The Session Description Protocol (SDP) Application Token Attribute
draft-even-mmusic-application-token-01
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| Authors | Roni Even , Jonathan Lennox , Qin Wu | ||
| Last updated | 2013-09-15 | ||
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draft-even-mmusic-application-token-01
MMUSIC WG R. Even
Internet-Draft Huawei Technologies
Intended status: Informational J. Lennox
Expires: March 19, 2014 Vidyo
Q. Wu
Huawei Technologies
September 15, 2013
The Session Description Protocol (SDP) Application Token Attribute
draft-even-mmusic-application-token-01.txt
Abstract
The RTP fixed header includes the payload type number and the SSRC
values of the RTP stream. RTP defines how to de-multiplex streams
within an RTP session, but in some use cases applications need
further identifiers in order to identify the application semantics
associated with particular streams within the session.
This document defines a mechanism to provide the mapping between the
SSRCs of RTP streams and the application semantics by defining
extensions to RTP and RTCP messages.
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-
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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 March 19, 2014.
Copyright Notice
Copyright (c) 2013 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
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(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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Proposal for Application ID . . . . . . . . . . . . . . . . . 5
3.1. appID token . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1. RTCP SDES message . . . . . . . . . . . . . . . . . . 8
3.1.2. RTP Header Extension . . . . . . . . . . . . . . . . 8
3.1.3. recv-appID . . . . . . . . . . . . . . . . . . . . . 9
4. Using Application ID token in Offer / Answer . . . . . . . . 10
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The RTP [RFC3550] header includes the payload type number and the
SSRC values of the RTP stream. RTP defines how to de-multiplex
streams within an RTP session, but in some use cases, applications
need further identifiers in order to identify semantics associated
with particular streams within the session.
SDP [RFC4566]can be used to describe multiple RTP media streams in
one or more m-lines that define a single SSRC multiplexed RTP session
(as specified in [RFC3550]). This addresses the WebRTC architecture
[I-D.ietf-rtcweb-overview].
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A Unified Plan for Using SDP with Large Numbers of Media Flows
[I-D.roach-mmusic-unified-plan] proposes that each m-line will
represent a media source [I-D.lennox-raiarea-rtp-grouping-taxonomy].
In the simple case a media source will be one video or audio RTP
stream. Media source description becomes more complicated when for
robust applications, techniques like RTX and FEC are used to protect
media. Also simulcast/layered coding can be used to provide support
to heterogeneous receivers. In these cases a media source may send
more than one RTP stream, for example, a video stream and a FEC
stream.
Some applications may require more information about the usage of the
RTP streams. For example, RTP streams from different cameras that
need to be identified by the application in order to render them
correctly, or a source that can send multiple versions of the same
stream in different resolutions (Simulcast
[I-D.westerlund-avtcore-rtp-simulcast]).
SDP provides in [RFC4574] a "label" attribute that contains a token
defined by an application and is used in its context. "Label" can be
attached to m-lines in multiple SDP documents allowing the
application to logically identify the media streams across SDP
sessions when necessary. The "label" attribute is a token and does
not provide any information about the content of the stream.
[RFC4796] defines the "content" attribute providing information about
the content of the stream, currently there is a small set of values
for the content attribute.
Both "label" and "content" attribute are SDP media-level attributes,
so when an SDP m-line supports multiple RTP streams, this value is
applicable to all RTP streamsdescribed by the SDP m-line.
There is a need to have a token that will allow the mapping between a
single RTP streams(identified by an SSRC) in an m-line to the
application logic. For example, SSRC1 is the RTP stream from the
left camera and SSRC2 is the RTP stream from the right camera
specifiedand SSRC3 is the FEC stream that protect both streams. Note
that there are cases where the SSRCs of the RTP streams are not known
or may change during the call..
Support of FEC, SVC and simulcast bring more requirements as
explained using the following examples.
The first example is of a unified plan
[I-D.roach-mmusic-unified-plan] offer of one audio source and one
video source. The video source includes two SVC RTP streams a base
layer and an enhancement layer. There are also two FEC options:
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>Base layer S1 is protected by FEC repair stream R1
Base Layer S1 and Enhancement S2 layers protected by FEC repair
stream R2.
This enables the answer to select the base layer with R1 or the Base
+ enhancement layers both protected by R2.
