One document matched: draft-pthatcher-mmusic-rid-00.txt
Network Working Group P. Thatcher
Internet-Draft Google
Intended status: Standards Track M. Zanaty
Expires: April 4, 2016 S. Nandakumar
Cisco Systems
A. Roach
Mozilla
B. Burman
Ericsson
B. Campen
Mozilla
October 02, 2015
RTP Payload Format Constraints
draft-pthatcher-mmusic-rid-00
Abstract
In this specification, we define a framework for identifying Source
RTP Streams with the constraints on its payload format in the Session
Description Protocol. This framework uses "rid" SDP attribute to: a)
effectively identify the Source 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 Source RTP Streams to their media
format specification in the SDP.
Note-1: The name 'rid' is not yet finalized. Please refer to
Section "Open Issues" for more details on the naming.
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 April 4, 2016.
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Copyright Notice
Copyright (c) 2015 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Key Words for Requirements . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. SDP 'rid' Media Level Attribute . . . . . . . . . . . . . . . 5
6. "rid-level' attributes . . . . . . . . . . . . . . . . . . . 6
7. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 7
7.1. Generating the Initial SDP Offer . . . . . . . . . . . . 7
7.2. Answerer processing the SDP Offer . . . . . . . . . . . . 7
7.2.1. 'rid' unaware Answerer . . . . . . . . . . . . . . . 7
7.2.2. 'rid' aware Answerer . . . . . . . . . . . . . . . . 8
7.3. Generating the SDP Answer . . . . . . . . . . . . . . . . 8
7.4. Offering Processing of the SDP Answer . . . . . . . . . . 9
7.5. Modifying the Session . . . . . . . . . . . . . . . . . . 9
8. Usage of 'rid' in RTP . . . . . . . . . . . . . . . . . . . . 9
8.1. RTP 'rid' Header Extension . . . . . . . . . . . . . . . 9
9. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . 10
10. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Many Bundled Streams using Many Codecs . . . . . . . . . 12
10.2. Simulcast . . . . . . . . . . . . . . . . . . . . . . . 14
10.3. Scalable Layers . . . . . . . . . . . . . . . . . . . . 14
10.4. Simulcast with Payload Types . . . . . . . . . . . . . . 15
11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Name of the identifier . . . . . . . . . . . . . . . . . 16
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
12.1. New RTP Header Extension URI . . . . . . . . . . . . . . 16
12.2. New SDP Media-Level attribute . . . . . . . . . . . . . 17
12.3. Registry for RID-Level Attributes . . . . . . . . . . . 17
13. Security Considerations . . . . . . . . . . . . . . . . . . . 18
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
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15.1. Normative References . . . . . . . . . . . . . . . . . . 18
15.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Payload Type (PT) in RTP provides mapping between the format of the
RTP payload and the media format description specified in the
signaling. For applications that use SDP for signaling, the
constructs rtpmap and/or fmtp describe the characteristics of the
media that is carried in the RTP payload, mapped to a given PT.
Recent advances in standards such as RTCWEB and NETVC have given rise
to rich multimedia applications requiring support for multiple RTP
Streams with in a RTP session
[I-D.ietf-mmusic-sdp-bundle-negotiation],
[I-D.ietf-mmusic-sdp-simulcast] or having to support multiple codecs,
for example. 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 underspecied payload format parameters,
o Ambiguity in mapping between the individual Source RTP Streams and
their equivalent format specification in the SDP.
This specification defines a new SDP framework for configuring and
identifying Source RTP Streams (Section 2.1.10
[I-D.ietf-avtext-rtp-grouping-taxonomy]) called "RTP Source Stream
Identifier (rid)" along with the SDP attributes to constrain their
payload formats in a codec-agnostic way. The "rid" framework can be
thought of as complementary extension to the way the media format
parameters are specified in SDP today, via the "a=fmtp" attribute.
This specification also proposes a new RTP header extension to carry
the "rid" value, to provide correlation between the RTP Packets and
their format specification in the SDP.
Note that the "rid" parameters only serve to further constrain the
parameters that are established on a PT format. They do not relax
any existing constraints.
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2. 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]
3. Terminology
The terms Source RTP Stream, Endpoint, RTP Session, and RTP Stream
are used as defined in [I-D.ietf-avtext-rtp-grouping-taxonomy].
[RFC4566] and [RFC3264] terminology is also used where appropriate.
