Internet DRAFT - draft-begen-mmusic-temporal-interleaving
draft-begen-mmusic-temporal-interleaving
MMUSIC A. Begen
Internet-Draft Y. Cai
Intended status: Standards Track H. Ou
Expires: September 12, 2012 Cisco
March 11, 2012
Delayed Duplication Attribute in the Session Description Protocol
draft-begen-mmusic-temporal-interleaving-04
Abstract
A straightforward approach to provide protection against packet
losses due to network outages with a longest duration of T time units
is to simply 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 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 September 12, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 4
3. The 'duplication-delay' Attribute . . . . . . . . . . . . . . . 4
4. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Registration of SDP Attributes . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Consider that a media sender 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. Alternatively, the second transmission may be hit by an
outage and gets dropped, and the original transmission completes
successfully. On the receiver side, both transmissions can also
arrive and in that case, the receiver (or the node that does the
duplicate suppression) needs to identify the duplicate packets and
discard them appropriately, 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. Furthermore, delayed
duplication must not be used 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 can make
congestion only worse.
One particular use case for delayed duplication is to improve the
reliability of real-time video feeds inside a core IP network
[IC2011]. Compared to other popular redundancy approaches such as
Forward Error Correction (FEC) [RFC6363] and redundant data encoding
(e.g., [RFC2198]), delayed duplication is quite easy to implement
since it does not require any special type of encoding or decoding.
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 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.
In this specification, we are not concerned about how the sender
should determine the duplication delay. We are not concerned about
how the receiver can suppress the duplicate packets and merge the
incoming streams to produce a hopefully loss-free and duplication-
free output stream (called stream merging), either. These
considerations are out of the scope for this specification. Rather,
our goal is simply to introduce a new attribute for the Session
Description Protocol (SDP) [RFC4566] that indicates that the media
stream is to be duplicated and sent two or more times, and also
indicates the relative delay for each additional duplication.
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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 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. Details on duplicating RTP streams are presented in
[I-D.begen-avtcore-rtp-duplication].
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].
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 *( ":" period)
period = 1*DIGIT ; in milliseconds
Figure 1: ABNF syntax for the 'interleaving-period' attribute
The 'duplication-delay' attribute is defined as both a media-level
and session-level attribute. It specifies the relative delay for
each duplication in milliseconds (ms). If used as a media-level
attribute, it MUST be used with the 'ssrc-group' attribute and "DUP"
grouping semantics as defined in
[I-D.begen-mmusic-redundancy-grouping]. If used as a session-level
attribute, it MUST be used with 'group' attribute and "DUP" grouping
semantics as defined in [I-D.begen-mmusic-redundancy-grouping].
4. SDP Examples
In the first example below, the multicast stream is duplicated with a
duplication delay of 100 ms. The streams have Synchronization
Sources (SSRC) of 1000 and 1010, and they are grouped together using
the 'ssrc-group' attribute defined in [RFC5576]. The "DUP" grouping
semantics are defined in [I-D.begen-mmusic-redundancy-grouping]. The
reason for using explicit grouping is that not all the media streams
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in the same "m" line are necessarily duplicates of each other.
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:ch1@example.com
a=ssrc:1010 cname:ch1@example.com
a=ssrc-group:DUP 1000 1010
a=duplication-delay:100
a=mid:Group1
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:ch1@example.com
a=ssrc:1010 cname:ch1@example.com
a=ssrc:1020 cname:ch1@example.com
a=ssrc-group:DUP 1000 1010 1020
a=duplication-delay:50:100
a=mid:Group1
In the third example below, the multicast UDP stream is duplicated
with a duplication delay of 50 ms. Both streams are sent in the same
source-specific multicast (SSM) session but they are sent to
different ports. The "DUP" grouping semantics
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[I-D.begen-mmusic-redundancy-grouping] are used to describe the
redundany relation.
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 233.252.0.1 198.51.100.1
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; and 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.
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. In
applications where this attribute is to be used, it is a good
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practice to put a hard limit both on the number of duplicate streams
and the total delay introduced due to duplication regardless of what
the SDP description specifies.
6. IANA Considerations
The following contact information shall be used for all registrations
in this document:
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: 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: [RFCXXXX]
Values: See [RFCXXXX]
7. Acknowledgements
Authors would like to thank Colin Perkins for his suggestions and
review.
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.
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[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.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[I-D.begen-mmusic-redundancy-grouping]
Begen, A., Cai, Y., and H. Ou, "Duplication Grouping
Semantics in the Session Description Protocol",
draft-begen-mmusic-redundancy-grouping-02 (work in
progress), October 2011.
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.
[I-D.begen-avtcore-rtp-duplication]
Begen, A. and C. Perkins, "Duplicating RTP Streams",
draft-begen-avtcore-rtp-duplication-00 (work in progress),
October 2011.
[IC2011] Evans, J., Begen, A., Greengrass, J., and C. Filsfils,
"Toward Lossless Video Transport (to appear in IEEE
Internet Computing)", 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.
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[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
Authors' Addresses
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
Email: abegen@cisco.com
Yiqun Cai
Cisco
170 W. Tasman Dr.
San Jose, CA 95134
USA
Email: ycai@cisco.com
Heidi Ou
Cisco
170 W. Tasman Dr.
San Jose, CA 95134
USA
Email: hou@cisco.com
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