



Internet Engineering Task Force (IETF)                         B. Burman
Request for Comments: 8853                                 M. Westerlund
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                            S. Nandakumar
                                                               M. Zanaty
                                                                   Cisco
                                                            January 2021


 Using Simulcast in Session Description Protocol (SDP) and RTP Sessions

Abstract

   In some application scenarios, it may be desirable to send multiple
   differently encoded versions of the same media source in different
   RTP streams.  This is called simulcast.  This document describes how
   to accomplish simulcast in RTP and how to signal it in the Session
   Description Protocol (SDP).  The described solution uses an RTP/RTCP
   identification method to identify RTP streams belonging to the same
   media source and makes an extension to SDP to indicate that those RTP
   streams are different simulcast formats of that media source.  The
   SDP extension consists of a new media-level SDP attribute that
   expresses capability to send and/or receive simulcast RTP streams.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc8853.

Copyright Notice

   Copyright (c) 2021 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.  Requirements Language
   3.  Use Cases
     3.1.  Reaching a Diverse Set of Receivers
     3.2.  Application-Specific Media Source Handling
     3.3.  Receiver Media-Source Preferences
   4.  Overview
   5.  Detailed Description
     5.1.  Simulcast Attribute
     5.2.  Simulcast Capability
     5.3.  Offer/Answer Use
       5.3.1.  Generating the Initial SDP Offer
       5.3.2.  Creating the SDP Answer
       5.3.3.  Offerer Processing the SDP Answer
       5.3.4.  Modifying the Session
     5.4.  Use with Declarative SDP
     5.5.  Relating Simulcast Streams
     5.6.  Signaling Examples
       5.6.1.  Single-Source Client
       5.6.2.  Multisource Client
       5.6.3.  Simulcast and Redundancy
   6.  RTP Aspects
     6.1.  Outgoing from Endpoint with Media Source
     6.2.  RTP Middlebox to Receiver
       6.2.1.  Media-Switching Mixer
       6.2.2.  Selective Forwarding Middlebox
     6.3.  RTP Middlebox to RTP Middlebox
   7.  Network Aspects
     7.1.  Bitrate Adaptation
   8.  Limitation
   9.  IANA Considerations
   10. Security Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Requirements
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Most of today's multiparty video-conference solutions make use of
   centralized servers to reduce the bandwidth and CPU consumption in
   the endpoints.  Those servers receive RTP streams from each
   participant and send some suitable set of possibly modified RTP
   streams to the rest of the participants, which usually have
   heterogeneous capabilities (screen size, CPU, bandwidth, codec,
   etc.).  One of the biggest issues is how to perform RTP stream
   adaptation to different participants' constraints with the minimum
   possible impact on both video quality and server performance.

   Simulcast is defined in this memo as the act of simultaneously
   sending multiple different encoded streams of the same media source
   -- e.g., the same video source encoded with different video-encoder
   types or image resolutions.  This can be done in several ways and for
   different purposes.  This document focuses on the case where it is
   desirable to provide a media source as multiple encoded streams over
   RTP [RFC3550] towards an intermediary so that the intermediary can
   provide the wanted functionality by selecting which RTP stream(s) to
   forward to other participants in the session, and more specifically
   how the identification and grouping of the involved RTP streams are
   done.

   The intended scope of the defined mechanism is to support negotiation
   and usage of simulcast when using SDP offer/answer and media
   transport over RTP.  The media transport topologies considered are
   point-to-point RTP sessions, as well as centralized multiparty RTP
   sessions, where a media sender will provide the simulcasted streams
   to an RTP middlebox or endpoint, and middleboxes may further
   distribute the simulcast streams to other middleboxes or endpoints.
   Simulcast could be used point to point between middleboxes as part of
   a distributed multiparty scenario.  Usage of multicast or broadcast
   transport is out of scope and left for future extensions.

   This document describes a few scenarios that motivate the use of
   simulcast and also defines the needed RTP/RTCP and SDP signaling for
   it.

2.  Definitions

2.1.  Terminology

   This document makes use of the terminology defined in "A Taxonomy of
   Semantics and Mechanisms for Real-Time Transport Protocol (RTP)
   Sources" [RFC7656] and "RTP Topologies" [RFC7667].  The following
   terms are especially noted or here defined:

   RTP mixer:  An RTP middlebox, in the wide sense of the term,
      encompassing Sections 3.6 to 3.9 of [RFC7667].

   RTP session:  An association among a group of participants
      communicating with RTP, as defined in [RFC3550] and amended by
      [RFC7656].

   RTP stream:  A stream of RTP packets containing media data, as
      defined in [RFC7656].

   RTP switch:  A common short term for the terms "switching RTP mixer",
      "source projecting middlebox", and "video switching Multipoint
      Control Unit (MCU)", as discussed in [RFC7667].

   Simulcast stream:  One encoded stream or dependent stream from a set
      of concurrently transmitted encoded streams and optional dependent
      streams, all sharing a common media source, as defined in
      [RFC7656].  For example, HD and thumbnail video simulcast versions
      of a single media source sent concurrently as separate RTP
      streams.

   Simulcast format:  Different formats of a simulcast stream serve the
      same purpose as alternative RTP payload types in nonsimulcast SDP:
      to allow multiple alternative media formats for a given RTP
      stream.  As for multiple RTP payload types on the "m=" line in
      offer/answer [RFC3264], any one of the negotiated alternative
      formats can be used in a single RTP stream at a given point in
      time, but not more than one (based on RTP timestamp).  What format
      is used can change dynamically from one RTP packet to another.

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Use Cases

   The use cases of simulcast described in this document relate to a
   multiparty communication session where one or more central nodes are
   used to adapt the view of the communication session towards
   individual participants and facilitate the media transport between
   participants.  Thus, these cases target the RTP mixer type of
   topology.

   There are two principal approaches for an RTP mixer to provide this
   adapted view of the communication session to each receiving
   participant:

   *  Transcoding (decoding and re-encoding) received RTP streams with
      characteristics adapted to each receiving participant.  This often
      includes mixing or composition of media sources from multiple
      participants into a mixed media source originated by the RTP
      mixer.  The main advantage of this approach is that it achieves
      close-to-optimal adaptation to individual receiving participants.
      The main disadvantages are that it can be very computationally
      expensive to the RTP mixer, typically degrades media Quality of
      Experience (QoE) such as creating end-to-end delay for the
      receiving participants, and requires the RTP mixer to have access
      to media content.

   *  Switching a subset of all received RTP streams or substreams to
      each receiving participant, where the used subset is typically
      specific to each receiving participant.  The main advantages of
      this approach are that it is computationally cheap to the RTP
      mixer, has very limited impact on media QoE, and does not require
      the RTP mixer to have (full) access to media content.  The main
      disadvantage is that it can be difficult to combine a subset of
      received RTP streams into a perfect fit for the resource situation
      of a receiving participant.  It is also a disadvantage that
      sending multiple RTP streams consumes more network resources from
      the sending participant to the RTP mixer.

   The use of simulcast relates to the latter approach, where it is more
   important to reduce the load on the RTP mixer and/or minimize QoE
   impact than to achieve an optimal adaptation of resource usage.

3.1.  Reaching a Diverse Set of Receivers

   The media sources provided by a sending participant potentially need
   to reach several receiving participants that differ in terms of
   available resources.  The receiver resources that typically differ
   include, but are not limited to:

   Codec:  This includes codec type (such as RTP payload format MIME
      type) and can include codec configuration.  A couple of codec
      resources that differ only in codec configuration will be
      "different" if they are somehow not "compatible", such as if they
      differ in video codec profile or the transport packetization
      configuration.

   Sampling:  This relates to how the media source is sampled, in
      spatial as well as temporal domain.  For video streams, spatial
      sampling affects image resolution, and temporal sampling affects
      video frame rate.  For audio, spatial sampling relates to the
      number of audio channels, and temporal sampling affects audio
      bandwidth.  This may be used to suit different rendering
      capabilities or needs at the receiving endpoints.

