Internet DRAFT - draft-ietf-avt-rtcp-port-for-ssm
draft-ietf-avt-rtcp-port-for-ssm
AVT A. Begen
Internet-Draft Cisco
Updates: 5760 (if approved) December 15, 2010
Intended status: Standards Track
Expires: June 18, 2011
RTP Control Protocol (RTCP) Port for Source-Specific Multicast (SSM)
Sessions
draft-ietf-avt-rtcp-port-for-ssm-04
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 18, 2011.
Copyright Notice
Copyright (c) 2010 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
3. The 'multicast-rtcp' Attribute . . . . . . . . . . . . . . . . 3
4. SDP Example . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Registration of SDP Attributes . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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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
(See Section 10.2 of [RFC5760]) by following the +1 rule. 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. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. 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
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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.
4. 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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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
5. 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] 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.
6. IANA Considerations
The following contact information shall be used for all registrations
in this document:
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Ali Begen
abegen@cisco.com
Note to the RFC Editor: In the following, replace "XXXX" with the
number of this document prior to publication as an RFC.
6.1. 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: [RFCXXXX]
Values: See [RFCXXXX]
7. Acknowledgments
Thanks to Colin Perkins amd 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.
8. References
8.1. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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
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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.
8.2. 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.
[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.
Author's Address
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
Email: abegen@cisco.com
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