Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 7197 Cisco
Category: Standards Track Y. Cai
ISSN: 2070-1721 Microsoft
H. Ou
Cisco
April 2014
Duplication Delay Attribute in the Session Description Protocol
Abstract
A straightforward approach to provide protection against packet
losses due to network outages with a longest duration of T time units
is to duplicate the original packets and send each copy separated in
time by at least T time units. This approach is commonly referred to
as "time-shifted redundancy", "temporal redundancy", or simply
"delayed duplication". This document defines an attribute to
indicate the presence of temporally redundant media streams and the
duplication delay in the Session Description Protocol.
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/rfc7197.
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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
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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 ...........................................4
3. The 'duplication-delay' Attribute ...............................5
4. SDP Examples ....................................................6
5. Security Considerations .........................................7
6. IANA Considerations .............................................8
6.1. Registration of SDP Attributes .............................9
7. Acknowledgements ................................................9
8. References ......................................................9
8.1. Normative References .......................................9
8.2. Informative References ....................................10
1. Introduction
Inside an IP network, packet delivery may be interrupted due to
failure of a physical link, interface, or device. To reduce the
impact of such interruptions, some networks are built in a resilient
manner, allowing for multiple alternative paths between two
endpoints. However, if there is no resiliency in the network or the
failure happens in a non-resilient part of the network, a temporary
outage will occur (i.e., packets will get dropped). The outage will
last until network reconvergence takes place (i.e., until
connectivity is restored) around the failure. Typically, network
reconvergence takes between tens and hundreds of milliseconds,
depending on the size and features of the network.
There are a number of network-reconvergence technologies available
today, such as IP Fast Convergence, MPLS Traffic Engineering Fast
Reroute, and Multicast Only Fast Reroute. These technologies can be
augmented by different types of application-layer loss-repair methods
such as Forward Error Correction (FEC), retransmission, temporal
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redundancy, and spatial redundancy to minimize (and sometimes totally
eliminate) the impact of outages. Each combination has its distinct
requirements in terms of bandwidth consumption and results in a
different network complexity. Thus, a network operator has to
carefully consider what combination to deploy for different parts of
a network (e.g., core vs. edge). A detailed overview of network-
convergence technologies and loss-repair methods is provided in
[IC2011].
One of the loss-repair methods is temporal redundancy, also known as
delayed duplication. A media sender using this method transmits an
original source packet and transmits its duplicate after a certain
delay following the original transmission. If a network outage hits
the original transmission, the expectation is that the second
transmission arrives at the receiver (with a high probability).
Alternatively, the second transmission may be hit by an outage and so
gets dropped, and the original transmission completes successfully.
Also, both transmissions can arrive on the receiver side; in that
case, the receiver (or the node that does the duplicate suppression)
needs to identify the duplicate packets and discard them
appropriately, thereby producing a duplicate-free stream.
Delayed duplication can be used in a variety of multimedia
applications where there is sufficient bandwidth for the duplicated
traffic and the application can tolerate the introduced delay.
However, it must be used with care, since it might easily result in a
new series of denial-of-service attacks. Delayed duplication is
harmful in cases where the primary cause of packet loss is
congestion, rather than a network outage due to a temporary link or
network element failure. Duplication should only be used by
endpoints that want to protect against network failures; protection
against congestion must be achieved through other means, as
duplication will only make congestion worse.
One particular use case for delayed duplication is to improve the
reliability of real-time video feeds inside a core IP network where
bandwidth is plentiful and maximum reliability (preferably zero loss)
is desired [IC2011]. Compared to other redundancy approaches such as
FEC [RFC6363] and redundant data encoding (e.g., [RFC2198]), delayed
duplication is easy to implement, since it does not require any
special type of encoding or decoding.
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For duplicate suppression, the receiver has to be able to identify
the identical packets. This is straightforward for media packets
that carry one or more unique identifiers such as the sequence number
field in the RTP header [RFC3550]. In non-RTP applications, the
receiver can use unique sequence numbers if available or other
alternative approaches to compare the incoming packets and discard
the duplicate ones.
This specification introduces a new Session Description Protocol
(SDP) [RFC4566] attribute for applications/services using the delayed
duplication method to indicate the relative delay for each additional
duplication. The attribute is used with the duplication grouping
semantics defined in [RFC7104].
