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
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publication of this document. Please review these documents
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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
