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Versions: (draft-lennox-raiarea-rtp-grouping-taxonomy) 00 01 02 03 04 05 06 07 08 RFC 7656

Network Working Group                                          J. Lennox
Internet-Draft                                                     Vidyo
Intended status: Informational                                  K. Gross
Expires: August 18, 2014                                             AVA
                                                           S. Nandakumar
                                                            G. Salgueiro
                                                           Cisco Systems
                                                               B. Burman
                                                                Ericsson
                                                       February 14, 2014


A Taxonomy of Grouping Semantics and Mechanisms for Real-Time Transport
                         Protocol (RTP) Sources
               draft-ietf-avtext-rtp-grouping-taxonomy-01

Abstract

   The terminology about, and associations among, Real-Time Transport
   Protocol (RTP) sources can be complex and somewhat opaque.  This
   document describes a number of existing and proposed relationships
   among RTP sources, and attempts to define common terminology for
   discussing protocol entities and their relationships.

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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Concepts  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Media Chain . . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Physical Stimulus . . . . . . . . . . . . . . . . . .   8
       2.1.2.  Media Capture . . . . . . . . . . . . . . . . . . . .   8
       2.1.3.  Raw Stream  . . . . . . . . . . . . . . . . . . . . .   8
       2.1.4.  Media Source  . . . . . . . . . . . . . . . . . . . .   9
       2.1.5.  Source Stream . . . . . . . . . . . . . . . . . . . .  10
       2.1.6.  Media Encoder . . . . . . . . . . . . . . . . . . . .  10
       2.1.7.  Encoded Stream  . . . . . . . . . . . . . . . . . . .  11
       2.1.8.  Dependent Stream  . . . . . . . . . . . . . . . . . .  11
       2.1.9.  Media Packetizer  . . . . . . . . . . . . . . . . . .  12
       2.1.10. Packet Stream . . . . . . . . . . . . . . . . . . . .  12
       2.1.11. Media Redundancy  . . . . . . . . . . . . . . . . . .  13
       2.1.12. Redundancy Packet Stream  . . . . . . . . . . . . . .  14
       2.1.13. Media Transport . . . . . . . . . . . . . . . . . . .  14
       2.1.14. Received Packet Stream  . . . . . . . . . . . . . . .  16
       2.1.15. Received Redundandy Packet Stream . . . . . . . . . .  16
       2.1.16. Media Repair  . . . . . . . . . . . . . . . . . . . .  16
       2.1.17. Repaired Packet Stream  . . . . . . . . . . . . . . .  17
       2.1.18. Media Depacketizer  . . . . . . . . . . . . . . . . .  17
       2.1.19. Received Encoded Stream . . . . . . . . . . . . . . .  17
       2.1.20. Media Decoder . . . . . . . . . . . . . . . . . . . .  17
       2.1.21. Received Source Stream  . . . . . . . . . . . . . . .  18
       2.1.22. Media Sink  . . . . . . . . . . . . . . . . . . . . .  18
       2.1.23. Received Raw Stream . . . . . . . . . . . . . . . . .  18
       2.1.24. Media Render  . . . . . . . . . . . . . . . . . . . .  18
     2.2.  Communication Entities  . . . . . . . . . . . . . . . . .  19
       2.2.1.  End Point . . . . . . . . . . . . . . . . . . . . . .  19
       2.2.2.  RTP Session . . . . . . . . . . . . . . . . . . . . .  19
       2.2.3.  Participant . . . . . . . . . . . . . . . . . . . . .  20
       2.2.4.  Multimedia Session  . . . . . . . . . . . . . . . . .  20
       2.2.5.  Communication Session . . . . . . . . . . . . . . . .  21
   3.  Relations at Different Levels . . . . . . . . . . . . . . . .  22
     3.1.  Media Source Relations  . . . . . . . . . . . . . . . . .  22
       3.1.1.  Synchronization Context . . . . . . . . . . . . . . .  22
       3.1.2.  End Point . . . . . . . . . . . . . . . . . . . . . .  23
       3.1.3.  Participant . . . . . . . . . . . . . . . . . . . . .  24



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       3.1.4.  WebRTC MediaStream  . . . . . . . . . . . . . . . . .  24
     3.2.  Packetization Time Relations  . . . . . . . . . . . . . .  24
       3.2.1.  Single and Multi-Session Transmission of SVC  . . . .  24
       3.2.2.  Multi-Channel Audio . . . . . . . . . . . . . . . . .  25
       3.2.3.  Redundancy Format . . . . . . . . . . . . . . . . . .  25
     3.3.  Packet Stream Relations . . . . . . . . . . . . . . . . .  26
       3.3.1.  Simulcast . . . . . . . . . . . . . . . . . . . . . .  27
       3.3.2.  Layered Multi-Stream  . . . . . . . . . . . . . . . .  28
       3.3.3.  Robustness and Repair . . . . . . . . . . . . . . . .  29
       3.3.4.  Packet Stream Separation  . . . . . . . . . . . . . .  32
     3.4.  Multiple RTP Sessions over one Media Transport  . . . . .  33
   4.  Topologies and Communication Entities . . . . . . . . . . . .  33
     4.1.  Point-to-Point Communication  . . . . . . . . . . . . . .  33
     4.2.  Centralized Conferencing  . . . . . . . . . . . . . . . .  34
     4.3.  Full Mesh Conferencing  . . . . . . . . . . . . . . . . .  37
     4.4.  Source-Specific Multicast . . . . . . . . . . . . . . . .  39
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  41
   6.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  41
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  41
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  42
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  42
   Appendix A.  Changes From Earlier Versions  . . . . . . . . . . .  44
     A.1.  Modifications Between WG Version -00 and -03  . . . . . .  44
     A.2.  Modifications Between Version -02 and -03 . . . . . . . .  44
     A.3.  Modifications Between Version -01 and -02 . . . . . . . .  44
     A.4.  Modifications Between Version -00 and -01 . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   The existing taxonomy of sources in RTP is often regarded as
   confusing and inconsistent.  Consequently, a deep understanding of
   how the different terms relate to each other becomes a real
   challenge.  Frequently cited examples of this confusion are (1) how
   different protocols that make use of RTP use the same terms to
   signify different things and (2) how the complexities addressed at
   one layer are often glossed over or ignored at another.

   This document attempts to provide some clarity by reviewing the
   semantics of various aspects of sources in RTP.  As an organizing
   mechanism, it approaches this by describing various ways that RTP
   sources can be grouped and associated together.

   All non-specific references to ControLling mUltiple streams for
   tElepresence (CLUE) in this document map to [I-D.ietf-clue-framework]




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   and all references to Web Real-Time Communications (WebRTC) map to
   [I-D.ietf-rtcweb-overview].

2.  Concepts

   This section defines concepts that serve to identify and name various
   transformations and streams in a given RTP usage.  For each concept
   an attempt is made to list any alternate definitions and usages that
   co-exist today along with various characteristics that further
   describes the concept.  These concepts are divided into two
   categories, one related to the chain of streams and transformations
   that media can be subject to, the other for entities involved in the
   communication.

2.1.  Media Chain

   In the context of this memo, Media is a sequence of synthetic or
   Physical Stimulus (Section 2.1.1) (sound waves, photons, key-
   strokes), represented in digital form.  Synthesized Media is
   typically generated directly in the digital domain.

   This section contains the concepts that can be involved in taking
   Media at a sender side and transporting it to a receiver, which may
   recover a sequence of physical stimulus.  This chain of concepts is
   of two main types, streams and transformations.  Streams are time-
   based sequences of samples of the physical stimulus in various
   representations, while transformations changes the representation of
   the streams in some way.

   The below examples are basic ones and it is important to keep in mind
   that this conceptual model enables more complex usages.  Some will be
   further discussed in later sections of this document.  In general the
   following applies to this model:

   o  A transformation may have zero or more inputs and one or more
      outputs.

   o  A Stream is of some type.

   o  A Stream has one source transformation and one or more sink
      transformation (with the exception of Physical Stimulus
      (Section 2.1.1) that can have no source or sink transformation).

   o  Streams can be forwarded from a transformation output to any
      number of inputs on other transformations that support that type.






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   o  If the output of a transformation is sent to multiple
      transformations, those streams will be identical; it takes a
      transformation to make them different.

   o  There are no formal limitations on how streams are connected to
      transformations, this may include loops if required by a
      particular transformation.

   It is also important to remember that this is a conceptual model.
   Thus real-world implementations may look different and have different
   structure.

   To provide a basic understanding of the relationships in the chain we
   below first introduce the concepts for the sender side (Figure 1).
   This covers physical stimulus until media packets are emitted onto
   the network.



































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      Physical Stimulus
             |
             V
   +--------------------+
   |    Media Capture   |
   +--------------------+
             |
        Raw Stream
             V
   +--------------------+
   |    Media Source    |<- Synchronization Timing
   +--------------------+
             |
       Source Stream
             V
   +--------------------+
   |   Media Encoder    |
   +--------------------+
             |
       Encoded Stream     +-----------+
             V            |           V
   +--------------------+ | +--------------------+
   |  Media Packetizer  | | |  Media Redundancy  |
   +--------------------+ | +--------------------+
             |            |           |
             +------------+ Redundancy Packet Stream
      Source Packet Stream            |
             V                        V
   +--------------------+   +--------------------+
   |  Media Transport   |   |  Media Transport   |
   +--------------------+   +--------------------+

             Figure 1: Sender Side Concepts in the Media Chain

   In Figure 1 we have included a branched chain to cover the concepts
   for using redundancy to improve the reliability of the transport.
   The Media Transport concept is an aggregate that is decomposed below
   in Section 2.1.13.

