Network Working Group                                      M. Westerlund
Internet-Draft                                                 B. Burman
Intended status: Informational                                  Ericsson
Expires: May January 3, 2018 2019                                      C. Perkins
                                                   University of Glasgow
                                                           H. Alvestrand
                                                                  Google
                                                                 R. Even
                                                                H. Zheng
                                                                  Huawei
                                                        October 30, 2017
                                                            July 2, 2018

    Guidelines for using the Multiplexing Features of RTP to Support
                         Multiple Media Streams
               draft-ietf-avtcore-multiplex-guidelines-05
               draft-ietf-avtcore-multiplex-guidelines-06

Abstract

   The Real-time Transport Protocol (RTP) is a flexible protocol that
   can be used in a wide range of applications, networks, and system
   topologies.  That flexibility makes for wide applicability, but can
   complicate the application design process.  One particular design
   question that has received much attention is how to support multiple
   media streams in RTP.  This memo discusses the available options and
   design trade-offs, and provides guidelines on how to use the
   multiplexing features of RTP to support multiple media streams.

Status of This Memo

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   This Internet-Draft will expire on May January 3, 2018. 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Subjects Out of Scope . . . . . . . . . . . . . . . . . .   5
   3.  RTP Multiplexing Overview . . . . . . . . . . . . . . . . . .   5
     3.1.  Reasons for Multiplexing and Grouping RTP Media Streams . . . .   5
     3.2.  RTP Multiplexing Points . . . . . . . . . . . . . . . . .   6
       3.2.1.  RTP Session . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Synchronisation Source (SSRC) . . . . . . . . . . . .   8
       3.2.3.  Contributing Source (CSRC)  . . . . . . . . . . . . .  10
       3.2.4.  RTP Payload Type  . . . . . . . . . . . . . . . . . .  10
     3.3.  Issues Related to RTP Topologies  . . . . . . . . . . . .  11
     3.4.  Issues Related to RTP and RTCP Protocol . . . . . . . . .  13  12
       3.4.1.  The RTP Specification . . . . . . . . . . . . . . . .  13
       3.4.2.  Multiple SSRCs in a Session . . . . . . . . . . . . .  15
       3.4.3.  Binding Related Sources . . . . . . . . . . . . . . .  15
       3.4.4.  Forward Error Correction  . . . . . . . . . . . . . .  17
   4.  Particular  Considerations for RTP Multiplexing . . . . . . . . . . . . .  17
     4.1.  Interworking Considerations . . . . . . . . . . . . . . .  17
       4.1.1.  Types of  Application Interworking  . . . . . . . . . . . . . . . .  17  18
       4.1.2.  RTP Translator Interworking . . . . . . . . . . . . .  18
       4.1.3.  Gateway Interworking  . . . . . . . . . . . . . . . .  18  19
       4.1.4.  Multiple SSRC Legacy Considerations . . . . . . . . .  19  20
     4.2.  Network Considerations  . . . . . . . . . . . . . . . . .  20
       4.2.1.  Quality of Service  . . . . . . . . . . . . . . . . .  20
       4.2.2.  NAT and Firewall Traversal  . . . . . . . . . . . . .  20  21
       4.2.3.  Multicast . . . . . . . . . . . . . . . . . . . . . .  22  23
     4.3.  Security and Key Management Considerations  . . . . . . .  23  24
       4.3.1.  Security Context Scope  . . . . . . . . . . . . . . .  24
       4.3.2.  Key Management for Multi-party session sessions . . . . . . .  24  25
       4.3.3.  Complexity Implications . . . . . . . . . . . . . . .  25

   5.  Archetypes  . . . . . . . . . .  RTP Multiplexing Design Choices . . . . . . . . . . . . . . .  25  26
     5.1.  Single SSRC per Session . Endpoint  . . . . . . . . . . . . . . . .  25  26
     5.2.  Multiple SSRCs of the Same Media Type . . . . . . . . . .  27
     5.3.  Multiple Sessions for one Media type  . . . . . . . . . .  28
     5.4.  Multiple Media Types in one Session . . . . . . . . . . .  30
     5.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  31
   6.  Summary considerations and guidelines . . . . . . . . . .  Guidelines  . .  31
     6.1.  Guidelines . . . . . . . . . . . . . . . . . . . . . . .  32  31
   7.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  33
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  34  33
   10. References Contributors  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     10.1.  Normative  33
   11. References  . . . . . . . . . . . . . . . . . .  34
     10.2.  Informative References . . . . . . .  33
     11.1.  Normative References . . . . . . . . . .  34
   Appendix A.  Dismissing Payload Type Multiplexing . . . . . . . .  38
   Appendix B.  Signalling considerations  33
     11.2.  Informative References . . . . . . . . . . . . .  40
     B.1.  Signalling Aspects . . . .  34
   Appendix A.  Dismissing Payload Type Multiplexing . . . . . . . .  38
   Appendix B.  Signalling Considerations  . . . . . . . . . . . . .  40
       B.1.1.
     B.1.  Session Oriented Properties . . . . . . . . . . . . . . .  40
       B.1.2.
     B.2.  SDP Prevents Multiple Media Types . . . . . . . . . . . .  41
       B.1.3.
     B.3.  Signalling Media Stream RTP stream Usage . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   The Real-time Transport Protocol (RTP) [RFC3550] is a commonly used
   protocol for real-time media transport.  It is a protocol that
   provides great flexibility and can support a large set of different
   applications.  RTP was from the beginning designed for multiple
   participants in a communication session.  It supports many topology
   paradigms
   of topologies and usages, as defined in [RFC7667].  RTP has several
   multiplexing points designed for different purposes.  These enable
   support of multiple media RTP streams and switching between different
   encoding or packetization of the media.  By using multiple RTP
   sessions, sets of media RTP streams can be structured for efficient
   processing or identification.  Thus  Thus, the question for any RTP
   application designer is how to best use the RTP session, the SSRC RTP
   stream identifier (SSRC), and the RTP payload type to meet the
   application's needs.

   There have been increased interest in more advanced usage of RTP, for RTP.
   For example, multiple RTP streams can occur be used when a single endpoint have
   has multiple media sources, like sources (like multiple cameras or microphones microphones)
   that need to be sent simultaneously.  Consequently, questions are
   raised regarding the most appropriate RTP usage.  The limitations in
   some implementations, RTP/RTCP extensions, and signalling has also
   been exposed.  The authors also hope that clarification on the
   usefulness of some functionalities in RTP will result in more
   complete implementations in the future.

   The purpose of this document is to provide clear information about
   the possibilities of RTP when it comes to multiplexing.  The RTP
   application designer needs to understand the implications that come
   from a particular usage of the RTP multiplexing points.  The document
   will recommend against some usages as being unsuitable, in general or
   for particular purposes.

   The document starts with some definitions and then goes into the
   existing RTP functionalities around multiplexing.  Both the desired
   behaviour and the implications of a particular behaviour depend on
   which topologies are used, which requires some consideration.  This
   is followed by a discussion of some choices in multiplexing behaviour
   and their impacts.  Some archetypes designs of RTP usage are discussed.
   Finally, some recommendations guidelines and examples are provided.

2.  Definitions

2.1.  Terminology

   The definitions in Section 3 of [RFC3550] are referenced normatively.

   The taxonomy defined in [RFC7656] is referenced normatively.

   The following terms and abbreviations are used in this document:

   Multiparty:  A communication situation including multiple endpoints.
      In this document document, it will be used to refer to situations where
      more than two endpoints communicate.

   RTP Source:  The originator or source of a particular Media Stream. RTP stream sent
      from an endpoint.  Identified using an SSRC in a particular RTP
      session.  An RTP source is the source of a single media RTP stream, and
      is associated with a single endpoint and a single Media Source. media source.
      An RTP Source is just called a Source in RFC 3550.  An endpoint
      can have multiple RTP sources.

   RTP Sink:  A recipient of a Media Stream.  An endpoint that receives RTP streams.  The Media RTP Sink is
      identified using one or more SSRCs.  The SSRCs used by an RTP sink
      can be both RTP source ones, as well as used soley to represent
      the RTP sink.  There can be more than one RTP Sink for one RTP
      source.

   Multiplexing:  The operation of taking multiple entities as input,
      aggregating them onto some common resource while keeping the
      individual entities addressable such that they can later be fully
      and unambiguously separated (de-multiplexed) again.

   RTP Session Group:  One or more RTP sessions that are used together
      to perform some function.  Examples are multiple RTP sessions used
      to carry different layers of a layered encoding.  In an RTP
      Session Group, CNAMEs are assumed to be valid across all RTP
      sessions, and designate synchronisation contexts that can cross
      RTP sessions. sessions; i.e. SSRCs that map to a common CNAME can be assumed
      to have RTCP SR timing information derived from a common clock
      such that they can be synchronised for playout.

   Signalling:  The process of configuring endpoints to participate in
      one or more RTP sessions.

2.2.  Subjects Out of Scope

   This document is focused on issues that affect RTP.  Thus, issues
   that involve signalling protocols, such as whether SIP, Jingle or
   some other protocol is in use for session configuration, the
   particular syntaxes used to define RTP session properties, or the
   constraints imposed by particular choices in the signalling
   protocols, are mentioned only as examples in order to describe the
   RTP issues more precisely.

   This document assumes the applications will use RTCP.  While there
   are such applications that don't send RTCP, they do not conform to the RTP
   specification, and thus can be regarded as reusing the RTP packet
   format but not implementing the RTP protocol.

3.  RTP Multiplexing Overview

3.1.  Reasons for Multiplexing and Grouping RTP Media Streams

   The

   There are several reasons why an endpoint might choose to send
   multiple media
   streams are widespread. streams.  In the below discussion, please keep in mind
   that the reasons for having multiple media RTP streams vary and include but
   are not limited to the following:

   o  Multiple Media Sources media sources

   o  Multiple Media Streams RTP streams might be needed to represent one Media
      Source media source
      (for instance when using layered encodings)

   o  A Retransmission retransmission stream might repeat some parts of the content of
      another Media
      Stream RTP stream

   o  An FEC stream might provide material that can be used to repair
      another Media Stream RTP stream

   o  Alternative Encodings, encodings, for instance using different codecs for the
      same audio stream

   o  Alternative formats, for instance multiple resolutions of the same
      video stream

   For each of these, these reasons, it is necessary to decide if each
   additional media RTP stream gets its own SSRC multiplexed is sent within a the same RTP session as the
   other RTP Session, streams, or if it is necessary to use additional RTP
   sessions to group the media RTP streams.  The choice between these made due to suitable for one reason
   reason, might not be the choice suitable for another reason.  The
   clearest understanding is associated with multiplexing multiple media
   sources of the same media type.  However, all reasons warrant
   discussion and clarification on how to deal with them.  As the
   discussion below will show, in reality we cannot choose a single one
   of the two SSRC or RTP session multiplexing solutions.  To utilise RTP well
   and as efficiently as possible, both are needed.  The real issue is
   finding the right guidance on when to create additional RTP sessions
   and when additional SSRCs RTP streams in an the same RTP session is the right
   choice.

