEC Framework architecture

   The content 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



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   corresponds to defining an RTP payload format for the specific FEC
   scheme.

   The use of RTP for repair packets is independent of the protocols
   used for source packets: if RTP is used for source packets, 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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4.  Procedural Overview

4.1.  General

   The mechanism defined in this document does not place any
   restrictions on the ADUs which can be protected together, except that
   the ADU is carried over a supported transport protocol (See
   Section 7).  The data can be from multiple source flows that are
   protected jointly.  The FEC Framework handles the source flows as a
   sequence of source blocks each consisting of a set of ADUs, possibly
   from multiple source flows which are to be protected together.  For
   example, each source block can be constructed from those ADUs 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.  First, it appends
   the Source FEC Payload IDs, if provided, to each of the ADUs, and
   sends the resulting packets, known as FEC source packets, to the
   receiver, and second it places the provided FEC repair packet
   payloads and corresponding Repair FEC Payload IDs appropriately to
   construct FEC repair packets and send them to the receiver.

   This document does not define how the sender determines which ADUs
   are included in which source blocks or the sending order and timing
   of FEC source and repair packets.  A specific CDP 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
   minimum length of time that it needs to 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 4.2.

   At the receiver, original ADUs 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 ADUs, plus their FEC Payload IDs to the FEC



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   scheme for possible decoding.

   If any ADUs related to a given source block have been lost, then the
   FEC scheme can perform FEC decoding to recover the missing ADUs
   (assuming sufficient FEC source and repair packets related to that
   source block have been received).

   Note that the receiver might 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 has to be able to deal with the severe
   re-ordering of packets that can occur.  However, such buffering is
   CDP and/or implementation-specific and is not specified here.  The
   receiver operation is described in more detail in Section 4.3.

   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 ADU.  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 schemes.  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 schemes.

   The FEC scheme MAY use different FEC Payload ID field formats for
   source and repair packets.

4.2.  Sender Operation

   It is assumed that the sender has constructed or received original
   data packets for the session.  These could be carrying any type of
   data.  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 source and
   repair flows:



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   1.  ADUs are provided by the application.

   2.  A source block is constructed as specified in Section 5.2.

   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 5.3 and FEC repair packets according to Section 5.4 using
       the FEC Payload IDs and repair packet payloads provided by the
       FEC scheme.

   7.  The FEC source and 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) ADUs
              |
              v
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |                |
   |                      |-------------------------->|   FEC Scheme   |
   |(2) Construct source  |(3) Source Block           |                |
   |    blocks            |                           |(4) FEC Encoding|
   |(6) Construct FEC     |<--------------------------|                |
   |    source and repair |                           |                |
   |    packets           |(5) Explicit Source FEC    |                |
   +----------------------+    Payload IDs            +----------------+
              |                Repair FEC Payload IDs
              |                Repair symbols
              |
              |(7) FEC source and repair packets
              v
   +----------------------+
   |   Transport Layer    |
   |     (e.g., UDP)      |
   +----------------------+

                        Figure 2: Sender operation
























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

             Figure 3: Sender operation with RTP repair flows

4.3.  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 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.

       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



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       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).

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

   3.  The Explicit Source FEC Payload IDs (if present), Repair FEC
       Payload IDs, FEC source and 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 can 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 ADUs to the FEC Framework in the form
       of source blocks containing received and decoded ADUs and
       indications of any ADUs which were missing and could not be
       decoded.

   6.  The FEC Framework passes the received and recovered ADUs to the
       application.

   The description above defines functionality responsibilities but does
   not imply a specific set of timing relationships.  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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   +----------------------+
   |     Application      |
   +----------------------+
              ^
              |
              |(6) ADUs
              |
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |                |
   |                      |<--------------------------|   FEC Scheme   |
   |(2)Extract FEC Payload|(5) ADUs                   |                |
   |   IDs and pass IDs & |                           |(4) FEC Decoding|
   |   payloads to FEC    |-------------------------->|                |
   |   scheme             |(3) Explicit Source FEC    |                |
   +----------------------+    Payload IDs            +----------------+
              ^                Repair FEC Payload IDs
              |                Source payloads
              |                Repair payloads
              |
              |(1) FEC source and repair packets
              |
   +----------------------+
   |   Transport Layer    |
   |     (e.g., UDP)      |
   +----------------------+