   Below we review a receiver media chain (Figure 2) matching the sender
   side to look at the inverse transformations and their attempts to
   recover possibly identical streams as in the sender chain.  Note that
   the streams out of a reverse transformation, like the Source Stream
   out the Media Decoder are in many cases not the same as the
   corresponding ones on the sender side, thus they are prefixed with a
   "Received" to denote a potentially modified version.  The reason for
   not being the same lies in the transformations that can be of
   irreversible type.  For example, lossy source coding in the Media



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   Encoder prevents the Source Stream out of the Media Decoder to be the
   same as the one fed into the Media Encoder.  Other reasons include
   packet loss or late loss in the Media Transport transformation that
   even Media Repair, if used, fails to repair.  It should be noted that
   some transformations are not always present, like Media Repair that
   cannot operate without Redundancy Packet Streams.

   +--------------------+   +--------------------+
   |  Media Transport   |   |  Media Transport   |
   +--------------------+   +--------------------+
             |                        |
   Received Packet Stream   Received Redundancy PS
             |                        |
             |    +-------------------+
             V    V
   +--------------------+
   |    Media Repair    |
   +--------------------+
             |
   Repaired Packet Stream
             V
   +--------------------+
   | Media Depacketizer |
   +--------------------+
             |
   Received Encoded Stream
             V
   +--------------------+
   |   Media Decoder    |
   +--------------------+
             |
   Received Source Stream
             V
   +--------------------+
   |     Media Sink     |--> Synchronization Information
   +--------------------+
             |
   Received Raw Stream
             V
   +--------------------+
   |   Media Renderer   |
   +--------------------+
             |
             V
     Physical Stimulus

            Figure 2: Receiver Side Concepts of the Media Chain




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2.1.1.  Physical Stimulus

   The physical stimulus is a physical event that can be measured and
   converted to digital form by an appropriate sensor or transducer.
   This include sound waves making up audio, photons in a light field
   that is visible, or other excitations or interactions with sensors,
   like keystrokes on a keyboard.

2.1.2.  Media Capture

   Media Capture is the process of transforming the Physical Stimulus
   (Section 2.1.1) into digital Media using an appropriate sensor or
   transducer.  The Media Capture performs a digital sampling of the
   physical stimulus, usually periodically, and outputs this in some
   representation as a Raw Stream (Section 2.1.3).  This data is due to
   its periodical sampling, or at least being timed asynchronous events,
   some form of a stream of media data.  The Media Capture is normally
   instantiated in some type of device, i.e. media capture device.
   Examples of different types of media capturing devices are digital
   cameras, microphones connected to A/D converters, or keyboards.

   Alternate usages:

   o  The CLUE WG uses the term "Capture Device" to identify a physical
      capture device.

   o  WebRTC WG uses the term "Recording Device" to refer to the locally
      available capture devices in an end-system.

   Characteristics:

   o  A Media Capture is identified either by hardware/manufacturer ID
      or via a session-scoped device identifier as mandated by the
      application usage.

   o  A Media Capture can generate an Encoded Stream (Section 2.1.7) if
      the capture device support such a configuration.

2.1.3.  Raw Stream

   The time progressing stream of digitally sampled information, usually
   periodically sampled and provided by a Media Capture (Section 2.1.2).
   A Raw Stream can also contain synthesized Media that may not require
   any explicit Media Capture, since it is already in an appropriate
   digital form.






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2.1.4.  Media Source

   A Media Source is the logical source of a reference clock
   synchronized, time progressing, digital media stream, called a Source
   Stream (Section 2.1.5).  This transformation takes one or more Raw
   Streams (Section 2.1.3) and provides a Source Stream as output.  This
   output has been synchronized with some reference clock, even if just
   a system local wall clock.

   The output can be of different types.  One type is directly
   associated with a particular Media Capture's Raw Stream.  Others are
   more conceptual sources, like an audio mix of multiple Raw Streams
   (Figure 3), a mixed selection of the three loudest inputs regarding
   speech activity, a selection of a particular video based on the
   current speaker, i.e. typically based on other Media Sources.

      Raw       Raw       Raw
     Stream    Stream    Stream
       |         |         |
       V         V         V
   +--------------------------+
   |        Media Source      |<-- Reference Clock
   |           Mixer          |
   +--------------------------+
                 |
                 V
           Source Stream

         Figure 3: Conceptual Media Source in form of Audio Mixer

   The CLUE WG uses the term "Media Capture" for this purpose.  A CLUE
   Media Capture is identified via indexed notation.  The terms Audio
   Capture and Video Capture are used to identify Audio Sources and
   Video Sources respectively.  Concepts such as "Capture Scene",
   "Capture Scene Entry" and "Capture" provide a flexible framework to
   represent media captured spanning spatial regions.

   The WebRTC WG defines the term "RtcMediaStreamTrack" to refer to a
   Media Source.  An "RtcMediaStreamTrack" is identified by the ID
   attribute.

   Typically a Media Source is mapped to a single m=line via the Session
   Description Protocol (SDP) [RFC4566] unless mechanisms such as
   Source-Specific attributes are in place [RFC5576].  In the latter
   cases, an m=line can represent either multiple Media Sources,
   multiple Packet Streams (Section 2.1.10), or both.

   Characteristics:



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   o  At any point, it can represent a physical captured source or
      conceptual source.

2.1.5.  Source Stream

   A time progressing stream of digital samples that has been
   synchronized with a reference clock and comes from particular Media
   Source (Section 2.1.4).

2.1.6.  Media Encoder

   A Media Encoder is a transform that is responsible for encoding the
   media data from a Source Stream (Section 2.1.5) into another
   representation, usually more compact, that is output as an Encoded
   Stream (Section 2.1.7).

   The Media Encoder step commonly includes pre-encoding
   transformations, such as scaling, resampling etc.  The Media Encoder
   can have a significant number of configuration options that affects
   the properties of the encoded stream.  This include properties such
   as bit-rate, start points for decoding, resolution, bandwidth or
   other fidelity affecting properties.  The actually used codec is also
   an important factor in many communication systems, not only its
   parameters.

   Scalable Media Encoders need special mentioning as they produce
   multiple outputs that are potentially of different types.  A scalable
   Media Encoder takes one input Source Stream and encodes it into
   multiple output streams of two different types; at least one Encoded
   Stream that is independently decodable and one or more Dependent
   Streams (Section 2.1.8) that requires at least one Encoded Stream and
   zero or more Dependent Streams to be possible to decode.  A Dependent
   Stream's dependency is one of the grouping relations this document
   discusses further in Section 3.3.2.

          Source Stream
                |
                V
   +--------------------------+
   |  Scalable Media Encoder  |
   +--------------------------+
      |         |   ...    |
      V         V          V
   Encoded  Dependent  Dependent
   Stream    Stream     Stream

            Figure 4: Scalable Media Encoder Input and Outputs




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   There are also other variants of encoders, like so-called Multiple
   Description Coding (MDC).  Such Media Encoder produce multiple
   independent and thus individually decodable Encoded Streams that are
   possible to combine into a Received Source Stream that is somehow a
   better representation of the original Source Stream than using only a
   single Encoded Stream.

   Alternate usages:

   o  Within the SDP usage, an SDP media description (m=line) describes
      part of the necessary configuration required for encoding
      purposes.

   o  CLUE's "Capture Encoding" provides specific encoding configuration
      for this purpose.

   Characteristics:

   o  A Media Source can be multiply encoded by different Media Encoders
      to provide various encoded representations.

2.1.7.  Encoded Stream

   A stream of time synchronized encoded media that can be independently
   decoded.

   Characteristics:

   o  Due to temporal dependencies, an Encoded Stream may have
      limitations in where decoding can be started.  These entry points,
      for example Intra frames from a video encoder, may require
      identification and their generation may be event based or
      configured to occur periodically.

2.1.8.  Dependent Stream

   A stream of time synchronized encoded media fragments that are
   dependent on one or more Encoded Streams (Section 2.1.7) and zero or
   more Dependent Streams to be possible to decode.

   Characteristics:

   o  Each Dependent Stream has a set of dependencies.  These
      dependencies must be understood by the parties in a multi-media
      session that intend to use a Dependent Stream.






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2.1.9.  Media Packetizer

   The transformation of taking one or more Encoded (Section 2.1.7) or
   Dependent Stream (Section 2.1.8) and put their content into one or
   more sequences of packets, normally RTP packets, and output Source
   Packet Streams (Section 2.1.10).  This step includes both generating
   RTP payloads as well as RTP packets.

   The Media Packetizer can use multiple inputs when producing a single
   Packet Stream.  One such example is SST packetization when using SVC
   (Section 3.2.1).

   The Media Packetizer can also produce multiple Packet Streams, for
   example when Encoded and/or Dependent Streams are distributed over
   multiple Packet Streams.  One example of this is MST packetization
   when using SVC (Section 3.2.1).

   Alternate usages:

   o  An RTP sender is part of the Media Packetizer.

   Characteristics:

   o  The Media Packetizer will select which Synchronization source(s)
      (SSRC) [RFC3550] in which RTP sessions that are used.

   o  Media Packetizer can combine multiple Encoded or Dependent Streams
      into one or more Packet Streams.