3.2.  RTP Multiplexing Points

   This section describes the multiplexing points present in the RTP
   protocol that can be used to distinguish media RTP streams and groups of
   media
   RTP streams.  Figure 1 outlines the process of demultiplexing
   incoming RTP streams:

                           |
                           | packets
           +--             v
           |        +------------+
           |        |   Socket   |   Transport Protocol Demultiplexing
           |        +------------+
           |            ||  ||
      RTP  |       RTP/ ||  |+-----> SCTP ( ...and any other protocols)
   Session |       RTCP ||  +------> STUN (multiplexed using same port)
           +--          ||
           +--          ||
           |      (split by SSRC)
           |      ||    ||    ||
           |      ||    ||    ||

     Media
     RTP   |     +--+  +--+  +--+
   Streams |     |PB|  |PB|  |PB| Jitter buffer, process RTCP, FEC, etc.
           |     +--+  +--+  +--+
           +--      |   |      |

           (pick rendering context based on PT)
           +--      |  /       |
           |        +---+        +----+     |
           |         /   |     |
   Payload |      +--+ +--+ +--+     +---+ +---+ +---+
   Formats |      |CR| |CR| |CR| Codecs and rendering     |Dec| |Dec| |Dec| Decoders
           |      +--+ +--+ +--+     +---+ +---+ +---+
           +--

                   Figure 1: RTP Demultiplexing Process

3.2.1.  RTP Session

   An RTP Session is the highest semantic layer in the RTP protocol, and
   represents an association between a group of communicating endpoints.
   The
   RTP does not contain a session identifier, yet RTP sessions must be
   possible to separate both across different endpoints and within a
   single endpoint.

   For RTP session separation across endpoints, the set of participants
   that form an RTP session is defined as those that share a single
   synchronisation source space [RFC3550].  That is, if a group of
   participants are each aware of the synchronisation source identifiers
   belonging to the other participants, then those participants are in a
   single RTP session.  A participant can become aware of a
   synchronisation source identifier by receiving an RTP packet
   containing it in the SSRC field or CSRC list, by receiving an RTCP
   packet mentioning it in an SSRC field, or through signalling (e.g.,
   the SDP Session Description Protocol (SDP) [RFC4566] "a=ssrc:" attribute). attribute
   [RFC5576]).  Thus, the scope of an RTP session is determined by the
   participants' network interconnection topology, in combination with
   RTP and RTCP forwarding strategies deployed by the endpoints and any
   middleboxes, and by the signalling.

   For RTP does not contain a session identifier.  Rather, it separation within a single endpoint, RTP relies on
   the underlying transport layer to separate different sessions, layer, and on the signalling to identify RTP
   sessions in a manner that is meaningful to the application.  A single
   endpoint can have one or more transport flows for the same RTP
   session.  The signalling layer might give RTP sessions an explicit
   identifier, or their the identification might be implicit based on the
   addresses and ports used.  Accordingly, a single RTP Session session can have
   multiple associated identifiers, explicit and implicit, belonging to
   different contexts.  For example, when running RTP on top of UDP/IP,
   an RTP endpoint can identify and delimit an RTP Session session from other
   RTP Sessions using sessions by receiving the multiple UDP flows used as identified
   based on their UDP source and destination IP addresses and UDP port
   numbers.  Another example is when using SDP grouping
   framework [RFC5888] which uses an identifier per "m="-line; if there
   is a one-to-one mapping between "m="-lines media descriptions (the "m=" line
   and the following associated lines) signals the transport flow and
   RTP sessions, that session configuration for the endpoints part of the RTP session.
   SDP grouping framework identifier will identify an RTP Session.
   [I-D.ietf-mmusic-sdp-bundle-negotiation] extends [RFC5888] allows labeling of the "m-"-line for
   bundled media, which adds complexity to demultiplexing media stream.
   Section 10.2 of [I-D.ietf-mmusic-sdp-bundle-negotiation] provides
   information about how RTP/RTCP streams are associated with SDP media
   description.
   descriptions, for example used so that RTP sessions are globally unique, but their identity Session Groups can only be
   determined by
   created.  With Negotiating Media Multiplexing Using the communication context at an endpoint of Session
   Description Protocol (SDP)[I-D.ietf-mmusic-sdp-bundle-negotiation],
   multiple media descriptions where each represents the
   session, RTP streams
   sent or by received for a middlebox that is aware media source are part of the session context.  The
   relationship between a common RTP sessions depending on the underlying
   application, transport, and signalling protocol. session.

   The RTP protocol makes no normative statements about the relationship
   between different RTP sessions, however the applications that use
   more than one RTP session will have some higher layer understanding
   of the relationship between the sessions they create.

3.2.2.  Synchronisation Source (SSRC)

   A synchronisation source (SSRC) identifies an RTP source or an RTP
   sink.  Every endpoint will have has at least one synchronisation source SSRC identifier, even if it
   does not send media (endpoints RTP packets.  RTP endpoints that are only RTP sinks
   still send RTCP, RTCP and use their synchronisation source
   identifier SSRC identifiers in the RTCP packets
   they send). send.  An endpoint can have multiple synchronisation sources SSRC identifiers if it
   contains multiple RTP sources (i.e., if it sends multiple media RTP
   streams).  Endpoints that are both RTP sources and RTP sinks use the
   same synchronisation
   sources SSRC in both roles.  At any given time, a an RTP source has one
   and only one SSRC - although that can change over the lifetime of the
   RTP source or sink.

   The synchronisation Source identifier SSRC is a 32-bit unsigned integer. identifier.  It is present in every RTP and RTCP
   packet header, and in the payload of some RTCP packet types.  It can
   also be present in SDP signalling.  Unless pre-signalled pre-signalled, e.g. using
   the SDP "a=ssrc:" attribute [RFC5576], the
   synchronisation source identifier SSRC is chosen at random.
   It is not dependent on the network address of the endpoint, and is
   intended to be unique within an RTP session.  Synchronisation source identifier  SSRC collisions can
   occur, and are handled as specified in [RFC3550] and [RFC5576],
   resulting in the synchronisation source identifier SSRC of the
   affecting colliding RTP sources and/or sinks
   changing.  An RTP source that changes its RTP Session identifier (e.g. source network transport address) address
   during a session has have to choose a new SSRC identifier to avoid being
   interpreted as looped source.

   Synchronisation source source, unless the transport layer mechansism,
   e.g ICE [RFC5245], handle such changes

   SSRC identifiers that belong to the same synchronisation context
   (i.e., that represent media RTP streams that can be synchronised using
   information in RTCP SR packets) are indicated
   by use of identical CNAME chunks in
   corresponding RTCP SDES packets.  SDP signalling can also be used to
   provide explicit SSRC grouping of
   synchronisation sources [RFC5576].

   In some cases, the same SSRC Identifier identifier value is used to relate
   streams in two different RTP Sessions, sessions, such as in Multi-Session
      Transmission of scalable video [RFC6190]. RTP retransmission
   [RFC4588].  This is to be avoided since there is no guarantee of uniqueness in that
   SSRC values are unique across RTP sessions.  For the RTP
   retransmission [RFC4588] case it is recommended to use explicit
   binding of the source RTP stream and the redundancy stream, e.g.
   using the RepairedRtpStreamId RTCP SDES item [I-D.ietf-avtext-rid].

   Note that RTP sequence number and RTP timestamp are scoped by the
   synchronisation source.
   SSRC.  Each RTP source will have a different
   synchronisation source, SSRC, and the
   corresponding media RTP stream will have a separate RTP sequence number and
   timestamp space.

   An SSRC identifier is used by different type of sources as well as
   sinks:

   Real Media Source:  Connected to a "physical" media source, for
      example a camera or microphone.

   Processed

   Conceptual Media Source:  A source with some attributed property
      generated by some network node, for example a filtering function
      in an RTP mixer that provides the most active speaker based on
      some criteria, or a mix representing a set of other sources.

   RTP Sink:  A source that does not generate any RTP media stream in
      itself (e.g. an endpoint or middlebox only receiving in an RTP
      session).  It still needs a sender an SSRC for use as source in RTCP
      reports.

   Note that an endpoint that generates more than one media type, e.g. a
   conference participant sending both audio and video, need not (and
   commonly does
   should not) use the same SSRC value across RTP sessions.  RTCP
   Compound packets containing the CNAME SDES item is the designated
   method to bind an SSRC to a CNAME, effectively cross-correlating
   SSRCs within and between RTP Sessions as coming from the same
   endpoint.  The main property attributed to SSRCs associated with the
   same CNAME is that they are from a particular synchronisation context
   and can be synchronised at playback.

   An RTP receiver receiving a previously unseen SSRC value will
   interpret it as a new source.  It might in fact be a previously
   existing source that had to change SSRC number due to an SSRC
   conflict.  However, the originator of the previous SSRC ought to have
   ended the conflicting source by sending an RTCP BYE for it prior to
   starting to send with the new SSRC, so the new SSRC is anyway
   effectively a new source.

3.2.3.  Contributing Source (CSRC)

   The Contributing Source (CSRC) is not a separate identifier.  Rather
   a synchronisation source
   an SSRC identifier is listed as a CSRC in the RTP header of a packet
   generated by an RTP mixer mixer, if the corresponding SSRC was in the
   header of one of the packets that contributed to the mix.

   It is not possible, in general, to extract media represented by an
   individual CSRC since it is typically the result of a media mixing
   (merge) operation by an RTP mixer on the individual media streams
   corresponding to the CSRC identifiers.  The exception is the case
   when only a single CSRC is indicated as this represent forwarding of
   a media
   an RTP stream, possibly modified.  The RTP header extension for
   Mixer-to-Client Audio Level Indication [RFC6465] expands on the
   receivers
   receiver's information about a packet with a CSRC list.  Due to these
   restrictions, CSRC will not be considered a fully qualified
   multiplexing point and will be disregarded in the rest of this
   document.

3.2.4.  RTP Payload Type

   Each Media Stream RTP stream utilises one or more RTP payload formats.  An RTP
   payload format describes how the output of a particular media codec
   is framed and encoded into RTP packets.  The payload format used is
   identified by the payload type (PT) field in the RTP data packet header.
   The combination of SSRC and PT therefore identifies a specific Media Stream RTP
   stream encoding format.  The format definition can be taken from
   [RFC3551] for statically allocated payload types, but ought to be
   explicitly defined in signalling, such as SDP, both for static and
   dynamic
   Payload Types. payload types.  The term "format" here includes whatever can
   be described by out-of-band signalling means.  In SDP, the term
   "format" includes media type, RTP timestamp sampling rate, codec,
   codec configuration, payload format configurations, and various
   robustness mechanisms such as redundant encodings [RFC2198].

