





Internet Engineering Task Force (IETF)                         M. Watson
Request for Comments: 6363                                 Netflix, Inc.
Category: Standards Track                                       A. Begen
ISSN: 2070-1721                                                    Cisco
                                                                 V. Roca
                                                                   INRIA
                                                            October 2011


                Forward Error Correction (FEC) Framework

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) that
   is compliant with various requirements defined in this document.
   Thus, Content Delivery Protocols can be defined that are not specific
   to a particular FEC scheme, and FEC schemes can be defined that are
   not specific to a particular Content Delivery Protocol.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6363.











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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   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 ....................................................3
   2. Definitions and Abbreviations ...................................5
   3. Architecture Overview ...........................................7
   4. Procedural Overview ............................................11
      4.1. General ...................................................11
      4.2. Sender Operation ..........................................13
      4.3. Receiver Operation ........................................15
   5. Protocol Specification .........................................19
      5.1. General ...................................................19
      5.2. Structure of the Source Block .............................19
      5.3. Packet Format for FEC Source Packets ......................19
           5.3.1. Generic Explicit Source FEC Payload ID .............21
      5.4. Packet Format for FEC Repair Packets ......................21
           5.4.1. Packet Format for FEC Repair Packets over RTP ......22
      5.5. FEC Framework Configuration Information ...................22
      5.6. FEC Scheme Requirements ...................................24
   6. Feedback .......................................................26
   7. Transport Protocols ............................................27
   8. Congestion Control .............................................27
      8.1. Motivation ................................................27
      8.2. Normative Requirements ....................................29
   9. Security Considerations ........................................29
      9.1. Problem Statement .........................................29
      9.2. Attacks against the Data Flows ............................31
           9.2.1. Access to Confidential Content .....................31
           9.2.2. Content Corruption .................................32
      9.3. Attacks against the FEC Parameters ........................33
      9.4. When Several Source Flows Are to Be Protected Together ....33
      9.5. Baseline Secure FEC Framework Operation ...................34
   10. Operations and Management Considerations ......................35
      10.1. What Are the Key Aspects to Consider? ....................35
      10.2. Operational and Management Recommendations ...............36
   11. IANA Considerations ...........................................39
   12. Acknowledgments ...............................................39
   13. References ....................................................40
      13.1. Normative References .....................................40
      13.2. Informative References ...................................40

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 that do not provide guaranteed packet
   delivery.  Primary examples are real-time, or streaming, media
   applications such as broadcast, multicast, or on-demand forms of
   audio, video, or multimedia.



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   Forward Error Correction (FEC) is a well-known technique for
   improving the reliability of packet transmission over networks that
   do not provide guaranteed packet delivery, especially in multicast
   and broadcast applications.  The FEC Building Block, defined in
   [RFC5052], provides a framework for the definition of Content
   Delivery Protocols (CDPs) for object delivery (including, primarily,
   file delivery) that 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 that 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 that
   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; rather, it 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 does it define how the details of the protected flows
   and the FEC streams that protect them are communicated from sender to
   receiver.  It is expected that any complete CDP specification that
   makes use of this framework will address these signaling
   requirements.  However, this document does specify the information
   that 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
   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.








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   Since RTP [RFC3550] 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 the 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).

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., the Real-Time Streaming
      Protocol (RTSP).

   Content Delivery Protocol (CDP): A complete application protocol
      specification that, 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 the definition of Content
      Delivery Protocols using FEC, such as the framework defined in
      this document.

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

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




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   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 that 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
      of FEC.

   Repair Flow: The packet flow carrying FEC data.

   Repair FEC Payload ID: A 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: A 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 the transport of the source
      and repair flows, e.g., UDP and the Datagram Congestion Control
      Protocol (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.





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

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 a 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 it defines
   the protocol fields and procedures used to identify packet payload
   data in the context of the FEC scheme.  The interface between the FEC



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   Framework and a FEC scheme, which is described in this document, is a
   logical one that 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
   that MUST be available to both sender and receiver(s).  For example,
   this information includes the specification of the ADU flows that are
   to be FEC protected, specification of the ADU flow(s) that 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 repair flow(s)).  The FEC Framework Configuration
   Information also includes information fields that are specific to the
   FEC scheme.  This information is analogous to the FEC Object
   Transmission Information defined in [RFC5052].

   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 that 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 that
   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, a FEC scheme has to define an RTP
   payload format for the repair data.






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   The architecture outlined above is illustrated in 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 that 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








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   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
   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.

4.  Procedural Overview

4.1.  General

   The mechanism defined in this document does not place any
   restrictions on the ADUs that can be protected together, except that
   the ADU be 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 that 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 a 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 a FEC-Scheme-Specific format).









