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Versions: 00

MMUSIC                                                          T. Reddy
Internet-Draft                                        Muthu A M. Perumal
Intended status: Standards Track                 Ram Mohan. Ravindranath
Expires: April 18, 2013                                          D. Wing
                                                                   Cisco
                                                        October 15, 2012


                 STUN Extensions for Firewall Traversal
              draft-reddy-mmusic-stun-auth-fw-traversal-00

Abstract

   Some networks deploy firewalls configured to block UDP traffic.  When
   SIP user agents or WebRTC endpoints are deployed behind such
   firewalls, media cannot be sent over UDP across the firewall, but
   must be sent using TCP (which causes a different user experience) or
   through a session border controller.

   This draft describes an alternate model wherein extensions to ICE
   connectivity checks can be examined by the firewall to permit
   outgoing UDP flows across the firewall.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 April 18, 2013.

Copyright Notice

   Copyright (c) 2012 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



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   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.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Different Components and the Trust model . . . . . . . . .  6
   5.  Usage and Processing . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Generating  FW-FLOWDATA Attribute  . . . . . . . . . . . .  6
     5.2.  Sending FW-FLOWDATA Attribute in Binding Request . . . . .  7
     5.3.  Firewalls processing FW-FLOWDATA Attribute . . . . . . . .  7
   6.  STUN Attribute Format  . . . . . . . . . . . . . . . . . . . .  9
   7.  Key Provisioning . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     11.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12























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1.  Introduction

   To protect networks using real-time communications, firewalls or
   session border controllers are typically deployed.

   Firewalls include Application Layer Gateway functionality, which
   intercepts and analyzes the session signaling traffic such as the
   Session Initiation Protocol (SIP) traffic and creates dynamic mapping
   to permit the media traffic.  In particular, firewall extracts the
   media transport addresses, transport protocol and ports from the
   session description and creates dynamic mapping for media to flow
   through.  This model has the following problems:

   1.  It does not work if the session signaling is end-to-end encrypted
       (say, using TLS).

   2.  It does not work if a non-standard session signaling is used that
       the firewall does not understand.

   3.  It does not work if the session signaling and media traverse
       different firewalls.

   When an enterprise deploys WebRTC, the above problems are relevant
   because:

   1.  The session signaling between the WebRTC application running in
       the browser and the web server could be using TLS.

   2.  WebRTC does not enforce a particular session signaling protocol
       to be used.  So, the firewall may not be able to understand it.

   3.  This session signaling and the peer-to-peer media may traverse
       different firewalls.

   As a result the firewall may block ICE connectivity checks and media
   traffic.

   Session Border Controllers (SBCs) are active participants with call
   signaling.  Like firewalls, they also create dynamic mappings to
   permit media traffic.  This forces call signaling and media through
   specific IP addresses, belonging to the SBC or an SBC-controlled
   media relay device.

   TURN is also used as an alternative to permit media traffic, i.e.
   Use TCP transport between the client and TURN server because
   Firewalls are configured to block UDP entirely.

   The use-case is explained in Section 4.2.4.1 of



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   [I-D.ietf-rtcweb-use-cases-and-requirements] refers to deploying a
   TURN server to audit all media sessions from inside the company
   premises to any external peer.

   Using TURN for all such communication has the following problems:

   o  Single TURN server will result in single point of failure.

   o  TURN server could increase media latency and high-end TURN server
      would be needed to cater to all such calls.

   o  TURN server is just providing the 5-tuple details (source IP
      address, destination IP address, protocol number, source port
      number, and destination port number) but no other details of the
      WebRTC server using which the call is initiated

   o  Enterprise firewalls would typically have granular policies to
      permit call initiated using selected WebRTC servers (Dr.Good) it
      trusts and block others (Dr.Evil).

   o  It comes at a high cost to the provider of the TURN server, since
      the server typically needs a high-bandwidth connection to the
      Internet.  As a consequence, it is best to use a TURN server only
      when a direct communication path cannot be found.  When the client
      and a peer use ICE to determine the communication path, ICE will
      use hole punching techniques to search for a direct path first and
      only use a TURN server when a direct path cannot be found.

   o  The value of the Diffserv field may not be preserved.

   o  The Explicit Congestion Notification (ECN) field may be reset.

