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Versions: (draft-begen-fecframe-sdp-elements) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 6364

FEC Framework                                                   A. Begen
Internet-Draft                                             Cisco Systems
Intended status:  Standards Track                       November 3, 2008
Expires:  May 7, 2009


                     SDP Elements for FEC Framework
                  draft-ietf-fecframe-sdp-elements-02

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on May 7, 2009.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document specifies the use of Session Description Protocol (SDP)
   to describe the parameters required to signal the Forward Error
   Correction (FEC) Framework Configuration Information between the
   sender(s) and receiver(s).  This document also provides examples that
   show the semantics for grouping multiple source and repair flows
   together for the applications that simultaneously use multiple
   instances of the FEC Framework.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  3
   3.  Forward Error Correction (FEC) and FEC Framework . . . . . . .  3
     3.1.  Forward Error Correction (FEC) . . . . . . . . . . . . . .  3
     3.2.  FEC Framework  . . . . . . . . . . . . . . . . . . . . . .  4
     3.3.  FEC Framework Configuration Information  . . . . . . . . .  5
   4.  SDP Descriptors for FEC Framework  . . . . . . . . . . . . . .  6
     4.1.  Transport Protocol Identifiers . . . . . . . . . . . . . .  6
     4.2.  Media Stream Grouping  . . . . . . . . . . . . . . . . . .  7
     4.3.  Source IP Addresses  . . . . . . . . . . . . . . . . . . .  9
     4.4.  Source Flows . . . . . . . . . . . . . . . . . . . . . . .  9
     4.5.  Repair Flows . . . . . . . . . . . . . . . . . . . . . . .  9
     4.6.  Repair Window  . . . . . . . . . . . . . . . . . . . . . . 11
     4.7.  Bandwidth Specification  . . . . . . . . . . . . . . . . . 11
   5.  Scenarios and Examples . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Session Announcement Considerations  . . . . . . . . . . . 12
     5.2.  Offer/Answer Considerations  . . . . . . . . . . . . . . . 12
     5.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 13
       5.3.1.  One Source Flow, One Repair Flow and One FEC Scheme  . 13
       5.3.2.  Two Source Flows, One Repair Flow and One FEC
               Scheme . . . . . . . . . . . . . . . . . . . . . . . . 14
       5.3.3.  Two Source Flows, Two Repair Flows and Two FEC
               Schemes  . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Transport Protocols  . . . . . . . . . . . . . . . . . . . 16
     7.2.  Attribute Names  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     9.1.  draft-ietf-fecframe-sdp-elements-02  . . . . . . . . . . . 18
     9.2.  draft-ietf-fecframe-sdp-elements-01  . . . . . . . . . . . 18
     9.3.  draft-ietf-fecframe-sdp-elements-00  . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     10.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21












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1.  Introduction

   The Forward Error Correction (FEC) Framework, described in
   [I-D.ietf-fecframe-framework], outlines a general framework for using
   FEC-based error recovery in packet flows carrying media content.
   While a continuous signaling between the sender(s) and receiver(s) is
   not required for a Content Delivery Protocol (CDP) that uses the FEC
   Framework, a set of parameters pertaining to the FEC Framework MUST
   be initially communicated between the sender(s) and receiver(s).  A
   signaling protocol (such as the one described in
   [I-D.ietf-fecframe-config-signaling]) is required to enable such
   communication and the parameters must be appropriately encoded so
   that they can be carried by the signaling protocol.

   One format to encode the parameters is the Session Description
   Protocol (SDP) [RFC4566].  SDP provides a simple text-based format
   for announcements and invitations to describe multimedia sessions.
   These SDP announcements and invitations include sufficient
   information for the sender(s) and receiver(s) to participate in the
   multimedia sessions.  SDP also provides a framework for capability
   negotiation, which MAY be used to negotiate all or a subset of the
   parameters pertaining to the individual sessions.

   The purpose of this document is to introduce the SDP elements that
   MUST be used by the CDPs using the FEC Framework that choose SDP
   [RFC4566] as their session description protocol.


