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rtcweb                                                      D. R. Worley
Internet-Draft                                                   Ariadne
Intended status: Standards Track                          March 15, 2013
Expires: September 16, 2013


 A Generic Bundle Mechanism for the Session Description Protocol (SDP)
                       draft-worley-sdp-bundle-06

Abstract

   This document defines a generic bundle mechanism for the Session
   Description Protocol (SDP) by which the media described by a number
   of media descriptions ("m= lines") are multiplexed and transmitted
   over a single transport association.  The single transport
   association is described by an additional media description, allowing
   SDP attributes to be applied to the aggregate, independently of
   attributes applied to the constituents.  In offer/answer usage, the
   bundle mechanism is backward compatible with SDP processors that do
   not understand the mechanism.  The mechanism is designed to be
   compatible with the limitations of the existing Internet
   infrastructure.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 16, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Desiderata  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Feature Desiderata  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Compatibility Desiderata  . . . . . . . . . . . . . . . .   8
   4.  Tutorial Examples . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  One Audio Stream and One Video Stream . . . . . . . . . .  10
       4.1.1.  Offer without Bundling  . . . . . . . . . . . . . . .  10
       4.1.2.  Offer with Bundling . . . . . . . . . . . . . . . . .  11
       4.1.3.  Answer from an Answerer that Supports Bundling  . . .  13
       4.1.4.  Answer from an Answerer that Does Not Support
               Bundling  . . . . . . . . . . . . . . . . . . . . . .  14
       4.1.5.  Fast-Start Offer  . . . . . . . . . . . . . . . . . .  16
     4.2.  Two Audio Streams and Two Video Streams . . . . . . . . .  17
     4.3.  Virtual Classroom with One Audio Stream, Two Video
           Streams, and a Group of Video Streams . . . . . . . . . .  19
     4.4.  One Audio Stream and Two SCTP Streams . . . . . . . . . .  21
   5.  Syntax and Semantics  . . . . . . . . . . . . . . . . . . . .  21
     5.1.  Constructing an Offer . . . . . . . . . . . . . . . . . .  21
     5.2.  Constructing an Answer  . . . . . . . . . . . . . . . . .  21
     5.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  22
     5.4.  RTCP, SSRC, and RTP Sessions  . . . . . . . . . . . . . .  22
     5.5.  ICE considerations  . . . . . . . . . . . . . . . . . . .  22
     5.6.  Encryption considerations . . . . . . . . . . . . . . . .  22
   6.  Compatibility Considerations  . . . . . . . . . . . . . . . .  22
     6.1.  Backward Compatibility during Offer/Answer  . . . . . . .  22
     6.2.  Backward Compatibility with Existing Devices  . . . . . .  22
   7.  Design Features and Comparison  . . . . . . . . . . . . . . .  22
     7.1.  Aggregation of Constituent Media Descriptions . . . . . .  24
     7.2.  Presence of a Bundle Media Description and Its Media Type  24
     7.3.  Immediate Update  . . . . . . . . . . . . . . . . . . . .  24
     7.4.  Effective Media Description Ports after Session
           Establishment . . . . . . . . . . . . . . . . . . . . . .  25
     7.5.  Payload Types in the Bundled Media Description  . . . . .  25
     7.6.  Relationship of Payload Types of Constituent Media
           Descriptions  . . . . . . . . . . . . . . . . . . . . . .  26
     7.7.  Basis of Demultiplexing . . . . . . . . . . . . . . . . .  26
     7.8.  Basis of Rejection of the Bundle MD . . . . . . . . . . .  26
     7.9.  How Constituent MDs Are Offered . . . . . . . . . . . . .  27



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   8.  Demultiplexing Considerations . . . . . . . . . . . . . . . .  30
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  33
   12. Revision History  . . . . . . . . . . . . . . . . . . . . . .  34
     12.1.  draft-worley-sdp-bundle-00 . . . . . . . . . . . . . . .  34
     12.2.  Changes from draft-worley-sdp-bundle-00 to draft-worley-
            sdp-bundle-01  . . . . . . . . . . . . . . . . . . . . .  34
     12.3.  Changes from draft-worley-sdp-bundle-01 to draft-worley-
            sdp-bundle-02  . . . . . . . . . . . . . . . . . . . . .  34
     12.4.  Changes from draft-worley-sdp-bundle-02 to draft-worley-
            sdp-bundle-03  . . . . . . . . . . . . . . . . . . . . .  34
     12.5.  Changes from draft-worley-sdp-bundle-03 to draft-worley-
            sdp-bundle-04  . . . . . . . . . . . . . . . . . . . . .  35
     12.6.  Changes from draft-worley-sdp-bundle-04 to draft-worley-
            sdp-bundle-05  . . . . . . . . . . . . . . . . . . . . .  35
     12.7.  Changes from draft-worley-sdp-bundle-05 to draft-worley-
            sdp-bundle-06  . . . . . . . . . . . . . . . . . . . . .  35
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     13.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Introduction

   The central idea of bundling is to multiplex the media that would be
   several transport flows into one transport flow, with particular
   emphasis on allowing one transport association to carry media that
   are presented to the higher, application layer, as multiple transport
   flows.

   At the interface between the SDP-configured layer and the lower,
   transport layer, the media are organized into a single transport
   flow.  The transport-related properties of the RTP session (e.g.,
   transport 5-tuple, encryption, ICE) are described by the transport-
   related attributes of a single media description.

   At the interface between the SDP-configured layer and the higher,
   application layer, the media are organized into several transport
   flows.  The application-related properties of the transport flow
   (e.g., media type and label) are described by the application-related
   attributes of separate media descriptions.

   (There are some attributes (e.g., bandwidth limitation) that can
   apply separately to both the bundled transport flow and the
   constituent transport flows.)





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   However, we do not include the payload type numbers as information
   available to the application; only the encoding name and its
   parameters are accessible to the application.  This gives the bundle
   mechanism freedom to place constraints on the use of payload types.

   The bundle is signaled in the session description by a "group"
   attribute with semantics "BUNDLE".  The first media description
   listed in the group is the "bundle" media description (MD), whose
   transport information describes the transport association via which
   the packets will be sent.  The remaining (zero or more) media
   descriptions listed in the group are the "constituent" MDs.  Packets
   (either RTP/RTCP/SRTP/SRTCP, SCTP, or SCTP-over-DTLS) received from
   the applications for these MDs are sent (unaltered) on the transport
   association for the bundle MD.  Packets received from the transport
   association for the bundle MD are demultiplexed (based on particular
   features of the constituent transport flow protocols and the payload
   types and SCTP ports specified in the constituent MDs) and sent to
   the applications for the constituent MDs.

   In offer/answer usage, we must arrange that the bundle mechanism is
   backward compatible with entities that do not understand the bundle
   mechanism.  This requirement drives many features of this solution.
   Section 6.1

   In addition, many devices in current usage (especially SBCs) apply
   more restrictions on the usage of SDP than one would expect from
   abstract consideration of their roles in the network.  Some features
   of this solution are constructed to avoid these restrictions.
   Section 6.2

2.  Terminology

   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].

   The important RFCs in this area use inconsistent terminology.  Here,
   we use:

   media  Media is (1) media content, considered in an abstract way,
      that is, without consideration of its particular encoding or the
      framing information around it, and (2) the particular bits and
      octets used to encode and transmit the abstract media content.

   media stream  (Taken from [RFC3550].)  A media stream is a set of RTP
      packets that are generated by and interpreted by one codec.  The
      RTP packets of a media stream are identified by a unique SSRC.




