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Network Working Group                                          S. CasnerRequest for Comments: 3556                                 Packet DesignCategory: Standards Track                                      July 2003Session Description Protocol (SDP) Bandwidth Modifiersfor RTP Control Protocol (RTCP) BandwidthStatus 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   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   <bandwidth-value> 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 inRFC 2327 which prescribes that the <bandwidth-value> 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                     [Page 3]

RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 20033.  Default values   If either or both of the RS and RR bandwidth specifiers are omitted,   the default values for these parameters are as specified in the RTP   profile in use for the session in question.  For the Audio/Video   Profile,RFC 3551 [2], the defaults follow the recommendations of the   RTP spec:      o  The total RTCP bandwidth is 5% of the session bandwidth.  If         one of these RTCP bandwidth specifiers is omitted, its value is         5% minus the value of the other one, but not less than zero.         If both are omitted, the sender and receiver RTCP bandwidths         are 1.25% and 3.75% of the session bandwidth, respectively.      o  At least RS/(RS+RR) of of the RTCP bandwidth is dedicated to         active data senders.  When the proportion of senders is greater         than RS/(RS+RR) of the participants, the senders get their         proportion of the sum of these parameters.   This memo does not impose limits on the values that may be specified   with the RR and RS modifiers, other than that they must be non-   negative.  However, the RTP specification and the appropriate RTP   profile may specify limits.4.  Precedence   An SDP description consists of a session-level description (details   that apply to the whole session and all media streams) and zero or   more media-level descriptions (details that apply only to a single   media stream).  Bandwidth specifiers may be present either at the   session level to specify the total bandwidth shared by all media, or   in the media sections to specify the bandwidth allocated to each   medium, or both.  This is true for the two RTCP bandwidth modifiers   defined here as well.   Since the bandwidth allocated to RTCP is a fraction of the session   bandwidth when not specified explicitly using the modifiers defined   here, there is an interaction between the session bandwidth and RTCP   bandwidth specifiers at the session and media levels of the SDP   description.  The precedence of these specifiers is as follows, with   (1) being the highest precedence:   1) Explicit RR or RS specifier at media level   2) Explicit RR or RS specifier at session level   3) Default based on session bandwidth specifier at media levelCasner                      Standards Track                     [Page 4]

RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 2003   4) Default based on session bandwidth specifier at session level   In particular, the relationship of (2) and (3) means that if the RR   bandwidth is specified as zero at the session level, that turns off   RTCP transmission for non-data-senders in all media.5.  Example   An example SDP description is:      v=0      o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4      s=SDP Seminar      i=A Seminar on the session description protocol      c=IN IP4 224.2.17.12/127      t=2873397496 2873404696      m=audio 49170 RTP/AVP 0      b=AS:64      b=RS:800      b=RR:2400      m=video 51372 RTP/AVP 31      b=AS:256      b=RS:800      b=RR:2400   In this example, the explicit RTCP bandwidths for the audio medium   are equal to the defaults and so could be omitted.  However, for the   video medium, the RTCP bandwidths have been set according to a data   bandwidth of 64 kb/s even though the maximum data bandwidth is   specified as 256 kb/s.  This is based on the assumption that the   video data bandwidth will automatically adapt to a lower value based   on network conditions.Casner                      Standards Track                     [Page 5]

RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 20036.  IANA ConsiderationsRFC 2327 [3] requires that new bandwidth modifiers be registered with   IANA by reference to a standards-track RFC specifying the semantics   of the bandwidth modifier precisely, indicating when it should be   used, and why the existing registered bandwidth specifiers do not   suffice.   This document is intended to satisfy those requirements.   In the "bwtype" table of the Session Description Protocol (SDP)   Parameters registry, the following two new bandwidth modifier names   have been registered:      RS      RR7.  Security Considerations   This memo defines bandwidth modifier keywords as an extension to SDP,   so the security considerations listed in the SDP specification apply   to session descriptions containing these modifiers as with any other.   The bandwidth value supplied with one of these modifiers could be   unreasonably large and cause the application to send RTCP packets at   an excessive rate, resulting in a denial of service.  This is similar   to the risk that an unreasonable bandwidth could be specified for the   media data, though encoders generally have a limited bandwidth range.   Applications should apply validity checks to all parameters received   in an SDP description, particularly one which is not authenticated.   This memo cannot specify limits because they are dependent on the RTP   profile and application.8.  References8.1  Normative References   [1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:       A Transport Protocol for Real-Time Applications,"RFC 3550, July       2003.   [2] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video       Conferences with Minimal Control",RFC 3551, July 2003.   [3] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",RFC 2327, April 1998.Casner                      Standards Track                     [Page 6]

RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 20038.2  Informative References   [4] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session       Description Protocol", Work in Progress.9.  Author's Address   Stephen L. Casner   Packet Design   3400 Hillview Avenue, Building 3   Palo Alto, CA 94304   United States   Phone: +1 650 739-1843   EMail: casner@acm.orgCasner                      Standards Track                     [Page 7]

RFC 3556       SDP Bandwidth Modifiers for RTCP Bandwidth      July 200310.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Casner                      Standards Track                     [Page 8]