2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Forward Error Correction (FEC) and FEC Framework

   This section gives a brief overview of FEC and the FEC Framework.

3.1.  Forward Error Correction (FEC)

   Any application that needs a reliable transmission over an unreliable
   packet network has to cope with packet losses.  FEC is an effective
   approach that provides reliable transmission particularly in
   multicast and broadcast applications where the feedback from the
   receiver(s) is potentially limited.

   In a nutshell, FEC groups source packets into blocks and applies



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   protection to generate a desired number of repair packets.  These
   repair packets may be sent on demand or independently of any receiver
   feedback.  The choice depends on the FEC scheme or the Content
   Delivery Protocol used by the application, the packet loss
   characteristics of the underlying network, the transport scheme
   (e.g., unicast, multicast and broadcast) and the application.  At the
   receiver side, lost packets can be recovered by erasure decoding
   provided that a sufficient number of source and repair packets have
   been received.

3.2.  FEC Framework

   The FEC Framework [I-D.ietf-fecframe-framework] outlines a general
   framework for using FEC codes in multimedia applications that stream
   audio, video or other types of multimedia content.  It defines the
   common components and aspects of Content Delivery Protocols (CDP).
   The FEC Framework also defines the requirements for the FEC schemes
   that need to be used within a CDP.  However, the details of the FEC
   schemes are not specified within the FEC Framework.  For example, the
   FEC Framework defines what configuration information has to be known
   at the sender and receiver(s) at minimum, but the FEC Framework
   neither specifies how the FEC repair packets are generated and used
   to recover missing source packets, nor dictates how the configuration
   information is communicated between the sender and receiver(s).
   These are rather specified by the individual FEC schemes or CDPs.

   For a proper operation, the information required by the FEC Framework
   and the details of an FEC scheme have to be communicated between the
   sender and receiver(s).  One way to provide this information is to
   use the Session Description Protocol (SDP) [RFC4566].  SDP provides a
   commonly used text-based format for announcements and invitations
   that describe multimedia sessions.  These SDP announcements and
   invitations include sufficient information for clients to participate
   in multimedia sessions.  By using the SDP capability negotiation
   framework, all or a subset of the parameters pertaining to the FEC
   Framework MAY also be negotiated between the sender and receiver(s).

   The purpose of this document is to introduce the SDP elements that
   MUST be used by the CDPs using the FEC Framework that choose SDP
   [RFC4566] as their session description protocol.

   Note that there are many similarities between the FEC Framework
   [I-D.ietf-fecframe-framework] and the FEC Building Block [RFC5052],
   which describes a framework that uses FEC codes to provide
   reliability to bulk data transfer applications running over IP
   multicast or broadcast.  See [I-D.ietf-fecframe-framework] for
   further details.




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3.3.  FEC Framework Configuration Information

   The FEC Framework defines a minimum set of information that MUST be
   communicated between the sender and receiver(s) for a proper
   operation of an FEC scheme.  This information is called the FEC
   Framework Configuration Information.  This information specifies how
   the sender applies protection to the source flow(s) and how the
   repair flow(s) can be used to recover lost data.  In other words,
   this information specifies the relationship(s) between the source and
   repair flows.

   The FEC Framework Configuration Information includes identifiers for
   unique identification of the source and repair flows that carry the
   source and repair packets, respectively.  For example, a packet flow
   that is transmitted over UDP is uniquely identified by the tuple of
   {Source IP Address, Destination IP Address, Source UDP Port,
   Destination UDP Port}.  However, an integer identifier MAY be used
   internally within the FEC scheme as a shorthand to identify this
   flow.

   Multiple instances of the FEC Framework MAY simultaneously exist at
   the sender and the receiver(s) for different source flows, for the
   same source flow, or for various combinations of source flows.  Each
   instance of the FEC Framework MUST provide the following FEC
   Framework Configuration Information:

   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 amongst all source flows that are
      protected by the same FEC repair flow.  The identifiers SHOULD be
      allocated starting from zero and increasing by one for each flow.