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   capture  (Taken from CLUE's work.)  A capture is a set of media
      streams that originate from one (physical or virtual) media source
      and should be composed to provide rendering of that source.  For
      example, media streams from one origin including layered
      encodings, forward error correction streams, recovery streams, and
      simulcasted media streams of varying bit rates compose one
      capture.

   transport association  (Taken from [I-D.alvestrand-mmusic-msid].)  A
      transport association is a single data path between two hosts,
      such as a TCP connection, or a pair of UDP ports that send packets
      to each other.  A transport association is identified by the
      identity of the protocol being used, the relevant host addresses,
      and the relevant port numbers.  In the case of unicast
      communications, these form a "5-tuple", namely, the protocol, the
      host addresses of the two hosts, and the port numbers used on the
      two hosts.  In the case of multicast sessions, these form a
      "3-tuple", namely, the protocol, the multicast address, and the
      port number.  In SDP, a transport association is specified by the
      address and port of a media description (and possibly the same
      information from the matching offer/answer SDP).  If a media
      description specifies multiple addresses or ports, each address or
      port specifies one transport association.

   transport flow  (Taken from
      [I-D.ietf-avtcore-multi-media-rtp-session].)  (This is called an
      "RTP session" by [RFC3264].)  A transport flow is the data that
      flows across a transport association.

   media description  (Taken from [RFC4566].)  A media description is
      one group of lines in a session description demarcated by an m=
      line.  By synecdoche, a media description is often referred to as
      "an m= line".

   transport association group  A transport association group is the set
      of transport associations denoted by one media description.
      Usually the m= line specifies only one port and the c= line
      specifies only one address, and so the media description's
      transport association group contains only one transport
      association.

   transport flow group  A transport flow group is the set of transport
      flows of the transport associations of a transport association
      group.

   session description  (Taken from [RFC4566] section 2.)  A session
      description is an SDP instance.




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   multimedia session  (Taken from [RFC4566] section 2.)  A multimedia
      session is the totality of the media that is transmitted/received
      as described by a particular session description.

   RTP session  (Taken from [RFC3550].)  An RTP session is a group of
      media streams which must not have duplicated SSRC values because
      the endpoints share RTCP reporting information.  Note that an RTP
      session may encompass more than one multimedia session.  RTP
      sessions are not fully described by session descriptions.

3.  Desiderata

   This section lists desiderata for the bundle mechanism in SDP.  (I
   use the term "desiderata" -- "things that are desired" -- rather than
   "requirements", because we may discover that we can't optimally
   satisfy all of these criteria at the same time.)  The first section
   lists desiderata that are arise from considering the ways
   applications may wish to bundling.  The second section lists
   desiderata that arise from compatibility with existing Internet
   infrastructure.

3.1.  Feature Desiderata

   These desiderata describe features that we would like the bundling
   mechanism to provide.

   DES F1  For each bundle, there is a group of media descriptions which
      describe the application-level RTP sessions.  This specification
      must allow the same granularity of description as when the media
      flows were not multiplexed.  This description includes identifiers
      which connect the media flows with the application and with each
      other.

   This requirement is taken from [I-D.jennings-mmusic-media-req].

   DES F2  For each bundle, there is a media description that describes
      the transport-level RTP session.

   DES F1 and DES F2 do not specify whether the transport-level media
   description may or may not also be one of the application-level media
   descriptions.

   DES F3  There must be a uniform way to deal with new SDP parameters,
      so that newly defined SDP parameters do not require a specific
      updating of the bundling procedures.

   This desideratum is taken from slides-interim-2013-rtcweb-1-10.pdf.




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   DES F4  Multiple separate bundles within one SDP must be supported.

   DES F5  Bundles may contain other bundles as constituents.

   Of course, no bundle may directly or indirectly contain itself.  (I
   don't expect any current implementation to implement bundles within
   bundles, but we should design the mechanism to allow this, as some
   day we will likely need it.)

   DES F6  A bundle may contain zero constituents.

   A bundle with no constituents serves no purpose for the transport of
   media, but we are likely to someday need to describe such a bundle.
   (Compare that an SDP m= line is syntactically constrained to specify
   at least one payload type.  When SDP was used only to specify
   multicast sessions, this constraint was common sense.  But once SDP
   offer/answer was invented, when a media description was rejected, the
   natural representation would be an m= line with a zero port and no
   payload types.  But a payload type was syntactically required, so we
   now have to provide at least one token payload type in rejected m=
   lines.)

   DES F7  If an answerer that does understand the bundle mechanism
      processes an offer that contains a bundle, it must be able to (1)
      accept the bundle and selectively accept or reject each
      constituent RTP session within it, (2) reject the bundle as a
      whole, or (3) reject the bundling and selectively accept or reject
      each constituent RTP session as separate RTP sessions.

   Presumably answer (3) resembles that which would be produced by an
   answerer that does not understand the bundle mechanism.  It is a
   lower priority that the answerer can distinguish between accepting
   the bundle while rejecting all of its constituents, and rejecting the
   bundle as a whole.  But those two conditions differ conceptually
   regarding whether any "framing" actions of the bundle are performed.

   DES F8  There must be a reliable way to demultiplex incoming RTP into
      the separate application-level RTP sessions.  Similarly, there
      must be a reliable way to demultiplex the associated RTCP
      information.

   The RTCP information for each media stream is tagged with the SSRC
   about which it reports, and the SSRC is used to correlate the RTCP
   reports with the RTP sessions containing media with the same SSRC.
   So regarding RTCP, this desideratum appears to be straightforward to
   satisfy.





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   DES F9  The specification must specify any needed additional
      procedures for handling SSRC collisions between media sources
      within different application-level RTP sessions, as those can now
      collide.

   In the terminology of [RFC3550], the constituent media descriptions
   are now part of one RTP session.

   DES F10  When an offer is constructed, the offerer must not need to
      preallocate TURN relays for constituent media descriptions.  When
      both endpoints support bundling, the mechanism must not require
      the offerer to allocate TURN relays for constituent media
      descriptions.

   This desideratum was suggested by Andrew Hutton.

   DES F11  It must be possible to add and remove one way video flows
      within the bundle without requiring an additional offer/answer
      cycle.

   Presumably this can be accomplished as it is now, with a single media
   description carrying multiple video flows that are distinguished only
   by their SSRCs.  This desideratum is taken from slides-
   interim-2013-rtcweb-1-10.pdf.

   DES F12  Bundling must not interfere with ICE usage, and in
      particular, ICE's ability to negotiate both IPv4 and IPv6
      addresses simultaneously.

   This desideratum was suggested by Andrew Hutton.

   DES F13  It is desirable for RTP media to appear on the wire as
      (encrypted) SRTP packets and for RTCP reports to appear on the
      wire as (authenticated but not encrypted) SRTCP packets.  Thus,
      encryption of RTP/RTCP via DTLS and encapsulation of multiplexed
      packets is not desirable.

   This desideratum was suggested by Martin Thompson (http://
   www.ietf.org/mail-archive/web/mmusic/current/msg10400.html) and Dan
   Wing (http://www.ietf.org/mail-archive/web/mmusic/current/
   msg10408.html).

3.2.  Compatibility Desiderata

   These desiderata describe compatibility of the bundling mechanism
   with with non-supporting endpoints or with existing entities in the
   Internet infrastructure.




