Network Working Group P. Thatcher
Internet-Draft Google
Intended status: Standards Track M. Zanaty
Expires: May 14, 2016 S. Nandakumar
Cisco Systems
B. Burman
Ericsson
A. Roach
B. Campen
Mozilla
November 11, 2015
RTP Payload Format Constraints
draft-ietf-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 12 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 May 14, 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
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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Key Words for Requirements . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. SDP 'rid' Media Level Attribute . . . . . . . . . . . . . . . 5
6. 'rid-level' constraints . . . . . . . . . . . . . . . . . . . 6
7. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 7
7.1. Generating the Initial SDP Offer . . . . . . . . . . . . 7
7.2. Answerer processing the SDP Offer . . . . . . . . . . . . 8
7.2.1. 'rid' unaware Answerer . . . . . . . . . . . . . . . 8
7.2.2. 'rid' aware Answerer . . . . . . . . . . . . . . . . 8
7.3. Generating the SDP Answer . . . . . . . . . . . . . . . . 9
7.4. Offering Processing of the SDP Answer . . . . . . . . . . 10
7.5. Modifying the Session . . . . . . . . . . . . . . . . . . 10
8. Usage of 'rid' in RTP and RTCP . . . . . . . . . . . . . . . 10
8.1. RTCP 'RID' SDES Extension . . . . . . . . . . . . . . . . 11
8.2. RTP 'rid' Header Extension . . . . . . . . . . . . . . . 11
9. Interaction with Other Techniques . . . . . . . . . . . . . . 12
10. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . 12
11. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1. Many Bundled Streams using Many Codecs . . . . . . . . . 14
11.2. Scalable Layers . . . . . . . . . . . . . . . . . . . . 16
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Name of the identifier . . . . . . . . . . . . . . . . . 16
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
13.1. New RTP Header Extension URI . . . . . . . . . . . . . . 17
13.2. New SDES item . . . . . . . . . . . . . . . . . . . . . 17
13.3. New SDP Media-Level attribute . . . . . . . . . . . . . 18
13.4. Registry for RID-Level Parameters . . . . . . . . . . . 18
14. Security Considerations . . . . . . . . . . . . . . . . . . . 19
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
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16. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
16.1. Normative References . . . . . . . . . . . . . . . . . . 20
16.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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 constraining
Source RTP Streams (Section 2.1.10
[I-D.ietf-avtext-rtp-grouping-taxonomy]), called "Restriction
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 RTCP SDES item to carry the
"rid" value, to provide correlation between the RTP Packets and their
format specification in the SDP. This SDES item also uses the header
extension mechanism [I-D.ietf-avtext-sdes-hdr-ext] to provide
correlation at stream startup, or stream changes where RTCP isn't
sufficient.
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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As described in Section 7.2.1, this mechanism achieves backwards
compatibility via the normal SDP processing rules, which require
unknown a= parameters to be ignored. This means that implementations
need to be prepared to handle successful offers and answers from
other implementations that neither indicate nor honor the constraints
requested by this mechanism.
Further, as described in Section 7 and its subsections, this
mechanism achieves extensibility by: (a) having offerers include all
supported constraints in their offer, abd (b) having answerers ignore
a=rid lines that specify unknown constraints.
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
constraints on payload configurations and to effectively map a
Source RTP Stream to its corresponding constraints.
1. Multi-source and Multi-stream Use Cases: Recently, there is a
rising trend with real-time multimedia applications supporting
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
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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". Roughly speaking, this attribute takes the following form
(see Section 10 for a formal definition).
a=rid:<rid-identifier> <direction> [pt=<fmt-list>;]<constraint>=<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" constraints 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' constraints
(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 13
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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6. 'rid-level' constraints
This section defines the 'rid-level' constraints that can be used to
constrain 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 during near resolution change boundaries.
All the constraints are optional and are subjected to negotiation
based on the SDP Offer/Answer rules described in Section 7
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This list is intended to be an initial set of constraints; future
documents may define additional constraints; see Section 13.4. While
this document doesn't 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 "rid-level" attributes defined in this
section.
7. SDP Offer/Answer Procedures
This section describes the SDP Offer/Answer [RFC3264] procedures when
using the 'rid' framework.
Note that 'rid's 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.
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 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) to which the constraints
expressed by the 'rid-level' attributes apply. Any Payload Types
chosen MUST be a valid payload type for the media section (that
is, it must be listed on the "m=" line).