      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.  Note that an application MAY wildcard
      some of the fields if only a subset of the fields of the tuple
      (e.g., {Destination IP Address, Destination UDP Port} ) is
      sufficient.

   3. The FEC Encoding ID that identifies the FEC scheme.






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

      This value MAY be zero indicating that no Explicit Source FEC
      Payload ID is used by the FEC scheme.  If it is nonzero, however,
      it means that the Explicit Source FEC Payload ID is used.  In this
      case, only one FEC scheme MUST be used for this source flow,
      unless the generic tag (defined in [I-D.ietf-fecframe-framework])
      is used by all of the FEC schemes protecting this source flow.

   5. An opaque container for the FEC-Scheme-Specific Information (FSSI)
      that is required by only the receiver or by both the receiver and
      sender.

   6. Another opaque container for the FSSI that is only required by the
      sender.  This is referred to as the Sender-Side FEC-Scheme-
      Specific Information (SS-FSSI).

   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 from the rest of the FSSI is to
   provide the receiver or the 3rd 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 opaque SS-FSSI and FSSI containers transmit the
   information in the form of an octet string.  The FEC schemes define
   the structure of this octet string, which MAY contain multiple
   distinct elements.  If the FEC scheme does not require any specific
   information, the FSSI MAY be null.  For the fully-specified FEC
   schemes, a full description of the encoded information in both
   containers MUST be provided.  See [I-D.ietf-fecframe-framework] for
   details.


4.  SDP Descriptors for FEC Framework

   This section defines the SDP elements that MUST be used to describe
   the FEC Framework Configuration Information in multimedia sessions by
   the CDPs that choose SDP [RFC4566] as their session description
   protocol.  Example SDP configurations can be found in Section 5.

4.1.  Transport Protocol Identifiers

   This specification defines a class of new transport protocol
   identifiers for SDP media descriptions.  For all existing identifiers
   <proto>, this specification defines the identifier 'FEC/<proto>'.



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   This identifier MAY be used as the transport protocol identifier in
   the media descriptions for the source data to indicate that the FEC
   Source Packet format defined in Section 6.3 of
   [I-D.ietf-fecframe-framework] is used, where the original transport
   payload field is formatted according to <proto>.  However, if the FEC
   scheme does not use the Explicit Source FEC Payload ID as described
   in Section 6.3 of [I-D.ietf-fecframe-framework], then the original
   transport protocol identifier MUST be used to support backward
   compatibility with the receivers that do not support FEC at all.

   This specification also defines another transport protocol
   identifier, 'UDP/FEC', to indicate the FEC Repair Packet format
   defined in Section 6.4 of [I-D.ietf-fecframe-framework].

4.2.  Media Stream Grouping

   The FEC Framework [I-D.ietf-fecframe-framework] states that multiple
   instances of the FEC Framework MAY exist at the sender and the
   receiver(s), and a source flow MAY be protected by multiple FEC
   Framework instances.  Furthermore, within a single FEC Framework
   instance, multiple source flows MAY be protected by multiple repair
   flows.  However, each repair flow MUST provide protection for a
   single FEC Framework instance.  An example scenario is shown in
   Figure 1.  Here, source flows 0 and 1 are grouped together and
   protected by repair flow 3; source flow 0 is also protected by repair
   flow 4; source flows 1 and 2 are grouped together and protected by
   repair flows 5, 6 and 7.

   The motivation behind grouping source flows before applying FEC
   protection is that a better coding performance may be achieved by
   doing so and many receivers may benefit from this grouping.  For
   example, consider a layered video source that consists of one base
   layer (e.g., source flow 0) and one enhancement layer (e.g., source
   flow 1), where each layer is carried in a separate flow.  Repair flow
   3 protects the combination of the base and enhancement layers for the
   receivers who receive both layers, and repair flow 4 protects the
   base layer only, for the receivers who want the base layer only, or
   who receive both layers but prefer FEC protection for the base layer
   only due to a bandwidth and/or processing-power limitation.