                       Figure 4: Receiver operation
























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   +----------------------+
   |     Application      |
   +----------------------+
              ^
              |
              |(6) ADUs
              |
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |                |
   |                      |<--------------------------|   FEC Scheme   |
   |(2)Extract FEC Payload|(5) ADUs                   |                |
   |   IDs and pass IDs & |                           |(4) FEC Decoding|
   |   payloads to FEC    |-------------------------->|                |
   |   scheme             |(3) Explicit Source FEC    |                |
   +----------------------+    Payload IDs            +----------------+
       ^             ^         Repair FEC Payload IDs
       |             |         Source payloads
       |             |         Repair payloads
       |             |
       |Source       |Repair payloads
       |packets      |
       |             |
   +-- |- -- -- -- -- -- -+
   |RTP| | RTP Processing |
   |   | +-- -- -- --|-- -+
   | +-- -- -- -- -- |--+ |
   | | RTP Demux        | |
   +-- -- -- -- -- -- -- -+
              ^
              |(1) FEC source and repair packets
              |
   +----------------------+
   |   Transport Layer    |
   |     (e.g., UDP)      |
   +----------------------+

            Figure 5: Receiver operation with RTP repair flows

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











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5.  Protocol Specification

5.1.  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 ADUs.  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 Configuration Information, which is 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.

5.2.  Structure of the Source Block

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

   o  A description of the source flow with which the ADU is associated
      with.

   o  The ADU itself.

   o  The length of the ADU.

5.3.  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 ADU 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.  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 a sequence number 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.

   In applications where avoidance of IP packet fragmentation is a goal,
   CDPs SHOULD consider the Explicit Source FEC Payload ID size when
   determining the size of ADUs that will be delivered using the FEC
   Framework.  This is because the addition of the Explicit Source FEC
   Payload ID increases the packet length.

   The Explicit Source FEC Payload ID is placed at the end of the packet
   so that in the case that Robust Header Compression (ROHC) [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 are used with this
   framework need to consider that FEC schemes can add this Explicit
   Source FEC Payload ID and thereby increase the packet size.

   In many applications, support for FEC is added to a pre-existing
   protocol and in this case use of the Explicit Source FEC Payload ID
   can break backwards compatibility, since source packets are modified.




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5.3.1.  Generic Explicit Source FEC Payload ID

   In order to apply FEC protection using multiple FEC schemes to a
   single source flow, all schemes have to 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 a length of two octets
   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 construction, 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 the maximum value which can be represented in two octets (65535)
   to 0).  Sequence numbers SHOULD NOT be reused until all values in the
   sequence number space have been used.

   Note that if the original packets of the source flow are already
   carrying a packet sequence number that is at least two bytes long,
   there is no need to add the generic Explicit Source FEC Payload ID
   and modify the packets.

5.4.  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.





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5.4.1.  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.


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

                Figure 8: Packet format for repair packets

5.5.  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 CDP 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 flows.  For example, in the case of UDP, each
   source flow is uniquely identified by a tuple {Source IP address,
   source UDP port, destination IP address, destination UDP port}.  In
   some applications some of these fields can contain wildcards, so that
   the flow is identified by a subset of the fields.  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 flows, by means of one or more
   packet flows consisting of repair packets.  The FEC Framework
   Configuration Information includes, for each instance of the FEC
   Framework:

   1.  Identification of the repair flows.

   2.  For each source flow protected by the repair flow(s):




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       A.  Definition of the source flow.

       B.  An integer identifier for this flow definition (i.e., tuple).
           This identifier MUST be unique amongst all source flows that
           are protected by the same FEC repair flow.  Integer
           identifiers can be allocated starting from zero and
           increasing by one for each flow.  However, any random (but
           still unique) allocation is also possible.  A source flow
           identifier need not be carried in source packets since source
           packets are directly associated with a flow by virtue of
           their packet headers.

   3.  The FEC Encoding ID, identifying the FEC scheme.

   4.  The length of the Explicit Source FEC Payload ID (in octets).

   5.  Zero or more FEC-Scheme-Specific Information (FSSI) 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, can be present
   at a sender or receiver.  A single instance of the FEC Framework
   protects packets of the source flows identified in (2) above, i.e.,
   all packets sent on those flows MUST be FEC source packets as defined
   in Section 5.3.  A single source flow can be protected by multiple
   instances of the FEC Framework.

   The integer flow identifier identified in (2b) above 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 can use it to identify the source
   flow with which a recovered packet is associated.  The integer flow
   identifier can 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 repair packets can provide protection for multiple
   source flows, repair packets would either not carry the identifier at
   all or can carry multiple identifiers.  However, in any case, the
   flow identifier associated with a particular source packet can be
   recovered from the repair packets as part of an FEC decoding
   operation.

   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



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   repair packets as defined in Section 5.4 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 can also include the RTP payload type to be
   used for repair packets.