SDP Offer:
v=0
o=- 20518 0 IN IP4 198.51.100.1
s=FEC Grouping Semantics for SSRC Multiplexing
t=0 0
c=IN IP4 203.0.113.1
a=group:BUNDLE m1 m2
m=audio 56600 RTP/SAVPF 0 109
a=msid:ma ta
a=mid:m1
a=ssrc:53280
a=rtpmap:0 PCMU/8000
a=rtpmap:109 opus/48000
m=video 56602 RTP/AVPF 100 101 110 111 - Main camera
a=msid:ma tb
a=mid:m2
a=rtpmap:100 H264/90000 - Base layer
a=rtpmap:101 H264-SVC/90000 - Enhancement layer.
a=depend:101 lay L1:100 - dependencies
a=rtpmap:110 1d-interleaved-parityfec/90000
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a=fmtp:110 L=5; D=10; repair-window=200000
a=rtpmap:111 1d-interleaved-parityfec/90000
a=fmtp:111 L=10; D=10; repair-window=400000
a=ssrc:1000 cname:MSTFEC@example.com
a=ssrc:1010 cname:MSTFEC@example.com
a=ssrc:2110 cname:MSTFEC@example.com
a=ssrc:2120 cname:MSTFEC@example.com
a=ssrc-group:FEC-FR 1000 2110
a=ssrc-group:FEC-FR 1000 1010 2120
a=ssrc-group:DDP 1000 1010
In this case all video streams are from the same source and can be
described using a single m-line. The grouping relations are
specified using the SSRCs values that need to be available in the
offer. It is also not clear based on the offer which SSRC is mapped
to each of the PT numbers.
2. Terminology
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[RFC2119] and
indicate requirement levels for compliant RTP implementations.
3. Proposal for Application ID
As we saw in the previous section, there are tokens defined that
could be used for the mapping, but they have existing usages and
semantics, and tend to apply at media-level or session level rather
than stream-level. In order to avoid overload of existing
attributes, it is better to have a new token attribute that can
identify a specific RTP stream corresponding to the application.
This document defines such new token, "AppID".
[I-D.roach-mmusic-unified-plan] describes a use case where for early
media it is important that the offer will include a token allowing
the media receiver to associate it with the correct m-line. This
requires that the appID will be the token of the received RTP stream
to be used by the sending side. On the other hand to specify the
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appIds of the source RTP stream and the protecting RTP streams there
may be a need to specify the sent appId since the relations between
the source and repair streams are for the send side and the
protection may not be symmetrical. Similar issue may exist for the
simulcast use case. This requires having a second optional attribute
for the recv-appId to be used for early media.
3.1. appID token
AppID is a general-purpose token associated with an RTP stream,
allowing the semantics of the stream with a token to be defined by
the application. This token may also be mapped, for example, to a
FEC stream , or to a specific resolution in a simulcast application
described in the SDP .
The token is chosen by the sender, and represents the RTP stream that
will be sent to the receiver.
The proposed token can be sent using SDP, RTCP SDES messages
[RFC3550], or an RTP header extension [RFC5285]
The SSRC mapping may be available to the receiver when receiving the
RTP stream through the RTP header extension, but may also be
available ahead of time via an RTCP SDES message conveyed before the
source started sending, even if the receiver has not seen any RTP
packets from this source like in a multipoint conference or in the
SDP description.
The receiver can receive new sources that may be of two kinds.
o A new RTP stream replacing an existing RTP stream, in which case
the AppID of the replaced RTP stream will be assigned to the new
SSRC.
o A new RTP stream requiring a different AppID, for example, when
adding a presentation stream to an existing call with two video
cameras from a room.
The solution supports an RTP session as described using SDP. The RTP
session may use Bundle [I-D.ietf-mmusic-sdp-bundle-negotiation]with
more than one m-lines. In this case, if the SSRCs of all RTP streams
are not known in advance, the AppIDs associated with each m-line need
to be available to the media receiver in order to map each SSRC to a
specific m-line configuration.
The document defines a new SDP media level attribute a=appID that can
be used to list all the appIDs that an application may use.
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The appID syntax provides a token identifier and optional SDP
attributes that describe the application usage if exists in SDP.
Application usage in SDP may be, for example, an image attribute
describing a simulcast application usage
[I-D.westerlund-avtcore-rtp-simulcast] or a FEC stream that protects
multiple RTP streams.
