<pre>Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 6364                                         Cisco
Category: Standards Track                                   October 2011
ISSN: 2070-1721


             <span class="h1">Session Description Protocol Elements for the</span>
                <span class="h1">Forward Error Correction (FEC) Framework</span>

Abstract

   This document specifies the use of the 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.

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 <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6364">http://www.rfc-editor.org/info/rfc6364</a>.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.




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   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.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Requirements Notation ...........................................<a href="#page-3">3</a>
   <a href="#section-3">3</a>. Forward Error Correction (FEC) and FEC Framework ................<a href="#page-3">3</a>
      <a href="#section-3.1">3.1</a>. Forward Error Correction (FEC) .............................<a href="#page-3">3</a>
      <a href="#section-3.2">3.2</a>. FEC Framework ..............................................<a href="#page-4">4</a>
      <a href="#section-3.3">3.3</a>. FEC Framework Configuration Information ....................<a href="#page-4">4</a>
   <a href="#section-4">4</a>. SDP Elements ....................................................<a href="#page-5">5</a>
      <a href="#section-4.1">4.1</a>. Transport Protocol Identifiers .............................<a href="#page-6">6</a>
      <a href="#section-4.2">4.2</a>. Media Stream Grouping ......................................<a href="#page-6">6</a>
      <a href="#section-4.3">4.3</a>. Source IP Addresses ........................................<a href="#page-6">6</a>
      <a href="#section-4.4">4.4</a>. Source Flows ...............................................<a href="#page-6">6</a>
      <a href="#section-4.5">4.5</a>. Repair Flows ...............................................<a href="#page-7">7</a>
      <a href="#section-4.6">4.6</a>. Repair Window ..............................................<a href="#page-8">8</a>
      <a href="#section-4.7">4.7</a>. Bandwidth Specification ....................................<a href="#page-9">9</a>
   <a href="#section-5">5</a>. Scenarios and Examples .........................................<a href="#page-10">10</a>
      <a href="#section-5.1">5.1</a>. Declarative Considerations ................................<a href="#page-10">10</a>
      <a href="#section-5.2">5.2</a>. Offer/Answer Model Considerations .........................<a href="#page-10">10</a>
   <a href="#section-6">6</a>. SDP Examples ...................................................<a href="#page-11">11</a>
      <a href="#section-6.1">6.1</a>. One Source Flow, One Repair Flow, and One FEC Scheme ......<a href="#page-11">11</a>
      <a href="#section-6.2">6.2</a>. Two Source Flows, One Repair Flow, and One FEC Scheme .....<a href="#page-12">12</a>
      <a href="#section-6.3">6.3</a>. Two Source Flows, Two Repair Flows, and Two FEC Schemes ...<a href="#page-13">13</a>
      <a href="#section-6.4">6.4</a>. One Source Flow, Two Repair Flows, and Two FEC Schemes ....<a href="#page-14">14</a>
   <a href="#section-7">7</a>. Security Considerations ........................................<a href="#page-15">15</a>
   <a href="#section-8">8</a>. IANA Considerations ............................................<a href="#page-15">15</a>
      <a href="#section-8.1">8.1</a>. Registration of Transport Protocols .......................<a href="#page-15">15</a>
      <a href="#section-8.2">8.2</a>. Registration of SDP Attributes ............................<a href="#page-16">16</a>
   <a href="#section-9">9</a>. Acknowledgments ................................................<a href="#page-16">16</a>
   <a href="#section-10">10</a>. References ....................................................<a href="#page-17">17</a>
      <a href="#section-10.1">10.1</a>. Normative References .....................................<a href="#page-17">17</a>
      <a href="#section-10.2">10.2</a>. Informative References ...................................<a href="#page-17">17</a>







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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   The Forward Error Correction (FEC) Framework, described in [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>],
   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 has to be initially
   communicated between the sender(s) and receiver(s).  A signaling
   protocol (such as the one described in [<a href="#ref-FECFRAME-CFG-SIGNAL">FECFRAME-CFG-SIGNAL</a>]) is
   required to enable such communication, and the parameters need to 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) [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].  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 can 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
   are used by the CDPs using the FEC Framework that choose SDP
   [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] for their multimedia sessions.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Requirements Notation</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Forward Error Correction (FEC) and FEC Framework</span>

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

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Forward Error Correction (FEC)</span>

   Any application that needs 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 either not available or quite limited.