   Bitrate:  This relates to the number of bits sent per second to
      transmit the media source as an RTP stream, which typically also
      affects the QoE for the receiving user.

   Letting the sending participant create a simulcast of a few
   differently configured RTP streams per media source can be a good
   trade-off when using an RTP switch as middlebox, instead of sending a
   single RTP stream and using an RTP mixer to create individual
   transcodings to each receiving participant.

   This requires that the receiving participants can be categorized in
   terms of available resources and that the sending participant can
   choose a matching configuration for a single RTP stream per category
   and media source.  For example, a set of receiving participants
   differ only in screen resolution; some are able to display video with
   at most 360p resolution, and some support 720p resolution.  A sending
   participant can then reach all receivers with best possible
   resolution by creating a simulcast of RTP streams with 360p and 720p
   resolution for each sent video media source.

   The maximum number of simulcasted RTP streams that can be sent is
   mainly limited by the amount of processing and uplink network
   resources available to the sending participant.

3.2.  Application-Specific Media Source Handling

   The application logic that controls the communication session may
   include special handling of some media sources.  It is, for example,
   commonly the case that the media from a sending participant is not
   sent back to itself.

   It is also common that a currently active speaker participant is
   shown in larger size or higher quality than other participants (the
   sampling or bitrate aspects of Section 3.1) in a receiving client.
   Many conferencing systems do not send the active speaker's media back
   to the sender itself, which means there is some other participant's
   media that instead is forwarded to the active speaker -- typically
   the previous active speaker.  This way, the previously active speaker
   is needed both in larger size (to current active speaker) and in
   small size (to the rest of the participants), which can be solved
   with a simulcast from the previously active speaker to the RTP
   switch.

3.3.  Receiver Media-Source Preferences

   The application logic that controls the communication session may
   allow receiving participants to state preferences on the
   characteristics of the RTP stream they like to receive, for example
   in terms of the aspects listed in Section 3.1.  Sending a simulcast
   of RTP streams is one way of accommodating receivers with conflicting
   or otherwise incompatible preferences.

4.  Overview

   This memo defines SDP [RFC4566] signaling that covers the above
   described simulcast use cases and functionalities.  A number of
   requirements for such signaling are elaborated in Appendix A.

   The Restriction Identifier (RID) mechanism, as defined in [RFC8851],
   enables an SDP offerer or answerer to specify a number of different
   RTP stream restrictions for a rid-id by using the "a=rid" line.
   Examples of such restrictions are maximum bitrate, maximum spatial
   video resolution (width and height), maximum video frame rate, etc.
   Each rid-id may also be restricted to use only a subset of the RTP
   payload types in the associated SDP media description.  Those RTP
   payload types can have their own configurations and parameters
   affecting what can be sent or received, using the "a=fmtp" line as
   well as other SDP attributes.

   A new SDP media-level attribute, "a=simulcast", is defined.  The
   attribute describes, independently for "send" and "receive"
   directions, the number of simulcast RTP streams as well as potential
   alternative formats for each simulcast RTP stream.  Each simulcast
   RTP stream, including alternatives, is identified using the RID
   identifier (rid-id), defined in [RFC8851].

   a=simulcast:send 1;2,3 recv 4

   If this line is included in an SDP offer, the "send" part indicates
   the offerer's capability and proposal to send two simulcast RTP
   streams.  Each simulcast stream is described by one or more RTP
   stream identifiers (rid-ids), and each group of rid-ids for a
   simulcast stream is separated by a semicolon (";").  When a simulcast
   stream has multiple rid-ids that are separated by a comma (","), they
   describe alternative representations for that particular simulcast
   RTP stream.  Thus, the "send" part shown above is interpreted as an
   intention to send two simulcast RTP streams.  The first simulcast RTP
   stream is identified and restricted according to rid-id 1.  The
   second simulcast RTP stream can be sent as two alternatives,
   identified and restricted according to rid-ids 2 and 3.  The "recv"
   part of the line shown here indicates that the offerer desires to
   receive a single RTP stream (no simulcast) according to rid-id 4.

   A more complete example SDP-offer media description is provided in
   Figure 1.

   m=video 49300 RTP/AVP 97 98 99
   a=rtpmap:97 H264/90000
   a=rtpmap:98 H264/90000
   a=rtpmap:99 VP8/90000
   a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
   a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
   a=fmtp:99 max-fs=240; max-fr=30
   a=rid:1 send pt=97;max-width=1280;max-height=720
   a=rid:2 send pt=98;max-width=320;max-height=180
   a=rid:3 send pt=99;max-width=320;max-height=180
   a=rid:4 recv pt=97
   a=simulcast:send 1;2,3 recv 4
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id

           Figure 1: Example Simulcast Media Description in Offer

   The SDP media description in Figure 1 can be interpreted at a high
   level to say that the offerer is capable of sending two simulcast RTP
   streams: one H.264 encoded stream in up to 720p resolution, and one
   additional stream encoded as either H.264 or VP8 with a maximum
   resolution of 320x180 pixels.  The offerer can receive one H.264
   stream with maximum 720p resolution.

   The receiver of this SDP offer can generate an SDP answer that
   indicates what it accepts.  It uses the "a=simulcast" attribute to
   indicate simulcast capability and specify what simulcast RTP streams
   and alternatives to receive and/or send.  An example of such an
   answering "a=simulcast" attribute, corresponding to the above offer,
   is:

   a=simulcast:recv 1;2 send 4

   With this SDP answer, the answerer indicates in the "recv" part that
   it wants to receive the two simulcast RTP streams.  It has removed an
   alternative that it doesn't support (rid-id 3).  The "send" part
   confirms to the offerer that it will receive one stream for this
   media source according to rid-id 4.  The corresponding, more complete
   example SDP answer media description could look like Figure 2.

   m=video 49674 RTP/AVP 97 98
   a=rtpmap:97 H264/90000
   a=rtpmap:98 H264/90000
   a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
   a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
   a=rid:1 recv pt=97;max-width=1280;max-height=720
   a=rid:2 recv pt=98;max-width=320;max-height=180
   a=rid:4 send pt=97
   a=simulcast:recv 1;2 send 4
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id

          Figure 2: Example Simulcast Media Description in Answer

   It is assumed that a single SDP media description is used to describe
   a single media source.  This is aligned with the concepts defined in
   [RFC7656] and will work in a WebRTC context, both with and without
   BUNDLE grouping of media descriptions [RFC8843].

   To summarize, the "a=simulcast" line describes "send"- and "receive"-
   direction simulcast streams separately.  Each direction can in turn
   describe one or more simulcast streams, separated by semicolons.  The
   identifiers describing simulcast streams on the "a=simulcast" line
   are rid-ids, as defined by "a=rid" lines in [RFC8851].  Each
   simulcast stream can be offered as a list of alternative rid-ids,
   with each alternative separated by a comma as shown in the example
   offer in Figure 1.  A detailed specification can be found in
   Section 5, and more detailed examples are outlined in Section 5.6.

5.  Detailed Description

   This section provides further details to the overview in Section 4.
   First, formal syntax is provided (Section 5.1), followed by the rest
   of the SDP attribute definition in Section 5.2.  "Relating Simulcast
   Streams" (Section 5.5) provides the definition of the RTP/RTCP
   mechanisms used.  The section concludes with a number of examples.

