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IETF RFC 7104
Last modified on Wednesday, January 22nd, 2014
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Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 7104 Cisco
Category: Standards Track Y. Cai
ISSN: 2070-1721 Microsoft
H. Ou
Cisco
January 2014
Duplication Grouping Semantics in the Session Description Protocol
 Abstract
Packet loss is undesirable for real-time multimedia sessions, but it
can occur due to congestion or other unplanned network outages. This
is especially true for IP multicast networks, where packet loss
patterns can vary greatly between receivers. One technique that can
be used to recover from packet loss without incurring unbounded delay
for all the receivers is to duplicate the packets and send them in
separate redundant streams. This document defines the semantics for
grouping redundant streams in the Session Description Protocol (SDP).
The semantics defined in this document are to be used with the SDP
Grouping Framework. Grouping semantics at the Synchronization Source
(SSRC) level are also defined in this document for RTP streams using
SSRC multiplexing.
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 7104.
Begen, et al. Standards Track PAGE 1 
RFC 7104 Duplication Grouping Semantics in SDP January 2014
Copyright Notice
Copyright (c) 2014 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. Requirements Notation ...........................................3
3. Duplication Grouping ............................................3
3.1. "DUP" Grouping Semantics ...................................3
3.2. Duplication Grouping for SSRC-Multiplexed RTP Streams ......3
3.3. SDP Offer/Answer Model Considerations ......................4
4. SDP Examples ....................................................5
4.1. Separate Source Addresses ..................................5
4.2. Separate Destination Addresses .............................6
4.3. Temporal Redundancy ........................................7
5. Security Considerations .........................................7
6. IANA Considerations .............................................8
7. Acknowledgments .................................................8
8. References ......................................................8
8.1. Normative References .......................................8
8.2. Informative References .....................................9
Begen, et al. Standards Track PAGE 2
RFC 7104 Duplication Grouping Semantics in SDP January 2014
1. Introduction
The Real-time Transport Protocol (RTP) [RFC 3550] is widely used today
for delivering IPTV traffic and other real-time multimedia sessions.
Many of these applications support very large numbers of receivers
and rely on intra-domain UDP/IP multicast for efficient distribution
of traffic within the network.
While this combination has proved successful, there does exist a
weakness. As [RFC 2354] noted, packet loss is not avoidable, even in
a carefully managed network. This loss might be due to congestion;
it might also be a result of an unplanned outage caused by a flapping
link, a link or interface failure, a software bug, or a maintenance
person accidentally cutting the wrong fiber. Since UDP/IP flows do
not provide any means for detecting loss and retransmitting packets,
it is left up to the RTP layer and the applications to detect, and
recover from, packet loss.
One technique to recover from packet loss without incurring unbounded
delay for all the receivers is to duplicate the packets and send them
in separate redundant streams. Variations on this idea have been
implemented and deployed today [IC2011]. [RTP-DUP] explains how
duplication can be achieved for RTP streams without breaking the RTP
and RTP Control Protocol (RTCP) functionality. In this document, we
describe the semantics needed in the Session Description Protocol
(SDP) [RFC 4566] to support this technique.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC 2119].
3. Duplication Grouping
3.1. "DUP" Grouping Semantics
Each "a=group" line is used to indicate an association relationship
between the redundant streams. The streams included in one "a=group"
line are called a "Duplication Group".
Using the SDP Grouping Framework in [RFC 5888], this document defines
"DUP" as the grouping semantics for redundant streams.
Begen, et al. Standards Track PAGE 3
RFC 7104 Duplication Grouping Semantics in SDP January 2014
The "a=group:DUP" semantics MUST be used to group the redundant
streams, except when the streams are specified in the same media
description, i.e., in the same "m" line (see Section 3.2). In an
"a=group:DUP" line, the order of the listed redundant streams does
not strictly indicate the order of transmission, although it is
RECOMMENDED that the stream listed first be sent first, with the
other stream(s) being the (time-delayed) duplicate(s).
3.2. Duplication Grouping for SSRC-Multiplexed RTP Streams
[RFC 5576] defines an SDP media-level attribute, called "ssrc-group",
for grouping the RTP streams that are SSRC multiplexed and carried in
the same RTP session. The grouping is based on the SSRC identifiers.
Since SSRC-multiplexed RTP streams are defined in the same "m" line,
the "group" attribute cannot be used.
This section explains how duplication is used with SSRC-multiplexed
streams using the "ssrc-group" attribute [RFC 5576].
The semantics of "DUP" for the "ssrc-group" attribute are the same as
the one defined for the "group" attribute, except that the SSRC
identifiers are used to designate the duplication grouping
associations: a=ssrc-group:DUP *(SP ssrc-id) [RFC 5576]. As above,
while in an "a=ssrc-group:DUP" line, the order of the listed
redundant streams does not necessarily indicate the order of
transmission, but it is RECOMMENDED that the stream listed first be
sent first, with the other stream(s) being the (time-delayed)
duplicate(s).
3.3. SDP Offer/Answer Model Considerations
When offering duplication grouping using SDP in an offer/answer model
[RFC 3264], the following considerations apply.
A node that is receiving an offer from a sender may or may not
understand line grouping. It is also possible that the node
understands line grouping but does not understand the "DUP"
semantics. From the viewpoint of the sender of the offer, these
cases are indistinguishable.
When a node is offered a session with the "DUP" grouping semantics
but it does not support line grouping or the duplication grouping
semantics, as per [RFC 5888], the node responds to the offer either
(1) with an answer that omits the grouping attribute or (2) with a
refusal to the request (e.g., "488 Not Acceptable Here" or "606 Not
Acceptable in SIP").