   The RTP payload type is scoped by the sending endpoint within an RTP Session.
   session.  PT has the same meaning across all RTP streams in an RTP
   session.  All synchronisation sources SSRCs sent from a single endpoint share the same
   payload types type definitions.  The RTP Payload Type payload type is designed such that
   only a single Payload Type payload type is valid at any time instant in the RTP
   source's RTP timestamp time line, effectively time-
   multiplexing time-multiplexing
   different Payload Types payload types if any change occurs.  The payload type used
   can change on a per-packet basis for an SSRC, for example a speech
   codec making use of generic comfort noise [RFC3389].  If there is a
   true need to send multiple Payload Types payload types for the same SSRC that are
   valid for the same instant, then redundant encodings [RFC2198] can be
   used.  Several additional constraints than the ones mentioned above
   need to be met to enable this use, one of which is that the combined
   payload sizes of the different Payload Types payload types ought not exceed the
   transport MTU.  If it is acceptable to send multiple formats of the
   same media source as separate RTP streams (with separate SSRC),
   simulcast [I-D.ietf-mmusic-sdp-simulcast] can be used.

   Other aspects of RTP payload format use are described in How to Write
   an RTP Payload
   HowTo Format [RFC8088].

   The payload type is not a multiplexing point at the RTP layer (see
   Appendix A for a detailed discussion of why using the payload type as
   an RTP multiplexing point does not work).  The RTP payload type is,
   however, used to determine how to render a media stream, consume and so can
   be viewed as selecting a rendering context. decode an RTP stream.
   The rendering context
   can be defined by the signalling, and the RTP payload type number is sometimes used to associate an RTP media
   stream with the signalling.
   This association signalling; this is possible provided unique RTP not recommended since a specific
   payload type numbers
   are used in each context.  For example, an RTP media stream value can be
   associated with an SDP "m=" line by comparing the RTP payload type
   numbers used by the media stream with payload types signalled in the
   "a=rtpmap:" lines in the media multiple bundled "m=" sections of the SDP.  If RTP media
   streams are being associated with signalling contexts based on the
   RTP payload type, then the assignment of
   [I-D.ietf-mmusic-sdp-bundle-negotiation].  This association is only
   possible if unique RTP payload type numbers
   needs to be unique across signalling contexts; if the same RTP
   payload format configuration is are used in multiple contexts, then a
   different RTP payload type number has to be assigned in each context
   to ensure uniqueness.  If the RTP payload type number is not being
   used to associated RTP media streams with a signalling context, then
   the same RTP payload type number can be used to indicate the exact
   same RTP payload format configuration in multiple contexts.  In case
   of bundled media, Section 10.2 of
   [I-D.ietf-mmusic-sdp-bundle-negotiation] provides more information on
   SDP signalling. context.

3.3.  Issues Related to RTP Topologies

   The impact of how RTP multiplexing is performed will in general vary
   with how the RTP Session session participants are interconnected, described
   by RTP Topology [RFC7667].

   Even the most basic use case, denoted Topo-Point-to-Point in
   [RFC7667], raises a number of considerations that are discussed in
   detail in following sections.  They range over such aspects as:

   o  Does my communication peer support RTP as defined with multiple
      SSRCs?
      SSRCs per RTP session?

   o  Do I need network differentiation in form of QoS?

   o  Can the application more easily process and handle the media
      streams if they are in different RTP sessions?

   o  Do I need to use additional media RTP streams for RTP retransmission or FEC.
      FEC?

   o  etc.

   For some Point point to Multi-point multi-point topologies (e.g.  Topo-ASM and Topo-SSM
   in [RFC7667]), multicast is used to interconnect the session
   participants.  Special considerations (documented in Section 4.2.3)
   need to be made as
   are then needed as multicast is a one to many one-to-many distribution system.

   Sometimes an RTP communication can end up in a situation when the
   peer it is
   communicating with is peers are not compatible with the other peer for various reasons:

   o  No common media codec for a media type thus requiring transcoding transcoding.

   o  Different support for multiple RTP sources and RTP sessions sessions.

   o  Usage of different media transport protocols, i.e RTP or other.

   o  Usage of different transport protocols, e.g.  UDP, DCCP, TCP TCP.

   o  Different security solutions, e.g.  IPsec, TLS, DTLS, SRTP with
      different keying mechanisms.

   In many situations this is resolved by the inclusion of a translator
   between the two peers, as described by Topo-PtP-Translator in
   [RFC7667].  The translator's main purpose is to make the peer peers look to
   the other peer like something it is
   compatible with. to each other.  There can also be other reasons than
   compatibility to insert a translator in the form of a middlebox or
   gateway, for example a need to monitor the
   media RTP streams.  If the
   stream transport characteristics are changed by the translator,
   appropriate media handling can require thorough understanding of the
   application logic, specifically any congestion control or media
   adaptation.

   The point to point topology can contain one to many RTP sessions with
   one to many media sources per session, each having one or more RTP
   sources per media source.

3.4.  Issues Related to RTP and RTCP Protocol

   Using multiple media RTP streams is a well supported well-supported feature of RTP.
   However, it can be unclear for most implementers or people writing RTP/RTCP
   applications or extensions attempting to apply multiple
   streams streams, it
   can be unclear when it is most appropriate to add an additional SSRC RTP
   stream in an existing RTP session and when it is better to use
   multiple RTP sessions.  This section tries to discuss discusses the various
   considerations needed.

3.4.1.  The RTP Specification

   RFC 3550 contains some recommendations and a bullet list with 5
   arguments for different aspects of RTP multiplexing.  Let's review
   Section 5.2 of [RFC3550], reproduced below:

   "For efficient protocol processing, the number of multiplexing points
   should be minimised, as described in the integrated layer processing
   design principle [ALF].  In RTP, multiplexing is provided by the
   destination transport address (network address and port number) which
   is different for each RTP session.  For example, in a teleconference
   composed of audio and video media encoded separately, each medium
   SHOULD be carried in a separate RTP session with its own destination
   transport address.

   Separate audio and video streams SHOULD NOT be carried in a single
   RTP session and demultiplexed based on the payload type or SSRC
   fields.  Interleaving packets with different RTP media types but
   using the same SSRC would introduce several problems:

   1.  If, say, two audio streams shared the same RTP session and the
       same SSRC value, and one were to change encodings and thus
       acquire a different RTP payload type, there would be no general
       way of identifying which stream had changed encodings.

   2.  An SSRC is defined to identify a single timing and sequence
       number space.  Interleaving multiple payload types would require
       different timing spaces if the media clock rates differ and would
       require different sequence number spaces to tell which payload
       type suffered packet loss.

   3.  The RTCP sender and receiver reports (see Section 6.4) can only
       describe one timing and sequence number space per SSRC and do not
       carry a payload type field.

   4.  An RTP mixer would not be able to combine interleaved streams of
       incompatible media into one stream.

   5.  Carrying multiple media in one RTP session precludes: the use of
       different network paths or network resource allocations if
       appropriate; reception of a subset of the media if desired, for
       example just audio if video would exceed the available bandwidth;
       and receiver implementations that use separate processes for the
       different media, whereas using separate RTP sessions permits
       either single- or multiple-process implementations.

   Using a different SSRC for each medium but sending them in the same
   RTP session would avoid the first three problems but not the last
   two.

   On the other hand, multiplexing multiple related sources of the same
   medium in one RTP session using different SSRC values is the norm for
   multicast sessions.  The problems listed above don't apply: an RTP
   mixer can combine multiple audio sources, for example, and the same
   treatment is applicable for all of them.  It might also be
   appropriate to multiplex streams of the same medium using different
   SSRC values in other scenarios where the last two problems do not
   apply."

   Let's consider one argument at a time.  The first is an argument is for
   using different SSRC for each individual media RTP stream, which is very
   applicable.
   basic.

   The second argument is advocating against using demultiplexing RTP streams
   within a session based on their RTP payload type
   multiplexing, numbers, which still
   stands as can been seen by the extensive list of issues found in
   Appendix A.

   The third argument is yet another argument against payload type
   multiplexing.

   The fourth is an argument is against multiplexing media streams RTP packets that require
   different handling into the same session.  As we saw in the
   discussion of RTP mixers, the RTP mixer has to must embed application logic in order
   to handle streams anyway; the separation of streams according to
   stream type is just another piece of application logic, which might
   or might not be appropriate for a particular application.
   A  One type
   of application that can mix different media sources "blindly" is the audio only
   audio-only "telephone" bridge; most other type types of application
   needs applications need
   application-specific logic to perform the mix correctly.

   The fifth argument discusses network aspects that we will discuss
   more below in Section 4.2.  It also goes into aspects of
   implementation, like decomposed Split Component Terminal (see Section 3.10 of
   [RFC7667]) endpoints where different processes or inter-connected
   devices handle different aspects of the whole multi-media session.

   A summary of RFC 3550's view on multiplexing is to use unique SSRCs
   for anything that is its own media/packet stream, and to use
   different RTP sessions for media streams that don't share a media
   type.  This document supports the first point; it is very valid.  The
   later is one thing which
   latter needs to be further discussed, discussion, as imposing a single solution on all
   usages of RTP is inappropriate.  Multiple Media Types in an RTP
   Session specification [I-D.ietf-avtcore-multi-media-rtp-session]
   provides a detailed analysis of the potential issues in having
   multiple media types in the same RTP session.  This document tries to provide an provides
   a wider scoped
   consideration regarding the usage of scope for an RTP session and considers multiple media types
   in one RTP session as a possible choice for the RTP application
   designer.

3.4.2.  Multiple SSRCs in a Session

   Using multiple SSRCs at one endpoint in an RTP session at one endpoint requires
   resolving some unclear aspects of the RTP specification.  These could
   potentially lead to some interoperability issues as well as some
   potential significant inefficiencies.  These are inefficiencies, as further discussed in "RTP
   Considerations for Endpoints Sending Multiple Media Streams"
   [RFC8108].  A  An RTP application designer needs to should consider these issues
   and the possible applicaiton impact availability or from lack of the appropriate RTP
   handling or optimization in the endpoints
   has on their application.

   If an application will become affected by the issues described, using
   Multiple peer endpoints.

   Using multiple RTP sessions can potentially mitigate these issues. application
   issues caused by multiple SSRCs in an RTP session.

3.4.3.  Binding Related Sources

   A common problem in a number of various RTP extensions has been how
   to bind related RTP sources and their media RTP streams together.  This
   issue is common to both using additional SSRCs and Multiple multiple RTP
   sessions.