4. Motivation
This section summarizes several motivations for proposing the "rid"
framework.
1. RTP PT Space Exhaustion: [RFC3550] defines payload type (PT) that
identifies the format of the RTP payload and determine its
interpretation by the application. [RFC3550] assigns 7 bits for
the PT in the RTP header. However, the assignment of static
mapping of payload codes to payload formats and multiplexing of
RTP with other protocols (such as RTCP) could result in limited
number of payload type numbers available for the application
usage. In scenarios where the number of possible RTP payload
configurations exceed the available PT space within a RTP
Session, there is need a way to represent the additional payload
configurations and also to effectively map a Source RTP Stream to
its configuration in the signaling.
2. Codec-Specific Media Format Specification in SDP: RTP Payload
configuration is typically specified using rtpmap and fmtp SDP
attributes. The rtpmap attribute provides the media format to
RTP PT mapping and the ftmp attribute describes the media format
specific parameters. The syntax for the fmtp attribute is
tightly coupled to a specific media format (such as H.264, H.265,
VP8). This has resulted in a myriad ways for defining the
attributes that are common across different media formats.
Additionally, with the advent of new standards efforts such as
NETVC, one can expect more media formats to be standardized in
the future. Thus, there is a need to define common media
characteristics in a codec-agnostic way in order to reduce the
duplicated efforts and to simplify the syntactic representation
across the different codec standards.
3. Multi-source and Multi-stream Use Cases: Recently, there is a
rising trend with real-time multimedia applications supporting
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multiple sources per endpoint with various temporal resolutions
(Scalable Video Codec) and spatial resolutions (Simulcast) per
source. These applications are being challenged by the limited
RTP PT space and/or by the underspecified SDP constructs for
exercising granular control on configuring the individual Source
RTP Streams.
5. SDP 'rid' Media Level Attribute
This section defines new SDP media-level attribute [RFC4566],
"a=rid".
a=rid:<rid-identifier> <direction> pt=<fmt-list> <rid-attribute>:<value> ...
A given "a=rid" SDP media attribute specifies constraints defining an
unique RTP payload configuration identified via the "rid-identifier".
A set of codec-agnostic "rid-level" attributes are defined
(Section 6) that describe the media format specification applicable
to one or more Payload Types speicified by the "a=rid" line.
The 'rid' framework MAY be used in combination with the 'a=fmtp' SDP
attribute for describing the media format parameters for a given RTP
Payload Type. However in such scenarios, the 'rid-level' attributes
(Section 6) further constrains the equivalent 'fmtp' attributes.
The 'direction' identifies the either 'send', 'recv' directionality
of the Source RTP Stream.
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.
The 'rid' media attribute MAY be used for any RTP-based media
transport. It is not defined for other transports.
Though the 'rid-level' attributes specified by the 'rid' property
follow the syntax similar to session-level and media-level
attributes, they are defined independently. All 'rid-level'
attributes MUST be registered with IANA, using the registry defined
in Section 12
Section 9 gives a formal Augmented Backus-Naur Form(ABNF) [RFC5234]
grammar for the "rid" attribute.
The "a=rid" media attribute is not dependent on charset.
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6. "rid-level' attributes
This section defines the 'rid-level' attributes that can be used to
constrain the RTP payload encoding format in a codec-agnostic way.
The following new SDP parameters shall be defined that represent
things common across video codecs.
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.
o max-br, for bit rate in bits per second. The exact means of
keeping withing 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
specific in "max-br."
o max-pps, for pixel rate in pixels per second. This value SHOULD
be handled identically to max-fps, after performing the following
conversion: max-fps = max-pps / (width * height). If the stream
resolution changes, this value is recalculated. Due to this
recalculation, excursions outside the specified maximum are
possible during near resolution change boundaries.
All the attributes are optional and are subjected to negotiation
based on the SDP Offer/Answer rules described in Section 7
Section 9 provides formal Augmented Backus-Naur Form(ABNF) [RFC5234]
grammar for each of the "rid-level" attributes defined in this
section.
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7. SDP Offer/Answer Procedures
This section describes the SDP Offer/Answer [RFC3264] procedures when
using the 'rid' framework.
