Internet Engineering Task Force (IETF) R. Ravindranath

Internet Engineering Task Force (IETF) R. Ravindranath
Request for Comments: 7879 T. Reddy
Category: Standards Track G. Salgueiro
ISSN: 2070-1721 Cisco
V. Pascual
Oracle
P. Ravindran
Nokia Networks
May 2016

DTLS-SRTP Handling in SIP Back-to-Back User Agents

Abstract

Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
exist on the signaling and media paths between the endpoints. This
document describes the behavior of B2BUAs when Secure Real-time
Transport (SRTP) security context is set up with the Datagram
Transport Layer Security (DTLS) 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 7841.

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/rfc7879.

Ravindranath, et al. Standards Track [Page 1]

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Copyright Notice

Copyright (c) 2016 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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Goals and Scope of this Document . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP . . . . . . . 5
4. Signaling-Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . 5
4.1. Proxy-B2BUAs . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Signaling-Only and SDP-Modifying Signaling-Only B2BUAs . 6
5. Media-Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . . . 6
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.1. Media Relay . . . . . . . . . . . . . . . . . . . . . 6
5.1.2. RTP- and RTCP-Aware Media-Aware B2BUA . . . . . . . . 8
6. Forking Considerations . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13

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1. Introduction

1.1. Overview

[RFC5763] describes how the Session Initiation Protocol (SIP)
[RFC3261] can be used to establish a Secure Real-time Transport
Protocol (SRTP) [RFC3711] security context with the Datagram
Transport Layer Security (DTLS) protocol [RFC6347]. It describes a
mechanism for transporting a certificate fingerprint using the
Session Description Protocol (SDP) [RFC4566]. The fingerprint
identifies the certificate that will be presented during the DTLS
handshake. DTLS-SRTP is currently defined for point-to-point media
sessions, in which there are exactly two participants. Each DTLS-
SRTP session (described in Section 3 of [RFC5764]) contains a single
DTLS connection (if RTP and RTP Control Protocol (RTCP) are
multiplexed) or two DTLS connections (if RTP and RTCP are not
multiplexed), and either two SRTP contexts (if media traffic is
flowing in both directions on the same 5-tuple) or one SRTP context
(if media traffic is only flowing in one direction).

In many SIP deployments, SIP Back-to-Back User Agents (B2BUA)
entities exist on the SIP-signaling path between the endpoints. As
described in [RFC7092], these B2BUAs can modify SIP and SDP
information. For example, as described in Section 3.1.3 of
[RFC7092], SDP-modifying signaling-only B2BUAs can potentially modify
the SDP. B2BUAs can also be present on the media path, in which case
they modify parts of the SDP information (like IP address, port) and
subsequently modify the RTP headers as well. Such B2BUAs are
referred to as "media-plane B2BUAs". [RFC7092] describes two
different categories of media-plane B2BUAs, according to the level of
activities performed on the media plane.

When B2BUAs are present in a call between two SIP User Agents (UAs),
they often make end-to-end DTLS-SRTP sessions impossible. An "end-
to-end DTLS-SRTP session" means that man-in-the-middle devices cannot
break the DTLS-SRTP session between the endpoints. In other words,
the man-in-the-middle device cannot create a separate DTLS-SRTP
session between the client and the middle device on one side, and the
middle device and the remote peer on the other side. B2BUAs may be
deployed for address hiding or media latching [RFC7362], although
Traversal Using Relays around NAT (TURN) and Interactive Connectivity
Establishment (ICE) are expected to be used more often for this
purpose as it provides better security properties. Such B2BUAs are
able to perform their functions without requiring termination of
DTLS-SRTP sessions, i.e., these B2BUAs need not act as DTLS proxy and
decrypt the RTP payload.

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1.2. Goals and Scope of this Document

A B2BUA could be deployed for address hiding or media latching as
described in [RFC7362]. Such B2BUAs only terminate the media plane
at the IP and transport (UDP/TCP) layers and may inspect the RTP
headers or RTP Control Protocol (RTCP) packets. The goal of this
specification is to provide guidance on how such B2BUAs function
without breaking the end-to-end DTLS-SRTP session. A B2BUA could
also terminate the media, or modify the RTP headers or RTP Control
Protocol (RTCP) packets. Such B2BUAs will not allow end-to-end DTLS-
SRTP. The recommendations made in this document are not expected to
be applied by B2BUAs terminating DTLS-SRTP sessions given deployment
reality.

This specification assumes that a B2BUA is not providing identity
assurance and is not authorized to terminate the DTLS-SRTP session.
A B2BUA that provides identity assurance on behalf of endpoints
behind it can modify any portion of SIP and SDP before it generates
the identity signature. As the B2BUA is generating the identity
signature, it is not possible to detect if a B2BUA has terminated the
DTLS-SRTP session. B2BUAs providing identity assurance and
terminating DTLS-SRTP sessions are out of scope of this document.

