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.
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) [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
Begen Standards Track [Page 2]
RFC 6128 RTCP Port for Multicast Sessions February 2011
(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.
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
[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
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: [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.
Begen Standards Track [Page 5]
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
Begen Standards Track [Page 6]