´╗┐

Internet Engineering Task Force (IETF)                     M. Westerlund
Request for Comments: 8872                                     B. Burman
Category: Informational                                         Ericsson
ISSN: 2070-1721                                               C. Perkins
                                                   University of Glasgow
                                                           H. Alvestrand
                                                                  Google
                                                                 R. Even
                                                          September 2020

    Guidelines for Using the Multiplexing Features of RTP to Support
                         Multiple Media Streams

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

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8872.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Definitions
     2.1.  Terminology
     2.2.  Focus of This Document
   3.  RTP Multiplexing Overview
     3.1.  Reasons for Multiplexing and Grouping RTP Streams
     3.2.  RTP Multiplexing Points
       3.2.1.  RTP Session
       3.2.2.  Synchronization Source (SSRC)
       3.2.3.  Contributing Source (CSRC)
       3.2.4.  RTP Payload Type
     3.3.  Issues Related to RTP Topologies
     3.4.  Issues Related to RTP and RTCP
       3.4.1.  The RTP Specification
       3.4.2.  Multiple SSRCs in a Session
       3.4.3.  Binding Related Sources
       3.4.4.  Forward Error Correction
   4.  Considerations for RTP Multiplexing
     4.1.  Interworking Considerations
       4.1.1.  Application Interworking
       4.1.2.  RTP Translator Interworking
       4.1.3.  Gateway Interworking
       4.1.4.  Legacy Considerations for Multiple SSRCs
     4.2.  Network Considerations
       4.2.1.  Quality of Service
       4.2.2.  NAT and Firewall Traversal
       4.2.3.  Multicast
     4.3.  Security and Key-Management Considerations
       4.3.1.  Security Context Scope
       4.3.2.  Key Management for Multi-party Sessions
       4.3.3.  Complexity Implications
   5.  RTP Multiplexing Design Choices
     5.1.  Multiple Media Types in One Session
     5.2.  Multiple SSRCs of the Same Media Type
     5.3.  Multiple Sessions for One Media Type
     5.4.  Single SSRC per Endpoint
     5.5.  Summary
   6.  Guidelines
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Dismissing Payload Type Multiplexing
   Appendix B.  Signaling Considerations
     B.1.  Session-Oriented Properties
     B.2.  SDP Prevents Multiple Media Types
     B.3.  Signaling RTP Stream Usage
   Acknowledgments
   Contributors
   Authors' Addresses

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.  From the beginning, RTP was designed for multiple
   participants in a communication session.  It supports many topology
   paradigms and usages, as defined in [RFC7667].  RTP has several
   multiplexing points designed for different purposes; these points
   enable support of multiple RTP streams and switching between
   different encoding or packetization techniques for the media.  By
   using multiple RTP sessions, sets of RTP streams can be structured
   for efficient processing or identification.  Thus, to meet an
   application's needs, an RTP application designer needs to understand
   how best to use the RTP session, the RTP stream identifier
   (synchronization source (SSRC)), and the RTP payload type.

   There has been increased interest in more-advanced usage of RTP.  For
   example, multiple RTP streams can be used when a single endpoint has
   multiple media sources (like multiple cameras or microphones) from
   which streams of media need to be sent simultaneously.  Consequently,
   questions are raised regarding the most appropriate RTP usage.  The
   limitations in some implementations, RTP/RTCP extensions, and
   signaling have also been exposed.  This document aims to clarify the
   usefulness of some functionalities in RTP that, hopefully, will
   result in future implementations that are more complete.

   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 arising
   from a particular usage of the RTP multiplexing points.  This
   document provides some guidelines and recommends against some usages
   as being unsuitable, in general or for particular purposes.

   This document starts with some definitions and then goes into
   existing RTP functionalities around multiplexing.  Both the desired
   behavior and the implications of a particular behavior depend on
   which topologies are used; therefore, this topic requires some
   consideration.  We then discuss some choices regarding multiplexing
   behavior and the impacts of those choices.  Some designs of RTP usage
   are also discussed.  Finally, some 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:

   Multi-party:
      Communication that includes multiple endpoints.  In this document,
      "multi-party" will be used to refer to scenarios where more than
      two endpoints communicate.

   Multiplexing:
      An operation that takes 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 (demultiplexed) again.

   RTP Receiver:
      An endpoint or middlebox receiving RTP streams and RTCP messages.
      It uses at least one SSRC to send RTCP messages.  An RTP receiver
      may also be an RTP sender.

   RTP Sender:
      An endpoint sending one or more RTP streams but also sending RTCP
      messages.

   RTP Session Group:
      One or more RTP sessions that are used together to perform some
      function.  Examples include 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 synchronization contexts that can cross RTP sessions;
      i.e., SSRCs that map to a common CNAME can be assumed to have RTCP
      Sender Report (SR) timing information derived from a common clock
      such that they can be synchronized for playout.

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

      |  Note: The above definitions of "RTP receiver" and "RTP sender"
      |  are consistent with the usage in [RFC3550].

2.2.  Focus of This Document

   This document is focused on issues that affect RTP.  Thus, issues
   that involve signaling protocols -- such as whether SIP [RFC3261],
   Jingle [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
   signaling protocols -- are mentioned only as examples in order to
   describe the RTP issues more precisely.

   This document assumes that the applications will use RTCP.  While
   there are 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 RTP.

3.  RTP Multiplexing Overview

3.1.  Reasons for Multiplexing and Grouping RTP Streams

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

   *  There might be multiple media sources.

   *  Multiple RTP streams might be needed to represent one media
      source, for example:

      -  To carry different layers of a scalable encoding of a media
         source

      -  Alternative encodings during simulcast, using different codecs
         for the same audio stream

      -  Alternative formats during simulcast, multiple resolutions of
         the same video stream

   *  A retransmission stream might repeat some parts of the content of
      another RTP stream.

   *  A Forward Error Correction (FEC) stream might provide material
      that can be used to repair another RTP stream.

   For each of these reasons, it is necessary to decide whether each
   additional RTP stream is sent within the same RTP session as the
   other RTP streams or it is necessary to use additional RTP sessions
   to group the RTP streams.  For a combination of reasons, the suitable
   choice for one situation might not be the suitable choice for another
   situation.  The choice is easiest when multiplexing multiple media
   sources of the same media type.  However, all reasons warrant
   discussion and clarification regarding how to deal with them.  As the
   discussion below will show, a single solution does not suit all
   purposes.  To utilize RTP well and as efficiently as possible, both
   are needed.  The real issue is knowing when to create multiple RTP
   sessions versus when to send multiple RTP streams in a single RTP
   session.