4. The Offerer then chooses the 'rid-level' constraints (Section 6)
to be applied for the rid, and adds them to the "a=rid" line. If
it wishes the answer to have the ability to specify a constraint,
but does not wish to set a value itself, it MUST include the name
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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 'rid-level' attributes will further
constrain the equivalent 'fmtp' parameters for the given Payload Type
for those streams associated with the 'rid'.
If a given codec would require "a=fmtp" line when used without
"a=rid" then the offer MUST include a valid corresponding "a=fmtp"
line even when using 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.
Section 7.1 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.
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,
and all "a=rid" lines with rid-identifiers that duplicate this
line, are rejected and MUST NOT be included in the SDP Answer.
2. If the "a=rid" line contains a "pt=" parameter, the list of
payload types is verified against the list of valid payload types
for the media section (that is, those listed on the "m=" line).
If there is no match for the Payload Type listed in the "a=rid"
line, then remove the "a=rid" line.
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3. The answerer ensures that "rid-level" parameters 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. Any lines that do
not are removed.
5. if the "a=rid" line contains a "pt=" parameter, the answerer
verifies that the attribute values provided in the "rid-level"
attributes are consistent with the corrsponding codecs and their
other parameters. See Section 9 for more detail. If the rid-
level parameters are incompatible with the other codec
properties, then the "a=rid" line is removed.
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 more constrained than the ones specified in the offer. The
answer MUST NOT include any constraints that were not present 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
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. This
includes situations in which the answerer does not understand one
or more of the constraints in the "a=rid" line that has an
associated value.
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 were sent in the offer.
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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.
4. If the "a=rid" line contains a "pt=" parameter, the offerer
verifies that the list of payload types is a subset of those sent
in the corresponding "a=rid" line in the offer.
5. If the "a=rid" line contains a "pt=" parameter, the offerer
verifies that the attribute values provided in the "rid-level"
attributes are consistent with the corrsponding codecs and their
other parameters. See Section 9 for more detail. If the rid-
level parameters are incompatible with the other codec
properties, then the "a=rid" line is removed.
7.5. Modifying the Session
Offers and answers inside an existing session follow the rules for
initial session negotiation. Such an offer MAY propose a change the
number of RIDs in use. To avoid race conditions with media, any RIDs
with proposed changes SHOULD use a new ID, rather than re-using one
from the previous offer/answer exchange. RIDs without proposed
changes SHOULD re-use the ID from the previous exchange.
8. Usage of 'rid' in RTP and RTCP
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 RTCP SDES item [RFC3550], 'RID',
which is used to carry rids within RTCP SDES packets. This makes it
possible for a receiver to associate received RTP packets
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(identifying the Source RTP Stream) with a media description having
the format constraint specified.
This specification also uses the RTP header extension for RTCP SDES
items [I-D.ietf-avtext-sdes-hdr-ext] to allow carrying RID
information in RTP packets to provide correlation at stream startup,
or after stream changes where the use of RTCP may not be sufficiently
responsive.
8.1. RTCP 'RID' SDES Extension
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RID=TBD | length | rid ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The rid payload is UTF-8 encoded and is not null-terminated.
RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.
8.2. RTP 'rid' Header Extension
Because recipients of RTP packets will typically need to know which
"a=rid" constraints they correspond to immediately upon receipt, this
specification also defines a means of carrying RID identifiers in RTP
extension headers, using the technique described in
[I-D.ietf-avtext-sdes-hdr-ext].
As described in that document, the header extension element can be
encoded using either the one-byte or two-byte header, and the
identification-tag payload is UTF-8 encoded, as in SDP.
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
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many cases, a one-byte tag will be sufficient; it is RECOMMENDED that
implementations use the shortest identifier that fits their purposes.
9. Interaction with Other Techniques
Historically, a number of other approaches have been defined that
allow constraining media streams via SDP parameters. These include:
o Codec-specific configuration set via format parameters ("a=fmtp");
for example, the H.264 "max-fs" format parameter
o Size restrictions imposed by image attribute attributes
("a=imgattr") [RFC6236]
When the mechanism described in this document is used in conjunction
with these other restricting mechanisms, it is intended to impose
additional restrictions beyond those communicated in other
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 7.2.2, this will
result in the corresponding "a=rid" line being ignored by the
recipient.