5.1.  Simulcast Attribute

   This document defines a new SDP media-level "a=simulcast" attribute,
   with value according to the syntax in Figure 3, which uses ABNF
   [RFC5234] and its update, "Case-Sensitive String Support in ABNF"
   [RFC7405]:

   sc-value     = ( sc-send [SP sc-recv] ) / ( sc-recv [SP sc-send] )
   sc-send      = %s"send" SP sc-str-list
   sc-recv      = %s"recv" SP sc-str-list
   sc-str-list  = sc-alt-list *( ";" sc-alt-list )
   sc-alt-list  = sc-id *( "," sc-id )
   sc-id-paused = "~"
   sc-id        = [sc-id-paused] rid-id
   ; SP defined in [RFC5234]
   ; rid-id defined in [RFC8851]

                     Figure 3: ABNF for Simulcast Value

   The "a=simulcast" attribute has a parameter in the form of one or two
   simulcast stream descriptions, each consisting of a direction ("send"
   or "recv"), followed by a list of one or more simulcast streams.
   Each simulcast stream consists of one or more alternative simulcast
   formats.  Each simulcast format is identified by a simulcast stream
   identifier (rid-id).  The rid-id MUST have the form of an RTP stream
   identifier, as described by "RTP Payload Format Restrictions"
   [RFC8851].

   In the list of simulcast streams, each simulcast stream is separated
   by a semicolon (";").  Each simulcast stream can, in turn, be offered
   in one or more alternative formats, represented by rid-ids, separated
   by commas (",").  Each rid-id can also be specified as initially
   paused [RFC7728], indicated by prepending a "~" to the rid-id.  The
   reason to allow separate initial pause states for each rid-id is that
   pause capability can be specified individually for each RTP payload
   type referenced by a rid-id.  Since pause capability specified via
   the "a=rtcp-fb" attribute applies only to specified payload types,
   and a rid-id specified by "a=rid" can refer to multiple different
   payload types, it is unfeasible to pause streams with rid-id where
   any of the related RTP payload type(s) do not have pause capability.

5.2.  Simulcast Capability

   Simulcast capability is expressed through a new media-level SDP
   attribute, "a=simulcast" (Section 5.1).  The use of this attribute at
   the session level is undefined.  Implementations of this
   specification MUST NOT use it at the session level and MUST ignore it
   if received at the session level.  Extensions to this specification
   may define such session-level usage.  Each SDP media description MUST
   contain at most one "a=simulcast" line.

   There are separate and independent sets of simulcast streams in the
   "send" and "receive" directions.  When listing multiple directions,
   each direction MUST NOT occur more than once on the same line.

   Simulcast streams using undefined rid-ids MUST NOT be used as valid
   simulcast streams by an RTP stream receiver.  The direction for a
   rid-id MUST be aligned with the direction specified for the
   corresponding RTP stream identifier on the "a=rid" line.

   The listed number of simulcast streams for a direction sets a limit
   to the number of supported simulcast streams in that direction.  The
   order of the listed simulcast streams in the "send" direction
   suggests a proposed order of preference, in decreasing order: the
   rid-id listed first is the most preferred, and subsequent streams
   have progressively lower preference.  The order of the listed rid-ids
   in the "recv" direction expresses which simulcast streams are
   preferred, with the leftmost being most preferred.  This can be of
   importance if the number of actually sent simulcast streams has to be
   reduced for some reason.

   rid-ids that have explicit dependencies [RFC5583] [RFC8851] to other
   rid-ids (even in the same media description) MAY be used.

   Use of more than a single, alternative simulcast format for a
   simulcast stream MAY be specified as part of the attribute parameters
   by expressing the simulcast stream as a comma-separated list of
   alternative rid-ids.  The order of the rid-id alternatives within a
   simulcast stream is significant; the rid-id alternatives are listed
   from (left) most preferred to (right) least preferred.  For the use
   of simulcast, this overrides the normal codec preference as expressed
   by format-type ordering on the "m=" line, using regular SDP rules.
   This is to enable a separation of general codec preferences and
   simulcast-stream configuration preferences.  However, the choice of
   which alternative to use per simulcast stream is independent, and
   there is currently no mechanism for the offerer to force the answerer
   to choose the same alternative for multiple simulcast streams.

   A simulcast stream can use a codec defined such that the same RTP
   synchronization source (SSRC) can change RTP payload type multiple
   times during a session, possibly even on a per-packet basis.  A
   typical example is a speech codec that makes use of formats for
   Comfort Noise [RFC3389] and/or dual-tone multifrequency (DTMF)
   [RFC4733].

   If RTP stream pause/resume [RFC7728] is supported, any rid-id MAY be
   prefixed by a "~" character to indicate that the corresponding
   simulcast stream is paused already from the start of the RTP session.
   In this case, support for RTP stream pause/resume MUST also be
   included under the same "m=" line where "a=simulcast" is included.
   All RTP payload types related to such an initially paused simulcast
   stream MUST be listed in the SDP as pause/resume capable as specified
   by [RFC7728] -- e.g., by using the "*" wildcard format for "a=rtcp-
   fb".

   An initially paused simulcast stream in the "send" direction for the
   endpoint sending the SDP MUST be considered equivalent to an
   unsolicited locally paused stream and handled accordingly.  Initially
   paused simulcast streams are resumed as described by the RTP pause/
   resume specification.  An RTP stream receiver that wishes to resume
   an unsolicited locally paused stream needs to know the SSRC of that
   stream.  The SSRC of an initially paused simulcast stream can be
   obtained from an RTP stream sender RTCP Sender Report (SR) or
   Receiver Report (RR) that includes both the desired SSRC as initial
   SSRC in the source description (SDES) chunk, optionally a MID SDES
   item [RFC8843] (if used and if rid-ids are not unique across "m="
   lines), and the rid-id value in an RtpStreamId RTCP SDES item
   [RFC8852].

   If the endpoint sending the SDP includes a "recv"-direction simulcast
   stream that is initially paused, then the remote RTP sender receiving
   the SDP SHOULD put its RTP stream in an unsolicited locally paused
   state.  The simulcast stream sender does not put the stream in the
   locally paused state if there are other RTP stream receivers in the
   session that do not mark the simulcast stream as initially paused.
   However, in centralized conferencing, the RTP sender usually does not
   see the SDP signaling from RTP receivers and cannot make this
   determination.  The reason for requiring that an initially paused
   "recv" stream be considered locally paused by the remote RTP sender
   instead of making it equivalent to implicitly sending a pause request
   is that the pausing RTP sender cannot know which receiving SSRC owns
   the restriction when Temporary Maximum Media Stream Bit Rate Request
   (TMMBR) and Temporary Maximum Media Stream Bit Rate Notification
   (TMMBN) are used for pause/resume signaling (Section 5.6 of
   [RFC7728]); this is because the RTP receiver's SSRC in the "send"
   direction is sometimes not yet known.

   Use of the redundant audio data format [RFC2198] could be seen as a
   form of simulcast for loss-protection purposes, but it is not
   considered conflicting with the mechanisms described in this memo and
   MAY therefore be used as any other format.  In this case, the "red"
   format, rather than the carried formats, SHOULD be the one to list as
   a simulcast stream on the "a=simulcast" line.

   The media formats and corresponding characteristics of simulcast
   streams SHOULD be chosen such that they are different -- e.g., as
   different SDP formats with differing "a=rtpmap" and/or "a=fmtp"
   lines, or as differently defined RTP payload format restrictions.  If
   this difference is not required, it is RECOMMENDED to use RTP
   duplication procedures [RFC7104] instead of simulcast.  To avoid
   complications in implementations, a single rid-id MUST NOT occur more
   than once per "a=simulcast" line.  Note that this does not eliminate
   use of simulcast as an RTP duplication mechanism, since it is
   possible to define multiple different rid-ids that are effectively
   equivalent.

5.3.  Offer/Answer Use

   Note:  The inclusion of "a=simulcast" or the use of simulcast does
      not change any of the interpretation or Offer/Answer procedures
      for other SDP attributes, such as "a=fmtp" or "a=rid".

5.3.1.  Generating the Initial SDP Offer

   An offerer wanting to use simulcast for a media description SHALL
   include one "a=simulcast" attribute in that media description in the
   offer.  An offerer listing a set of receive simulcast streams and/or
   alternative formats as rid-ids in the offer MUST be prepared to
   receive RTP streams for any of those simulcast streams and/or
   alternative formats from the answerer.