3.2.  RTP Multiplexing Points

   This section describes the multiplexing points present in RTP that
   can be used to distinguish RTP streams and groups of RTP streams.
   Figure 1 outlines the process of demultiplexing incoming RTP streams,
   starting with one or more sockets representing the reception of one
   or more transport flows, e.g., based on the UDP destination port.  It
   also demultiplexes RTP/RTCP from any other protocols, such as Session
   Traversal Utilities for NAT (STUN) [RFC5389] and DTLS-SRTP [RFC5764]
   on the same transport as described in [RFC7983].  The Processing and
   Buffering (PB) step in Figure 1 terminates RTP/RTCP and prepares the
   RTP payload for input to the decoder.

                      |   |   |
                      |   |   | packets
           +--        v   v   v
           |        +------------+
           |        |  Socket(s) |   Transport Protocol Demultiplexing
           |        +------------+
           |            ||  ||
      RTP  |       RTP/ ||  |+-----> DTLS (SRTP keying, SCTP, etc.)
   Session |       RTCP ||  +------> STUN (multiplexed using same port)
           +--          ||
           +--          ||
           |      ++(split by SSRC)-++---> Identify SSRC collision
           |      ||    ||    ||    ||
           | (associate with signaling by MID/RID)
           |      vv    vv    vv    vv
     RTP   |     +--+  +--+  +--+  +--+ Jitter buffer,
   Streams |     |PB|  |PB|  |PB|  |PB| process RTCP, etc.
           |     +--+  +--+  +--+  +--+
           +--     |    |      |    |
             (select decoder based on payload type (PT))
           +--     |   /       |  /
           |       +-----+     | /
           |         /   |     |/
   Payload |        v    v     v
   Formats |     +---+ +---+ +---+
           |     |Dec| |Dec| |Dec| Decoders
           |     +---+ +---+ +---+
           +--

                    Figure 1: RTP Demultiplexing Process

3.2.1.  RTP Session

   An RTP session is the highest semantic layer in RTP and represents an
   association between a group of communicating endpoints.  RTP does not
   contain a session identifier, yet different RTP sessions must be
   possible to identify both across a set of different endpoints and
   from the perspective of 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 SSRC
   space [RFC3550].  That is, if a group of participants are each aware
   of the SSRC identifiers belonging to the other participants, then
   those participants are in a single RTP session.  A participant can
   become aware of an SSRC identifier by receiving an RTP packet
   containing the identifier in the SSRC field or contributing source
   (CSRC) list, by receiving an RTCP packet listing it in an SSRC field,
   or through signaling (e.g., the Session Description Protocol (SDP)
   [RFC4566] "a=ssrc:" 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 signaling.

   For RTP session separation within a single endpoint, RTP relies on
   the underlying transport layer and the signaling 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, and a single RTP session can span multiple transport-layer
   flows even if all endpoints use a single transport-layer flow per
   endpoint for that RTP session.  The signaling layer might give RTP
   sessions an explicit identifier, or the identification might be
   implicit based on the addresses and ports used.  Accordingly, a
   single RTP session can have multiple associated identifiers, explicit
   and implicit, belonging to different contexts.  For example, when
   running RTP on top of UDP/IP, an endpoint can identify and delimit an
   RTP session from other RTP sessions by their UDP source and
   destination IP addresses and their UDP port numbers.  A single RTP
   session can be using multiple IP/UDP flows for receiving and/or
   sending RTP packets to other endpoints or middleboxes, even if the
   endpoint does not have multiple IP addresses.  Using multiple IP
   addresses only makes it more likely that multiple IP/UDP flows will
   be required.  Another example is SDP media descriptions (the "m="
   line and the subsequent associated lines) that signal the transport
   flow and RTP session configuration for the endpoint's part of the RTP
   session.  The SDP grouping framework [RFC5888] allows labeling of the
   media descriptions to be used so that RTP Session Groups can be
   created.  Through the use of "Negotiating Media Multiplexing Using
   the Session Description Protocol (SDP)" [RFC8843], multiple media
   descriptions become part of a common RTP session where each media
   description represents the RTP streams sent or received for a media
   source.

   RTP makes no normative statements about the relationship between
   different RTP sessions; however, applications that use more than one
   RTP session need to understand how the different RTP sessions that
   are created
   they create relate to one another.

3.2.2.  Synchronization Source (SSRC)

   An SSRC identifies a source of an RTP stream, or an RTP receiver when
   sending RTCP.  Every endpoint has at least one SSRC identifier, even
   if it does not send RTP packets.  RTP endpoints that are only RTP
   receivers still send RTCP and use their SSRC identifiers in the RTCP
   packets they send.  An endpoint can have multiple SSRC identifiers if
   it sends multiple RTP streams.  Endpoints that function as both RTP
   sender and RTP receiver use the same SSRC(s) in both roles.

   The SSRC is a 32-bit 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 signaling.  Unless presignaled, e.g., using
   the SDP "a=ssrc:" attribute [RFC5576], the 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.  SSRC collisions can
   occur and are handled as specified in [RFC3550] and [RFC5576],
   resulting in the SSRC of the colliding RTP streams or receivers
   changing.  An endpoint that changes its network transport address
   during a session has to choose a new SSRC identifier to avoid being
   interpreted as a looped source, unless a mechanism providing a
   virtual transport (such as Interactive Connectivity Establishment
   (ICE) [RFC8445]) abstracts the changes.

   SSRC identifiers that belong to the same synchronization context
   (i.e., that represent RTP streams that can be synchronized using
   information in RTCP SR packets) use identical CNAME chunks in
   corresponding RTCP source description (SDES) packets.  SDP signaling
   can also be used to provide explicit SSRC grouping [RFC5576].

   In some cases, the same SSRC identifier value is used to relate
   streams in two different RTP sessions, such as in RTP retransmission
   [RFC4588].  This is to be avoided, since there is no guarantee that
   SSRC values are unique across RTP sessions.  In the case of RTP
   retransmission [RFC4588], it is recommended to use explicit binding
   of the source RTP stream and the redundancy stream, e.g., using the
   RepairedRtpStreamId RTCP SDES item [RFC8852].  The
   RepairedRtpStreamId is a rather recent mechanism, so one cannot
   expect older applications to follow this recommendation.

   Note that the RTP sequence number and RTP timestamp are scoped by the
   SSRC and are thus specific per RTP stream.

   Different types of entities use an SSRC to identify themselves, as
   follows:

   *  A real media source uses the SSRC to identify a "physical" media
      source.

   *  A conceptual media source uses the SSRC to identify the result of
      applying some filtering function in a 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.

   *  An RTP receiver uses the SSRC to identify itself as the source of
      its RTCP reports.

   An endpoint that generates more than one media type, e.g., a
   conference participant sending both audio and video, need not (and,
   indeed, should not) use the same SSRC value across RTP sessions.
   Using RTCP compound packets containing the CNAME SDES item is the
   designated method for binding 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 synchronization
   context and can be synchronized 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 its SSRC number due to an SSRC
   conflict.  Using the media identification (MID) extension [RFC8843]
   helps to identify which media source the new SSRC represents, and
   using the restriction identifier (RID) extension [RFC8851] helps to
   identify what encoding or redundancy stream it represents, even
   though the SSRC changed.  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, making the
   new SSRC a new source.

