The Session Description Protocol (SDP) Application Token Attribute
draft-even-mmusic-application-token-02
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| Authors | Roni Even , Jonathan Lennox , Qin Wu | ||
| Last updated | 2014-01-03 | ||
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draft-even-mmusic-application-token-02
MMUSIC WG R. Even
Internet-Draft Huawei Technologies
Intended status: Informational J. Lennox
Expires: July 7, 2014 Vidyo
Q. Wu
Huawei Technologies
January 3, 2014
The Session Description Protocol (SDP) Application Token Attribute
draft-even-mmusic-application-token-02.txt
Abstract
The RTP fixed header includes the payload type number and the SSRC
values of the RTP stream. RTP defines how to de-multiplex streams
within an RTP session, but in some use cases applications need
further identifiers in order to identify the application semantics
associated with particular streams within the session as conveyed in
the signaling.
This document defines a mechanism to provide the mapping between the
SSRCs of RTP streams and the application semantics by defining
extensions to RTP and RTCP messages.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 7, 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Proposal for Application ID . . . . . . . . . . . . . . . . . 5
3.1. appId token . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1. RTCP SDES message . . . . . . . . . . . . . . . . . . 8
3.1.2. RTP Header Extension . . . . . . . . . . . . . . . . . 9
3.1.3. recv-appId . . . . . . . . . . . . . . . . . . . . . . 10
3.2. The "appId-group" media attribute . . . . . . . . . . . . 10
3.3. appId attributes . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. The "pt" appId attribute . . . . . . . . . . . . . . . 11
4. Using Application ID token in Offer / Answer . . . . . . . . . 11
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
The RTP [RFC3550] header includes the payload type number and the
SSRC values of the RTP stream. RTP defines how to de-multiplex
streams within an RTP session, but in some use cases, applications
need further identifiers in order to identify semantics associated
with particular streams within the session.
SDP [RFC4566] can be used to describe multiple RTP media streams in
one or more m-lines that define a single SSRC multiplexed RTP session
(as specified in [RFC3550]). This addresses the WebRTC architecture
[I-D.ietf-rtcweb-overview].
A Unified Plan for Using SDP with Large Numbers of Media Flows
[I-D.roach-mmusic-unified-plan] proposes that each m-line will
represent a media source [I-D.ietf-avtext-rtp-grouping-taxonomy]. In
the simple case a media source will be one video or audio RTP stream.
Media source description becomes more complicated when, for robust
applications, techniques like retransmission (RTX) and Forward Error
Correction (FEC) are used to protect media, or simulcast or layered
coding can be used to provide support to heterogeneous receivers. In
these cases a media source may send more than one RTP stream: for
example, a scalable video stream base layer, an enhancement layer and
a FEC stream.
Multiple SDP m-lines can be multiplexed to a single RTP session using
[I-D.ietf-mmusic-sdp-bundle-negotiation]. The same payload type
number can be used in multiple bundled m-lines.
Some applications may require more information about the usage of the
RTP streams: for example, RTP streams from different cameras that
need to be identified by the application in order to render them
correctly, or a source that can send multiple versions of the same
stream in different resolutions (i.e. simulcast
[I-D.westerlund-avtcore-rtp-simulcast]).
A single RTP media stream can be identified in SDP by using the SSRC
attribute [RFC5576]. Relations between RTP streams in the same
session can be specified using the ssrc-group attribute [RFC5576].
Using the SSRC to identify RTP streams in an SDP session assumes that
this information is available to the SDP signaling layer. The SSRC
is RTP layer information and may change in the RTP layer during a
session.
Support of FEC, SVC and simulcast brings more requirements as
explained using the following examples.
The following example is of a unified plan
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[I-D.roach-mmusic-unified-plan] offer of one audio source and one
video source. The video source includes two SVC RTP streams a base
layer and an enhancement layer. There are also two FEC options:
Base layer S1 is protected by FEC repair stream R1
Base Layer S1 and Enhancement layer S2 are protected by FEC repair
stream R2.
This enables the answer to select the base layer with R1 or the Base
+ enhancement layers both protected by R2.
This example uses the SSRC and SSRC-GROUP attributes which requires
the pre-knowledge of the SSRCs that are RTP layer values.
