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RTP Stream Identifier Source Description (SDES)
RFC 8852

Document Type RFC - Proposed Standard (January 2021)
Authors Adam Roach , Suhas Nandakumar , Peter Thatcher
Last updated 2021-01-19
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Ben Campbell
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RFC 8852

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
                                                            January 2021

            RTP Stream Identifier Source Description (SDES)


   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

Copyright Notice

   Copyright (c) 2021 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
   ( 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.  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
   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
   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

   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 "" and may also be present in a stream with
   a CNAME of "", 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

   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

   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

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

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

   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 <>
   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-
   Description:  RTP Repaired Stream Identifier
   Contact:  Adam Roach <>
   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

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, <>.

   [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,

   [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, <>.

   [RFC8285]  Singer, D., Desineni, H., and R. Even, Ed., "A General
              Mechanism for RTP Header Extensions", RFC 8285,
              DOI 10.17487/RFC8285, October 2017,

   [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,

6.2.  Informative References

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,

   [RFC8830]  Alvestrand, H., "WebRTC MediaStream Identification in the
              Session Description Protocol", RFC 8830,
              DOI 10.17487/RFC8830, January 2021,


   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

Authors' Addresses

   Adam Roach


   Suhas Nandakumar
   Cisco Systems


   Peter Thatcher