The following sections describe the behavior B2BUAs can follow to
avoid breaking end-to-end DTLS-SRTP sessions.

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].

Transport Address: The combination of an IP address and port number.

The following generalized terms are defined in [RFC3261], Section 6.

B2BUA: A SIP Back-to-Back User Agent, which is the logical
combination of a User Agent Server (UAS) and a User Agent Client
(UAC).

UAS: A SIP User Agent Server.

UAC: A SIP User Agent Client.

All of the pertinent B2BUA terminology and taxonomy used in this
document are based on [RFC7092].

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It is assumed the reader is already familiar with the fundamental
concepts of the RTP protocol [RFC3550] and its taxonomy [RFC7656], as
well as those of SRTP [RFC3711] and DTLS [RFC6347].

3. B2BUAs Procedures to Allow End-to-End DTLS-SRTP

A B2BUA MUST follow the rules mentioned below to allow end-to-end
DTLS-SRTP sessions.

1. B2BUAs MUST forward the certificate fingerprint and SDP setup
attribute it receives from one endpoint unmodified towards the
other endpoint and vice versa.

2. The enhancements described in [RFC4474] provide a means for
signing portions of SIP requests in order to provide identity
assurance and certificate pinning by providing an identity
signature over the SDP that carries the fingerprint of keying for
DTLS-SRTP [RFC5763]. B2BUAs can identify that the enhancements
in [RFC4474] are used for identity assurance if the SIP request
contains both Identity and Identity-Info headers. In cases where
endpoints use [RFC4474], B2BUAs MUST ensure that it does not
modify any of the information used to construct the identity
signature. This includes the entire SDP body and portions of the
SIP header as described in [RFC4474]. In this case, a B2BUA
cannot act as a media-relay B2BUA.

3. [SIP-ID] is introduced to overcome the limitations of [RFC4474]
(discussed in Section 1 of [SIP-ID]). Unlike [RFC4474], [SIP-ID]
does not generate an identity signature over material that
intermediaries in the field commonly alter. In this case, a
B2BUA can act as a media-relay B2BUA. B2BUAs can identify that
[SIP-ID] is used for identity assurance if the SIP request
contains an Identity header but does not include an Identity-Info
header. The Identity-Info header is deprecated in [SIP-ID]. A
B2BUA MUST ensure that it does not modify any of the headers used
to construct the identity signature.

4. Both media relays and media-aware relays MUST NOT modify the
authenticated portion of RTP and RTCP packets, and MUST NOT
modify the authentication tag in the RTP and RTCP packets.

4. Signaling-Plane B2BUA Handling of DTLS-SRTP

Section 3.1 of [RFC7092] describes different categories of signaling-
plane B2BUAs. This section explains how these B2BUAs are expected to
comply with the recommendations in Section 3.

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4.1. Proxy-B2BUAs

Proxy-B2BUAs, as defined in Section 3.1.1 of [RFC7092], modify only
the Via and Record-Route SIP headers. These B2BUAs can continue to
perform their function and still allow end-to-end DTLS-SRTP sessions
since none of the headers used to construct the identity signature
are modified.

4.2. Signaling-Only and SDP-Modifying Signaling-Only B2BUAs

These categories of B2BUAs are likely to modify headers that are used
to construct the identity signature. For example, a signaling-only
B2BUA can modify the Contact URI. Such B2BUAs are likely to violate
rule 2 or rule 3 in Section 3. Depending upon the application
requirements, such a B2BUA may be able to limit modification of
header fields to those allowed to be modified by [RFC4474] or
[SIP-ID].

5. Media-Plane B2BUA Handling of DTLS-SRTP

5.1. General

This section describes how the different types of media-plane B2BUAs
defined in [RFC7092] are expected to comply with the recommendations
in Section 3.

5.1.1. Media Relay

From an application-layer point of view, a media relay (as defined in
Section 3.2.1 of [RFC7092]) forwards all packets it receives on a
negotiated connection, without inspecting or modifying the packet
contents. A media relay only modifies the transport layer (UDP/TCP)
and IP headers.

A media-relay B2BUA follows rule 1 mentioned in Section 3 and
forwards the certificate fingerprint and SDP setup attribute it
receives from one endpoint unmodified towards the other endpoint and
vice versa. The following example shows a SIP call establishment
flow, with both SIP endpoints (user agents) using DTLS-SRTP, and a
media-relay B2BUA.