This specification does not explain how to select the duplication
delay that a sender should use; the selection technique depends on
the underlying network and the reconvergence technologies used inside
such a network. This specification does not explain how the receiver
should suppress the duplicate packets and merge the incoming streams
to produce a loss-free and duplication-free output stream (a process
commonly called "stream merging"), either. An application or a
transport service that will use the delayed duplication method must
determine its own rules about stream merging.
In practice, more than two redundant streams are unlikely to be used,
since the additional delay and increased overhead are not easily
justified. However, we define the new attribute in a general way so
that it could be used with more than two redundant streams (i.e.,
multiple duplications), if needed. While the primary focus in this
specification is the RTP-based transport, the new attribute is
applicable to both RTP and non-RTP streams. Protocol issues and
details on duplicating RTP streams are presented in [RFC7198].
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
[RFC2119].
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3. The 'duplication-delay' Attribute
The following ABNF [RFC5234] syntax formally describes the
'duplication-delay' attribute:
delaying-attribute = "a=duplication-delay:" periods CRLF
periods = period *( SP period)
period = 1*DIGIT ; in milliseconds
ABNF Syntax for the 'duplication-delay' Attribute
The 'duplication-delay' attribute is defined as both a media-level
and session-level attribute. It specifies the relative delay with
respect to the previous transmission of each duplication in
milliseconds (ms) at the time of transmission. The following rules
apply:
o If used as a media-level attribute, it MUST be used with the
'ssrc-group' attribute and "DUP" grouping semantics as defined in
[RFC7104]. When used as a media-level attribute, the relative
delay value(s) it specifies SHALL apply to every Synchronization
Source (SSRC)-based duplication grouping in the same media
description. In other words, one cannot specify different
duplication delay values for different duplication groups in the
same media description.
o If used as a session-level attribute, it MUST be used with 'group'
attribute and "DUP" grouping semantics as defined in [RFC7104].
When used as a session-level attribute, the relative delay
value(s) it specifies SHALL apply to every duplication grouping in
the same SDP description. In other words, one cannot specify
different duplication delay values for different duplication
groups in the same SDP description. If one needs to specify
different duplication delay values for different duplication
groups, then one MUST use different SDP descriptions for each or
MUST use the 'duplication-delay' attribute at the media level. In
that case, the 'duplication-delay' attribute MUST NOT be used at
the session level.
o For offer/answer model considerations, refer to [RFC7104].
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4. SDP Examples
In the first example below, the multicast stream consists of two RTP
streams, each duplicated once, resulting in two sets of two-stream
groups. The same duplication delay of 100 ms is applied to each
grouping. The first set's streams have SSRCs of 1000 and 1010, and
the second set's streams have SSRCs of 1020 and 1030.
v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=Delayed Duplication
t=0 0
m=video 30000 RTP/AVP 100 101
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=rtpmap:101 MP2T/90000
a=ssrc:1020 cname:ch1b@example.com
a=ssrc:1030 cname:ch1b@example.com
a=ssrc-group:DUP 1020 1030
a=duplication-delay:100
a=mid:Ch1
Note that in actual use, SSRC values, which are random 32-bit
numbers, could be much larger than the ones shown in this example.
In the second example below, the multicast stream is duplicated
twice. 50 ms after the original transmission, the first duplicate is
transmitted, and 100 ms after that, the second duplicate is
transmitted. In other words, the same packet is transmitted three
times over a period of 150 ms.
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:ch1c@example.com
a=ssrc:1010 cname:ch1c@example.com
a=ssrc:1020 cname:ch1c@example.com
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a=ssrc-group:DUP 1000 1010 1020
a=duplication-delay:50 100
a=mid:Ch1
In the third example below, the multicast UDP stream is duplicated
with a duplication delay of 50 ms. Redundant streams are sent in
separate source-specific multicast (SSM) sessions, so the receiving
host has to join both SSM sessions if it wants to receive both
streams.
v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=Delayed Duplication
t=0 0
a=group:DUP S1a S1b
a=duplication-delay:50
m=audio 30000 udp mp4
c=IN IP4 233.252.0.1/127
a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
a=mid:S1a
m=audio 40000 udp mp4
c=IN IP4 233.252.0.2/127
a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
a=mid:S1b
5. Security Considerations
The 'duplication-delay' attribute is not believed to introduce any
significant security risk to multimedia applications. A malevolent
third party could use this attribute to misguide the receiver(s)
about the duplication delays and/or the number of redundant streams.