Each value of the AppID maps to one SSRC at a time. When a new SSRC
is mapped to an existing AppID using an RTP header extension or SDES
message, it replaces the previous RTP stream for this application
usage.
The formal representation of the appID token is:
appid-attribute = "appID:" token [SP attribute]
; The base definition of "attribute" is in [RFC4566].
; (It is the content of "a=" lines.)
Examples:
The SSRC of the stream is not known when the SDP offer is sent, an
appID is specified and can be used for mapping to specific SSRCs in
the application.
m=video 49200 RTP/AVP 99
a=rtpmap:99 H264/90000
a=appID:2
The second example is when the application usage of the RTP steam is
specified using SDP to provide different image resolutions. The
media receiver can map the received SSRC to the specific resolution
based on the appId.
Note:This example is using a separate m-line for each offered
resolution on the send direction grouped using SCR option
[I-D.westerlund-avtcore-rtp-simulcast] It uses the same msid for all
grouped image attribute. Other options will be added based on the
work done on [I-D.westerlund-avtcore-rtp-simulcast]
a=group:SCR 1 2
m=video 49200 RTP/AVP 98
a=rtpmap:98 H264/90000
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imageattr:98 send [x=640,y=360] recv[[x=640,y=360] [x=320,y=180]
a=msid:ma ta
a=appID:2
a=mid:1
m=video 49200 RTP/AVP 99
a=rtpmap:99 H264/90000
imageattr:99 send [x=320,y=180]
a=msid:ma ta
a=appID:3
a=mid:2
a=sendonly
3.1.1. RTCP SDES message
The document specify a new RTCP SDES message
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AppID = XXX | length |AppID token
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ....
This AppID is the same token as defined in the new SDP attribute and
will also be used in the RTP header extension.
This SDES message MAY be sent in a compound RTCP packet based on the
application need.
3.1.2. RTP Header Extension
The Application ID could be carried within the RTP header extension
field, using [RFC5285] two bytes header extension.
This is negotiated within the SDP i.e.
a=extmap:1 urn:ietf:params:rtp-hdrext:App-ID
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Packets tagged by the sender with the AppID will then contain a
header extension as shown below
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=1 | Len-1 | AppID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AppID .............. |
+-+-+-+-+-+-+-+-+
To add or modify the AppID by an intermediary can be an expensive
operation, particularly if SRTP is used to authenticate the packet.
Modification to the contents of the RTP header requires a re-
authentication of the complete packet, and this could prove to be a
limiting factor in the throughput of a multipoint device.
There is no need to send the AppID header extension with all RTP
packets. Senders MAY choose to send it only when a new SSRC is sent,
or when an SSRC changes its association to an AppID. If such a mode
is being used, the header extension SHOULD be sent in the first few
RTP packets to reduce the risk of losing it due to packet loss. For
codecs with decoder refresh points (such as I-Frames in video
codecs), senders also SHOULD send the AppID header extension along
with the packets carrying the decoder refresh.
3.1.3. recv-appID
An offer may include a recv-appID attribute allowing the offerer to
request from the answerer to use this token for the RTP stream sent
from the answerer for a sendrecv or recvonly RTP stream. This is
important in order to support early media from the answerer that may
be received by the offerer before the answer SDP arrives.
The formal representation of the appID token is:
appid-attribute = "recv-appID:" token
; The base definition of "attribute" is in [RFC4566].
; (It is the content of "a=" lines.)
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4. Using Application ID token in Offer / Answer
The appId may be used in offer answer. Some use cases are provided.
They only show part of the SDP that can demonstrate the usage.
the simple case is when each media source describes one RTP stream.
In this case the SSRC may be used for the mapping if known but having
appId address the case where the SSRC changes. The recv-appID is
offered to allow for early media synchronization.
The offer is:
m=video 49200 RTP/AVP 99
a=rtpmap:99 H264/90000
a=appID 2
a=recv-appId 10
a=ssrc:20010 CNAME:v1@example.com
m=video 49200 RTP/AVP 100
a=rtpmap:100 H264/90000
a=appID 3
a=recv-appId 20
a=ssrc:20010 CNAME:v2@example.com
In this example a three camera system sending three RTP streams
protected by a single FEC stream. (note that the full offer may also
include a FEC stream for each of the three RTP streams and the
answerer may choose which FEC scheme he prefers).