   FSSI includes the information that is specific to the FEC scheme used
   by the CDP.  FSSI is used to communicate the information that cannot
   be adequately represented otherwise and is essential for proper FEC
   encoding and decoding operations.  The motivation behind separating
   the FSSI required only by the sender (which is carried in Sender-Side
   FEC-Scheme-Specific Information (SS-FSSI) container) from the rest of
   the FSSI is to provide the receiver or the third party entities a
   means of controlling the FEC operations at the sender.  Any FSSI
   other than the one solely required by the sender MUST be communicated
   via the FSSI container.

   The variable-length SS-FSSI and FSSI containers transmit the
   information in textual representation and contain zero or more
   distinct elements, whose descriptions are provided by the fully-
   specified FEC schemes.

   For the CDPs that choose the Session Description Protocol (SDP)
   [RFC4566] as their session description protocol, the ABNF [RFC5234]
   syntax for the SS-FSSI and FSSI containers is provided in Section 4.5
   of [I-D.ietf-fecframe-sdp-elements].

5.6.  FEC Scheme Requirements

   In order to be used with this framework, an FEC scheme MUST be
   capable of processing data arranged into blocks of ADUs (source
   blocks).

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

   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 Information elements.

   4.  A full specification of the FEC code.




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       This specification MUST precisely define the valid FEC-Scheme-
       Specific 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
       symbols).

       Furthermore, given a source block as defined in Section 5.2,
       valid values of the FEC-Scheme-Specific Information, a valid
       Repair FEC Payload ID value and a valid packet payload size, the
       specification MUST uniquely define the values of the encoding
       symbols to be included in the repair packet payload of a packet
       with the given Repair FEC 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 Information, a valid Repair FEC Payload
       ID value and a valid packet payload size as input produces the
       exact value of the encoding symbols 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 has to 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 has to 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 can 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>







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   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>

      2.3 FEC Framework Configuration Information  <Define the names,
         types and text value encoding formats of the FEC-Scheme-
         Specific 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 IDs and FEC-Scheme-Specific Information>

   4. FEC Code Specification  <Provide a complete specification of the
      FEC Code>

   Specifications can include additional sections including 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 can 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
   can be specified within the same document.





















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6.  Feedback

   Many applications require some kind of feedback on transport
   performance.  E.g., how much data arrived at the receiver, at what
   rate and when?  When FEC is added to such applications, feedback
   mechanisms can also need to be enhanced to report on the performance
   of the FEC.  E.g., 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 the sense that each block is transmitted
   over a relatively small period of time).  Thus, feedback information
   that is averaged over longer periods of time will likely not provide
   sufficient information for engineering purposes.  More detailed
   feedback over shorter time scales might be preferred.  For example,
   for applications using RTP transport, 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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7.  Transport Protocols

   This framework is intended to be used to define CDPs that operate
   over transport protocols providing an unreliable datagram service,
   including in particular the User Datagram Protocol (UDP) and the
   Datagram Congestion Control Protocol (DCCP).













































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8.  Congestion Control

   This section starts with some informative background on the
   motivation of the normative requirements for congestion control,
   which are spelled out in Section 8.2.

8.1.  Motivation

   o  The enforcement of congestion control principles has gained a lot
      of momentum in the IETF over the recent years.  While the need for
      congestion control 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 heterogeneous 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 of link-layer errors and independent of the network load.

   o  The authors of this document 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, e.g., 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 heterogeneous networks.  In other
      cases, application reliability requirements can be so high that
      the required end-to-end reliability will be difficult to achieve.
      Furthermore, the end-to-end network reliability is not necessarily
      known in advance.

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

   o  In order to prevent such mis-use, one approach is to leave
      standardization 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.

   o  Another approach is to write a clear applicability statement.  For
      example, one could restrict the use of this framework to networks
      with certain loss characteristics (e.g., wireless links).
      However, there can be applications where the use of FEC is



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      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 a traffic
      generated as an emergency signal.

   o  A third approach 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.

   o  Taking above considerations into account, Section 8.2 specifies a
      small set of constraints for the FEC, which are mandatory for all
      senders compliant with this FEC Framework.  Further restrictions
      can be imposed by certain CDPs.

8.2.  Normative Requirements

   o  The bandwidth of FEC repair data MUST NOT exceed the bandwidth of
      the original source data being protected (without the possible
      addition of an Explicit Source FEC Payload ID).  This disallows
      the (static or dynamic) use of excessively strong FEC to combat
      high packet loss rates, which can otherwise be chosen by naively
      implemented dynamic FEC-strength selection mechanisms.  We
      acknowledge that there are a few exotic applications, e.g., IP
      traffic from space-based senders, or senders in certain hardened
      military devices, which could warrant a higher FEC strength.
      However, in this specification we give preference to the overall
      stability and network friendliness of average applications.

   o  Whenever the source data rate is adapted due to the operation of
      congestion control mechanisms, the FEC repair data rate MUST be
      similarly adapted.















