RFC 3556 (rfc3556) - Page 2 of 8

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

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=:

   where  is a single alphanumeric word giving the meaning of
   the bandwidth figure, and where the default units for  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:

      b=RR:

   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