   The solutions can be divided into some groups, a few groups:

   o  RTP/RTCP based, based

   o  Signalling based (SDP), (SDP)

   o  grouping related RTP sessions, and sessions

   o  grouping SSRCs within an RTP session. session

   Most solutions are explicit, but some implicit methods have also been
   applied to the problem.

   The SDP-based signalling solutions are:

   SDP Media Description Grouping:  The SDP Grouping Framework [RFC5888]
      uses various semantics to group any number of media descriptions.
      These has previously been considered primarily as grouping RTP
      sessions, [I-D.ietf-mmusic-sdp-bundle-negotiation] groups multiple
      media descriptors descriptions as a single RTP session.

   SDP SSRC grouping:  Source-Specific Media Attributes in SDP [RFC5576]
      includes a solution for grouping SSRCs the same way as the
      Grouping framework groups Media Descriptions.

   SDP MSID grouping:  Media Stream Identifiers [I-D.ietf-mmusic-msid]
      includes
      specifies a solution Session Description Protocol (SDP) Grouping mechanism
      for grouping SSRCs RTP streams that can be used to specify relations between RTP
      streams.  This mechanism is independent used to signal the association between
      the SDP concept of
      their allocation "media description" and the WebRTC concept of
      "MediaStream" / "MediaStreamTrack" (Corresponds to RTP sessions. the [RFC7656]
      term "Source Stream") using SDP signalling.

   This supports a lot of use cases.  All these solutions have
   shortcomings in cases where the session's dynamic properties are such
   that it is difficult or resource consuming to keep the list of
   related SSRCs up to date.

   Within RTP/RTCP based solutions when binding

   An RTP/RTCP-based solution is to an endpoint or
   synchronization context, i.e. use the RTCP SDES CNAME has not been sufficient and
   one way to bind related the
   RTP streams in multiple to an endpoint or synchronization context.  For
   applications with a single RTP stream per type (Media, Source or
   Redundancy) this is sufficient independent if one or more RTP
   sessions has been are used.  However, some applications choose not to use it
   because of perceived complexity or a desire not to implement RTCP and
   instead use the same SSRC value to bind related RTP streams across all the
   multiple RTP sessions.  RTP Retransmission [RFC4588] is in multiple RTP
   session mode, mode and Generic FEC
   [RFC5109], as well as the RTP payload format for Scalable Video
   Coding [RFC6190] in Multi Session Transmission (MST) mode uses [RFC5109] both use this method.  This
   method clearly works may work but might have some downside downsides in RTP sessions with
   many participating SSRCs.  The birthday paradox
   ensures that if you populate a single session with 9292 SSRCs at
   random, the chances are approximately 1% that at least one collision
   will occur.  When a an SSRC collision occur occurs, this will
   force one to change SSRC in all RTP sessions and thus resynchronizing resynchronize
   all of them instead of only the single media stream having the
   collision.  Therefore  Therefore, it is not recommended to use such method.  Using [RFC7656] streams from
   the same media source should use the same RTP session.

   It can be noted that Section 8.3 of the RTP Specification [RFC3550]
   recommends using a single identical SSRC space across all
   values to relate RTP sessions for
   layered coding. streams.

   Another solution that has been applied to binding bind SSRCs has been is an implicit method used by RTP
   Retransmission [RFC4588] when doing retransmissions in the same RTP
   session as the source RTP media stream.  This  The receiver missing a packet
   issues an RTP retransmission request, and then await awaits a new SSRC
   carrying the RTP retransmission payload and where that SSRC is from
   the same CNAME.  This limits a requestor to having only one
   outstanding request on any new source SSRCs per endpoint.

   RTP Payload Format Restrictions [I-D.ietf-mmusic-rid] provides an
   RTP/RTCP based mechanism capable of
   supporting explicit association to unambiguously identify the RTP streams
   within an RTP session. session and restrict the streams' payload format
   parameters in a codec-agnostic way beyond what is provided with the
   regular Payload Types.  The mapping is done by specifying an "a=rid"
   value in the SDP offer/answer signalling and having the corresponding
   "rtp-stream-id" value as an SDES item and an RTP header extension.

   The RID solution also includes a solution for binding redundancy RTP
   streams to their original source RTP streams, given that those use
   RID identifiers.

   It can be noted that Section 8.3 of the RTP Specification [RFC3550]
   recommends using a single SSRC space across all RTP sessions for
   layered coding.  Based on the experience so far however, we recommend
   to use a solution doing explicit binding between the RTP streams so
   what the used SSRC values are do not matter.  That way solutions
   using multiple RTP streams in a single RTP session and multiple RTP
   sessions uses the same solution.

3.4.4.  Forward Error Correction

   There exist a number of Forward Error Correction (FEC) based schemes
   for how to reduce the packet loss of the original streams.  Most of
   the FEC schemes will protect a single source flow.  The protection is
   achieved by transmitting a certain amount of redundant information
   that is encoded such that it can repair one or more packet losses
   over the set of packets they protect. the redundant information protects.  This
   sequence of redundant information also needs to be transmitted as its
   own media stream, or in some cases cases, instead of the original media
   stream.  Thus  Thus, many of these schemes create a need for binding
   related flows as discussed above.  Looking at the history of these
   schemes, there are schemes using multiple SSRCs and schemes using
   multiple RTP sessions, and some schemes that support both modes of
   operation.

   Using multiple RTP sessions supports the case where some set of
   receivers might not be able to utilise the FEC information.  By
   placing it in a separate RTP session, it session and if separating RTP sessions
   on transport level, FEC can easily be ignored. ignored already on transport
   level.

   In usages involving multicast, having the FEC information on its own
   multicast group allows for similar flexibility.  This is especially
   useful when receivers see very heterogeneous packet loss rates.
   Those receivers that are not seeing packet loss don't need to join
   the multicast group with the FEC data, and so avoid the overhead of
   receiving unnecessary FEC packets, for example.

4.  Particular  Considerations for RTP Multiplexing

4.1.  Interworking Considerations

   There are several different kinds of interworking, and this section
   discusses two related ones.  The two; interworking between different applications and including
   the implications of potentially different choices of
   usage of RTP's RTP multiplexing points.  The second topic relates to what point
   choices and limitations that have to be considered when working with
   some legacy applications.

4.1.1.  Types of  Application Interworking

   It is not uncommon that applications or services of similar but not
   identical usage, especially the ones intended for interactive
   communication, encounter a situation where one want to interconnect
   two or more of these applications.

   In these cases cases, one ends up in a situation where one might use a
   gateway to interconnect applications.  This gateway must then needs to either
   change the multiplexing structure or adhere to the respective
   limitations in each application.

   There are two fundamental approaches to gatewaying: RTP Translator
   interworking (RTP bridging), where the gateway acts as an RTP
   Translator, and with the two applications are being members of the same RTP
   session, and Gateway Interworking (with RTP termination), where there
   are independent RTP sessions running from each interconnected
   application to the gateway.

4.1.2.  RTP Translator Interworking

   From an RTP perspective the RTP Translator approach could work if all
   the applications are using the same codecs with the same payload
   types, have made the same multiplexing choices, and have the same
   capabilities in number of simultaneous media RTP streams combined with the
   same set of RTP/RTCP extensions being supported.  Unfortunately  Unfortunately, this
   might not always be true.

   When one is gatewaying via an RTP Translator, a natural requirement an important
   consideration is that if the two applications being interconnected need to
   use the same approach to multiplexing.  If one side is using RTP
   session multiplexing and the other is using SSRC multiplexing with
   bundle, the mapping of SDP "m=" lines between both sides requires
   that the order in bundled and not bundled sides will be the same to
   allow routing without mapping, it is possible for the RTP translator
   to map the RTP streams between both sides.  There are also challenges
   with SSRC collision handling since there may be a collision on the
   SSRC multiplexing side but the RTP session multiplexing side will not
   be aware of any collision unless SSRC translation is applied on the
   RTP translator.  Furthermore, if one of the applications is capable
   of working in several modes (such as being able to use Additional SSRCs additional RTP
   streams in one RTP session or Multiple multiple RTP sessions at will), and the
   other one is not, successful interconnection depends on locking the
   more flexible application into the operating mode where
   interconnection can be successful, even if no participants are using
   the less flexible application are present when the RTP sessions are being
   created.

4.1.3.  Gateway Interworking

   When one terminates RTP sessions at the gateway, there are certain
   tasks that the gateway has to carry out:

   o  Generating appropriate RTCP reports for all media RTP streams (possibly
      based on incoming RTCP reports), originating from SSRCs controlled
      by the gateway.

   o  Handling SSRC collision resolution in each application's RTP
      sessions.

   o  Signalling, choosing and policing appropriate bit-rates for each
      session.

   For applications that uses any security mechanism, e.g. in the form
   of SRTP, then the gateway needs to be able to decrypt incoming packets and
   re-encrypt them in the other application's security context.  This is
   necessary even if all that's needed is a simple remapping of SSRC
   numbers.  If this is done, the gateway also needs to be a member of
   the security contexts of both sides, of course.

   Other tasks a gateway might need to apply include transcoding (for
   incompatible codec types), media-level adaptations that cannot be
   solved through media negotiation (such as rescaling (for for incompatible
   video size requirements), suppression of content that is known not to
   be handled in the destination application, or the addition or removal
   of redundancy coding or scalability layers to fit the need needs of the
   destination domain.

   From the above, we can see that the gateway needs to have an intimate
   knowledge of the application requirements; a gateway is by its nature
   application specific, not a commodity product.

   This fact reveals the potential for these gateways to block
   application evolution
   of the applications by blocking unknown RTP and RTCP extensions that the regular application has
   applications have been extended with. with but that are unknown to the
   gateway.

   If one uses security functions, like SRTP, they can and as can be seen above from
   above, they incur both additional risk due to the gateway needing requirement to be have
   the gateway in the security association between the endpoints, unless the endpoints (unless
   the gateway is on the transport level, level), and additional complexities
   in form of the decrypt-encrypt cycles needed for each forwarded
   packet.  SRTP, due to its keying structure, also requires that each
   RTP session needs different master keys, as use of the same key in
   two RTP sessions can for some ciphers can result in two-time pads that
   completely breaks the confidentiality of the packets.

4.1.4.  Multiple SSRC Legacy Considerations

   Historically, the most common RTP use cases have been point to point
   Voice over IP (VoIP) or streaming applications, commonly with no more
   than one media source per endpoint and media type (typically audio
   and video).  Even in conferencing applications, especially voice voice-
   only, the conference focus or bridge has provided a single stream
   with a mix of the other participants to each participant.  It is also
   common to have individual RTP sessions between each endpoint and the
   RTP mixer, meaning that the mixer functions as an RTP-terminating
   gateway.

