FECFRAME Working Group Rajiv Asati
Internet Draft Cisco Systems
Intended status: Standards Track
Expires: July 2010
February 24, 2010
Methods to convey FEC Framework Configuration Information
draft-ietf-fecframe-config-signaling-02.txt
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Abstract
FEC Framework document [FECARCH] defines the FEC Framework
Configuration Information necessary for the FEC framework operation.
This document describes how to use existing signaling protocols to
determine and dynamically communicate the Configuration information
between sender(s) and receiver(s).
Table of Contents
1. Introduction...................................................3
2. Terminology/Abbreviations......................................3
3. FEC Framework Configuration Information........................4
3.1. Encoding Format...........................................5
4. Signaling Protocol Usage.......................................6
4.1. Signaling Protocol for Multicasting.......................7
4.1.1. Sender Procedure.....................................9
4.1.2. Receiver Procedure..................................11
4.2. Signaling Protocol for Unicasting........................12
4.2.1. SIP.................................................13
4.2.2. RSTP................................................13
5. Security Considerations.......................................14
6. IANA Considerations...........................................14
7. Acknowledgments...............................................14
8. References....................................................16
8.1. Normative References.....................................16
8.2. Informative References...................................16
Author's Addresses...............................................17
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1. Introduction
FEC Framework document [FECARCH] defines the FEC Framework
Configuration Information that governs the overall FEC framework
operation common to any FEC scheme. This information MUST be
available at both sender and reciever(s).
This document describes how to use various signaling protocols to
communicate the Configuration information between sender and
receiver(s). The configuration information may be encoded in any
compatible format such as SDP [RFC4566], XML etc. A signaling
protocol could be utilised by any FEC scheme and/or any Content
Delivery Protocol (CDP).
This document doesn't describe any FEC scheme specific information
(FSSI) (for example, how source blocks are constructed) or any sender
or receiver side operation for a particular FEC scheme (for example,
whether the receiver makes use of one or more repair flows that are
received). Such FEC scheme specifics SHOULD be covered in separate
document(s). This document doesn't mandate a particular encoding
format for the configuration information either.
This document is structured such that Section 2 describes the terms
used in this document, section 3 describes the FEC Framework
Configuration Information, section 4 describes how to use signaling
protocol for the multicast applications, section 5 describes the
signaling protocol for the unicast applications, and section 6
describes security consideration.
2. Terminology/Abbreviations
This document makes use of the terms/abbreviations defined in the FEC
Framework document [FECARCH]. Additionally, it defines the following:
o Media Sender - Node performing the Media encoding and producing
the original media flow(s) to the 'FEC Sender'
o Media Receiver - Node performing the Media decoding;
o FEC Sender - Node performing the FEC encoding on the original
stream to produce the FEC stream(s)
o FEC Receiver - Node performing the FEC decoding, as needed, and
providing the original media flow(s) to the Media receiver.
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o Sender - Same as FEC Sender
o Receiver - Same as FEC Receiver
o (Media) Stream - A single media instance i.e., an audio stream or
a video stream.
This document deliberately refers to the 'FEC Sender' and 'FEC
Receiver' as the 'Sender' and 'Receiver' respectively.
3. FEC Framework Configuration Information
The FEC Framework [FECARCH] defines a minimum set of information that
MUST be communicated between the sender and receiver(s) for a proper
operation of an FEC scheme. This information is referred to as "FEC
Framework Configuration Information". This is the information that
the FEC Framework needs in order to apply FEC protection to the
transport flows.
A single instance of the FEC Framework provides FEC protection for
all packets of a specified set of source packet flows, by means of
one or more packet flows consisting of repair packets. As per the FEC
Framework document [FECARCH] section 6.5, the FEC Framework
Configuration Information includes the following for each FEC
Framework instance:
1. Identification of Source Flow(s)
2. Identification of the repair flow(s)
3. Identification of FEC Scheme
4. Length of Source FEC payload ID
5. FEC Scheme Specific Information (FSSI)
FSSI basically provides an opaque container to encode FEC scheme
specific configuration information such as buffer size, decoding
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wait-time etc. Please refer to the FEC Framework document [FECARCH]
for more details.
The usage of signaling protocols described in this document requires
that the application layer responsible for the FEC Framework instance
provide the value for each of the configuration information parameter
(listed above) encoded as per the chosen encoding format. Failure to
receive the complete information, the signaling protocol module MUST
return an error for the Operation, Administration and Maintenance
(OAM) purposes and optionally convey to the application layer. Please
refer to the figure 1 of the FEC Framework document [FECARCH] for
further illustration.