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   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.  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 they 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
   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 that 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



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   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 that 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 in Figure 3 for the case of RTP repair flows,
   describe a possible way to generate compliant source and repair
   flows:

   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.





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   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).

   +----------------------+
   |     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
   Figures 4 and 5 for the case of UDP and RTP repair flows,
   respectively, when receiving a 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) are 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, and 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 that 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
   that are correctly received and those that 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.

   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 backward 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 that 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 FEC 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 that 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 FEC Repair Packets over RTP

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:









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   1.  Identification of the repair flows.

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

       A.  Definition of the source flow.

       B.  An integer identifier for this flow definition (i.e., tuple).
           This identifier MUST be unique among 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 either would 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 a FEC decoding
   operation.



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   A single FEC repair flow provides repair packets for a single
   instance of the FEC Framework.  Other packets MUST NOT be sent within
   this flow; i.e., all packets in the FEC repair flow MUST be FEC
   repair packets as defined in Section 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 a 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] for their multimedia sessions, the ABNF [RFC5234] syntax
   for the SS-FSSI and FSSI containers is provided in Section 4.5 of
   [RFC6364].

5.6.  FEC Scheme Requirements

   In order to be used with this framework, a 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.




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   4.  A full specification of the FEC code.

       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 that -- 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 are both
       possible and practical.

   FEC scheme specifications MAY additionally define the following:

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

   Whenever a FEC scheme specification defines an 'encoding format' for
   an element, this has to be defined in terms of a sequence of bytes
   that can be embedded within a protocol.  The length of the encoding
   format either MUST 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.











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

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

   2.  Formats and Codes

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

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

       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 that 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 a 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.

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 may also need to be enhanced to report on the performance
   of the FEC, e.g., how much lost data was recovered by the FEC?






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   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 that use 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 information calculated both 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 that would give an inaccurate
   picture of congestion conditions.

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

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).

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 recent years.  While the need for
      congestion control over the open Internet is unquestioned, and the
      goal of TCP friendliness is generally agreed upon for most (but
      not all) applications, the problem of congestion detection and
      measurement in heterogeneous networks can hardly be considered
      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.



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   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 as, nor is it intended to be, a
      quality-of-service (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 misuse, 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 the Digital Video Broadcasting (DVB)
      Project, the Third Generation Partnership Project (3GPP), and
      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
      justified to combat congestion-induced packet losses --
      particularly in lightly loaded networks, where congestion is the
      result of relatively rare random peaks in instantaneous traffic
      load -- thereby intentionally violating congestion control
      principles.  One possible example for such an application could be
      a no-matter-what, brute-force FEC protection of traffic generated
      as an emergency signal.

   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 that the application alone would
      not itself cause; i.e., the use of this framework must not make
      things worse.







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   o  Taking the above considerations into account, Section 8.2
      specifies a small set of constraints for FEC; these constraints
      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, that 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.

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 contrary,
   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, the CDP, or the network itself, with
   completely different consequences, particularly in terms of the
   number of impacted nodes.

   Attacks can have several goals:

   o  They can try to give access to confidential content (e.g., in the
      case of 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 denial-of-service
      (DoS) attack.



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   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)
   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
   does not necessarily fully control this 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 the 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 outside of the context of the IETF, that can
   be proprietary solutions.  Otherwise, the Secure Real-Time Transport
   Protocol (SRTP) [RFC3711] and IPsec/Encapsulating Security Payload
   (IPsec/ESP) [RFC4303] 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 generate RTP source flows and, when applied below the



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   FEC Framework, that both the FEC source and repair packets be regular
   RTP packets.  Therefore, SRTP is not considered to be 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 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 be 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 be applied
   either on the source data before the FEC protection or, if done after
   the FEC protection, on both the FEC source packets and repair packets
   (and an encryption at least as 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 the 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 from getting hold of the plaintext, all
   processing, once deciphering has taken place, MUST occur in a
   protected environment.  If encryption is applied after FEC protection





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   at the sending side (i.e., below the 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
   such cases, 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 on the traffic.  In that case, it is
   preferable 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:

   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, when
      associated with 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, with TESLA, checking a packet
      requires a small delay (a second or more) after its reception.
      Whether or not this extra delay, both in terms of startup delay at
      the client and end-to-end delay, is appropriate 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.







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   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 be used.

   Note that when integrity protection is applied, it is RECOMMENDED
   that it take place on both FEC source and repair packets.  The
   motivation is to keep corrupted packets from being considered during
   decoding, as such packets 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.

   Therefore, it is RECOMMENDED that security measures be taken to
   guarantee the integrity of the FEC Framework Configuration
   Information.  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.

   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 a FEC source or repair
   packet will lead a receiver to assign this packet to a wrong block.