   When establishing RTP sessions that can contain endpoints that aren't
   updated to handle multiple streams following these recommendations, a
   particular application can have issues with multiple SSRCs within a
   single session.  These issues include:

   1.  Need to handle more than one stream simultaneously rather than
       replacing an already existing stream with a new one.

   2.  Be capable of decoding multiple streams simultaneously.

   3.  Be capable of rendering multiple streams simultaneously.

   This indicates that gateways attempting to interconnect to this class
   of devices has to make sure that only one media RTP stream of each type
   gets delivered to the endpoint if it's expecting only one, and that
   the multiplexing format is what the device expects.  It is highly
   unlikely that RTP translator-based interworking can be made to
   function successfully in such a context.

4.2.  Network Considerations

   The RTP multiplexing choice has impact on network level mechanisms
   that need to be considered by the implementer.

4.2.1.  Quality of Service

   When it comes to Quality of Service mechanisms, they are either flow
   based or packet marking based.  RSVP [RFC2205] is an example of a
   flow based mechanism, while Diff-Serv [RFC2474] is an example of a
   packet marking based one.  For a packet marking based scheme, the
   method of multiplexing will not affect the possibility to use QoS.

   However, for a flow based scheme there is a clear difference between
   the multiplexing methods.  Additional SSRC will result in all media RTP
   streams being part of the same 5-tuple (protocol, source address,
   destination address, source port, destination port) which is the most
   common selector for flow based QoS.

   It must also needs to be noted that packet marking based QoS mechanisms can
   have limitations.  A general observation is that different DSCP
   Differentiated Services Code Points (DSCP) can be assigned to
   different packets within a flow as well as within an RTP Media Stream. stream.
   However, care needs to must be taken when considering which forwarding
   behaviours that are applied on path due to these DSCPs.  In some
   cases the forwarding behaviour can result in packet reordering.  For
   more discussion of this see [RFC7657].

   More specific to the choice between using one or more RTP session can
   be the

   The method for assigning marking to packets. packets can impact what number of
   RTP sessions to choose.  If this marking is done using a network
   ingress function, it can have issues discriminating the different RTP media
   streams.  The network API on the endpoint also needs to be capable of
   setting the marking on a per packet per-packet basis to reach the full
   functionality.

4.2.2.  NAT and Firewall Traversal

   In today's network there exist a large number of middleboxes.  The
   ones that normally have most impact on RTP are Network Address
   Translators (NAT) and Firewalls (FW).

   Below we analyse and comment on the impact of requiring more
   underlying transport flows in the presence of NATs and Firewalls:

   End-Point Port Consumption:  A given IP address only has 65536
      available local ports per transport protocol for all consumers of
      ports that exist on the machine.  This is normally never an issue
      for an end-user machine.  It can become an issue for servers that
      handle large number of simultaneous streams.  However, if the
      application uses ICE to authenticate STUN requests, a server can
      serve multiple endpoints from the same local port, and use the
      whole 5-tuple (source and destination address, source and
      destination port, protocol) as identifier of flows after having
      securely bound them to the remote endpoint address using the STUN
      request.  In theory the minimum number of media server ports
      needed are the maximum number of simultaneous RTP Sessions a
      single endpoint can use.  In practice, implementation will
      probably benefit from using more server ports to simplify
      implementation or avoid performance bottlenecks.

   NAT State:  If an endpoint sits behind a NAT, each flow it generates
      to an external address will result in a state that has to be kept
      in the NAT.  That state is a limited resource.  In home or Small
      Office/Home Office (SOHO) NATs, memory or processing are usually
      the most limited resources.  For large scale NATs serving many
      internal endpoints, available external ports are likely the scarce
      resource.  Port limitations is primarily a problem for larger
      centralised NATs where endpoint independent mapping requires each
      flow to use one port for the external IP address.  This affects
      the maximum number of internal users per external IP address.
      However, it is worth pointing out that a real-time video
      conference session with audio and video is likely using less than
      10 UDP flows, compared to certain web applications that can use
      100+ TCP flows to various servers from a single browser instance.

   NAT Traversal Excess Time: Extra Delay:  Performing the NAT/FW traversal takes a
      certain amount of time for each flow.  It also takes time in a
      phase of communication between accepting to communicate and the
      media path being established which is fairly critical.  The best
      case scenario for how much extra time it takes after finding the
      first valid candidate pair following the specified ICE procedures
      are: 1.5*RTT + Ta*(Additional_Flows-1), where Ta is the pacing
      timer, which ICE specifies to be no smaller than 20 ms.
      timer.  That assumes a message in one direction, and then an
      immediate triggered check back.  The reason it isn't more, is that
      ICE first finds one candidate pair that works prior to attempting
      to establish multiple flows.  Thus, there is no extra time until
      one has found a working candidate pair.  Based on that working
      pair the needed extra time is to in parallel establish the, in
      most cases 2-3, additional flows.  However, packet loss causes
      extra delays, at least 100 ms, which is the minimal retransmission
      timer for ICE.

   NAT Traversal Failure Rate:  Due to the need to establish more than a
      single flow through the NAT, there is some risk that establishing
      the first flow succeeds but that one or more of the additional
      flows fail.  The risk that this happens is hard to quantify, but
      ought to be fairly low as one flow from the same interfaces has
      just been successfully established.  Thus only rare events such as
      NAT resource overload, or selecting particular port numbers that
      are filtered etc., ought to be reasons for failure.

   Deep Packet Inspection and Multiple Streams:  Firewalls differ in how
      deeply they inspect packets.  There exist some potential that
      deeply inspecting firewalls will have similar legacy issues with
      multiple SSRCs as some stack implementations.

   Additional SSRC keeps the

   Using additional media RTP streams within one in the same RTP
   Session session and transport
   flow and does not introduce any additional NAT traversal complexities per media
   RTP stream.  This can be compared with normally one or two additional
   transport flows per RTP session when using multiple RTP sessions.
   Additional lower layer transport flows will be needed, unless an
   explicit de-multiplexing layer is added between RTP and the transport
   protocol.  At time of writing no such mechanism was defined.

4.2.3.  Multicast

   Multicast groups provides a powerful semantics tool for a number of real-
   time real-time
   applications, especially the ones that desire broadcast-like
   behaviours with one endpoint transmitting to a large number of
   receivers, like in IPTV.  There are also the RTP/RTCP extension to
   better support Source Specific Multicast (SSM) [RFC5760].  Another
   application is the Many to Many communication, which RTP [RFC3550]
   was originally built to support.  But that same the multicast semantics do
   result in a certain number of limitations.

   One limitation is that for any group, sender side adaptation to the
   actual receiver properties causes degradation for all participants to
   what is supported by the receiver with the worst conditions among the
   group participants.  In most cases  For broadcast type of applications this is not
   acceptable.  Instead  Instead, various receiver based receiver-based solutions are employed
   to ensure that the receivers achieve best possible performance.  By
   using scalable encoding and placing each scalability layer in a
   different multicast group, the receiver can control the amount of
   traffic it receives.  To have each scalability layer on a different
   multicast group, one RTP session per multicast group is used.

   In addition, the transport flow considerations in multicast are a bit
   different from unicast; NATs with port translation are not useful in
   the multicast environment, meaning that the entire port range of each
   multicast address is available for distinguishing between RTP
   sessions.

   Thus

   Thus, when using broadcast applications it appears easiest and most
   straightforward to use multiple RTP sessions for sending different
   media flows used for adapting to network conditions.  It is also
   common that streams that improve transport robustness are sent in
   their own multicast group to allow for interworking with legacy or to
   support different levels of protection.

   For many to many applications there is different needs.  Here are some common behaviours for it will
   depend on how the actual application is realized what is the most
   appropriate choice.  With sender side congestion control there might
   not exist any benefit with using multiple RTP session.

   The properties of a broadcast application using RTP multicast:

   1.  Multicast applications use  Uses a group of RTP sessions, not one.  Each endpoint will need
       to be a member of a number of RTP sessions in order to perform
       well.

   2.  Within each RTP session, the number of RTP Sinks sinks is likely to be
       much larger than the number of RTP sources.

   3.  Multicast  The applications need signalling functions to identify the
       relationships between RTP sessions.

   4.  Multicast  The applications need signalling or RTP/RTCP functions to
       identify the relationships between SSRCs in different RTP sessions.

   All
       sessions when needs beyond CNAME exists.

   Both broadcast and many to many multicast configurations applications do share a
   signalling requirement; all of the participants will need to have the
   same RTP and payload type configuration.  Otherwise, A could for
   example be using payload type 97 as the video codec H.264 while B
   thinks it is MPEG-2.  It is to be noted that SDP offer/answer
   [RFC3264] is not appropriate for ensuring this property. property in broadcast/
   multicast context.  The signalling aspects of multicast broadcast/multicast are
   not explored further in this memo.

   Security solutions for this type of group communications are also
   challenging.  First of all  First, the key-management and the security protocol
   needs to support group communication.  Source  Second, source authentication
   requires special solutions.  For more discussion on this please
   review Options for Securing RTP Sessions [RFC7201].

4.3.  Security and Key Management Considerations

   When dealing with point-to-point, 2-member RTP sessions only, there
   are few security issues that are relevant to the choice of having one
   RTP session or multiple RTP sessions.  However, there are a few
   aspects of multiparty sessions that might warrant consideration.  For
   general information of possible methods of securing RTP, please
   review RTP Security Options [RFC7201].

4.3.1.  Security Context Scope

   When using SRTP [RFC3711] the security context scope is important and
   can be a necessary differentiation in some applications.  As SRTP's
   crypto suites are (so far) are built around symmetric keys, the receiver
   will need to have the same key as the sender.  This results in that
   no one in a multi-party session can be certain that a received packet
   really was sent by the claimed sender or and not by another party having
   access to the key.  At least unless TESLA source authentication
   [RFC4383], which adds delay to achieve source authentication.  In
   most cases this is a symmetric ciphers provide sufficient security
   property, properties,
   but there are a few cases where this does create issues.

   The first case is when someone leaves a multi-party session and one
   wants to ensure that the party that left can no longer access the
   media RTP
   streams.  This requires that everyone re-keys without disclosing the
   keys to the excluded party.

   A second case is when using security as an enforcing mechanism for
   differentiation.  Take for example a scalable layer or a high quality
   simulcast version which that only premium users are allowed to access.  The
   mechanism preventing a receiver from getting the high quality stream
   can be based on the stream being encrypted with a key that user can't
   access without paying premium, having the key-management limit access
   to the key.

   SRTP [RFC3711] has no special functions for dealing with different
   sets of master keys for different SSRCs.  The key-management
   functions have different capabilities to establish different set sets of
   keys, normally on a per endpoint basis.  For example, DTLS-SRTP
   [RFC5764] and Security Descriptions [RFC4568] establish different
   keys for outgoing and incoming traffic from an endpoint.  This key
   usage has to be written into the cryptographic context, possibly
   associated with different SSRCs.