   This draft has a solution where an authorized server (could be a Call
   Agent or a WebRTC server ) generate a cryptographic token which is
   passed to the endpoints.  The endpoint includes the token in its ICE
   connectivity checks.  The firewall intercepts the ICE connectivity
   checks containing the token, validates it, and permits the ICE
   connectivity checks and the subsequent media flow through the
   firewall.


2.  Notational Conventions

   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].

   This note uses terminology defined in [RFC5245].



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3.  Problem Statement

   In the below topology, an webRTC Server is deployed in the enterprise
   Data Center.  Alice makes a webRTC call to Bob. For the two endpoints
   to successfully establish media sessions, firewalls FW1 and FW2 need
   to permit the ICE connectivity checks and media traffic.  In such
   scenarios the mechanism described in this draft proposes a new
   comprehension-optional FW-FLOWDATA STUN attribute to be included in
   STUN Bind requests sent during ICE connectivity checks so that
   firewalls will permit media traffic between internal peers.  This
   STUN attribute is created by the trusted WebRTC server and sent to
   the endpoints to be propagated by the respective ICE agents during
   ICE connectivity checks.


               =========================
               |  WebRTC Server        |
               =========================
                         |  Data Center
                         |
                         |
                 ==================
                 |    WAN         |-----+-+-------+---+----+-+-----+
                 ==================                        |
                           |              Branch office 2  |
  Branch office 1          |                               |
                           |                               |
                   +-------+-------+               +----+-------+
                   | Firewall 1    |               | Firewall 2 |
                   |               |               |            |
                   +-------+-------+               +----+-------+
                           |                               |
                           |                               |
                           |                               |
   ---+-+-----+-----------+-+-----+--------         -----+-+-----+------
                           |                               |
                        +-+------+                     +--------+
                        | Alice  |                     | Bob    |
                        +--------+                     +--------+

          Figure 1: WebRTC service in enterprise - internal call


4.  Solution Overview

   This section gives an overview of the solution and the different
   components involved in the solution and the role of each component.




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4.1.  Different Components and the Trust model

   Figure 1 above shows typical components involved in a WebRTC call
   scenario.  As part of the call setup, the WebRTC endpoint would have
   to gather its candidates from a STUN/TURN server, send the candidates
   in the offer to the peer endpoint.  On receiving the answer from the
   peer endpoint it starts the ICE connectivity checks.  As discussed in
   the problem statement, firewalls would typically block these ICE
   connectivity checks and media flowing there after.  To allow this
   traffic a firewall needs to authorize the flow.

   o  A new comprehensive optional STUN attribute called FW-FLOWDATA is
      defined as part of this draft.  This is used by WebRTC endpoints
      requiring firewall traversal.

   o  This STUN attribute FW-FLOWDATA is generated by the WebRTC server
      in co-ordination with the WebRTC endpoint.

   o  Once the WebRTC session ends the firewall's dynamic mappings are
      closed after timeout.

   o  DISCUSSION: Could we could have a FW-FLOWDATA attribute sent in a
      STUN message to close the dynamic mappings in the firewalls?


5.  Usage and Processing

   An RTP endpoint which generates media can include the FW-FLOWDATA
   attribute in its STUN Binding requests used in ICE connectivity
   checks, to inform on-path firewalls to permit the flow.

5.1.  Generating  FW-FLOWDATA Attribute

   The WebRTC server after processing the OFFER/ANSWER sends the FW-
   FLOWDATA STUN attribute to both the peers to be included in the ICE
   connectivity checks.  The Authentication Tag field in the FW-FLOWDATA
   attribute contains the digest of the FW-FLOWDATA attribute for data
   origin authentication and integrity protection.  The server first
   selects the candidate address info based on OFFER/ANSWER exchange and
   generates other fields of this attribute.  The server then computes a
   digest for the FW-FLOWDATA attribute using HMAC-SHA1.  The key for
   HMAC-SHA1 is provisioned using the technique in Section 7.  The
   result of which is truncated to 96 bits (retaining the left most
   bits) to produce HMAC-SHA-1-96 and input into the Authentication Tag
   field.  The mechanism to send FW-FLOWDATA attribute from the WebRTC
   server to the cient is outside the scope of this draft.  But it is
   assumed that signalling protocols used for WebRTC call setup will be
   enhanced to deliver this new attribute to the WebRTC client.  The



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   WebRTC server MUST provide a new FW-FLOWDATA to allow the media
   session to continue before Lifetime expires.