7.1. Generating the Initial SDP Offer
For each 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
the below steps:
1. It MUST generate rid-identifier' unique with in a media
description
2. It MUST set the direction for the 'rid-identifier' to one of
'send' or 'recv'
3. A listing of SDP format tokens (usually corresponding to RTP
payload types) MUST be included to which the constraints
expressed by the 'rid-level' attributes apply. The Payload Types
chosen MUST either be defined as part of "a=rtpmap" or "a=fmtp"
attributes.
4. The Offerer then chooses the 'rid-level' attributes (Section 6)
to be applied for the SDP format tokens listed.
5. If an 'a=fmtp' attribute is also used to provide media-format-
specific parameters, then the 'rid-level' attributes will further
constrain the equivalent 'fmtp' parameters for the given Payload
Type for those streams associated with the 'rid'.
7.2. 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:
7.2.1. 'rid' unaware Answerer
If the receiver doesn't support the 'rid' framework proposed in this
specification, the entire "a=rid" line is ignored following the
standard [RFC3264] Offer/Answer rules. If a given codec would
require 'a=fmtp' line when used without "a-rid" then the offer still
needs to include that even when using RID.
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7.2.2. 'rid' aware Answerer
If the answerer supports 'rid' framework, the following 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
is rejected and MUST not be included in the SDP Answer.
2. As a next step, the list of payload types are verified against
the list obtained from "a=rtpmap" and/or "a=fmtp" attributes. If
there is no match for the Payload Type listed in the "a=rid"
line, then remove the "a=rid" line. The exception being when '*'
is used for identifying the media format, where in the "a=rid"
line applies to all the formats in a given media description.
3. On verifying the Payload Type(s) matches, the answerer shall
ensure that "rid-level" attributes listed are supported and
syntactically well formed. In the case of a syntax error or an
unsupported parameter, the "a=rid" line is removed.
4. If the 'depend' rid-level attribute is included, the answerer
MUST make sure that the rid-identifiers listed unambiguously
match the rid-identifiers in the SDP offer.
5. If the media description contains an "a=fmtp" attribute, the
answerer verifies that the attribute values provided in the "rid-
level" attributes are within the scope of their fmtp equivalents
for a given media format.
7.3. Generating the SDP Answer
Having performed the verification of the SDP offer as described, the
answerer shall perform the following steps to generate the SDP
answer.
For each "a=rid" line:
1. The answerer MAY choose to modify specific 'rid-level' attribute
value in the answer SDP. In such a case, the modified value MUST
be lower (more constrained) than the ones specified in the offer.
2. The answerer MUST NOT modify the 'rid-identifier' present in the
offer.
3. 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
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media format tokens from an "a=rid" line, the answerer MUST
remove the entire "a=rid" line.
4. 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.
7.4. Offering Processing of the SDP Answer
The offerer shall follow the steps similar to answerer's offer
processing with the following exceptions
1. The offerer MUST ensure that the 'rid-identifiers' aren't changed
between the offer and the answer. If so, the offerer MUST
consider the corresponding 'a=rid' line as rejected.
2. If there exist changes in the 'rid-level' attribute values, the
offerer MUST ensure that the modifications can be supported or
else consider the "a=rid" line as rejected.
3. If the SDP answer contains any "rid-identifier" that doesn't
match with the offer, the offerer MUST ignore the corresponding
"a=rid" line.
7.5. Modifying the Session
TODO
8. Usage of 'rid' in RTP
The RTP fixed header includes the payload type number and the SSRC
values of the RTP stream. RTP defines how you de-multiplex streams
within an RTP session, but in some use cases applications need
further identifiers in order to effectively map the individual RTP
Streams to their equivalent payload configurations in the SDP.
This specification defines a new RTP header extension to include the
'rid-identifier'. This makes it possible for a receiver to associate
received RTP packets (identifying the Source RTP Stream) with a media
description having the format constraint specificied.
8.1. RTP 'rid' Header Extension
The payload, containing the identification-tag, of the RTP 'rid-
identifier' header extension element can be encoded using either the
one-byte or two-byte header [RFC5285]. The identification-tag
payload is UTF-8 encoded, as in SDP.
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As the identification-tag is included in an RTP header extension,
there should be some consideration about the packet expansion caused
by the identification-tag. To avoid Maximum Transmission Unit (MTU)
issues for the RTP packets, the header extension's size needs to be
taken into account when the encoding media. Note that set of header
extensions included in the packet needs to be padded to the next
32-bit boundary using zero bytes [RFC5285]
It is recommended that the identification-tag is kept short. Due to
the properties of the RTP header extension mechanism, when using the
one-byte header, a tag that is 1-3 bytes will result in that a
minimal number of 32-bit words are used for the RTP header extension,
in case no other header extensions are included at the same time. In
many cases, a one-byte tag will be sufficient; it is RECOMMENDED that
implementations use the shortest tag that fits their purposes.