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9.  Security Considerations

   First of all, it must be clear that the application of FEC protection
   to a stream does not provide any kind of security.  On the opposite,
   the FEC Framework itself could be subject to attacks, or could pose
   new security risks.  The goals of this section are to state the
   problem, discuss the risks and identify solutions when feasible.  It
   also defines a mandatory to implement (but not mandatory to use)
   security scheme.

9.1.  Problem Statement

   A content delivery system is potentially subject to many attacks.
   Attacks can target the content, or the CDP, or the network itself,
   with completely different consequences, in particular in terms of the
   number of impacted nodes.

   Attacks can have several goals:

   o  They can try to give access to a confidential content (e.g., in
      case of a non-free content).

   o  They can try to corrupt the source flows (e.g., to prevent a
      receiver from using them), which is a form of DoS attack.

   o  They can try to compromise the receiver's behavior (e.g., by
      making the decoding of an object computationally expensive), which
      is another form of DoS attack.

   o  They can try to compromise the network's behavior (e.g., by
      causing congestion within the network), which potentially impacts
      a large number of nodes.

   These attacks can be launched either against the source and/or repair
   flows (e.g., by sending fake FEC source and/or repair packets) or
   against the FEC parameters that are sent either in-band (e.g., in the
   Repair FEC Payload ID or in the Explicit Source FEC Payload ID) or
   out-of-band (e.g., in the FEC Framework Configuration Information).

   Several dimensions to the problem need to be considered.  The first
   one is the way the FEC Framework is used.  The FEC Framework can be
   used end-to-end, i.e., it can be included in the final end-device
   where the upper application runs; or the FEC Framework can be used in
   middleboxes, for instance, to globally protect several source flows
   exchanged between two or more distant sites.

   A second dimension is the threat model.  When the FEC Framework
   operates in the end-device, this device (e.g., a personal computer)



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   might be subject to attacks.  Here, the attacker is either the end-
   user (who might want to access confidential content) or somebody
   else.  In all cases the attacker has access to the end-device, but
   not necessarily to the full control of the end-device (a secure
   domain can exist).  Similarly, when the FEC Framework operates in a
   middlebox, this middlebox can be subject to attacks or the attacker
   can gain access to it.  The threats can also concern the end-to-end
   transport (e.g., through Internet).  Here, examples of threats
   include the transmission of fake FEC source or repair packets, the
   replay of valid packets, the drop, delay or misordering of packets,
   and of course traffic eavesdropping.

   The third dimension consists in the desired security services.  Among
   them, the content integrity and sender authentication services are
   probably the most important features.  We can also mention DoS
   mitigation, anti-replay protection or content confidentiality.

   Finally, the fourth dimension consists in the security tools
   available.  This is the case of the various Digital Rights Management
   (DRM) systems, defined out of the context of the IETF and that can be
   proprietary solutions.  Otherwise SRTP and IPsec/ESP are two tools
   that can turn out to be useful in the context of the FEC Framework.
   Note that using SRTP requires that the application generates RTP
   source flows and, when applied below the FEC Framework, that both the
   FEC source and repair packets to be regular RTP packets.  Therefore
   SRTP is not considered as a universal solution applicable in all use
   cases.

   In the following sections, we further discuss security aspects
   related to the use of the FEC Framework.

9.2.  Attacks Against the Data Flows

9.2.1.  Access to Confidential Content

   Access control to the source flow being transmitted is typically
   provided by means of encryption.  This encryption can be done by the
   content provider itself, or within the application (for instance by
   using the Secure Real-time Transport Protocol (SRTP) [RFC3711]), or
   at the network layer, on a per-packet basis when IPsec/ESP is used
   [RFC4303].  If confidentiality is a concern, it is RECOMMENDED that
   one of these solutions is used.  Even if we mention these attacks
   here, they are neither related to nor facilitated by the use of FEC.

   Note that when encryption is applied, this encryption MUST either be
   applied on the source data before the FEC protection, or if done
   after the FEC protection, then both the FEC source packets and repair
   packets MUST be encrypted (and an encryption at least as



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   cryptographically secure as the encryption applied on the FEC source
   packets MUST be used for the FEC repair packets).  Otherwise, if
   encryption were to be performed only on the FEC source packets after
   FEC encoding, a non-authorized receiver could be able to recover the
   source data after decoding the FEC repair packets provided that a
   sufficient number of such packets were available.

   The following considerations apply when choosing where to apply
   encryption (and more generally where to apply security services
   beyond encryption).  Once decryption has taken place, the source data
   is in plaintext.  The full path between the output of the deciphering
   module and the final destination (e.g., the TV display in case of a
   video) MUST be secured, in order to prevent any unauthorized access
   to the source data.

