RTP Payload Format Constraints
draft-ietf-mmusic-rid-05
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8851.
|
|
|---|---|---|---|
| Authors | Peter Thatcher , Mo Zanaty , Suhas Nandakumar , Bo Burman , Adam Roach , Byron Campen | ||
| Last updated | 2016-03-28 | ||
| Replaces | draft-pthatcher-mmusic-rid | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
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by Robert Sparks
Ready w/nits
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||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Flemming Andreasen | ||
| IESG | IESG state | Became RFC 8851 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | "Flemming Andreasen" <fandreas@cisco.com> |
draft-ietf-mmusic-rid-05
Network Working Group P. Thatcher
Internet-Draft Google
Updates: 4855 (if approved) M. Zanaty
Intended status: Standards Track S. Nandakumar
Expires: September 26, 2016 Cisco Systems
B. Burman
Ericsson
A. Roach
B. Campen
Mozilla
March 25, 2016
RTP Payload Format Constraints
draft-ietf-mmusic-rid-05
Abstract
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.
This specification updates RFC4855 to give additional guidance on
choice of Format Parameter (fmtp) names, and on their relation to the
constraints defined by this document.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 26, 2016.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Key Words for Requirements . . . . . . . . . . . . . . . . . 4
4. SDP "a=rid" Media Level Attribute . . . . . . . . . . . . . . 5
5. "a=rid" constraints . . . . . . . . . . . . . . . . . . . . . 6
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. "a=rid"-unaware Answerer . . . . . . . . . . . . . . 8
6.2.2. "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 . . . . . . . . . . . . . . . . . . 12
8. Interaction with Other Techniques . . . . . . . . . . . . . . 12
8.1. Interaction with VP8 Format Parameters . . . . . . . . . 13
8.1.1. max-fr - Maximum Framerate . . . . . . . . . . . . . 13
8.1.2. max-fs - Maximum Framesize, in VP8 Macroblocks . . . 13
8.2. Interaction with H.264 Format Parameters . . . . . . . . 14
8.2.1. profile-level-id and max-recv-level - Negotiated Sub-
Profile . . . . . . . . . . . . . . . . . . . . . . . 15
8.2.2. max-br / MaxBR - Maximum Video Bitrate . . . . . . . 15
8.2.3. max-fs / MaxFS - Maximum Framesize, in H.264
Macroblocks . . . . . . . . . . . . . . . . . . . . . 15
8.2.4. max-mbps / MaxMBPS - Maximum Macroblock Processing
Rate . . . . . . . . . . . . . . . . . . . . . . . . 16
8.2.5. max-smbps - Maximum Decoded Picture Buffer . . . . . 16
9. Format Parameters for Future Payloads . . . . . . . . . . . . 16
10. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . 16
11. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Many Bundled Streams using Many Codecs . . . . . . . . . 18
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11.2. Scalable Layers . . . . . . . . . . . . . . . . . . . . 19
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12.1. New SDP Media-Level attribute . . . . . . . . . . . . . 20
12.2. Registry for RID-Level Parameters . . . . . . . . . . . 20
13. Security Considerations . . . . . . . . . . . . . . . . . . . 21
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
15.1. Normative References . . . . . . . . . . . . . . . . . . 22
15.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Terminology
The terms "Source RTP Stream", "Endpoint", "RTP Session", and "RTP
Stream" are used as defined in [RFC7656].
The term "RID RTP Stream" is used as defined in
[I-D.roach-avtext-rid].
[RFC4566] and [RFC3264] terminology is also used where appropriate.
2. Introduction
Payload Type (PT) in RTP provides a mapping between the RTP payload
format and the associated SDP media description. The SDP rtpmap 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.
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
configurations,
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
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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 a need for 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 [RFC7656]), along with the SDP
attributes to constrain 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 constraints on individual streams are indicated with a
new "a=rid" SDP attribute. Note that the constraints communicated
via this attribute only serve to further constrain the parameters
that are established on a PT format. They do not relax any existing
restrictions.