   Using multiple FEC Framework instances for a single source flow
   provides flexibility to the receivers.  Different instances may offer
   repair flows that are generated by different FEC schemes, and
   receivers choose receiving the appropriate repair flow(s) that they
   can support and decode.  Alternatively, different instances (whether
   they use the same FEC scheme or not) may use larger and smaller
   source block sizes, which accommodate the receivers that have looser
   and tighter latency requirements, respectively.  In addition,



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   different instances may also provide FEC protection at different
   redundancy levels.  This is particularly useful in multicast
   scenarios where different receivers might experience different packet
   loss rates and each receiver can choose the repair flow that is
   tailored to its needs.
                                          ____| FEC FRAMEWORK
                                         /    | INSTANCE
                                        /     | 3: Repair Flow
                                       /
                  SOURCE FLOWS        /       | FEC FRAMEWORK
                  0: Source Flow |___/ |------| INSTANCE
              __| 1: Source Flow |            | 4: Repair Flow
             |  | 2: Source Flow
             |                          | FEC FRAMEWORK
             |__________________________| INSTANCE
                                        | 5: Repair Flow
                                        | 6: Repair Flow
                                        | 7: Repair Flow

     Figure 1: Example scenario with multiple FEC Framework instances

   The 'group' attribute and the FEC grouping semantics defined in
   [RFC4756] are used to associate source and repair flows together with
   the following additional requirement:

   In the case that the Explicit Source FEC Payload ID is used, then
   only one FEC scheme MUST be used for this source flow, unless the
   generic tag is used by all of the FEC schemes for the Source FEC
   Payload ID, as defined in [I-D.ietf-fecframe-framework].

   The 'group' attribute MAY be used to group multiple repair flows with
   one or more source flows.  Note that [RFC3388] prohibits an "m" line
   identified by its 'mid' attribute from appearing in more than one
   "a=group:FEC" line.  Thus, [RFC3388] mandates us to write

        a=group:FEC 0 1 2 3 4 5 6 7

   for the scenario sketched in Figure 1.  This limitation prevents us
   from indicating particular associations between the source and repair
   flows by using an "a=group:FEC" line per FEC Framework instance
   [RFC4756].

   Editor's note:  The FEC grouping and flow association issues are
   currently under discussion in FECFRAME and MMUSIC WGs (See
   [I-D.begen-mmusic-rfc4756bis]).  This section will be updated once a
   decision is made.

   The FEC Framework also supports additivity among the repair flows,



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   meaning that multiple repair flows MAY be decoded jointly to improve
   the recovery chances of the missing packets.  In addition, the sender
   MAY assign different levels of priority to each repair flow.  See
   Section 4.5 for details.

4.3.  Source IP Addresses

   The 'source-filter' attribute of SDP ("a=source-filter") as defined
   in [RFC4570] is used to express the source addresses or fully
   qualified domain names in the FEC Framework.

   Editor's note:  Additional requirements or exceptions regarding
   source filters are TBD.

4.4.  Source Flows

   The FEC Framework allows that multiple source flows MAY be grouped
   and protected together by a single or multiple FEC Framework
   instances.  For this reason, as described in Section 3.3, individual
   source flows MUST be identified with unique identifiers.  For this
   purpose, we introduce the attribute 'fec-source-flow'.

   The syntax for the new attribute in ABNF [RFC5234] is as follows:

        fec-source-flow-line = "a=fec-source-flow:" source-id
             [";" SP tag-length] CRLF

        source-id = "id=" src-id
        src-id = 1*DIGIT

        tag-length = "tag-len=" tlen
        tlen = *DIGIT

   The MANDATORY parameter 'id' is used to identify the source flow.
   Note that the parameter 'id' MUST be an integer.

   The OPTIONAL 'tag-len' parameter is used to specify the length of the
   Explicit Source FEC Payload ID field (in bytes) and MUST be used
   according to the requirements listed in Section 4.2.  If no value is
   specified for the 'tag-len' parameter, it indicates a value of zero.

