path: root/doc/rfc/rfc8851.txt

Internet Engineering Task Force (IETF)                   A.B. Roach, Ed.
Request for Comments: 8851                                       Mozilla
Updates: 4855                                               January 2021
Category: Standards Track                                               
ISSN: 2070-1721


                    RTP Payload Format Restrictions

Abstract

   In this specification, we define a framework for specifying
   restrictions on RTP streams in the Session Description Protocol
   (SDP).  This framework defines a new "rid" ("restriction identifier")
   SDP attribute 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.

   This specification updates RFC 4855 to give additional guidance on
   choice of Format Parameter (fmtp) names and their relation to the
   restrictions defined by this document.

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/rfc8851.

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.  Terminology
   2.  Introduction
   3.  Key Words for Requirements
   4.  SDP "a=rid" Media Level Attribute
   5.  "a=rid" Restrictions
   6.  SDP Offer/Answer Procedures
     6.1.  Generating the Initial SDP Offer
     6.2.  Answerer Processing the SDP Offer
       6.2.1.  "a=rid"-Unaware Answerer
       6.2.2.  "a=rid"-Aware Answerer
     6.3.  Generating the SDP Answer
     6.4.  Offerer Processing of the SDP Answer
     6.5.  Modifying the Session
   7.  Use with Declarative SDP
   8.  Interaction with Other Techniques
     8.1.  Interaction with VP8 Format Parameters
       8.1.1.  max-fr - Maximum Frame Rate
       8.1.2.  max-fs - Maximum Frame Size, in VP8 Macroblocks
     8.2.  Interaction with H.264 Format Parameters
       8.2.1.  profile-level-id and max-recv-level - Negotiated
               Subprofile
       8.2.2.  max-br / MaxBR - Maximum Video Bitrate
       8.2.3.  max-fs / MaxFS - Maximum Frame Size, in H.264
               Macroblocks
       8.2.4.  max-mbps / MaxMBPS - Maximum Macroblock Processing Rate
       8.2.5.  max-smbps - Maximum Decoded Picture Buffer
     8.3.  Redundancy Formats and Payload Type Restrictions
   9.  Format Parameters for Future Payloads
   10. Formal Grammar
   11. SDP Examples
     11.1.  Many Bundled Streams Using Many Codecs
     11.2.  Scalable Layers
   12. IANA Considerations
     12.1.  New SDP Media-Level Attribute
     12.2.  Registry for RID-Level Parameters
   13. Security Considerations
   14. References
     14.1.  Normative References
     14.2.  Informative References
   Acknowledgements
   Contributors
   Author's Address

1.  Terminology

   The terms "source RTP stream", "endpoint", "RTP session", and "RTP
   stream" are used as defined in [RFC7656].

   [RFC4566] and [RFC3264] terminology is also used where appropriate.

2.  Introduction

   The payload type (PT) field in RTP provides a mapping between the RTP
   payload format and the associated SDP media description.  For a given
   PT, the SDP rtpmap and/or fmtp attributes are used to describe the
   properties of the media that is carried in the RTP payload.

   Recent advances in standards have given rise to rich multimedia
   applications requiring support for either multiple RTP streams within
   an RTP session [RFC8843] [RFC8853] or a large number of codecs.
   These demands have unearthed challenges inherent with:

   *  The restricted RTP PT space in specifying the various payload
      configurations

   *  The codec-specific constructs for the payload formats in SDP

   *  Missing or underspecified payload format parameters

   *  Overloading of PTs to indicate not just codec configurations, but
      individual streams within an RTP session

   To expand on these points: [RFC3550] assigns 7 bits for the PT in the
   RTP header.  However, the assignment of static mapping of RTP payload
   type numbers to payload formats and multiplexing of RTP with other
   protocols (such as the RTP Control Protocol (RTCP)) could result in a
   limited number of payload type numbers available for application
   usage.  In scenarios where the number of possible RTP payload
   configurations exceeds the available PT space within an RTP session,
   there is a need for a way to represent the additional restrictions on
   payload configurations and effectively map an RTP stream to its
   corresponding restrictions.  This issue is exacerbated by the
   increase in techniques -- such as simulcast and layered codecs --
   that introduce additional streams into RTP sessions.

   This specification defines a new SDP framework for restricting source
   RTP streams (Section 2.1.10 of [RFC7656]), along with the SDP
   attributes to restrict payload formats in a codec-agnostic way.  This
   framework can be thought of as a complementary extension to the way
   the media format parameters are specified in SDP today, via the
   "a=fmtp" attribute.

   The additional restrictions on individual streams are indicated with
   a new "a=rid" ("restriction identifier") SDP attribute.  Note that
   the restrictions communicated via this attribute only serve to
   further restrict the parameters that are established on a PT format.
   They do not relax any restrictions imposed by other mechanisms.

   This specification makes use of the RTP Stream Identifier Source
   Description (SDES) RTCP item defined in [RFC8852] to provide
   correlation between the RTP packets and their format specification in
   the SDP.

   As described in Section 6.2.1, this mechanism achieves backwards
   compatibility via the normal SDP processing rules, which require
   unknown "a=" lines to be ignored.  This means that implementations
   need to be prepared to handle successful offers and answers from
   other implementations that neither indicate nor honor the
   restrictions requested by this mechanism.

   Further, as described in Section 6 and its subsections, this
   mechanism achieves extensibility by: (a) having offerers include all
   supported restrictions in their offer, and (b) having answerers
   ignore "a=rid" lines that specify unknown restrictions.

3.  Key Words for Requirements

   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.

