RFC 3556 (rfc3556) - Page 3 of 8

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

RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 2003


   For the RTP A/V Profile [2], which specifies that the default RTCP
   interval algorithm defined in the RTP spec [1] is to be used, at
   least RS/(RS+RR) of the RTCP bandwidth is dedicated to active data
   senders.  If the proportion of senders to total participants is less
   than or equal to RS/(RS+RR), each sender gets RS divided by the
   number of senders.  When the proportion of senders is greater than
   RS/(RS+RR), the senders get their proportion of the sum of these
   parameters, which means that a sender and a non-sender each get the
   same allocation.  Therefore, it is not possible to constrain the data
   senders to use less RTCP bandwidth than is allowed for non-senders.
   A few special cases are worth noting:

      o  If RR is zero, then the proportion of participants that are
         senders can never be greater than RS/(RS+RR), and therefore
         non-senders never get any RTCP bandwidth independent of the
         number of senders.

      o  Setting RS to zero does not mean that data senders are not
         allowed to send RTCP packets, it only means that they are
         treated the same as non-senders.  The proportion of senders (if
         there are any) would always be greater than RS/(RS+RR) if RR is
         non-zero.

      o  If RS and RR are both zero, it would be unwise to attempt
         calculation of the fraction RS/(RS+RR).

   The bandwidth allocation specified by the RS and RR modifiers applies
   to the total bandwidth consumed by all RTCP packet types, including
   SR, RR, SDES, BYE, APP and any new types defined in the future.  The
    for these modifiers is in units of bits per second
   with an integer value.

      NOTE:  This specification was in conflict with the initial SDP
      spec in RFC 2327 which prescribes that the  for
      all bandwidth modifiers should be an integer number of kilobits
      per second.  This discrepancy was forced by the fact that the
      desired RTCP bandwidth setting may be less than 1 kb/s.

      At the 44th IETF meeting in Minneapolis, two solutions were
      considered: allow fractional values, or specify that the units for
      these particular modifiers would be in bits per second.  The
      second choice was preferred so that the syntax would not be
      changed.  The SDP spec is being modified [4] to advance to Draft
      Standard, and will allow this change in semantics.







Casner                      Standards Track