4.5.  Repair Flows

   A repair flow MUST contain only repair packets formatted as described
   in [I-D.ietf-fecframe-framework] for a single FEC Framework instance.
   In other words, packets belonging to source flows or other repair
   flows from a different FEC Framework instance MUST NOT be sent within
   this flow.  We introduce the attribute 'fec-repair-flow' to describe



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   the repair flows.

   The syntax for the new attribute in ABNF is as follows:

        fec-repair-flow-line = "a=fec-repair-flow:" fec-encoding-id
             [";" SP flow-priority] [";" SP sender-side-scheme-specific]
             [";" SP scheme-specific] CRLF

        fec-encoding-id = "encoding-id=" enc-id
        enc-id = 1*DIGIT ; FEC Encoding ID

        flow-priority = "priority=" priority-of-the-flow
        priority-of-the-flow = *DIGIT

        sender-side-scheme-specific = "ss-fssi=" sender-info
        sender-info = *CHAR

        scheme-specific = "fssi=" scheme-info
        scheme-info = *CHAR

   The MANDATORY parameter 'encoding-id' is used to identify the FEC
   scheme used to generate this repair flow.  These identifiers MUST be
   registered with IANA by the FEC schemes that use the FEC Framework.

   The OPTIONAL parameter 'priority' is used to indicate the priorities
   of the repair flows.  The exact usage of the parameter 'priority' and
   the pertaining rules MAY be defined by the FEC scheme or the CDP.  If
   no value is specified for the parameter 'priority', it means that the
   receiver(s) MAY receive and use the repair flows in any order.
   However, if a priority is assigned to the repair flow(s), the
   receivers MUST follow the specified order in receiving and using the
   repair flow(s).

   The OPTIONAL parameters 'ss-fssi' and 'fssi' are opaque containers to
   convey the FEC-Scheme-Specific Information (FSSI) that includes the
   information that is specific to the FEC scheme used by the CDP and is
   necessary for proper FEC encoding and decoding operations.  The FSSI
   required only by the sender (called Sender-Side FSSI) MUST be
   communicated in the container specified by the parameter 'ss-fssi'.
   Any other FSSI MUST be communicated in the container specified by the
   parameter 'fssi'.  In both containers, FSSI is transmitted in the
   form of an octet string.  The FEC schemes define the structure of
   this octet string, which MAY contain multiple distinct elements.  If
   the FEC scheme does not require any specific information, the 'ss-
   fssi' and 'fssi' parameters MAY be null and ignored.






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4.6.  Repair Window

   An FEC encoder processes a block of source packets and generates a
   number of repair packets, which are then transmitted within a certain
   duration.  At the receiver side, the FEC decoder tries to decode all
   the packets received within the repair window to recover the missing
   packets, if there are any.  Repair window stands for the time that
   spans the source packets and the corresponding repair packets.
   Assuming that there is no issue of delay variation, the FEC decoder
   SHOULD NOT wait longer than the repair window since additional
   waiting would not help the recovery process.

   This document specifies a new attribute to describe the size of the
   repair window in milliseconds and microseconds.

   The syntax for the attribute in ABNF is as follows:

        repair-window-line = "a=repair-window:" window-size
             [SP unit] CRLF

        window-size = 1*DIGIT

        unit = ms / us

   <unit> is the unit of time the repair window size is specified with.
   Currently, two units are defined:  "ms", which stands for
   milliseconds and "us", which stands for microseconds.  The default
   unit is "ms".  Alternative units MAY be defined in the future by
   registering them with IANA.

   The 'a=repair-window' attribute is a media-level attribute since each
   repair flow MAY have a different repair window value.

4.7.  Bandwidth Specification

   The bandwidth specification as defined in [RFC4566] denotes the
   proposed bandwidth to be used by the session or media.  The
   specification of bandwidth is OPTIONAL.