2.1.10.  Packet Stream

   A stream of RTP packets containing media data, source or redundant.
   The Packet Stream is identified by an SSRC belonging to a particular
   RTP session.  The RTP session is identified as discussed in
   Section 2.2.2.

   A Source Packet Stream is a packet stream containing at least some
   content from an Encoded Stream.  Source material is any media
   material that is produced for transport over RTP without any
   additional redundancy applied to cope with network transport losses.
   Compare this with the Redundancy Packet Stream (Section 2.1.12).

   Alternate usages:

   o  The term "Stream" is used by the CLUE WG to define an encoded
      Media Source sent via RTP.  "Capture Encoding", "Encoding Groups"
      are defined to capture specific details of the encoding scheme.




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   o  RFC3550 [RFC3550] uses the terms media stream, audio stream, video
      stream and streams of (RTP) packets interchangeably.  It defines
      the SSRC as the "The source of a stream of RTP packets, ...".

   o  The equivalent mapping of a Packet Stream in SDP [RFC4566] is
      defined per usage.  For example, each Media Description (m=line)
      and associated attributes can describe one Packet Stream OR
      properties for multiple Packet Streams OR for an RTP session (via
      [RFC5576] mechanisms for example).

   Characteristics:

   o  Each Packet Stream is identified by a unique Synchronization
      source (SSRC) [RFC3550] that is carried in every RTP and RTP
      Control Protocol (RTCP) packet header in a specific RTP session
      context.

   o  At any given point in time, a Packet Stream can have one and only
      one SSRC.  SSRC collision is a valid reason to change SSRC for a
      Packet Stream, since the Packet Stream itself is not changed in
      any way, only the identifying SSRC number.

   o  Each Packet Stream defines a unique RTP sequence numbering and
      timing space.

   o  Several Packet Streams may map to a single Media Source via the
      source transformations.

   o  Several Packet Streams can be carried over a single RTP Session.

2.1.11.  Media Redundancy

   Media redundancy is a transformation that generates redundant or
   repair packets sent out as a Redundancy Packet Stream to mitigate
   network transport impairments, like packet loss and delay.

   The Media Redundancy exists in many flavors; they may be generating
   independent Repair Streams that are used in addition to the Source
   Stream (RTP Retransmission [RFC4588] and some FEC [RFC5109]), they
   may generate a new Source Stream by combining redundancy information
   with source information (Using XOR FEC [RFC5109] as a redundancy
   payload [RFC2198]), or completely replace the source information with
   only redundancy packets.








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2.1.12.  Redundancy Packet Stream

   A Packet Stream (Section 2.1.10) that contains no original source
   data, only redundant data that may be combined with one or more
   Received Packet Stream (Section 2.1.14) to produce Repaired Packet
   Streams (Section 2.1.17).

2.1.13.  Media Transport

   A Media Transport defines the transformation that the Packet Streams
   (Section 2.1.10) are subjected to by the end-to-end transport from
   one RTP sender to one specific RTP receiver (an RTP session may
   contain multiple RTP receivers per sender).  Each Media Transport is
   defined by a transport association that is identified by a 5-tuple
   (source address, source port, destination address, destination port,
   transport protocol).  Each transport association normally contains
   only a single RTP session, although a proposal exists for sending
   multiple RTP sessions over one transport association
   [I-D.westerlund-avtcore-transport-multiplexing].

   Characteristics:

   o  Media Transport transmits Packet Streams of RTP Packets from a
      source transport address to a destination transport address.

   The Media Transport concept sometimes needs to be decomposed into
   more steps to enable discussion of what a sender emits that gets
   transformed by the network before it is received by the receiver.
   Thus we provide also this Media Transport decomposition (Figure 5).






















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         Packet Stream
                |
                V
   +--------------------------+
   |  Media Transport Sender  |
   +--------------------------+
                |
         Sent Packet Stream
                V
   +--------------------------+
   |    Network Transport     |
   +--------------------------+
                |
    Transported Packet Stream
                V
   +--------------------------+
   | Media Transport Receiver |
   +--------------------------+
                |
                V
       Received Packet Stream

                Figure 5: Decomposition of Media Transport

2.1.13.1.  Media Transport Sender

   The first transformation within the Media Transport (Section 2.1.13)
   is the Media Transport Sender, where the sending End-Point
   (Section 2.2.1) takes a Packet Stream and emits the packets onto the
   network using the transport association established for this Media
   Transport thus creating a Sent Packet Stream (Section 2.1.13.2).  In
   this process it transforms the Packet Stream in several ways.  First,
   it gains the necessary protocol headers for the transport
   association, for example IP and UDP headers, thus forming IP/UDP/RTP
   packets.  In addition, the Media Transport Sender may queue, pace or
   otherwise affect how the packets are emitted onto the network.  Thus
   adding delay, jitter and inter packet spacings that characterize the
   Sent Packet Stream.

2.1.13.2.  Sent Packet Stream

   The Sent Packet Stream is the Packet Stream as entering the first hop
   of the network path to its destination.  The Sent Packet Stream is
   identified using network transport addresses, like for IP/UDP the
   5-tuple (source IP address, source port, destination IP address,
   destination port, and protocol (UDP)).





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2.1.13.3.  Network Transport

   Network Transport is the transformation that the Sent Packet Stream
   (Section 2.1.13.2) is subjected to by traveling from the source to
   the destination through the network.  These transformations include,
   loss of some packets, varying delay on a per packet basis, packet
   duplication, and packet header or data corruption.  These
   transformations produces a Transported Packet Stream
   (Section 2.1.13.4) at the exit of the network path.

2.1.13.4.  Transported Packet Stream

   The Packet Stream that is emitted out of the network path at the
   destination, subjected to the Network Transport's transformation
   (Section 2.1.13.3).

2.1.13.5.  Media Transport Receiver

   The receiver End-Point's (Section 2.2.1) transformation of the
   Transported Packet Stream (Section 2.1.13.4) by its reception process
   that result in the Received Packet Stream (Section 2.1.14).  This
   transformation includes transport checksums being verified and if
   non-matching, causing discarding of the corrupted packet.  Other
   transformations can include delay variations in receiving a packet on
   the network interface and providing it to the application.

2.1.14.  Received Packet Stream

   The Packet Stream (Section 2.1.10) resulting from the Media
   Transport's transformation, i.e. subjected to packet loss, packet
   corruption, packet duplication and varying transmission delay from
   sender to receiver.

2.1.15.  Received Redundandy Packet Stream

   The Redundancy Packet Stream (Section 2.1.12) resulting from the
   Media Transport's transformation, i.e. subjected to packet loss,
   packet corruption, and varying transmission delay from sender to
   receiver.

2.1.16.  Media Repair

   A Transformation that takes as input one or more Source Packet
   Streams (Section 2.1.10) as well as Redundancy Packet Streams
   (Section 2.1.12) and attempts to combine them to counter the
   transformations introduced by the Media Transport (Section 2.1.13) to
   minimize the difference between the Source Stream (Section 2.1.5) and
   the Received Source Stream (Section 2.1.21) after Media Decoder



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   (Section 2.1.20).  The output is a Repaired Packet Stream
   (Section 2.1.17).

2.1.17.  Repaired Packet Stream

   A Received Packet Stream (Section 2.1.14) for which Received
   Redundancy Packet Stream (Section 2.1.15) information has been used
   to try to re-create the Packet Stream (Section 2.1.10) as it was
   before Media Transport (Section 2.1.13).

2.1.18.  Media Depacketizer

   A Media Depacketizer takes one or more Packet Streams
   (Section 2.1.10) and depacketizes them and attempts to reconstitute
   the Encoded Streams (Section 2.1.7) or Dependent Streams
   (Section 2.1.8) present in those Packet Streams.

   It should be noted that in practical implementations, the Media
   Depacketizer and the Media Decoder may be tightly coupled and share
   information to improve or optimize the overall decoding process in
   various ways.  It is however not expected that there would be any
   benefit in defining a taxonomy for those detailed (and likely very
   implementation-dependent) steps.

2.1.19.  Received Encoded Stream

   The received version of an Encoded Stream (Section 2.1.7).

2.1.20.  Media Decoder

   A Media Decoder is a transformation that is responsible for decoding
   Encoded Streams (Section 2.1.7) and any Dependent Streams
   (Section 2.1.8) into a Source Stream (Section 2.1.5).

   It should be noted that in practical implementations, the Media
   Decoder and the Media Depacketizer may be tightly coupled and share
   information to improve or optimize the overall decoding process in
   various ways.  It is however not expected that there would be any
   benefit in defining a taxonomy for those detailed (and likely very
   implementation-dependent) steps.

   Alternate usages:

   o  Within the context of SDP, an m=line describes the necessary
      configuration and identification (RTP Payload Types) required to
      decode either one or more incoming Media Streams.

   Characteristics:



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   o  A Media Decoder is the entity that will have to deal with any
      errors in the encoded streams that resulted from corruptions or
      failures to repair packet losses.  This as a media decoder
      generally is forced to produce some output periodically.  It thus
      commonly includes concealment methods.

2.1.21.  Received Source Stream

   The received version of a Source Stream (Section 2.1.5).