9. Formal Grammar
This section gives a formal Augmented Backus-Naur Form (ABNF)
[RFC5234] grammar for each of the new media and rid-level attributes
defined in this document.
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rid-syntax = "a=rid:" rid-identifier SP rid-dir SP rid-fmt-list SP rid-attr-list
rid-identifier = 1*(alpha-numeric / "-" / "_")
rid-dir = "send" / "recv"
rid-fmt-list = "pt=" rid-fmt *( ";" rid-fmt )
rid-fmt = "*" ; wildcard: applies to all formats
/ fmt
rid-attr-list = rid-width-param
/ rid-height-param
/ rid-fps-param
/ rid-fs-param
/ rid-br-param
/ rid-pps-param
/ rid-depend-param
rid-width-param = "max-width=" param-val
rid-height-param = "max-height=" param-val
rid-fps-param = "max-fps=" param-val
rid-fs-param = "max-fs=" param-val
rid-br-param = "max-br=" param-val
rid-pps-param = "max-pps=" param-val
rid-depend-param = "depend=" rid-list
rid-list = rid-identifier *( ";" rid-identifier )
param-val = byte-string
; WSP defined in {{RFC5234}}
; fmt defined in {{RFC4566}}
; byte-string in {{RFC4566}}
10. SDP Examples
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10.1. Many Bundled Streams using Many Codecs
In this scenario, the offerer supports the Opus, G.722, G.711 and
DTMF audio codecs, and VP8, VP9, H.264 (CBP/CHP, mode 0/1), H.264-SVC
(SCBP/SCHP) and H.265 (MP/M10P) for video. An 8-way video call (to a
mixer) is supported (send 1 and receive 7 video streams) by offering
7 video media sections (1 sendrecv at max resolution and 6 recvonly
at smaller resolutions), all bundled on the same port, using 3
different resolutions. The resolutions include:
o 1 receive stream of 720p resolution is offered for the active
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.
Expressing all these codecs and resolutions using 32 dynamic PTs (2
audio + 10x3 video) would exhaust the primary dynamic space (96-127).
RIDs are used to avoid PT exhaustion and express the resolution
constraints.
Note: The SDP given below skips few lines to keep the example
short and focussed, as indicated by either the "..." or the
comments inserted.
Example 1
Offer:
...
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
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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 pt=*; max-width=1280; max-height=720; max-fps=30
a=rid:2 recv pt=*; 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 pt=*; 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 pt=*; 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 pt=*; 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)...
...
Answer:
...same as offer but swap send/recv...
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10.2. Simulcast
Adding simulcast to the above example allows the mixer to selectively
forward streams like an SFU rather than transcode high resolutions to
lower ones. Simulcast encodings can be expressed using PTs or RIDs.
Using PTs can exhaust the primary dynamic space even faster in
simulcast scenarios. So RIDs are used to avoid PT exhaustion and
express the encoding constraints. In the example below, 3
resolutions are offered to be sent as simulcast to a mixer/SFU.
Example 2
Offer:
...
m=audio ... same as from Example 1 ..
...
m=video ...same as from Example 1 ...
...same rtpmap/fmtp as above...
a=sendrecv
a=mid:v1 (max resolution)
a=rid:1 send pt=*; max-width=1280; max-height=720; max-fps=30
a=rid:2 recv pt=*; max-width=1280; max-height=720; max-fps=30
a=rid:5 send pt=*; max-width=640; max-height=360; max-fps=15
a=rid:6 send pt=*; max-width=320; max-height=180; max-fps=15
a=simulcast: send rid=1;5;6 recv rid=2
...
...same m=video sections as Example 1 for mid:v2-v7...
...
Answer:
...same as offer but swap send/recv...
10.3. Scalable Layers
Adding scalable layers to the above simulcast example gives the SFU
further flexibility to selectively forward packets from a source that
best match the bandwidth and capabilities of diverse receivers.
Scalable encodings have dependencies between layers, unlike
independent simulcast streams. RIDs can be used to express these
dependencies using the "depend" parameter. In the example below, the
highest resolution is offered to be sent as 2 scalable temporal
layers (using MRST).
