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Versions: (draft-begen-mmusic-rfc4756bis) 00 01 02 03 04 05 06 07 08 09 10 RFC 5956

MMUSIC                                                          A. Begen
Internet-Draft                                             Cisco Systems
Obsoletes:  4756                                          April 30, 2009
(if approved)
Intended status:  Standards Track
Expires:  November 1, 2009


   Forward Error Correction Grouping Semantics in Session Description
                                Protocol
                    draft-ietf-mmusic-rfc4756bis-02

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Abstract

   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 was 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 does not support
   describing additive repair flows.  This document addresses these
   issues by introducing new FEC grouping semantics.  SSRC-level
   grouping semantics is also introduced in this document for Real-time
   Transport Protocol (RTP) streams using SSRC multiplexing.


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
   4.  FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  New Grouping Semantics . . . . . . . . . . . . . . . . . .  6
     4.2.  SDP Example  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Grouping for SSRC-Multiplexed RTP Streams  . . . . . . . .  8
     4.4.  Offer-Answer Model Considerations  . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12

















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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, 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 exclusive OR (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 provides 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
   purpose.

   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.

   It should be noted that the FEC Framework requires the source and



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   repair packets to be carried in different streams.  When Real-time
   Transport Protocol (RTP) [RFC3550] is used to carry the source and
   repair streams, the FEC Framework recommends that each stream is
   carried in its own RTP session.  This provides flexibility in using
   FEC in a backward-compatible manner.  However, in some scenarios, a
   single RTP session may be desired to carry multiple RTP streams via
   SSRC multiplexing in order to reduce the port usage.  For such
   scenarios, an appropriate grouping semantics is also required.

   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 layers
   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 any other limitation.

   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 may 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   requirements.

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
   schemes.

   For example, in Figure 3, 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 3: 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


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.  If there are more than one
   repair flows included in an FEC group, they are considered to be
   additive.  Repair flows that are in different FEC groups are non-
   additive.

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



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   Framework.

   The "a=group:FEC-XR" semantics MUST always be used to associate the
   source and repair flows except when the source and repair flows are
   specified in the same media description, i.e., in the same "m" line.

4.2.  SDP Example

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

        v=0
        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=New FEC Grouping Semantics
        t=0 0
        a=group:FEC-XR S1 R1
        a=group:FEC-XR S1 S2 R2
        m=video 30000 RTP/AVP 100
        c=IN IP4 233.252.0.1/127
        a=rtpmap:100 MP2T/90000
        a=mid:S1
        m=video 30000 RTP/AVP 101
        c=IN IP4 233.252.0.2/127
        a=rtpmap:101 MP2T/90000
        a=mid:S2
        m=application 30000 RTP/AVP 110
        c=IN IP4 233.252.0.3/127
        a=rtpmap:110 1d-interleaved-parityfec/90000
        a=fmtp:110 L:5; D:10; repair-window: 200000
        a=mid:R1
        m=application 30000 RTP/AVP 111
        c=IN IP4 233.252.0.4/127
        a=rtpmap:111 1d-interleaved-parityfec/90000
        a=fmtp:111 L:10; D:10; repair-window: 400000
        a=mid:R2

   In this example, the source and repair flows are carried in their own
   RTP sessions and the grouping is achieved through the "a=group:
   FEC-XR" lines.

   For the additivity issues, let us consider the scenario sketched in
   Figure 3.  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 MUST write

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




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   If none of the repair flows are additive, we MUST write

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

   Note that additivity is not necessarily a transitive relation.  Thus,
   each set of additive repair flows MUST be stated explicitly.

4.3.  Grouping for SSRC-Multiplexed RTP Streams

   [I-D.ietf-mmusic-sdp-source-attributes] defines a grouping attribute,
   called 'ssrc-group', for the RTP streams that are SSRC multiplexed
   and carried in the same RTP session.  The grouping is based on the
   Synchronization Source (SSRC) identifiers.  Since SSRC-multiplexed
   RTP streams are defined in the same "m" line, the 'group' attribute
   cannot be used.  Instead, the 'ssrc-group' attribute MUST be used.

   Per [RFC3550], the SSRC identifiers for the RTP streams that are
   carried in the same RTP session MUST be unique.  However, the SSRC
   identifiers are not guaranteed to be unique among different RTP
   sessions.  Thus, the 'ssrc-group' attribute MUST only be used at the
   media level [I-D.ietf-mmusic-sdp-source-attributes].  The semantics
   of "FEC-XR" for the 'ssrc-group' attribute is exactly the same as the
   one defined for the 'group' attribute.

   Let us consider the following scenario where there are two source
   flows (e.g., one video and one audio) and a single repair flow that
   protects only one of the source flows (e.g., video).  Suppose that
   all these flows are separate RTP streams that are SSRC multiplexed in
   the same RTP session.