SDP Offer:
v=0
o=- 20518 0 IN IP4 198.51.100.1
s=FEC Grouping Semantics for SSRC Multiplexing
t=0 0
c=IN IP4 203.0.113.1
a=group:BUNDLE m1 m2
m=audio 56600 RTP/SAVPF 0 109
a=msid:ma ta
a=mid:m1
a=ssrc:53280
a=rtpmap:0 PCMU/8000
a=rtpmap:109 opus/48000
m=video 56602 RTP/AVPF 100 101 110 111 - Main camera
a=msid:ma tb
a=mid:m2
a=rtpmap:100 H264/90000 - Base layer
a=rtpmap:101 H264-SVC/90000 - Enhancement layer.
a=depend:101 lay L1:100 - dependencies
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=rtpmap:111 1d-interleaved-parityfec/90000
a=fmtp:111 L=10; D=10; repair-window=400000
a=ssrc:1000 cname:MSTFEC@example.com
a=ssrc:1010 cname:MSTFEC@example.com
a=ssrc:2110 cname:MSTFEC@example.com
a=ssrc:2120 cname:MSTFEC@example.com
a=ssrc-group:FEC-FR 1000 2110
a=ssrc-group:FEC-FR 1000 1010 2120
a=ssrc-group:DDP 1000 1010
In this case all video streams are from the same source and can be
described using a single m-line. The grouping relations are
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specified using the SSRCs values that need to be available in the
offer. It is also not clear based on the offer which rtpmap line
corresponds to each of the a=ssrc lines, e.g. which rtpmap line will
be sent using ssrc = 2110. The answerer can deduce the information
based on analyzing the ssrc-group information but there can be case
that it will not be possible..
There are cases where the SSRCs of the RTP streams may not be
available.
A selective forwarding middlebox as described in RTP topologies
section 3.7 [topologies] may project the RTP stream from the source
to destination and forward new SSRCs without any signaling.
A three camera telepresence system may send two video media stream of
the two recent active speakers to a system with two monitors. In
this case it may send first the video from the left and center camera
(this will cause the video from the center camera to be displayed on
the right) and later the video from the center and right camera (this
will cause the video from the center camera to be displayed on the
left). The SSRC of the video stream from the center camera will
remain the same but the mapping to the stream description will
change.
As discussed in [I-D.roach-mmusic-unified-plan] during call
establishment, circumstances may arise under which an endpoint can
send an offer to receive a new stream, and begin receiving media for
that media stream prior to receiving the SDP that correlates its SSRC
to the m-line. For such cases, the endpoint will not know how to
handle the media, and will most probably be forced to discard it.
2. Terminology
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[RFC2119] and
indicate requirement levels for compliant RTP implementations.
3. Proposal for Application ID
As we saw in the introduction, the SSRC of the RTP stream which
should be created by the RTP layer is used by the SDP in the offer to
map an RTP stream description to the SDP or application logic. There
are cases where the SSRCs of the RTP streams may not be available or
do not provide all the required information.
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There is a need to have a token that will allow the mapping between a
single RTP stream in an m-line to the application logic and to the
actual RTP media stream. For example, stream1 is the RTP stream from
the left camera and stream2 is the RTP stream from the right camera
and stream3 is the FEC stream that protects both streams.
This token can be used for defining group semantics inside an SDP
m-line. There is also a need for a token that will allow the offerer
to correlate a received RTP stream to the application logic before
receiving the answer from the remote side.
In order to address these requirements this document defines an SDP
token attribute appId that provides a level of indirection for the
binding. The actual binding is done in RTP by associating the appId
with an SSRC using a new RTCP SDES message and a new RTP header
extension that define the mapping from appId to a specific SSRC.
Having the binding in RTP/RTCP allows the RTP layer to change the
SSRC of a media stream by sending a new binding message (SDES an RTP
header extension) without a need to have an SDP level offer/answer
exchange.
The document also defines an appId-group attribute that has similar
semantics to SSRC-group but uses the appId instead of SSRC to specify
the different RTP streams in the group.
For the case when the offerer receives an RTP stream before the SDP
answer, we define a new optional attribute recv-appId to be used for
correlating this received RTP stream.
3.1. appId token
AppId is a general-purpose token associated with an RTP stream,
allowing the binding of the SDP representation to an SSRC. This
allows the semantics of the stream with the token to be defined by
the application and mapped to an RTP stream without having to know
its SSRC in the application.
The token is chosen by the sender, and represents the RTP stream that
will be sent to the receiver.
The proposed token can be sent using SDP, RTCP SDES messages
[RFC3550], or an RTP header extension [RFC5285]
The SSRC mapping may be available to the receiver when receiving the
RTP stream through the RTP header extension, but may also be
available ahead of time via an RTCP SDES message conveyed before the
source starts sending, even if the receiver has not seen any RTP
packets from this source (as in a multipoint conference).
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The receiver can receive new sources that may be of two kinds.
o A new RTP stream replacing an existing RTP stream, in which case
the AppId of the replaced RTP stream will be assigned to the new
SSRC.
o A new RTP stream requiring a different AppId, for example, when
adding a presentation stream to an existing call with two video
cameras from a room.