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   DES C1  In offer/answer usage, an endpoint using the bundle mechanism
      must interwork correctly with an endpoint that does not understand
      the bundle mechanism.

   DES C2  Interworking must continue when SDP endpoints are replaced
      with other endpoints during a sequence of offer/answer exchanges
      (such as happens in 3PCC or call transfers "behind an SBC"),
      including when a supporting endpoint is replaced by a non-
      supporting endpoint or vice-versa.

   SDP features (e.g., the codec set and ICE) are generally designed so
   that an offerer always offers every facility it is willing to support
   in the current situation, regardless of whether it was agreed to by
   the answerer in a preceding exchange.  Thus, if the current answerer
   is a different endpoint than the previous answerer, the new answerer
   will negotiate a compatible set of facilities without needing
   knowledge of its predecessor's SDP.  The offerer will smoothly
   transition to the new facilities.  This property is required to
   support 3PCC situations (e.g., [RFC3725] and
   [I-D.worley-service-example]).  This desideratum was suggested by
   Richard Ejzak.

   DES C3  Avoid using media types in m= lines other than audio and
      video unless required for user media, as some SBCs reject SDP that
      uses other media types.

   This desideratum was suggested by Hadriel Kaplan.

   DES C4  Any additional m= lines prescribed by the bundle mechanism
      should be ordered after the constituent m= lines.

   Many devices that have only one audio or video channel accept the
   first m= line with that media type and reject any further ones

   non-DES C5  SBCs generally pass through attributes that they do not
      understand.  SBCs generally pass through codec specifications that
      they do not understand, even if they are configured to transcode
      certain specific codecs.

   This non-desideratum was suggested by Hadriel Kaplan.

   DES C6  After offer/answer processing is finished, if the exchanged
      SDP is examined by a non-supporting SBC, the set of transport
      associations that it sees being specified for media exchange
      should be the set that are actually used for media transfer.

   This is needed because SBCs monitor the packet traffic on the
   transport associations and if no media is seen on one of the



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   associations for a significant period of time, the SBC will tear down
   the call.  This desideratum was suggested by Hadriel Kaplan.

   DES C7  In a session description, no endpoint of a transport
      association may be used multiple times.

   Such duplication is not defined by [RFC4566].  Some SBCs do not
   support such duplication (ultimately, because it was not supported by
   [RFC2327]), and they reject SDP specifying duplicated transport
   association endpoints.  This desideratum was suggested by Cullen
   Jennings.

   DES C8  Offer/answer processing between supporting processors must be
      completed in one exchange.  When interworking between supporting
      and non-supporting processors, it is less desirable but admissible
      that a second offer/answer exchange may be needed to complete
      configuring the multimedia session.

   DES C9  If an intermediate entity provides transcoding between
      codecs, and if the offer/answer does not specify encryption of a
      media stream, the media stream should be able to take advantage of
      the transcoding facility.

   The non-encrypted case is not expected to be very common.  Encrypted
   media can't be transcoded by an intermediate entity.

4.  Tutorial Examples

   This section is non-normative.  (This section was suggested by
   Charles Eckel.)

   This is an introduction to SDP bundling via a series of examples of
   offer/answer processing.  Some mandatory SDP lines have been omitted
   from the examples for brevity.  Long SDP lines have been folded by
   using trailing backslashes.  Blank lines have been inserted for
   clarity.

4.1.  One Audio Stream and One Video Stream

4.1.1.  Offer without Bundling

   Here is a typical, non-bundled SDP example with both audio and video
   media:

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

   This SDP media description (MD) provides the transport information



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   about the audio and also identifies the role of the audio from the
   application's point of view.  In this case, the fact that it is the
   first audio m= line suffices to tell the application how to treat it.
   In more complex cases, label or content attributes might be used to
   communicate the proper handling to the application.

           m=audio 10000 RTP/AVP 0 8 97
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31000 typ relay

   This MD provides the transport information about the video and also
   identifies the role of the video from the application's point of view.

           m=video 10002 RTP/AVP 31 32
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31002 typ relay


   We call the RTP that is described by each media description (MD) a
   transport flow (TF).  The audio and video are carried in separate
   TFs, which each have a separate transport association (address/port).

4.1.2.  Offer with Bundling

   With SDP bundling, we add an additional MD to describe a single
   "bundle" TF to carry both the audio and video information, and a
   group attribute to show the association of the bundle MD with the
   constituent MDs:

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

   The following group attribute declares which MDs are included in the
   multiplexed MD:  mid:con1 and mid:con2 are the constituent MDs whose
   TFs (from the application point of view) will be carried by the TF of
   the first-designed MD, mid:bundle, which is the bundle MD.  In order
   to allow demultiplexing of the packets on the bundle TF, the
   constituent MDs must use disjoint sets of payload types.




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           a=group:BUNDLE bundle con1 con2

   This MD provides the application-level description of the audio TF.
   As in the previous example, it is the first audio m= line.  It
   includes any attributes which apply to the audio media from the
   application point of view, including the payload type definitions.
   When interpreted by a supporting processor, the transport information
   is ignored.  When interpreted by a processor that does not support
   bundling, the transport information sets up the transport association
   for the audio TF.  But to avoid the overhead of preallocating TURN
   relays that will probably not be used, ICE relay candidates are not
   provided.

           m=audio 10000 RTP/AVP 0 8 97
           a=mid:con1
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx

   This MD provides the application-level description of the video TF.
   As in the previous example, it is the first video m= line.  It
   includes any attributes which apply to the video media from the
   application point of view.  As in the audio MD, the association
   information is used only by a processor that does not support
   bundling.

           m=video 10002 RTP/AVP 31 32
           a=mid:con2
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx

   This MD provides the transport information for the bundle TF,
   including any attributes which apply to the transport.  We use RTCP
   multiplexing [RFC5761], so only one set of ICE candidates (and only
   one TURN relay) is needed for this MD.

   The MD is artificially given the media type "audio" (which is ugly,
   but it avoids rejection by SBCs) and it is placed after all of the
   constituent MDs so as to not affect their positions as "first audio
   MD", etc.  The MD has a proto value of "bundle" to describe the packet
   traffic (which in general can be a mixture of RTP/SRTP/RTCP/SRTCP,
   SCTP, and DTLS).  A single, dummy fmt value of 0 is used.  The proto



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   value ensures that an answerer that does not support bundling will not
   accept this MD.

   The packets sent on this transport flow are the packets provided by
   the applications for the constituent transport flows.

           m=audio 10004 bundle 0
           a=mid:bundle
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10004 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51004 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31004 typ relay


   If this SDP bundle is accepted, RTP provided by the application for
   the audio TF will be sent from port 10004.  RTP provided by the
   application for the video TF will be also be sent from port 10004.

   RTP that is received on port 10004 is interpreted according to the
   payload type number.  Since the payload type numbers used in the two
   constituent transport flows are disjoint, incoming RTP packets can be
   directed to the proper applications based on payload type number.

4.1.3.  Answer from an Answerer that Supports Bundling

   If the answerer supports SDP bundling, and desires to accept the
   offered bundle and its constituent MDs, the answerer signals that it
   accepts the SDP bundling by providing a matching group:BUNDLE
   attribute in the answer.  As always in offer/answer, the MDs in the
   answer correspond to the MDs in the offer by ordinal position.