5.3.2.  Creating the SDP Answer

   An answerer that does not understand the concept of simulcast will
   also not know the attribute and will remove it in the SDP answer, as
   defined in existing SDP offer/answer procedures [RFC3264].  Since SDP
   session-level simulcast is undefined in this memo, an answerer that
   receives an offer with the "a=simulcast" attribute on the SDP session
   level SHALL remove it in the answer.  An answerer that understands
   the attribute but receives multiple "a=simulcast" attributes in the
   same media description SHALL disable use of simulcast by removing all
   "a=simulcast" lines for that media description in the answer.

   An answerer that does understand the attribute and wants to support
   simulcast in an indicated direction SHALL reverse directionality of
   the unidirectional direction parameters -- "send" becomes "recv" and
   vice versa -- and include it in the answer.

   An answerer that receives an offer with simulcast containing an
   "a=simulcast" attribute listing alternative rid-ids MAY keep all the
   alternative rid-ids in the answer, but it MAY also choose to remove
   any nondesirable alternative rid-ids in the answer.  The answerer
   MUST NOT add any alternative rid-ids in the "send" direction in the
   answer that were not present in the offer receive direction.  The
   answerer MUST be prepared to receive any of the receive-direction
   rid-id alternatives and MAY send any of the "send"-direction
   alternatives that are part of the answer.

   An answerer that receives an offer with simulcast that lists a number
   of simulcast streams MAY reduce the number of simulcast streams in
   the answer, but it MUST NOT add simulcast streams.

   An answerer that receives an offer without RTP stream pause/resume
   capability MUST NOT mark any simulcast streams as initially paused in
   the answer.

   An RTP stream answerer capable of pause/resume that receives an offer
   with RTP stream pause/resume capability MAY mark any rid-ids that
   refer to pause/resume capable formats as initially paused in the
   answer.

   An answerer that receives indication in an offer of a rid-id being
   initially paused SHOULD mark that rid-id as initially paused also in
   the answer, regardless of direction, unless it has good reason for
   the rid-id not being initially paused.  One reason to remove an
   initial pause in the answer compared to the offer could be, for
   example, that all "receive"-direction simulcast streams for a media
   source the answerer accepts in the answer would otherwise be paused.

5.3.3.  Offerer Processing the SDP Answer

   An offerer that receives an answer without "a=simulcast" MUST NOT use
   simulcast towards the answerer.  An offerer that receives an answer
   with "a=simulcast" without any rid-id in a specified direction MUST
   NOT use simulcast in that direction.

   An offerer that receives an answer where some rid-id alternatives are
   kept MUST be prepared to receive any of the kept "send"-direction
   rid-id alternatives and MAY send any of the kept "receive"-direction
   rid-id alternatives.

   An offerer that receives an answer where some of the rid-ids are
   removed compared to the offer MAY release the corresponding resources
   (codec, transport, etc) in its "receive" direction and MUST NOT send
   any RTP packets corresponding to the removed rid-ids.

   An offerer that offered some of its rid-ids as initially paused and
   receives an answer that does not indicate RTP stream pause/resume
   capability MUST NOT initially pause any simulcast streams.

   An offerer with RTP stream pause/resume capability that receives an
   answer where some rid-ids are marked as initially paused SHOULD
   initially pause those RTP streams, even if they were marked as
   initially paused also in the offer, unless it has good reason for
   those RTP streams not being initially paused.  One such reason could
   be, for example, that the answerer would otherwise initially not
   receive any media of that type at all.

5.3.4.  Modifying the Session

   Offers inside an existing session follow the same rules as for
   initial SDP offer, with these additions:

   1.  rid-ids marked as initially paused in the offerer's "send"
       direction SHALL reflect the offerer's opinion of the current
       pause state at the time of creating the offer.  This is purely
       informational, and RTP stream pause/resume signaling [RFC7728] in
       the ongoing session SHALL take precedence in case of any conflict
       or ambiguity.

   2.  rid-ids marked as initially paused in the offerer's "receive"
       direction SHALL (as in an initial offer) reflect the offerer's
       desired rid-id pause state.  Except for the case where the
       offerer already paused the corresponding RTP stream through RTP
       stream pause/resume [RFC7728] signaling, this is identical to the
       conditions at an initial offer.

   Creation of SDP answers and processing of SDP answers inside an
   existing session follow the same rules as described above for initial
   SDP offer/answer.

   Session modification restrictions in Section 6.5 of "RTP Payload
   Format Restrictions" [RFC8851] also apply.

5.4.  Use with Declarative SDP

   This document does not define the use of "a=simulcast" in declarative
   SDP, partly because use of the simulcast format identification
   [RFC8851] is not defined for use in declarative SDP.  If concrete use
   cases for simulcast in declarative SDP are identified in the future,
   the authors of this memo expect that additional specifications will
   address such use.

5.5.  Relating Simulcast Streams

   Simulcast RTP streams MUST be related on the RTP level through
   RtpStreamId [RFC8852], as specified in the SDP "a=simulcast"
   attribute (Section 5.2) parameters.  This is sufficient as long as
   there is only a single media source per SDP media description.  When
   using BUNDLE [RFC8843], where multiple SDP media descriptions jointly
   specify a single RTP session, the SDES MID (Media Identification)
   mechanism in BUNDLE allows relating RTP streams back to individual
   media descriptions, after which the RtpStreamId relations described
   above can be used.  Use of the RTP header extension for the RTCP
   source description items [RFC7941] for both MID and RtpStreamId
   identifications can be important to ensure rapid initial reception,
   required to correctly interpret and process the RTP streams.
   Implementers of this specification MUST support the RTCP source
   description (SDES) item method and SHOULD support RTP header
   extension method to signal RtpStreamId on the RTP level.

   NOTE:  For the case where it is clear from SDP that the RTP PT
      uniquely maps to a corresponding RtpStreamId, an RTP receiver can
      use RTP PT to relate simulcast streams.  This can sometimes enable
      decoding even in advance of receiving RtpStreamId information in
      RTCP SDES and/or RTP header extensions.

   RTP streams MUST only use a single alternative rid-id at a time
   (based on RTP timestamps) but MAY change format (and rid-id) on a
   per-RTP packet basis.  This corresponds to the existing
   (nonsimulcast) SDP offer/answer case when multiple formats are
   included on the "m=" line in the SDP answer, enabling per-RTP packet
   change of RTP payload type.

5.6.  Signaling Examples

   These examples describe a client-to-video-conference service, using a
   centralized media topology with an RTP mixer.

                    +---+      +-----------+      +---+
                    | A |<---->|           |<---->| B |
                    +---+      |           |      +---+
                               |   Mixer   |
                    +---+      |           |      +---+
                    | F |<---->|           |<---->| J |
                    +---+      +-----------+      +---+

                Figure 4: Four-Party Mixer-Based Conference

5.6.1.  Single-Source Client

   Alice is calling in to the mixer with a simulcast-enabled client
   capable of a single media source per media type.  The client can send
   a simulcast of 2 video resolutions and frame rates: HD 1280x720p
   30fps and thumbnail 320x180p 15fps.  This is defined below using the
   "imageattr" [RFC6236].  In this example, only the "pt" "a=rid"
   parameter is used to describe simulcast stream formats, effectively
   achieving a 1:1 mapping between RtpStreamId and media formats (RTP
   payload types).  Alice's Offer:

   v=0
   o=alice 2362969037 2362969040 IN IP4 192.0.2.156
   s=Simulcast-Enabled Client
   c=IN IP4 192.0.2.156
   t=0 0
   m=audio 49200 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   m=video 49300 RTP/AVP 97 98
   a=rtpmap:97 H264/90000
   a=rtpmap:98 H264/90000
   a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
   a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
   a=imageattr:97 send [x=1280,y=720] recv [x=1280,y=720]
   a=imageattr:98 send [x=320,y=180] recv [x=320,y=180]
   a=rid:1 send pt=97
   a=rid:2 send pt=98
   a=rid:3 recv pt=97
   a=simulcast:send 1;2 recv 3
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id

                  Figure 5: Single-Source Simulcast Offer

   The only thing in the SDP that indicates simulcast capability is the
   line in the video media description containing the "simulcast"
   attribute.  The included "a=fmtp" and "a=imageattr" parameters
   indicate that sent simulcast streams can differ in video resolution.
   The RTP header extension for RtpStreamId is offered to avoid issues
   with the initial binding between RTP streams (SSRCs) and the
   RtpStreamId identifying the simulcast stream and its format.

