Network Working Group                                        P. Thatcher
Internet-Draft                                                    Google
Intended status: Standards Track                               M. Zanaty
Expires: August 4, 6, 2016                                    S. Nandakumar
                                                           Cisco Systems
                                                               B. Burman
                                                                A. Roach
                                                               B. Campen
                                                       February 01, 03, 2016

                     RTP Payload Format Constraints


   In this specification, we define a framework for identifying RTP
   Streams with the constraints on its payload format in the Session
   Description Protocol.  This framework defines a new "rid" SDP
   attribute to: a) effectively identify the RID RTP Streams within a
   RTP Session, b) constrain their payload format parameters in a codec-
   agnostic way beyond what is provided with the regular Payload Types
   and c) enable unambiguous mapping between the RID RTP Streams to
   their media format specification in the SDP.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 4, 6, 2016.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Key Words for Requirements  Introduction  . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . .   3
   3.  Key Words for Requirements  . . . . . . . . . . . . . . . . .   4
   4.  SDP "a=rid" Media Level Attribute . . . . . . . . . . . . . .   4
   5.  "a=rid" constraints . . . . . . . . . . . . . . . . . . . . .   5
   6.  SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . .   7
     6.1.  Generating the Initial SDP Offer  . . . . . . . . . . . .   7
     6.2.  Answerer processing the SDP Offer . . . . . . . . . . . .   8
       6.2.1.  'rid' unaware  "a=rid"-unaware Answerer  . . . . . . . . . . . . . . .   8
       6.2.2.  'rid' aware  "a=rid"-aware Answerer  . . . . . . . . . . . . . . . .   8
     6.3.  Generating the SDP Answer . . . . . . . . . . . . . . . .   9
     6.4.  Offerer Processing of the SDP Answer  . . . . . . . . . .  10
     6.5.  Modifying the Session . . . . . . . . . . . . . . . . . .  11
   7.  Use with Declarative SDP  . . . . . . . . . . . . . . . . . .  11
   8.  Interaction with Other Techniques . . . . . . . . . . . . . .  11  12
   9.  Formal Grammar  . . . . . . . . . . . . . . . . . . . . . . .  12
   10. SDP Examples  . . . . . . . . . . . . . . . . . . . . . . . .  13  14
     10.1.  Many Bundled Streams using Many Codecs . . . . . . . . .  13  14
     10.2.  Scalable Layers  . . . . . . . . . . . . . . . . . . . .  15  16
   11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Declarative SDP  . . . . . . . . . . . . . . . . . . . .  16
     11.2.  Definition of bitrate  . . . . . . . . . . . . . . . . .  16
     11.3.  Escaping new constraint values . . . . . . . . . . . . .  16  17
     11.4.  Utility of max-width and max height  . . . . . . . . . .  17
     11.5.  Definition of max-fps  . . . . . . . . . . . . . . . . .  17  18
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  New SDP Media-Level attribute  . . . . . . . . . . . . .  18
     12.2.  Registry for RID-Level Parameters  . . . . . . . . . . .  18
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19  20
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19  20
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  19  20
     15.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Terminology

   The terms "Source RTP Stream", "Endpoint", "RTP Session", and "RTP
   Stream" are used as defined in

   The term "RID RTP Stream" is used as defined in

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

2.  Introduction

   Payload Type (PT) in RTP provides a mapping between the format of the RTP payload
   format and the associated SDP media format description specified in the
   signaling.  For applications that use description.  The SDP for signaling, the
   constructs rtpmap and/or and/
   or fmtp attributes are used, for a given PT, to the describe the
   characteristics of the media that is carried in the RTP payload, mapped to a given PT. payload.