   In the context of the FEC Framework, the bandwidth specification can
   be used to express the bandwidth of the repair flows or the bandwidth
   of the session.  If included in the SDP, it SHALL adhere to the
   following rules:

   The session-level bandwidth for an FEC Framework instance MAY be
   specified.  In this case, it is RECOMMENDED to use the Transport
   Independent Application Specific (TIAS) bandwidth modifier [RFC3890]
   and the 'a=maxprate' attribute for the session.



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   The media-level bandwidth for the individual repair flows MAY also be
   specified.  In this case, it is RECOMMENDED to use the TIAS bandwidth
   modifier [RFC3890].

   The Application Specific (AS) bandwidth modifier [RFC4566] MAY be
   used instead of TIAS, however, this is NOT RECOMMENDED since TIAS
   allows the calculation of the bitrate according to the IP version and
   transport protocol, whereas AS does not.  Thus, in TIAS-based bitrate
   calculations, the packet size SHALL include all headers and payload,
   excluding the IP and UDP headers.  In AS-based bitrate calculations,
   the packet size SHALL include all headers and payload, plus the IP
   and UDP headers.

   For the ABNF syntax information of the TIAS and AS, refer to
   [RFC3890] and [RFC4566], respectively.


5.  Scenarios and Examples

   This section discusses the considerations for session announcement
   and offer/answer models.  SDP examples that can be used by the FEC
   Framework are also provided.

5.1.  Session Announcement Considerations

   In multicast-based applications, the FEC Framework Configuration
   Information pertaining to all FEC protection options available at the
   sender MAY be advertised to the receivers as a part of a session
   announcement.  This way, the sender can let the receivers know all
   available options for FEC protection.  Based on their needs, the
   receivers MAY choose protection provided by one or more FEC Framework
   instances and subscribe to the respective multicast group(s) to
   receive the repair flow(s).  Unless explicitly required by the CDP,
   the receivers SHOULD NOT send an answer back to the sender specifying
   their choices.

5.2.  Offer/Answer Considerations

   In unicast-based applications, a sender and receiver MAY adopt the
   offer/answer model [RFC3264] to set the FEC Framework Configuration
   Information.  In this case, the sender offers all available options
   to the receiver and the receiver answers back to the sender with its
   choice(s).  Note that some FEC protection options MAY be offered to
   only a particular set of (e.g., premium) receivers.

   Receivers supporting the SDP Capability Negotiation Framework
   [I-D.ietf-mmusic-sdp-capability-negotiation] MAY also use this
   framework to negotiate all or a subset of the FEC Framework



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   parameters.

   The backward compatibility in offer/answer model is handled as
   specified in [RFC3388].  If a receiver receives an offer containing
   FEC grouping and it does not understand the FEC grouping semantics,
   it MAY respond with an answer that ignores the grouping attribute or
   MAY refuse the request.  In the first case, the offerer MUST
   establish the connection without FEC.  In the second case, if the
   offerer still wishes to establish the session, it SHOULD retry the
   request with an offer without FEC.

5.3.  Examples

   Editor's note:  More examples showing the usage of multiple FEC
   Framework instances, additivity of the repair flows and
   prioritization of the repair flows will be provided once the issues
   related to FEC grouping and flow association are resolved.

   Editor's note:  As of now, no FEC Encoding ID has been registered
   with IANA.  In the examples below, an FEC Encoding ID of zero will be
   used for illustrative purposes.  Artificial content for the SS-FSSI
   and FSSI will also be provided.

   [RFC3388] defines the media stream identification attribute ('mid')
   as a token in ABNF.  In contrast, the identifiers for the source
   flows MUST be integers and SHOULD be allocated starting from zero and
   increasing by one for each flow.  To avoid any ambiguity, using the
   same values for identifying the media streams and source flows is NOT
   RECOMMENDED, even when 'mid' values are integers.