   When the FEC Framework endpoint is the end system (i.e., where the
   upper application runs) and if the threat model includes the
   possibility that an attacker has access to this end system, then the
   end system architecture is very important.  More precisely, in order
   to prevent an attacker to get hold of the plaintext, all processings,
   once deciphering has taken place, MUST occur in a protected
   environment.  If encryption is applied after FEC protection at the
   sending side (i.e., below FEC Framework), it means that FEC decoding
   MUST take place in the protected environment.  With certain use
   cases, this MAY be complicated or even impossible.  In that case
   applying encryption before FEC protection is preferred.

   When the FEC Framework endpoint is a middlebox, the recovered source
   flow, after FEC decoding, SHOULD NOT be sent in plaintext to the
   final destination(s) if the threat model includes the possibility
   that an attacker eavesdrops the traffic.  In that case also it is
   preferred to apply encryption before FEC protection.

   In some cases, encryption could be applied both before and after the
   FEC protection.  The considerations described above still apply in
   such cases.

9.2.2.  Content Corruption

   Protection against corruptions (e.g., against forged FEC source/
   repair packets) is achieved by means of a content integrity
   verification/source authentication scheme.  This service is usually
   provided at the packet level.  In this case, after removing all the
   forged packets, the source flow might sometimes be recovered.
   Several techniques can provide this content integrity/source
   authentication service:





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   o  At the application layer, SRTP [RFC3711] provides several
      solutions to check the integrity and authenticate the source of
      RTP and RTCP messages, among other services.  For instance,
      associated to the Timed Efficient Stream Loss-Tolerant
      Authentication (TESLA) [RFC4383], SRTP is an attractive solution
      that is robust to losses, provides a true authentication/integrity
      service, and does not create any prohibitive processing load or
      transmission overhead.  Yet, checking a packet requires a small
      delay (a second or more) after its reception with TESLA.  Whether
      this extra delay, both in terms of startup delay at the client and
      end-to-end delay, is appropriate or not depends on the target use
      case.  In some situations, this might degrade the user experience.
      In other situations, this will not be an issue.  Other building
      blocks can be used within SRTP to provide content integrity/
      authentication services.

   o  At the network layer, IPsec/ESP [RFC4303] offers (among other
      services) an integrity verification mechanism that can be used to
      provide authentication/content integrity services.

   It is up to the developer and the person in charge of deployment, who
   know the security requirements and features of the target application
   area, to define which solution is the most appropriate.  Nonetheless
   it is RECOMMENDED that at least one of these techniques is used.

   Note that when integrity protection is applied, it is RECOMMENDED
   that it takes place on both FEC source and repair packets.  The
   motivation is to avoid corrupted packets to be considered during
   decoding, which would often lead to a decoding failure or result in a
   corrupted decoded source flow.

9.3.  Attacks Against the FEC Parameters

   Attacks on these FEC parameters can prevent the decoding of the
   associated object.  For instance, modifying the finite field size of
   a Reed-Solomon FEC scheme (when applicable) will lead a receiver to
   consider a different FEC code.

   It is therefore RECOMMENDED that security measures are taken to
   guarantee the FEC Framework Configuration Information integrity.
   Since the FEC Framework does not define how the FEC Framework
   Configuration Information is communicated from sender to receiver, we
   cannot provide further recommendations on how to guarantee its
   integrity.  However, any complete CDP specification MUST give
   recommendations on how to achieve it.  When the FEC Framework
   Configuration Information is sent out-of-band, e.g., in a session
   description, it SHOULD be protected, for instance, by digitally
   signing it.



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   Attacks are also possible against some FEC parameters included in the
   Explicit Source FEC Payload ID and Repair FEC Payload ID.  For
   instance, modifying the Source Block Number of an FEC source or
   repair packet will lead a receiver to assign this packet to a wrong
   block.

   It is therefore RECOMMENDED that security measures are taken to
   guarantee the Explicit Source FEC Payload ID and Repair FEC Payload
   ID integrity.  To that purpose, one of the packet-level source
   authentication/content integrity techniques of Section 9.2.2 can be
   used.

9.4.  When Several Source Flows are to be Protected Together

   When several source flows, with different security requirements, need
   to be FEC protected jointly, within a single FEC Framework instance,
   then each flow MAY be processed appropriately, before the protection.
   For instance, source Flows that require access control MAY be
   encrypted before they are FEC protected.

   There are also situations where the only insecure domain is the one
   over which the FEC Framework operates.  In that case, this situation
   MAY be addressed at the network layer, using IPsec/ESP (see
   Section 9.5), even if only a subset of the source flows have strict
   security requirements.