4.3.2.  Key Management for Multi-party session sessions

   Performing key-management for multi-party session can be a challenge.
   This section considers some of the issues.

   Multi-party sessions, such as transport translator based sessions and
   multicast sessions, cannot use Security Description [RFC4568] nor
   DTLS-SRTP [RFC5764] without an extension as each endpoint provides
   its set of keys.  In centralised conferences, the signalling
   counterpart is a conference server and the media plane unicast
   counterpart (to which DTLS messages would be sent) is the transport
   translator.  Thus  Thus, an extension like Encrypted Key Transport
   [I-D.ietf-avt-srtp-ekt] is needed
   [I-D.ietf-perc-srtp-ekt-diet] or a MIKEY [RFC3830] based solution
   that allows for keying all session participants with the same master
   key.
   key is needed.

4.3.3.  Complexity Implications

   The usage of security functions can surface complexity implications
   of
   from the choice of multiplexing and topology.  This becomes
   especially evident in RTP topologies having any type of middlebox
   that processes or modifies RTP/RTCP packets.  Where there is very
   small overhead for an RTP translator or mixer to rewrite an SSRC
   value in the RTP packet of an unencrypted session, the cost of doing it is higher
   when using cryptographic security functions is higher. functions.  For example example, if using
   SRTP [RFC3711], the actual security context and exact crypto key are
   determined by the SSRC field value.  If one changes it, SSRC, the
   encryption and authentication tag needs to be performed using must use another key.  Thus  Thus, changing
   the SSRC value implies a decryption using the old SSRC and its
   security context context, followed by an encryption using the new one.

5.  Archetypes  RTP Multiplexing Design Choices

   This section discusses some archetypes of how some RTP multiplexing design choices can
   be used in applications to achieve certain goals goals, and a summary of their
   implications.
   the implications of such choices.  For each archetype design there is
   discussion of benefits and downsides.

5.1.  Single SSRC per Session Endpoint

   In this archetype design each endpoint in a point-to-point session has only a
   single SSRC, thus the RTP session contains only two SSRCs, one local
   and one remote.  This session can be used both unidirectional, i.e.
   only a single media RTP stream or bi-directional, i.e. both endpoints have
   one media RTP stream each.  If the application needs additional media flows
   between the endpoints, they will have to establish additional RTP
   sessions.

   The Pros:

   1.  This archetype design has great legacy interoperability potential as it
       will not tax any RTP stack implementations.

   2.  The signalling has good possibilities to negotiate and describe
       the exact formats and bit-rates for each media RTP stream, especially
       using today's tools in SDP.

   3.  It does not matter if usage or purpose of the media stream is
       signalled on media stream level or session level as there is no
       difference.

   4.  It is possible to control security association per RTP media stream
       with current key-management, since each media RTP stream is directly
       related to an RTP session, and the most used keying mechanisms
       operates on a per-session basis.

   The Cons:

   a.  The number of RTP sessions grows directly in proportion with the
       number of media RTP streams, which has the implications:

       *  Linear growth of the amount of NAT/FW state with number of
          media RTP
          streams.

       *  Increased delay and resource consumption from NAT/FW
          traversal.

       *  Likely larger signalling message and signalling processing
          requirement due to the amount of session related information.

       *  Higher potential for a single media RTP stream to fail during
          transport between the endpoints.

   b.  When the number of RTP sessions grows, the amount of explicit
       state for relating media RTP stream also grows, linearly or possibly
       exponentially, linearly, depending on
       how the application needs to relate
       media RTP streams.

   c.  The port consumption might become a problem for centralised
       services, where the central node's port consumption grows rapidly
       with the number of sessions.

   d.  For applications where the media streams are RTP stream usage is highly dynamic in
       their usage, dynamic,
       i.e. entering and leaving, the amount of signalling can grow
       high.  Issues arising can also arise from the timely establishment of
       additional RTP sessions can also arise. sessions.

   e.  Cross session RTCP requests might be needed, and  If, against the fact that
       they're impossible can cause issues.

   f.  If recommendation, the same SSRC value is reused in
       multiple RTP sessions rather than being randomly chosen,
       interworking with applications that
       uses another use a different multiplexing
       structure than this application will require SSRC translation.

   g.  Cannot be used with Any Source Multicast (ASM) as one cannot
       guarantee that only two endpoints participate as packet senders.
       Using SSM, it is possible to restrict to these requirements if no
       RTCP feedback is injected back into the SSM group.

   h.  For most security mechanisms, each RTP session or transport flow
       requires individual key-management and security association
       establishment thus increasing the overhead.

   RTP applications that need to inter-work interwork with legacy RTP
   applications, like most deployed VoIP and video conferencing
   solutions, applications
   can potentially benefit from this structure.  However, a large number
   of media descriptions in SDP can also run into issues with existing
   implementations.  For any application needing a larger number of
   media flows, the overhead can become very significant.  This
   structure is also not suitable for multi-party sessions, as any given media
   RTP stream from each participant, although having same usage in the
   application, needs its own RTP session.  In addition, the dynamic
   behaviour that can arise in multi-party applications can tax the
   signalling system and make timely media establishment more difficult.

5.2.  Multiple SSRCs of the Same Media Type

   In this archetype, design, each RTP session serves only a single media type.
   The RTP session can contain multiple media RTP streams, either from a
   single endpoint or from multiple endpoints.  This commonly creates a
   low number of RTP sessions, typically only one for audio and one for
   video, with a corresponding need for two listening ports when using
   RTP/RTCP multiplexing.

   The Pros:

   1.  Low number of RTP sessions needed compared to single Single SSRC per
       Endpoint case.  This implies:

       *  Reduced NAT/FW state

       *  Lower NAT/FW Traversal Cost in both processing and delay.

   2.  Allows for early de-multiplexing in the processing chain in RTP
       applications where all media streams of the same type have the
       same usage in the application.

   3.  Works well with Split Component Terminal (see Section 3.10 of
       [RFC7667]) where the split is per media type de-composite endpoints.

   4. type.

   3.  Enables Flow-based QoS with different prioritisation between
       media types.

   5.

   4.  For applications with dynamic usage of media RTP streams, i.e. they
       come
       frequently added and go frequently, removed, having much of the state associated
       with the RTP session rather than an per individual SSRC can avoid
       the need for in-session signalling of meta-information about each
       SSRC.

   6.

   5.  Low overhead for security association establishment.

   The Cons:

   a.  May have some need for cross session RTCP requests overhead for things
       that affect both media types in an asynchronous way.

   b. security association establishment.

   The Cons:

   a.  Some potential for concern with legacy implementations that does
       not don't
       support the RTP specification fully when it comes to handling
       multiple SSRC per endpoint.

   c.  Will not be able

   b.  Not possible to control security association for sets of
       media RTP
       streams within the same media type with today's key- management
       mechanisms, unless these are split into different RTP sessions.

   For RTP applications where all media RTP streams of the same media type
   share same usage, this structure provides efficiency gains in amount
   of network state used and provides more fate sharing with other media
   flows of the same type.  At the same time, it is still maintaining
   almost all functionalities when it comes to negotiation in the
   signalling of the properties for the individual media type type, and also
   enabling
   enables flow based QoS prioritisation between media types.  It
   handles multi-party session well, independently of multicast or
   centralised transport distribution, as additional sources can
   dynamically enter and leave the session.

5.3.  Multiple Sessions for one Media type

   In this archetype one

   This design goes one step further than in the above (Section 5.2) by using
   multiple RTP sessions also for a single media
   type, but still not as far as having a single SSRC per RTP session. type.  The main reason
   for going in this direction is that the RTP application needs
   separation of the media RTP streams due to their usage.  Some typical
   reasons for going to this archetype design are scalability over multicast,
   simulcast, need for extended QoS prioritisation of media RTP streams due to
   their usage in the application, or the need for fine- grained
   signalling using today's tools.

   The Pros:

   1.  More suitable for Multicast multicast usage where receivers can
       individually select which RTP sessions they want to participate
       in, assuming each RTP session has its own multicast group.

   2.  Indication of the application's  The application can indicate its usage of the media stream, where RTP streams on RTP
       session level, in case multiple different usages exist.

   3.  Less need for SSRC specific explicit signalling for each media
       stream and thus reduced need for explicit and timely signalling.

   4.  Enables detailed QoS prioritisation for flow based flow-based mechanisms.

   5.  Works well with de-composite endpoints.

   6.  Handles dynamic usage of media streams well.

   7.  For transport translator based multi-party sessions, this
       structure allows for improved control of which type Split Component Terminal (see Section 3.10 of media
       streams an endpoint receives.

   8.
       [RFC7667]).

   6.  The scope for who is included in a security association can be
       structured around the different RTP sessions, thus enabling such
       functionality with existing key-management.

   The Cons:

   a.  Increases the amount of RTP sessions compared to Multiple SSRCs
       of the Same Media Type.

   b.  Increased amount of session configuration state.

   c.  May need synchronised cross-session RTCP requests and require
       some consideration due to this.

   d.  For media RTP streams that are part of scalability, simulcast or
       transport robustness it will be needed robustness, a method to bind sources, which
       need to support sources across multiple
       RTP sessions.

   e. sessions is needed.

   d.  Some potential for concern with legacy implementations that does
       not support the RTP specification fully when it comes to handling
       multiple SSRC per endpoint.

   f.

   e.  Higher overhead for security association establishment.

   g. establishment due to the
       increased number of RTP sessions.

   f.  If the applications need finer control than on media type RTP session level
       over which session participants that are included in different sets of
       security associations, most of today's key-management will have
       difficulties establishing such a session.

   For more complex RTP applications that have several different usages
   for media RTP streams of the same media type and / or uses scalability or
   simulcast, this solution can enable those functions at the cost of
   increased overhead associated with the additional sessions.  This
   type of structure is suitable for more advanced applications as well
   as multicast based multicast-based applications requiring differentiation to
   different participants.

5.4.  Multiple Media Types in one Session

   This archetype is to use design uses a single RTP session for multiple different media
   types, like audio and video, and possibly also transport robustness
   mechanisms like FEC or Retransmission.  Each media stream
   will use its own SSRC and a given SSRC value from a particular  An endpoint will never use the SSRC for can have zero,
   one or more than media sources per media type.  Resulting in a single number of
   RTP streams of various media types and both source and redundancy
   type.

   The Pros:

   1.  Single RTP session which implies:

       *  Minimal NAT/FW state.

       *  Minimal NAT/FW Traversal Cost.

       *  Fate-sharing for all media flows.

   2.  Enables separation of the different media types based on the
       payload types so media type specific endpoint or central
       processing can still be supported despite single session.

   3.  Can handle dynamic allocations of media RTP streams well on an RTP
       level.  Depends on the application's needs for explicit
       indication of the stream usage and how timely that can be
       signalled.

