Network Working Group A. Roach
Internet-Draft Mozilla
Intended status: Standards Track S. Nandakumar
Expires: December 23, 2016 Cisco Systems
P. Thatcher
Google
June 21, 2016
RTP Stream Identifier Source Description (SDES)
draft-ietf-avtext-rid-04
Abstract
This document defines and registers two new RTCP SDES items. One,
named RtpStreamId, is used for unique identification of RTP streams.
The other, RepairedRtpStreamId, can be used to identify which stream
a redundancy RTP stream is to be used to repair.
Status of This Memo
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Usage of RtpStreamId and RepairedRtpStreamId in RTP and RTCP 3
3.1. RTCP 'RtpStreamId' SDES Extension . . . . . . . . . . . . 4
3.2. RTCP 'RepairedRtpStreamId' SDES Extension . . . . . . . . 4
3.3. RTP 'RtpStreamId' and 'RepairedRtpStreamId' Header
Extensions . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4.1. New RtpStreamId SDES item . . . . . . . . . . . . . . . . 5
4.2. New RepairRtpStreamId SDES item . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
RTP sessions frequently consist of multiple streams, each of which is
identified at any given time by its SSRC; however, the SSRC
associated with a stream is not guaranteed to be stable over its
lifetime. Within a session, these streams can be tagged with a
number of identifiers, including CNAMEs and MSIDs
[I-D.ietf-mmusic-msid]. Unfortunately, none of these have the proper
ordinality to refer to an individual stream; all such identifiers can
appear in more than one stream at a time. While approaches that use
unique Payload Types (PTs) per stream have been used in some
applications, this is a semantic overloading of that field, and one
for which its size is inadequate: in moderately complex systems that
use PT to uniquely identify every potential combination of codec
configuration and unique stream, it is possible to simply run out of
values.
To address this situation, we define a new RTCP SDES identifier,
RtpStreamId, that uniquely identifies a single RTP stream. A key
motivator for defining this identifier is the ability to
differentiate among different encodings of a single Source Stream
that are sent simultaneously (i.e., simulcast). This need for unique
identification extends to dependent streams (e.g., where layers used
by a layered codec are transmitted on separate streams).
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At the same time, when redundancy RTP streams are in use, we also
need an identifier that connects such streams to the RTP stream for
which they are providing redundancy. For this purpose, we define an
additional SDES identifier, RepairedRtpStreamId. This identifier can
appear only in packets associated with a redundancy RTP stream. They
carry the same value as the RtpStreamId of the RTP stream that the
redundant RTP stream is correcting.
2. Terminology
In this document, the terms "source stream", "encoded stream," "RTP
stream", "source RTP stream", "dependent stream", "received RTP
stream", and "redundancy RTP stream" are used as defined in
[RFC7656].
3. Usage of RtpStreamId and RepairedRtpStreamId 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; however, 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 two new RTCP SDES items [RFC3550]. The
first item is 'RtpStreamId', which is used to carry RTP stream
identifiers within RTCP SDES packets. This makes it possible for a
receiver to associate received RTP packets (identifying the RTP
stream) with a media description having the format constraint
specified. The second is 'RepairedRtpStreamId', which can be used in
redundancy RTP streams to indicate the RTP stream repaired by a
redundancy RTP stream.
To be clear: the value carried in a RepairedRtpStreamId will always
match the RtpStreamId value from another RTP stream in the same
session. For example, if a source RTP stream is identified by
RtpStreamId "A", then any redundancy RTP stream that repairs that
source RTP stream will contain a RepairedRtpStreamId of "A" (if this
mechanism is being used to perform such correlation). These
redundant RTP streams may also contain their own unique RtpStreamId.
This specification also uses the RTP header extension for RTCP SDES
items [I-D.ietf-avtext-sdes-hdr-ext] to allow carrying RtpStreamId
and RepairedRtpStreamId values in RTP packets. This allows
correlation at stream startup, or after stream changes where the use
of RTCP may not be sufficiently responsive.
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3.1. RTCP 'RtpStreamId' 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RtpStreamId=TBD| length | RtpStreamId ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RtpStreamId payload is UTF-8 encoded and is not null-terminated.
RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.
3.2. RTCP 'RepairedRtpStreamId' 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Repaired...=TBD| length | RepairRtpStreamId ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RepairedRtpStreamId payload is UTF-8 encoded and is not null-
terminated.
RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.
3.3. RTP 'RtpStreamId' and 'RepairedRtpStreamId' Header Extensions
Because recipients of RTP packets will typically need to know which
streams they correspond to immediately upon receipt, this
specification also defines a means of carrying RtpStreamId and
RepairedRtpStreamId 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 identifier is included in an RTP header extension, there
should be some consideration given to the packet expansion caused by
the identifier. 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
[RFC5285].
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In many cases, a one-byte identifier will be sufficient to
distinguish streams in a session; implementations are strongly
encouraged to use the shortest identifier that fits their purposes.
4. IANA Considerations
4.1. New RtpStreamId 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 RtpStreamId SDES item to the IANA "RTCP SDES
item types" registry as follows:
Value: TBD
Abbrev.: RtpStreamId
Name: RTP Stream Identifier
Reference: RFCXXXX
4.2. New RepairRtpStreamId 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 RepairedRtpStreamId SDES item to the IANA
"RTCP SDES item types" registry as follows:
Value: TBD
Abbrev.: RepairedRtpStreamId
Name: Repaired RTP Stream Identifier
Reference: RFCXXXX
5. Security Considerations
The actual identifiers used for RtpStreamIds (and therefore
RepairedRtpStreamIds) are expected to be opaque. As such, they are
not expected to contain information that would be sensitive, were it
observed by third-parties.
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6. Acknowledgements
Many thanks for review and input from Cullen Jennings, Magnus
Westerlund, Colin Perkins, Peter Thatcher, Jonathan Lennox, and Paul
Kyzivat.
7. References
7.1. 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-07 (work in progress), June
2016.
[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>.
[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>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
7.2. Informative References
[I-D.ietf-mmusic-msid]
Alvestrand, H., "WebRTC MediaStream Identification in the
Session Description Protocol", draft-ietf-mmusic-msid-14
(work in progress), June 2016.
Authors' Addresses
Adam Roach
Mozilla
Email: adam@nostrum.com
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Suhas Nandakumar
Cisco Systems
Email: snandaku@cisco.com
Peter Thatcher
Google
Email: pthatcher@google.com
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