   Since the use of FEC Framework should not add any additional threat,
   it is RECOMMENDED that the FEC Framework aggregate flow be in line
   with the maximum security requirements of the individual source
   flows.  For instance, if denial-of-service (DoS) protection is
   required, an integrity protection SHOULD be provided below the FEC
   Framework, using for instance IPsec/ESP.

   Generally speaking, whenever feasible, it is RECOMMENDED to avoid FEC
   protecting flows with totally different security requirements.
   Otherwise, an important processing would be added to protect the
   source flows that do not need it.

9.5.  Baseline Secure FEC Framework Operation

   This section describes a baseline mode of secure FEC Framework
   operation based on the application of the IPsec security protocol,
   which is one possible solution to solve or mitigate the security
   threats introduced by the use of the FEC Framework.

   Two related documents are of interest.  First, Section 5.1 of
   [RFC5775] defines a baseline secure ALC operation for sender-to-group
   transmissions, assuming the presence of a single sender and a source-



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   specific multicast (SSM) or SSM-like operation.  The proposed
   solution, based on IPsec/ESP, can be used to provide a baseline FEC
   Framework secure operation, for the downstream source flow.

   Second, Section 7.1 of [RFC5740] defines a baseline secure NORM
   operation, for sender-to-group transmissions as well as unicast
   feedbacks from receivers.  Here, it is also assumed there is a single
   sender.  The proposed solution is also based on IPsec/ESP.  However,
   the difference with respect to the first document relies on the
   management of IPsec Security Associations (SA) and corresponding
   Security Policy Database (SPD) entries, since NORM requires a second
   set of SA and SPD to be defined to protect unicast feedbacks from
   receivers.

   The FEC Framework has been defined in such a way to be independent
   from the application that generates source flows.  Some applications
   might use purely unidirectional flows, while other applications might
   also use unicast feedbacks from the receivers.  For instance, this is
   the case when considering RTP/RTCP based source flows.  Depending on
   the specific situation, it is RECOMMENDED that the baseline secure
   FEC Framework operation inherits from [RFC5775] in case of purely
   unidirectional sender-to-group flows, or [RFC5740] in case both
   sender-to-group and unicast feedbacks flows are needed.

   Note that the IPsec/ESP requirements profiles outlined in [RFC5775]
   and [RFC5740] are commonly available on many potential hosts.  They
   can form the basis of a secure mode of operation.  One potential
   limitation, however, is the need for privileged user authorization.
   However, automated key management implementations are typically
   configured with the privileges necessary to affect system IPsec
   configuration.




















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10.  Operations and Management Considerations

   The question of operating and managing the FEC Framework and the
   associated FEC scheme(s) is of high practical importance.  The goals
   of this section are to discuss the general requirements, aspects
   related to a specific deployment and solutions whenever possible.

   In particular, this section discusses the questions of
   interoperability across vendors/use cases and whether defining
   mandatory to implement (but not mandatory to use) solutions is
   beneficial.

10.1.  What are the Key Aspects to Consider?

   Several aspects need to be considered since they will directly impact
   the way the FEC Framework and the associated FEC schemes can be
   operated and managed.

   This section lists them as follows:

   o  A Single Small Generic Component within a Larger (and Often
      Legacy) Solution: The FEC Framework is one component within a
      larger solution which includes both one or several upper-layer
      applications (that generate one or several ADU flows) and an
      underlying protocol stack.  A key design principle is that the FEC
      Framework should be able to work without making any assumption
      with respect to either the upper-layer application(s) or the
      underlying protocol stack, even if there are special cases where
      assumptions are made.

   o  One-to-One with Feedback vs. One-to-Many with Feedback vs. One-to-
      Many without Feedback Scenarios: The FEC Framework can be used in
      use cases that completely differ from one another.  Some use cases
      are one-way (e.g., in broadcast networks), with either a one-to-
      one, one-to-many or many-to-many transmission model, and the
      receiver(s) cannot send any feedback to the sender(s).  Other use
      cases follow a bidirectional one-to-one, one-to-many, or many-to-
      many scenario, and the receiver(s) can send feedback to the
      sender(s).

   o  Non-FEC Framework Capable Receivers: With the one-to-many and
      many-to-many use cases, the receiver population might have
      different capabilities with respect to the FEC Framework itself
      and the supported FEC schemes.  Some receivers might not be
      capable of decoding the repair packets belonging to a particular
      FEC scheme, while some other receivers might not be supporting the
      FEC Framework at all.




