Internet Engineering Task Force (IETF) A.B. Roach
Request for Comments: 8852 Mozilla
Category: Standards Track S. Nandakumar
ISSN: 2070-1721 Cisco Systems
P. Thatcher
Google
January 2021
RTP Stream Identifier Source Description (SDES)
Abstract
This document defines and registers two new Real-time Transport
Control Protocol (RTCP) Stream Identifier Source Description (SDES)
items. One, named RtpStreamId, is used for unique identification of
RTP streams. The other, RepairedRtpStreamId, can be used to identify
which stream is to be repaired using a redundancy RTP stream.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8852.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction
2. Terminology
3. Usage of RtpStreamId and RepairedRtpStreamId in RTP and RTCP
3.1. RTCP "RtpStreamId" SDES Extension
3.2. RTCP "RepairedRtpStreamId" SDES Extension
3.3. RTP "RtpStreamId" and "RepairedRtpStreamId" Header
Extensions
4. IANA Considerations
4.1. New RtpStreamId SDES Item
4.2. New RepairRtpStreamId SDES Item
4.3. New RtpStreamId Header Extension URI
4.4. New RepairRtpStreamId Header Extension URI
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
RTP sessions frequently consist of multiple streams, each of which is
identified at any given time by its synchronization source (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 MediaStream
Identification (MSID) [RFC8830]. 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 Stream Identifier
Source Description (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).
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", "RTP stream", "source
RTP stream", "dependent stream", "received RTP stream", and
"redundancy RTP stream" are used as defined in [RFC7656].
The following acronyms are also used:
* CNAME: Canonical Endpoint Identifier, defined in [RFC3550]
* MID: Media Identification, defined in [RFC8843]
* MSID: MediaStream Identification, defined in [RFC8830]
* RTCP: Real-time Transport Control Protocol, defined in [RFC3550]
* RTP: Real-time Transport Protocol, defined in [RFC3550]
* SDES: Source Description, defined in [RFC3550]
* SSRC: Synchronization Source, defined in [RFC3550]
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 to demultiplex 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 [RFC7941] 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. This speed of response is necessary since, in many
cases, the stream cannot be properly processed until it can be
identified.
RtpStreamId and RepairedRtpStreamId values are scoped by source
identifier (e.g., CNAME) and by media session. When the media is
multiplexed using the BUNDLE extension [RFC8843], these values are
further scoped by their associated MID values. For example: an
RtpStreamId of "1" may be present in the stream identified with a
CNAME of "1234@example.com" and may also be present in a stream with
a CNAME of "5678@example.org", and these would refer to different
streams. Similarly, an RtpStreamId of "1" may be present with an MID
of "A", and again with a MID of "B", and also refer to two different
streams.
Note that the RepairedRtpStreamId mechanism is limited to indicating
one repaired stream per redundancy stream. If systems require
correlation for schemes in which a redundancy stream contains
information used to repair more than one stream, they will have to
use a more complex mechanism than the one defined in this
specification.
As with all SDES items, RtpStreamId and RepairedRtpStreamId are
limited to a total of 255 octets in length. RtpStreamId and
RepairedRtpStreamId are constrained to contain only alphanumeric
characters. For avoidance of doubt, the only allowed byte values for
these IDs are decimal 48 through 57, 65 through 90, and 97 through
122.
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=12 | length | RtpStreamId ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RtpStreamId payload is ASCII encoded and is not null terminated.
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...=13 | length | RepairRtpStreamId ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RepairedRtpStreamId payload is ASCII encoded and is not null
terminated.
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 [RFC7941].
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 ASCII encoded.
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 encoding media. Note that the set of header extensions
included in the packet needs to be padded to the next 32-bit boundary
[RFC8285].
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.
Implementors are warned, in particular, not to include any
information in the identifier that is derived from potentially user-
identifying information, such as user ID or IP address. To avoid
identification of specific implementations based on their pattern of
tag generation, implementations are encouraged to use a simple scheme
that starts with the ASCII digit "1", and increments by one for each
subsequent identifier.