3.2.3.  Contributing Source (CSRC)

   The CSRC is not a separate identifier.  Rather, an SSRC identifier is
   listed as a CSRC in the RTP header of a packet generated by an RTP
   mixer or video Multipoint Control Unit (MCU) / switch, if the
   corresponding SSRC was in the header of one of the packets that
   contributed to the output.

   It is not possible, in general, to extract media represented by an
   individual CSRC, since it is typically the result of a media merge
   (e.g., mix) operation on the individual media streams corresponding
   to the CSRC identifiers.  The exception is the case where only a
   single CSRC is indicated, as this represents the forwarding of an RTP
   stream that might have been modified.  The RTP header extension ("A
   Real-time Transport Protocol (RTP) Header Extension for
   Mixer-to-Client Audio Level Indication" [RFC6465]) expands on the
   receiver's information about a packet with a CSRC list.  Due to these
   restrictions, a 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 RTP stream utilizes 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 is
   identified by the payload type (PT) field in the RTP packet header.
   The combination of SSRC and PT therefore identifies a specific RTP
   stream in a specific encoding format.  The format definition can be
   taken from [RFC3551] for statically allocated payload types but ought
   to be explicitly defined in signaling, such as SDP, for both static
   and dynamic payload types.  The term "format" here includes those
   aspects described by out-of-band signaling 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.  PT has the same meaning across all RTP streams in an RTP
   session.  All SSRCs sent from a single endpoint share the same
   payload type definitions.  The RTP payload type is designed such that
   only a single payload type is valid at any instant in time in the RTP
   stream's timestamp timeline, effectively time-multiplexing different
   payload types if any change occurs.  The payload type 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 for the same SSRC that are valid for the
   same instant, then redundant encodings [RFC2198] can be used.
   Several additional constraints, other than those mentioned above,
   need to be met to enable this usage, one of which is that the
   combined payload sizes of the different payload types ought not
   exceed the transport MTU.

   Other aspects of using the RTP payload format are described in "How
   to Write an RTP Payload 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 consume and decode an RTP stream.
   The RTP payload type number is sometimes used to associate an RTP
   stream with the signaling, which in general requires that unique RTP
   payload type numbers be used in each context.  Using MID, e.g., when
   bundling "m=" sections [RFC8843], can replace the payload type as a
   signaling association, and unique RTP payload types are then no
   longer required for that purpose.

3.3.  Issues Related to RTP Topologies

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

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

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

   *  Do I need network differentiation in the form of QoS
      (Section 4.2.1)?

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

   *  Do I need to use additional RTP streams for RTP retransmission or
      FEC?

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

   Sometimes, an RTP communication session can end up in a situation
   where the communicating peers are not compatible, for various
   reasons:

   *  No common media codec for a media type, thus requiring
      transcoding.

   *  Different support for multiple RTP streams and RTP sessions.

   *  Usage of different media transport protocols (i.e., one peer uses
      RTP, but the other peer uses a different transport protocol).

   *  Usage of different transport protocols, e.g., UDP, the Datagram
      Congestion Control Protocol (DCCP), or TCP.

   *  Different security solutions (e.g., IPsec, TLS, DTLS, or the
      Secure Real-time Transport Protocol (SRTP)) with different keying
      mechanisms.

   These compatibility issues can often be resolved by the inclusion of
   a translator between the two peers -- the Topo-PtP-Translator, as
   described in [RFC7667].  The translator's main purpose is to make the
   peers look compatible to each other.  There can also be reasons other
   than compatibility for inserting a translator in the form of a
   middlebox or gateway -- for example, a need to monitor the RTP
   streams.  Beware that changing the stream transport characteristics
   in the translator can require a thorough understanding of aspects
   ranging from congestion control and media-level adaptations to
   application-layer semantics.

   Within the uses enabled by the RTP standard, the point-to-point
   topology can contain one or more RTP sessions with one or more media
   sources per session, each having one or more RTP streams per media
   source.

3.4.  Issues Related to RTP and RTCP

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

3.4.1.  The RTP Specification

   RFC 3550 contains some recommendations and a numbered list
   (Section 5.2 of [RFC3550]) of five arguments regarding different
   aspects of RTP multiplexing.  Please review Section 5.2 of [RFC3550].
   Five important aspects are quoted below.

   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.

       This argument advocates the use of different SSRCs for each
       individual RTP stream, as this is fundamental to RTP operation.

   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.

       This argument advocates against demultiplexing RTP streams within
       a session based only on their RTP payload type numbers; it still
       stands, as can be seen by the extensive list of issues discussed
       in Appendix A.

   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.

       This argument is yet another argument against payload type
       multiplexing.

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

       This argument advocates against multiplexing RTP packets that
       require different handling into the same session.  In most cases,
       the RTP mixer must embed application logic to handle streams; 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.  One type of
       application that can mix different media sources blindly is the
       audio-only telephone bridge, although the ability to do that
       comes from the well-defined scenario that is aided by the use of
       a single media type, even though individual streams may use
       incompatible codec types; most other types of applications need
       application-specific logic to perform the mix correctly.

   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.

       This argument discusses network aspects that are described in
       Section 4.2.  It also goes into aspects of implementation, like
       split component terminals (see Section 3.10 of [RFC7667]) --
       endpoints where different processes or interconnected devices
       handle different aspects of the whole multimedia session.

   To summarize, RFC 3550's view on multiplexing is to use unique SSRCs
   for anything that is its own media/packet stream and 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 latter
   needs further discussion, as imposing a single solution on all usages
   of RTP is inappropriate.  "Sending Multiple Types of Media in a
   Single RTP Session" [RFC8860] updates RFC 3550 to allow multiple
   media types in an RTP session and provides a detailed analysis of the
   potential benefits and issues related to having multiple media types
   in the same RTP session.  Thus, [RFC8860] provides a wider 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 requires that
   some unclear aspects of the RTP specification be resolved.  These
   items could potentially lead to some interoperability issues as well
   as some potential significant inefficiencies, as further discussed in
   "Sending Multiple RTP Streams in a Single RTP Session" [RFC8108].  An
   RTP application designer should consider these issues and the
   application's possible impact caused by a lack of appropriate RTP
   handling or optimization in the peer endpoints.

   Using multiple RTP sessions can potentially mitigate 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 streams together.  This issue is common to both
   using additional SSRCs and multiple RTP sessions.

   The solutions can be divided into a few groups:

   *  RTP/RTCP based

   *  Signaling based, e.g., SDP

   *  Grouping related RTP sessions

   *  Grouping SSRCs within an RTP session

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

   The SDP-based signaling solutions are:

   SDP media description grouping:
      The SDP grouping framework [RFC5888] uses various semantics to
      group any number of media descriptions.  SDP media description
      grouping has primarily been used to group RTP sessions, but in
      combination with [RFC8843], it can also group multiple media
      descriptions within a single RTP session.