This document does not make any assumption that the 'FEC sender' and
'Media Source/Sender' functionalities are implemented on the same
device, though that may be the case. Similarly, this document does
not make any assumption that 'FEC receiver' and 'Media Receiver'
functionalities are implemented on the same device, though that may
be the case.
3.1. Encoding Format
The FEC Framework Configuration Information (listed above in section
3) may be encoded in any format such as SDP, XML etc. as chosen or
prefered by a particular FEC Framework instance. The selection of
such encoding format or syntax is independent of the signaling
protocol and beyond the scope of this document.
Whatever encoding format is selected for a particular FEC framework
instance, it MUST be known to the signaling protocol. This is to
provide a means (e.g. a field such as Payload Type) in the signaling
protocol message(s) to convey the chosen encoding format for the
configuration information so that the Payload i.e., configuration
information can be correctly parsed as per the semantics of the
chosen encoding format at the receiver. Please note that the encoding
format is not a negotiated parameter, but rather a property of a
particular FEC Framework instance and/or its implementation.
Additionally, the encoding format for each FEC Framework
configuration parameter MUST be defined in terms of a sequence of
octets that can be embedded within the payload of the signaling
protocol message(s). The length of the encoding format MUST either
be fixed, or derived by examining the encoded octets themselves. For
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example, the initial octets may include some kind of length
indication.
Independent of what all encoding formats supported by an FEC scheme,
each instance of the FEC Framework MUST use a single encoding format
to describe e.g. encode all of the configuration information
associated with that instance. The signaling protocol specified in
this document SHOULD not validate the encoded information, though it
may validate the syntax or length of the encoded information.
The reader may refer to the SDP elements document [FECSDP], which
describes the usage of 'SDP' encoding format as an example encoding
format for FEC Framework Configuration Information.
4. Signaling Protocol Usage
FEC Framework [FECARCH] requires certain FEC Framework Configuration
Information to be available to both sender and receiver(s). This
configuration information is almost always formulated at the sender
(or on behalf of a sender), and somehow made available at the
receiver(s). While one may envision a static method to populate the
configuration information at both sender and receiver(s), it would
not be optimal since it would (i) require the knowledge of every
receiver in advance, (b) require the time and means to configure each
receiver and sender, and (c) increase the misconfiguration
possibility. Hence, there is a benefit in using a dynamic method
i.e., signaling protocol to convey the configuration information
between sender and one or more receivers.
Since the configuration information may be needed at a particular
receiver versus many receivers (depending on the multimedia stream
being unicast e.g. Video on Demand, or multicast e.g. Broadcast or
IPTV), we need two types of signaling protocols - one to deliver the
configuration information to many receivers via multicasting
(described in section 4.1), and the other to deliver the
configuration information to one and only one receiver via unicasting
(described in section 4.2).
Figure 1 below illustrates a sample topology showing the FEC sender
and FEC receiver (that may or may not be the Media Sender and Media
Receiver respectively) such that FEC_Sender1 is serving
FEC_Reciver11,12,13 via the multicast signaling protocol, whereas the
FEC_Sender2 is serving only FEC_Reciever2 via the unicast signaling
protocol.
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FEC_Sender2---------| |--------FEC_Receiver2
| |
FEC_Sender1-----IP/MPLS network
|-----------FEC_Receiver11
|-----------FEC_Receiver12
|-----------FEC_Receiver13
Figure 1 Topology using Sender and Receiver
The rest of the section continues to use the terms 'Sender' and
'Receiver' to refer to the 'FEC Sender' and 'FEC Receiver'
respectively.
4.1. Signaling Protocol for Multicasting
This specification describes using Session Announcement Protocol
(SAP) version 2 [RFC2974] as the signaling protocol to multicast the
configuration information from one sender to many receivers. The
apparent advantage is that the server doesn't need to maintain any
state for any receiver using SAP.
At the high level, a sender, acting as the SAP announcer, signals the
FEC Framework Configuration Information for each FEC Framework
instance available at the sender, using the SAP message(s). The
configuration information, encoded in a suitable format as per the
section 3.1, is carried in the Payload of the SAP message(s). A
receiver, acting as the SAP listener, listens on a well known UDP
port and at least one well known multicast group IP address (as
explained in the section 4.1.1). This enables the receiver to receive
the SAP message(s) and obtains the FEC Framework Configuration
Information for each FEC Framework Instance.
