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Internet Engineering Task Force (IETF)                          A. BegenRequest for Comments: 6128                                         CiscoUpdates:5760                                              February 2011Category: Standards TrackISSN: 2070-1721RTP Control Protocol (RTCP) Port forSource-Specific Multicast (SSM) SessionsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6128.Begen                        Standards Track                    [Page 1]

RFC 6128            RTCP Port for Multicast Sessions       February 2011Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................22. The 'multicast-rtcp' Attribute ..................................33. SDP Example .....................................................34. Security Considerations .........................................45. IANA Considerations .............................................45.1. Registration of SDP Attributes .............................56. Acknowledgments .................................................57. References ......................................................57.1. Normative References .......................................57.2. Informative References .....................................51.  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 descriptionBegen                        Standards Track                    [Page 2]

RFC 6128            RTCP Port for Multicast Sessions       February 2011   (seeSection 10.2 of [RFC5760]) by following the +1 rule (seeSection11 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 inSection 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'                                 attribute4.  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.comBegen                        Standards Track                    [Page 4]

RFC 6128            RTCP Port for Multicast Sessions       February 20115.1.  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.  References7.1.  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.comBegen                        Standards Track                    [Page 6]