   4.

   3.  Minimal overhead for security association establishment.

   The Cons:

   a.  Less suitable for interworking with other applications that uses
       individual RTP sessions per media type or multiple sessions for a
       single media type, due to the potential need of SSRC translation.

   b.  Negotiation of bandwidth for the different media types is
       currently not only possible using RID [I-D.ietf-mmusic-rid] in SDP.  This requires SDP extensions to
       enable payload or source specific bandwidth.  Likely to be a
       problem due to media type asymmetry in needed bandwidth.

   c.  Not suitable for de-composite endpoints. Split Component Terminal (see Section 3.10 of
       [RFC7667]).

   d.  Flow based  Flow-based QoS cannot provide separate treatment to some media of RTP streams
       compared to others in the single RTP session.

   e.  If there is significant asymmetry between the media RTP streams' RTCP
       reporting needs, there are some challenges in configuration and
       usage to avoid wasting RTCP reporting on the media RTP stream that does
       not need that frequent reporting.

   f.  Not suitable for applications where some receivers like to
       receive only a subset of the media RTP streams, especially if multicast
       or transport translator is being used.

   g.  Additional concern with legacy implementations that do not
       support the RTP specification fully when it comes to handling
       multiple SSRC per endpoint, as also multiple simultaneous media
       types needs to be handled.

   h.  If the applications need finer control over which session
       participants that are included in different sets of security
       associations, most key-management will have difficulties
       establishing such a session.

5.5.  Summary

   There are some clear relations similarities between these archetypes. designs.  Both the
   "single
   "Single SSRC per RTP session" Endpoint" and the "multiple media types "Multiple Media Types in one
   session"
   Session" are cases which that require full explicit signalling of the media
   stream relations.  However, they operate on two different levels
   where the first primarily enables session level binding, and the
   second needs to do it all on SSRC level. level binding.  From another perspective, the two
   solutions are the two extreme points when it comes to number of RTP
   sessions needed.

   The two other archetypes designs "Multiple SSRCs of the Same Media Type" and
   "Multiple Sessions for one Media Type" are examples of two other
   cases examples that first of all
   primarily allows for some implicit mapping of the role or usage of
   the media RTP streams based on which RTP session they appear in.  It thus
   potentially allows for less signalling and in particular
   reduced reduces the
   need for real-time signalling in dynamic sessions.  They also
   represent points in between the first two designs when it comes to
   amount of RTP sessions established, i.e. representing an attempt to reduce
   balance the amount of RTP sessions as much as possible without compromising with the functionality the
   communication session provides both on network level and on
   signalling level.

6.  Summary considerations and guidelines
6.1.  Guidelines

   This section contains a number of recommendations multi-stream guidelines for
   implementers or specification writers when it comes to handling multi-stream. writers.

   Do not Require use the same SSRC across Sessions: RTP sessions:  As discussed in
      Section 3.4.3 there exist drawbacks in using the same SSRC in
      multiple RTP sessions as a mechanism to bind related media RTP streams
      together.  It is instead suggested that recommended to use a mechanism to
      explicitly signal the relation is used, relation, either in RTP/RTCP or in the used
      signalling mechanism that establishes used to establish the RTP session(s).

   Use additional SSRCs RTP streams for additional Media Sources: media sources:  In the
      cases where an RTP endpoint needs to transmit additional media RTP
      streams of the same media type in the application, with the same
      processing requirements at the network and RTP layers, it is
      suggested to send them as additional SSRCs in the same RTP session.  For example a
      telepresence room where there are three cameras, and each camera
      captures 2 persons sitting at the table, sending each camera as
      its own SSRC RTP stream within a single RTP session is suggested.

   Use additional RTP sessions for streams with different requirements:

      When media RTP streams have different processing requirements from the
      network or the RTP layer at the endpoints, it is suggested that
      the different types of streams are put in different RTP sessions.
      This includes the case where different participants want different
      subsets of the set of RTP streams.

   When using multiple RTP Sessions use grouping:  When using Multiple
      RTP session solutions, it is suggested to explicitly group the
      involved RTP sessions when needed using the a signalling mechanism,
      for example The Session Description Protocol (SDP) Grouping
      Framework.
      Framework [RFC5888], using some appropriate grouping semantics.

   RTP/RTCP Extensions May Support Additional SSRCs Multiple RTP Streams as well as Multiple
   RTP sessions:
      When defining an RTP or RTCP extension, the creator needs to
      consider if this extension is applicable to usage use with additional
      SSRCs and Multiple multiple RTP sessions.  Any extension intended to be
      generic is suggested to must support both.  Applications  Extensions that are not as generally
      applicable will have to consider if interoperability is better
      served by defining a single solution or providing both options.

   Transport Support Extensions:  When defining new RTP/RTCP extensions
      intended for transport support, like the retransmission or FEC
      mechanisms, they are expected to must include support for both
      additional SSRCs and multiple RTP
      streams in the same RTP sessions so and multiple RTP sessions, such
      that application developers can choose freely from the set of
      mechanisms without concerning themselves with which of the
      multiplexing choices a particular solution supports.

7.  Open Issues

   There are currently some issues that needs to be resolved before this
   document is ready to be published:

   1.  Use of RFC 2119 language is section on SSRC (3.2.2)

   2.  Better align source and sink terminolgy with Taxonomy
       (Section 3.2.2)

   3.  Section on Binding Related Sources (Section 3.4.3) needs more
       text on usage of the RID and other SDES based mechanisms created.

   4.  Does the MSID text need to be updated and clarified based on the
       evoulsion of MSID since previous version.  Section 3.4.3.

   5.  Section 4.1.2 (RTP Translator Interworking) needs to be updated.
       It is not obvious that it is a natural requirement that the same
       multiplexing is used.  This needs better discussion.

   6.  Refernce to Ta for ICE being 20 ms will be resolved before this
   document is ready to be published:

   1.  Does the MSID text need to be updated due to
       ICE update.

   7.  In Section 4.3.2 (Key Management for Multi-party session) and clarified based on the
       reference to EKT
       evolution of MSID since previous version.  Section 3.4.3.

   2.  Changed definitions needs to be updated, question is if draft-ietf-
       perc-ekt-diet is appropriate here?

   8.  Can we find a more approriate term than archetypes?

   9. review and consideration.

8.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section can be removed on publication as an
   RFC.

9.  Security Considerations

   The security considerations of the RTP specification [RFC3550] and
   any applicable RTP profile [RFC3551],[RFC4585],[RFC3711], the
   extensions for sending multiple media types in a single RTP session
   [I-D.ietf-avtcore-multi-media-rtp-session], MSID
   [I-D.ietf-mmusic-msid], RID [I-D.ietf-mmusic-rid], BUNDLE
   [I-D.ietf-mmusic-sdp-bundle-negotiation], [RFC5760], [RFC5761], apply
   if selected and thus needs to be considered in the evaluation.

   There is discussion of the security implications of choosing multiple
   SSRC vs
   Multiple multiple RTP session sessions in Section 4.3.

10.  Contributors

   Hui Zheng (Marvin) from Huawei contributed to WG draft versions -04
   and -05 of the document.

11.  References

10.1.

11.1.  Normative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <https://www.rfc-editor.org/info/rfc7656>.

10.2.

11.2.  Informative References

   [ALF]      Clark, D. and D. Tennenhouse, "Architectural
              Considerations for a New Generation of Protocols", SIGCOMM
              Symposium on Communications Architectures and
              Protocols (Philadelphia, Pennsylvania), pp. 200--208, IEEE
              Computer Communications Review, Vol. 20(4), September
              1990.

   [I-D.ietf-avt-srtp-ekt]
              Wing, D., McGrew, D., and K. Fischer, "Encrypted Key
              Transport for Secure RTP", draft-ietf-avt-srtp-ekt-03
              (work in progress), October 2011.

   [I-D.ietf-avtcore-multi-media-rtp-session]
              Westerlund, M., Perkins, C., and J. Lennox, "Sending
              Multiple Types of Media in a Single RTP Session", draft-
              ietf-avtcore-multi-media-rtp-session-13 (work in
              progress), December 2015.

   [I-D.ietf-avtext-rid]
              Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
              Identifier Source Description (SDES)", draft-ietf-avtext-
              rid-09 (work in progress), October 2016.

   [I-D.ietf-mmusic-msid]
              Alvestrand, H., "WebRTC MediaStream Identification in the
              Session Description Protocol", draft-ietf-mmusic-msid-16
              (work in progress), February 2017.

   [I-D.ietf-mmusic-rid]
              Thatcher, P., Zanaty, M., Nandakumar, S., Burman, B.,
              Roach, A., and B. Campen, "RTP Payload Format Restrictions", draft-ietf-mmusic-rid-11 draft-ietf-
              mmusic-rid-15 (work in progress), July 2017. May 2018.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-39
              negotiation-52 (work in progress), August 2017.

   [I-D.lennox-mmusic-sdp-source-selection]
              Lennox, J. May 2018.

   [I-D.ietf-mmusic-sdp-simulcast]
              Burman, B., Westerlund, M., Nandakumar, S., and H. Schulzrinne, "Mechanisms for Media
              Source Selection M. Zanaty,
              "Using Simulcast in the Session Description Protocol
              (SDP)", draft-lennox-mmusic-sdp-source-selection-05 SDP and RTP Sessions", draft-ietf-
              mmusic-sdp-simulcast-13 (work in progress), October 2012. June 2018.

   [I-D.ietf-perc-srtp-ekt-diet]
              Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", draft-ietf-perc-srtp-ekt-diet-07 (work in progress),
              March 2018.

   [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,
              DOI 10.17487/RFC2198, September 1997,
              <https://www.rfc-editor.org/info/rfc2198>.

   [RFC2205]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, DOI 10.17487/RFC2205,
              September 1997, <https://www.rfc-editor.org/info/rfc2205>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
              October 2000, <https://www.rfc-editor.org/info/rfc2974>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3389]  Zopf, R., "Real-time Transport Protocol (RTP) Payload for
              Comfort Noise (CN)", RFC 3389, DOI 10.17487/RFC3389,
              September 2002, <https://www.rfc-editor.org/info/rfc3389>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <https://www.rfc-editor.org/info/rfc3551>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              DOI 10.17487/RFC3830, August 2004,
              <https://www.rfc-editor.org/info/rfc3830>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              DOI 10.17487/RFC4383, February 2006,
              <https://www.rfc-editor.org/info/rfc4383>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <https://www.rfc-editor.org/info/rfc4568>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4568, 4585,
              DOI 10.17487/RFC4568, 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4568>.
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <https://www.rfc-editor.org/info/rfc4588>.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
              February 2008, <https://www.rfc-editor.org/info/rfc5104>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <https://www.rfc-editor.org/info/rfc5109>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <https://www.rfc-editor.org/info/rfc5245>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <https://www.rfc-editor.org/info/rfc5576>.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760,
              DOI 10.17487/RFC5760, February 2010,
              <https://www.rfc-editor.org/info/rfc5760>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <https://www.rfc-editor.org/info/rfc5888>.