                  SOURCE FLOWS             | FEC FRAMEWORK INSTANCE #1
                  S5: Source Flow |--------| R8: Repair Flow
                  S6: Source Flow

    Figure 4: Example scenario with one FEC Framework instance, where a
         single repair flow protects only one of the source flows

   The following SDP describes the scenario sketched in Figure 4.











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        v=0
        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=New FEC Grouping Semantics for SSRC Multiplexing
        t=0 0
        m=video 30000 RTP/AVP 100 101 110
        c=IN IP4 233.252.0.1/127
        a=rtpmap:100 JPEG/90000
        a=rtpmap:101 L16/32000/2
        a=rtpmap:110 1d-interleaved-parityfec/90000
        a=fmtp:110 L:5; D:10; repair-window: 200000
        a=ssrc:1000 cname:fec@example.com
        a=ssrc:1010 cname:fec@example.com
        a=ssrc:2110 cname:fec@example.com
        a=ssrc-group:FEC-XR 1000 2110
        a=mid:Group1

   Note that in actual use, SSRC values, which are random 32-bit
   numbers, may be much larger than the ones shown in this example.
   Also note that before receiving an RTP packet for each stream, the
   receiver cannot know which SSRC identifier is associated with which
   payload type.

   The additivity of the repair flows is handled in the same way as
   described in Section 4.2.  In other words, the repair flows that are
   included in an "a=ssrc-group" line are additive.  Repair flows that
   are in different "a=ssrc-group" lines are non-additive.

4.4.  Offer-Answer Model Considerations

   When offering FEC grouping using SDP in an Offer/Answer model
   [RFC3264], the following considerations apply.

   A node that is receiving an offer from a sender may or may not
   understand line grouping.  It is also possible that the node
   understands line grouping but it does not understand the "FEC-XR"
   semantics.  From the viewpoint of the sender of the offer, these
   cases are indistinguishable.

   When a node is offered a session with the "FEC-XR" grouping semantics
   but it does not support line grouping or the FEC grouping semantics,
   the node SHOULD respond to the offer either:

   o  With an answer that ignores the grouping attribute.

      In this case, the original sender of the offer MUST first check
      whether using the FEC grouping semantics of [RFC4756] will create
      any ambiguity or not, while keeping in mind the limitations
      explained in Section 3.1.  If using the "FEC" semantics rather



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      than the "FEC-XR" semantics still provides an exact association
      among the source and repair flows, the sender of the offer MUST
      send a new offer using the "FEC" semantics.  However, if an exact
      association cannot be described, the sender MUST send a new offer
      without FEC.

   o  With a refusal to the request (e.g., 488 Not Acceptable Here or
      606 Not Acceptable in SIP).

      In this case, if the sender of the offer still wishes to establish
      the session, it MUST first check whether using the FEC grouping
      semantics of [RFC4756] will create any ambiguity or not, while
      keeping in mind the limitations explained in Section 3.1.  If
      using the "FEC" semantics rather than the "FEC-XR" semantics still
      provides an exact association among the source and repair flows,
      the sender of the offer SHOULD send a new offer using the "FEC"
      semantics.  However, if an exact association cannot be described,
      the sender SHOULD send a new offer without FEC.

   Note that in both cases described above, when the sender of the offer
   sends a new offer with the "FEC" semantics, and the node understands
   it, the session will be established and the rules pertaining to
   [RFC4756] will be valid.

   However, if the node does not understand the "FEC" semantics, it
   SHOULD respond to the offer either (1) with an answer that ignores
   the grouping attribute, or (2) with a refusal to the request.  In the
   first case, the sender MUST send a new offer without FEC.  In the
   second case, if the sender of the offer still wishes to establish the
   session, it SHOULD retry the request with an offer without FEC.


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
   sources.


6.  IANA Considerations

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




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   Note to the RFC Editor:  In the following, please replace "XXXX" with
   the number of this document prior to publication as an RFC.

   Semantics                    Token   Reference
   ---------------------------  ------  ---------
   Forward Error Correction XR  FEC-XR  [RFCXXXX]

   This document also registers the following semantics with IANA in
   Semantics for the 'ssrc-group' SDP Attribute under SDP Parameters:

   Semantics                    Token   Reference
   ---------------------------  ------  ---------
   Forward Error Correction XR  FEC-XR  [RFCXXXX]


7.  Acknowledgments

   Some parts of this document are based on [RFC4756].  Thus, the author
   would like to thank those who contributed to [RFC4756].  Also, thanks
   to Jonathan Lennox who has contributed to Section 4.3.


8.  References

8.1.  Normative References

   [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.

   [I-D.ietf-mmusic-rfc3388bis]
              Camarillo, G., "The SDP (Session Description Protocol)
              Grouping Framework", draft-ietf-mmusic-rfc3388bis-02 (work
              in progress), January 2009.

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

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

   [I-D.ietf-mmusic-sdp-source-attributes]
              Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol



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              (SDP)", draft-ietf-mmusic-sdp-source-attributes-02 (work
              in progress), October 2008.

8.2.  Informative References

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

   [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.


Author's Address

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

   Email:  abegen@cisco.com
























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