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

 Abstract

   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
   http://www.rfc-editor.org/info/RFC 6128.














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RFC 6128            RTCP Port for Multicast Sessions       February 2011


 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
   (http://trustee.ietf.org/license-info) 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) [RFC 4566] has an attribute
   that allows RTP applications [RFC 3550] to specify an address and a
   port associated with the RTP Control Protocol (RTCP) traffic
   [RFC 3605].  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 [RFC 5760].  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 [RFC 5760].  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 [RFC 5760]) by following the +1 rule (see Section
   11 of [RFC 3550]).  However, [RFC 3550] lifted the requirement for the
   +1 rule since it imposed an unnecessary restriction on RTCP port
   selection.

   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
   [RFC 5760].

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 [RFC 5761], 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 [RFC 5760].

   The following ABNF [RFC 5234] 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
   [RFC 4566].

   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 [RFC 3550] 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
   198.51.100.1) to the multicast destination address of 233.252.0.2 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 192.0.2.1 and
   port of 43000 is specified by the 'rtcp' attribute.



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RFC 6128            RTCP Port for Multicast Sessions       February 2011


           v=0
           o=ali 1122334455 1122334466 IN IP4 ssm.example.com
           s='multicast-rtcp' Example
           t=0 0
           a=rtcp-unicast:rsi
           m=video 41000 RTP/AVPF 98
           i=Multicast Stream
           c=IN IP4 233.252.0.2/255
           a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
           a=rtpmap:98 MP2T/90000
           a=multicast-rtcp:42000
           a=rtcp:43000 IN IP4 192.0.2.1
           a=mid:1

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

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
   [RFC 5652] [RFC 5751] when SDP is used in a signaling packet using MIME
   types (application/sdp).  Alternatively, HTTPS [RFC 2818] or the
   authentication method in the Session Announcement Protocol (SAP)
   [RFC 2974] could be used as well.

5.  IANA Considerations

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

   Ali Begen
   abegen@cisco.com









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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:          [RFC 6128]
        Values:             See [RFC 6128]

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
   [RFC 3605] and [RFC 5760].  So, also thanks to those who contributed to
   those specifications.

7.  References

7.1.  Normative References

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

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

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

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

7.2.  Informative References

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

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



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RFC 6128            RTCP Port for Multicast Sessions       February 2011


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

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

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

   [RFC 5751]  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
   Cisco
   181 Bay Street
   Toronto, ON  M5J 2T3
   Canada

   EMail: abegen@cisco.com






























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

RFC TOTAL SIZE: 11544 bytes
PUBLICATION DATE: Saturday, February 12th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)