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
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (https://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.  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 "[email protected]" and may also be present in a stream with
  a CNAME of "[email protected]", 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 <[email protected]>
  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 <[email protected]>
  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: [email protected]


  Suhas Nandakumar
  Cisco Systems

  Email: [email protected]


  Peter Thatcher
  Google

  Email: [email protected]

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