MMUSIC                                                          A. Begen
Internet-Draft                                             Cisco Systems
Intended status:  Standards Track                       October 27, 2008
Expires:  April 30, 2009

   Forward Error Correction Grouping Semantics in Session Description

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   Copyright (C) The IETF Trust (2008).


   The Session Description Protocol (SDP) supports grouping media lines.
   SDP also has semantics defined for grouping the associated source and
   Forward Error Correction (FEC)-based repair flows.  However, the
   semantics that were defined in RFC 4756 generally fail to provide the
   specific grouping relationships between the source and repair flows
   when there are more than one source and/or repair flows in the same
   group.  Furthermore, the existing semantics also do not support

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   additive repair flows and prioritization among the repair flows
   protecting one or more source flows.  This document addresses these
   issues by introducing new FEC grouping semantics.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Requirements and Issues with RFC 4756  . . . . . . . . . . . .  5
     3.1.  Source and Repair Flow Association . . . . . . . . . . . .  5
     3.2.  Support for Additivity . . . . . . . . . . . . . . . . . .  6
     3.3.  Support for Prioritization . . . . . . . . . . . . . . . .  6
   4.  FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  New Grouping Semantics . . . . . . . . . . . . . . . . . .  7
     4.2.  Offer / Answer Consideration . . . . . . . . . . . . . . .  7
     4.3.  Example of FEC Grouping  . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

   Any application that needs a reliable transmission over an unreliable
   packet network has to cope with packet losses.  Forward Error
   Correction (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
   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.

   For example, one of the most basic FEC schemes is the parity codes,
   where an XOR operation is applied to a group of packets (i.e., source
   block) to generate a single repair packet.  At the receiver side,
   this scheme achieves a full recovery if only one packet is lost
   within the source block and the repair packet is received.  There are
   various other ways of generating repair packets, possibly with
   different loss-recovery capabilities.

   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.  The FEC Framework
   specification states that source and repair packets MUST be carried
   in different streams, which are referred to as the source and repair
   flows, respectively.  At the receiver side, the receivers should know
   which flows are the source flows and which flows are the repair
   flows.  The receivers should also know the exact association of the
   source and repair flows so that they can use the correct data to
   repair the original content in case there is a packet loss.
   Currently, SDP [RFC4566] uses [RFC3388] and [RFC4756] for this

   In order to provide applications more flexibility, the FEC Framework
   [I-D.ietf-fecframe-framework] allows a source flow to be protected by
   multiple FEC schemes, each of which requires an instance of the FEC
   Framework.  Thus, multiple instances of the FEC Framework MAY exist
   at the sender and the receiver(s).  Furthermore, within a single FEC
   Framework instance, multiple source flows MAY be grouped and
   protected by one or more repair flows.

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   A basic example scenario is shown in Figure 1.  Here, source flow S1
   is protected by repair flow R1.  Also, source flows S1 and S2 are
   grouped and protected together by repair flow R2.

               SOURCE FLOWS             | FEC FRAMEWORK INSTANCE #1
             | S1: Source Flow |--------| R1: Repair Flow
         |   | S2: Source Flow
         +______________________________| FEC FRAMEWORK INSTANCE #2
                                        | R2: Repair Flow

   Figure 1: Example scenario with two FEC Framework instances where R1
            protects S1, and R2 protects the group of S1 and S2

   Grouping source flows before applying FEC protection may allow us to
   achieve a better coding performance.  As a typical scenario, suppose
   that source flows S1 and S2 in Figure 1 correspond to the base and
   enhancement layers in a layered video content, respectively.  Repair
   flow R2 protects the combination of the base and enhancement layer
   for the receivers who receive both layers, and repair flow R1
   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

   Using multiple FEC Framework instances for a single source flow
   provides flexibility to the receivers.  An example scenario is
   sketched in Figure 2.  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, 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.