4. IANA Considerations
4.1. New RtpStreamId SDES Item
This document adds the RtpStreamId SDES item to the IANA "RTP SDES
Item Types" registry as follows:
Value: 12
Abbrev.: RtpStreamId
Name: RTP Stream Identifier
Reference: RFC 8852
4.2. New RepairRtpStreamId SDES Item
This document adds the RepairedRtpStreamId SDES item to the IANA "RTP
SDES Item Types" registry as follows:
Value: 13
Abbrev.: RepairedRtpStreamId
Name: Repaired RTP Stream Identifier
Reference: RFC 8852
4.3. New RtpStreamId Header Extension URI
This document defines a new extension URI in the "RTP SDES Compact
Header Extensions" subregistry of the "RTP Compact Header Extensions"
subregistry, as follows:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
Description: RTP Stream Identifier
Contact: Adam Roach <adam@nostrum.com>
Reference: RFC 8852
4.4. New RepairRtpStreamId Header Extension URI
This document defines a new extension URI in the "RTP SDES Compact
Header Extensions" subregistry of the "RTP Compact Header Extensions"
subregistry, as follows:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-
id
Description: RTP Repaired Stream Identifier
Contact: Adam Roach <adam@nostrum.com>
Reference: RFC 8852
5. Security Considerations
Although the identifiers defined in this document are limited to be
strictly alphanumeric, SDES items have the potential to carry any
string. As a consequence, there exists a risk that they might carry
privacy-sensitive information. Implementations need to take care
when generating identifiers so that they do not contain information
that can identify the user or allow for long-term tracking of the
device. Following the generation recommendations in Section 3.3 will
result in non-instance-specific labels, with only minor
fingerprinting possibilities in the total number of used RtpStreamIds
and RepairedRtpStreamIds.
Even if the SDES items are generated to convey as little information
as possible, implementors are strongly encouraged to encrypt SDES
items -- both in RTCP and RTP header extensions -- so as to preserve
privacy against third parties.
As the SDES items are used for identification of the RTP streams for
different application purposes, it is important that the intended
values are received. An attacker, either a third party or malicious
RTP middlebox, that removes or changes the values for these SDES
items can severely impact the application. The impact can include
failure to decode or display the media content of the RTP stream. It
can also result in incorrectly attributing media content to
identifiers of the media source, such as incorrectly identifying the
speaker. To prevent this from occurring due to third-party attacks,
integrity and source authentication is needed.
"Options for Securing RTP Sessions" [RFC7201] discusses options for
how encryption, integrity, and source authentication can be
accomplished.
6. References
6.1. Normative References
[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, <https://www.rfc-editor.org/info/rfc3550>.
[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,
<https://www.rfc-editor.org/info/rfc7656>.
[RFC7941] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
Header Extension for the RTP Control Protocol (RTCP)
Source Description Items", RFC 7941, DOI 10.17487/RFC7941,
August 2016, <https://www.rfc-editor.org/info/rfc7941>.
[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017,
<https://www.rfc-editor.org/info/rfc8285>.
[RFC8843] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 8843,
DOI 10.17487/RFC8843, January 2021,
<https://www.rfc-editor.org/info/rfc8843>.
6.2. Informative References
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<https://www.rfc-editor.org/info/rfc7201>.
[RFC8830] Alvestrand, H., "WebRTC MediaStream Identification in the
Session Description Protocol", RFC 8830,
DOI 10.17487/RFC8830, January 2021,
<https://www.rfc-editor.org/info/rfc8830>.
Acknowledgements
Many thanks to Cullen Jennings, Magnus Westerlund, Colin Perkins,
Jonathan Lennox, and Paul Kyzivat for review and input. Magnus
Westerlund provided nearly all of the Security Considerations
section.
Authors' Addresses
Adam Roach
Mozilla
Email: adam@nostrum.com
Suhas Nandakumar
Cisco Systems
Email: snandaku@cisco.com
Peter Thatcher
Google
Email: pthatcher@google.com
