|Title||RTP Control Protocol (RTCP) Port for Source-Specific Multicast (SSM)
Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6128 Cisco
Updates: 5760 February 2011
Category: Standards Track
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
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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
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
(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
The following ABNF [RFC5234] syntax formally describes the
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
198.51.100.1) to the multicast destination address of 184.108.40.206 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.
o=ali 1122334455 1122334466 IN IP4 ssm.example.com
m=video 41000 RTP/AVPF 98
c=IN IP4 220.127.116.11/255
a=source-filter:incl IN IP4 18.104.22.168 198.51.100.1
a=rtcp:43000 IN IP4 192.0.2.1
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:
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
Values: See [RFC6128]
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
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,
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[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.
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