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Example 3
Offer:
...
m=audio ...same as Example 1 ...
...
m=video ...same as Example 1 ...
...same rtpmap/fmtp as Example 1...
a=sendrecv
a=mid:v1 (max resolution)
a=rid:0 send pt=*; max-width=1280; max-height=720; max-fps=15
a=rid:1 send pt=*; max-width=1280; max-height=720; max-fps=30; depend=0
a=rid:2 recv pt=*; max-width=1280; max-height=720; max-fps=30
a=rid:5 send pt=*; max-width=640; max-height=360; max-fps=15
a=rid:6 send pt=*; 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 Example1 for mid:v2-v7...
...
Answer:
...same as offer but swap send/recv...
10.4. Simulcast with Payload Types
This example shows a simulcast Offer SDP that uses rid framework to
identify:
o 1 send stream at max resolution,
o 1 recv stream at max resolution,
o 1 recv stream at low resolution
and includes 2 "a=simulcast" lines to identify the simulcast streams
with the Payload Types and rid-identifier respectively.
Note: The exact rules for the usage of rid framework with
simulcast is still a work in progress.
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Example 4
Offer:
m=video 10000 RTP/AVP 97 98
a=rtpmap:97 VP8/90000
a=rtpmap:98 VP8/90000
a=fmtp:97 max-fs=3600
a=fmtp:98 max-fs=3600
a=rid:1 send pt=97; max-br=; max-height=720;
a=rid:2 recv pt=97; max-width=1280; max-height=720
a=rid:3 recv pt=98; max-width=320; max-height=180
a=simulcast send pt=97 recv pt=*
a=simulcast: send rid=1 recv rid=2;3
11. Open Issues
11.1. Name of the identifier
The name 'rid' is provisionally used and is open for further
discussion.
Here are the few options that were considered while writing this
draft
o ESID: Encoded Stream ID, does not align well with taxonomy which
defines Encoded Stream as before RTP packetization.
o RSID or RID: RTP Stream ID, aligns better with taxonomy but very
vague.
o LID: Layer ID, aligns well for SVC with each layer in a separate
stream, but not for other SVC layerings or independent simulcast
which is awkward to view as layers.
o EPT or XPT: EXtended Payload Type, conveys XPT.PT usage well, but
may be confused with PT, for example people may mistakenly think
they can use it in other places where PT would normally be used.
12. IANA Considerations
12.1. New RTP Header Extension URI
This document defines a new extension URI in the RTP Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:
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Extension URI: urn:ietf:params:rtp-hdrext:rid
Description: RTP Stream Identifier
Contact: <name@email.com>
Reference: RFCXXXX
12.2. 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 XXXX.
12.3. Registry for RID-Level Attributes
This specification creates a new IANA registry named "att-field (rid
level)" within the SDP parameters registry. The rid-level attributes
MUST be registered with IANA and documented under the same rules as
for SDP session-level and media-level attributes as specified in
[RFC4566].
New attribute 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 attribute name (as it will appear in SDP)
o long-form attribute name in English
o whether the attribute value is subject to the charset attribute
o a one-paragraph explanation of the purpose of the attribute
o a specification of appropriate attribute values for this attribute
The initial set of rid-level attribute names, with definitions in
Section XXXX of this document, is given below
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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]
depend [RFCXXXX]
13. Security Considerations
TODO
14. Acknowledgements
Many thanks to review from Cullen Jennings, Magnus Westerlund.
15. References
15.1. Normative References
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, "A Taxonomy of Semantics and Mechanisms for
Real-Time Transport Protocol (RTP) Sources", draft-ietf-
avtext-rtp-grouping-taxonomy-08 (work in progress), July
2015.
[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>.
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[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>.
[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>.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
15.2. Informative References
[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-23 (work in progress), July 2015.
[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-01 (work in progress), July 2015.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, DOI
10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.
Authors' Addresses
Peter Thatcher
Google
Email: pthatcher@google.com
Mo Zanaty
Cisco Systems
Email: mzanaty@cisco.com
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Suhas Nandakumar
Cisco Systems
Email: snandaku@cisco.com
Adam Roach
Mozilla
Email: adam@nostrum.com
Bo Burman
Ericsson
Email: bo.burman@ericsson.com
Byron Campen
Mozilla
Email: bcampen@mozilla.com
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