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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
+
+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.
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+Begen Standards Track [Page 1]
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+RFC 6128 RTCP Port for Multicast Sessions February 2011
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+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]
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+RFC 6128 RTCP Port for Multicast Sessions February 2011
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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
+ 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.
+
+
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+Begen Standards Track [Page 3]
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+RFC 6128 RTCP Port for Multicast Sessions February 2011
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+ 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
+
+
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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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+Begen Standards Track [Page 5]
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+RFC 6128 RTCP Port for Multicast Sessions February 2011
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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
+ Cisco
+ 181 Bay Street
+ Toronto, ON M5J 2T3
+ Canada
+
+ EMail: abegen@cisco.com
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