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.
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 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, and (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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]
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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 10 for a formal definition).
a=rid:<rid-id> <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"
field. This value binds the restriction to the RID RTP Stream
identified by its RID SDES item [I-D.roach-avtext-rid].
The "direction" field identifies the directionality of the RID RTP
Stream; it may be either "send" or "recv".
The optional "pt=<fmt-list>" lists one or more PT values that can be
used in the associated RID 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 constraints (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 constraints specified by the "rid" constraints follow a
syntax similar to session-level and media-level parameters, they are
defined independently. All "rid" constraints 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.
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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
frames.
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 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.
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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 10 provides formal Augmented Backus-Naur Form(ABNF) [RFC5234]
grammar for each of the "a=rid" constraints 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
description
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
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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 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" constraints will further
restrict 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. "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. "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,
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and all "a=rid" lines with rid-identifiers that duplicate this
line, are discarded and MUST NOT be included in the SDP Answer.
2. 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 removed from the "pt=".
3. The answerer ensures that the "a=rid" line is syntactically well
formed. In the case of a syntax error, the "a=rid" line is
removed.
4. If the "direction" field is "recv", The answerer ensures that
"a=rid" constraints are supported. In the case of an unsupported
constraint, the "a=rid" line is removed.
5. If the "depend" constraint 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
removed.
6. The answerer verifies that the constraints 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=", 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" constraints are incompatible with the other codec
properties for all codecs, then the "a=rid" line is removed.
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 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" field 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
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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
offer.
4. If the "a=rid" line contains a "pt=", 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=" 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 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
"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-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 discard the "a=rid" 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 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
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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.
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"
constraints are incompatible with the other codec properties,
then the offerer SHALL discard the "a=rid" line.
7. The offerer verifies that the constraints 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=", 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" constraints 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 constraints on a RID RTP Stream. The offerer MAY
still apply any constraints 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, 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 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
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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
sending.
In declarative use, the "direction" field 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
capabilities.
8. Interaction with Other Techniques
Historically, a number of other approaches have been defined that
allow constraining media streams via SDP. These include:
o Codec-specific configuration set via format parameters ("a=fmtp");
for example, the H.264 "max-fs" format parameter [RFC6184]
o Size restrictions imposed by image attribute attributes
("a=imageattr") [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 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
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 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
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"a=imageattr"), 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.
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 constraints on receiver capabilities, and never
indicate sending constraints.
8.1.1. max-fr - Maximum Framerate
The VP8 "max-fr" format parameter corresponds to the "max-fps"
constraint defined in this specification. If an RTP sender is
generating a stream using a format defined with this format
parameter, and the sending constraints defined via "a=rid" include a
"max-fps" parameter, then the sent stream is will conform to the
smaller of the two values.
8.1.2. max-fs - Maximum Framesize, in VP8 Macroblocks
The VP8 "max-fs" format parameter corresponds to the "max-fs"
constraint 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 constraints 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 are also required to 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:
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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
constraints:
o deint-buf-cap
o in-band-parameter-sets
o level-asymmetry-allowed
o max-rcmd-nalu-size
o max-cpb
o max-dpb
o packetization-mode
o redundant-pic-cap
o sar-supported
o sar-understood
o sprop-deint-buf-req
o sprop-init-buf-time
o sprop-interleaving-depth
o sprop-level-parameter-sets
o sprop-max-don-diff
o sprop-parameter-sets
o use-level-src-parameter-sets
Note that the max-cpb and max-dpb format parameters for H.264
correspond to constraints on the stream, but they are specific to the
way the H.264 codec operates, and do not have codec-independent
equivalents.
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The following codec format parameters correspond to constraints on
receiver capabilities, and never indicate sending constraints.