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   In a nutshell, FEC groups source packets into blocks and applies
   protection to generate a desired number of repair packets.  These
   repair packets can 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
   itself.  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.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  FEC Framework</span>

   The FEC Framework [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>] 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 (CDPs).  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 a 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.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  FEC Framework Configuration Information</span>

   The FEC Framework [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>] defines a minimum set of information that
   has to be communicated between the sender and receiver(s) for proper
   operation of a FEC scheme.  This information is called the "FEC
   Framework Configuration Information".  This information includes
   unique identifiers for the source and repair flows that carry the
   source and repair packets, respectively.  It also specifies how the
   sender applies protection to the source flow(s) and how the repair
   flow(s) can be used to recover lost data.

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








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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.

   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.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  SDP Elements</span>

   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 [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] for their multimedia sessions.
   Example SDP descriptions can be found in <a href="#section-6">Section 6</a>.





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<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Transport Protocol Identifiers</span>

   This specification defines a new transport protocol identifier for
   the FEC schemes that take a UDP-formatted input stream and append an
   Explicit Source FEC Payload ID, as described in <a href="./rfc6363#section-5.3">Section&nbsp;5.3 of
   [RFC6363]</a>, to generate a source flow.  This new protocol identifier
   is called 'FEC/UDP'.  To use input streams that are formatted
   according to another &lt;proto&gt; (as listed in the table for the 'proto'
   field in the "Session Description Protocol (SDP) Parameters"
   registry), the corresponding 'FEC/&lt;proto&gt;' transport protocol
   identifier MUST be registered with IANA by following the instructions
   specified in [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].

   Note that if a FEC scheme does not use the Explicit Source FEC
   Payload ID as described in <a href="./rfc6363#section-4.1">Section&nbsp;4.1 of [RFC6363]</a>, 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 <a href="./rfc6363#section-5.4">Section&nbsp;5.4 of [RFC6363]</a>.  For detailed registration
   information, refer to <a href="#section-8.1">Section 8.1</a>.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Media Stream Grouping</span>

   In the FEC Framework, the 'group' attribute and the FEC grouping
   semantics defined in [<a href="./rfc5888" title="&quot;The Session Description Protocol (SDP) Grouping Framework&quot;">RFC5888</a>] and [<a href="./rfc5956" title="&quot;Forward Error Correction Grouping Semantics in the Session Description Protocol&quot;">RFC5956</a>], respectively, are used
   to associate source and repair flows.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Source IP Addresses</span>

   The 'source-filter' attribute of SDP ("a=source-filter") as defined
   in [<a href="./rfc4570" title="&quot;Session Description Protocol (SDP) Source Filters&quot;">RFC4570</a>] is used to express the source addresses or fully
   qualified domain names in the FEC Framework.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Source Flows</span>

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








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   The syntax for the new attribute in ABNF [<a href="./rfc5234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC5234</a>] is as follows:

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

        source-id = "id=" src-id
        src-id = 1*DIGIT ; Represented as 32-bit non-negative
                         ; integers, and leading zeros are ignored

        tag-length = "tag-len=" tlen
        tlen = %x31-39 *DIGIT

   The REQUIRED parameter 'id' is used to identify the source flow.
   Parameter 'id' MUST be an integer.

   The 'tag-len' parameter is used to specify the length of the Explicit
   Source FEC Payload ID field (in octets).  In the case that an
   Explicit Source FEC Payload ID is used, the 'tag-len' parameter MUST
   exist and indicate its length.  Otherwise, the 'tag-len' parameter
   MUST NOT exist.

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Repair Flows</span>

   A repair flow MUST contain only repair packets formatted as described
   in [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>] for a single FEC Framework instance; i.e., packets
   belonging to source flows or other repair flows from a different FEC
   Framework instance cannot be sent within this flow.  We introduce the
   attribute 'fec-repair-flow' to describe the repair flows.

   The syntax for the new attribute in ABNF is as follows (CHAR and CTL
   are defined in [<a href="./rfc5234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC5234</a>]):

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

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

      flow-preference = "preference-lvl=" preference-level-of-the-flow
      preference-level-of-the-flow = 1*DIGIT









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      sender-side-scheme-specific = "ss-fssi=" sender-info
      sender-info = element *( "," element )
      element     = name ":" value
      name        = token
      token       = 1*&lt;any CHAR except CTLs or separators&gt;
      value       = *&lt;any CHAR except CTLs or separators&gt;
      separator   = "(" / ")" / "&lt;" / "&gt;" / "@"
                     / "," / ";" / ":" / "\" / DQUOTE
                     / "/" / "[" / "]" / "?" / "="
                     / "{" / "}" / SP / HTAB

      scheme-specific = "fssi=" scheme-info
      scheme-info = element *( "," element )

   The REQUIRED parameter 'encoding-id' is used to identify the FEC
   scheme used to generate this repair flow.  These identifiers (in the
   range of [0 - 255]) are registered by the FEC schemes that use the
   FEC Framework and are maintained by IANA.