   The answerer provides the necessary transport information for the
   bundle MD.  The answerer understands that MDs mid:con1 and mid:con2
   are incorporated into MD mid:bundle, and ignores their transport
   information.  It accepts each constituent MD as part of the bundle by
   providing an answer MD for each of them that specifies an attribute
   "a=bundleaccept" and a port number of 0 (so intermediate entities see
   the MD as being rejected).

   Note that the constituent MDs, despite having zero port numbers, are
   still incorporated in the "a=group:BUNDLE" attribute.  This
   contravenes [RFC5888] section 9.2, and so this proposal requires an
   extension of RFC 5888.

           o=- 2890844526 2890844526 IN IP4 answer.example.com
           c=IN IP4 answer.example.com

           a=group:BUNDLE bundle con1 con2




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           m=audio 0 RTP/AVP 0 8 97
           a=mid:con1
           a=bundleaccept
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000

           m=video 0 RTP/AVP 31 32
           a=mid:con2
           a=bundleaccept
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000

           m=audio 20000 bundle 0
           a=mid:bundle
           a=candidate:0 1 UDP 2113601791 10.0.2.1 20000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.35 51090 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 32000 typ relay


4.1.4.  Answer from an Answerer that Does Not Support Bundling

   SDP bundling allows for backward compatibility in case the answerer
   does not understand bundling.  If the answerer does not understand
   bundling, it ignores the group attribute, and effectively sees the
   offer as:

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

           m=audio 10000 RTP/AVP 0 8 97
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx

           m=video 10002 RTP/AVP 31 32
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx

           m=audio 10004 bundle 0



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           a=candidate:0 1 UDP 2113601791 10.0.1.1 10004 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51004 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31004 typ relay


   If the answerer wishes to accept the first audio and video streams,
   it assembles this answer:

           o=- 2890844526 2890844526 IN IP4 answer.example.com
           c=IN IP4 answer.example.com

   The absence of the group attribute informs the offerer that bundling
   was rejected.

   The audio MD is accepted.  Transport information is provided,
   including ICE candidates.

           m=audio 20000 RTP/AVP 0 8 97
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000
           a=candidate:0 1 UDP 2113601791 10.0.2.1 20000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.128 52000 typ srflx

   The video MD is accepted.  Transport information (using a different
   port) is provided.

           m=audio 20002 RTP/AVP 31 32
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000
           a=candidate:0 1 UDP 2113601791 10.0.2.1 20002 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.128 52002 typ srflx

   The bundle MD is rejected by the answerer because the proto value is
   "bundle", and the answerer does not implement it.

           m=audio 0 bundle 0


   Because the group attribute is not present in the response, the
   offerer knows that the answerer does not support bundling (or does
   not want to consider the offered bundle).  The offerer knows that the
   answerer wants to establish one audio TF and one video TF, and
   formally, that has been done.  But if transport requires ICE relay
   candidates on the offerer's side, the offerer must send an updated
   offer containing those ICE candidates for the constituent MDs:



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           o=- 2890844526 2890844527 IN IP4 host.example.com
           c=IN IP4 host.example.com

   No group attribute is included, to ensure that this update only
   revises transport attributes, and does not trigger bundle-supporting
   behavior if the answering entity has changed in the meantime.

   Provide ICE relay candidates for the audio MD.

           m=audio 10000 RTP/AVP 0 8 97
           a=mid:con1
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31000 typ relay

   Provide a separate TURN relay for the video MD.

           m=video 10002 RTP/AVP 31 32
           a=mid:con2
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31002 typ relay

   The bundle MD must still be listed, but it is disabled.

           m=audio 0 bundle 0
           a=mid:bundle


   The answerer provides the same answer as before.

   The ICE renegotiation proceeds, the transport associations are
   established, and RTP flows.

4.1.5.  Fast-Start Offer

   The baseline procedure requires the offerer to update its offer when
   it discovers that the answerer does not support SDP bundling if TURN
   relays are needed to support the constituent MDs.  The offerer can
   avoid this delay by providing TURN relays for the constituent MDs as
   well as for the bundle MD.  The penalty is that the offerer must



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   preallocate TURN relays for both the constituent MDs as well as the
   bundle MD.

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

           a=group:BUNDLE bundle con1 con2

           m=audio 10000 RTP/AVP 0 8 97
           a=mid:con1
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31000 typ relay

           m=video 10002 RTP/AVP 31 32
           a=mid:con2
           a=rtcp-mux
           a=rtpmap:31 H261/90000
           a=rtpmap:32 MPV/90000
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31002 typ relay

           m=audio 10004 bundle 0
           a=mid:bundle
           a=rtcp-mux
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10004 typ host
           a=candidate:1 1 UDP 1694194431 198.51.100.32 51004 typ srflx
           a=candidate:2 1 UDP 1006633215 10.0.100.1 31004 typ relay


   If the answerer understands bundling and accepts the bundle, it
   accepts the constituent MDs within the bundle (with "a=bundleaccept"
   and port 0) and accepts the bundle MD.  If the answerer does not
   understand bundling, it accepts the constituent MDs and rejects the
   bundle MD.  In either case, ICE relay candidates are in place and ICE
   negotiation proceeds.

4.2.  Two Audio Streams and Two Video Streams








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   In this example, a presentation involves four media roles: the
   speaker's audio, the floor microphone, the video of the speaker, and
   the video of the speaker's slides.  We use separate MDs for each
   media stream because each TF has a different role; the application
   will handle each of them in distinctly different ways.

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

           a=group:BUNDLE b c1 c2 c3 c4

           m=audio 10000 RTP/AVP 0 8 97
           a=mid:c1
           a=label:speaker-audio
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000

   Note that different constituent MDs must use different payload types
   (even for the same codec), because incoming RTP is demultiplexed based
   on payload type.

           m=audio 10002 RTP/AVP 98 99 100
           a=mid:c2
           a=label:floor-mic
           a=rtcp-mux
           a=rtpmap:98 PCMU/8000
           a=rtpmap:99 PCMA/8000
           a=rtpmap:100 G722

           m=video 10004 RTP/AVP 101 102
           a=mid:c3
           a=label:speaker-video
           a=rtcp-mux
           a=rtpmap:101 H261/90000
           a=rtpmap:102 MPV/90000

           m=video 10006 RTP/AVP 103 104
           a=mid:c4
           a=label:slides
           a=rtcp-mux
           a=rtpmap:103 H261/90000
           a=rtpmap:104 MPV/90000

           m=multipart 10008 bundle 0
           a=mid:b




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4.3.  Virtual Classroom with One Audio Stream, Two Video Streams, and a
      Group of Video Streams

   This example is the teacher's connection to a virtual classroom
   server.  The media descriptions are tagged using the "content"
   attribute.  [RFC4796]  The media comprises:

   1.  1.  one audio channel, for sending the teacher's voice and
       receiving the voice of a selected student

   2.  2.  one video channel, for sending the teacher's presentation

   3.  3.  one video channel, for sending the teacher's face

   4.  4.  one video channel, for receiving a dynamically varying set of
       students' faces

   The fourth TF (for students' faces) contains a large and dynamically
   varying set of video captures.  These can be handled by a single TF
   because they all have essentially similar roles -- the application
   will process them as a set.  As Adam Roach would say, "no control
   surfaces are necessary to talk about and/or manipulate the individual
   streams".  In particular, this allows a large number of captures to
   be handled without mentioning them in the SDP, at the expense of not
   allowing the SDP to describe any of them individually.  Similarly,
   the number of captures can vary without having to renegotiate the
   SDP.