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+-------+ +-------------------+ +-----+
| Alice | | Media-Relay B2BUA | | Bob |
+-------+ +-------------------+ +-----+
|(1) INVITE | (3) INVITE |
| a=setup:actpass | a=setup:actpass |
| a=fingerprint1 | a=fingerprint1 |
| (Alice's IP/port) | (B2BUAs IP/port) |
|------------------------>|-------------------------->|
| | |
| (2) 100 trying | |
|<------------------------| |
| | (4) 100 trying |
| |<--------------------------|
| | |
| | (5) 200 OK |
| | a=setup:active |
| | a=fingerprint2 |
| | (Bob's IP/port) |
|<------------------------|<--------------------------|
| (6) 200 OK | |
| a=setup:active | |
| a=fingerprint2 | |
| B2BUAs IP/port | |
| (7, 8) ClientHello + use_srtp |
|<----------------------------------------------------|
|(B2BUA changes transport(UDP/TCP) and IP header) |
| | |
| | |
| (9,10) ServerHello + use_srtp |
|---------------------------------------------------->|
|(B2BUA changes transport(UDP/TCP) and IP header) |
| | |
| | |
| (11) | |
| [Certificate exchange between Alice and Bob over |
| DTLS ] | |
| | |
| (12) | |
|<---------SRTP/SRTCP-----------SRTP/SRTCP----------->|
| [B2BUA changes transport(UDP/TCP) and IP headers] |

Figure 1: INVITE with SDP Call Flow for Media-Relay B2BUA

Note: For brevity, the entire value of the SDP fingerprint attribute
is not shown. The example here shows only one DTLS connection for
the sake of simplicity. In reality, depending on whether the RTP and
RTCP flows are multiplexed or demultiplexed, there will be one or two
DTLS connections.

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RFC 7879 DTLS-SRTP Handling in SIP B2BUA May 2016

If RTP and RTCP traffic is multiplexed on a single port as described
in [RFC5761], then only a single DTLS connection is required between
the peers. If RTP and RTCP are not multiplexed, then the peers would
have to establish two DTLS connections. In this case, after
receiving an INVITE request, Bob triggers the establishment of a DTLS
connection. Note that the DTLS handshake and the sending of the
INVITE response can happen in parallel; thus, the B2BUA has to be
prepared to receive DTLS, Session Traversal Utilities for NAT (STUN),
and media on the ports it advertised to Bob in the SDP offer before
it receives an SDP answer from Bob. Since a media-relay B2BUA does
not differentiate between a DTLS message, RTP, or any packet it
receives, it only changes the transport layer (UDP/TCP) and IP
headers and forwards the packet towards the other endpoint. The
B2BUA cannot decrypt the RTP payload, as the payload is encrypted
using the SRTP keys derived from the DTLS connection setup between
Alice and Bob.

If the endpoints use [RFC4474], a B2BUA cannot function as a media-
relay without violating rule 2 in Section 3. If [SIP-ID] is used, a
B2BUA can modify the IP address in the c= line and the port in the m=
line in the SDP as long as it does not otherwise violate rule 3 in
Section 3.

5.1.2. RTP- and RTCP-Aware Media-Aware B2BUA

Unlike the media relay discussed in Section 5.1.1, a media-aware
relay as defined in Section 3.2.2 of [RFC7092] is aware of the type
of media traffic it is receiving. There are two types of media-aware
relays, those that merely inspect the RTP headers and unencrypted
portions of RTCP packets, and those that inspect and modify the RTP
headers and unencrypted portions of RTCP packets.

5.1.2.1. RTP Header and RTCP Packets Inspection

An RTP-/RTCP-aware media relay does not modify the RTP headers and
RTCP packets but only inspects the packets. Such B2BUAs follow rule
4 in Section 3 and can continue to do their function while allowing
end-to-end DTLS-SRTP. Inspection by the B2BUA will not reveal the
clear-text for encrypted parts of the SRTP/SRTCP packets.

5.1.2.2. RTP Header and RTCP Packet Modification

A B2BUA cannot modify RTP headers or RTCP packets, as to do so it
would need to act as a DTLS endpoint, terminate the DTLS-SRTP
session, and decrypt/re-encrypt RTP packets. If a B2BUA modifies
unencrypted or encrypted portions of the RTP or RTCP packets, then
the integrity check will fail and the packet will be dropped by the
endpoint. The unencrypted and encrypted portions of the RTP or RTCP

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packets are integrity protected using the HMAC algorithm negotiated
during the DTLS handshake (discussed in Section 4.1.2 of [RFC5764]).
B2BUAs have to follow the rules in Section 3 to avoid breaking the
integrity of SRTP/SRTCP streams.