   SDP media multiplexing:
      "Negotiating Media Multiplexing Using the Session Description
      Protocol (SDP)" [RFC8843] uses information taken from both SDP and
      RTCP to associate RTP streams to SDP media descriptions.  This
      allows both SDP and RTCP to group RTP streams belonging to an SDP
      media description and group multiple SDP media descriptions into a
      single RTP session.

   SDP SSRC grouping:
      "Source-Specific Media Attributes in the Session Description
      Protocol (SDP)" [RFC5576] includes a solution for grouping SSRCs
      in the same way that the grouping framework groups media
      descriptions.

   The above grouping constructs support many use cases.  Those
   solutions have shortcomings in cases where the session's dynamic
   properties are such that it is difficult or a drain on resources to
   keep the list of related SSRCs up to date.

   One RTP/RTCP-based grouping solution is to use the RTCP SDES CNAME to
   bind related RTP streams to an endpoint or a synchronization context.
   For applications with a single RTP stream per type (media, source, or
   redundancy stream), the CNAME is sufficient for that purpose,
   independent of whether one or more RTP sessions are used.  However,
   some applications choose not to use a CNAME because of perceived
   complexity or a desire not to implement RTCP and instead use the same
   SSRC value to bind related RTP streams across multiple RTP sessions.
   RTP retransmission [RFC4588], when configured to use multiple RTP
   sessions, and generic FEC [RFC5109] both use the CNAME method to
   relate the RTP streams, which may work but might have some downsides
   in RTP sessions with many participating SSRCs.  It is not recommended
   to use identical SSRC values across RTP sessions to relate RTP
   streams; when an SSRC collision occurs, this will force a change of
   that SSRC in all RTP sessions and will thus resynchronize all of the
   streams instead of only the single media stream experiencing the
   collision.

   Another method for implicitly binding SSRCs is used by RTP
   retransmission [RFC4588] when using the same RTP session as the
   source RTP stream for retransmissions.  A receiver that is missing a
   packet issues an RTP retransmission request and then awaits a new
   SSRC carrying the RTP retransmission payload, where that SSRC is from
   the same CNAME.  This limits a requester to having only one
   outstanding retransmission request on any new SSRCs per endpoint.

   "RTP Payload Format Restrictions" [RFC8851] provides an RTP/RTCP-
   based mechanism to unambiguously identify the RTP streams within an
   RTP 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 signaling and having the corresponding RtpStreamId value
   as an SDES item and an RTP header extension. extension [RFC8852].  The RID
   solution also includes a solution for binding redundancy RTP streams
   to their original source RTP streams, given that those streams use
   RID identifiers.  The redundancy stream uses the RepairedRtpStreamId
   SDES item and RTP header extension to declare the RtpStreamId value
   of the source stream to create the binding.

   Experience has shown that an explicit binding between the RTP
   streams, agnostic of SSRC values, behaves well.  That way, solutions
   using multiple RTP streams in a single RTP session and in multiple
   RTP sessions will use the same type of binding.

3.4.4.  Forward Error Correction

   There exist a number of FEC-based schemes designed to mitigate packet
   loss in the original streams.  Most of the FEC schemes protect a
   single source flow.  This protection is achieved by transmitting a
   certain amount of redundant information that is encoded such that it
   can repair one or more instances of packet loss over the set of
   packets the redundant information protects.  This sequence of
   redundant information needs to be transmitted as its own media stream
   or, in some cases, instead of the original media stream.  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 utilize the FEC information.  By
   placing it in a separate RTP session and if separating RTP sessions
   at the transport level, FEC can easily be ignored at the transport
   level, without considering any RTP-layer information.

   In usages involving multicast, sending FEC information in a separate
   multicast group allows for similar flexibility.  This is especially
   useful when receivers see heterogeneous packet loss rates.  A
   receiver can decide, based on measurement of experienced packet loss
   rates, whether to join a multicast group with suitable FEC data
   repair capabilities.

4.  Considerations for RTP Multiplexing

4.1.  Interworking Considerations

   There are several different kinds of interworking, and this section
   discusses two: interworking directly between different applications
   and the interworking of applications through an RTP translator.  The
   discussion includes the implications of potentially different RTP
   multiplexing point choices and limitations that have to be considered
   when working with some legacy applications.

4.1.1.  Application Interworking

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

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

   There are two fundamental approaches to building a gateway: using RTP
   translator interworking (RTP bridging), where the gateway acts as an
   RTP translator with the two interconnected applications being members
   of the same RTP session; or using gateway interworking
   (Section 4.1.3) with RTP termination, where there are independent RTP
   sessions between each interconnected application and the gateway.

   For interworking to be feasible, any security solution in use needs
   to be compatible and capable of exchanging keys with either the peer
   or the gateway under the trust model being used.  Secondly, the
   applications need to use media streams in a way that makes sense in
   both applications.

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 regarding the number of simultaneous RTP streams
   combined with the same set of RTP/RTCP extensions being supported.
   Unfortunately, this might not always be true.

   When a gateway is implemented via an RTP translator, an important
   consideration is 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 [RFC8843], it may be possible for the RTP translator to map
   the RTP streams between both sides using some method, e.g., based on
   the number and order of SDP "m=" lines from each side.  There are
   also challenges related to SSRC collision handling, since, unless
   SSRC translation is applied on the RTP translator, there may be a
   collision on the SSRC multiplexing side that the RTP session
   multiplexing side will not be aware of.  Furthermore, if one of the
   applications is capable of working in several modes (such as being
   able to use additional RTP streams in one RTP session or 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
   none of the participants are using the less flexible application 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:

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

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

   *  Signaling, choosing, and policing appropriate bitrates for each
      session.

   For applications that use any security mechanism, e.g., in the form
   of SRTP, the gateway needs to be able to decrypt and verify source
   integrity of the incoming packets and then re-encrypt, integrity
   protect, and sign the packets as the peer 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 and thus
   a trusted entity.

   The gateway might also need to apply transcoding (for incompatible
   codec types), media-level adaptations that cannot be solved through
   media negotiation (such as rescaling 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 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 and not a commodity product.

   These gateways might therefore potentially block application
   evolution by blocking RTP and RTCP extensions that the applications
   have been extended with but that are unknown to the gateway.

   If one uses a security mechanism like SRTP, the gateway and the
   necessary trust in it by the peers pose an additional risk to
   communication security.  The gateway also incurs additional
   complexities in the form of the decrypt-encrypt cycles needed for
   each forwarded packet.  SRTP, due to its keying structure, also
   requires that each RTP session need different master keys, as the use
   of the same key in two RTP sessions can, for some ciphers, result in
   a reuse of a one-time pad that completely breaks the confidentiality
   of the packets.