   (In contrast, the third TF (the teacher's face) is a separate TF
   because it is processed in a different role than that of the
   students' faces.)

   In unbundled usage, there would be one transport association for the
   fourth TF.  Incoming RTP from that association would be demultiplexed
   by the application based on the SSRC values, which would be unique
   for each student.  With bundling, once the single transport TF is
   demultiplexed based on the RTP payload type, packets destined for the
   fourth TF (index = 3) would be further demultiplexed by their SSRC
   values.  The demultiplexing by SSRC is considered to be an
   application layer function in the context of SDP bundling.

   The offered SDP is:

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

           a=group:BUNDLE b c1 c2 c3 c4




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   The audio channel is send/receive.

           m=audio 10000 RTP/AVP 0 8 97
           a=mid:c1
           a=label:speaker-audio
           a=content:speaker
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000

   The teacher's face and presentation are send-only.

           m=video 10002 RTP/AVP 103 104
           a=mid:c2
           a=label:speaker-video
           a=content:speaker
           a=sendonly
           a=rtcp-mux
           a=rtpmap:103 H261/90000
           a=rtpmap:104 MPV/90000

           m=video 10004 RTP/AVP 105 106
           a=mid:c3
           a=label:presentation
           a=content:slides
           a=sendonly
           a=rtcp-mux
           a=rtpmap:105 H261/90000
           a=rtpmap:106 MPV/90000

   The student video input is receive-only and is limited to 24
   simultaneous SSRCs.

           m=video 10006 RTP/AVP 107 108
           a=mid:c4
           a=label:student-thumbnails
           a=recvonly
           a=max-recv-ssrc:* 24
           a=rtcp-mux
           a=rtpmap:107 H261/90000
           a=rtpmap:108 MPV/90000

           m=audio 10000 RTP/AVP 127
           a=mid:b
           a=rtcp-mux
           a=rtpmap:127 bundle
           a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host



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           a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx


4.4.  One Audio Stream and Two SCTP Streams

   This example contains one audio MD and two SCTP MDs, which are used
   for Webrtc datachannels.

           o=- 2890844526 2890844526 IN IP4 host.example.com
           c=IN IP4 host.example.com

           a=group:BUNDLE bundle con1 con2 con3

           m=audio 10000 RTP/AVP 0 8 97
           a=mid:con1
           a=rtcp-mux
           a=rtpmap:0 PCMU/8000
           a=rtpmap:8 PCMA/8000
           a=rtpmap:97 iLBC/8000

   These MDs provides the the SCTP TFs.  Because packets are not
   encapsulated, the two SCTP TFs must use different (nominal) SCTP ports
   to allow their packets to be distinguished.

           m=application 10002 DTLS/SCTP 5000
           a=mid:con2
           a=sctpmap:5000 webrtc-datachannel 16

           m=application 10004 DTLS/SCTP 5001
           a=mid:con3
           a=sctpmap:5001 webrtc-datachannel 16

           m=audio 10006 bundle 0
           a=mid:bundle


5.  Syntax and Semantics

   TBD (Here lies the real description.)

5.1.  Constructing an Offer

   TBD

5.2.  Constructing an Answer

   TBD




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5.3.  Offer/Answer Considerations

   TBD

5.4.  RTCP, SSRC, and RTP Sessions

   TBD

5.5.  ICE considerations

   TBD

5.6.  Encryption considerations

   TBD Need to discuss here how the encryption associations are set up.
   For SRTP/SRTCP, it would be possible to have either one association
   for all multiplexed streams, or one for each constituent MD, because
   SRTP preserves the PTs.  (Have to verify that and check whether SRTCP
   preserves SSRCs.)  But SCTP-over-DTLS can't be demultiplexed before
   it's decrypted, so there can be only one DTLS crypto association.

6.  Compatibility Considerations

6.1.  Backward Compatibility during Offer/Answer

   TBD

6.2.  Backward Compatibility with Existing Devices

   TBD

7.  Design Features and Comparison

   Key:

         x = feature present in proposal
         - = feature not present in proposal
         . = feature not discussed in proposal
       N/A = feature is not relevant because of another feature choice

                   worley-sdp-bundle-06
                   |   ietf-mmusic-sdp-bundle-negotiation-03 (BUNDLE)
                   |   |   holmberg-mmusic-sdp-mmt-negotiation-00 (MMT)
                   |   |   |   alvestrand-one-rtp-02 (TOGETHER)
                   |   |   |   |   ejzak-mmusic-bundle-alternatives-01
                   |   |   |   |   |   Roach alternative 1a
                   |   |   |   |   |   |(roach-mmusic-mlines-00)
                   |   |   |   |   |   |   Roach alternative 1b



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                   |   |   |   |   |   |   |   Roach alternative 2
                   |   |   |   |   |   |   |   |   westerlund-avtcore-
                   |   |   |   |   |   |   |   |   |transport-multiplexing-05
                   |   |   |   |   |   |   |   |   |(SHIM)
                   V   V   V   V   V   V   V   V   V
   MD grouping:
       one         -   -   -   -   -   -   x   -   -
       per type    -   -   -   -   -   x   -   -   -
       none        x   x   x   x   x   -   -   x   x

   Separate bundle MD:
       no          -   x   -   x   x   x   x   x   x
       m=anymedia  -   -   x   -   -   -   -   -   -
       m=audio     x   -   -   -   -   -   -   -   -
       m=multipart -   -   -   -   -   -   -   -   -

   Immediate update:
       none        -   -   x   x   x   x   x   x   x
       for support -   x   -   -   -   -   -   -   -
       for compat. x   -   -   -   -   -   -   -   -

   Constituent MD ports after establishment:
       N/A         -   -   -   -   -   x   x   -   -
       same        -   x   -   x   -   -   -   x   x
       different   -   -   -   -   -   -   -   -   -
       null        -   -   -   -   -   -   -   -   -
       rejected    x   -   x   -   x   -   -   -   -

   Bundle MD payload types:
       N/A         x   -   -   -   -   x   x   -   -
       one MD      -   x   -   x   .   -   -   x   x
       all MDs     -   -   x   -   .   -   -   -   -
       one value   -   -   -   -   .   -   -   -   -

   Constituent MD payload types:
       N/A         -   -   -   -   -   x   x   -   -
       overlapping -   -   -   -   .   -   -   x   x
       distinct    x   x   x   x   .   -   -   -   -

   Demultiplexing based on:
       N/A         -   -   -   -   -   x   x   -   -
       PT          x   x   x   x   .   -   -   x   -
       encap.      -   -   -   -   .   -   -   -   x

   Rejection of bundle MD based on:
       N/A         -   x   -   x   x   x   x   x   x
       media type  -   -   x   -   -   -   -   -   -
       proto       x   -   -   -   -   -   -   -   -



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       codec       -   -   -   -   -   -   -   -   -

   Addresses/ports in constituent MDs in offer:
       N/A         -   -   -   -   -   x   x   -   -
       NA,ZP       -   -   -   -   -   -   -   -   -
       NA,NZP      -   -   -   -   x   -   -   -   -
       RA,ZP       -   -   -   -   -   -   -   -   -
       RA,NZP,U    x   x   x   -   x   -   -   -   -
       RA,NZP,S    -   x   -   x   -   -   -   x   x
   NA = null address, RA = real address
   ZP = zero port, NZP = non-zero port
   U = unique ports, S = shared port


7.1.  Aggregation of Constituent Media Descriptions

   Are the constituent media descriptions combined into grouped media
   descriptions?

   o  All media are combined into one media description.

   o  All media of each media type are combined into one media
      description (which has that type).

   o  Each constituent media description is separate in the session
      description.