4.  SDP "a=rid" Media Level Attribute

   This section defines new SDP media-level attribute [RFC4566],
   "a=rid", used to communicate a set of restrictions to be applied to
   an identified RTP stream.  Roughly speaking, this attribute takes the
   following form (see Section 10 for a formal definition):

   a=rid:<rid-id> <direction> [pt=<fmt-list>;]<restriction>=<value>...

   An "a=rid" SDP media attribute specifies restrictions defining a
   unique RTP payload configuration identified via the "rid-id" field.
   This value binds the restriction to the RTP stream identified by its
   RTP Stream Identifier Source Description (SDES) item [RFC8852].
   Implementations that use the "a=rid" parameter in SDP MUST support
   the RtpStreamId SDES item described in [RFC8852].  Such
   implementations MUST send that SDES item for all streams in an SDP
   media description ("m=") that have "a=rid" lines remaining after
   applying the rules in Section 6 and its subsections.

   Implementations that use the "a=rid" parameter in SDP and make use of
   redundancy RTP streams [RFC7656] -- e.g., RTP RTX [RFC4588] or
   Forward Error Correction (FEC) [RFC5109] -- for any of the source RTP
   streams that have "a=rid" lines remaining after applying the rules in
   Section 6 and its subsections MUST support the RepairedRtpStreamId
   SDES item described in [RFC8852] for those redundancy RTP streams.
   RepairedRtpStreamId MUST be used for redundancy RTP streams to which
   it can be applied.  Use of RepairedRtpStreamId is not applicable for
   redundancy formats that directly associate RTP streams through shared
   synchronization sources (SSRCs) -- for example, [RFC8627] -- or other
   cases that RepairedRtpStreamId cannot support, such as referencing
   multiple source streams.

   RepairedRtpStreamId is used to provide the binding between the
   redundancy RTP stream and its source RTP stream by setting the
   RepairedRtpStreamId value for the redundancy RTP stream to the
   RtpStreamId value of the source RTP stream.  The redundancy RTP
   stream MAY (but need not) have an "a=rid" line of its own, in which
   case the RtpStreamId SDES item value will be different from the
   corresponding source RTP stream.

   It is important to note that this indirection may result in the
   temporary inability to correctly associate source and redundancy data
   when the SSRC associated with the RtpStreamId or RepairedRtpStreamId
   is dynamically changed during the RTP session.  This can be avoided
   if all RTP packets, source and repair, include their RtpStreamId or
   RepairedRtpStreamId, respectively, after the change.  To maximize the
   probability of reception and utility of redundancy information after
   such a change, all the source packets referenced by the first several
   repair packets SHOULD include such information.  It is RECOMMENDED
   that the number of such packets is large enough to give a high
   probability of actual updated association.  Section 4.1.1 of
   [RFC8285] provides relevant guidance for RTP header extension
   transmission considerations.  Alternatively, to avoid this issue,
   redundancy mechanisms that directly reference its source data may be
   used, such as [RFC8627].

   The "direction" field identifies the direction of the RTP stream
   packets to which the indicated restrictions are applied.  It may be
   either "send" or "recv".  Note that these restriction directions are
   expressed independently of any "inactive", "sendonly", "recvonly", or
   "sendrecv" attributes associated with the media section.  It is, for
   example, valid to indicate "recv" restrictions on a "sendonly"
   stream; those restrictions would apply if, at a future point in time,
   the stream were changed to "sendrecv" or "recvonly".

   The optional "pt=<fmt-list>" lists one or more PT values that can be
   used in the associated RTP stream.  If the "a=rid" attribute contains
   no "pt", then any of the PT values specified in the corresponding
   "m=" line may be used.

   The list of zero or more codec-agnostic restrictions (Section 5)
   describes the restrictions that the corresponding RTP stream will
   conform to.

   This framework MAY be used in combination with the "a=fmtp" SDP
   attribute for describing the media format parameters for a given RTP
   payload type.  In such scenarios, the "a=rid" restrictions
   (Section 5) further restrict the equivalent "a=fmtp" attributes.

   A given SDP media description MAY have zero or more "a=rid" lines
   describing various possible RTP payload configurations.  A given
   "rid-id" MUST NOT be repeated in a given media description ("m="
   section).

   The "a=rid" media attribute MAY be used for any RTP-based media
   transport.  It is not defined for other transports, although other
   documents may extend its semantics for such transports.

   Though the restrictions specified by the "rid" restrictions follow a
   syntax similar to session-level and media-level parameters, they are
   defined independently.  All "rid" restrictions MUST be registered
   with IANA, using the registry defined in Section 12.

   Section 10 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234]
   grammar for the "rid" attribute.  The "a=rid" media attribute is not
   dependent on charset.

5.  "a=rid" Restrictions

   This section defines the "a=rid" restrictions that can be used to
   restrict the RTP payload encoding format in a codec-agnostic way.
   Please also see the preceding section for a description of how the
   "pt" parameter is used.

   The following restrictions are intended to apply to video codecs in a
   codec-independent fashion.

   *  *max-width*, for spatial resolution in pixels.  In the case that
      stream-orientation signaling is used to modify the intended
      display orientation, this attribute refers to the width of the
      stream when a rotation of zero degrees is encoded.

   *  *max-height*, for spatial resolution in pixels.  In the case that
      stream-orientation signaling is used to modify the intended
      display orientation, this attribute refers to the height of the
      stream when a rotation of zero degrees is encoded.

   *  *max-fps*, for frame rate in frames per second.  For encoders that
      do not use a fixed frame rate for encoding, this value is used to
      restrict the minimum amount of time between frames: the time
      between any two consecutive frames SHOULD NOT be less than 1/max-
      fps seconds.