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           SOURCE FLOWS              | FEC FRAMEWORK INSTANCE #1
           S3: Source Flow |---------| R3: Repair Flow
                           |---------| FEC FRAMEWORK INSTANCE #2
                                     | R4: Repair Flow

     Figure 2: Example scenario with two FEC Framework instances, each
       with a single repair flow protecting the same source flow S3

   To summarize, the FEC Framework supports the following:

   1.  A source flow MAY be protected by multiple different FEC schemes.

   2.  An FEC scheme MAY generate multiple repair flows.

   3.  Source flows MAY be grouped prior to FEC protection.  That is,
       one or more repair flows MAY protect a group of source flows.

   To fully benefit from the flexibility provided by the FEC Framework,
   the grouping semantics for FEC MUST support these features.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Requirements and Issues with RFC 4756

3.1.  Source and Repair Flow Association

   Currently, the 'group' attribute and the FEC grouping semantics
   defined in [RFC3388] and [RFC4756], respectively, are used to
   associate source and repair flows together.

   The 'group' attribute is used to group multiple repair flows with one
   or more source flows.  However, [RFC3388] prohibits an "m" line
   identified by its 'mid' attribute from appearing in more than one
   "a=group" line using the same semantics.  This limitation prevents us
   from indicating specific associations between the source and repair
   flows by using an "a=group:FEC" line per FEC Framework instance.  For
   example, for the scenario sketched in Figure 1, [RFC3388] mandates us
   to write

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        a=group:FEC S1 S2 R1 R2

   Clearly, this "a=group:FEC" line does not say anything specific about
   which repair flows are protecting which source flows.

   A new work ([I-D.ietf-mmusic-rfc3388bis]) is currently in progress in
   the MMUSIC WG to remove this limitation in [RFC3388].  However,
   [RFC4756] also needs to be updated according to the FEC Framework

3.2.  Support for Additivity

   The FEC Framework also supports additive repair flows.  Additivity
   among the repair flows means that multiple repair flows may be
   decoded jointly to improve the recovery chances of the missing
   packets in a single or the same set of source flows.  Additive repair
   flows can be generated by the same FEC scheme or different FEC

   For example, in Figure 4, repair flows R5 and R6 may be additive
   within the FEC Framework instance #1.  Alternatively, all three
   repair flows R5, R6 and R7 could be additive, too.

           SOURCE FLOWS              | FEC FRAMEWORK INSTANCE #1
           S4: Source Flow |---------| R5: Repair Flow
                           |         | R6: Repair Flow
                           |---------| FEC FRAMEWORK INSTANCE #2
                                     | R7: Repair Flow

    Figure 4: Example scenario with two FEC Framework instances, where
    two repair flows in the first instance and a single repair flow in
            the second instance protect the same source flow S4

3.3.  Support for Prioritization

   When there are multiple repair flows protecting a single or the same
   set of source flows, the sender MAY assign different levels of
   priority to each repair flow.  The sender uses prioritization to let
   the receivers know in which order the receivers MUST receive and
   decode the repair flows.  Note that the repair flows that are
   assigned a priority level may be additive or non-additive.

   A possible scenario where prioritization can be used is that the
   sender generates two repair flows and sends them in two separate
   multicast groups.  It might be highly desirable to keep all the
   receivers in the same multicast group to save from the network
   traffic.  The sender can achieve this by assigning a higher priority

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   to one of the repair flows.  Another scenario is where one of the
   repair flows is more expensive in terms of complexity, cost, etc. and
   the sender wants every receiver to use the less expensive repair flow
   first.  This is again possible by using prioritization.  For example,
   in Figure 4, the sender MAY assign a higher priority to repair flow
   R5 and a lower priority to repair flow R6.

   Currently, there is no SDP semantics for indicating the priority
   levels of the individual flows that are grouped in an "a=group" line.
   Instead, within the FEC Framework, the priority of a repair flow is
   described with the optional parameter 'priority' in the "a=fec-
   repair-flow" line [I-D.ietf-fecframe-sdp-elements], which is used to
   describe the individual repair flows.  However, there might be other
   benefits of defining more general semantics for flow prioritization
   in SDP that can be used by other applications and frameworks besides
   the FEC Framework.