   The answer from the server indicates that it, too, is simulcast
   capable.  Should it not have been simulcast capable, the
   "a=simulcast" line would not have been present, and communication
   would have started with the media negotiated in the SDP.  Also, the
   usage of the RtpStreamId RTP header extension is accepted.

   v=0
   o=server 823479283 1209384938 IN IP4 192.0.2.2
   s=Answer to Simulcast-Enabled Client
   c=IN IP4 192.0.2.43
   t=0 0
   m=audio 49672 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   m=video 49674 RTP/AVP 97 98
   a=rtpmap:97 H264/90000
   a=rtpmap:98 H264/90000
   a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
   a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
   a=imageattr:97 send [x=1280,y=720] recv [x=1280,y=720]
   a=imageattr:98 send [x=320,y=180] recv [x=320,y=180]
   a=rid:1 recv pt=97
   a=rid:2 recv pt=98
   a=rid:3 send pt=97
   a=simulcast:recv 1;2 send 3
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id

                  Figure 6: Single-Source Simulcast Answer

   Since the server is the simulcast media receiver, it reverses the
   direction of the "simulcast" and "rid" attribute parameters.

5.6.2.  Multisource Client

   Fred is calling in to the same conference as in the example above
   with a two-camera, two-display system, thus capable of handling two
   separate media sources in each direction, where each media source is
   simulcast enabled in the "send" direction.  Fred's client is
   restricted to a single media source per media description.

   The first two simulcast streams for the first media source use
   different codecs, H264-SVC [RFC6190] and H264 [RFC6184].  These two
   simulcast streams also have a temporal dependency.  Two different
   video codecs, VP8 [RFC7741] and H264, are offered as alternatives for
   the third simulcast stream for the first media source.  Only the
   highest-fidelity simulcast stream is sent from start, the lower-
   fidelity streams being initially paused.

   The second media source is offered with three different simulcast
   streams.  All video streams of this second media source are loss
   protected by RTP retransmission [RFC4588].  In addition, all but the
   highest-fidelity simulcast stream are initially paused.  Note that
   the lower resolution is more prioritized than the medium-resolution
   simulcast stream.

   Fred's client is also using BUNDLE to send all RTP streams from all
   media descriptions in the same RTP session on a single media
   transport.  Although using many different simulcast streams in this
   example, the use of RtpStreamId as simulcast stream identification
   enables use of a low number of RTP payload types.  Note that when
   using both BUNDLE [RFC8843] and "a=rid" [RFC8851], it is recommended
   to use the RTP header extension for the RTCP source descriptions
   items [RFC7941] for carrying these RTP stream-identification fields,
   which is consequently also included in the SDP.  Note also that for
   "a=rid", the corresponding RtpStreamId SDES attribute RTP header
   extension is named rtp-stream-id [RFC8852].

   v=0
   o=fred 238947129 823479223 IN IP6 2001:db8::c000:27d
   s=Offer from Simulcast-Enabled Multi-Source Client
   c=IN IP6 2001:db8::c000:27d
   t=0 0
   a=group:BUNDLE foo bar zen
   m=audio 49200 RTP/AVP 99
   a=mid:foo
   a=rtpmap:99 G722/8000
   m=video 49600 RTP/AVPF 100 101 103
   a=mid:bar
   a=rtpmap:100 H264-SVC/90000
   a=rtpmap:101 H264/90000
   a=rtpmap:103 VP8/90000
   a=fmtp:100 profile-level-id=42400d;max-fs=3600;max-mbps=216000; \
       mst-mode=NI-TC
   a=fmtp:101 profile-level-id=42c00d;max-fs=3600;max-mbps=108000
   a=fmtp:103 max-fs=900; max-fr=30
   a=rid:1 send pt=100;max-width=1280;max-height=720;max-fps=60;depend=2
   a=rid:2 send pt=101;max-width=1280;max-height=720;max-fps=30
   a=rid:3 send pt=101;max-width=640;max-height=360
   a=rid:4 send pt=103;max-width=640;max-height=360
   a=depend:100 lay bar:101
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
   a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   a=rtcp-fb:* ccm pause nowait
   a=simulcast:send 1;2;~4,3
   m=video 49602 RTP/AVPF 96 104
   a=mid:zen
   a=rtpmap:96 VP8/90000
   a=fmtp:96 max-fs=3600; max-fr=30
   a=rtpmap:104 rtx/90000
   a=fmtp:104 apt=96;rtx-time=200
   a=rid:1 send max-fs=921600;max-fps=30
   a=rid:2 send max-fs=614400;max-fps=15
   a=rid:3 send max-fs=230400;max-fps=30
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
   a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
   a=rtcp-fb:* ccm pause nowait
   a=simulcast:send 1;~3;~2

                Figure 7: Fred's Multisource Simulcast Offer

5.6.3.  Simulcast and Redundancy

   The example in this section looks at applying simulcast with audio
   and video redundancy formats.  The audio media description uses codec
   and bitrate restrictions, combined with the RTP payload for redundant
   audio data [RFC2198] for enhanced packet-loss resilience.  The video
   media description applies both resolution and bitrate restrictions,
   combined with Forward Error Correction (FEC) in the form of flexible
   FEC [RFC8627] and RTP retransmission [RFC4588].

   The audio source is offered to be sent as two simulcast streams.  The
   first simulcast stream is encoded with Opus, restricted to 64 kbps
   (rid-id=1), and the second simulcast stream (rid-id=2) is encoded
   with either G.711, or G.711 combined with linear predictive coding
   (LPC) for redundancy and explicit comfort noise (CN).  Both simulcast
   streams include telephone-event capability.  In this example, stand-
   alone LPC is not offered as a possible payload type for the second
   simulcast stream's RID, which could be motivated by, for example, not
   providing sufficient quality.

   The video source is offered to be sent as two simulcast streams, both
   with two alternative simulcast formats.  Redundancy and repair are
   offered in the form of both flexible FEC and RTP retransmission.  The
   flexible FEC is not bound to any particular RTP streams and is
   therefore able to be used across all RTP streams that are being sent
   as part of this media description.

   o=fred 238947129 823479223 IN IP6 2001:db8::c000:27d
   s=Offer from Simulcast-Enabled Client using Redundancy
   c=IN IP6 2001:db8::c000:27d
   t=0 0
   a=group:BUNDLE foo bar
   m=audio 49200 RTP/AVP 97 98 99 100 101 102
   a=mid:foo
   a=rtpmap:97 G711/8000
   a=rtpmap:98 LPC/8000
   a=rtpmap:99 OPUS/48000/1
   a=rtpmap:100 RED/8000/1
   a=rtpmap:101 CN/8000
   a=rtpmap:102 telephone-event/8000
   a=fmtp:99 useinbandfec=1;usedtx=0
   a=fmtp:100 97/98
   a=fmtp:102 0-15
   a=ptime:20
   a=maxptime:40
   a=rid:1 send pt=99,102;max-br=64000
   a=rid:2 send pt=100,97,101,102
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
   a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   a=simulcast:send 1;2
   m=video 49600 RTP/AVPF 103 104 105 106 107
   a=mid:bar
   a=rtpmap:103 H264/90000
   a=rtpmap:104 VP8/90000
   a=rtpmap:105 rtx/90000
   a=rtpmap:106 rtx/90000
   a=rtpmap:107 flexfec/90000
   a=fmtp:103 profile-level-id=42c00d;max-fs=3600;max-mbps=108000
   a=fmtp:104 max-fs=3600; max-fr=30
   a=fmtp:105 apt=103;rtx-time=200
   a=fmtp:106 apt=104;rtx-time=200
   a=fmtp:107 repair-window=100000
   a=rid:1 send pt=103;max-width=1280;max-height=720;max-fps=30
   a=rid:2 send pt=104;max-width=1280;max-height=720;max-fps=30
   a=rid:3 send pt=103;max-width=640;max-height=360;max-br=300000
   a=rid:4 send pt=104;max-width=640;max-height=360;max-br=300000
   a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
   a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
   a=rtcp-fb:* ccm pause nowait
   a=simulcast:send 1,2;3,4