8.2.1. profile-level-id and max-recv-level - Negotiated Sub-Profile
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 constraints defined in
this document. [RFC6184] also defines formate 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 constraints 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" constraint 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 constraints defined via
"a=rid" include a "max-fps" parameter, then the sent stream is 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 Framesize, in H.264 Macroblocks
The H.264 "MaxFs" parameter (and its equiavelent "max-fs" format
parameter) corresponds roughly to the "max-fs" constraint defined in
this document, by way of a conversion factor of the number of pixels
per macroblock (typically 16 or 64).
As the size of H.264 macroblocks can change on a per-macroblock basis
for certain H.264 profiles, a direct mathematical conversion between
H.264's "max-fs" format parameter and the "a=rid" "max-fs" constraint
cannot be expressed. If an RTP sender is generating a stream using a
format defined with this format parameter, and the sending
constraints defined via "a=rid" include a "max-fs" parameter, then
the sent stream will conform to both signaled capabilities
simultaneously.
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8.2.4. max-mbps / MaxMBPS - Maximum Macroblock Processing Rate
The H.264 "MaxMBPS" parameter (and its equiavelent "max-mbps" format
parameter) corresponds roughly to the "max-pps" constraint defined in
this document, by way of a conversion factor of the number of pixels
per macroblock (typically 16 or 64).
As the size of H.264 macroblocks can change on a per-macroblock basis
for certain H.264 profiles, a direct mathematical conversion between
H.264's "max-mbps" format parameter and the "a=rid" "max-pps"
constraint cannot be expressed. If an RTP sender is generating a
stream using a format defined with this format parameter, and the
sending constraints defined via "a=rid" include a "max-pps"
parameter, then the sent stream will conform to both signaled
capabilities simultaneously.
8.2.5. max-smbps - Maximum Decoded Picture Buffer
The H.264 "max-smbps" format parameter operates the same way as the
"max-mpbs" 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.
9. Format Parameters for Future Payloads
Registrations of future RTP payload format specifications that define
media types that have parameters matching the RID constraints
specified in this memo SHOULD name those parameters in a manner that
matches the names of those RID constraints, and SHOULD explicitly
state what media type parameters are constrained by what RID
constraints.
10. 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)
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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
11. SDP Examples
Note: see [I-D.ietf-mmusic-sdp-simulcast] for examples of RID used in
simulcast scenarios.
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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:
o 1 receive stream of 720p resolution is offered for the active
speaker.
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
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=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
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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
...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
...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
...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
...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" constraint. In the example below, the highest resolution is
offered to be sent as 2 scalable temporal layers (using MRST).
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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 on their relation to RID
constraints.
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 10.
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" constraints
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, constraints always
have to restrict the definition to be a subset of what is otherwise
allowable, and never expand it.
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New constraint 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 constraint name (as it will appear in SDP)
o long-form constraint name in English
o whether the constraint value is subject to the charset attribute
o an explanation of the purpose of the constraint
o a specification of appropriate attribute values for this
constraint
o an ABNF definition of the constraint
The initial set of "a=rid" constraint 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
constraints 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 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
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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. Thanks to Colin Perkins for input on future payload
type handing..
15. References
15.1. Normative References
[I-D.roach-avtext-rid]
Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
Identifier (RID) Source Description (SDES)", draft-roach-
avtext-rid-02 (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,
<http://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,
<http://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, <http://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, <http://www.rfc-editor.org/info/rfc4566>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<http://www.rfc-editor.org/info/rfc4855>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
15.2. Informative References
[H264] ITU-T Recommendation H.264, "Advanced video coding for
generic audiovisual services (V9)", February 2014,
<http://www.itu.int/rec/T-REC-H.264-201304-I>.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-27 (work in progress), February 2016.
[I-D.ietf-mmusic-sdp-simulcast]
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), 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,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184, DOI 10.17487/
RFC6184, May 2011,
<http://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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7741>.
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Authors' Addresses
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
Adam Roach
Mozilla
Email: adam@nostrum.com
Byron Campen
Mozilla
Email: bcampen@mozilla.com
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