   *  *max-fs*, for frame size in pixels per frame.  This is the product
      of frame width and frame height, in pixels, for rectangular
      frames.

   *  *max-br*, for bitrate in bits per second.  The restriction applies
      to the media payload only and does not include overhead introduced
      by other layers (e.g., RTP, UDP, IP, or Ethernet).  The exact
      means of keeping within this limit are left up to the
      implementation, and instantaneous excursions outside the limit are
      permissible.  For any given one-second sliding window, however,
      the total number of bits in the payload portion of RTP SHOULD NOT
      exceed the value specified in "max-br."

   *  *max-pps*, for pixel rate in pixels per second.  This value SHOULD
      be handled identically to max-fps, after performing the following
      conversion: max-fps = max-pps / (width * height).  If the stream
      resolution changes, this value is recalculated.  Due to this
      recalculation, excursions outside the specified maximum are
      possible near resolution-change boundaries.

   *  *max-bpp*, for maximum number of bits per pixel, calculated as an
      average of all samples of any given coded picture.  This is
      expressed as a floating point value, with an allowed range of
      0.0001 to 48.0.  These values MUST NOT be encoded with more than
      four digits to the right of the decimal point.

   *  *depend*, to identify other streams that the stream depends on.
      The value is a comma-separated list of rid-ids.  These rid-ids
      identify RTP streams that this stream depends on in order to allow
      for proper interpretation.  The mechanism defined in this document
      allows for such dependencies to be expressed only when the streams
      are in the same media section.

   All the restrictions are optional and subject to negotiation based on
   the SDP offer/answer rules described in Section 6.

   This list is intended to be an initial set of restrictions.  Future
   documents may define additional restrictions; see Section 12.2.
   While this document does not define restrictions for audio codecs or
   any media types other than video, there is no reason such
   restrictions should be precluded from definition and registration by
   other documents.

   Section 10 provides formal Augmented Backus-Naur Form (ABNF)
   [RFC5234] grammar for each of the "a=rid" restrictions defined in
   this section.

6.  SDP Offer/Answer Procedures

   This section describes the SDP offer/answer procedures [RFC3264] when
   using this framework.

   Note that "rid-id" values are only required to be unique within a
   media section ("m=" line); they do not necessarily need to be unique
   within an entire RTP session.  In traditional usage, each media
   section is sent on its own unique 5-tuple (that is: combination of
   sending address, sending port, receiving address, receiving port, and
   transport protocol), which provides an unambiguous scope.  Similarly,
   when using BUNDLE [RFC8843], Media Identification (MID) values
   associate RTP streams uniquely to a single media description.  When
   restriction identifier (RID) is used with the BUNDLE mechanism,
   streams will be associated with both MID and RID SDES items.

6.1.  Generating the Initial SDP Offer

   For each RTP media description in the offer, the offerer MAY choose
   to include one or more "a=rid" lines to specify a configuration
   profile for the given set of RTP payload types.

   In order to construct a given "a=rid" line, the offerer must follow
   these steps:

   1.  It MUST generate a "rid-id" that is unique within a media
       description.

   2.  It MUST set the direction for the "rid-id" to one of "send" or
       "recv".

   3.  It MAY include a listing of SDP media formats (usually
       corresponding to RTP payload types) allowed to appear in the RTP
       stream.  Any payload type chosen MUST be a valid payload type for
       the media section (that is, it must be listed on the "m=" line).
       The order of the listed formats is significant; the alternatives
       are listed from (left) most preferred to (right) least preferred.
       When using RID, this preference overrides the normal codec
       preference as expressed by format type ordering on the "m=" line,
       using regular SDP rules.

   4.  The offerer then chooses zero or more "a=rid" restrictions
       (Section 5) to be applied to the RTP stream and adds them to the
       "a=rid" line.

   5.  If the offerer wishes the answerer to have the ability to specify
       a restriction but does not wish to set a value itself, it
       includes the name of the restriction in the "a=rid" line, but
       without any indicated value.

   Note: If an "a=fmtp" attribute is also used to provide media-format-
   specific parameters, then the "a=rid" restrictions will further
   restrict the equivalent "a=fmtp" parameters for the given payload
   type for the specified RTP stream.

   If a given codec would require an "a=fmtp" line when used without
   "a=rid", then the offer MUST include a valid corresponding "a=fmtp"
   line even when using "a=rid".

6.2.  Answerer Processing the SDP Offer

6.2.1.  "a=rid"-Unaware Answerer

   If the receiver doesn't support the framework defined in this
   specification, the entire "a=rid" line is ignored following the
   standard offer/answer rules [RFC3264].

   Section 6.1 requires the offer to include a valid "a=fmtp" line for
   any media formats that otherwise require it (in other words, the
   "a=rid" line cannot be used to replace "a=fmtp" configuration).  As a
   result, ignoring the "a=rid" line is always guaranteed to result in a
   valid session description.

6.2.2.  "a=rid"-Aware Answerer

   If the answerer supports the "a=rid" attribute, the following
   verification steps are executed, in order, for each "a=rid" line in a
   received offer:

   1.  The answerer ensures that the "a=rid" line is syntactically well
       formed.  In the case of a syntax error, the "a=rid" line is
       discarded.

   2.  The answerer extracts the rid-id from the "a=rid" line and
       verifies its uniqueness within a media section.  In the case of a
       duplicate, the entire "a=rid" line, and all "a=rid" lines with
       rid-ids that duplicate this line, are discarded and MUST NOT be
       included in the SDP answer.

   3.  If the "a=rid" line contains a "pt=", the list of payload types
       is verified against the list of valid payload types for the media
       section (that is, those listed on the "m=" line).  Any PT missing
       from the "m=" line is discarded from the set of values in the
       "pt=".  If no values are left in the "pt=" parameter after this
       processing, then the "a=rid" line is discarded.