5.3.1.  One Source Flow, One Repair Flow and One FEC Scheme

                 SOURCE FLOWS             | INSTANCE #1
                 0: Source Flow |---------| 1: Repair Flow

                           Figure 6: Scenario #1

   In this example, we have one source video flow (mid:S1) and one FEC
   repair flow (mid:R1).  We form one FEC group with the "a=group:FEC S1
   R1" line.  The source and repair flows are sent to the same port on
   different multicast groups.  The repair window is set to 150 ms.










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        v=0
        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=FEC Framework Examples
        t=0 0
        a=group:FEC S1 R1
        m=video 30000 RTP/AVP 100
        c=IN IP4 224.1.1.1/127
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        a=mid:S1
        m=application 30000 udp/fec
        c=IN IP4 224.1.2.1/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=1Q2A3Z; fssi=4W5S6X
        a=repair-window: 150
        a=mid:R1

5.3.2.  Two Source Flows, One Repair Flow and One FEC Scheme

                 SOURCE FLOWS             | INSTANCE #1
                 0: Source Flow |_________| 2: Repair Flow
                 1: Source Flow |

                           Figure 8: Scenario #2

   In this example, we have two source video flows (mid:S1 and mid:S2)
   and one FEC repair flow (mid:R1), protecting both source flows.  We
   form one FEC group with the "a=group:FEC S1 S2 R1" line.  The source
   and repair flows are sent to the same port on different multicast
   groups.  The repair window is set to 150500 us.






















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        v=0
        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=FEC Framework Examples
        t=0 0
        a=group:FEC S1 S2 R1
        m=video 30000 RTP/AVP 100
        c=IN IP4 224.1.1.1/127
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        a=mid:S1
        m=video 30000 RTP/AVP 101
        c=IN IP4 224.1.1.2/127
        a=rtpmap:101 MP2T/90000
        a=fec-source-flow: id=1
        a=mid:S2
        m=application 30000 udp/fec
        c=IN IP4 224.1.2.1/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=1Q2A3Z; fssi=4W5S6X
        a=repair-window: 150500 us
        a=mid:R1

5.3.3.  Two Source Flows, Two Repair Flows and Two FEC Schemes

                 SOURCE FLOWS             | INSTANCE #1
                 0: Source Flow |---------| 2: Repair Flow
                 1: Source Flow |_
                                  \-------| INSTANCE #2
                                          | 3: Repair Flow

                          Figure 10: Scenario #3

   In this example, we have two source video flows (mid:S1 and mid:S2)
   and two FEC repair flows (mid:R1 and mid:R2).  The source flows
   mid:S1 and mid:S2 are protected by the repair flows mid:R1 and
   mid:R2, respectively.  We form two FEC groups with the "a=group:FEC
   S1 R1" and "a=group:FEC S2 R2" lines.  The source and repair flows
   are sent to the same port on different multicast groups.  The repair
   window is set to 200 ms and 400 ms for the first and second FEC
   group, respectively.












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        v=0
        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=FEC Framework Examples
        t=0 0
        a=group:FEC S1 R1
        a=group:FEC S2 R2
        m=video 30000 RTP/AVP 100
        c=IN IP4 224.1.1.1/127
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        a=mid:S1
        m=video 30000 RTP/AVP 101
        c=IN IP4 224.1.1.2/127
        a=rtpmap:101 MP2T/90000
        a=fec-source-flow: id=1
        a=mid:S2
        m=application 30000 udp/fec
        c=IN IP4 224.1.2.1/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=1Q2A3Z; fssi=4W5S6X
        a=repair-window: 200 ms
        a=mid:R1
        m=application 30000 udp/fec
        c=IN IP4 224.1.2.2/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=123QAZ; fssi=456WSX
        a=repair-window: 400 ms
        a=mid:R2


6.  Security Considerations

   For the general security considerations related to SDP, refer to
   [RFC4566].  For the security considerations related to source/FEC
   media stream grouping in SDP and use of source address filters in
   SDP, refer to [RFC4756] and [RFC4570], respectively.


7.  IANA Considerations

7.1.  Transport Protocols

   The 'proto' sub-field of the media description line ("m=") describes
   the transport protocol used.  This document registers the following
   values:








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        UDP/FEC

   Editor's note:  Additional transport protocols to be registered are
   TBD.