The solution supports an RTP session as described using SDP. The RTP
session may use Bundle [I-D.ietf-mmusic-sdp-bundle-negotiation] with
more than one m-line. In this case, if the SSRCs of all RTP streams
are not known in advance, the AppIds associated with each m-line need
to be available to the media receiver in order to map each SSRC to a
specific m-line configuration. The appIds MUST be unique in an SDP
session.
Editor Note (is this required?): It is preferable that they will be
unique in an RTP session which is not a problem in a point to point
call or in a multipoint conference with a central signaling point.
The document defines a new SDP media level attribute a=appId that can
be used to list all the appIds that an application may use.
The appId syntax provides a token identifier. Each value of the
AppId maps to one SSRC at a time. When a new SSRC is mapped to an
existing AppId using an RTP header extension or SDES message, it
replaces the previous RTP stream for this application usage.
The definition is
a=appId:token
a=appId:token <attribute>
a=appId:token <attribute>:<value>
The formal representation of the appId token is:
appId-attribute = "appId:" token *[WSP attribute]
attribute =/ appId-attr
; The base definition of "attribute" is in [RFC4566].
; (It is the content of "a=" lines.)
; WSP and DIGIT defined in [RFC5234]
; token defined in [RFC4566]
Examples:
The SDP offer specifies an appId that will be used for mapping to
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specific SSRCs. The example shows two RTP streams having different
content [RFC4796] with the same payload type number. The appId can
be used to identify the content of the RTP stream.
a=group:BUNDLE m1 m2
m=video 49200 RTP/AVP 99
a=rtpmap:99 H264/90000
a=mid:m1
a=content:main
a=appId:2
m=video 49200 RTP/AVP 99
a=rtpmap:99 H264/90000
a=mid:m2
a=content:alt
a=appId:3
The second example is when the application usage of the RTP stream is
specified using SDP to specify different content [RFC4796] , and each
RTP stream has a Retransmission stream. The media receiver can map
the received SSRC of the RTP stream or the retransmission to the
specific content based on the appId.
a=group:BUNDLE m1 m2
m=video 49200 RTP/AVP 97,98
a=rtpmap:98 H264/90000
a=mid:m1
a=content:main
a=rtpmap:97 rtx/90000
a=fmtp:97 apt=98;rtx-time=3000
a=appId:2
a=appId:3
m=video 49200 RTP/AVP 97,98
a=rtpmap:98 H264/90000
a=mid:m2
a=content:alt
a=rtpmap:97 rtx/90000
a=fmtp:97 apt=98;rtx-time=3000
a=appId:4
a=appId:5
3.1.1. RTCP SDES message
This document specifies a new RTCP SDES message
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AppId = XXX | length |AppId token
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ....
This AppId is the same token as defined in the new SDP attribute and
is also used in the RTP header extension.
This SDES message MAY be sent in a compound RTCP packet based on the
application need.
Editor Note: Need guidance on how often the SDES message should be
sent.
3.1.2. RTP Header Extension
The Application ID is carried within the RTP header extension field,
using [RFC5285] two bytes header extension.
Support is negotiated within the SDP, i.e.
a=extmap:1 urn:ietf:params:rtp-hdrext:App-ID
Packets tagged by the sender with the AppId then contain a header
extension as shown below
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=1 | Len-1 | AppId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AppID .............. |
+-+-+-+-+-+-+-+-+
To add or modify the AppId by an intermediary can be an expensive
operation, particularly if SRTP is used to authenticate the packet.
Modification to the contents of the RTP header requires a re-
authentication of the complete packet, and this could prove to be a
limiting factor in the throughput of a multipoint device.
There is no need to send the AppId header extension with all RTP
packets. Senders MAY choose to send it only when a new SSRC is sent,
or when an SSRC changes its association to an AppId. If such a mode
is being used, the header extension SHOULD be sent in the first few
RTP packets to reduce the risk of losing it due to packet loss. For
codecs with decoder refresh points (such as I-Frames in video
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codecs), senders also SHOULD send the AppId header extension along
with the packets carrying the decoder refresh.
3.1.3. recv-appId
An offer may include a recv-appId attribute allowing the offerer to
request from the answerer to use this token for the RTP stream sent
from the answerer for a sendrecv or recvonly RTP stream. This is
important in order to support early media from the answerer that may
be received by the offerer before the answer SDP arrives.
The formal representation of the appId token is:
appId-attribute = "recv-appId:" token
; The base definition of "attribute" is in [RFC4566].
; (It is the content of "a=" lines.)
3.2. The "appId-group" media attribute
a=appId-group:<semantics> <appId> ...
The SDP media attribute "appId-group" expresses a relationship among
several media sources specified in the same SDP m-line. It is
analogous to the "group" session-level attribute [RFC3388], which
expresses a relationship among media streams in an SDP multimedia
session (i.e., a relationship among several logically related RTP
sessions). As media sources are already identified by their appId,
no analogous property to the "mid" attribute is necessary.