   4.  If the "direction" field is "recv", the answerer ensures that the
       specified "a=rid" restrictions are supported.  In the case of an
       unsupported restriction, the "a=rid" line is discarded.

   5.  If the "depend" restriction is included, the answerer MUST make
       sure that the listed rid-ids unambiguously match the rid-ids in
       the media description.  Any "depend" "a=rid" lines that do not
       are discarded.

   6.  The answerer verifies that the restrictions are consistent with
       at least one of the codecs to be used with the RTP stream.  If
       the "a=rid" line contains a "pt=", it contains the list of such
       codecs; otherwise, the list of such codecs is taken from the
       associated "m=" line.  See Section 8 for more detail.  If the
       "a=rid" restrictions are incompatible with the other codec
       properties for all codecs, then the "a=rid" line is discarded.

   Note that the answerer does not need to understand every restriction
   present in a "send" line: if a stream sender restricts the stream in
   a way that the receiver does not understand, this causes no issues
   with interoperability.

6.3.  Generating the SDP Answer

   Having performed verification of the SDP offer as described in
   Section 6.2.2, the answerer shall perform the following steps to
   generate the SDP answer.

   For each "a=rid" line that has not been discarded by previous
   processing:

   1.  The value of the "direction" field is reversed: "send" is changed
       to "recv", and "recv" is changed to "send".

   2.  The answerer MAY choose to modify specific "a=rid" restriction
       values in the answer SDP.  In such a case, the modified value
       MUST be more restrictive than the ones specified in the offer.
       The answer MUST NOT include any restrictions that were not
       present in the offer.

   3.  The answerer MUST NOT modify the "rid-id" present in the offer.

   4.  If the "a=rid" line contains a "pt=", the answerer is allowed to
       discard one or more media formats from a given "a=rid" line.  If
       the answerer chooses to discard all the media formats from an
       "a=rid" line, the answerer MUST discard the entire "a=rid" line.
       If the offer did not contain a "pt=" for a given "a=rid" line,
       then the answer MUST NOT contain a "pt=" in the corresponding
       line.

   5.  In cases where the answerer is unable to support the payload
       configuration specified in a given "a=rid" line with a direction
       of "recv" in the offer, the answerer MUST discard the
       corresponding "a=rid" line.  This includes situations in which
       the answerer does not understand one or more of the restrictions
       in an "a=rid" line with a direction of "recv".

   Note: In the case that the answerer uses different PT values to
   represent a codec than the offerer did, the "a=rid" values in the
   answer use the PT values that are present in its answer.

6.4.  Offerer Processing of the SDP Answer

   The offerer SHALL follow these steps when processing the answer:

   1.  The offerer matches the "a=rid" line in the answer to the "a=rid"
       line in the offer using the "rid-id".  If no matching line can be
       located in the offer, the "a=rid" line is ignored.

   2.  If the answer contains any restrictions that were not present in
       the offer, then the offerer SHALL discard the "a=rid" line.

   3.  If the restrictions have been changed between the offer and the
       answer, the offerer MUST ensure that the modifications are more
       restrictive than they were in the original offer and that they
       can be supported; if not, the offerer SHALL discard the "a=rid"
       line.

   4.  If the "a=rid" line in the answer contains a "pt=" but the offer
       did not, the offerer SHALL discard the "a=rid" line.

   5.  If the "a=rid" line in the answer contains a "pt=" and the offer
       did as well, the offerer verifies that the list of payload types
       is a subset of those sent in the corresponding "a=rid" line in
       the offer.  Note that this matching must be performed
       semantically rather than on literal PT values, as the remote end
       may not be using symmetric PTs.  For the purpose of this
       comparison: for each PT listed on the "a=rid" line in the answer,
       the offerer looks up the corresponding "a=rtpmap" and "a=fmtp"
       lines in the answer.  It then searches the list of "pt=" values
       indicated in the offer and attempts to find one with an
       equivalent set of "a=rtpmap" and "a=fmtp" lines in the offer.  If
       all PTs in the answer can be matched, then the "pt=" values pass
       validation; otherwise, it fails.  If this validation fails, the
       offerer SHALL discard the "a=rid" line.  Note that this semantic
       comparison necessarily requires an understanding of the meaning
       of codec parameters, rather than a rote byte-wise comparison of
       their values.

   6.  If the "a=rid" line contains a "pt=", the offerer verifies that
       the attribute values provided in the "a=rid" attributes are
       consistent with the corresponding codecs and their other
       parameters.  See Section 8 for more detail.  If the "a=rid"
       restrictions are incompatible with the other codec properties,
       then the offerer SHALL discard the "a=rid" line.

   7.  The offerer verifies that the restrictions are consistent with at
       least one of the codecs to be used with the RTP stream.  If the
       "a=rid" line contains a "pt=", it contains the list of such
       codecs; otherwise, the list of such codecs is taken from the
       associated "m=" line.  See Section 8 for more detail.  If the
       "a=rid" restrictions are incompatible with the other codec
       properties for all codecs, then the offerer SHALL discard the
       "a=rid" line.

   Any "a=rid" line present in the offer that was not matched by step 1
   above has been discarded by the answerer and does not form part of
   the negotiated restrictions on an RTP stream.  The offerer MAY still
   apply any restrictions it indicated in an "a=rid" line with a
   direction field of "send", but it is not required to do so.

   It is important to note that there are several ways in which an offer
   can contain a media section with "a=rid" lines, although the
   corresponding media section in the response does not.  This includes
   situations in which the answerer does not support "a=rid" at all or
   does not support the indicated restrictions.  Under such
   circumstances, the offerer MUST be prepared to receive a media stream
   to which no restrictions have been applied.

