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rfc:rfc6128

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/rfc6128.

Begen Standards Track [Page 1] 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) [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

Begen Standards Track [Page 2] RFC 6128 RTCP Port for Multicast Sessions February 2011

 (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
 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
 [RFC5760].

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
 [RFC4566].
 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
 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.

Begen Standards Track [Page 3] 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
 [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
 abegen@cisco.com

Begen Standards Track [Page 4] RFC 6128 RTCP Port for Multicast Sessions February 2011

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.

Begen Standards Track [Page 5] RFC 6128 RTCP Port for Multicast Sessions February 2011

 [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
 Cisco
 181 Bay Street
 Toronto, ON  M5J 2T3
 Canada
 EMail: abegen@cisco.com

Begen Standards Track [Page 6]

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