   This proposal does not aggregate constituent MDs so that attributes
   can be provided directly for each constituent MD.

7.2.  Presence of a Bundle Media Description and Its Media Type

   Is there a separate bundle media description, and if so, what media
   type does it have?

   o  There is no separate bundle media description.

   o  There is a separate bundle media description of type "anymedia".

   o  There is a separate bundle media description of type "audio".

   o  There is a separate bundle media description of type "multipart".

   This proposal has a separate bundle MD so that attributes can be
   provided for the bundle MD independently of any constituent MD.

7.3.  Immediate Update




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   Is an immediate updated offer/answer used during session
   establishment?

   o  No.

   o  Yes, when establishing a session using bundling.

   o  Yes, when establishing a session not using bundling.

   This proposal can require immediate updates for non-bundle-supporting
   answers to provide ICE TURN candidates, if the offerer has not
   preallocated them.

7.4.  Effective Media Description Ports after Session Establishment

   What are the effective port numbers in MDs after the session is
   established?

   o  There are not multiple media descriptions because constituent MDs
      are combined.

   o  Port numbers in the MDs are the same.

   o  Port numbers in the MDs are different.

   o  All but one MD have null addresses.

   o  All but one MD have a zero port number (and thus are formally
      rejected).

   This proposal uses a zero port number for constituent MDs to prevent
   intermediate entities from expecting to see media for the constituent
   transport associations.

7.5.  Payload Types in the Bundled Media Description

   What payload types are listed for the bundled MD?

   o  There is no MD describing the bundle as a whole.

   o  The bundle MD lists the payload types of one constituent MD.

   o  The bundle MD lists the payload types of all constituent MDs.

   o  The bundle MD lists one payload type.

   This proposal uses a distinct proto value that does not use payload
   type numbers.



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7.6.  Relationship of Payload Types of Constituent Media Descriptions

   What is the relationship between the payload types of the constituent
   MDs?

   o  There are not multiple media descriptions because constituent MDs
      are combined.

   o  Different constituent MDs may have overlapping payload type
      numbers.

   o  Different constituent MDs may not have overlapping payload type
      numbers.

   This proposal requires constituent MDs to use distinct payload types
   in order to demultiplex the constituent MDs.

7.7.  Basis of Demultiplexing

   What is the basis for the demultiplexing of RTP?

   o  Demultiplexing is not done because incoming RTP is not attributed
      to specific constituent MDs (possibly because constituent MDs are
      combined).

   o  Demultiplexing is done based on payload type numbers.

   o  Demultiplexing is done based on data carried in an encapsulation.

   This proposal does demultiplexing based on information in the
   encapsulated payload format.

7.8.  Basis of Rejection of the Bundle MD

   We must ensure that the bundle MD is rejected by non-supporting
   endpoints.  What method is used to ensure rejection?

   o  There is no bundle MD.

   o  The bundle MD uses a special media type value.

   o  The bundle MD uses a special proto value.

   o  The bundle MD uses (only) a special codec name.

   This proposal uses a distinct proto type in the bundle media
   description to ensure that answerers that do not implement bundling
   reject this MD.



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7.9.  How Constituent MDs Are Offered

   There are a number of alternative ways that the offerer can configure
   the constituent media descriptions.















































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   +----------+-----+------+------+-------+-------+------+------+------+
   | Method   | 1   | 2    | 3    | 4     | 5     | 6    | 7    | 8    |
   +----------+-----+------+------+-------+-------+------+------+------+
   | Coded in | NA, | NA,N | RA,Z | RA,NZ | RA,NZ | RA,N | RA,N | RA,N |
   | chart as | ZP  | ZP   | P    | P,U   | P,U   | ZP,U | ZP,U | ZP,S |
   |          |     |      |      |       |       |      |      |      |
   | Offered  | nul | null | real | real  | real  | real | real | real |
   | address  | l   |      |      |       |       |      |      |      |
   |          |     |      |      |       |       |      |      |      |
   | Offered  | zer | non- | zero | non-  | non-  | non- | non- | non- |
   | port     | o   | zero |      | zero, | zero, | zero | zero | zero |
   |          |     |      |      | uniqu | uniqu | , un | , un | , sh |
   |          |     |      |      | e     | e     | ique | ique | ared |
   |          |     |      |      |       |       |      |      |      |
   | TURN can | N/A | N/A  | N/A  | no    | yes   | no   | yes  | yes  |
   | didates? |     |      |      |       |       |      |      |      |
   |          |     |      |      |       |       |      |      |      |
   | Supporti | yes | yes  | yes  | yes   | yes   | no   | no   | yes  |
   | ng       |     |      |      |       |       |      |      |      |
   | answerer |     |      |      |       |       |      |      |      |
   | accepts? |     |      |      |       |       |      |      |      |
   |          |     |      |      |       |       |      |      |      |
   | Update   | no  | no   | poss | yes   | yes   | no   | no   | no   |
   | needed   |     |      | ibly |       |       |      |      |      |
   | for supp |     |      |      |       |       |      |      |      |
   | orting a |     |      |      |       |       |      |      |      |
   | nswerer? |     |      |      |       |       |      |      |      |
   |          |     |      |      |       |       |      |      |      |
   | Non-supp | no  | prob | no   | yes   | yes   | yes  | yes  | yes  |
   | orting   |     | ably |      |       |       |      |      |      |
   | answerer |     |      |      |       |       |      |      |      |
   | accepts? |     |      |      |       |       |      |      |      |
   |          |     |      |      |       |       |      |      |      |
   | Update   | yes | yes  | yes  | yes   | no    | yes  | no   | no   |
   | needed   |     |      |      |       |       |      |      |      |
   | for non- |     |      |      |       |       |      |      |      |
   | supporti |     |      |      |       |       |      |      |      |
   | ng answe |     |      |      |       |       |      |      |      |
   | rer?     |     |      |      |       |       |      |      |      |
   |          |     |      |      |       |       |      |      |      |
   | Disadvan | ACE | CD   | ABCE | BC    | BG    | C    | G    | BFG  |
   | tages    |     |      |      |       |       |      |      |      |
   +----------+-----+------+------+-------+-------+------+------+------+


   Offered address  This is the address offered for the MD.  There are
      two choices, a null address (0.0.0.0 for IPv4 or "invalid."  for
      IPv6 or mixed IPv4/IPv6) or a real address of the offerer.



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   Offered port  This is the port that is offered.  It can be either
      non-zero or zero (which indicates a stream that is not being
      offered).  If the offered port is non-zero and the offered address
      is real, the port can be either unique, or shared with the other
      media descriptions in the bundle.

   TURN candidates?  If the offered address is real and the offered port
      is non-zero, are TURN candidates for the address provided (if they
      are needed)?

   Supporting answerer accepts?  If the answerer supports bundling, does
      it accept the constituent media description (with a non-zero port
      in the answer)?  If not, the answerer must have some other method
      of indicating acceptance of the constituent of the bundle.

   Update needed for supporting answerer?  Is an SDP update needed to
      complete session setup if the answerer supports bundling?  If an
      update is needed, it is needed to inform intermediaries that there
      will be no media sent based on the connection information in this
      media description; the update is not needed to establish
      communications and does not delay the application.

