RTP Payload Format Restrictions
RFC 8851

Document Type RFC - Proposed Standard (January 2021; No errata)
Updates RFC 4855
Author Adam Roach 
Last updated 2021-01-19
Replaces draft-pthatcher-mmusic-rid
Stream Internet Engineering Task Force (IETF)
Formats plain text html xml pdf htmlized (tools) htmlized bibtex
Stream WG state Submitted to IESG for Publication
Document shepherd Flemming Andreasen
Shepherd write-up Show (last changed 2018-01-26)
IESG IESG state RFC 8851 (Proposed Standard)
Action Holders
Consensus Boilerplate Yes
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Responsible AD Ben Campbell
Send notices to "Flemming Andreasen" <fandreas@cisco.com>
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Internet Engineering Task Force (IETF)                   A.B. Roach, Ed.
Request for Comments: 8851                                       Mozilla
Updates: 4855                                               January 2021
Category: Standards Track                                               
ISSN: 2070-1721

                    RTP Payload Format Restrictions


   In this specification, we define a framework for specifying
   restrictions on RTP streams in the Session Description Protocol
   (SDP).  This framework defines a new "rid" ("restriction identifier")
   SDP attribute to unambiguously identify the RTP streams within an RTP
   session and restrict the streams' payload format parameters in a
   codec-agnostic way beyond what is provided with the regular payload

   This specification updates RFC 4855 to give additional guidance on
   choice of Format Parameter (fmtp) names and their relation to the
   restrictions defined by this document.

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
   (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.  Terminology
   2.  Introduction
   3.  Key Words for Requirements
   4.  SDP "a=rid" Media Level Attribute
   5.  "a=rid" Restrictions
   6.  SDP Offer/Answer Procedures
     6.1.  Generating the Initial SDP Offer
     6.2.  Answerer Processing the SDP Offer
       6.2.1.  "a=rid"-Unaware Answerer
       6.2.2.  "a=rid"-Aware Answerer
     6.3.  Generating the SDP Answer
     6.4.  Offerer Processing of the SDP Answer
     6.5.  Modifying the Session
   7.  Use with Declarative SDP
   8.  Interaction with Other Techniques
     8.1.  Interaction with VP8 Format Parameters
       8.1.1.  max-fr - Maximum Frame Rate
       8.1.2.  max-fs - Maximum Frame Size, in VP8 Macroblocks
     8.2.  Interaction with H.264 Format Parameters
       8.2.1.  profile-level-id and max-recv-level - Negotiated
       8.2.2.  max-br / MaxBR - Maximum Video Bitrate
       8.2.3.  max-fs / MaxFS - Maximum Frame Size, in H.264
       8.2.4.  max-mbps / MaxMBPS - Maximum Macroblock Processing Rate
       8.2.5.  max-smbps - Maximum Decoded Picture Buffer
     8.3.  Redundancy Formats and Payload Type Restrictions
   9.  Format Parameters for Future Payloads
   10. Formal Grammar
   11. SDP Examples
     11.1.  Many Bundled Streams Using Many Codecs
     11.2.  Scalable Layers
   12. IANA Considerations
     12.1.  New SDP Media-Level Attribute
     12.2.  Registry for RID-Level Parameters
   13. Security Considerations
   14. References
     14.1.  Normative References
     14.2.  Informative References
   Author's Address

1.  Terminology

   The terms "source RTP stream", "endpoint", "RTP session", and "RTP
   stream" are used as defined in [RFC7656].

   [RFC4566] and [RFC3264] terminology is also used where appropriate.

2.  Introduction

   The payload type (PT) field in RTP provides a mapping between the RTP
   payload format and the associated SDP media description.  For a given
   PT, the SDP rtpmap and/or fmtp attributes are used to describe the
   properties of the media that is carried in the RTP payload.

   Recent advances in standards have given rise to rich multimedia
   applications requiring support for either multiple RTP streams within
   an RTP session [RFC8843] [RFC8853] or a large number of codecs.
   These demands have unearthed challenges inherent with:

   *  The restricted RTP PT space in specifying the various payload

   *  The codec-specific constructs for the payload formats in SDP

   *  Missing or underspecified payload format parameters

   *  Overloading of PTs to indicate not just codec configurations, but
      individual streams within an RTP session
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