6.5.  Modifying the Session

   Offers and answers inside an existing session follow the rules for
   initial session negotiation.  Such an offer MAY propose a change in
   the number of RIDs in use.  To avoid race conditions with media, any
   RIDs with proposed changes SHOULD use a new ID rather than reusing
   one from the previous offer/answer exchange.  RIDs without proposed
   changes SHOULD reuse the ID from the previous exchange.

7.  Use with Declarative SDP

   This document does not define the use of a RID in declarative SDP.
   If concrete use cases for RID in declarative SDP use are identified
   in the future, we expect that additional specifications will address
   such use.

8.  Interaction with Other Techniques

   Historically, a number of other approaches have been defined that
   allow restricting media streams via SDP.  These include:

   *  Codec-specific configuration set via format parameters ("a=fmtp")
      -- for example, the H.264 "max-fs" format parameter [RFC6184]

   *  Size restrictions imposed by the "a=imageattr" attribute [RFC6236]

   When the mechanism described in this document is used in conjunction
   with these other restricting mechanisms, it is intended to impose
   additional restrictions beyond those communicated in other
   techniques.

   In an offer, this means that "a=rid" lines, when combined with other
   restrictions on the media stream, are expected to result in a non-
   empty intersection.  For example, if image attributes are used to
   indicate that a PT has a minimum width of 640, then specification of
   "max-width=320" in an "a=rid" line that is then applied to that PT is
   nonsensical.  According to the rules of Section 6.2.2, this will
   result in the corresponding "a=rid" line being ignored by the
   recipient.

   In an answer, the "a=rid" lines, when combined with the other
   restrictions on the media stream, are also expected to result in a
   non-empty intersection.  If the implementation generating an answer
   wishes to restrict a property of the stream below that which would be
   allowed by other parameters (e.g., those specified in "a=fmtp" or
   "a=imageattr"), its only recourse is to discard the "a=rid" line
   altogether, as described in Section 6.3.  If it instead attempts to
   restrict the stream beyond what is allowed by other mechanisms, then
   the offerer will ignore the corresponding "a=rid" line, as described
   in Section 6.4.

   The following subsections demonstrate these interactions using
   commonly used video codecs.  These descriptions are illustrative of
   the interaction principles outlined above and are not normative.

8.1.  Interaction with VP8 Format Parameters

   [RFC7741] defines two format parameters for the VP8 codec.  Both
   correspond to restrictions on receiver capabilities and never
   indicate sending restrictions.

8.1.1.  max-fr - Maximum Frame Rate

   The VP8 "max-fr" format parameter corresponds to the "max-fps"
   restriction defined in this specification.  If an RTP sender is
   generating a stream using a format defined with this format
   parameter, and the sending restrictions defined via "a=rid" include a
   "max-fps" parameter, then the sent stream will conform to the smaller
   of the two values.

8.1.2.  max-fs - Maximum Frame Size, in VP8 Macroblocks

   The VP8 "max-fs" format parameter corresponds to the "max-fs"
   restriction defined in this document, by way of a conversion factor
   of the number of pixels per macroblock (typically 256).  If an RTP
   sender is generating a stream using a format defined with this format
   parameter, and the sending restrictions defined via "a=rid" include a
   "max-fs" parameter, then the sent stream will conform to the smaller
   of the two values; that is, the number of pixels per frame will not
   exceed:

     min(rid_max_fs, fmtp_max_fs * macroblock_size)

   This fmtp parameter also has bearing on the max-height and max-width
   parameters.  Section 6.1 of [RFC7741] requires that the width and
   height of the frame in macroblocks be less than int(sqrt(fmtp_max_fs
   * 8)).  Accordingly, the maximum width of a transmitted stream will
   be limited to:

     min(rid_max_width, int(sqrt(fmtp_max_fs * 8)) * macroblock_width)

   Similarly, the stream's height will be limited to:

     min(rid_max_height, int(sqrt(fmtp_max_fs * 8)) * macroblock_height)

8.2.  Interaction with H.264 Format Parameters

   [RFC6184] defines format parameters for the H.264 video codec.  The
   majority of these parameters do not correspond to codec-independent
   restrictions:

   *  deint-buf-cap

   *  in-band-parameter-sets

   *  level-asymmetry-allowed

   *  max-rcmd-nalu-size

   *  max-cpb

   *  max-dpb

   *  packetization-mode

   *  redundant-pic-cap

   *  sar-supported

   *  sar-understood

   *  sprop-deint-buf-req

   *  sprop-init-buf-time

   *  sprop-interleaving-depth

   *  sprop-level-parameter-sets

   *  sprop-max-don-diff

   *  sprop-parameter-sets

   *  use-level-src-parameter-sets

   Note that the max-cpb and max-dpb format parameters for H.264
   correspond to restrictions on the stream, but they are specific to
   the way the H.264 codec operates, and do not have codec-independent
   equivalents.

   The [RFC6184] codec format parameters covered in the following
   sections correspond to restrictions on receiver capabilities and
   never indicate sending restrictions.

8.2.1.  profile-level-id and max-recv-level - Negotiated Subprofile

   These parameters include a "level" indicator, which acts as an index
   into Table A-1 of [H264].  This table contains a number of
   parameters, several of which correspond to the restrictions defined
   in this document.  [RFC6184] also defines format parameters for the
   H.264 codec that may increase the maximum values indicated by the
   negotiated level.  The following sections describe the interaction
   between these parameters and the restrictions defined by this
   document.  In all cases, the H.264 parameters being discussed are the
   maximum of those indicated by [H264] Table A-1 and those indicated in
   the corresponding "a=fmtp" line.