2.1.22.  Media Sink

   The Media Sink receives a Source Stream (Section 2.1.5) that
   contains, usually periodically, sampled media data together with
   associated synchronization information.  Depending on application,
   this Source Stream then needs to be transformed into a Raw Stream
   (Section 2.1.3) that is sent in synchronization with the output from
   other Media Sinks to a Media Render (Section 2.1.24).  The media sink
   may also be connected with a Media Source (Section 2.1.4) and be used
   as part of a conceptual Media Source.

   Characteristics:

   o  The Media Sink can further transform the Source Stream into a
      representation that is suitable for rendering on the Media Render
      as defined by the application or system-wide configuration.  This
      include sample scaling, level adjustments etc.

2.1.23.  Received Raw Stream

   The received version of a Raw Stream (Section 2.1.3).

2.1.24.  Media Render

   A Media Render takes a Raw Stream (Section 2.1.3) and converts it
   into Physical Stimulus (Section 2.1.1) that a human user can
   perceive.  Examples of such devices are screens, D/A converters
   connected to amplifiers and loudspeakers.

   Characteristics:

   o  An End Point can potentially have multiple Media Renders for each
      media type.








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2.2.  Communication Entities

   This section contains concept for entities involved in the
   communication.

2.2.1.  End Point

   A single addressable entity sending or receiving RTP packets.  It may
   be decomposed into several functional blocks, but as long as it
   behaves as a single RTP stack entity it is classified as a single
   "End Point".

   Alternate usages:

   o  The CLUE Working Group (WG) uses the terms "Media Provider" and
      "Media Consumer" to describes aspects of End Point pertaining to
      sending and receiving functionalities.

   Characteristics:

   o  End Points can be identified in several different ways.  While
      RTCP Canonical Names (CNAMEs) [RFC3550] provide a globally unique
      and stable identification mechanism for the duration of the
      Communication Session (see Section 2.2.5), their validity applies
      exclusively within a Synchronization Context (Section 3.1.1).
      Thus one End Point can have multiple CNAMEs.  Therefore,
      mechanisms outside the scope of RTP, such as application defined
      mechanisms, must be used to ensure End Point identification when
      outside this Synchronization Context.

2.2.2.  RTP Session

   An RTP session is an association among a group of participants
   communicating with RTP.  It is a group communications channel which
   can potentially carry a number of Packet Streams.  Within an RTP
   session, every participant can find meta-data and control information
   (over RTCP) about all the Packet Streams in the RTP session.  The
   bandwidth of the RTCP control channel is shared between all
   participants within an RTP Session.

   Alternate usages:

   o  Within the context of SDP, a singe m=line can map to a single RTP
      Session or multiple m=lines can map to a single RTP Session.  The
      latter is enabled via multiplexing schemes such as BUNDLE
      [I-D.ietf-mmusic-sdp-bundle-negotiation], for example, which
      allows mapping of multiple m=lines to a single RTP Session.




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   Characteristics:

   o  Typically, an RTP Session can carry one ore more Packet Streams.

   o  An RTP Session shares a single SSRC space as defined in RFC3550
      [RFC3550].  That is, the End Points participating in an RTP
      Session can see an SSRC identifier transmitted by any of the other
      End Points.  An End Point can receive an SSRC either as SSRC or as
      a Contributing source (CSRC) in RTP and RTCP packets, as defined
      by the endpoints' network interconnection topology.

   o  An RTP Session uses at least two Media Transports
      (Section 2.1.13), one for sending and one for receiving.
      Commonly, the receiving one is the reverse direction of the same
      one as used for sending.  An RTP Session may use many Media
      Transports and these define the session's network interconnection
      topology.  A single Media Transport can normally not transport
      more than one RTP Session, unless a solution for multiplexing
      multiple RTP sessions over a single Media Transport is used.  One
      example of such a scheme is Multiple RTP Sessions on a Single
      Lower-Layer Transport
      [I-D.westerlund-avtcore-transport-multiplexing].

   o  Multiple RTP Sessions can be related.

2.2.3.  Participant

   A participant is an entity reachable by a single signaling address,
   and is thus related more to the signaling context than to the media
   context.

   Characteristics:

   o  A single signaling-addressable entity, using an application-
      specific signaling address space, for example a SIP URI.

   o  A participant can have several Multimedia Sessions
      (Section 2.2.4).

   o  A participant can have several associated transport flows,
      including several separate local transport addresses for those
      transport flows.

2.2.4.  Multimedia Session

   A multimedia session is an association among a group of participants
   engaged in the communication via one or more RTP Sessions




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   (Section 2.2.2).  It defines logical relationships among Media
   Sources (Section 2.1.4) that appear in multiple RTP Sessions.

   Alternate usages:

   o  RFC4566 [RFC4566] defines a multimedia session as a set of
      multimedia senders and receivers and the data streams flowing from
      senders to receivers.

   o  RFC3550 [RFC3550] defines it as set of concurrent RTP sessions
      among a common group of participants.  For example, a video
      conference (which is a multimedia session) may contain an audio
      RTP session and a video RTP session.

   Characteristics:

   o  A Multimedia Session can be composed of several parallel RTP
      Sessions with potentially multiple Packet Streams per RTP Session.

   o  Each participant in a Multimedia Session can have a multitude of
      Media Captures and Media Rendering devices.

2.2.5.  Communication Session

   A Communication Session is an association among group of participants
   communicating with each other via a set of Multimedia Sessions.

   Alternate usages:

   o  The Session Description Protocol (SDP) [RFC4566] defines a
      multimedia session as a set of multimedia senders and receivers
      and the data streams flowing from senders to receivers.  In that
      definition it is however not clear if a multimedia session
      includes both the sender's and the receiver's view of the same RTP
      Packet Stream.

   Characteristics:

   o  Each participant in a Communication Session is identified via an
      application-specific signaling address.

   o  A Communication Session is composed of at least one Multimedia
      Session per participant, involving one or more parallel RTP
      Sessions with potentially multiple Packet Streams per RTP Session.

   For example, in a full mesh communication, the Communication Session
   consists of a set of separate Multimedia Sessions between each pair
   of Participants.  Another example is a centralized conference, where



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   the Communication Session consists of a set of Multimedia Sessions
   between each Participant and the conference handler.

3.  Relations at Different Levels

   This section uses the concepts from previous section and look at
   different types of relationships among them.  These relationships
   occur at different levels and for different purposes.  The section is
   organized such as to look at the level where a relation is required.
   The reason for the relationship may exist at another step in the
   media handling chain.  For example, using Simulcast (discussed in
   Section 3.3.1) needs to determine relations at Packet Stream level,
   however the reason to relate Packet Streams is that multiple Media
   Encoders use the same Media Source, i.e. to be able to identify a
   common Media Source.

3.1.  Media Source Relations

   Media Sources (Section 2.1.4) are commonly grouped and related to an
   End Point (Section 2.2.1) or a Participant (Section 2.2.3).  This
   occurs for several reasons; both application logic as well as media
   handling purposes.  These cases are further discussed below.

3.1.1.  Synchronization Context

   A Synchronization Context defines a requirement on a strong timing
   relationship between the Media Sources, typically requiring alignment
   of clock sources.  Such relationship can be identified in multiple
   ways as listed below.  A single Media Source can only belong to a
   single Synchronization Context, since it is assumed that a single
   Media Source can only have a single media clock and requiring
   alignment to several Synchronization Contexts (and thus reference
   clocks) will effectively merge those into a single Synchronization
   Context.

   A single Multimedia Session can contain media from one or more
   Synchronization Contexts.  An example of that is a Multimedia Session
   containing one set of audio and video for communication purposes
   belonging to one Synchronization Context, and another set of audio
   and video for presentation purposes (like playing a video file) with
   a separate Synchronization Context that has no strong timing
   relationship and need not be strictly synchronized with the audio and
   video used for communication.








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3.1.1.1.  RTCP CNAME

   RFC3550 [RFC3550] describes Inter-media synchronization between RTP
   Sessions based on RTCP CNAME, RTP and Network Time Protocol (NTP)
   [RFC5905] formatted timestamps of a reference clock.  As indicated in
   [I-D.ietf-avtcore-clksrc], despite using NTP format timestamps, it is
   not required that the clock be synchronized to an NTP source.

3.1.1.2.  Clock Source Signaling

   [I-D.ietf-avtcore-clksrc] provides a mechanism to signal the clock
   source in SDP both for the reference clock as well as the media
   clock, thus allowing a Synchronization Context to be defined beyond
   the one defined by the usage of CNAME source descriptions.

3.1.1.3.  CLUE Scenes

   In CLUE "Capture Scene", "Capture Scene Entry" and "Captures" define
   an implied Synchronization Context.

3.1.1.4.  Implicitly via RtcMediaStream

   The WebRTC WG defines "RtcMediaStream" with one or more
   "RtcMediaStreamTracks".  All tracks in a "RtcMediaStream" are
   intended to be possible to synchronize when rendered.

3.1.1.5.  Explicitly via SDP Mechanisms

   RFC5888 [RFC5888] defines m=line grouping mechanism called "Lip
   Synchronization (LS)" for establishing the synchronization
   requirement across m=lines when they map to individual sources.

   RFC5576 [RFC5576] extends the above mechanism when multiple media
   sources are described by a single m=line.

3.1.2.  End Point

   Some applications requires knowledge of what Media Sources originate
   from a particular End Point (Section 2.2.1).  This can include such
   decisions as packet routing between parts of the topology, knowing
   the End Point origin of the Packet Streams.