                 Figure 8: Simulcast and Redundancy Example

6.  RTP Aspects

   This section discusses what the different entities in a simulcast
   media path can expect to happen on the RTP level.  This is explored
   from source to sink by starting in an endpoint with a media source
   that is simulcasted to an RTP middlebox.  That RTP middlebox sends
   media sources to other RTP middleboxes (cascaded middleboxes), as
   well as selecting some simulcast format of the media source and
   sending it to receiving endpoints.  Different types of RTP
   middleboxes and their usage of the different simulcast formats
   results in several different behaviors.

6.1.  Outgoing from Endpoint with Media Source

   The most straightforward simulcast case is the RTP streams being
   emitted from the endpoint that originates a media source.  When
   simulcast has been negotiated in the sending direction, the endpoint
   can transmit up to the number of RTP streams needed for the
   negotiated simulcast streams for that media source.  Each RTP stream
   (SSRC) is identified by associating it (Section 5.5) with an
   RtpStreamId SDES item, transmitted in RTCP and possibly also as an
   RTP header extension.  In cases where multiple media sources have
   been negotiated for the same RTP session and thus BUNDLE [RFC8843] is
   used, the MID SDES item will also be sent, similarly to the
   RtpStreamId.

   Each RTP stream might not be continuously transmitted due to any of
   the following reasons: temporarily paused using Pause/Resume
   [RFC7728], sender-side application logic temporarily pausing it, or
   lack of network resources to transmit this simulcast stream.
   However, all simulcast streams that have been negotiated have active
   and maintained SSRCs (at least in regular RTCP reports), even if no
   RTP packets are currently transmitted.  The relation between an RTP
   stream (SSRC) and a particular simulcast stream is not expected to
   change, except in exceptional situations such as SSRC collisions.  At
   SSRC changes, the usage of MID and RtpStreamId should enable the
   receiver to correctly identify the RTP streams even after an SSRC
   change.

6.2.  RTP Middlebox to Receiver

   RTP streams in a multiparty RTP session can be used in multiple
   different ways when the session utilizes simulcast at least on the
   media-source-to-middlebox legs.  This is to a large degree due to the
   different RTP middlebox behaviors, but also the needs of the
   application.  This text assumes that the RTP middlebox will select a
   media source and choose which simulcast stream for that media source
   to deliver to a specific receiver.  In many cases, at most one
   simulcast stream per media source will be forwarded to a particular
   receiver at any instant in time, even if the selected simulcast
   stream may vary.  For cases where this does not hold due to
   application needs, the RTP stream aspects will fall under the
   middlebox-to-middlebox case (Section 6.3).

   The selection of which simulcast streams to forward towards the
   receiver is application specific.  However, in conferencing
   applications, active speaker selection is common.  In case the number
   of media sources possible to forward, N, is less than the total
   number of media sources available in a multimedia session, the
   current and previous speakers (up to N in total) are often the ones
   forwarded.  To avoid the need for media-specific processing to
   determine the current speaker(s) in the RTP middlebox, the endpoint
   providing a media source may include metadata, such as the RTP header
   extension for client-to-mixer audio level indication [RFC6464].

   The possibilities for stream switching are media type specific, but
   for media types with significant interframe dependencies in the
   encoding, like most video coding, the switching needs to be made at
   suitable switching points in the media stream that breaks or
   otherwise deals with the dependency structure.  Even if switching
   points can be included periodically, it is common to use mechanisms
   like Full Intra Requests [RFC5104] to request switching points from
   the endpoint performing the encoding of the media source.

   Inclusion of the RtpStreamId SDES item for an SSRC in the middlebox-
   to-receiver direction should only occur when use of RtpStreamId has
   been negotiated in that direction.  It is worth noting that one can
   signal multiple RtpStreamIds when simulcast signaling indicates only
   a single simulcast stream, allowing one to use all of the
   RtpStreamIds as alternatives for that simulcast stream.  One reason
   for including the RtpStreamId in the middlebox-to-receiver direction
   for an RTP stream is to let the receiver know which restrictions
   apply to the currently delivered RTP stream.  In case the RtpStreamId
   is negotiated to be used, it is important to remember that the used
   identifiers will be specific to each signaling session.  Even if the
   central entity can attempt to coordinate, it is likely that the
   RtpStreamIds need to be translated to the leg-specific values.  The
   below cases will assume that RtpStreamId is not used in the mixer to
   receiver direction.

6.2.1.  Media-Switching Mixer

   This section discusses the behavior in cases where the RTP middlebox
   behaves like the media-switching mixer in RTP topologies
   (Section 3.6.2 of [RFC7667]).  The fundamental aspect here is that
   the media sources delivered from the middlebox will be the mixer's
   conceptual or functional ones.  For example, one media source may be
   the main speaker in high-resolution video, while a number of other
   media sources are thumbnails of each participant.

   The above results in the RTP stream produced by the mixer being one
   that switches between a number of received incoming RTP streams for
   different media sources and in different simulcast versions.  The
   mixer selects the media source to be sent as one of the RTP streams
   and then selects among the available simulcast streams for the most
   appropriate one.  The selection criteria include available bandwidth
   on the mixer-to-receiver path and restrictions based on the
   functional usage of the RTP stream delivered to the receiver.  As an
   example of the latter, it is unnecessary to forward a full HD video
   to a receiver if the display area is just a thumbnail.  Thus,
   restrictions may exist to not allow some simulcast streams to be
   forwarded for some of the mixer's media sources.

   This will result in a single RTP stream being used for each of the
   RTP mixer's media sources.  At any point in time, this RTP stream is
   a selection of one particular RTP stream arriving to the mixer, where
   the RTP header-field values are rewritten to provide a consistent,
   single RTP stream.  If the RTP mixer doesn't receive any incoming
   stream matched to this media source, the SSRC will not transmit but
   be kept alive using RTCP.  The SSRC and thus RTP stream for the
   mixer's media source is expected to be long-term stable.  It will
   only be changed by signaling or other disruptive events.  Note that
   although the above talks about a single RTP stream, there can in some
   cases be multiple RTP streams carrying the selected simulcast stream
   for the originating media source, including redundancy or other
   auxiliary RTP streams.

   The mixer may communicate the identity of the originating media
   source to the receiver by including the Contributing Source (CSRC)
   field with the originating media source's SSRC value.  Note that due
   to the possibility that the RTP mixer switches between simulcast
   versions of the media source, the CSRC value may change, even if the
   media source is kept the same.

   It is important to note that any MID SDES item from the originating
   media source needs to be removed and not be associated with the RTP
   stream's SSRC.  That is, there is nothing in the signaling between
   the mixer and the receiver that is structured around the originating
   media sources, only the mixer's media sources.  If they were
   associated with the SSRC, the receiver would likely believe that
   there has been an SSRC collision and the RTP stream is spurious,
   because it doesn't carry the identifiers used to relate it to the
   correct context.  However, this is not true for CSRC values, as long
   as they are never used as an SSRC.  In these cases, one could provide
   CNAME and MID as SDES items.  A receiver could use this to determine
   which CSRC values that are associated with the same originating media
   source.