   Recent advances in standards have given rise to rich multimedia
   applications requiring support for multiple RTP Streams within a RTP
   session [I-D.ietf-mmusic-sdp-bundle-negotiation],
   [I-D.ietf-mmusic-sdp-simulcast] or having to support a large number
   of codecs.  These demands have unearthed challenges inherent with:

   o  The restricted RTP PT space in specifying the various payload

   o  The codec-specific constructs for the payload formats in SDP,

   o  Missing or underspecified payload format parameters,

   o  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 RTCP) could result in limited number of payload
   type numbers available for the application usage.  In scenarios where
   the number of possible RTP payload configurations exceed the
   available PT space within a RTP Session, there is need a way to
   represent the additional constraints on payload configurations and to
   effectively map a RID RTP Stream to its corresponding constraints.
   This issue is exacerbated by the increase in techniques such as
   simulcast and layered codecs, which introduce additional streams into
   RTP Sessions.

   This specification defines a new SDP framework for constraining
   Source RTP Streams (Section 2.1.10
   [I-D.ietf-avtext-rtp-grouping-taxonomy]), along with the SDP
   attributes to constrain payload formats in a codec-agnostic way.
   This framework can be thought of as complementary extension to the
   way the media format parameters are specified in SDP today, via the
   "a=fmtp" attribute.

   This specification makes use of the RTP Stream Identifier SDES RTCP
   item defined in [I-D.roach-avtext-rid]  to provide correlation
   between the RTP Packets and their format specification in the SDP.

   The additional constraints on individual streams are indicated with a
   new "a=rid" SDP attribute.  Note that the parameters communicated via
   this attribute only serve to further constrain the parameters that
   are established on a PT format.  They do not relax any existing

   As described in Section 6.2.1, this mechanism achieves backwards
   compatibility via the normal SDP processing rules, which require
   unknown a= parameters 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 constraints
   requested by this mechanism.

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


3.  Key Words for Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119]

3.  Terminology

   The terms "Source RTP Stream", "Endpoint", "RTP Session", and "RTP
   Stream" are used as defined in

   The term "RID RTP Stream" is used as defined in

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

4.  SDP "a=rid" Media Level Attribute

   This section defines new SDP media-level attribute [RFC4566],
   "a=rid".  Roughly speaking, this attribute takes the following form
   (see Section 9 for a formal definition).

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

   An "a=rid" SDP media attribute specifies constraints defining a
   unique RTP payload configuration identified via the "rid-identifier".
   This value binds the restriction to the RID RTP Stream identified by
   its RID SDES item [I-D.roach-avtext-rid].

   The "direction" parameter identifies the directionality of the RID
   RTP Stream; it may be either "send" or "recv".

   The optional "pt" parameter lists one or more PT values that can be
   used in the associated RID RTP Stream.  If the parameter is absent,
   then any of the PT values specified in the corresponding "m=" line
   may be used.

   The list of zero or more codec-agnostic "constraint" parameters
   (Section 5) describe the restrictions that the corresponding RID 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" constraints (Section 5)
   further constrain 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-identifier" 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 parameters specified by the "rid" constraints follow a
   syntax similar to session-level and media-level attributes, they are
   defined independently.  All "rid" parameters MUST be registered with
   IANA, using the registry defined in Section 12

   Section 9 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" constraints

   This section defines the "a=rid" constraints that can be used to
   restrict the RTP payload encoding format in a codec-agnostic way.

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

   o  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.

   o  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 width of the
      stream when a rotation of zero degrees is encoded.

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

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

   o  max-br, for bit rate 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."

   o  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 during near resolution change boundaries.

   o  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.

   All the constraints are optional and are subject to negotiation based
   on the SDP Offer/Answer rules described in Section 6.

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

   Section 9 provides formal Augmented Backus-Naur Form(ABNF) [RFC5234]
   grammar for each of the "a=rid" attributes parameters defined in this section.

6.  SDP Offer/Answer Procedures

   This section describes the SDP Offer/Answer [RFC3264] procedures when
   using this framework.