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   o  Internet vs. non-Internet Networks: The FEC Framework can be
      useful in many use cases that use a transport network that is not
      the public Internet (e.g., with IPTV or Mobile TV).  In such
      networks, the operational and management considerations can be
      achieved through an open or proprietary solution, which is
      specified outside of the IETF.

   o  Congestion Control Considerations: See Section 8 for a discussion
      on whether congestion control is needed or not, and its
      relationships with the FEC Framework.

   o  Within End-Systems vs. within Middleboxes: The FEC Framework can
      be used within end-systems, very close to the upper-layer
      application, or within dedicated middleboxes, for instance when it
      is desired to protect one or several flows while they cross a
      lossy channel between two or more remote sites.

   o  Protecting a Single Flow vs. Several Flows Globally: The FEC
      Framework can be used to protect a single flow, or several flows
      globally.

10.2.  Operational and Management Recommendations

   Overall, from the discussion of Section 10.1, it is clear that the
   CDPs and FEC schemes compatible with the FEC Framework widely differ
   in their capabilities, application and deployment scenarios such that
   a common operation and management method or protocol that works well
   for all of them would be too complex to define.  Thus, as a design
   choice, the FEC Framework does not dictate the use of any particular
   technology or protocol for transporting FEC data, managing the hosts,
   signaling the configuration information or encoding the configuration
   information.  This provides flexibility and is one of the main goals
   of the FEC Framework.  However, this section gives some RECOMMENDED
   guidelines.

   o  A Single Small Generic Component within a Larger (and Often
      Legacy) Solution: It is anticipated that the FEC Framework will
      often be used to protect one or several RTP streams.  Therefore,
      implementations SHOULD make feedback information accessible via
      RTCP to enable users to take advantage of the tools using (or used
      by) RTCP to operate and manage the FEC Framework instance along
      with the associated FEC schemes.

   o  One-to-One with Feedback vs. One-to-Many with Feedback vs. One-to-
      Many without Feedback Scenarios: With use cases that are one-way,
      the FEC Framework sender does not have any way to gather feedback
      from receivers.  With use cases that are bidirectional, the FEC
      Framework sender can collect detailed feedback (e.g., in case of a



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      one-to-one scenario) or at least occasional feedback (e.g., in
      case of a multicast, one-to-many scenario).  All these
      applications have naturally different operational and management
      aspects.  If any, they also have different requirements or
      features for collecting feedback, processing it and acting on it.
      The data structures for carrying the feedback also vary.

      Implementers SHOULD make feedback available using either an in-
      band or out-of-band asynchronous reporting mechanism.  When an
      out-of-band solution is preferred, a standardized reporting
      mechanism, such as Syslog [RFC5424] or SNMP notifications
      [RFC3411], is RECOMMENDED.  When required, a mapping mechanism
      between the Syslog and SNMP reporting mechanisms could be used, as
      described in [RFC5675] and [RFC5676].

   o  Non-FEC Framework Capable Receivers: Section 5.3 gives
      recommendations on how to provide backward compatibility in
      presence of receivers that cannot support the FEC scheme being
      used, or the FEC Framework itself: basically the use of Explicit
      Source FEC Payload ID is banned.  Additionally, a non-FEC
      Framework capable receiver MUST also have a means not to receive
      the repair packets that it will not be able to decode in the first
      place or a means to identify and discard them appropriately upon
      receiving them.  This SHOULD be achieved by sending repair packets
      on a different transport-layer flow.  In case of RTP transport and
      if both source and repair packets will be sent on the same
      transport-layer flow, this SHOULD be achieved by using an RTP
      framing for FEC repair packets with a different payload type.  It
      is the responsibility of the sender to select the appropriate
      mechanism when needed.

   o  Within End-Systems vs. within Middleboxes: When the FEC Framework
      is used within middleboxes, it is RECOMMENDED that the paths
      between the hosts where the sending applications run and the
      middlebox that performs FEC encoding be as reliable as possible,
      i.e., are not prone to packet loss, packet reordering, or varying
      delays in delivering packets.

      Similarly, it is RECOMMENDED that the paths between the
      middleboxes that perform FEC decoding and the end-systems where
      the receiving applications operate, in situations where this is a
      different host, be as reliable as possible.


   o  Management of Communication Issues Before Reaching the Sending
      FECFRAME Instance: Let us consider situations where the FEC
      Framework is used within middleboxes.  At the sending side, the
      general reliability recommendation for the path between the