4.1.4.  Legacy Considerations for Multiple SSRCs

   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 or
   video).  Even in conferencing applications, especially voice-only,
   the conference focus or bridge provides to each participant a single
   stream containing a mix of the other participants.  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.

   Applications and systems that aren't updated to handle multiple
   streams following these recommendations can have issues with
   participating in RTP sessions containing multiple SSRCs within a
   single session, such as:

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

   2.  Being capable of decoding multiple streams simultaneously.

   3.  Being capable of rendering multiple streams simultaneously.

   This indicates that gateways attempting to interconnect to this class
   of devices have to make sure that only one RTP stream of each media
   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 implementer needs to consider that the RTP multiplexing
   choice also impacts network-level mechanisms.

4.2.1.  Quality of Service

   QoS mechanisms are either flow based or packet marking based.  RSVP
   [RFC2205] is an example of a flow-based mechanism, while Diffserv
   [RFC2474] is an example of a packet-marking-based mechanism.

   For a flow-based scheme, additional SSRCs will receive the same QoS
   as all other 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.

   For a packet-marking-based scheme, the method of multiplexing will
   not affect the possibility of using QoS.  Different Differentiated
   Services Code Points (DSCPs) can be assigned to different packets
   within a transport flow (5-tuple) as well as within an RTP stream,
   assuming the usage of UDP or other transport protocols that do not
   have issues with packet reordering within the transport flow
   (5-tuple).  To avoid packet-reordering issues, packets belonging to
   the same RTP flow should limit their use of DSCPs to packets whose
   corresponding Per-Hop Behavior (PHB) do not enable reordering.  If
   the transport protocol being used assumes in-order delivery of
   packets (e.g., TCP and the Stream Control Transmission Protocol
   (SCTP)), then a single DSCP should be used.  For more discussion on
   this topic, see [RFC7657].

   The method for assigning marking to 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
   streams.  The network API on the endpoint also needs to be capable of
   setting the marking on a per-packet basis to reach full
   functionality.

4.2.2.  NAT and Firewall Traversal

   In today's networks, there exist a large number of middleboxes.
   Those that normally have the most impact on RTP are Network Address
   Translators (NATs) and Firewalls (FWs).

   Below, we analyze and comment on the impact of requiring more
   underlying transport flows in the presence of NATs and FWs:

   Endpoint 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 a 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 the
      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 is the maximum number of
      simultaneous RTP sessions a single endpoint can use.  In practice,
      implementations 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, the most limited resource is
      memory or processing.  For large-scale NATs serving many internal
      endpoints, available external ports are likely the scarce
      resource.  Port limitations are primarily a problem for larger
      centralized 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, as a comparison, a real-time video conference session
      with audio and video likely uses less than 10 UDP flows, compared
      to certain web applications that can use 100+ TCP flows to various
      servers from a single browser instance.

   Extra Delay Added by NAT Traversal:
      Performing the NAT/FW traversal takes a certain amount of time for
      each flow.  The best-case scenario for additional NAT/FW traversal
      time after finding the first valid candidate pair following the
      specified ICE procedures is 1.5*RTT + Ta*(Additional_Flows-1),
      where Ta is the pacing timer.  That assumes a message in one
      direction, immediately followed by a return message in the
      opposite direction to confirm reachability.  It isn't more,
      because 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 extra time is needed to establish the additional
      flows (two or three, in most cases) in parallel.  However, packet
      loss causes extra delays of at least 500 ms (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 will
      succeed but one or more of the additional flows will fail.  The
      risk of this happening is hard to quantify but should be fairly
      low, as one flow from the same interfaces has just been
      successfully established.  Thus, only such rare events as NAT
      resource overload, selecting particular port numbers that are
      filtered, etc., ought to be reasons for failure.

   Deep Packet Inspection and Multiple Streams:
      FWs differ in how deeply they inspect packets.  Previous
      experience using FWs and Session Border Gateways (SBGs) with RTP
      shows that there is a significant risk that the FWs and SBGs will
      reject RTP sessions that use multiple SSRCs.

   Using additional RTP streams in the same RTP session and transport
   flow does not introduce any additional NAT traversal complexities per
   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 demultiplexing layer is added between RTP and the
   transport protocol.  At the time of this writing, no such mechanism
   was defined.

4.2.3.  Multicast

   Multicast groups provide a powerful tool for a number of real-time
   applications, especially those that desire broadcast-like behaviors
   with one endpoint transmitting to a large number of receivers, like
   in IPTV.  An RTP/RTCP extension to better support Source-Specific
   Multicast (SSM) [RFC5760] is also available.  Many-to-many
   communication, which RTP [RFC3550] was originally built to support,
   has several limitations in common with multicast.

   One limitation is that, for any group, sender-side adaptations with
   the intent to suit all receivers would have to adapt to the most
   limited receiver experiencing the worst conditions among the group
   participants, which imposes degradation for all participants.  For
   broadcast-type applications with a large number of receivers, this is
   not acceptable.  Instead, various receiver-based solutions are
   employed to ensure that the receivers achieve the 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 in 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, 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 improving transport robustness are sent in their
   own multicast group to allow for interworking with legacy
   applications or to support different levels of protection.

   Many-to-many applications have different needs, and the most
   appropriate multiplexing choice will depend on how the actual
   application is realized.  Multicast applications that are capable of
   using sender-side congestion control can avoid the use of multiple
   multicast sessions and RTP sessions that result from the use of
   receiver-side congestion control.

   The properties of a broadcast application using RTP multicast are as
   follows:

   1.  The application uses a group of RTP sessions -- not just 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 receivers is likely to
       be much larger than the number of RTP senders.

   3.  The application needs signaling functions to identify the
       relationships between RTP sessions.

   4.  The application needs signaling or RTP/RTCP functions to identify
       the relationships between SSRCs in different RTP sessions when
       more complex relations than those that can be expressed by the
       CNAME exist.

   Both broadcast and many-to-many multicast applications share a
   signaling requirement; all of the participants need 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.  SDP offer/answer [RFC3264] is not appropriate for ensuring
   this property in a broadcast/multicast context.  The signaling
   aspects of broadcast/multicast are not explored further in this memo.

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

4.3.  Security and Key-Management Considerations

   When dealing with point-to-point two-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 multi-party sessions that might warrant consideration.
   For general information regarding possible methods of securing RTP,
   please review [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) built around symmetric keys, the
   receiver will need to have the same key as the sender.  As a result,
   no one in a multi-party session can be certain that a received packet
   was really sent by the claimed sender and not by another party having
   access to the key.  The single SRTP algorithm not having this
   property is Timed Efficient Stream Loss-Tolerant Authentication
   (TESLA) source authentication [RFC4383].  However, TESLA adds delay
   to achieve source authentication.  In most cases, symmetric ciphers
   provide sufficient security properties, but in a few cases they can
   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 RTP
   streams.  This requires that everyone rekey without disclosing the
   new keys to the excluded party.