8.2.2.  max-br / MaxBR - Maximum Video Bitrate

   The H.264 "MaxBR" parameter (and its equivalent "max-br" format
   parameter) corresponds to the "max-bps" restriction defined in this
   specification, by way of a conversion factor of 1000 or 1200; see
   [RFC6184] for details regarding which factor gets used under
   differing circumstances.

   If an RTP sender is generating a stream using a format defined with
   this format parameter, and the sending restrictions defined via
   "a=rid" include a "max-fps" parameter, then the sent stream will
   conform to the smaller of the two values -- that is:

     min(rid_max_br, h264_MaxBR * conversion_factor)

8.2.3.  max-fs / MaxFS - Maximum Frame Size, in H.264 Macroblocks

   The H.264 "MaxFs" parameter (and its equivalent "max-fs" format
   parameter) corresponds roughly to the "max-fs" restriction defined in
   this document, by way of a conversion factor of 256 (the number of
   pixels per macroblock).

   If an RTP sender is generating a stream using a format defined with
   this format parameter, and the sending restrictions defined via
   "a=rid" include a "max-fs" parameter, then the sent stream will
   conform to the smaller of the two values -- that is:

     min(rid_max_fs, h264_MaxFs * 256)

8.2.4.  max-mbps / MaxMBPS - Maximum Macroblock Processing Rate

   The H.264 "MaxMBPS" parameter (and its equivalent "max-mbps" format
   parameter) corresponds roughly to the "max-pps" restriction defined
   in this document, by way of a conversion factor of 256 (the number of
   pixels per macroblock).

   If an RTP sender is generating a stream using a format defined with
   this format parameter, and the sending restrictions defined via
   "a=rid" include a "max-pps" parameter, then the sent stream will
   conform to the smaller of the two values -- that is:

     min(rid_max_pps, h264_MaxMBPS * 256)

8.2.5.  max-smbps - Maximum Decoded Picture Buffer

   The H.264 "max-smbps" format parameter operates the same way as the
   "max-mbps" format parameter, under the hypothetical assumption that
   all macroblocks are static macroblocks.  It is handled by applying
   the conversion factor described in Section 8.1 of [RFC6184], and the
   result of this conversion is applied as described in Section 8.2.4.

8.3.  Redundancy Formats and Payload Type Restrictions

   Section 4 specifies that redundancy formats using redundancy RTP
   streams bind the redundancy RTP stream to the source RTP stream with
   either the RepairedRtpStreamId SDES item or other mechanisms.
   However, there exist redundancy RTP payload formats that result in
   the redundancy being included in the source RTP stream.  An example
   of this is "RTP Payload for Redundant Audio Data" [RFC2198], which
   encapsulates one source stream with one or more redundancy streams in
   the same RTP payload.  Formats defining the source and redundancy
   encodings as regular RTP payload types require some consideration for
   how the "a=rid" restrictions are defined.  The "a=rid" line "pt="
   parameter can be used to indicate whether the redundancy RTP payload
   type and/or the individual source RTP payload type(s) are part of the
   restriction.

   Example (SDP excerpt):

      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:5 send pt=99,102;max-br=64000
      a=rid:6 send pt=100,97,101,102

   The RID with ID=6 restricts the payload types for this RID to 100
   (the redundancy format), 97 (G.711), 101 (Comfort Noise), and 102
   (dual-tone multi-frequency (DTMF) tones).  This means that RID 6 can
   either contain the Redundant Audio Data (RED) format, encapsulating
   encodings of the source media stream using payload type 97 and 98, 97
   without RED encapsulation, Comfort noise, or DTMF tones.  Payload
   type 98 is not included in the RID, and can thus not be sent except
   as redundancy information in RED encapsulation.  If 97 were to be
   excluded from the pt parameter, it would instead mean that payload
   types 97 and 98 are only allowed via RED encapsulation.

9.  Format Parameters for Future Payloads

   Registrations of future RTP payload format specifications that define
   media types that have parameters matching the RID restrictions
   specified in this memo SHOULD name those parameters in a manner that
   matches the names of those RID restrictions and SHOULD explicitly
   state what media-type parameters are restricted by what RID
   restrictions.

10.  Formal Grammar

   This section gives a formal Augmented Backus-Naur Form (ABNF)
   [RFC5234] grammar, with the case-sensitive extensions described in
   [RFC7405], for each of the new media and "a=rid" attributes defined
   in this document.

   rid-syntax        = %s"a=rid:" rid-id SP rid-dir
                       [ rid-pt-param-list / rid-param-list ]

   rid-id            = 1*(alpha-numeric / "-" / "_")

   alpha-numeric     = < as defined in [RFC4566] >

   rid-dir           = %s"send" / %s"recv"

   rid-pt-param-list = SP rid-fmt-list *(";" rid-param)

   rid-param-list    = SP rid-param *(";" rid-param)

   rid-fmt-list      = %s"pt=" fmt *( "," fmt )

   fmt               = < as defined in [RFC4566] >

   rid-param         = rid-width-param
                       / rid-height-param
                       / rid-fps-param
                       / rid-fs-param
                       / rid-br-param
                       / rid-pps-param
                       / rid-bpp-param
                       / rid-depend-param
                       / rid-param-other

   rid-width-param   = %s"max-width" [ "=" int-param-val ]

   rid-height-param  = %s"max-height" [ "=" int-param-val ]

   rid-fps-param     = %s"max-fps" [ "=" int-param-val ]

   rid-fs-param      = %s"max-fs" [ "=" int-param-val ]

   rid-br-param      = %s"max-br" [ "=" int-param-val ]

   rid-pps-param     = %s"max-pps" [ "=" int-param-val ]

   rid-bpp-param     = %s"max-bpp" [ "=" float-param-val ]

   rid-depend-param  = %s"depend=" rid-list

   rid-param-other   = 1*(alpha-numeric / "-") [ "=" param-val ]

   rid-list          = rid-id *( "," rid-id )

   int-param-val     = 1*DIGIT

   float-param-val   = 1*DIGIT "." 1*DIGIT

   param-val         = *(%x20-3A / %x3C-7E)
                       ; Any printable character except semicolon

11.  SDP Examples

   Note: See [RFC8853] for examples of RID used in simulcast scenarios.

