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RFC 6128

Proposed STD
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RTP Control Protocol (RTCP) Port for Source-Specific Multicast (SSM) Sessions

Updates:    5760

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Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 6128                                         Cisco
Updates: 5760                                              February 2011
Category: Standards Track
ISSN: 2070-1721

                  RTP Control Protocol (RTCP) Port for
                Source-Specific Multicast (SSM) Sessions


   The Session Description Protocol (SDP) has an attribute that allows
   RTP applications to specify an address and a port associated with the
   RTP Control Protocol (RTCP) traffic.  In RTP-based source-specific
   multicast (SSM) sessions, the same attribute is used to designate the
   address and the RTCP port of the Feedback Target in the SDP
   description.  However, the RTCP port associated with the SSM session
   itself cannot be specified by the same attribute to avoid ambiguity,
   and thus, is required to be derived from the "m=" line of the media
   description.  Deriving the RTCP port from the "m=" line imposes an
   unnecessary restriction.  This document removes this restriction by
   introducing a new SDP attribute.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Page 2 
Copyright Notice

   Copyright (c) 2011 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
   ( 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. Introduction ....................................................2
   2. The 'multicast-rtcp' Attribute ..................................3
   3. SDP Example .....................................................3
   4. Security Considerations .........................................4
   5. IANA Considerations .............................................4
      5.1. Registration of SDP Attributes .............................5
   6. Acknowledgments .................................................5
   7. References ......................................................5
      7.1. Normative References .......................................5
      7.2. Informative References .....................................5

1.  Introduction

   The Session Description Protocol (SDP) [RFC4566] has an attribute
   that allows RTP applications [RFC3550] to specify an address and a
   port associated with the RTP Control Protocol (RTCP) traffic
   [RFC3605].  This attribute is called 'rtcp'.

   Now consider a network where one or more media senders send RTP
   packets to a distribution source, which then multicasts these RTP
   packets to multicast receivers using a source-specific multicast
   (SSM) arrangement [RFC5760].  The distribution source also multicasts
   the forward RTCP traffic (i.e., RTCP sender reports and receiver
   reports or their summaries) to the receivers in the same SSM session.

   In RTP-based SSM sessions, the 'rtcp' attribute is used to designate
   the address and the RTCP port of the Feedback Target in the SDP
   description [RFC5760].  However, the RTCP port associated with the
   SSM session itself cannot be specified by the same attribute since it
   could potentially cause ambiguity.  Thus, the multicast RTCP port is
   required to be derived from the "m=" line of the media description

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   (see Section 10.2 of [RFC5760]) by following the +1 rule (see Section
   11 of [RFC3550]).  However, [RFC3550] lifted the requirement for the
   +1 rule since it imposed an unnecessary restriction on RTCP port

   In this specification, we introduce a new SDP attribute to remove
   this restriction.  The new attribute allows the multicast sender to
   use its desired port in the RTCP session.  This document updates

2.  The 'multicast-rtcp' Attribute

   In RTP-based SSM sessions, the distribution source can use different
   multicast RTP and RTCP ports to send the RTP and RTCP packets,
   respectively.  Alternatively, the distribution source can use RTP/
   RTCP port muxing [RFC5761], in which case the RTP and RTCP packets
   are sent to the same destination port in the SSM session.

   For the cases when the distribution source does not want to use the
   one higher port for the RTCP traffic, this document defines a new SDP
   attribute, called 'multicast-rtcp'.  By using this attribute, the
   distribution source uses a desired port for the SSM RTCP session.  In
   the absence of the 'multicast-rtcp' attribute, the +1 rule applies
   following [RFC5760].

   The following ABNF [RFC5234] syntax formally describes the
   'multicast-rtcp' attribute:

               rtcp-attribute = "a=multicast-rtcp:" port CRLF

         Figure 1: ABNF syntax for the 'multicast-rtcp' attribute

   Here, the 'port' token is defined as specified in Section 9 of

   The 'multicast-rtcp' attribute is defined as both a media-level and
   session-level attribute.  Except where stated otherwise in this
   document, the rules of [RFC3550] apply.

3.  SDP Example

   In the session description shown in Figure 2, a source stream is
   multicast from a distribution source (with a source IP address of to the multicast destination address of and
   port 41000.  The forward RTCP traffic is multicast in the same
   multicast group but to port 42000 as specified by the "a=multicast-
   rtcp:42000" line.  A feedback target with an address of and
   port of 43000 is specified by the 'rtcp' attribute.

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           o=ali 1122334455 1122334466 IN IP4
           s='multicast-rtcp' Example
           t=0 0
           m=video 41000 RTP/AVPF 98
           i=Multicast Stream
           c=IN IP4
           a=source-filter:incl IN IP4
           a=rtpmap:98 MP2T/90000
           a=rtcp:43000 IN IP4

       Figure 2: Example SDP showing the use of the 'multicast-rtcp'

4.  Security Considerations

   The 'multicast-rtcp' attribute is not believed to introduce any
   significant security risk to multimedia applications.  A malevolent
   third party could use this attribute to redirect the RTCP traffic,
   but this requires intercepting and rewriting the packets carrying the
   SDP description; and if an interceptor can do that, many more attacks
   are possible, including a wholesale change of the addresses and port
   numbers at which the media will be sent.

   In order to avoid attacks of this sort, the SDP description needs to
   be integrity protected and provided with source authentication.  This
   can, for example, be achieved on an end-to-end basis using S/MIME
   [RFC5652] [RFC5751] when SDP is used in a signaling packet using MIME
   types (application/sdp).  Alternatively, HTTPS [RFC2818] or the
   authentication method in the Session Announcement Protocol (SAP)
   [RFC2974] could be used as well.

5.  IANA Considerations

   The following contact information shall be used for all registrations
   in this document:

   Ali Begen

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5.1.  Registration of SDP Attributes

   This document registers a new attribute name in SDP.

        SDP Attribute ("att-field"):
        Attribute name:     multicast-rtcp
        Long form:          Port in the multicast RTCP session
        Type of name:       att-field
        Type of attribute:  Media or session level
        Subject to charset: No
        Purpose:            Specifies the port for the SSM RTCP session
        Reference:          [RFC6128]
        Values:             See [RFC6128]

6.  Acknowledgments

   Thanks to Colin Perkins and Magnus Westerlund for suggesting the name
   for the 'multicast-rtcp' attribute and providing text for portions of
   this specification.  Some parts of this specification are based on
   [RFC3605] and [RFC5760].  So, also thanks to those who contributed to
   those specifications.

7.  References

7.1.  Normative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760, February 2010.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

7.2.  Informative References

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              October 2003.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

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   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

Author's Address

   Ali Begen
   181 Bay Street
   Toronto, ON  M5J 2T3