   In RTP, this identification has been overloaded with the
   Synchronization Context through the usage of the source description
   CNAME item.  This works for some usages, but sometimes it breaks
   down.  For example, if an End Point has two sets of Media Sources
   that have different Synchronization Contexts, like the audio and
   video of the human participant as well as a set of Media Sources of



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   audio and video for a shared movie.  Thus, an End Point may have
   multiple CNAMEs.  The CNAMEs or the Media Sources themselves can be
   related to the End Point.

3.1.3.  Participant

   In communication scenarios, it is commonly needed to know which Media
   Sources that originate from which Participant (Section 2.2.3).  Thus
   enabling the application to for example display Participant Identity
   information correctly associated with the Media Sources.  This
   association is currently handled through the signaling solution to
   point at a specific Multimedia Session where the Media Sources may be
   explicitly or implicitly tied to a particular End Point.

   Participant information becomes more problematic due to Media Sources
   that are generated through mixing or other conceptual processing of
   Raw Streams or Source Streams that originate from different
   Participants.  This type of Media Sources can thus have a dynamically
   varying set of origins and Participants.  RTP contains the concept of
   Contributing Sources (CSRC) that carries such information about the
   previous step origin of the included media content on RTP level.

3.1.4.  WebRTC MediaStream

   An RtcMediaStream, in addition to requiring a single Synchronization
   Context as discussed above, is also an explicit grouping of a set of
   Media Sources, as identified by RtcMediaStreamTracks, within the
   RtcMediaStream.

3.2.  Packetization Time Relations

   At RTP Packetization time, there exists a possibility for a number of
   different types of relationships between Encoded Streams
   (Section 2.1.7), Dependent Streams (Section 2.1.8) and Packet Streams
   (Section 2.1.10).  These are caused by grouping together or
   distributing these different types of streams into Packet Streams.
   This section will look at such relationships.

3.2.1.  Single and Multi-Session Transmission of SVC

   Scalable Video Coding [RFC6190] has a mode of operation called Single
   Session Transmission (SST), where Encoded Streams and Dependent
   Streams from the SVC Media Encoder are sent in a single RTP Session
   (Section 2.2.2) using the SVC RTP Payload format.  There is another
   mode of operation where Encoded Streams and Dependent Streams are
   distributed across multiple RTP Sessions, called Multi-Session
   Transmission (MST).  Regardless if used with SST or MST, as they are




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   defined, each of those RTP Sessions may contain one or more Packet
   Streams (SSRC) per Media Source.

   To elaborate, what could be called SST-SingleStream (SST-SS) uses a
   single Packet Stream in a single RTP Session to send all Encoded and
   Dependent Streams.  Similarly, SST-MultiStream (SST-MS) uses multiple
   Packet Streams in a single RTP Session to send the Encoded and
   Dependent Streams.  MST-SS uses a single Packet Stream in each of
   multiple RTP Sessions and MST-MS uses multiple Packet Streams in each
   of the multiple RTP Sessions:

   +-----------------------+--------------------+----------------------+
   |                       | Single RTP Session | Multiple RTP         |
   |                       |                    | Sessions             |
   +-----------------------+--------------------+----------------------+
   | Single Packet Stream  | SST-SS             | MST-SS               |
   | Multiple Packet       | SST-MS             | MST-MS               |
   | Streams               |                    |                      |
   +-----------------------+--------------------+----------------------+

3.2.2.  Multi-Channel Audio

   There exist a number of RTP payload formats that can carry multi-
   channel audio, despite the codec being a mono encoder.  Multi-channel
   audio can be viewed as multiple Media Sources sharing a common
   Synchronization Context.  These are independently encoded by a Media
   Encoder and the different Encoded Streams are then packetized
   together in a time synchronized way into a single Source Packet
   Stream using the used codec's RTP Payload format.  Example of such
   codecs are, PCMA and PCMU [RFC3551], AMR [RFC4867], and G.719
   [RFC5404].

3.2.3.  Redundancy Format

   The RTP Payload for Redundant Audio Data [RFC2198] defines how one
   can transport redundant audio data together with primary data in the
   same RTP payload.  The redundant data can be a time delayed version
   of the primary or another time delayed Encoded Stream using a
   different Media Encoder to encode the same Media Source as the
   primary, as depicted below in Figure 6.











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   +--------------------+
   |    Media Source    |
   +--------------------+
             |
        Source Stream
             |
             +------------------------+
             |                        |
             V                        V
   +--------------------+   +--------------------+
   |   Media Encoder    |   |   Media Encoder    |
   +--------------------+   +--------------------+
             |                        |
             |                 +------------+
       Encoded Stream          | Time Delay |
             |                 +------------+
             |                        |
             |     +------------------+
             V     V
   +--------------------+
   |  Media Packetizer  |
   +--------------------+
             |
             V
      Packet Stream

   Figure 6: Concept for usage of Audio Redundancy with different Media
                                 Encoders

   The Redundancy format is thus providing the necessary meta
   information to correctly relate different parts of the same Encoded
   Stream, or in the case depicted above (Figure 6) relate the Received
   Source Stream fragments coming out of different Media Decoders to be
   able to combine them together into a less erroneous Source Stream.

3.3.  Packet Stream Relations

   This section discusses various cases of relationships among Packet
   Streams.  This is a common relation to handle in RTP due to that
   Packet Streams are separate and have their own SSRC, implying
   independent sequence numbers and timestamp spaces.  The underlying
   reasons for the Packet Stream relationships are different, as can be
   seen in the cases below.  The different Packet Streams can be handled
   within the same RTP Session or different RTP Sessions to accomplish
   different transport goals.  This separation of Packet Streams is
   further discussed in Section 3.3.4.





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3.3.1.  Simulcast

   A Media Source represented as multiple independent Encoded Streams
   constitutes a simulcast of that Media Source.  Figure 7 below
   represents an example of a Media Source that is encoded into three
   separate and different Simulcast streams, that are in turn sent on
   the same Media Transport flow.  When using Simulcast, the Packet
   Streams may be sharing RTP Session and Media Transport, or be
   separated on different RTP Sessions and Media Transports, or be any
   combination of these two.  It is other considerations that affect
   which usage is desirable, as discussed in Section 3.3.4.

                            +----------------+
                            |  Media Source  |
                            +----------------+
                     Source Stream  |
             +----------------------+----------------------+
             |                      |                      |
             v                      v                      v
    +------------------+   +------------------+   +------------------+
    |  Media Encoder   |   |  Media Encoder   |   |  Media Encoder   |
    +------------------+   +------------------+   +------------------+
             | Encoded              | Encoded              | Encoded
             | Stream               | Stream               | Stream
             v                      v                      v
    +------------------+   +------------------+   +------------------+
    | Media Packetizer |   | Media Packetizer |   | Media Packetizer |
    +------------------+   +------------------+   +------------------+
             | Source               | Source               | Source
             | Packet               | Packet               | Packet
             | Stream               | Stream               | Stream
             +-----------------+    |    +-----------------+
                               |    |    |
                               V    V    V
                          +-------------------+
                          |  Media Transport  |
                          +-------------------+

                Figure 7: Example of Media Source Simulcast

   The simulcast relation between the Packet Streams is the common Media
   Source.  In addition, to be able to identify the common Media Source,
   a receiver of the Packet Stream may need to know which configuration
   or encoding goals that lay behind the produced Encoded Stream and its
   properties.  This to enable selection of the stream that is most
   useful in the application at that moment.





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3.3.2.  Layered Multi-Stream

   Layered Multi-Stream (LMS) is a mechanism by which different portions
   of a layered encoding of a Source Stream are sent using separate
   Packet Streams (sometimes in separate RTP Sessions).  LMSs are useful
   for receiver control of layered media.

   A Media Source represented as an Encoded Stream and multiple
   Dependent Streams constitutes a Media Source that has layered
   dependencies.  The figure below represents an example of a Media
   Source that is encoded into three dependent layers, where two layers
   are sent on the same Media Transport using different Packet Streams,
   i.e. SSRCs, and the third layer is sent on a separate Media
   Transport, i.e. a different RTP Session.

                            +----------------+
                            |  Media Source  |
                            +----------------+
                                    |
                                    |
                                    V
       +---------------------------------------------------------+
       |                      Media Encoder                      |
       +---------------------------------------------------------+
               |                    |                     |
        Encoded Stream       Dependent Stream     Dependent Stream
               |                    |                     |
               V                    V                     V
       +----------------+   +----------------+   +----------------+
       |Media Packetizer|   |Media Packetizer|   |Media Packetizer|
       +----------------+   +----------------+   +----------------+
               |                    |                     |
         Packet Stream         Packet Stream        Packet Stream
               |                    |                     |
               +------+      +------+                     |
                      |      |                            |
                      V      V                            V
                +-----------------+              +-----------------+
                | Media Transport |              | Media Transport |
                +-----------------+              +-----------------+

           Figure 8: Example of Media Source Layered Dependency

   As an example, the SVC MST (Section 3.2.1) relation needs to identify
   the common Media Encoder origin for the Encoded and Dependent
   Streams.  The SVC RTP Payload RFC is not particularly explicit about
   how this relation is to be implemented.  When using different RTP
   Sessions, thus different Media Transports, and as long as there is



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   only one Packet Stream per Media Encoder and a single Media Source in
   each RTP Session (MST-SS (Section 3.2.1)), common SSRC and CNAMEs can
   be used to identify the common Media Source.  When multiple Packet
   Streams are sent from one Media Encoder in the same RTP Session (SST-
   MS), then CNAME is the only currently specified RTP identifier that
   can be used.  In cases where multiple Media Encoders use multiple
   Media Sources sharing Synchronization Context, and thus having a
   common CNAME, additional heuristics need to be applied to create the
   MST relationship between the Packet Streams.