11.1.  Many Bundled Streams Using Many Codecs

   In this scenario, the offerer supports the Opus, G.722, G.711, and
   DTMF audio codecs and VP8, VP9, H.264 (CBP/CHP, mode 0/1), H.264-SVC
   (SCBP/SCHP), and H.265 (MP/M10P) for video.  An 8-way video call (to
   a mixer) is supported (send 1 and receive 7 video streams) by
   offering 7 video media sections (1 sendrecv at max resolution and 6
   recvonly at smaller resolutions), all bundled on the same port, using
   3 different resolutions.  The resolutions include:

   *  1 receive stream of 720p resolution is offered for the active
      speaker.

   *  2 receive streams of 360p resolution are offered for the prior 2
      active speakers.

   *  4 receive streams of 180p resolution are offered for others in the
      call.

   NOTE: The SDP given below skips a few lines to keep the example short
   and focused, as indicated by either the "..." or the comments
   inserted.

   The offer for this scenario is shown below.

   ...
   m=audio 10000 RTP/SAVPF 96 9 8 0 123
   a=rtpmap:96 OPUS/48000
   a=rtpmap:9 G722/8000
   a=rtpmap:8 PCMA/8000
   a=rtpmap:0 PCMU/8000
   a=rtpmap:123 telephone-event/8000
   a=mid:a1
   ...
   m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107
   a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   a=rtpmap:98 VP8/90000
   a=fmtp:98 max-fs=3600; max-fr=30
   a=rtpmap:99 VP9/90000
   a=fmtp:99 max-fs=3600; max-fr=30
   a=rtpmap:100 H264/90000
   a=fmtp:100 profile-level-id=42401f; packetization-mode=0
   a=rtpmap:101 H264/90000
   a=fmtp:101 profile-level-id=42401f; packetization-mode=1
   a=rtpmap:102 H264/90000
   a=fmtp:102 profile-level-id=640c1f; packetization-mode=0
   a=rtpmap:103 H264/90000
   a=fmtp:103 profile-level-id=640c1f; packetization-mode=1
   a=rtpmap:104 H264-SVC/90000
   a=fmtp:104 profile-level-id=530c1f
   a=rtpmap:105 H264-SVC/90000
   a=fmtp:105 profile-level-id=560c1f
   a=rtpmap:106 H265/90000
   a=fmtp:106 profile-id=1; level-id=93
   a=rtpmap:107 H265/90000
   a=fmtp:107 profile-id=2; level-id=93
   a=sendrecv
   a=mid:v1 (max resolution)
   a=rid:1 send max-width=1280;max-height=720;max-fps=30
   a=rid:2 recv max-width=1280;max-height=720;max-fps=30
   ...
   m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107
   a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   ...same rtpmap/fmtp as above...
   a=recvonly
   a=mid:v2 (medium resolution)
   a=rid:3 recv max-width=640;max-height=360;max-fps=15
   ...
   m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107
   a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   ...same rtpmap/fmtp as above...
   a=recvonly
   a=mid:v3 (medium resolution)
   a=rid:3 recv max-width=640;max-height=360;max-fps=15
   ...
   m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107
   a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   ...same rtpmap/fmtp as above...
   a=recvonly
   a=mid:v4 (small resolution)
   a=rid:4 recv max-width=320;max-height=180;max-fps=15
   ...
   m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107
   a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
   ...same rtpmap/fmtp as above...
   ...same rid:4 as above for mid:v5,v6,v7 (small resolution)...
   ...

11.2.  Scalable Layers

   Adding scalable layers to a session within a multiparty conference
   gives a selective forwarding unit (SFU) further flexibility to
   selectively forward packets from a source that best match the
   bandwidth and capabilities of diverse receivers.  Scalable encodings
   have dependencies between layers, unlike independent simulcast
   streams.  RIDs can be used to express these dependencies using the
   "depend" restriction.  In the example below, the highest resolution
   is offered to be sent as 2 scalable temporal layers (using Multiple
   RTP Streams on a Single Media Transport (MRST)).  See [RFC8853] for
   additional detail about simulcast usage.

   Offer:
   ...
   m=audio ...same as previous example ...
   ...
   m=video ...same as previous example ...
   ...same rtpmap/fmtp as previous example ...
   a=sendrecv
   a=mid:v1 (max resolution)
   a=rid:0 send max-width=1280;max-height=720;max-fps=15
   a=rid:1 send max-width=1280;max-height=720;max-fps=30;depend=0
   a=rid:2 recv max-width=1280;max-height=720;max-fps=30
   a=rid:5 send max-width=640;max-height=360;max-fps=15
   a=rid:6 send max-width=320;max-height=180;max-fps=15
   a=simulcast: send rid=0;1;5;6 recv rid=2
   ...
   ...same m=video sections as previous example for mid:v2-v7...
   ...

12.  IANA Considerations

   This specification updates [RFC4855] to give additional guidance on
   choice of Format Parameter (fmtp) names and their relation to RID
   restrictions.