   Non-supporting answerer accepts?  If the answerer does not support
      bundling, does it accept this media description (without a further
      update)?

   Update needed for non-supporting answerer?  Is an SDP update needed
      to complete session setup if the answerer does not support
      bundling?  If an update is needed, in all cases, it is needed to
      establish communication.

   Flaws  The disadvantages of each alternative:

      A     Media descriptions that are rejected (have zero ports) are
            not allowed to be members of a group (in the
            offer).[RFC5888]

      B     An SDP update is needed for a supporting answerer to prevent
            intermediaries from timing out the multimedia session.

      C     An SDP update is needed for a non-supporting answerer to
            establish communications.

      D     Although a media description with a non-zero port but a null
            address is formally offered (although shown as on-hold via
            the old method), it is possible that the answerer will not
            consider it to be offered and many not accept it even if it
            otherwise wood.



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      E     If the offered port is zero, the media description is not
            formally offered and the answerer should not accept it.

      F     SDP offers with multiple media descriptions that use the
            same port number (for the same real address) may be rejected
            by intermediaries.

      G     A TURN relay must be allocated for the constituent media
            description before the offer is sent.

   In my estimation, the worst disadvantages are A (zero port in offer),
   E (acceptance of offer with zero port), and F (duplicate port
   numbers), because they involve violations of [RFC4566] or are known
   to trigger limitations of large numbers of intermediate devices.
   Disadvantage D (offering a MD with a null address) is nearly as
   severe, as we expect it to cause undesired behavior in many non-
   supporting answerers.  Disadvantages C (update needed to communicate
   with non-supporting answerer) and G (TURN relay must be preallocated)
   are moderate, and disadvantage B (updated needed to prevent
   intermediaries from timing out) is the least severe (because it never
   delays the establishment of communication).

   Applying these priorities to the possible solutions, methods 6 and 7
   (offer real address, non-zero unique port, with/without TURN
   candidates, answer has zero port for constituent MDs) are tied for
   the best choices, with the choice made based on the relative
   importance of minimizing preallocation of TURN relays compared to
   quickly establishing communication with non-supporting answerers.

8.  Demultiplexing Considerations

   This section discusses the constraints regarding demultiplexing
   datagrams from multiple protocols that are presented on one transport
   flow.  This is an expansion of the analysis in [RFC5764] section
   5.1.2.

   The first octets of datagrams generated by particular protocols are:

   +--------------+------------+--------------+-----------+------------+
   | Protocol     | First      | Second octet | Third     | Fourth     |
   |              | octet      |              | octet     | octet      |
   +--------------+------------+--------------+-----------+------------+
   | STUN         | 0x00, 0x01 | 0x00, 0x01   |           |            |
   |              |            |              |           |            |
   | RTP          | 0x80 to    | 0x00 to      |           |            |
   |              | 0xBF       | 0xC7, 0xCD   |           |            |
   |              |            | to 0xFF      |           |            |
   |              |            |              |           |            |



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   | RTCP         | 0x80 to    | 0xC8 to 0xCC |           |            |
   |              | 0xBF       |              |           |            |
   |              |            |              |           |            |
   | RTP/RTCPv3   | 0xC0 to    |              |           |            |
   |              | 0xFF       |              |           |            |
   |              |            |              |           |            |
   | DTLS         | 0x14 to    | 0x03         | 0x03      |            |
   |              | 0x17       |              |           |            |
   |              |            |              |           |            |
   | SCTP         | source     | source port  | dest.     | dest. port |
   |              | port high  | low          | port high | low        |
   +--------------+------------+--------------+-----------+------------+


   TBD RFC 5764 specifies that the first octet of a DTLS packet is in
   the range 0x14 to 0x3F.  RFC 5246 and RFC 6374 together specify the
   first octet is a "ContentType", with the range 0x14 to 0x17.  Are
   additional octet values reserved for expansion?  What is the range
   that should be reserved in practice?

   The most generalized stack of protocols we consider is this:

   Layer 6:  ... application interfaces ...
              |||      |||     |||     |||
               V        V       V       V
               |        |       |       |
               |        |       |       |
   Layer 5:    |        |       |      SCTP
               |        |       |       |
               |        |       |       |
              RTP     SRTP      |       |
   Layer 4:   RTCP    SRTCP    SCTP    DTLS   [ STUN ]
                \       |       |       |        /
                 --------------- ----------------
                                V
                                |
                                |
   Layer 3:              [ encapsulation      STUN ]
                         [      \              /   ]
                         [       ---- ---------    ]
                         [           V             ]
                                     |
                                     |
   Layer 2:                       [ DTLS               STUN ]
                                  [   \                 /   ]
                                  [    ------- ---------    ]
                                  [           V             ]
                                              |



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                                              |
   Layer 1:                                [ TURN ]
                                              |
                                              |
   Layer 0:                                  UDP


   Layer 0:  UDP  This is the base layer of this analysis, where packets
      are carried by UDP.

   Layer 1:  TURN (optional)  If a packet arrives from a TURN relay for
      which we are a client, the TURN encapsulation must be removed
      before further processing.  This need can be detected because the
      packet arrives from the client-facing address/port of a TURN
      server of which this entity is a client.

   Layer 2:  DTLS applied to the bundle transport flow                    (optional)
      If DTLS is applied to the bundle transport flow as a whole, that
      use of DTLS will have been negotiated.  However, before
      deciphering, STUN packets need to be separated.  STUN packets can
      be distinguished from DTLS packets by their first or second
      octets.

   Layer 3:  Decapsulation (depending on the bundling method  If the
      bundling method uses encapsulation, decapsulation is done at this
      point in the protocol stack.  However, before decapsulating, STUN
      packets need to be separated.  The encapsulation method must allow
      encapsulated packets to be distinguished from STUN packets, if
      STUN packets have not been demultiplexed at layer 2 (due to DTLS
      encryption of the entire transport stream).

      All proposed encapsulation techniques ensure the first octet of
      the encapsulation is in the range allowed for the first octet of
      RTP, 0x80 to 0xBF, and thus is distinguishable from STUN.

   Layer 4:  Core demultiplexing  At this layer, most of the protocols
      are demultiplexed.  RTP/SRTP, SRTP/SRTCP, DTLS, and STUN are
      distinguished by the values of the first two octets of the packet.
      SRTP/SRTCP is distinguished from RTP/RTCP by negotiation with the
      other endpoint -- SRTP/SRTCP is never multiplexed with RTP/RTCP.

      SCTP can be distinguished from the other protocols if the other
      endpoint agrees to use only SCTP port numbers in the range 0x4000
      to 0x7FFF.  This restriction can be specified here, because this
      limitation is only needed when bundling is implemented, which
      happens only when when both endpoints support bundling.  (This
      particular range is chosen to avoid collision with a future RTP/
      RTCP version 3.  The range of SCTP ports can be chosen arbitrarily



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      because the SCTP ports are not used to route packets to this
      entity, as they are encapsulated in UDP.)

   Layer 5:  SCTP over DTLS  If the layer 4 protocol is DTLS, the
      protocol above it will always be SCTP.

   Layer 6:  Application interface demultiplexing  The method used to
      demultiplex the various application interface streams varies
      depending whether encapsulation is used and if not, on the layer 4
      /5 protocol.  If the layer 4 protocol is encapsulated, the
      encapsulation determines which constituent media description is to
      be assigned to this packet, and then application demultiplexing is
      done as normal for for that particular media description.
      Otherwise, the constituent media description must be determined by
      a method that is specific to the layer 4/5 protocol:

      RTP/SRTP  An RTP/SRTP packet is assigned to the constituent media
            description which specifies its payload type number.  (If
            encapsulation is not used, the constituent media
            descriptions must specify distinct payload type numbers.)