   A second case is when security is used as an enforcing mechanism for
   stream access differentiation between different receivers.  Take, for
   example, a scalable layer or a high-quality simulcast version that
   only users paying a premium 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 users can't
   access without paying a premium, using the key-management mechanism
   to limit access to the key.

   As specified in [RFC3711], SRTP uses unique keys per SSRC; however,
   the original assumption was a single-session master key from which
   SSRC-specific RTP and RTCP keys were derived.  However, that
   assumption was proven incorrect, as the application usage and the
   developed key-management mechanisms have chosen many different
   methods for ensuring unique keys per SSRC.  The key-management
   functions have different abilities to establish different 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.  Thus, limitations do exist,
   depending on the chosen key-management method and due to the
   integration of particular implementations of the key-management
   method and SRTP.

4.3.2.  Key Management for Multi-party Sessions

   The capabilities of the key-management method combined with the RTP
   multiplexing choices affect the resulting security properties,
   control over the secured media, and who has access to it.

   Multi-party sessions contain at least one RTP stream from each active
   participant.  Depending on the multi-party topology [RFC7667], each
   participant can both send and receive multiple RTP streams.
   Transport translator-based sessions (Topo-Trn-Translator) and
   multicast sessions (Topo-ASM) can use neither Security Descriptions
   [RFC4568] nor DTLS-SRTP [RFC5764] without an extension, because each
   endpoint provides its own set of keys.  In centralized conferences,
   the signaling counterpart is a conference server, and the transport
   translator is the media-plane unicast counterpart (to which DTLS
   messages would be sent).  Thus, an extension like Encrypted Key
   Transport [RFC8870] or a solution based on Multimedia Internet KEYing
   (MIKEY) [RFC3830] that allows for keying all session participants
   with the same master key is needed.

   Privacy-Enhanced RTP Conferencing (PERC) also enables a different
   trust model with semi-trusted media-switching RTP middleboxes
   [RFC8871].

4.3.3.  Complexity Implications

   There can be complex interactions between the choice of multiplexing
   and topology and the security functions.  This becomes especially
   evident in RTP topologies having any type of middlebox that processes
   or modifies RTP/RTCP packets.  While the overhead of an RTP
   translator or mixer rewriting an SSRC value in the RTP packet of an
   unencrypted session is low, the cost is higher when using
   cryptographic security functions.  For example, if using SRTP
   [RFC3711], the actual security context and exact crypto key are
   determined by the SSRC field value.  If one changes the SSRC value,
   the encryption and authentication must use another key.  Thus,
   changing the SSRC value implies a decryption using the old SSRC and
   its security context, followed by an encryption using the new one.

5.  RTP Multiplexing Design Choices

   This section discusses how some RTP multiplexing design choices can
   be used in applications to achieve certain goals and summarizes the
   implications of such choices.  The benefits and downsides of each
   design are also discussed.

5.1.  Multiple Media Types in One Session

   This 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.  An endpoint can send zero,
   one, or multiple media sources per media type, resulting in a number
   of RTP streams of various media types for both source and redundancy
   streams.

   Advantages:

   1.  Only a single RTP session is used, which implies:

       *  Minimal need to keep NAT/FW state.

       *  Minimal NAT/FW traversal cost.

       *  Fate-sharing for all media flows.

       *  Minimal overhead for security association establishment.

   2.  Dynamic allocation of RTP streams can be handled almost entirely
       at the RTP level.  The extent to which this allocation can be
       kept at the RTP level depends on the application's needs for an
       explicit indication of stream usage and in how timely a fashion
       that information can be signaled.

   Disadvantages:

   1.  It is less suitable for interworking with other applications that
       use individual RTP sessions per media type or multiple sessions
       for a single media type, due to the risk of SSRC collisions and
       thus a potential need for SSRC translation.

   2.  Negotiation of individual bandwidths for the different media
       types is currently only possible in SDP when using RID [RFC8851].

   3.  It is not suitable for split component terminals (see
       Section 3.10 of [RFC7667]).

   4.  Flow-based QoS cannot be used to provide separate treatment of
       RTP streams compared to others in the single RTP session.

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

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

   7.  There are some additional concerns regarding legacy
       implementations that do not support the RTP specification fully
       when it comes to handling multiple SSRCs per endpoint, as
       multiple simultaneous media types are sent as separate SSRCs in
       the same RTP session.

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

5.2.  Multiple SSRCs of the Same Media Type

   In this design, each RTP session serves only a single media type.
   The RTP session can contain multiple RTP streams, from either a
   single endpoint or 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 [RFC5761].

   Advantages:

   1.  It works well with split component terminals (see Section 3.10 of
       [RFC7667]) where the split is per media type.

   2.  It enables flow-based QoS with different prioritization levels
       between media types.

   3.  For applications with dynamic usage of RTP streams (i.e., streams
       are frequently added and removed), having much of the state
       associated with the RTP session rather than per individual SSRC
       can avoid the need for in-session signaling of meta-information
       about each SSRC.  In simple cases, this allows for unsignaled RTP
       streams where session-level information and an RTCP SDES item
       (e.g., CNAME) are sufficient.  In the more complex cases where
       more source-specific metadata needs to be signaled, the SSRC can
       be associated with an intermediate identifier, e.g., the MID
       conveyed as an SDES item as defined in Section 15 of [RFC8843].

   4.  The overhead of security association establishment is low.

   Disadvantages:

   1.  A slightly higher number of RTP sessions are needed, compared to
       multiple media types in one session (Section 5.1).  This implies
       the following:

       *  More NAT/FW state is needed.

       *  The cost of NAT/FW traversal is increased in terms of both
          processing and delay.

   2.  There is some potential for concern regarding legacy
       implementations that don't support the RTP specification fully
       when it comes to handling multiple SSRCs per endpoint.

   3.  It is not possible to control security associations for sets of
       RTP streams within the same media type with today's key-
       management mechanisms, unless these are split into different RTP
       sessions (Section 5.3).

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

5.3.  Multiple Sessions for One Media Type

   This design goes one step further than the design discussed in
   Section 5.2 by also using multiple RTP sessions for a single media
   type.  The main reason for going in this direction is that the RTP
   application needs separation of the RTP streams according to their
   usage, such as, for example, scalability over multicast, simulcast,
   the need for extended QoS prioritization, or the need for fine-
   grained signaling using RTP session-focused signaling tools.

   Advantages:

   1.  This design is more suitable for multicast usage where receivers
       can individually select which RTP sessions they want to
       participate in, assuming that each RTP session has its own
       multicast group.

   2.  When multiple different usages exist, the application can
       indicate its usage of the RTP streams at the RTP session level.

   3.  There is less need for SSRC-specific explicit signaling for each
       media stream and thus a reduced need for explicit and timely
       signaling when RTP streams are added or removed.