   If RtpStreamIds are used in the scenario described by this section,
   it should be noted that the RtpStreamId on a particular SSRC will
   change based on the actual simulcast stream selected for switching.
   These RtpStreamId identifiers will be local to this leg's signaling
   context.  In addition, the defined RtpStreamIds and their parameters
   need to cover all the media sources and simulcast streams received by
   the RTP mixer that can be switched into this media source, sent by
   the RTP mixer.

6.2.2.  Selective Forwarding Middlebox

   This section discusses the behavior in cases where the RTP middlebox
   behaves like the Selective Forwarding Middlebox in RTP topologies
   (Section 3.7 of [RFC7667]).  Applications for this type of RTP
   middlebox result in each originating media source having a
   corresponding media source on the leg between the middlebox and the
   receiver.  A Selective Forwarding Middlebox (SFM) could go as far as
   exposing all the simulcast streams for a media source; however, this
   section will focus on having a single simulcast stream that can
   contain any of the simulcast formats.  This section will assume that
   the SFM projection mechanism works on the media-source level and maps
   one of the media source's simulcast streams onto one RTP stream from
   the SFM to the receiver.

   This usage will result in the individual RTP stream(s) for one media
   source being able to switch between being active and paused, based on
   the subset of media sources the SFM wants to provide the receiver for
   the moment.  With SFMs, there exist no reasons to use CSRC to
   indicate the originating stream, as there is a one-to-one media-
   source mapping.  If the application requires knowing the simulcast
   version received to function well, then RtpStreamId should be
   negotiated on the SFM to receiver leg.  Which simulcast stream that
   is being forwarded is not made explicit unless RtpStreamId is used on
   the leg.

   Any MID SDES items being sent by the SFM to the receiver are only
   those agreed between the SFM and the receiver, and no MID values from
   the originating side of the SFM are to be forwarded.

   An SFM could expose corresponding RTP streams for all the media
   sources and their simulcast streams and then, for any media source
   that is to be provided, forward one selected simulcast stream.
   However, this is not recommended, as it would unnecessarily increase
   the number of RTP streams and require the receiver to timely detect
   switching between simulcast streams.  The above usage requires the
   same SFM functionality for switching, while avoiding the
   uncertainties of timely detecting that an RTP stream ends.  The
   benefit would be that the received simulcast stream would be
   implicitly provided by which RTP stream would be active for a media
   source.  However, using RtpStreamId to make this explicit also
   exposes which alternative format is used.  The conclusion is that
   using one RTP stream per simulcast stream is unnecessary.  The issue
   with timely detecting end of streams, independent of whether they are
   stopped temporarily or long term, is that there is no explicit
   indication that the transmission has intentionally been stopped.  The
   RTCP-based pause and resume mechanism [RFC7728] includes a PAUSED
   indication that provides the last RTP sequence number transmitted
   prior to the pause.  Due to usage, the timeliness of this solution
   depends on when delivery using RTCP can occur in relation to the
   transmission of the last RTP packet.  If no explicit information is
   provided at all, then detection based on nonincreasing RTCP SR field
   values and timers need to be used to determine pause in RTP packet
   delivery.  As a result, when the last RTP packet arrives (if it
   arrives), one usually cannot determine that this will be the last.
   That it was the last is something that one learns later.

6.3.  RTP Middlebox to RTP Middlebox

   This relates to the transmission of simulcast streams between RTP
   middleboxes or other usages where one wants to enable the delivery of
   multiple simultaneous simulcast streams per media source, but the
   transmitting entity is not the originating endpoint.  For a
   particular direction between middleboxes A and B, this looks very
   similar to the originating-to-middlebox case on a media-source basis.
   However, in this case, there are usually multiple media sources,
   originating from multiple endpoints.  This can create situations
   where limitations in the number of simultaneously received media
   streams can arise -- for example, due to limitation in network
   bandwidth.  In this case, a subset of not only the simulcast streams
   but also media sources can be selected.  As a result, individual RTP
   streams can become paused at any point and later be resumed based on
   various criteria.

   The MIDs used between A and B are the ones agreed between these two
   identities in signaling.  The RtpStreamId values will also be
   provided to ensure explicit information about which simulcast stream
   they are.  The RTP-stream-to-MID and -RtpStreamId associations should
   here be long-term stable.

7.  Network Aspects

   Simulcast is in this memo defined as the act of sending multiple
   alternative encoded streams of the same underlying media source.
   Transmitting multiple independent streams that originate from the
   same source could potentially be done in several different ways using
   RTP.  A general discussion on considerations for use of the different
   RTP multiplexing alternatives can be found in "Guidelines for Using
   the Multiplexing Features of RTP to Support Multiple Media Streams"
   [RFC8872].  Discussion and clarification on how to handle multiple
   streams in an RTP session can be found in [RFC8108].

   The network aspects that are relevant for simulcast are:

   Quality of Service (QoS):  When using simulcast, it might be of
      interest to prioritize a particular simulcast stream, rather than
      applying equal treatment to all streams.  For example, lower-
      bitrate streams may be prioritized over higher-bitrate streams to
      minimize congestion or packet losses in the low-bitrate streams.
      Thus, there is a benefit to using a simulcast solution with good
      QoS support.

   NAT/FW Traversal (Network Address Translator / Firewall
   Traversal):  Using multiple RTP sessions incurs more cost for NAT/FW
      traversal unless they can reuse the same transport flow, which can
      be achieved by multiplexing negotiation using SDP port numbers
      [RFC8843].


7.1.  Bitrate Adaptation

   Use of multiple simulcast streams can require a significant amount of
   network resources.  The aggregate bandwidth for all simulcast streams
   for a media source (and thus SDP media description) is bounded by any
   SDP "b=" line applicable to that media source.  It is assumed that a
   suitable congestion-control mechanism is used by the application to
   ensure that it doesn't cause persistent congestion.  If the amount of
   available network resources varies during an RTP session such that it
   does not match what is negotiated in SDP, the bitrate used by the
   different simulcast streams may have to be reduced dynamically.  When
   a simulcasting media source uses a single media transport for all of
   the simulcast streams, it is likely that a joint congestion control
   across all simulcast streams is used for that media source.  What
   simulcast streams to prioritize when allocating available bitrate
   among the simulcast streams in such adaptation SHOULD be taken from
   the simulcast stream order on the "a=simulcast" line and ordering of
   alternative simulcast formats (Section 5.2).  Simulcast streams that
   have pause/resume capability and that would be given such low bitrate
   by the adaptation process that they are considered not really useful
   can be temporarily paused until the limiting condition clears.

8.  Limitation

   The chosen approach has a limitation that relates to the use of a
   single RTP session for all simulcast formats of a media source, which
   comes from sending all simulcast streams related to a media source
   under the same SDP media description.

   It is not possible to use different simulcast streams on different
   media transports, which limits the possibilities for applying
   different QoS to different simulcast streams.  When using unicast,
   QoS mechanisms based on individual packet marking are feasible, since
   they do not require separation of simulcast streams into different
   RTP sessions to apply different QoS.

   It is also not possible to separate different simulcast streams into
   different multicast groups to allow a multicast receiver to pick the
   stream it wants, rather than receive all of them.  In this case, the
   only reasonable implementation is to use different RTP sessions for
   each multicast group so that reporting and other RTCP functions
   operate as intended.  Such simulcast usage in a multicast context is
   out of scope for the current document and would require additional
   specification.

9.  IANA Considerations

   This document registers a new media-level SDP attribute, "simulcast",
   in the "att-field (media level only)" registry within the "Session
   Description Protocol (SDP) Parameters" registry, according to the
   procedures of [RFC4566] and [RFC8859].

   Contact name, email:  The IESG (iesg@ietf.org)

   Attribute name:  simulcast

   Long-form attribute name:  Simulcast stream description

   Charset dependent:  No

   Attribute value:  sc-value; see Section 5.1 of RFC 8853.