      RTCP/SRTCP  An RTCP/SRTCP sub-packet is dispatched to the
            application which receives the RTP media stream containing
            the SSRC that is carried in the RTCP sub-packet.

      SCTP  An SCTP packet is assigned to the constituent media
            description which specifies its destination port number.
            (If encapsulation is not used, the constituent media
            descriptions must specify distinct SCTP port numbers.)

9.  Security Considerations

   If an SBC wishes to prevent positively the transport of certain media
   types or codecs, and enforces that by examining the content of RTP
   packets, the use of kumquat encoding may defeat the examination.

   TBD

10.  IANA Considerations

   TBD

11.  Acknowledgments

   Many people have provided input for this proposal regarding both the
   technical aspects and the organization of the presentation.  Chief
   among them are the authors of the predecessor proposals
   ([I-D.alvestrand-one-rtp] (TOGETHER),



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   [I-D.holmberg-mmusic-sdp-mmt-negotiation] (MMT), and
   [I-D.ietf-mmusic-sdp-bundle-negotiation] (BUNDLE)): Harald
   Alvestrand, Jonathan Lennox, and Christer Holmberg.  In addition,
   input was provided by Charles Eckel, Andrew Hutton, Cullen Jennings,
   Hadriel Kaplan, Paul Kyzivat, Adam Roach, and Robert Sparks.

12.  Revision History

   Note to RFC Editor: Please remove this section before publication.

12.1.  draft-worley-sdp-bundle-00

   Initial version.

12.2.  Changes from draft-worley-sdp-bundle-00 to draft-worley-sdp-
       bundle-01

   Thoroughly revise the text and structure of the document.

12.3.  Changes from draft-worley-sdp-bundle-01 to draft-worley-sdp-
       bundle-02

   Heavily revise Terminology regarding media flows.

   Revise Desiderata, including adding that multiple separate bundles
   must be possible, and noninterference with ICE negotiation.

   Add section on ICE considerations.

   Change "fusion" to "bundle".

   Use a=rtcp-mux in examples to be more realistic (and to shorten the
   examples).

   Correct the use of ICE in answers; ICE candidates are not provided if
   an offered MD does not contain ICE candidates.

   Add an example of a fast-start offer.

12.4.  Changes from draft-worley-sdp-bundle-02 to draft-worley-sdp-
       bundle-03

   Add design comparison Section 7.

   Use bibxml references.

   Add DES C9, regarding continued usage of transcoding facilities
   offered by intermediate entities.



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12.5.  Changes from draft-worley-sdp-bundle-03 to draft-worley-sdp-
       bundle-04

   Add demultiplexing considerations Section 8.

12.6.  Changes from draft-worley-sdp-bundle-04 to draft-worley-sdp-
       bundle-05

   Change recommendation for SCTP port numbers from 0xC000-0xFFFF to
   0x4000-0x7FFF to avoid collision with a future RTP/RTCP version 3.

   Add the transport flow index to the KUMQUAT encapsulation.

   Add section on choices for offering constituent MDs Section 7.9.
   Revise the examples to show offering "real address, non-zero port, no
   ICE candidates".

   Add note to DES C9 (support intermediate transcoding facilities)
   saying that intermediate transcoding facilities are not expected to
   be very useful, given that encryption will be the normal use case.

   Add an exampleSection 4.4 with SCTP MDs.

   Add a section for encryption considerations.Section 5.6

   Revise generalized demultiplexing diagram to make explicit the
   optional RTP encapsulation layer.

   Update comparison chartSection 7 for draft-ejzak-mmusic-bundle-
   alternatives-01[I-D.ejzak-mmusic-bundle-alternatives] and draft-
   westerlund-avtcore-transport-
   multiplexing-05[I-D.westerlund-avtcore-transport-multiplexing].

   Update comparison chartSection 7 to discuss alternative address/port/
   candidate policies for offering constituent MDs.

12.7.  Changes from draft-worley-sdp-bundle-05 to draft-worley-sdp-
       bundle-06

   Add desideratum that we want the RTP media to be visible on the wire
   as SRTP.

   Remove the encapsulation.

   Change acceptance-within-bundle in an answer to use a zero port
   number with a=bundleaccept attribute.  Revise the the table
   Section 7.9 to include this method.




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   Change c= lines in examples to use host names, which is what would be
   done in a dual-stack environment.  (ICE candidates carry the actual
   addresses used.)

   Add TURN ICE candidates to the examples with ICE candidates, as
   described by the existing text.

13.  References

13.1.  Normative References

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

   [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.

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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.

13.2.  Informative References

   [RFC2327]  Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, April 2004.

   [RFC4796]  Hautakorpi, J. and G. Camarillo, "The Session Description
              Protocol (SDP) Content Attribute", RFC 4796, February
              2007.





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   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.

   [I-D.alvestrand-mmusic-msid]
              Alvestrand, H., "Cross Session Stream Identification in
              the Session Description Protocol", draft-alvestrand-
              mmusic-msid-02 (work in progress), December 2012.

   [I-D.alvestrand-one-rtp]
              Alvestrand, H., "SDP Grouping for Single RTP Sessions",
              draft-alvestrand-one-rtp-02 (work in progress), September
              2011.

   [I-D.ejzak-mmusic-bundle-alternatives]
              Ejzak, R., "Alternatives to BUNDLE", draft-ejzak-mmusic-
              bundle-alternatives-01 (work in progress), February 2013.

   [I-D.holmberg-mmusic-sdp-mmt-negotiation]
              Holmberg, C., Alvestrand, H., and J. Lennox, "Multiplexed
              Media Types (MMT) Using Session Description Protocol (SDP)
              Port Numbers", draft-holmberg-mmusic-sdp-mmt-
              negotiation-00 (work in progress), October 2012.

   [I-D.ietf-avtcore-multi-media-rtp-session]
              Westerlund, M., Perkins, C., and J. Lennox, "Multiple
              Media Types in an RTP Session", draft-ietf-avtcore-multi-
              media-rtp-session-02 (work in progress), February 2013.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Multiplexing Negotiation Using Session Description
              Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
              bundle-negotiation-03 (work in progress), February 2013.

   [I-D.jennings-mmusic-media-req]
              Jennings, C., Uberti, J., and E. Rescorla, "Requirements
              from various WG for MMUSIC", draft-jennings-mmusic-media-
              req-00 (work in progress), February 2013.

   [I-D.roach-mmusic-mlines]




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              Roach, A., "Thoughts on syntax for representing multiple
              media streams", draft-roach-mmusic-mlines-00 (work in
              progress), January 2013.

   [I-D.westerlund-avtcore-transport-multiplexing]
              Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a
              Single Lower-Layer Transport", draft-westerlund-avtcore-
              transport-multiplexing-05 (work in progress), February
              2013.

   [I-D.worley-service-example]
              Worley, D., "Session Initiation Protocol Service Example
              -- Music on Hold", draft-worley-service-example-11 (work
              in progress), February 2013.

Author's Address

   Dale R. Worley
   Ariadne Internet Services, Inc.
   738 Main St.
   Waltham, MA  02451
   US

   Phone: +1 781 647 9199
   Email: worley@ariadne.com

























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