   4.  It enables detailed QoS prioritization for flow-based mechanisms.

   5.  It works well with split component terminals (see Section 3.10 of
       [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 mechanisms.

   Disadvantages:

   1.  There is an increased amount of session configuration state
       compared to multiple SSRCs of the same media type (Section 5.2),
       due to the increased amount of RTP sessions.

   2.  For RTP streams that are part of scalability, simulcast, or
       transport robustness, a method for binding sources across
       multiple RTP sessions is needed.

   3.  There is some potential for concern regarding legacy
       implementations that don't support the RTP specification fully
       when it comes to handling multiple SSRCs per endpoint.

   4.  The overhead of security association establishment is higher, due
       to the increased number of RTP sessions.

   5.  If the applications need finer control over which participants in
       a given RTP session are included in different sets of security
       associations, most of today's key-management mechanisms will have
       difficulties establishing such a session.

   For more-complex RTP applications that have several different usages
   for RTP streams of the same media type or that use 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 applications requiring differentiation to
   different participants.

5.4.  Single SSRC per Endpoint

   In this 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 either
   unidirectionally (i.e., one SSRC sends an RTP stream that is received
   by the other SSRC) or bidirectionally (i.e., the two SSRCs both send
   an RTP stream and receive the RTP stream sent by the other endpoint).
   If the application needs additional media flows between the
   endpoints, it will have to establish additional RTP sessions.

   Advantages:

   1.  This design has great potential for interoperability with legacy
       applications, as it will not tax any RTP stack implementations.

   2.  The signaling system makes it possible to negotiate and describe
       the exact formats and bitrates for each RTP stream, especially
       using today's tools in SDP.

   3.  It is possible to control security associations per RTP stream
       with current key-management functions, since each RTP stream is
       directly related to an RTP session and the most commonly used
       keying mechanisms operate on a per-session basis.

   Disadvantages:

   1.  The amount of NAT/FW state grows linearly with the number of RTP
       streams.

   2.  NAT/FW traversal increases delay and resource consumption.

   3.  There are likely more signaling message and signaling processing
       requirements due to the increased amount of session-related
       information.

   4.  There is higher potential for a single RTP stream to fail during
       transport between the endpoints, due to the need for a separate
       NAT/FW traversal for every RTP stream, since there is only one
       stream per session.

   5.  The amount of explicit state for relating RTP streams grows,
       depending on how the application relates RTP streams.

   6.  Port consumption might become a problem for centralized services,
       where the central node's port or 5-tuple filter consumption grows
       rapidly with the number of sessions.

   7.  For applications where RTP stream usage is highly dynamic, i.e.,
       entities frequently enter and leave sessions, the amount of
       signaling can become high.  Issues can also arise from the need
       for timely establishment of additional RTP sessions.

   8.  If, against the recommendation in [RFC3550], the same SSRC value
       is reused in multiple RTP sessions rather than being randomly
       chosen, interworking with applications that use a different
       multiplexing structure will require SSRC translation.

   RTP applications with a strong need to interwork with legacy RTP
   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 non-mixed multi-party
   sessions, as any given RTP stream from each participant, although
   having the same usage in the application, needs its own RTP session.
   In addition, the dynamic behavior that can arise in multi-party
   applications can tax the signaling system and make timely media
   establishment more difficult.

5.5.  Summary

   Both the "single SSRC per endpoint" (Section 5.4) and "multiple media
   types in one session" (Section 5.1) cases require full explicit
   signaling of the media stream relationships.  However, they operate
   on two different levels, where the first primarily enables session-
   level binding and the second needs SSRC-level binding.  From another
   perspective, the two solutions are the two extremes when it comes to
   the number of RTP sessions needed.

   The two other designs -- multiple SSRCs of the same media type
   (Section 5.2) and multiple sessions for one media type (Section 5.3)
   -- are two examples that primarily allow for some implicit mapping of
   the role or usage of the RTP streams based on which RTP session they
   appear in.  Thus, they potentially allow for less signaling and, in
   particular, reduce the need for real-time signaling in sessions with
   a dynamically changing number of RTP streams.  They also represent
   points between the first two designs when it comes to the amount of
   RTP sessions established, i.e., they represent an attempt to balance
   the amount of RTP sessions with the functionality the communication
   session provides at both the network level and the signaling level.

6.  Guidelines

   This section contains a number of multi-stream guidelines for
   implementers, system designers, and specification writers.

   Do not require the use of the same SSRC value across RTP sessions:
      As discussed in Section 3.4.3, there are downsides to using the
      same SSRC in multiple RTP sessions as a mechanism to bind related
      RTP streams together.  It is instead recommended to use a
      mechanism to explicitly signal the relationship, in either
      RTP/RTCP or the signaling mechanism used to establish the RTP
      session(s).

   Use additional RTP streams for additional media sources:
      In the cases where an RTP endpoint needs to transmit additional
      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 in the same RTP session.  For example, in
      the case of a telepresence room where there are three cameras and
      each camera captures two persons sitting at the table, we suggest
      that each camera send its own RTP stream within a single RTP
      session.

   Use additional RTP sessions for streams with different
   requirements:
      When 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 be put in different RTP sessions.
      This includes the case where different participants want different
      subsets of the set of RTP streams.

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

   Ensure that RTP/RTCP extensions support 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 for use with additional
      SSRCs and multiple RTP sessions.  Any extension intended to be
      generic must support both.  Extensions that are not as generally
      applicable will have to consider whether interoperability is
      better served by defining a single solution or providing both
      options.

   Provide adequate extensions for transport support:
      When defining new RTP/RTCP extensions intended for transport
      support, like the retransmission or FEC mechanisms, they must
      include support for both multiple RTP streams in the same RTP
      session 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.  IANA Considerations

   This document has no IANA actions.

8.  Security Considerations

   The security considerations discussed in the RTP specification
   [RFC3550]; any applicable RTP profile [RFC3551] [RFC4585] [RFC3711];
   and the extensions for sending multiple media types in a single RTP
   session [RFC8860], RID [RFC8851], BUNDLE [RFC8843], [RFC5760], and
   [RFC5761] apply if selected and thus need to be considered in the
   evaluation.

   Section 4.3 discusses the security implications of choosing multiple
   SSRCs vs. multiple RTP sessions.

9.  References

9.1.  Normative References

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

   [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>.

   [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 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [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>.

   [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>.

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

   [RFC8843]  Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", RFC 8843,
              DOI 10.17487/RFC8843, September 2020,
              <https://www.rfc-editor.org/info/rfc8843>.

   [RFC8851]  Roach, A.B., Ed., "RTP Payload Format Restrictions",
              RFC 8851, DOI 10.17487/RFC8851, September 2020,
              <https://www.rfc-editor.org/info/rfc8851>.

   [RFC8852]  Roach, A.B., Nandakumar, S., and P. Thatcher, "RTP Stream
              Identifier Source Description (SDES)", RFC 8852,
              DOI 10.17487/RFC8852, September 2020,
              <https://www.rfc-editor.org/info/rfc8852>.