   Note that "rid-identifier" 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, which provides an
   unambiguous scope.  Similarly, when using BUNDLE
   [I-D.ietf-mmusic-sdp-bundle-negotiation], MID values associate RTP
   streams uniquely to a single media description.

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-identifier" that is unique within a media

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

   3.  It MAY include a listing of SDP format tokens (usually
       corresponding to RTP payload types) allowed to appear in the RID
       RTP Stream.  Any Payload Types 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" constraints
       (Section 5) to be applied to the rid, RID RTP Stream, and adds them to
       the "a=rid" line.

   5.  If the offerer wishes the answerer to have the ability to specify
       a constraint, but does not wish to set a value itself, it MUST
       include the name of the constraint 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" attributes will further
   constrain the equivalent "a=fmtp" parameters for the given Payload
   Type for the specified RID 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

   For each media description in the offer, and for each "a=rid"
   attribute in the media description, the receiver of the offer will
   perform the following steps:

6.2.1.  'rid' unaware  "a=rid"-unaware Answerer

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

   Section 6.1 requires the offer to include a valid "a=fmtp" line for
   any codecs 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.  'rid' aware  "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
   given media description:

   1.  Extract the rid-identifier from the "a=rid" line and verify its
       uniqueness.  In the case of a duplicate, the entire "a=rid" line,
       and all "a=rid" lines with rid-identifiers that duplicate this
       line, are rejected discarded and MUST NOT be included in the SDP Answer.

   2.  If the "a=rid" line contains a "pt=" parameter, 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 removed from the "pt="

   3.  The answerer ensures that "a=rid" parameters listed are
       syntactically well formed.  In the case of a syntax error, the
       "a=rid" line is removed.

   4.  If the "direction" parameter is "recv", The answerer ensures that
       "a=rid" parameters are supported.  In the case of an unsupported
       parameter, the "a=rid" line is removed.

   5.  If the "depend" parameter is included, the answerer MUST make
       sure that the listed rid-identifiers unambiguously match the rid-
       identifiers in the SDP offer.  Any "a=rid" lines that do not are

   6.  The answerer verifies that the constraining parameters are
       consistent with at least one of the codecs to be used with the
       RID RTP Stream.  If the "a=rid" line contains a "pt=" parameter,
       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" parameters are incompatible with the
       other codec properties for all codecs, then the "a=rid" line is

   Note that the answerer does not need to understand every constraint
   present in a "send" line: if a stream sender constrains 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, described in
   Section 6.2.2, the answerer shall perform the following steps to
   generate the SDP answer.

   For each "a=rid" line:

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

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

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

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

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

   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 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-identifier".  If no matching
       line can be located in the offer, the "a=rid" line is ignored.

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

   3.  If the constraints have been changed between the offer and the
       answer, the offerer MUST ensure that the modifications can be
       supported; if they cannot, the offerer SHALL consider discard the "a=rid" line as

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

   5.  If the "a=rid" line in the answer contains a "pt=" parameter 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.  If not, the offerer SHALL consider discard the
       "a=rid" line as rejected. line.

   6.  If the "a=rid" line contains a "pt=" parameter, 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"
       parameters are incompatible with the other codec properties, then
       the offerer SHALL discard the "a=rid" line is removed. line.

   7.  The offerer verifies that the constraining parameters are
       consistent with at least one of the codecs to be used with the
       RID RTP Stream.  If the "a=rid" line contains a "pt=" parameter,
       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" parameters are incompatible with the
       other codec properties for all codecs, then the "a=rid" line offerer SHALL be considered rejected
       discard the "a=rid" line.

   Any "a=rid" line present in the offer that was not matched by step 1
   above SHALL has been discarded by the answerer, and does not form part of
   the negotiated constraints on a RID RTP Stream.  The offerer MAY
   still apply any constraints it indicated in an "a=rid" line with a
   direction parameter 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, but 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 constraints.  Under such circumstances, the
   offerer MUST be considered as rejected. prepared to receive a media stream to which no
   constraints 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 re-using
   one from the previous offer/answer exchange.  RIDs without proposed
   changes SHOULD re-use the ID from the previous exchange.