6. Forking Considerations

Due to forking [RFC3261], a SIP request carrying an SDP offer sent by
an endpoint (offerer) can reach multiple remote endpoints. As a
result, multiple DTLS-SRTP sessions can be established, one between
the endpoint that sent the SIP request and each of the remote
endpoints that received the request. B2BUAs have to follow rule 1 in
Section 3 while handling offer/answer and forward the certificate
fingerprints and SDP setup attributes it received in the SDP answer
from each endpoint (answerer) unmodified towards the offerer. Since
each DTLS connection is set up on a unique 5-tuple, B2BUA replaces
the answerer's transport addresses in each answer with its unique
transport addresses so that the offerer can establish a DTLS
connection with each answerer. The B2BUA, acting as a media relay
here, follows rule 4 mentioned in Section 3.

Bob (192.0.2.1:6666)
/
/
/ DTLS-SRTP=XXX
/
/
DTLS-SRTP=XXX v
<-----------> (192.0.2.3:7777)
Alice (192.0.2.0:5555) B2BUA
<-----------> (192.0.2.3:8888)
DTLS-SRTP=YYY ^
\
\ DTLS-SRTP=YYY
\
\
\
Charlie (192.0.2.2:6666)

Figure 2: B2BUA Handling Multiple Answers

For instance, as shown in Figure 2, Alice sends a request with an
offer and the request is forked. Alice receives answers from both
Bob and Charlie. The B2BUA advertises different B2BUA transport
addresses in each answer, as shown in Figure 2, where XXX and YYY
represent different DTLS-SRTP sessions. The B2BUA replaces Bob's
transport address (192.0.2.1:6666) in the answer with its transport
address (192.0.2.3:7777) and Charlie's transport address

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(192.0.2.2:6666) in the answer with its transport address
(192.0.2.3:8888). The B2BUA tracks the remote sources (Bob and
Charlie) and associates them to the local sources that are used to
send packets to Alice.

7. Security Considerations

This document describes the behavior B2BUAs must follow to avoid
breaking end-to-end DTLS-SRTP. Media relays that modify RTP or RTCP,
or modify SIP header fields or SDP fields that are protected by the
identity signature, are incompatible with end-to-end DTLS-SRTP. Such
relays are out of scope for this document. Security considerations
discussed in [RFC5763] are also applicable to this document. In
addition, the B2BUA behaviors outlined in this document do not impact
the security and integrity of a DTLS-SRTP session or the data
exchanged over it. A malicious B2BUA can try to break into the DTLS
connection, but such an attack can be prevented using the identity
validation mechanism discussed in [RFC4474] or [SIP-ID]. Either the
endpoints or the authentication service proxies involved in the call
can use the identity validation mechanisms discussed in [RFC4474] or
[SIP-ID] to validate the identity of peers and detect malicious
B2BUAs that can attempt to terminate the DTLS connection to decrypt
the RTP payload.

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RFC 7879 DTLS-SRTP Handling in SIP B2BUA May 2016

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.

[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>.

[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
<http://www.rfc-editor.org/info/rfc5764>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.

8.2. Informative References

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.

[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.

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RFC 7879 DTLS-SRTP Handling in SIP B2BUA May 2016

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.

[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.

[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<http://www.rfc-editor.org/info/rfc7092>.

[RFC7362] Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
Traversal (HNT) for Media in Real-Time Communication",
RFC 7362, DOI 10.17487/RFC7362, September 2014,
<http://www.rfc-editor.org/info/rfc7362>.

[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.

[SIP-ID] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", Work in Progress,
draft-ietf-stir-rfc4474bis-09, May 2016

Acknowledgments

Special thanks to Lorenzo Miniero, Ranjit Avarsala, Hadriel Kaplan,
Muthu Arul Mozhi, Paul Kyzivat, Peter Dawes, Brett Tate, Dan Wing,
Charles Eckel, Simon Perreault, Albrecht Schwarz, Jens Guballa,
Christer Holmberg, Colin Perkins, Ben Campbell, and Alissa Cooper for
their constructive comments, suggestions, and early reviews that were
critical to the formulation and refinement of this document. The
authors would also like to thank Dan Romascanu, Vijay K. Gurbani,
Francis Dupont, Paul Wouters, and Stephen Farrell for their review
and feedback of this document.

Contributors

Rajeev Seth provided substantial contributions to this document.

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Authors' Addresses

Ram Mohan Ravindranath
Cisco
Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
India

Email: [email protected]

Tirumaleswar Reddy
Cisco
Cessna Business Park
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India

Email: [email protected]

Gonzalo Salgueiro
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
United States

Email: [email protected]

Victor Pascual
Oracle
Barcelona, Spain

Email: [email protected]

Parthasarathi Ravindran
Nokia Networks
Bangalore, Karnataka
India

Email: [email protected]

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