   [RFC8860]  Westerlund, M., Perkins, C., and J. Lennox, "Sending
              Multiple Types of Media in a Single RTP Session",
              RFC 8860, DOI 10.17487/RFC8860, September 2020,
              <https://www.rfc-editor.org/info/rfc8860>.

   [RFC8870]  Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", RFC 8870, DOI 10.17487/RFC8870, September 2020,
              <https://www.rfc-editor.org/info/rfc8870>.

9.2.  Informative References

   [JINGLE]   Ludwig, S., Beda, J., Saint-Andre, P., McQueen, R., Egan,
              S., and J. Hildebrand, "XEP-0166: Jingle", September 2018,
              <https://xmpp.org/extensions/xep-0166.html>.

   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J.C., 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>.

   [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", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <https://www.rfc-editor.org/info/rfc4568>.

   [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>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <https://www.rfc-editor.org/info/rfc5389>.

   [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>.

   [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>.

   [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>.

   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,
              <https://www.rfc-editor.org/info/rfc7983>.

   [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>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8852]  Roach, A.B., Nandakumar, S., and P. Thatcher, "RTP Stream
              Identifier Source Description (SDES)", RFC 8852,
              DOI 10.17487/RFC8852, September 2020,
              <https://www.rfc-editor.org/info/rfc8852>.

   [RFC8871]  Jones, P., Benham, D., and C. Groves, "A Solution
              Framework for Private Media in Privacy-Enhanced RTP
              Conferencing (PERC)", RFC 8871, DOI 10.17487/RFC8871,
              September 2020, <https://www.rfc-editor.org/info/rfc8871>.

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 issues related to
   multiple RTP streams.  Attempting to use payload type multiplexing
   beyond its defined usage has well-known negative effects on RTP, as
   discussed below.  To use the payload type as the single discriminator
   for multiple streams implies that all the different RTP streams are
   being sent with the same SSRC, thus using the same timestamp and
   sequence number space.  The many effects of using payload type
   multiplexing are as follows:

   1.   Constraints are placed on the RTP timestamp rate for the
        multiplexed media.  For example, 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 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, 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 be done relatively easily on the sender side
        by ensuring that the fragments of each 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
        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 RTP stream before being used with
        payload type multiplexing.

   4.   Sending multiple media streams in the same sequence number space
        makes it impossible to determine which media stream lost a
        packet.  Such a scenario causes difficulties, since the receiver
        cannot determine to which stream it should apply packet-loss
        concealment or other stream-specific loss-mitigation mechanisms.

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

   6.   The current RTCP feedback mechanisms are built around providing
        feedback on 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 is
        reported, so sending feedback for a specific RTP payload type is
        difficult without extending existing RTCP reporting.

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

   8.   The number of payload types is inherently limited.  Accordingly,
        using 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.  This is
        currently possible for RTP sessions and SSRCs, but there is no
        defined way to group payload types.

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

   11.  Most existing SDP media-level attributes cannot be applied on a
        per-payload-type basis and would require redefinition in that
        context.

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

Appendix B.  Signaling Considerations

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

   We document some issues here that need to be addressed when using
   some form of signaling to establish RTP sessions.  These issues
   cannot be addressed by simply tweaking, extending, or profiling RTP;
   rather, they require a dedicated and in-depth look at the signaling
   primitives that set up the RTP sessions.

   There exist various signaling solutions for establishing RTP
   sessions.  Many are based on SDP [RFC4566]; however, SDP
   functionality is also dependent on the signaling protocols carrying
   the SDP.  The Real-Time Streaming Protocol (RTSP) [RFC7826] and the
   Session Announcement Protocol (SAP) [RFC2974] both use SDP in a
   declarative fashion, while SIP [RFC3261] uses SDP with the additional
   definition of offer/answer [RFC3264].  The impact on signaling, and
   especially on SDP, needs to be considered, as it can greatly affect
   how to deploy a certain multiplexing point choice.

B.1.  Session-Oriented Properties

   One aspect of existing signaling protocols is that they are focused
   on RTP sessions or, in the case of SDP, the concept of media
   descriptions.  A number of things are signaled at the media
   description level, but those are not necessarily strictly bound to an
   RTP session and could be of interest for signaling, especially for a
   particular RTP stream (SSRC) within the session.  The following
   properties have been identified as being potentially useful for
   signaling, and not only at the RTP session level:

   *  Bitrate and/or bandwidth can be specified today only as an
      aggregate limit, or as a common "any RTP stream" limit, unless
      either codec-specific bandwidth limiting or RTCP signaling using
      Temporary Maximum Media Stream Bit Rate Request (TMMBR) messages
      [RFC5104] is used.

   *  Which SSRC will use which RTP payload type (this information will
      be visible in the first media packet but is sometimes useful to
      have before the packet arrives).

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

   For some parameters, such as RTP payload type, resolution, and frame
   rate, an SSRC-linked mechanism has been proposed in [RFC8851].

B.2.  SDP Prevents Multiple Media Types

   SDP uses the "m=" line to both delineate an RTP session and specify
   the top-level 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 "a=rtpmap:" attribute.  This binding has to be loosened in
   order to use SDP to describe RTP sessions containing multiple top-
   level media types.

   [RFC8843] describes how to let multiple SDP media descriptions use a
   single underlying transport in SDP, which allows the definition of
   one RTP session with different top-level media types.

B.3.  Signaling RTP Stream Usage

   RTP streams being transported in RTP have a particular usage in an
   RTP application.  In many applications to date, this usage of the RTP
   stream is implicitly signaled.  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
   RTP streams, there will be more than a single usage or purpose among
   the set of RTP streams being sent or received.  RTP applications will
   need to somehow signal this usage.  The signaling that is used will
   have to identify the RTP streams affected by their RTP-level
   identifiers, which means that they have to be identified by either
   their session or their SSRC + session.

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

   If all 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, signaling needs to identify both the session
   and the SSRC.

   If this signaling 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 signaling to
   know how to treat RTP streams with different usages in the right
   fashion.

Acknowledgments

   The authors would like to acknowledge and thank Cullen Jennings, Dale
   R. Worley, Huang Yihong (Rachel), Benjamin Kaduk, Mirja K├╝hlewind,
   and Vijay Gurbani for review and comments.

Contributors

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

Authors' Addresses

   Magnus Westerlund
   Ericsson
   Torshamnsgatan 23
   SE-164 80 Kista
   Sweden

   Phone: +46 10 714 82 87

   Email: magnus.westerlund@ericsson.com

   Bo Burman
   Ericsson
   Gronlandsgatan 31
   SE-164 60 Kista
   Sweden

   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
   SE-11122 Stockholm
   Sweden

   Email: harald@alvestrand.no

   Roni Even

   Email: ron.even.tlv@gmail.com