Similarly, an answer the a=rid lines, when combined with the other
restrictions on the media stream, are also expected to result in a
non-empty union. If the implementation generating an answer wishes
to restrict a property of the stream below that which would be
allowed by other parameters (e.g., those specified in "a=fmtp" or
"a=imgattr"), its only recourse is to remove the "a=rid" line
altogether, as described in Section 7.3. 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 7.4.
10. 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
[ rid-pt-param-list / rid-param-list ]
rid-identifier = 1*(alpha-numeric / "-" / "_")
rid-dir = "send" / "recv"
rid-pt-param-list = SP rid-fmt-list *(";" rid-param)
rid-param-list = SP rid-param *(";" rid-param)
rid-fmt-list = "pt=" fmt *( "," fmt )
; fmt defined in {{RFC4566}}
rid-param = rid-width-param
/ rid-height-param
/ rid-fps-param
/ rid-fs-param
/ rid-br-param
/ rid-pps-param
/ rid-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-depend-param = "depend=" rid-list
rid-param-other = 1*(alpha-numeric / "-") [ "=" param-val ]
rid-list = rid-identifier *( "," rid-identifier )
int-param-val = 1*DIGIT
param-val = *( %x20-58 / %x60-7E )
; Any printable character except semicolon
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11. SDP Examples
Note: see [I-D.ietf-mmusic-sdp-simulcast] 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:
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 focused, 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
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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
...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)...
...
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Answer:
...same as offer but swap send/recv...
11.2. 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).
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 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 Example1 for mid:v2-v7...
...
Answer:
...same as offer but swap send/recv...
12. Open Issues
12.1. 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
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o CID: Constraint ID, which is a rather precise description of what
we are attempting to accomplish.
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.
13. IANA Considerations
13.1. 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:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:rid
Description: RTP Stream Restriction Identifier
Contact: <mmusic@ietf.org>
Reference: RFCXXXX
13.2. New SDES item
RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of
this document.
RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.
This document adds the MID SDES item to the IANA "RTCP SDES item
types" registry as follows:
Value: TBD
Abbrev.: RID
Name: Restriction Identification
Reference: RFCXXXX
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13.3. 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.
13.4. Registry for RID-Level Parameters
This specification creates a new IANA registry named "att-field (rid
level)" within the SDP parameters registry. The rid-level parameters
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, parameters always
have to restrict the definition to be a subset of what is otherwise
allowable, and never expand it.
New parameter registrations are accepted according to the
"Specification Required" policy of [RFC5226], provided that the
specification includes the following information:
o contact name, email address, and telephone number
o parameter name (as it will appear in SDP)
o long-form parameter name in English
o whether the parameter value is subject to the charset attribute
o an explanation of the purpose of the parameter
o a specification of appropriate attribute values for this parameter
o an ABNF definition of the parameter
The initial set of rid-level parameter names, with definitions in
Section 6 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]
It is conceivable that a future document wants to define a RID-level
parameter that contains string values. These extensions need to take
care to conform to the ABNF defined for rid-param-other. In
particular, this means that such extensions will need to define
escaping mechanisms if they want to allow semicolons, unprintable
characters, or byte values greater than 127 in the string.
OPEN ITEM: Do we need to do more than this regarding escaping?
14. Security Considerations
As with most SDP parameters, a failure to provide integrity
protection over the a=rid attributes provides attackers a way to
modify the session in potentially unwanted ways. This could result
in an implementation sending greater amounts of data than a recipient
wishes to receive. In general, however, since the "a=rid" attribute
can only restrict a stream to be a subset of what is otherwise
allowable, modification of the value cannot result in a stream that
is of higher bandwidth than would be sent to an implementation that
does not support this mechanism.
The actual identifiers used for RIDs are expected to be opaque. As
such, they are not expected to contain information that would be
sensitive, were it observed by third-parties.
15. Acknowledgements
Many thanks to review from Cullen Jennings, Magnus Westerlund, and
Paul Kyzivat.
16. References
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16.1. Normative References
[I-D.ietf-avtext-sdes-hdr-ext]
Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
Header Extension for RTCP Source Description Items",
draft-ietf-avtext-sdes-hdr-ext-02 (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>.
[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>.
16.2. Informative 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.
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[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-03 (work in progress), October 2015.
[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>.
[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>.
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
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Adam Roach
Mozilla
Email: adam@nostrum.com
Byron Campen
Mozilla
Email: bcampen@mozilla.com
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