One may refer to 'Requirements for IP Multicast Session Announcement
in the Internet' document [SAP-REQ] to know about the SAP
limitations.
Using the configuration information, the receiver becomes aware of
available FEC protection options, corresponding multicast trees (S,G
or *,G addresses) etc. The receiver may subsequently subscribe to one
or more multicast trees to receive the FEC streams using out-of-band
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multicasting techniques such as PIM [RFC4601]. This, however, is
outside the scope of this document.
SAP message is carried over UDP over IP. The destination UDP port
MUST be 9875 and source UDP port may be any available number. The SAP
message(s) SHOULD contain an authentication header and MAY employ
cryptography. One cryptography method suggested by this specification
is the usage of Group Cryptography as specified in GDOI [RFC3547].
Figure 2 below illustrates the SAP packet format (it is reprinted
from the RFC2974) -
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=1 |A|R|T|E|C| auth len | msg id hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: originating source (32 or 128 bits) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional authentication data |
: .... :
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
| optional payload type |
+ +-+- - - - - - - - - -+
| |0| |
+ - - - - - - - - - - - - - - - - - - - - +-+ |
| |
: payload :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 SAP Message format
While the RFC2974 includes explanation for each field, it is worth
discussing the 'Payload' and 'Payload Type' fields. The 'Payload'
field MUST be used to carry the FEC Framework Configuration
Information. Subsequently, the optional 'Payload Type' field, which
is a MIME content type specifier, MUST describe the encoding format
used to encode the Payload. For example, the 'Payload Type' field may
be application/sdp if the FEC Framework Configuration Information is
encoded in SDP format and carried in the SAP payload. Similarly, it
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would be application/xml if the FEC Framework Configuration
Information was encoded in XML format.
4.1.1. Sender Procedure
The sender signals the FEC framework configuration for each FEC
framework instance in a periodic SAP announcement message [RFC2974].
The SAP announcement message is sent to a well known multicast IP
address and UDP port, as specified in [RFC2974]. The announcement is
multicast with the same scope as the session being announced.
The SAP module at the sender obtains the FEC Framework Configuration
Information per Instance from the 'FEC Framework' module and places
that in the SAP payload accordingly. A single SAP (announcement)
message MUST carry the FEC Framework Configuration Information for a
single FEC Framework Instance. The SAP message is then sent over UDP
over IP.
While it is possible to aggregate multiple SAP (announcement)
messages in a single UDP datagram as long as the resulting UDP
datagram length is less than the IP MTU of the outgoing interface,
this specification does not recommend it since there is no length
field in the SAP header to identify SAP message boundary. Hence,
this specification recommends single SAP announcement message to be
sent in a UDP datagram.
The IP packet carrying the SAP message MUST be sent to destination IP
address of one of the following depending on the selected scope:
- 224.2.127.254 (if IPv4 global scope 224.0.1.0-238.255.255.255
is selected for the FEC stream), or
- FF0X:0:0:0:0:0:2:7FFE (if IPv6 multicasting is selected for the
FEC stream, where X is the 4-bit scope value), or
- the highest multicast address (239.255.255.255, for example) in
the relevant administrative scope zone (if IPv4 administrative
scope 239.0.0.0-239.255.255.255 is selected for the FEC stream)
The destination UDP port MUST be 9875 and source UDP port may be any
available number. The default IP TTL value (or Hop Limit value)
SHOULD be 255, though the implementation may allow it to be any other
value to implicitly create the multicast boundary for SAP
announcements. The IP DSCP field may be set to any value that
indicates a desired QoS treatment in the IP network.
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The IP packet carrying the SAP message MUST be sent with source IP
address that is reachable by the receiver. The sender may assign the
same IP address in the "originating source" field of the SAP message,
as the one used in the source IP address of the IP packet.
Furthermore, the FEC Framework Configuration Information MUST NOT
include any of the reserved multicast group IP addresses for the FEC
streams (i.e., source or repair flows), though it may use the same IP
address as the 'originating source' address to identify the FEC
streams (i.e., source or repair flows). Please refer to IANA
assignments for multicast addresses.
The sender MUST periodically send the 'SAP announcement' message to
ensure that the receiver doesn't purge the cached entry(s) from the
database and doesn't trigger the deletion of FEC Framework
Configuration Information.
Please note that the deletion of FEC Framework Configuration
Information SHOULD not mean that the receiver stops its FEC
processing for the instance for which it had already got the
configuration information.