   The OPTIONAL parameter 'preference-lvl' is used to indicate the
   preferred order for using the repair flows.  The exact usage of the
   parameter 'preference-lvl' and the pertaining rules MAY be defined by
   the FEC scheme or the CDP.  If the parameter 'preference-lvl' does
   not exist, it means that the receiver(s) MAY receive and use the
   repair flows in any order.  However, if a preference level is
   assigned to the repair flow(s), the receivers are encouraged to
   follow the specified order in receiving and using the repair flow(s).

   The OPTIONAL parameters 'ss-fssi' and 'fssi' are 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 (the 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 textual representation and MAY contain multiple distinct
   elements.  If the FEC scheme does not require any specific
   information, the 'ss-fssi' and 'fssi' parameters MUST NOT exist.

<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>.  Repair Window</span>

   The repair window is the time that spans a FEC block, which consists
   of the source block and the corresponding repair packets.

   At the sender side, the FEC encoder processes a block of source
   packets and generates a number of repair packets.  Then, both the
   source and repair packets are transmitted within a certain duration



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   not larger than the value of the repair window.  The value of the
   repair window impacts the maximum number of source packets that can
   be included in a FEC block.

   At the receiver side, the FEC decoder should wait at least for the
   duration of the repair window after getting the first packet in a FEC
   block, to allow all the repair packets to arrive.  (The waiting time
   can be adjusted if there are missing packets at the beginning of the
   FEC block.)  The FEC decoder can start decoding the already received
   packets sooner; however, it SHOULD NOT register a FEC decoding
   failure until it waits at least for the duration of the repair
   window.

   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 unit CRLF

        window-size = %x31-39 *DIGIT ; Represented as
                                     ; 32-bit non-negative integers

        unit = "ms" / "us"

   &lt;unit&gt; is the unit of time specified for the repair window size.  Two
   units are defined here: 'ms', which stands for milliseconds; and
   'us', which stands for microseconds.

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

   Specifying the repair window size in an absolute time value does not
   necessarily correspond to an integer number of packets or exactly
   match with the clock rate used in RTP (in the case of RTP transport),
   causing mismatches among subsequent repair windows.  However, in
   practice, this mismatch does not break anything in the FEC decoding
   process.

<span class="h3"><a class="selflink" id="section-4.7" href="#section-4.7">4.7</a>.  Bandwidth Specification</span>

   The bandwidth specification as defined in [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] denotes the
   proposed bandwidth to be used by the session or media.  The
   specification of bandwidth is OPTIONAL.







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   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 a FEC Framework instance or the
   media-level bandwidth for the individual repair flows MAY be
   specified.  In this case, it is RECOMMENDED that the Transport
   Independent Application Specific (TIAS) bandwidth modifier [<a href="./rfc3890" title="&quot;A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP)&quot;">RFC3890</a>]
   and the 'a=maxprate' attribute be used, unless the Application-
   Specific (AS) bandwidth modifier [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] is used.  The use of the
   AS bandwidth modifier 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
   [<a href="./rfc3890" title="&quot;A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP)&quot;">RFC3890</a>] and [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>], respectively.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Scenarios and Examples</span>

   This section discusses the considerations for Session Announcement
   and Offer/Answer Models.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Declarative Considerations</span>

   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 can choose protection provided by one or more FEC Framework
   instances and subscribe to the respective multicast session(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, since this can easily overwhelm the sender,
   particularly in large-scale multicast applications.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Offer/Answer Model Considerations</span>

   In unicast-based applications, a sender and receiver MAY adopt the
   Offer/Answer Model [<a href="./rfc3264" title="&quot;An Offer/Answer Model with Session Description Protocol (SDP)&quot;">RFC3264</a>] to set the FEC Framework Configuration
   Information.  In this case, the sender offers the options available
   to this particular receiver, and the receiver answers back to the
   sender with its choice(s).



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   Receivers supporting the SDP Capability Negotiation Framework
   [<a href="./rfc5939" title="&quot;Session Description Protocol (SDP) Capability Negotiation&quot;">RFC5939</a>] MAY also use this framework to negotiate all, or a subset,
   of the FEC Framework parameters.

   The backward compatibility in the Offer/Answer Model is handled as
   specified in [<a href="./rfc5956" title="&quot;Forward Error Correction Grouping Semantics in the Session Description Protocol&quot;">RFC5956</a>].