3.3.3.  Robustness and Repair

   Packet Streams may be protected by Redundancy Packet Streams during
   transport.  Several approaches listed below can achieve the same
   result;

   o  Duplication of the original Packet Stream

   o  Duplication of the original Packet Stream with a time offset,

   o  Forward Error Correction (FEC) techniques, and

   o  Retransmission of lost packets (either globally or selectively).

3.3.3.1.  RTP Retransmission

   The figure below (Figure 9) represents an example where a Media
   Source's Source Packet Stream is protected by a retransmission (RTX)
   flow [RFC4588].  In this example the Source Packet Stream and the
   Redundancy Packet Stream share the same Media Transport.





















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          +--------------------+
          |    Media Source    |
          +--------------------+
                    |
                    V
          +--------------------+
          |   Media Encoder    |
          +--------------------+
                    |                              Retransmission
              Encoded Stream     +--------+     +---- Request
                    V            |        V     V
          +--------------------+ | +--------------------+
          |  Media Packetizer  | | | RTP Retransmission |
          +--------------------+ | +--------------------+
                    |            |           |
                    +------------+  Redundancy Packet Stream
             Source Packet Stream            |
                    |                        |
                    +---------+    +---------+
                              |    |
                              V    V
                       +-----------------+
                       | Media Transport |
                       +-----------------+

          Figure 9: Example of Media Source Retransmission Flows

   The RTP Retransmission example (Figure 9) helps illustrate that this
   mechanism works purely on the Source Packet Stream.  The RTP
   Retransmission transform buffers the sent Source Packet Stream and
   upon requests emits a retransmitted packet with some extra payload
   header as a Redundancy Packet Stream.  The RTP Retransmission
   mechanism [RFC4588] is specified so that there is a one to one
   relation between the Source Packet Stream and the Redundancy Packet
   Stream.  Thus a Redundancy Packet Stream needs to be associated with
   its Source Packet Stream upon being received.  This is done based on
   CNAME selectors and heuristics to match requested packets for a given
   Source Packet Stream with the original sequence number in the payload
   of any new Redundancy Packet Stream using the RTX payload format.  In
   cases where the Redundancy Packet Stream is sent in a separate RTP
   Session from the Source Packet Stream, these sessions are related,
   e.g. using the SDP Media Grouping's [RFC5888] FID semantics.

3.3.3.2.  Forward Error Correction

   The figure below (Figure 10) represents an example where two Media
   Sources' Source Packet Streams are protected by FEC.  Source Packet
   Stream A has a Media Redundancy transformation in FEC Encoder 1.



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   This produces a Redundancy Packet Stream 1, that is only related to
   Source Packet Stream A. The FEC Encoder 2, however takes two Source
   Packet Streams (A and B) and produces a Redundancy Packet Stream 2
   that protects them together, i.e. Redundancy Packet Stream 2 relate
   to two Source Packet Streams (a FEC group).  FEC decoding, when
   needed due to packet loss or packet corruption at the receiver,
   requires knowledge about which Source Packet Streams that the FEC
   encoding was based on.

   In Figure 10 all Packet Streams are sent on the same Media Transport.
   This is however not the only possible choice.  Numerous combinations
   exist for spreading these Packet Streams over different Media
   Transports to achieve the communication application's goal.

       +--------------------+                +--------------------+
       |   Media Source A   |                |   Media Source B   |
       +--------------------+                +--------------------+
                 |                                     |
                 V                                     V
       +--------------------+                +--------------------+
       |   Media Encoder A  |                |   Media Encoder B  |
       +--------------------+                +--------------------+
                 |                                     |
           Encoded Stream                        Encoded Stream
                 V                                     V
       +--------------------+                +--------------------+
       | Media Packetizer A |                | Media Packetizer B |
       +--------------------+                +--------------------+
                 |                                     |
       Source Packet Stream A                Source Packet Stream B
                 |                                     |
           +-----+-------+-------------+       +-------+------+
           |             V             V       V              |
           |    +---------------+  +---------------+          |
           |    | FEC Encoder 1 |  | FEC Encoder 2 |          |
           |    +---------------+  +---------------+          |
           |             |                 |                  |
           |     Redundancy PS 1    Redundancy PS 2           |
           V             V                 V                  V
       +----------------------------------------------------------+
       |                    Media Transport                       |
       +----------------------------------------------------------+

                      Figure 10: Example of FEC Flows

   As FEC Encoding exists in various forms, the methods for relating FEC
   Redundancy Packet Streams with its source information in Source
   Packet Streams are many.  The XOR based RTP FEC Payload format



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   [RFC5109] is defined in such a way that a Redundancy Packet Stream
   has a one to one relation with a Source Packet Stream.  In fact, the
   RFC requires the Redundancy Packet Stream to use the same SSRC as the
   Source Packet Stream.  This requires to either use a separate RTP
   session or to use the Redundancy RTP Payload format [RFC2198].  The
   underlying relation requirement for this FEC format and a particular
   Redundancy Packet Stream is to know the related Source Packet Stream,
   including its SSRC.

3.3.4.  Packet Stream Separation

   Packet Streams can be separated exclusively based on their SSRCs or
   at the RTP Session level or at the Multi-Media Session level as
   explained below.

   When the Packet Streams that have a relationship are all sent in the
   same RTP Session and are uniquely identified based on their SSRC
   only, it is termed an SSRC-Only Based Separation.  Such streams can
   be related via RTCP CNAME to identify that the streams belong to the
   same End Point.  [RFC5576]-based approaches, when used, can
   explicitly relate various such Packet Streams.

   On the other hand, when Packet Streams that are related but are sent
   in the context of different RTP Sessions to achieve separation, it is
   known as RTP Session-based separation.  This is commonly used when
   the different Packet Streams are intended for different Media
   Transports.

   Several mechanisms that use RTP Session-based separation rely on it
   to enable an implicit grouping mechanism expressing the relationship.
   The solutions have been based on using the same SSRC value in the
   different RTP Sessions to implicitly indicate their relation.  That
   way, no explicit RTP level mechanism has been needed, only signaling
   level relations have been established using semantics from Grouping
   of Media lines framework [RFC5888].  Examples of this are RTP
   Retransmission [RFC4588], SVC Multi-Session Transmission [RFC6190]
   and XOR Based FEC [RFC5109].  RTCP CNAME explicitly relates Packet
   Streams across different RTP Sessions, as explained in the previous
   section.  Such a relationship can be used to perform inter-media
   synchronization.

   Packet Streams that are related and need to be associated can be part
   of different Multimedia Sessions, rather than just different RTP
   sessions within the same Multimedia Session context.  This puts
   further demand on the scope of the mechanism(s) and its handling of
   identifiers used for expressing the relationships.





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3.4.  Multiple RTP Sessions over one Media Transport

   [I-D.westerlund-avtcore-transport-multiplexing] describes a mechanism
   that allow several RTP Sessions to be carried over a single
   underlying Media Transport.  The main reasons for doing this are
   related to the impact of using one or more Media Transports.  Thus
   using a common network path or potentially have different ones.
   There is reduced need for NAT/FW traversal resources and no need for
   flow based QoS.

   However, Multiple RTP Sessions over one Media Transport makes it
   clear that a single Media Transport 5-tuple is not sufficient to
   express which RTP Session context a particular Packet Stream exists
   in.  Complexities in the relationship between Media Transports and
   RTP Session already exist as one RTP Session contains multiple Media
   Transports, e.g. even a Peer-to-Peer RTP Session with RTP/RTCP
   Multiplexing requires two Media Transports, one in each direction.
   The relationship between Media Transports and RTP Sessions as well as
   additional levels of identifiers need to be considered in both
   signaling design and when defining terminology.

4.  Topologies and Communication Entities

   This section reviews some communication topologies and looks at the
   relationship among the communication entities that are defined in
   Section 2.2.  It does not deal with discussions about the streams and
   their relation to the transport.  Instead, it covers the aspects that
   enable the transport of those streams.  For example, the Media
   Transports (Section 2.1.13) that exists between the End Points
   (Section 2.2.1) that are part of an RTP session (Section 2.2.2) and
   their relationship to the Multi-Media Session (Section 2.2.4) between
   Participants (Section 2.2.3) and the established Communication
   session (Section 2.2.5) are explained.

   The text provided below is neither any exhaustive listing of possible
   topologies, nor does it cover all topologies described in
   [I-D.ietf-avtcore-rtp-topologies-update].

4.1.  Point-to-Point Communication

   Figure 11 shows a very basic point-to-point communication session
   between A and B. It uses two different audio and video RTP sessions
   between A's and B's end points.  Assume that the Multi-media session
   shared by the participants is established using SIP (i.e., there is a
   SIP Dialog between A and B).  The high level representation of this
   communication scenario can be demonstrated using Figure 11.