While the time interval between repetitions of an announcement can be
calculated as per the very sophisticated but complex method explained
in [RFC2974], the preferred and simpler method recommended by this
specification is to let the user specify the time interval from the
range of 1-200 seconds with suggested default being 60 seconds. The
time interval MUST be chosen to ensure that SAP announcement message
is sent out before the corresponding multicast routing entry e.g.
(S,G) or (*,G) (corresponding to the SAP multicast tree(s)) on the
router doesn't time out. (It is worth noting that the default time-
out period for the multicast routing entry is 210 seconds, per the
PIM specification [RFC4601], though the time-out value may be set to
another value as allowed by the implementation.)
The SAP implementation MAY also support the complex method for
determining the SAP announcement time interval, and provide the
option to select it over the simpler method.
When simpler method is used, the 'time interval' may be signaled in
the SAP message payload e.g. within the FEC Framework Configuration
Information.
Note that SAP doesn't allow the time-interval to be signaled in the
SAP header. Hence, the usage of simpler method desires the time-
interval to be included in the FEC Framework Configuration
Information, if the default time interval (=60 seconds) for SAP
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message repeations is not deemed enough. For example, the usage of
"r=" (repeat time) field in SDP to convey the time-interval value,
if SDP encoding format is used.
The sender may choose to delete the announced FEC Framework
Configuration Information by sending a 'SAP deletion' message. This
deletion may be useful if the sender no longer desired to send any
FEC streams. If the sender needs to modify the announced FEC
Framework Configuration Information for one or more FEC instances,
then the sender MUST send a new announcement message with a different
'Message Identifier Hash' value as per the rules described in section
5 of RFC2974 [RFC2974]. Such announcement message SHOULD be sent
immediately (without having to wait for the time-interval) to ensure
that the modifications are received by the receiver as soon as
possible. The sender MUST send the SAP deletion message to delete the
previous SAP announcement message (i.e., with the previous 'Message
Identifier Hash' value).
4.1.2. Receiver Procedure
The receiver MUST listen on UDP port 9875 for packets arriving with
IP destination address of either 224.2.127.254 (if IPv4 global scope
session is used for the FEC stream), or FF0X:0:0:0:0:0:2:7FFE (if
IPv6 is selected, where X is the 4-bit scope value), or the highest
IP address (239.255.255.255, for example) in the relevant
administrative scope zone (if IPv4 administrative scope 239.0.0.0-
239.255.255.255 is selected for the FEC stream). These IP addresses
are mandated for SAP usage by RFC2974 [RFC2974].
The receiver, upon receiving a SAP announcement message, creates an
entry, if it doesn't already exist, in a local database and passes
the FEC Framework Configuration Information from the SAP Payload
field to the 'FEC Framework' module. Each entry also maintains a
time-out value, which is (re)set to the five times the time-interval
value, which is either the default = 60 seconds, or the value
signaled by the sender.
Note that SAP doesn't allow the time-interval to be signaled in the
SAP header. Hence, the time-interval SHOULD be included in the FEC
Framework Configuration Information. For example, the usage of "r="
(repeat time) field in SDP to convey the time-interval value, if
SDP encoding format is used.
The time-out value associated with each entry is reset when the
corresponding annoucement (please see section 5 of [RFC2974]) is
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received. If the time-out value for any entry reaches zero, then the
entry is deleted from the database.
The receiver, upon receiving a SAP delete message, MUST delete the
matching SAP entry in its database. This SHOULD result in the
receiver no longer using the relevant FEC Framework Configuration
Information for the corresponding instance, and SHOULD no longer
subscribe to any related FEC streams.
4.2. Signaling Protocol for Unicasting
This document describes leveraging any signaling protocol that is
already used by the unicast application, for exchanging the FEC
Framework Configuration Information between two nodes.
For example, a multimedia (VoD) client may send a request via
unicasting for a particular content to the multimedia (VoD) server,
which may offer various options such as encodings, bitrates,
transport etc. for the content. The client selects the suitable
options and answers to the server, paving the way for the content to
be unicast on the chosen transport from server to the client. This
offer/answer signaling, described in [RFC3264], is commonly utilized
by many application protocols such as SIP, RTSP etc.
The fact that two nodes desiring unicast communication almost always
rely on an application to first exchange the application related
parameters via the signaling protocol, it is logical to enhance such
signaling protocol(s) to (a) convey the desire for the FEC protection
and (b) subsequently also exchange FEC parameters i.e., FEC Framework
Configuration Information. This enables the node acting as the
offerer to offer 'FEC Framework Configuration Information' for each
of available FEC instances, and the node acting as the answerer
conveying the chosen FEC Framework instance(s) to the offerer. The
usage of FEC framework instance is explained the FEC Framework
document [FECARCH].
