Internet DRAFT - draft-begen-avt-rtcp-port-for-ssm
draft-begen-avt-rtcp-port-for-ssm
AVT A. Begen
Internet-Draft Cisco
Updates: 5760 (if approved) April 3, 2010
Intended status: Standards Track
Expires: October 5, 2010
RTP Control Protocol (RTCP) Port for Multicast Sessions
draft-begen-avt-rtcp-port-for-ssm-01
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. Similarly, in any-source multicast (ASM) sessions,
there is no explicit way to specify the destination RTCP port.
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
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
3. The 'multicast-rtcp' Attribute . . . . . . . . . . . . . . . . 3
3.1. SDP Example . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Registration of SDP Attributes . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. 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. Similar to SSM sessions,
in any-source multicast (ASM) sessions, there is no explicit way to
specify the destination RTCP port, either, and the new SDP attribute
is equally applicable in ASM sessions as well.
If approved, this document intends to update [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 may use different
multicast RTP and RTCP ports to send the RTP and RTCP packets,
respectively. Alternatively, the distribution source may use RTP/
RTCP port muxing [I-D.ietf-avt-rtp-and-rtcp-mux], in which case the
RTP and RTCP packets are sent to the same destination port in the SSM
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session. For the former case, this document defines a new SDP
attribute, called 'multicast-rtcp'. By using this attribute, the
distribution source MAY use a desired port for the SSM RTCP session.
The formal description of the 'multicast-rtcp' attribute is defined
by the following ABNF [RFC5234] syntax:
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 MAY be used as a media-level
attribute; it MUST NOT be used as a session-level attribute.
3.1. SDP Example
In the SDP 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
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. Therefore, as usual
adequate security measures are RECOMMENDED to ensure the integrity
and authenticity of the SDP descriptions so that transport addresses
of the media senders, distribution sources, feedback targets as well
as other session-specific information can be authenticated.
5. IANA Considerations
The following contact information shall be used for all registrations
in this document:
Ali Begen
abegen@cisco.com
170 West Tasman Drive
San Jose, CA 95134 USA
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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 level
Subject to charset: No
Purpose: See this document
Reference: This document
Values: See this document
6. Acknowledgments
Thanks to Colin Perkins amd Magnus Westerlund for suggesting the name
for the 'multicast-rtcp' attribute. Some parts of this specification
are based on [RFC3605] and [RFC5760]. So, thanks to those who
contributed to those specifications, too.
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.
[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.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
October 2003.
[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.
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7.2. Informative References
[I-D.ietf-avt-rtp-and-rtcp-mux]
Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port",
draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
August 2007.
Author's Address
Ali Begen
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
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