7.  Use with Declarative SDP

   Although designed predominantly with session negotiation in mind, the
   "a=rid" attribute can also be used in declarative SDP situations.
   When used with declarative SDP, any constraints applied to a RID
   indicate how the sender intends to constrain the stream they are

   In declarative use, the "direction" parameter MUST be set to "send"
   in all "a=rid" lines.

   Recipients of declarative SDP may use the indicated constraints to
   select an RID RTP Stream to decode, based on their needs and

8.  Interaction with Other Techniques

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

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

   o  Size restrictions imposed by image attribute attributes
      ("a=imgattr") [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

   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 union.  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

   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 union.  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=imgattr"), its only recourse is to remove the "a=rid" line
   altogether, as described in Section 6.3.  If it instead attempts to
   constrain 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.

9.  Formal Grammar

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

   rid-syntax        = "a=rid:" rid-identifier SP rid-dir
                       [ rid-pt-param-list / rid-param-list ]

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

   rid-dir           = "send" / "recv"

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

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

   rid-fmt-list      = "pt=" fmt *( "," fmt )
                        ; fmt 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   = "max-width" [ "=" int-param-val ]

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

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

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

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

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

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

   rid-depend-param  = "depend=" rid-list

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

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

   int-param-val     = 1*DIGIT

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

   param-val         = *( %x20-58 / %x60-7E )
                       ; Any printable character except semicolon

10.  SDP Examples

   Note: see [I-D.ietf-mmusic-sdp-simulcast] for examples of RID used in
   simulcast scenarios.

10.1.  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:

   o  1 receive stream of 720p resolution is offered for the active

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

   o  4 receive streams of 180p resolution are offered for others in the

   Expressing all these codecs and resolutions using 32 dynamic PTs (2
   audio + 10x3 video) would exhaust the primary dynamic space (96-127).
   RIDs are used to avoid PT exhaustion and express the resolution

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

   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
   m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107
   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=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
   ...same rtpmap/fmtp as above...
   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
   ...same rtpmap/fmtp as above...
   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
   ...same rtpmap/fmtp as above...
   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
   ...same rtpmap/fmtp as above...
   ...same rid:4 as above for mid:v5,v6,v7 (small resolution)...

10.2.  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" parameter.  In the example below, the highest resolution is
   offered to be sent as 2 scalable temporal layers (using MRST).

   m=audio ...same as previous example ...
   m=video ...same as previous example ...
   ...same rtpmap/fmtp as previous example ...
   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...

11.  Open Issues

11.1.  Declarative SDP

   Section 7 describes the use of "a=rid" for declarative SDP.  This is
   a pretty small amount of work, and the use of this mechanism to
   describe how a sender is going to constrain a stream does have some
   amount of utility.  Is the text sufficient?  If not, do we want to
   invest the work needed to make RID work with declarative use cases?

   PROPOSAL: Keep the current text.

11.2.  Definition of bitrate

   Some questions have been raised as to whether we need a more formal
   description of bitrate than we currently use.

   If I read correctly, Magnus indicated that the definition in the
   document is consistent with TIAS, and believes it is sufficiently
   well defined.

   PROPOSAL: keep current definition that exists in description of "max-

11.3.  Escaping new constraint values

   The parameters on an "a=rid:" line are extensible.  The syntax for
   these is:

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

   param-val         = *( %x20-58 / %x60-7E )
                       ; Any printable character except semicolon

   If an extension has values that can contain semicolons, they need an
   escaping mechanism.  Note that this is not an issue for any currently
   defined parameters, as they all take numeric values only.