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      sending applications and the middlebox is important but it may not
      guarantee that a loss, reordering or important delivery delay
      cannot happen, for whatever reason.  If such a rare event happens,
      this event SHOULD NOT compromise the operation of the FECFRAME
      instances, neither at the sending side nor receiving side.  This
      is particularly important with FEC schemes that do not modify the
      ADU for backward compatibility purposes (i.e., do not use any
      Explicit Source FEC Payload ID) and rely for instance on the RTP
      sequence number field to identify FEC source packets within their
      source block.  In this case, packet loss or packet reordering
      leads to a gap in the RTP sequence number space seen by the
      FECFRAME instance.  Similarly, varying delay in delivering packets
      over this path can lead to significant timing issues.  With FEC
      schemes that indicate in the Repair FEC Payload ID, for each
      source block, the base RTP sequence number and number of
      consecutive RTP packets that belong to this source block, a
      missing ADU or an ADU delivered out of order could cause the
      FECFRAME sender to switch to a new source block.  However, some
      FEC schemes and/or receivers may not necessarily handle such
      varying source block sizes.  In this case, one could consider
      duplicating the last ADU received before the loss, or inserting
      zero'ed ADU(s), depending on the ADU flow nature.  Implementers
      SHOULD consider the consequences of such alternative approaches
      based on their use cases.

   o  Protecting a Single Flow vs. Several Flows Globally: In the
      general case, the various ADU flows that are globally protected
      can have different features, and in particular different real-time
      requirements (in case of real-time flows).  The process of
      globally protecting these flows SHOULD take into account the
      requirements of each individual flow.  In particular, it would be
      counter-productive to add repair traffic to a real-time flow for
      which the FEC decoding delay at a receiver makes decoded ADUs for
      this flow useless because they do not satisfy the associated real-
      time constraints.  From a practical point of view, this means that
      the source block creation process at the sending FEC Framework
      instance, SHOULD consider the most stringent real-time
      requirements of the ADU flows being globally protected.

   o  ADU Flow Bundle Definition and Flow Delivery: By design a repair
      flow might enable a receiver to recover the ADU flow(s) that it
      protects even if none of the associated FEC source packets are
      received.  Therefore, when defining the bundle of ADU flows that
      are globally protected and when defining which receiver receives
      which flow, the sender SHOULD make sure that the ADU flow(s) and
      repair flow(s) of that bundle will only be received by receivers
      that are authorized to receive all the ADU flows of that bundle.
      See Section 9.4 for additional recommendations for situations



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      where a strict access control to ADU flows is needed.

      Additionally when multiple ADU flows are globally protected, a
      receiver who wants to benefit from FECFRAME loss protection SHOULD
      receive all the ADU flows of the bundle.  Otherwise, the missing
      FEC source packets would be considered as lost which might
      significantly reduce the efficiency of the FEC scheme.












































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11.  IANA Considerations

   FEC schemes for use with this framework are identified in protocols
   using FEC Encoding IDs.  Values of FEC Encoding IDs are subject to
   IANA registration.  For this purposes, this document creates a new
   registry called FEC Framework (FECFRAME) FEC Encoding IDs.

   The values that can be assigned within the FEC Framework (FECFRAME)
   FEC Encoding ID registry are numeric indexes in the range (0, 255).
   Values of 0 and 255 are reserved.  Assignment requests are granted on
   an IETF Consensus basis as defined in [RFC5226].  Section 5.6 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 SA Working
   Group 4.  Further thanks are due to the members of the FECFRAME
   Working Group for their comments and reviews.










































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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.

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

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.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.



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

   [I-D.ietf-fecframe-sdp-elements]
              Begen, A., "Session Description Protocol Elements for FEC
              Framework", draft-ietf-fecframe-sdp-elements-11 (work in
              progress), October 2010.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC5740]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "NACK-Oriented Reliable Multicast (NORM) Transport
              Protocol", RFC 5740, November 2009.

   [RFC5675]  Marinov, V. and J. Schoenwaelder, "Mapping Simple Network
              Management Protocol (SNMP) Notifications to SYSLOG
              Messages", RFC 5675, October 2009.

   [RFC5676]  Schoenwaelder, J., Clemm, A., and A. Karmakar,
              "Definitions of Managed Objects for Mapping SYSLOG
              Messages to Simple Network Management Protocol (SNMP)
              Notifications", RFC 5676, October 2009.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              February 2006.

   [RFC5775]  Luby, M., Watson, M., and L. Vicisano, "Asynchronous
              Layered Coding (ALC) Protocol Instantiation", RFC 5775,
              April 2010.

   [MBMSTS]   3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
              Protocols and codecs", 3GPP TS 26.346, April 2005.











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Internet-Draft                FEC Framework                    June 2011


Authors' Addresses

   Mark Watson
   Netflix, Inc.
   100 Winchester Circle
   Los Gatos, CA  95032
   USA

   Email: watsonm@netflix.com


   Ali Begen
   Cisco
   181 Bay Street
   Toronto, ON  M5J 2T3
   Canada

   Email: abegen@cisco.com


   Vincent Roca
   INRIA
   655, av. de l'Europe
   Inovallee; Montbonnot
   ST ISMIER cedex  38334
   France

   Email: vincent.roca@inria.fr
   URI:   http://planete.inrialpes.fr/people/roca/






















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