Begen, et al. Standards Track PAGE 4
RFC 7104 Duplication Grouping Semantics in SDP January 2014
In the first case, the original sender of the offer must send a new
offer without any duplication grouping. In the second case, if the
sender of the offer still wishes to establish the session, it should
retry the request with an offer without the duplication grouping.
This behavior is specified in [RFC 5888].
4. SDP Examples
4.1. Separate Source Addresses
In this example, the redundant streams use the same IP destination
address (232.252.0.1), but they are sourced from different addresses
(198.51.100.1 and 198.51.100.2). Thus, the receiving host needs to
join both source-specific multicast (SSM) sessions separately.
v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=DUP Grouping Semantics
t=0 0
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 198.51.100.2
a=rtpmap:100 MP2T/90000
a=ssrc:1000 cname:ch1@example.com
a=ssrc:1010 cname:ch1@example.com
a=ssrc-group:DUP 1000 1010
a=mid:Ch1
Note that in actual use, SSRC values, which are random 32-bit
numbers, can be much larger than the ones shown in this example.
Also, note that this SDP description does not use the "duplication-
delay" attribute (defined in [DELAYED-DUP]) since the sender does not
apply any delay between the redundant streams upon transmission.
Alternatively, one MAY explicitly insert an "a=duplication-delay:0"
line before the "a=mid:Ch1" line for informational purposes.
Begen, et al. Standards Track PAGE 5
RFC 7104 Duplication Grouping Semantics in SDP January 2014
4.2. Separate Destination Addresses
In this example, the redundant streams have different IP destination
addresses. The example shows the same UDP port number and IP source
address for each stream, but either or both could have been different
for the two streams.
v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=DUP Grouping Semantics
t=0 0
a=group:DUP S1a S1b
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
a=rtpmap:100 MP2T/90000
a=mid:S1a
m=video 30000 RTP/AVP 101
c=IN IP4 233.252.0.2/127
a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
a=rtpmap:101 MP2T/90000
a=mid:S1b
Optionally, one could be more explicit and insert an
"a=duplication-delay:0" line before the first "m" line.
Begen, et al. Standards Track PAGE 6
RFC 7104 Duplication Grouping Semantics in SDP January 2014
4.3. Temporal Redundancy
In this example, the redundant streams have the same IP source and
destination addresses (i.e., they are transmitted in the same SSM
session). Due to the same source and destination addresses, the
packets in both streams will be routed over the same path. To
provide resiliency against packet loss, the duplicate of an original
packet is transmitted 50 milliseconds (ms) later as indicated by the
"duplication-delay" attribute (defined in [DELAYED-DUP]).
v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=Delayed Duplication
t=0 0
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
a=rtpmap:100 MP2T/90000
a=ssrc:1000 cname:ch1a@example.com
a=ssrc:1010 cname:ch1a@example.com
a=ssrc-group:DUP 1000 1010
a=duplication-delay:50
a=mid:Ch1
5. Security Considerations
In general, the security considerations of [RFC 4566] apply to this
document as well.
There is a weak threat for the receiver that the duplication grouping
can be modified to indicate relationships that do not exist. Such
attacks might result in failure of the duplication mechanisms and/or
mishandling of the media streams by the receivers.
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 the 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. As for the confidentiality, if it
is desired, it can be useful to use a secure, encrypted transport
method to carry the SDP description.
Begen, et al. Standards Track PAGE 7
RFC 7104 Duplication Grouping Semantics in SDP January 2014
6. IANA Considerations
This document registers the following semantics with IANA in the
"Semantics for the "group" SDP Attribute" subregistry (under the
"Session Description Protocol (SDP) Parameters" registry:
Semantics Token Reference
------------------------------------- ------ ---------
Duplication DUP [RFC 7104]
This document also registers the following semantics with IANA in the
"Semantics for the "ssrc-group" SDP Attribute" subregistry under the
"Session Description Protocol (SDP) Parameters" registry:
Token Semantics Reference
------- ----------------------------- ---------
DUP Duplication [RFC 7104]
7. Acknowledgments
The authors would like to thank Colin Perkins, Bill Ver Steeg, Dave
Oran, and Toerless Eckert for their input and suggestions.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[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 5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC 5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
Begen, et al. Standards Track PAGE 8
RFC 7104 Duplication Grouping Semantics in SDP January 2014
8.2. Informative References
[DELAYED-DUP]
Begen, A., Cai, Y., and H. Ou, "Delayed Duplication
Attribute in the Session Description Protocol", Work in
Progress, December 2013.
[IC2011] Evans, J., Begen, A., Greengrass, J., and C. Filsfils,
"Toward Lossless Video Transport, IEEE Internet Computing,
vol. 15/6, pp. 48-57", November 2011.
[RFC 2354] Perkins, C. and O. Hodson, "Options for Repair of
Streaming Media", RFC 2354, June 1998.
[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 5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC 5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RTP-DUP] Begen, A. and C. Perkins, "Duplicating RTP Streams", Work
in Progress, October 2013.
Begen, et al. Standards Track PAGE 9
RFC 7104 Duplication Grouping Semantics in SDP January 2014
Authors' Addresses
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
EMail: abegen@cisco.com
Yiqun Cai
Microsoft
1065 La Avenida
Mountain View, CA 94043
USA
EMail: yiqunc@microsoft.com
Heidi Ou
Cisco
170 W. Tasman Dr.
San Jose, CA 95134
USA
EMail: hou@cisco.com
Begen, et al. Standards Track PAGE 10
RFC TOTAL SIZE: 17719 bytes
PUBLICATION DATE: Wednesday, January 22nd, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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