   Therefore, it is RECOMMENDED that security measures be taken to
   guarantee the integrity of the Explicit Source FEC Payload ID and
   Repair FEC Payload ID.  To that purpose, one of the packet-level
   source authentication/content integrity techniques described in
   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.




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   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 has strict
   security requirements.

   Since the use of the 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 that FEC
   protecting flows with totally different security requirements be
   avoided.  Otherwise, significant processing overhead would be added
   to protect source flows that do not need it.

9.5.  Baseline Secure FEC Framework Operation

   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 feedback from the receivers.  For instance, this is
   the case when considering RTP/RTCP-based source flows.

   This section describes a baseline mode of secure FEC Framework
   operation based on the application of the IPsec 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 Asynchronous Layered Coding (ALC)
   operation for sender-to-group transmissions, assuming the presence of
   a single sender and a source-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 NACK-
   Oriented Reliable Multicast (NORM) operation, for sender-to-group
   transmissions as well as unicast feedback 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
   [RFC5775] relies on the management of IPsec Security Associations
   (SAs) and corresponding Security Policy Database (SPD) entries, since
   NORM requires a second set of SAs and SPD entries to be defined to
   protect unicast feedback from receivers.



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   Note that the IPsec/ESP requirement profiles outlined in [RFC5775]
   and [RFC5740] are commonly available on many potential hosts.  They
   can form the basis of a secure mode of operation.  Configuration and
   operation of IPsec typically require privileged user authorization.
   Automated key management implementations are typically configured
   with the privileges necessary to allow the needed system IPsec
   configuration.

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 aspects and recommendations related to
   specific deployments and solutions.

   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:

   1.  A Single Small Generic Component within a Larger (and Often
       Legacy) Solution: The FEC Framework is one component within a
       larger solution that includes 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.

   2.  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).




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   3.  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 support the FEC
       Framework at all.

   4.  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.

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

   6.  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).

   7.  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 in Section 10.1, it is clear that the
   CDPs and FEC schemes compatible with the FEC Framework differ widely
   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.









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   1.  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.

   2.  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
       the case of a one-to-one scenario) or at least occasional
       feedback (e.g., in the case of a multicast, one-to-many
       scenario).  All these applications have naturally different
       operational and management aspects.  They also have different
       requirements or features, if any, 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 Simple Network Management
       Protocol (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].

   3.  Non-FEC Framework Capable Receivers: Section 5.3 gives
       recommendations on how to provide backward compatibility in the
       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 the 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.






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   4.  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., not be prone to packet loss, packet reordering, or varying
       delays in delivering packets.

       Similarly, when the FEC Framework is used within middleboxes, it
       is RECOMMENDED that the paths be as reliable as possible between
       the middleboxes that perform FEC decoding and the end-systems
       where the receiving applications operate.

   5.  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
       sending applications and the middlebox is important, but it may
       not guarantee that a loss, reordering, or long delivery delay
       cannot happen, for whatever reason.  If such a rare event
       happens, this event SHOULD NOT compromise the operation of the
       FECFRAME instances, at either the sending side or the 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 on, for
       instance, 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
       zeroed ADU(s), depending on the nature of the ADU flow.
       Implementers SHOULD consider the consequences of such alternative
       approaches, based on their use cases.

   6.  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 the 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
       counterproductive to add repair traffic to a real-time flow for



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       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.

   7.  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
       where strict access control for ADU flows is needed.

       Additionally, when multiple ADU flows are globally protected, a
       receiver that 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 lost, which might
       significantly reduce the efficiency of the FEC scheme.

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 purpose, this document creates a new
   registry called the "FEC Framework (FECFRAME) FEC Encoding IDs".

   The values that can be assigned within the "FEC Framework (FECFRAME)
   FEC Encoding IDs" registry are numeric indexes in the range (0, 255).
   Values of 0 and 255 are reserved.  Assignment requests are granted on
   an IETF Review basis as defined in [RFC5226].  Section 5.6 defines
   explicit requirements that documents defining new FEC Encoding IDs
   should meet.

12.  Acknowledgments

   This document is based in part on [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.

   [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.

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

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

   [RFC5234]  Crocker, D., Ed., 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.

13.2.  Informative References

   [FEC-SF]   Watson, M., Luby, M., Westerlund, M., and S. Wenger,
              "Forward Error Correction (FEC) Streaming Framework", Work
              in Progress, July 2005.

   [MBMSTS]   3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
              Protocols and codecs", 3GPP TS 26.346, March 2009,
              <http://ftp.3gpp.org/specs/html-info/26346.htm>.

   [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.

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






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RFC 6363                      FEC Framework                 October 2011


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

   [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.

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

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

   [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.

   [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.

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

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

   [RFC6364]  Begen, A., "Session Description Protocol Elements for FEC
              Framework", RFC 6364, October 2011.









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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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