   [RFC6190]  Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
              "RTP Payload Format for Scalable Video Coding", RFC 6190,
              DOI 10.17487/RFC6190, May 2011,
              <https://www.rfc-editor.org/info/rfc6190>.

   [RFC6465]  Ivov, E., Ed., Marocco, E., Ed., and J. Lennox, "A Real-
              time Transport Protocol (RTP) Header Extension for Mixer-
              to-Client Audio Level Indication", RFC 6465,
              DOI 10.17487/RFC6465, December 2011,
              <https://www.rfc-editor.org/info/rfc6465>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <https://www.rfc-editor.org/info/rfc7201>.

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,
              <https://www.rfc-editor.org/info/rfc7657>.

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,
              <https://www.rfc-editor.org/info/rfc7667>.

   [RFC7826]  Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, Ed., "Real-Time Streaming Protocol
              Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
              2016, <https://www.rfc-editor.org/info/rfc7826>.

   [RFC8088]  Westerlund, M., "How to Write an RTP Payload Format",
              RFC 8088, DOI 10.17487/RFC8088, May 2017,
              <https://www.rfc-editor.org/info/rfc8088>.

   [RFC8108]  Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
              "Sending Multiple RTP Streams in a Single RTP Session",
              RFC 8108, DOI 10.17487/RFC8108, March 2017,
              <https://www.rfc-editor.org/info/rfc8108>.

Appendix A.  Dismissing Payload Type Multiplexing

   This section documents a number of reasons why using the payload type
   as a multiplexing point is unsuitable for most things related to
   multiple streams
   is unsuitable. RTP streams.  If one attempts to use Payload type
   multiplexing beyond it's its defined usage, that has well known negative
   effects on RTP.  To use Payload payload type as the single discriminator for
   multiple streams implies that all the different media RTP streams are being
   sent with the same SSRC, thus using the same timestamp and sequence
   number space.  This has many effects:

   1.   Putting restraint on RTP timestamp rate for the multiplexed
        media.  For example, media RTP streams that use different RTP
        timestamp rates cannot be combined, as the timestamp values need
        to be consistent across all multiplexed media frames.  Thus
        streams are forced to use the same RTP timestamp rate.  When
        this is not possible, Payload Type payload type multiplexing cannot be used.

   2.   Many RTP payload formats can fragment a media object over
        multiple RTP packets, like parts of a video frame.  These
        payload formats need to determine the order of the fragments to
        correctly decode them.  Thus  Thus, it is important to ensure that all
        fragments related to a frame or a similar media object are
        transmitted in sequence and without interruptions within the
        object.  This can relatively simple be solved on the sender side
        by ensuring that the fragments of each media RTP stream are sent in
        sequence.

   3.   Some media formats require uninterrupted sequence number space
        between media parts.  These are media formats where any missing
        RTP sequence number will result in decoding failure or invoking
        of
        a repair mechanism within a single media context.  The text/
        T140 payload format [RFC4103] is an example of such a format.
        These formats will need a sequence numbering abstraction
        function between RTP and the individual media RTP stream before being
        used with Payload Type payload type multiplexing.

   4.   Sending multiple streams in the same sequence number space makes
        it impossible to determine which Payload Type and thus payload type, which stream a
        packet loss relates to. to, and thus to which stream to potentially
        apply packet loss concealment or other stream-specific loss
        mitigation mechanisms.

   5.   If RTP Retransmission [RFC4588] is used and there is a loss, it
        is possible to ask for the missing packet(s) by SSRC and
        sequence number, not by Payload Type. payload type.  If only some of the
        Payload Type
        payload type multiplexed streams are of interest, there is no
        way of telling which missing packet(s) belong to the interesting
        stream(s) and all lost packets need be requested, wasting
        bandwidth.

   6.   The current RTCP feedback mechanisms are built around providing
        feedback on media RTP streams based on stream ID (SSRC), packet
        (sequence numbers) and time interval (RTP Timestamps).  There is
        almost never a field to indicate which Payload Type payload type is reported,
        so sending feedback for a specific media stream RTP payload type is difficult
        without extending existing RTCP reporting.

   7.   The current RTCP media control messages [RFC5104] specification
        is oriented around controlling particular media flows, i.e.
        requests are done addressing a particular SSRC.  Such mechanisms
        would need to be redefined to support Payload Type payload type multiplexing.

   8.   The number of payload types are inherently limited.
        Accordingly, using Payload Type payload type multiplexing limits the number
        of streams that can be multiplexed and does not scale.  This
        limitation is exacerbated if one uses solutions like RTP and
        RTCP multiplexing [RFC5761] where a number of payload types are
        blocked due to the overlap between RTP and RTCP.

   9.   At times, there is a need to group multiplexed streams and this
        is currently possible for RTP Sessions sessions and for SSRC, but there
        is no defined way to group Payload Types. payload types.

   10.  It is currently not possible to signal bandwidth requirements
        per media RTP stream when using Payload Type Multiplexing. payload type multiplexing.

   11.  Most existing SDP media level attributes cannot be applied on a
        per Payload Type payload type level and would require re-definition in that
        context.

   12.  A legacy endpoint that does not understand the indication that
        different RTP payload types are different media RTP streams might be
        slightly confused by the large amount of possibly overlapping or
        identically defined RTP Payload Types. payload types.

Appendix B.  Signalling considerations Considerations

   Signalling is not an architectural consideration for RTP itself, so
   this discussion has been moved to an appendix.  However, it is hugely
   important for anyone building complete applications, so it is
   deserving of discussion.

   The issues raised here need to be addressed in the WGs that deal with
   signalling; they cannot be addressed by tweaking, extending or
   profiling RTP.

B.1.  Signalling Aspects

   There exist various signalling solutions for establishing RTP
   sessions.  Many are SDP [RFC4566] based, however SDP functionality is
   also dependent on the signalling protocols carrying the SDP.  Where  RTSP
   [RFC7826] and SAP [RFC2974] both use SDP in a declarative fashion,
   while SIP [RFC3261] uses SDP with the additional definition of Offer/Answer Offer/
   Answer [RFC3264].  The impact on signalling and especially SDP needs
   to be considered as it can greatly affect how to deploy a certain
   multiplexing point choice.

B.1.1.

B.1.  Session Oriented Properties

   One aspect of the existing signalling is that it is focused around
   RTP sessions, or at least in the case of SDP the media description.
   There are a number of things that are signalled on a session level/ media description
   level but those are not necessarily strictly bound to an RTP session
   and could be of interest to signal specifically for a particular media RTP
   stream (SSRC) within the session.  The following properties have been
   identified as being potentially useful to signal not only on RTP
   session level:

   o  Bitrate/Bandwidth exist today only at aggregate or as a common any
      media stream
      "any RTP stream" limit, unless either codec-specific bandwidth
      limiting or RTCP signalling using TMMBR is used.

   o  Which SSRC that will use which RTP Payload Types payload types (this will be
      visible from the first media packet, but is sometimes useful to
      know before packet arrival).

   Some of these issues are clearly SDP's problem rather than RTP
   limitations.  However, if the aim is to deploy an solution using
   additional SSRCs that contains several sets of media RTP streams with
   different properties (encoding/packetization parameter, bit-rate,
   etc.), putting each set in a different RTP session would directly
   enable negotiation of the parameters for each set.  If insisting on
   additional SSRC only, a number of signalling extensions are needed to
   clarify that there are multiple sets of media RTP streams with different
   properties and that they need in fact be kept different, since a
   single set will not satisfy the application's requirements.

   For some parameters, such as RTP payload type, resolution and
   framerate, a SSRC-linked mechanism has been proposed:
   [I-D.lennox-mmusic-sdp-source-selection].

B.1.2. proposed in
   [I-D.ietf-mmusic-rid]

B.2.  SDP Prevents Multiple Media Types

   SDP chose to use the m= line both to delineate an RTP session and to
   specify the top level of the MIME media type; audio, video, text,
   image, application.  This media type is used as the top-level media
   type for identifying the actual payload format and is bound to a
   particular payload type using the rtpmap attribute.  This binding has
   to be loosened in order to use SDP to describe RTP sessions
   containing multiple MIME top level types.

   There is an accepted WG item in the MMUSIC WG to define

   [I-D.ietf-mmusic-sdp-bundle-negotiation] describes how to let
   multiple SDP media lines describe descriptions use a single underlying transport
   [I-D.ietf-mmusic-sdp-bundle-negotiation] and thus it becomes possible in SDP
   SDP, which allows to define one RTP session with media types having
   different MIME top level types.

B.1.3.

B.3.  Signalling Media Stream RTP stream Usage

   Media

   RTP streams being transported in RTP has some particular usage in an
   RTP application.  This usage of the media RTP stream is in many
   applications so far implicitly signalled.  For example, an
   application might choose to take all incoming audio RTP streams, mix
   them and play them out.  However, in more advanced applications that
   use multiple media RTP streams there will be more than a single usage or
   purpose among the set of media RTP streams being sent or received.  RTP
   applications will need to signal this usage somehow.  The signalling
   used will have to identify the media RTP streams affected by their RTP-
   level identifiers, which means that they have to be identified either
   by their session or by their SSRC + session.

   In some applications, the receiver cannot utilise the media RTP stream at
   all before it has received the signalling message describing the
   media RTP
   stream and its usage.  In other applications, there exists a default
   handling that is appropriate.

   If all media RTP streams in an RTP session are to be treated in the same
   way, identifying the session is enough.  If SSRCs in a session are to
   be treated differently, signalling needs to identify both the session
   and the SSRC.

   If this signalling affects how any RTP central node, like an RTP
   mixer or translator that selects, mixes or processes streams, treats
   the streams, the node will also need to receive the same signalling
   to know how to treat media RTP streams with different usage in the right
   fashion.

Authors' Addresses

   Magnus Westerlund
   Ericsson
   Torshamsgatan 23
   SE-164 80 Kista
   Sweden

   Phone: +46 10 714 82 87
   Email: magnus.westerlund@ericsson.com

   Bo Burman
   Ericsson
   Farogatan 6
   Gronlandsgatan 31
   SE-164 80 Kista
   Sweden

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

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow G12 8QQ
   United Kingdom

   Email: csp@csperkins.org
   Harald Tveit Alvestrand
   Google
   Kungsbron 2
   Stockholm 11122
   Sweden

   Email: harald@alvestrand.no

   Roni Even
   Huawei

   Email: roni.even@huawei.com

   Hui Zheng
   Huawei

   Email: marvin.zhenghui@huawei.com