While enhancing an application's signaling protocol to exchange FEC
parameters is one method (briefly explained above), another method
would be to have a unicast based generic protocol that could be used
by two nodes independent of the application's signaling protocol. The
latter method is under investigation and may be covered by a separate
document.
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The remainder of this section provides example signaling protocols
and explains how they can be used to exchange FEC Framework
Configuration Information.
4.2.1. SIP
SIP [RFC3261] is an application-level signaling protocol to create,
modify, and terminate multimedia sessions with one or more
participants. SIP also enables the participants to discover one
another and to agree on a characterization of a multimedia session
they would like to share. SIP runs on either TCP or UDP or SCTP
transport, and uses SDP as the encoding format to describe multmedia
session attributes.
SIP already uses an offer/answer model with SDP, described in
[RFC3264], to exchange the information between two nodes to establish
unicast sessions between them. This document extends the usage of
this model for exchanging the FEC Framework Configuration
Information, explained in section 3. Any SDP specific enhancements to
accommodate the FEC Framework are covered in the SDP Elements
specification [FECSDP].
4.2.2. RSTP
RTSP [RFC2326] is an application-level signaling protocol for control
over the delivery of data with real-time properties. RTSP provides an
extensible framework to enable controlled, on-demand delivery of
real-time data, such as audio and video. RTSP runs on either TCP or
UDP transports.
RTSP already provides an ability to extend the existing method with
new parameters. This specification defines 'FEC Protection Required'
option-tag (please see section 6 for IANA Considerations) and
prescribes including it in the Require (or Proxy-Require) header of
SETUP (method) request message, so as to request for FEC protection
for the data.
The node receiving such request either responds with "200 OK" message
that includes offers i.e., available FEC options (e.g. FEC Framework
Configuration Information for each Instnace) or "551 Option not
supported" message. A sample of related message exchange is shown
below -
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Node1->Node2: SETUP < ... > RTSP/1.0
CSeq: 1
Transport: <omitted for simplicity>
Require: FECprotectionRequired
Node2->Node1: RTSP/1.0 200 OK
CSeq: 1
Transport: <omitted for simplicity>
The requesting node (Node1) may then send a new SETUP message to
convey the selected FEC protection to Node2, and proceed with regular
RTSP messaging.
Suffice to say, if the requesting node (Node1) received '551 Option
not supported' response from Node2, then the requesting node (Node1)
may send the SETUP message without using the Require header.
5. Security Considerations
There is no additional security consideration other than what's
already covered in [RFC2974] for SAP, [RFC2326] for RTSP, and
[RFC3261] for SIP.
6. IANA Considerations
This document requests IANA to register a new option-tag for FEC
protection required, as described in section 4.2.2, and provides the
following information in compliance with section 3.8.1 in [RFC2326]:
. Name of option = FECprotectionRequired
. Change of Control = IETF
7. Acknowledgments
Thanks to Colin Perkins for pointing out the issue with the time-
interval for the SAP messages. Additionally, thanks to Mark Watson,
Ali Begen and Ulas Kozat for solidifying this proposal.
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This document was prepared using 2-Word-v2.0.template.dot.
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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.
[FECARCH] Watson, M., "Forward Error Correction (FEC) Framework",
draft-ietf-fecframe-framework-05 (work in progress), Jan
2010.
[FECSDP] Begen, A., "SDP Elements for FEC Framework", draft-ietf-
fecframe-sdp-elements-04 (work in progress), Feb 2010.
[RFC2974] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
8.2. Informative References
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC2326] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC3261] Handley, M., Schulzrinne, H., Schooler, E. and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4601] Fenner, etc., "Protocol Independent Multicast - Sparse Mode
(PIM-SM): Protocol Specification", RFC 4601, August 2006.
[RFC3547] Baugher, etc., "The Group Domain of Interpretation", RFC
3547, July 2003.
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[SAP-REQ] Asaeda, etc., "Requirements for IP Multicast Session
Announcement in the Internet, draft-ietf-mboned-session-
announcement-req-02, April 2010.
Author's Addresses
Rajiv Asati
Cisco Systems,
7025-6 Kit Creek Rd, RTP, NC, 27709-4987
Email: rajiva@cisco.com
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