   Purpose:  Signals simulcast capability for a set of RTP streams

   Mux category:  NORMAL

10.  Security Considerations

   The simulcast capability, configuration attributes, and parameters
   are vulnerable to attacks in signaling.

   A false inclusion of the "a=simulcast" attribute may result in
   simultaneous transmission of multiple RTP streams that would
   otherwise not be generated.  The impact is limited by the media
   description joint bandwidth, shared by all simulcast streams
   irrespective of their number.  However, there may be a large number
   of unwanted RTP streams that will impact the share of bandwidth
   allocated for the originally wanted RTP stream.

   A hostile removal of the "a=simulcast" attribute will result in
   simulcast not being used.

   Integrity protection and source authentication of all SDP signaling,
   including simulcast attributes, can mitigate the risks of such
   attacks that attempt to alter signaling.

   Security considerations related to the use of "a=rid" and the
   RtpStreamId SDES item are covered in [RFC8851] and [RFC8852].  There
   are no additional security concerns related to their use in this
   specification.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [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>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC7728]  Burman, B., Akram, A., Even, R., and M. Westerlund, "RTP
              Stream Pause and Resume", RFC 7728, DOI 10.17487/RFC7728,
              February 2016, <https://www.rfc-editor.org/info/rfc7728>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8843]  Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", RFC 8843,
              DOI 10.17487/RFC8843, January 2021,
              <https://www.rfc-editor.org/info/rfc8843>.

   [RFC8851]  Roach, A.B., Ed., "RTP Payload Format Restrictions",
              RFC 8851, DOI 10.17487/RFC8851, January 2021,
              <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, January 2021,
              <https://www.rfc-editor.org/info/rfc8852>.

   [RFC8859]  Nandakumar, S., "A Framework for Session Description
              Protocol (SDP) Attributes When Multiplexing", RFC 8859,
              DOI 10.17487/RFC8859, January 2021,
              <https://www.rfc-editor.org/info/rfc8859>.

11.2.  Informative References

   [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>.

   [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>.

   [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>.

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              DOI 10.17487/RFC4733, December 2006,
              <https://www.rfc-editor.org/info/rfc4733>.

   [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>.

   [RFC5583]  Schierl, T. and S. Wenger, "Signaling Media Decoding
              Dependency in the Session Description Protocol (SDP)",
              RFC 5583, DOI 10.17487/RFC5583, July 2009,
              <https://www.rfc-editor.org/info/rfc5583>.

   [RFC6184]  Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184,
              DOI 10.17487/RFC6184, May 2011,
              <https://www.rfc-editor.org/info/rfc6184>.

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

   [RFC6236]  Johansson, I. and K. Jung, "Negotiation of Generic Image
              Attributes in the Session Description Protocol (SDP)",
              RFC 6236, DOI 10.17487/RFC6236, May 2011,
              <https://www.rfc-editor.org/info/rfc6236>.

   [RFC6464]  Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
              Transport Protocol (RTP) Header Extension for Client-to-
              Mixer Audio Level Indication", RFC 6464,
              DOI 10.17487/RFC6464, December 2011,
              <https://www.rfc-editor.org/info/rfc6464>.

   [RFC7104]  Begen, A., Cai, Y., and H. Ou, "Duplication Grouping
              Semantics in the Session Description Protocol", RFC 7104,
              DOI 10.17487/RFC7104, January 2014,
              <https://www.rfc-editor.org/info/rfc7104>.

   [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>.

   [RFC7741]  Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
              Galligan, "RTP Payload Format for VP8 Video", RFC 7741,
              DOI 10.17487/RFC7741, March 2016,
              <https://www.rfc-editor.org/info/rfc7741>.

   [RFC7941]  Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
              Header Extension for the RTP Control Protocol (RTCP)
              Source Description Items", RFC 7941, DOI 10.17487/RFC7941,
              August 2016, <https://www.rfc-editor.org/info/rfc7941>.

   [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>.

   [RFC8627]  Zanaty, M., Singh, V., Begen, A., and G. Mandyam, "RTP
              Payload Format for Flexible Forward Error Correction
              (FEC)", RFC 8627, DOI 10.17487/RFC8627, July 2019,
              <https://www.rfc-editor.org/info/rfc8627>.

   [RFC8872]  Westerlund, M., Burman, B., Perkins, C., Alvestrand, H.,
              and R. Even, "Guidelines for Using the Multiplexing
              Features of RTP to Support Multiple Media Streams",
              RFC 8872, DOI 10.17487/RFC8872, January 2021,
              <https://www.rfc-editor.org/info/rfc8872>.

Appendix A.  Requirements

   The following requirements are met by the defined solution to support
   the use cases (Section 3):

   REQ-1:  Identification:

      REQ-1.1:  It must be possible to identify a set of simulcasted RTP
         streams as originating from the same media source in SDP
         signaling.

      REQ-1.2:  An RTP endpoint must be capable of identifying the
         simulcast stream that a received RTP stream is associated with,
         knowing the content of the SDP signaling.

   REQ-2:  Transport usage.  The solution must work when using:

      REQ-2.1:  Legacy SDP with separate media transports per SDP media
         description.

      REQ-2.2:  Bundled [RFC8843] SDP media descriptions.

   REQ-3:  Capability negotiation.  The following must be possible:

      REQ-3.1:  The sender can express capability of sending simulcast.

      REQ-3.2:  The receiver can express capability of receiving
         simulcast.

      REQ-3.3:  The sender can express the maximum number of simulcast
         streams that can be provided.

      REQ-3.4:  The receiver can express the maximum number of simulcast
         streams that can be received.

      REQ-3.5:  The sender can detail the characteristics of the
         simulcast streams that can be provided.

      REQ-3.6:  The receiver can detail the characteristics of the
         simulcast streams that it prefers to receive.

   REQ-4:  Distinguishing features.  It must be possible to have
      different simulcast streams use different codec parameters, as can
      be expressed by SDP format values and RTP payload types.

   REQ-5:  Compatibility.  It must be possible to use simulcast in
      combination with other RTP mechanisms that generate additional RTP
      streams:

      REQ-5.1:  RTP retransmission [RFC4588].

      REQ-5.2:  RTP Forward Error Correction [RFC5109].

      REQ-5.3:  Related payload types such as audio Comfort Noise and/or
         DTMF.

      REQ-5.4:  A single simulcast stream can consist of multiple RTP
         streams, to support codecs where a dependent stream is
         dependent on a set of encoded and dependent streams, each
         potentially carried in their own RTP stream.

   REQ-6:  Interoperability.  The solution must be possible to use in:

      REQ-6.1:  Interworking with nonsimulcast legacy clients using a
         single media source per media type.

      REQ-6.2:  WebRTC environment with a single media source per SDP
         media description.

Acknowledgements

   The authors would like to thank Bernard Aboba, Thomas Belling, Roni
   Even, Adam Roach, Iñaki Baz Castillo, Paul Kyzivat, and Arun
   Arunachalam for the feedback they provided during the development of
   this document.

Contributors

   Morgan Lindqvist and Fredrik Jansson, both from Ericsson, have
   contributed with important material to the first draft versions of
   this document.  Robert Hanton and Cullen Jennings from Cisco, Peter
   Thatcher from Google, and Adam Roach from Mozilla contributed
   significantly to subsequent versions.

Authors' Addresses

   Bo Burman
   Ericsson
   Gronlandsgatan 31
   SE-164 60 Stockholm
   Sweden

   Email: bo.burman@ericsson.com


   Magnus Westerlund
   Ericsson
   Torshamnsgatan 23
   SE-164 83 Stockholm
   Sweden

   Email: magnus.westerlund@ericsson.com


   Suhas Nandakumar
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134
   United States of America

   Email: snandaku@cisco.com


   Mo Zanaty
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134
   United States of America

   Email: mzanaty@cisco.com