4.  FEC Grouping

4.1.  New Grouping Semantics

   Each "a=group" line is used to indicate an association relationship
   between the source and repair flows.  The flows included in one
   "a=group" line are called an "FEC Group."

   By extending [I-D.ietf-mmusic-rfc3388bis] we define "FEC-XR" as the
   new grouping semantics that can support the features of the FEC

4.2.  Offer / Answer Consideration

   The backward compatibility in offer / answer is generally handled as
   specified in [RFC3388].

   Depending on the implementation, a node that does not understand FEC
   grouping (either does not understand line grouping at all, or just
   does not understand the FEC semantics) SHOULD respond to an offer
   containing FEC grouping either (1) with an answer that ignores the
   grouping attribute or (2) with a refusal to the request (e.g., 488
   Not acceptable here or 606 Not acceptable in SIP).

   In the first case, the original sender of the offer MUST establish
   the connection without FEC.  In the second case, if the sender of the
   offer still wishes to establish the session, it SHOULD re-try the
   request with an offer without FEC.

   Editor's note:  This section may need to be updated according to

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4.3.  Example of FEC Grouping

   For the scenario sketched in Figure 1, we can write the following

        o=ali 1122334455 1122334466 IN IP4
        s=New FEC Grouping Semantics
        t=0 0
        a=group:FEC-XR S1 R1
        a=group:FEX-XR S1 S2 R2
        m=video 30000 RTP/AVP 100
        c=IN IP4
        a=rtpmap:100 MP2T/90000
        m=video 30000 RTP/AVP 101
        c=IN IP4
        a=rtpmap:101 MP2T/90000
        m=application 30000 RTP/AVP 110
        c=IN IP4
        a=rtpmap:110 1d-interleaved-parityfec/90000
        a=fmtp:110 L:5; D:10; repair-window: 200000
        m=application 30000 RTP/AVP 111
        c=IN IP4
        a=rtpmap:111 1d-interleaved-parityfec/90000
        a=fmtp:111 L:10; D:10; repair-window: 200000

   For the additivity and prioritization issues, let us consider the
   scenario sketched in Figure 4.  Suppose that repair flows R5 and R6
   are additive but repair flow R7 is not additive with any of the other
   repair flows.  In this case, we can write

        a=group:FEC-XR S4 R5 R6
        a=group:FEC-XR S4 R7

   If none of the repair flows were additive, we could have written

        a=group:FEC-XR S4 R5
        a=group:FEC-XR S4 R6
        a=group:FEC-XR S4 R7

   If the sender wants to assign priority levels to the repair flows,
   the priority order may be indicated with the order of the 'mid'

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   values of the repair flows.  However, this approach is potentially
   prone to errors and SHOULD be evaluated carefully whether it covers
   the corner cases or not.  One example is the case when priorities are
   assigned to a mix of cross-instance additive and non-additive repair

5.  Security Considerations

   There is a weak threat for the receiver that the FEC grouping can be
   modified to indicate FEC relationships that do not exist.  Such
   attacks may result in failure of FEC to protect, and/or mishandling
   of other media payload streams.  It is recommended that the receiver
   SHOULD do integrity check on SDP and follow the security
   considerations of SDP [RFC4566] to only trust SDP from trusted

6.  IANA Considerations

   This document registers the following semantics with IANA in
   Semantics for the "group" SDP Attribute under SDP Parameters.

   Semantics                    Token   Reference
   ---------------------------  ------  -------------
   Forward Error Correction XR  FEC-XR  This document

7.  Acknowledgments

   Some parts of this document is based on [RFC4756].  Thus, the author
   would like to thank those who contributed to [RFC4756].

8.  References

8.1.  Normative References

              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.

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              Camarillo, G., "The SDP (Session Description Protocol)
              Grouping Framework", draft-ietf-mmusic-rfc3388bis-01 (work
              in progress), July 2008.

8.2.  Informative References

   [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.

              Begen, A., "SDP Elements for FEC Framework",
              draft-ietf-fecframe-sdp-elements-01 (work in progress),
              July 2008.

Author's Address

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


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