Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth
RFC 3556

Document Type RFC - Proposed Standard (July 2003; No errata)
Last updated 2013-03-02
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IESG IESG state RFC 3556 (Proposed Standard)
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Network Working Group                                          S. Casner
Request for Comments: 3556                                 Packet Design
Category: Standards Track                                      July 2003

         Session Description Protocol (SDP) Bandwidth Modifiers
               for RTP Control Protocol (RTCP) Bandwidth

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines an extension to the Session Description
   Protocol (SDP) to specify two additional modifiers for the bandwidth
   attribute.  These modifiers may be used to specify the bandwidth
   allowed for RTP Control Protocol (RTCP) packets in a Real-time
   Transport Protocol (RTP) session.

1.  Introduction

   The Real-time Transport Protocol (RTP), RFC 3550 [1], includes a
   control protocol RTCP which provides synchronization information from
   data senders and feedback information from data receivers.

   Normally, the amount of bandwidth allocated to RTCP in an RTP session
   is 5% of the session bandwidth.  For some applications, it may be
   appropriate to specify the RTCP bandwidth independently of the
   session bandwidth.  Using a separate parameter allows rate-adaptive
   applications to set an RTCP bandwidth consistent with a "typical"
   data bandwidth that is lower than the maximum bandwidth specified by
   the session bandwidth parameter.  That allows the RTCP bandwidth to
   be kept under 5% of the data bandwidth when the rate has been adapted
   downward.

   On the other hand, there may be applications that send data at very
   low rates but need to communicate extra RTCP information, such as APP
   packets.  These applications may need to specify RTCP bandwidth that
   is higher than 5% of the data bandwidth.

Casner                      Standards Track                     [Page 1]
RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 2003

   The RTP specification allows a profile to specify that the RTCP
   bandwidth may be divided into two separate session parameters for
   those participants which are active data senders and those which are
   not.  Using two parameters allows RTCP reception reports to be turned
   off entirely for a particular session by setting the RTCP bandwidth
   for non-data-senders to zero while keeping the RTCP bandwidth for
   data senders non-zero so that sender reports can still be sent for
   inter-media synchronization.  Turning off RTCP reception reports is
   not recommended because they are needed for the functions listed in
   the RTP specification, particularly reception quality feedback and
   congestion control.  However, doing so may be appropriate for systems
   operating on unidirectional links or for sessions that do not require
   feedback on the quality of reception or liveness of receivers and
   that have other means to avoid congestion.

   This memo defines an extension to the Session Description Protocol
   (SDP) [3] to specify RTCP bandwidth for senders and non-senders
   (receivers).

2.  SDP Extensions

   The Session Description Protocol includes an optional bandwidth
   attribute with the following syntax:

      b=<modifier>:<bandwidth-value>

   where <modifier> is a single alphanumeric word giving the meaning of
   the bandwidth figure, and where the default units for <bandwidth-
   value> are kilobits per second.  This attribute specifies the
   proposed bandwidth to be used by the session or media.

   A typical use is with the modifier "AS" (for Application Specific
   Maximum) which may be used to specify the total bandwidth for a
   single media stream from one site (source).

   This memo defines two additional bandwidth modifiers:

      b=RS:<bandwidth-value>

      b=RR:<bandwidth-value>

   where "RS" indicates the RTCP bandwidth allocated to active data
   senders (as defined by the RTP spec) and "RR" indicates the RTCP
   bandwidth allocated to other participants in the RTP session (i.e.,
   receivers).  The exact behavior induced by specifying these bandwidth
   modifiers depends upon the algorithm used to calculate the RTCP
   reporting interval.  Different algorithms may be specified by
   different RTP profiles.

Casner                      Standards Track                     [Page 2]
RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 2003
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