For example, if the malevolent third party increases the value of the
duplication delay, the receiver(s) will unnecessarily incur a longer
delay, since they will have to wait for the entire period. Or, if
the duplication delay is reduced by the malevolent third party, the
receiver(s) might not wait long enough for the duplicated
transmission and incur unnecessary packet losses. However, these
require intercepting and rewriting the packets carrying the SDP
description; if an interceptor can do that, many more attacks are
also possible.
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.
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Another security risk is due to possible software misconfiguration or
a software bug where a large number of duplicates could be
unwillingly signaled in the 'duplication-delay' attribute.
Similarly, an attacker can use this attribute to start a denial-of-
service attack by signaling and sending too many duplicated streams.
In applications where this attribute is to be used, it is a good
practice to put a hard limit on both the number of duplicate streams
and the total delay introduced due to duplication, regardless of what
the SDP description specifies.
Since this mechanism causes duplication of media packets, if those
packets are also cryptographically protected (e.g., encrypted) then
such duplication could act as an accelerator if any Million Message
[RFC3218] or similar attack such as Lucky 13 [Lucky13] exists against
the security mechanism that is in use. Such acceleration could turn
an otherwise infeasible attack into one that is practical; however,
assuming that the amount of duplication is small and that such weak
or broken security mechanisms should really not be used, the overall
security impact of the duplication should be minimal. If, however, a
bad actor were in control of the SDP but did not have access to the
keying material used for media, then such a bad actor could
potentially use the SDP to cause the media handling to use a weak or
broken mechanism with a lot of duplication, in which case the
duplication could be significant. Deployments where the SDP is
controlled by an actor who should not have access to the media keying
material should therefore be cautious in their use of this
duplication mechanism.
If this mechanism were used in conjunction with a source description
(SDES) and if the key being used for media protection is derived from
a human-memorable or otherwise dictionary-attackable secret, then the
duplication done here could allow for a more efficient dictionary
attack against the media. The right countermeasure is to use proper
keying or, if using an SDES, to ensure that the keys used are not
dictionary-attackable.
6. IANA Considerations
The following contact information shall be used for the registration
in this document:
Ali Begen
abegen@cisco.com
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6.1. Registration of SDP Attributes
This document registers a new attribute name in SDP.
SDP Attribute ("att-field"):
Attribute name: duplication-delay
Long form: Duplication delay for temporally redundant
streams
Type of name: att-field
Type of attribute: Media or session level
Subject to charset: No
Purpose: Specifies the relative duplication delay(s)
for redundant stream(s)
Reference: [RFC7197]
Values: See [RFC7197]
7. Acknowledgements
The authors would like to thank Colin Perkins, Paul Kyzivat, and
Stephen Farrell for their suggestions and reviews.
8. References
8.1. Normative References
[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.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7104] Begen, A., Cai, Y., and H. Ou, "Duplication Grouping
Semantics in the Session Description Protocol", RFC 7104,
January 2014.
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8.2. Informative References
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, October 2011.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997.
[RFC7198] Begen, A. and C. Perkins, "Duplicating RTP Streams",
RFC 7198, April 2014.
[IC2011] Evans, J., Begen, A., Greengrass, J., and C. Filsfils,
"Toward Lossless Video Transport", IEEE Internet
Computing, Vol. 15, No. 6, pp. 48-57, November 2011.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[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.
[RFC3218] Rescorla, E., "Preventing the Million Message Attack on
Cryptographic Message Syntax", RFC 3218, January 2002.
[Lucky13] AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
the TLS and DTLS Record Protocols", IEEE Symposium on
Security and Privacy, May 2013,
<http://ieeexplore.ieee.org/xpl/articleDetails.jsp?
tp=&arnumber=6547131&queryText%3DLucky+Thirteen>.
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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 11]