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  SDP Examples</span>

   This section provides SDP examples that can be used by the FEC
   Framework.

   [<a id="ref-RFC5888">RFC5888</a>] defines the media stream identification attribute ('mid')
   as a token in ABNF.  In contrast, the identifiers for the source
   flows are integers and can 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.

   In the examples below, random FEC Encoding IDs will be used for
   illustrative purposes.  Artificial content for the SS-FSSI and FSSI
   will also be provided.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  One Source Flow, One Repair Flow, and One FEC Scheme</span>

                 SOURCE FLOWS             | INSTANCE #1
                 S1: Source Flow |--------| R1: Repair Flow
                                          |

                           Figure 1: 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-FR 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-FR S1 R1
        m=video 30000 RTP/AVP 100
        c=IN IP4 233.252.0.1/127
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        a=mid:S1
        m=application 30000 UDP/FEC
        c=IN IP4 233.252.0.2/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5
        a=repair-window:150ms
        a=mid:R1

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Two Source Flows, One Repair Flow, and One FEC Scheme</span>

                SOURCE FLOWS
                S2: Source Flow |         | INSTANCE #1
                                |---------| R2: Repair Flow
                S3: Source Flow |

                           Figure 2: Scenario #2

   In this example, we have two source video flows (mid:S2 and mid:S3)
   and one FEC repair flow (mid:R2) protecting both source flows.  We
   form one FEC group with the "a=group:FEC-FR S2 S3 R2" 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-FR S2 S3 R2
        m=video 30000 RTP/AVP 100
        c=IN IP4 233.252.0.1/127
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        a=mid:S2
        m=video 30000 RTP/AVP 101
        c=IN IP4 233.252.0.2/127
        a=rtpmap:101 MP2T/90000
        a=fec-source-flow: id=1
        a=mid:S3
        m=application 30000 UDP/FEC
        c=IN IP4 233.252.0.3/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5
        a=repair-window:150500us
        a=mid:R2

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Two Source Flows, Two Repair Flows, and Two FEC Schemes</span>

                 SOURCE FLOWS             | INSTANCE #1
                 S4: Source Flow |--------| R3: Repair Flow

                 S5: Source Flow |--------| INSTANCE #2
                                          | R4: Repair Flow

                           Figure 3: Scenario #3

   In this example, we have two source video flows (mid:S4 and mid:S5)
   and two FEC repair flows (mid:R3 and mid:R4).  The source flows
   mid:S4 and mid:S5 are protected by the repair flows mid:R3 and
   mid:R4, respectively.  We form two FEC groups with the
   "a=group:FEC-FR S4 R3" and "a=group:FEC-FR S5 R4" 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-FR S4 R3
        a=group:FEC-FR S5 R4
        m=video 30000 RTP/AVP 100
        c=IN IP4 233.252.0.1/127
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        a=mid:S4
        m=video 30000 RTP/AVP 101
        c=IN IP4 233.252.0.2/127
        a=rtpmap:101 MP2T/90000
        a=fec-source-flow: id=1
        a=mid:S5
        m=application 30000 UDP/FEC
        c=IN IP4 233.252.0.3/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5
        a=repair-window:200ms
        a=mid:R3
        m=application 30000 UDP/FEC
        c=IN IP4 233.252.0.4/127
        a=fec-repair-flow: encoding-id=0; ss-fssi=n:14,k:10
        a=repair-window:400ms
        a=mid:R4

<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  One Source Flow, Two Repair Flows, and Two FEC Schemes</span>

                 SOURCE FLOWS             | INSTANCE #1
                 S6: Source Flow |--------| R5: Repair Flow
                                 |
                                 |--------| INSTANCE #2
                                          | R6: Repair Flow

                           Figure 4: Scenario #4

   In this example, we have one source video flow (mid:S6) and two FEC
   repair flows (mid:R5 and mid:R6) with different preference levels.
   The source flow mid:S6 is protected by both of the repair flows.  We
   form two FEC groups with the "a=group:FEC-FR S6 R5" and
   "a=group:FEC-FR S6 R6" lines.  The source and repair flows are sent
   to the same port on different multicast groups.  The repair window is
   set to 200 ms for both FEC groups.