This is the SDP offer for the video sources:
v=0
o=- 20518 0 IN IP4 198.51.100.1
s=FEC Grouping Semantics for SSRC Multiplexing
t=0 0
c=IN IP4 203.0.113.1
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a=group:BUNDLE m1 m2 m3 m4 R1
a=group:FEC-FR m2 m3 m4 R1
m=audio 56600 RTP/SAVPF 109
a=mid:m1
a=msid:ma ta
a=appID 1
a=ssrc:53280
a=rtpmap:109 opus/48000
m=video 56602 RTP/AVPF 100 - left camera
a=mid:m2
a=msid:ma tb
a=appID 2
a=rtpmap:100 H264/90000
a=ssrc:1000 cname:MSTFEC@example.com
m=video 56602 RTP/AVPF 101- Middle camera
a=mid:m3
a=msid:ma tc
a=appID 3
a=rtpmap:101 H264/90000
a=ssrc:1010 cname:MSTFEC@example.com
m=video 56602 RTP/AVPF 102 - Right camera
a=mid:m4
a=msid:ma td
a=appID 4
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a=rtpmap:102 H264/90000
a=ssrc:1020 cname:MSTFEC@example.com
m=video 56602 RTP/AVP 110
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=mid:R1
a=appID 5
The FEC stream is specified in a separate SDP m-line even though it
is not a media source but it does not have any msid so it is not a
media stream track. The appID is used to identify this stream as the
FEC stream
In the CLUE WG case the mapping is from a media source represented by
an SDP m-line to a CLUE Capture encoding specified in the CLUE
framework [I-D.ietf-clue-framework]. The mapping may be done using
the label attribute.
Example of an offer that offers three CLUE individual encodes. The
CLUE config message can be used to map an individual encode to a CLUE
media capture [I-D.kyzivat-clue-signaling]. The label value is used
in the CLUE protocol to identify CLUE individual encodes. The appId
is used to identify the stream by the receiver:
a=group:CLUE 4 5 6
...
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
a=sendrecv
a=mid:2
a=appID 9
...
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m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:4
a=appID 8
a=label:enc1
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:5
a=appID 7
a=label:enc2
m=video 6002 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42e016
a=sendonly
a=mid:6
a=appID 6
a=label:enc3
5. Acknowledgements
Place Holder
6. IANA Considerations
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TBD
7. Security Considerations
TBD.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
8.2. Informative References
[I-D.ietf-clue-framework]
Duckworth, M., Pepperell, A., and S. Wenger, "Framework
for Telepresence Multi-Streams", draft-ietf-clue-
framework-10 (work in progress), May 2013.
[I-D.ietf-mmusic-msid]
Alvestrand, H., "Cross Session Stream Identification in
the Session Description Protocol", draft-ietf-mmusic-
msid-00 (work in progress), February 2013.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
bundle-negotiation-04 (work in progress), June 2013.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-06 (work
in progress), February 2013.
[I-D.kyzivat-clue-signaling]
Kyzivat, P., Xiao, L., Groves, C., and R. Hansen, "CLUE
Signaling", draft-kyzivat-clue-signaling-04 (work in
progress), July 2013.
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[I-D.lennox-raiarea-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
"A Taxonomy of Grouping Semantics and Mechanisms for Real-
Time Transport Protocol (RTP) Sources", draft-lennox-
raiarea-rtp-grouping-taxonomy-01 (work in progress), July
2013.
[I-D.roach-mmusic-unified-plan]
Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for
Using SDP with Large Numbers of Media Flows", draft-roach-
mmusic-unified-plan-00 (work in progress), July 2013.
[I-D.westerlund-avtcore-rtp-simulcast]
Westerlund, M., Lindqvist, M., and F. Jansson, "Using
Simulcast in RTP Sessions", draft-westerlund-avtcore-rtp-
simulcast-02 (work in progress), February 2013.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4574] Levin, O. and G. Camarillo, "The Session Description
Protocol (SDP) Label Attribute", RFC 4574, August 2006.
[RFC4796] Hautakorpi, J. and G. Camarillo, "The Session Description
Protocol (SDP) Content Attribute", RFC 4796, February
2007.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
Authors' Addresses
Roni Even
Huawei Technologies
Tel Aviv
Israel
Email: roni.even@mail01.huawei.com
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Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
Qin Wu
Huawei Technologies
Email: bill.wu@huawei.com
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