7.2.  Attribute Names

   As recommended by [RFC4566], the following attribute names should be
   registered with IANA.

   The contact information for the registrations is:

   Ali Begen
   abegen@cisco.com

   SDP Attribute ("att-field"):
        Attribute name:     fec-source-flow
        Long form:          Pointer to FEC Source Flow
        Type of name:       att-field
        Type of attribute:  Media level
        Subject to charset: No
        Purpose:            See this document
        Reference:          This document
        Values:             See this document

   SDP Attribute ("att-field"):
        Attribute name:     fec-repair-flow
        Long form:          Pointer to FEC Repair Flow
        Type of name:       att-field
        Type of attribute:  Media level
        Subject to charset: No
        Purpose:            See this document
        Reference:          This document
        Values:             See this document

   SDP Attribute ("att-field"):
        Attribute name:     repair-window
        Long form:          Repair Window Size
        Type of name:       att-field
        Type of attribute:  Media level
        Subject to charset: No
        Purpose:            See this document
        Reference:          This document
        Values:             See this document







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8.  Acknowledgments

   The author would like to thank the FEC Framework Design Team for
   their inputs, suggestions and contributions.


9.  Change Log

9.1.  draft-ietf-fecframe-sdp-elements-02

   The following are the major changes compared to version 01:

   o  Clarified the definitions for the FSSI fields.

   o  Hostnames in the SDP examples are fixed.

9.2.  draft-ietf-fecframe-sdp-elements-01

   The following are the major changes compared to version 00:

   o  Additive repair flows can now be from different instances.  The
      sender may also assign different levels of priorities to each
      repair flow regardless of whether the repair flows are additive or
      not.

   o  SDP examples are fixed.

   o  Comments posted in the mailing list are incorporated.

9.3.  draft-ietf-fecframe-sdp-elements-00

   This is the initial version, which is based on an earlier individual
   submission.  The following are the major changes compared to that
   document:

   o  The opaque container in the FEC Framework Configuration
      Information (FEC-Scheme-Specific Information) is now divided into
      two parts:  information needed only by the sender and information
      needed by the receiver.  The repair flow descriptors are also
      updated accordingly.

   o  "Minimum Buffer Size" is now called "Repair Window."  Its size can
      also be specified in microseconds in addition to milliseconds.

   o  Simple examples with complete SDPs are included.

   o  "Scheme ID" is changed to "Encoding ID" to be consistent with the
      framework draft.



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   o  Some other editorial changes.


10.  References

10.1.  Normative References

   [I-D.ietf-fecframe-framework]
              Watson, M., "Forward Error Correction (FEC) Framework",
              draft-ietf-fecframe-framework-03 (work in progress),
              October 2008.

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

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

   [RFC4570]  Quinn, B. and R. Finlayson, "Session Description Protocol
              (SDP) Source Filters", RFC 4570, July 2006.

   [RFC4756]  Li, A., "Forward Error Correction Grouping Semantics in
              Session Description Protocol", RFC 4756, November 2006.

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.

   [RFC3890]  Westerlund, M., "A Transport Independent Bandwidth
              Modifier for the Session Description Protocol (SDP)",
              RFC 3890, September 2004.

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

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

10.2.  Informative References

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

   [I-D.ietf-fecframe-config-signaling]
              Asati, R., "Methods to convey FEC Framework Configuration
              Information", draft-ietf-fecframe-config-signaling-01
              (work in progress), November 2008.



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   [I-D.begen-mmusic-rfc4756bis]
              Begen, A., "Forward Error Correction Grouping Semantics in
              Session Description Protocol",
              draft-begen-mmusic-rfc4756bis-00 (work in progress),
              October 2008.

   [I-D.ietf-mmusic-sdp-capability-negotiation]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-09 (work in
              progress), July 2008.


Author's Address

   Ali Begen
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  abegen@cisco.com






























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Full Copyright Statement

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