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   +---+         +---+
   | A |<------->| B |
   +---+         +---+

                  Figure 11: Point to Point Communication

   However, this picture gets slightly more complex when redrawn using
   the communication entities concepts defined earlier in this document.

   +-----------------------------------------------------------+
   | Communication Session                                     |
   |                                                           |
   | +----------------+                     +----------------+ |
   | | Participant A  |   +-------------+   | Participant B  | |
   | |                |   | Multi-Media |   |                | |
   | | +-------------+|<=>| Session     |<=>|+-------------+ | |
   | | | End Point A ||   |(SIP Dialog) |   || End Point B | | |
   | | |             ||   +-------------+   ||             | | |
   | | | +-----------++---------------------++-----------+ | | |
   | | | | RTP Session|                     |            | | | |
   | | | | Audio      |---Media Transport-->|            | | | |
   | | | |            |<--Media Transport---|            | | | |
   | | | +-----------++---------------------++-----------+ | | |
   | | |             ||                     ||             | | |
   | | | +-----------++---------------------++-----------+ | | |
   | | | | RTP Session|                     |            | | | |
   | | | | Video      |---Media Transport-->|            | | | |
   | | | |            |<--Media Transport---|            | | | |
   | | | +-----------++---------------------++-----------+ | | |
   | | +-------------+|                     |+-------------+ | |
   | +----------------+                     +----------------+ |
   +-----------------------------------------------------------+

   Figure 12: Point to Point Communication Session with two RTP Sessions

   Figure 12 shows the two RTP Sessions only exist between the two End
   Points A and B and over their respective Media Transports.  The
   Multi-Media Session establishes the association between the two
   Participants and configures these RTP sessions and the Media
   Transports that are used.

4.2.  Centralized Conferencing

   This section looks at the centralized conferencing communication
   topology, where a number of participants, like A, B, C, and D in
   Figure 13, communicate using an RTP mixer.





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   +---+      +------------+      +---+
   | A |<---->|            |<---->| B |
   +---+      |            |      +---+
              |   Mixer    |
   +---+      |            |      +---+
   | C |<---->|            |<---->| D |
   +---+      +------------+      +---+

          Figure 13: Centralized Conferincing using an RTP Mixer

   In this case each of the Participants establish their Multi-media
   session with the Conference Bridge.  Thus, negotiation for the
   establishment of the used RTP sessions and their configuration
   happens between these entities.  The participants have their End
   Points (A, B, C, D) and the Conference Bridge has the host running
   the RTP mixer, referred to as End Point M in Figure 14.  However,
   despite the individual establishment of four Multi-Media Sessions and
   the corresponding Media Transports for each of the RTP sessions
   between the respective End Points and the Conference Bridge, there is
   actually only two RTP sessions.  One for audio and one for Video, as
   these RTP sessions are, in this topology, shared between all the
   Participants.





























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   +-------------------------------------------------------------------+
   | Communication Session                                             |
   |                                                                   |
   | +----------------+                             +----------------+ |
   | | Participant A  |       +-------------+       | Conference     | |
   | |                |       | Multi-Media |       | Bridge         | |
   | | +-------------+|<=====>| Session A   |<=====>|+-------------+ | |
   | | | End Point A ||       |(SIP Dialog) |       || End Point M | | |
   | | |             ||       +-------------+       ||             | | |
   | | | +-----------++-----------------------------++-----------+ | | |
   | | | | RTP Session|                             |            | | | |
   | | | | Audio      |-------Media Transport------>|            | | | |
   | | | |            |<------Media Transport-------|            | | | |
   | | | +-----------++-----------------------------++------+    | | | |
   | | |             ||                             ||      |    | | | |
   | | | +-----------++-----------------------------++----+ |    | | | |
   | | | | RTP Session|                             |     | |    | | | |
   | | | | Video      |-------Media Transport------>|     | |    | | | |
   | | | |            |<------Media Transport-------|     | |    | | | |
   | | | +-----------++-----------------------------++    | |    | | | |
   | | +-------------+|                             ||    | |    | | | |
   | +----------------+                             ||    | |    | | | |
   |                                                ||    | |    | | | |
   | +----------------+                             ||    | |    | | | |
   | | Participant B  |       +-------------+       ||    | |    | | | |
   | |                |       | Multi-Media |       ||    | |    | | | |
   | | +-------------+|<=====>| Session B   |<=====>||    | |    | | | |
   | | | End Point B ||       |(SIP Dialog) |       ||    | |    | | | |
   | | |             ||       +-------------+       ||    | |    | | | |
   | | | +-----------++-----------------------------++    | |    | | | |
   | | | | RTP Session|                             |     | |    | | | |
   | | | | Video      |-------Media Transport------>|     | |    | | | |
   | | | |            |<------Media Transport-------|     | |    | | | |
   | | | +-----------++-----------------------------++----+ |    | | | |
   | | |             ||                             ||      |    | | | |
   | | | +-----------++-----------------------------++------+    | | | |
   | | | | RTP Session|                             |            | | | |
   | | | | Audio      |-------Media Transport------>|            | | | |
   | | | |            |<------Media Transport-------|            | | | |
   | | | +-----------++-----------------------------++-----------+ | | |
   | | +-------------+|                             |+-------------+ | |
   | +----------------+                             +----------------+ |
   +-------------------------------------------------------------------+

     Figure 14: Centralized Conferencing with Two Participants A and B
                  communicating over a Conference Bridge





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   It is important to stress that in the case of Figure 14, it might
   appear that the Multi-Media Sessions context is scoped between A and
   B over M. This might not be always true and they can have contexts
   that extend further.  In this case the RTP session, its common SSRC
   space goes beyond what occurs between A and M and B and M
   respectively.

4.3.  Full Mesh Conferencing

   This section looks at the case where the three Participants (A, B and
   C) wish to communicate.  They establish individual Multi-Media
   Sessions and RTP sessions between themselves and the other two peers.
   Thus, each providing two copies of their media to every other
   participant.  Figure 15 shows a high level representation of such a
   topology.

   +---+      +---+
   | A |<---->| B |
   +---+      +---+
     ^         ^
      \       /
       \     /
        v   v
        +---+
        | C |
        +---+

   Figure 15: Full Mesh Conferencing with three Participants A, B and C

   In this particular case there are two aspects worth noting.  The
   first is there will be multiple Multi-Media Sessions per
   Communication Session between the participants.  This, however,
   hasn't been true in the earlier examples; the Centralized
   Conferencing inSection 4.2 being the exception.  The second aspect is
   consideration of whether one needs to maintain relationships between
   entities and concepts, for example Media Sources, between these
   different Multi-Media Sessions and between Packet Streams in the
   independent RTP sessions configured by those Multi-Media Sessions.













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                          +-----------------------------------------+
                          | Participant A                           |
      +----------+        | +--------------------------------------+|
      | Multi-   |        | | End Point A                          ||
      | Media    |<======>| |                                      ||
      | Session  |        | |+-------+     +-------+     +-------+ ||
      | 1        |        | || RTP 1 |<----| MS A1 |---->| RTP 2 | ||
      +----------+        | ||       |     +-------+     |       | ||
          ^^              | +|-------|-------------------|-------|-+|
          ||              +--|-------|-------------------|-------|--+
          ||                 |       |          ^^       |       |
          VV                 |       |          ||       |       |
   +-------------------------|-------|----+     ||       |       |
   | Participant B           |       |    |     VV       |       |
   | +-----------------------|-------|---+| +----------+ |       |
   | | End Point B    +----->|       |   || | Multi-   | |       |
   | |                |      +-------+   || | Media    | |       |
   | | +-------+      |      +-------+   || | Session  | |       |
   | | | MS B1 |------+----->| RTP 3 |   || | 2        | |       |
   | | +-------+             |       |   || +----------+ |       |
   | +-----------------------|-------|---+|     ^^       |       |
   +-------------------------|-------|----+     ||       |       |
          ^^                 |       |          ||       |       |
          ||                 |       |          VV       |       |
          ||              +--|-------|-------------------|-------|--+
          VV              |  |       | Participant C     |       |  |
      +----------+        | +|-------|-------------------|-------|-+|
      | Multi-   |        | ||       | End Point C       |       | ||
      | Media    |<======>| |+-------+                   +-------+ ||
      | Session  |        | |    ^         +-------+         ^     ||
      | 3        |        | |    +---------| MS C1 |---------+     ||
      +----------+        | |              +-------+               ||
                          | +--------------------------------------+|
                          +-----------------------------------------+

   Figure 16: Full Mesh Conferencing between three Participants A, B and
                                     C

   For the sake of clarity, Figure 16 above does not include all these
   concepts.  The Media Sources (MS) from a given End Point is sent to
   the two peers.  This requires encoding and Media Packetization to
   enable the Packet Streams to be sent over Media Transports in the
   context of the RTP sessions depicted.  The RTP sessions 1, 2, and 3
   are independent, and established in the context of each of the Multi-
   Media Sessions 1, 2 and 3.  The joint communication session the full
   figure represents (not shown here as it was Figure 14 in order to
   save space), however, combines the received representations of the
   peers' Media Sources and plays them back.