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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-FR S6 R5
     a=group:FEC-FR S6 R6
     m=video 30000 RTP/AVP 100
     c=IN IP4 233.252.0.1/127
     a=rtpmap:100 MP2T/90000
     a=fec-source-flow: id=0
     a=mid:S6
     m=application 30000 UDP/FEC
     c=IN IP4 233.252.0.3/127
     a=fec-repair-flow: encoding-id=0; preference-lvl=0; ss-fssi=n:7,k:5
     a=repair-window:200ms
     a=mid:R5
     m=application 30000 UDP/FEC
     c=IN IP4 233.252.0.4/127
     a=fec-repair-flow: encoding-id=1; preference-lvl=1; ss-fssi=t:3
     a=repair-window:200ms
     a=mid:R6

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   There is a weak threat if the SDP is modified in a way that it shows
   an incorrect association and/or grouping of the source and repair
   flows.  Such attacks can result in failure of FEC protection and/or
   mishandling of other media streams.  It is RECOMMENDED that the
   receiver perform an integrity check on SDP to only trust SDP from
   trusted sources.  The receiver MUST also follow the security
   considerations of SDP [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].  For other general security
   considerations related to SDP, refer to [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>].  For the security
   considerations related to the use of source address filters in SDP,
   refer to [<a href="./rfc4570" title="&quot;Session Description Protocol (SDP) Source Filters&quot;">RFC4570</a>].

   The security considerations for the FEC Framework also apply.  Refer
   to [<a href="./rfc6363" title="&quot;Forward Error Correction (FEC) Framework&quot;">RFC6363</a>] for details.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Registration of Transport Protocols</span>

   This specification updates the "Session Description Protocol (SDP)
   Parameters" registry as defined in <a href="./rfc4566#section-8.2.2">Section&nbsp;8.2.2 of [RFC4566]</a>.
   Specifically, it adds the following values to the table for the
   'proto' field.





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      Type            SDP Name             Reference
      ------          ----------           -----------
      proto           FEC/UDP              [<a href="./rfc6364">RFC6364</a>]
      proto           UDP/FEC              [<a href="./rfc6364">RFC6364</a>]

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Registration of SDP Attributes</span>

   This document registers new attribute names in SDP.

   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:            Provide parameters for a FEC source flow
        Reference:          [<a href="./rfc6364">RFC6364</a>]
        Values:             See [<a href="./rfc6364">RFC6364</a>]

   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:            Provide parameters for a FEC repair flow
        Reference:          [<a href="./rfc6364">RFC6364</a>]
        Values:             See [<a href="./rfc6364">RFC6364</a>]

   SDP Attribute ("att-field"):
        Attribute name:     repair-window
        Long form:          Pointer to FEC Repair Window
        Type of name:       att-field
        Type of attribute:  Media level
        Subject to charset: No
        Purpose:            Indicate the size of the repair window
        Reference:          [<a href="./rfc6364">RFC6364</a>]
        Values:             See [<a href="./rfc6364">RFC6364</a>]

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Acknowledgments</span>

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








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<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  References</span>

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>.  Normative References</span>

   [<a id="ref-RFC6363">RFC6363</a>]  Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", <a href="./rfc6363">RFC 6363</a>, October 2011.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC4566">RFC4566</a>]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", <a href="./rfc4566">RFC 4566</a>, July 2006.

   [<a id="ref-RFC4570">RFC4570</a>]  Quinn, B. and R. Finlayson, "Session Description Protocol
              (SDP) Source Filters", <a href="./rfc4570">RFC 4570</a>, July 2006.

   [<a id="ref-RFC5888">RFC5888</a>]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", <a href="./rfc5888">RFC 5888</a>, June 2010.

   [<a id="ref-RFC5956">RFC5956</a>]  Begen, A., "Forward Error Correction Grouping Semantics in
              the Session Description Protocol", <a href="./rfc5956">RFC 5956</a>,
              September 2010.

   [<a id="ref-RFC3890">RFC3890</a>]  Westerlund, M., "A Transport Independent Bandwidth
              Modifier for the Session Description Protocol (SDP)",
              <a href="./rfc3890">RFC 3890</a>, September 2004.

   [<a id="ref-RFC5234">RFC5234</a>]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>,
              January 2008.

   [<a id="ref-RFC3264">RFC3264</a>]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", <a href="./rfc3264">RFC 3264</a>,
              June 2002.

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>.  Informative References</span>

   [<a id="ref-FECFRAME-CFG-SIGNAL">FECFRAME-CFG-SIGNAL</a>]
              Asati, R., "Methods to convey FEC Framework Configuration
              Information", Work in Progress, September 2011.

   [<a id="ref-RFC5939">RFC5939</a>]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", <a href="./rfc5939">RFC 5939</a>, September 2010.








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Author's Address

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

   EMail: abegen@cisco.com










































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</pre>