   1.  Change extension syntax to only allow numeric values

   2.  Define a universal escaping mechanism for all extensions to use

   3.  Leave this problem for the first extension parameter - if any -
       to define value in a way that might allow a semicolon.  Note that
       this approach would allow the use of percent-style escaping
       (e.g., "%3B") but not backslash-style escaping (e.g., "\;"), as
       parsers that do not support the new constraint would interpret
       the embedded semicolon as a separator.

   PROPOSAL: Option #3

11.4.  Utility of max-width and max height

   Comment from Stephan Wenger: Are max-width and max-height actually
   useful controls?  Shouldn't max-fs be sufficient for any plausible

   PROPOSAL: Keep max-height and max-width.  Implementation is well-
   defined and easily implemented.  At least one participant expressed
   support for these parameters at IETF 94 face-to-face meeting.

11.5.  Definition of max-fps

   Comment from Stephan Wenger: Would it be better to define max-fps as
   constraining the average over a second rather than the inverse of the
   smallest allowed interval between frames?

   PROPOSAL: Keep as currently defined.  The difference is subtle.  The
   only kinds of cases allowed by an average that aren't allowed by a
   minimum interframe interval are those such as sending no packets for
   most of a second, followed by a burst of 30 frames 1 ms apart, as
   part of a stream constrained to 30 fps.  Such cases seem undesirable.

12.  IANA Considerations

12.1.  New SDP Media-Level attribute

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

   The "rid" attribute is used to identify characteristics of RTP stream
   with in a RTP Session.  Its format is defined in Section 9.

12.2.  Registry for RID-Level Parameters

   This specification creates a new IANA registry named "att-field (rid
   level)" within the SDP parameters registry.  The "a=rid" parameters
   MUST be registered with IANA and documented under the same rules as
   for SDP session-level and media-level attributes as specified in

   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, parameters always
   have to restrict the definition to be a subset of what is otherwise
   allowable, and never expand it.

   New parameter registrations are accepted according to the
   "Specification Required" policy of [RFC5226], provided that the
   specification includes the following information:

   o  contact name, email address, and telephone number

   o  parameter name (as it will appear in SDP)

   o  long-form parameter name in English
   o  whether the parameter value is subject to the charset attribute

   o  an explanation of the purpose of the parameter

   o  a specification of appropriate attribute values for this parameter

   o  an ABNF definition of the parameter

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

      Type            SDP Name                     Reference
      ----            ------------------           ---------
      att-field       (rid level)
                      max-width                     [RFCXXXX]
                      max-height                    [RFCXXXX]
                      max-fps                       [RFCXXXX]
                      max-fs                        [RFCXXXX]
                      max-br                        [RFCXXXX]
                      max-pps                       [RFCXXXX]
                      max-bpp                       [RFCXXXX]
                      depend                        [RFCXXXX]

   It is conceivable that a future document wants to define a RID-level
   parameter that contains 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 provides 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.  Acknowledgements

   Many thanks to review from Cullen Jennings, Magnus Westerlund, and
   Paul Kyzivat.

15.  References

15.1.  Normative References

              Roach, A., Nandakumar, S., and P. Thatcher, "RTP Payload
              Format Constraints", draft-roach-avtext-rid-01 (work in
              progress), February 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, DOI
              10.17487/RFC3264, June 2002,

   [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, <>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
              2008, <>.

15.2.  Informative References

              Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, "A Taxonomy of Semantics and Mechanisms for
              Real-Time Transport Protocol (RTP) Sources", draft-ietf-
              avtext-rtp-grouping-taxonomy-08 (work in progress), July

              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-25 (work in progress), January 2016.

              Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
              "Using Simulcast in SDP and RTP Sessions", draft-ietf-
              mmusic-sdp-simulcast-04 (work in progress), October 2015. February 2016.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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,

Authors' Addresses

   Peter Thatcher


   Mo Zanaty
   Cisco Systems


   Suhas Nandakumar
   Cisco Systems


   Bo Burman

   Adam Roach


   Byron Campen