Editor note: Since the appId is unique in an SDP session the app-Id
group can be used also at the session level - do we want it?
The <semantics> parameter is taken from the specification of the
"group" attribute [RFC3388]. The initial semantic values defined for
the "appId-group" attribute are FID (Flow Identification) [RFC3388]
and FEC (Forward Error Correction) [RFC4756]. In each case, the
relationship among the grouped sources is the same as the
relationship among corresponding sources in media streams grouped
using the SDP "group" attribute.
Though the "appId-group" semantic values follow the same syntax as
"group" semantic values, they are defined independently. All "appId-
group" semantic values MUST be registered with IANA, using the
registry defined in Section 6
The "appId-group" attribute indicates the sources in a group by
listing the appIds of the sources in the group. It MUST list at
least one appId for a group and MAY list any number of additional
ones. Every appId listed in an "appId-group" attribute MUST be
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defined by a corresponding "appId" line in the same media
description.
3.3. appId attributes
3.3.1. The "pt" appId attribute
The SDP offer example in the introduction demonstrated that when
there are multiple RTP streams in the offer each have a different pt
number it it is not clear which SSRC specified using a=ssrc: is
correlated to each of the rtpmap lines. In order to provide the
mapping we define an appId attribute "pt".
a=appId:token pt:value
appId-attrib = "pt:" pt-value
pt-value = 1*3DIGIT
4. Using Application ID token in Offer / Answer
The appId may be used in offer answer. Some use cases are provided.
They only show part of the SDP that can demonstrate the usage.
A simple case is when each SDP m-line describes one RTP stream and
the m-lines are bundled . The recv-appId is offered so when the
offerer sees an RTP stream with appId token value 10 it knows it is
the main video.
The offer is:
a=group:BUNDLE m1 m2
m=video 49200 RTP/AVP 98
a=rtpmap:98 H264/90000
a=mid:m1
a=content:main
a=appId:2
a=recv-appId:10
m=video 49200 RTP/AVP 99
a=rtpmap:99 H264/90000
a=mid:m2
a=content:alt
a=appId:3
a=recv-appId:20
A second example is using the same case as in section one (SVC with
FEC) This example shows how to use the appId optional pt parameter to
map to a specific stream description.
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v=0
o=- 20518 0 IN IP4 198.51.100.1
s=FEC Grouping Semantics for SSRC Multiplexing
t=0 0
c=IN IP4 203.0.113.1
a=group:BUNDLE m1 m2
m=audio 56600 RTP/SAVPF 0 109
a=mid:m1
a=rtpmap:0 PCMU/8000
a=rtpmap:109 opus/48000
a=appId:10
m=video 56602 RTP/AVPF 100 101 110 111 - Main camera
a=mid:m2
a=rtpmap:100 H264/90000 - Base layer
a=rtpmap:101 H264-SVC/90000 - Enhancement layer.
a=depend:101 lay L1:100 - dependencies
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=rtpmap:111 1d-interleaved-parityfec/90000
a=fmtp:111 L=10; D=10; repair-window=400000
a=appId:1000 pt=100
a=appId:1010 pt=101
a=appId:2110 pt=110
a=appId:2120 pt=111
a=appId-group:FEC-FR 1000 2110
a=appId-group:FEC-FR 1000 1010 2120
a=appId-group:DDP 1000 1010
5. Acknowledgements
Place Holder
6. IANA Considerations
TBD
7. Security Considerations
TBD.
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
8.2. Informative References
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
"A Taxonomy of Grouping Semantics and Mechanisms for Real-
Time Transport Protocol (RTP) Sources",
draft-ietf-avtext-rtp-grouping-taxonomy-00 (work in
progress), November 2013.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers",
draft-ietf-mmusic-sdp-bundle-negotiation-05 (work in
progress), October 2013.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-08 (work
in progress), September 2013.
[I-D.roach-mmusic-unified-plan]
Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for
Using SDP with Large Numbers of Media Flows",
draft-roach-mmusic-unified-plan-00 (work in progress),
July 2013.
[I-D.westerlund-avtcore-rtp-simulcast]
Westerlund, M., Lindqvist, M., and F. Jansson, "Using
Simulcast in RTP Sessions",
draft-westerlund-avtcore-rtp-simulcast-03 (work in
progress), October 2013.
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[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
[RFC4796] Hautakorpi, J. and G. Camarillo, "The Session Description
Protocol (SDP) Content Attribute", RFC 4796,
February 2007.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
Authors' Addresses
Roni Even
Huawei Technologies
Tel Aviv,
Israel
Email: roni.even@mail01.huawei.com
Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
Qin Wu
Huawei Technologies
Email: bill.wu@huawei.com
Even, et al. Expires July 7, 2014 [Page 14]