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   It is noteworthy that the full mesh conferencing topologies described
   here have the potential for creating loops.  For example, if one
   compares the above full mesh with a mixing three party communication
   session as depicted in (Figure 17).  In this example A's Media Source
   A1 is sent to B over a Multi-Media Session (A-B).  In B the Media
   Source A1 is mixed with Media Source B1 and the resulting Media
   Source (MS AB) is sent to C over a Multi-Media Session (B-C).  If C
   and A would establish a Multi-Media Session (A-C) and C would act in
   the same role as B, then A would receive a Media Source from C that
   contains a mix of A, B and C's individual Media Sources.  This would
   result in A playing out a time delay version of its own signal (i.e.,
   the system has created an echo path).

   +--------------+    +--------------+    +--------------+
   | A            |    | B +-------+  |    | C            |
   |              |    |   | MS B1 |  |    |              |
   |              |    |   +-------+  |    |              |
   | +-------+    |    |     |        |    |              |
   | | MS A1 |----|--->|-----+ MS AB -|--->|              |
   | +-------+    |    |              |    |              |
   +--------------+    +--------------+    +--------------+

            Figure 17: Mixing Three Party Communication Session

   The looping issue can be avoided, detected or prevented using two
   general methods.  The first method is to use great care when setting
   up and establishing the communication session if participants have
   any mixing or forwarding capacity, so that one doesn't end up getting
   back a partial or full representation of one's own media believing it
   is someone else's. The other method is to maintain some unique
   identifiers at the communication session level for all Media Sources
   and ensure that any Packet Streams received identify those Media
   Sources that contributed to the content of the Packet Stream.

4.4.  Source-Specific Multicast

   In one-to-many media distribution cases (e.g., IPTV), where one Media
   Sender or a set of Media Senders is allowed to send Packet Streams on
   a particular Source-Specific Multicast (SSM) group to many receivers
   (R), there are some different aspects to consider.  Figure 18
   presents a high level SSM system for RTP/RTCP defined in [RFC5760].
   In this case, several Media Senders sends their Packet Streams to the
   Distribution Source, which is the only one allowed to send to the SSM
   group.  The Receivers joining the SSM group can provide RTCP feedback
   on its reception by sending unicast feedback to a Feedback Target
   (FT).





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   +--------+       +-----+
   |Media   |       |     |       Source-Specific
   |Sender 1|<----->| D S |       Multicast (SSM)
   +--------+       | I O |  +--+----------------> R(1)
                    | S U |  |  |                    |
   +--------+       | T R |  |  +-----------> R(2)   |
   |Media   |<----->| R C |->+  |           :   |    |
   |Sender 2|       | I E |  |  +------> R(n-1) |    |
   +--------+       | B   |  |  |          |    |    |
       :            | U   |  +--+--> R(n)  |    |    |
       :            | T +-|          |     |    |    |
       :            | I | |<---------+     |    |    |
   +--------+       | O |F|<---------------+    |    |
   |Media   |       | N |T|<--------------------+    |
   |Sender M|<----->|   | |<-------------------------+
   +--------+       +-----+       RTCP Unicast

   FT = Feedback Target

        Figure 18: Source-Specific Multicast Communication Topology

   Here the Media Transport from the Distribution Source to all the SSM
   receivers (R) have the same 5-tuple, but in reality have different
   paths.  Also, the Multi-Media Sessions between the Distribution
   Source and the individual receivers are normally identical.  This is
   due to one-way communication from the Distribution Source to the
   receiver of configuration information.  This is information typically
   embedded in Electronic Program Guides (EPGs), distributed by the
   Session Announcement Protocol (SAP) [RFC2974] or other one-way
   protocols.  In some cases load balancing occurs, for example, by
   providing the receiver with a set of Feedback Targets and then it
   randomly selects one out of the set.

   This scenario varies significantly from previously described
   communication topologies due to the asymmetric nature of the RTP
   Session context across the Distribution Source.  The Distribution
   Source forms a focal point in collecting the unicasted RTCP feedback
   from the receivers and then re-distributing it to the Media Senders.
   Each Media Sender and the Distribution Source establish their own
   Multi-Media Session Context for the underlying RTP Sessions but with
   shared RTCP context across all the receivers.

   To improve the readability,Figure 18 intentionally hides the details
   of the various entities . Expanding on this, one can think of Media
   Senders being part of one or more Multi-Media Sessions grouped under
   a Communication Session.  The Media Sender in this scenario refers to
   the Media Packetizer transformation Section 2.1.9.  The Packet Stream
   generated by such a Media Sender can be part of its own RTP Session



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   or can be multiplexed with other Packet Streams within an End Point.
   The latter case requires careful consideration since the re-
   distributed RTCP packets now correspond to a single RTP Session
   Context across all the Media Senders.

5.  Security Considerations

   This document simply tries to clarify the confusion prevalent in RTP
   taxonomy because of inconsistent usage by multiple technologies and
   protocols making use of the RTP protocol.  It does not introduce any
   new security considerations beyond those already well documented in
   the RTP protocol [RFC3550] and each of the many respective
   specifications of the various protocols making use of it.

   Hopefully having a well-defined common terminology and understanding
   of the complexities of the RTP architecture will help lead us to
   better standards, avoiding security problems.

6.  Acknowledgement

   This document has many concepts borrowed from several documents such
   as WebRTC [I-D.ietf-rtcweb-overview], CLUE [I-D.ietf-clue-framework],
   Multiplexing Architecture
   [I-D.westerlund-avtcore-transport-multiplexing].  The authors would
   like to thank all the authors of each of those documents.

   The authors would also like to acknowledge the insights, guidance and
   contributions of Magnus Westerlund, Roni Even, Paul Kyzivat, Colin
   Perkins, Keith Drage, Harald Alvestrand, and Alex Eleftheriadis.

7.  Contributors

   Magnus Westerlund has contributed the concept model for the media
   chain using transformations and streams model, including rewriting
   pre-existing concepts into this model and adding missing concepts.
   The first proposal for updating the relationships and the topologies
   based on this concept was also performed by Magnus.

8.  IANA Considerations

   This document makes no request of IANA.

9.  References








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9.1.  Normative References

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

9.2.  Informative References

   [I-D.ietf-avtcore-clksrc]
              Williams, A., Gross, K., Brandenburg, R., and H. Stokking,
              "RTP Clock Source Signalling", draft-ietf-avtcore-
              clksrc-09 (work in progress), December 2013.

   [I-D.ietf-avtcore-rtp-topologies-update]
              Westerlund, M. and S. Wenger, "RTP Topologies", draft-
              ietf-avtcore-rtp-topologies-update-01 (work in progress),
              October 2013.

   [I-D.ietf-clue-framework]
              Duckworth, M., Pepperell, A., and S. Wenger, "Framework
              for Telepresence Multi-Streams", draft-ietf-clue-
              framework-14 (work in progress), February 2014.

   [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-05 (work in progress), October 2013.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for Brower-
              based Applications", draft-ietf-rtcweb-overview-08 (work
              in progress), September 2013.

   [I-D.westerlund-avtcore-transport-multiplexing]
              Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP
              Sessions onto a Single Lower-Layer Transport", draft-
              westerlund-avtcore-transport-multiplexing-07 (work in
              progress), October 2013.

   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
              Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
              September 1997.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.




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

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

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

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

   [RFC4867]  Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
              "RTP Payload Format and File Storage Format for the
              Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
              (AMR-WB) Audio Codecs", RFC 4867, April 2007.

   [RFC5109]  Li, A., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, December 2007.

   [RFC5404]  Westerlund, M. and I. Johansson, "RTP Payload Format for
              G.719", RFC 5404, January 2009.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, June 2009.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760, February 2010.

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

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6190]  Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
              "RTP Payload Format for Scalable Video Coding", RFC 6190,
              May 2011.

   [RFC6222]  Begen, A., Perkins, C., and D. Wing, "Guidelines for
              Choosing RTP Control Protocol (RTCP) Canonical Names
              (CNAMEs)", RFC 6222, April 2011.



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Appendix A.  Changes From Earlier Versions

   NOTE TO RFC EDITOR: Please remove this section prior to publication.

A.1.  Modifications Between WG Version -00 and -03

   o  WG version -00 text is identical to individual draft -03

   o  Amended description of SVC SST and MST encodings with respect to
      concepts defined in this text

   o  Removed UML as normative reference, since the text no longer uses
      any UML notation

   o  Removed a number of level 4 sections and moved out text to the
      level above

A.2.  Modifications Between Version -02 and -03

   o  Section 4 rewritten (and new communication topologies added) to
      reflect the major updates to Sections 1-3

   o  Section 8 removed (carryover from initial -00 draft)

   o  General clean up of text, grammar and nits

A.3.  Modifications Between Version -01 and -02

   o  Section 2 rewritten to add both streams and transformations in the
      media chain.

   o  Section 3 rewritten to focus on exposing relationships.

A.4.  Modifications Between Version -00 and -01

   o  Too many to list

   o  Added new authors

   o  Updated content organization and presentation

Authors' Addresses









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   Jonathan Lennox
   Vidyo, Inc.
   433 Hackensack Avenue
   Seventh Floor
   Hackensack, NJ  07601
   US

   Email: jonathan@vidyo.com


   Kevin Gross
   AVA Networks, LLC
   Boulder, CO
   US

   Email: kevin.gross@avanw.com


   Suhas Nandakumar
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   US

   Email: snandaku@cisco.com


   Gonzalo Salgueiro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   US

   Email: gsalguei@cisco.com


   Bo Burman
   Ericsson
   Farogatan 6
   SE-164 80 Kista
   Sweden

   Phone: +46 10 714 13 11
   Email: bo.burman@ericsson.com







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