12.1.  New SDP Media-Level Attribute

   This document defines "rid" as an SDP media-level attribute.  This
   attribute has been registered by IANA under "Session Description
   Protocol (SDP) Parameters" under "att-field (media level only)".

   The "rid" attribute is used to identify the properties of an RTP
   stream within an RTP session.  Its format is defined in Section 10.

   The formal registration information for this attribute follows.

   Contact name, email address, and telephone number
      IETF MMUSIC Working Group
      mmusic@ietf.org
      +1 510 492 4080

   Attribute name (as it will appear in SDP)
      rid

   Long-form attribute name in English
      Restriction Identifier

   Type of attribute (session level, media level, or both)
      Media Level

   Whether the attribute value is subject to the charset attribute
      The attribute is not dependent on charset.

   A one-paragraph explanation of the purpose of the attribute
      The "rid" SDP attribute is used 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.

   A specification of appropriate attribute values for this attribute
      Valid values are defined by the ABNF in RFC 8851

   Multiplexing (Mux) Category
      SPECIAL

12.2.  Registry for RID-Level Parameters

   This specification creates a new IANA registry named "RID Attribute
   Parameters" within the SDP parameters registry.  The "a=rid"
   restrictions MUST be registered with IANA and documented under the
   same rules as for SDP session-level and media-level attributes as
   specified in [RFC4566].

   Parameters for "a=rid" lines that modify the nature of encoded media
   MUST be of the form that the result of applying the modification to
   the stream results in a stream that still complies with the other
   parameters that affect the media.  In other words, restrictions
   always have to restrict the definition to be a subset of what is
   otherwise allowable, and never expand it.

   New restriction registrations are accepted according to the
   "Specification Required" policy of [RFC8126].  The registration MUST
   contain the RID parameter name and a reference to the corresponding
   specification.  The specification itself must contain the following
   information (not all of which appears in the registry):

   *  restriction name (as it will appear in SDP)

   *  an explanation of the purpose of the restriction

   *  a specification of appropriate attribute values for this
      restriction

   *  an ABNF definition of the restriction

   The initial set of "a=rid" restriction names, with definitions in
   Section 5 of this document, is given below:

                    +====================+===========+
                    | RID Parameter Name | Reference |
                    +====================+===========+
                    | pt                 | RFC 8851  |
                    +--------------------+-----------+
                    | max-width          | RFC 8851  |
                    +--------------------+-----------+
                    | max-height         | RFC 8851  |
                    +--------------------+-----------+
                    | max-fps            | RFC 8851  |
                    +--------------------+-----------+
                    | max-fs             | RFC 8851  |
                    +--------------------+-----------+
                    | max-br             | RFC 8851  |
                    +--------------------+-----------+
                    | max-pps            | RFC 8851  |
                    +--------------------+-----------+
                    | max-bpp            | RFC 8851  |
                    +--------------------+-----------+
                    | depend             | RFC 8851  |
                    +--------------------+-----------+

                    Table 1: "a=rid" restriction names

   It is conceivable that a future document will want to define RID-
   level restrictions that contain string values.  These extensions need
   to take care to conform to the ABNF defined for rid-param-other.  In
   particular, this means that such extensions will need to define
   escaping mechanisms if they want to allow semicolons, unprintable
   characters, or byte values greater than 127 in the string.

13.  Security Considerations

   As with most SDP parameters, a failure to provide integrity
   protection over the "a=rid" attributes gives attackers a way to
   modify the session in potentially unwanted ways.  This could result
   in an implementation sending greater amounts of data than a recipient
   wishes to receive.  In general, however, since the "a=rid" attribute
   can only restrict a stream to be a subset of what is otherwise
   allowable, modification of the value cannot result in a stream that
   is of higher bandwidth than would be sent to an implementation that
   does not support this mechanism.

   The actual identifiers used for RIDs are expected to be opaque.  As
   such, they are not expected to contain information that would be
   sensitive, were it observed by third parties.

14.  References

14.1.  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>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://www.rfc-editor.org/info/rfc4855>.

   [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>.

   [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>.

   [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>.

14.2.  Informative References

   [H264]     International Telecommunication Union, "Advanced video
              coding for generic audiovisual services", ITU-T
              Recommendation H.264, June 2019,
              <https://www.itu.int/rec/T-REC-H.264>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8285]  Singer, D., Desineni, H., and R. Even, Ed., "A General
              Mechanism for RTP Header Extensions", RFC 8285,
              DOI 10.17487/RFC8285, October 2017,
              <https://www.rfc-editor.org/info/rfc8285>.

   [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>.

   [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>.

   [RFC8853]  Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
              "Using Simulcast in Session Description Protocol (SDP) and
              RTP Sessions", RFC 8853, DOI 10.17487/RFC8853, January
              2021, <https://www.rfc-editor.org/info/rfc8853>.

Acknowledgements

   Many thanks to Cullen Jennings, Magnus Westerlund, and Paul Kyzivat
   for reviewing.  Thanks to Colin Perkins for input on future payload
   type handling.

Contributors

   The following individuals have contributed significant text to this
   document.

   Peter Thatcher
   Google

   Email: pthatcher@google.com


   Mo Zanaty
   Cisco Systems

   Email: mzanaty@cisco.com


   Suhas Nandakumar
   Cisco Systems

   Email: snandaku@cisco.com


   Bo Burman
   Ericsson

   Email: bo.burman@ericsson.com


   Byron Campen
   Mozilla

   Email: bcampen@mozilla.com


Author's Address

   Adam Roach (editor)
   Mozilla

   Email: adam@nostrum.com