5.2.  Sending FW-FLOWDATA Attribute in Binding Request

   Once a WebRTC endpoint receives the FW-FLOWDATA, it is responsible
   for generating the STUN message and retransmitting the transactions
   per the STUN specification.  The FW-FLOWDATA attribute should be
   placed before the FINGERPRINT attribute (if present) and after the
   MESSAGE-INTEGRITY attribute.  The STUN length field is adjusted to
   point to the new end of the STUN message; that is, the STUN length
   field always accurately indicates the length of the STUN message
   (including the MESSAGE-INTEGRITY, FINGERPRINT, and FW-FLOWDATA
   attributes).  This does not interfere with 3rd party receivers of the
   STUN message, as they will adjust the STUN length field to point to
   the end of the MESSAGE-INTEGRITY field.  Receivers that do not
   understand the FW-FLOWDATA will ignore it.

   FW-FLOWDATA attribute received by the WebRTC client is passed to the
   web browser's ICE agent (API to be added in in W3C WebRTC-API
   specification [I.D.w3c-webrtc]).  The ICE agent includes the FW-
   FLOWDATA attribute with all ICE connectivity checks, so that on-path
   firewalls can validate and permit the ICE connectivity checks and
   forthcoming media.  The token MUST included in the ICE binding
   indication packets (keepalive) (In case the lifetime expires)

   For the FW-FLOWDATA attribute to be visible to the firewalls between
   the client and the TURN server, the FW-FLOWDATA should be included in
   the ALLOCATE request, channel bind or refresh messages going to the
   TURN server.  This is to avoid firewalls having to look for STUN
   packets within STUN (TURN) packets.

5.3.  Firewalls processing FW-FLOWDATA Attribute

   Firewalls can reliably determine a UDP message is a STUN message
   because all STUN messages sent as ICE connectivity checks include the
   32-bit STUN magic cookie and the FINGERPRINT attribute.  STUN
   messages which are authenticated also include a MESSAGE-INTEGRITY
   attribute which authenticates the fields prior to the MESSAGE-
   INTEGRITY.

   When the firewall receives a STUN binding request with FW-FLOWDATA
   attribute it stores the Authentication Tag in the FW-FLOWDATA
   attribute.  The firewall then generates a digest for the FW-FLOWDATA
   attribute using HMAC-SHA1.  The result of which is truncated to 96
   bits (retaining the left most bits) to produce HMAC-SHA-1-96.  If the
   value of the newly generated digest HMAC-SHA-1-96 is identical to the
   stored one, the firewall can ensure that the FW-FLOWDATA attribute



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   has not been tampered with.  Otherwise the packet is discarded.

   To facilitate timestamp checking for replay attacks, each firewall
   should perform the following check for each message:

   When a message is received, the received timestamp, TSnew, is
   checked, and the packet is accepted if the timestamp is recent enough
   to the reception time of the packet, RDnew:

   Lifetime + Delta > (RDnew - TSnew)

   The recommended value for the allowed Delta is 30 seconds.  If the
   timestamp is NOT within the boundaries then discard the STUN message.

   The firewall also performs the following checks:

   o  Ensures that the source IP address and UDP port of the packet
      matches with one of the local CAI entries in the payload except
      for peer-reflexive cases.

   o  Ensures the destination IP address and UDP port of the packet
      matches with one of the local CAI entries in the packet payload
      except for peer-reflexive cases.

   o  Firewall if located after NAT(peer-reflexive cases) can skip CAI
      processing (It can be configurable option).  For peer-reflexive
      case, destination CAI MUST match in case of outgoing STUN packet
      and source CAI MUST match incase of incoming STUN packet

   If all the above checks pass then the firewall creates the 5-tuple
   dynamic mapping using the local candidate IP address, local candidate
   port, remote candidate IP address, remote candidate port, transport
   protocol.  The session time of the dynamic mapping will be set to a
   short lifetime (default value of 60 seconds).

   If the initial ICE connectivity check includes the ICE-CONTROLLING
   attribute but does not include USE-CANDIDATE, ICE connectivity check
   is successful and a subsequent ICE connectivity check includes both
   these attributes, the firewall can determine that the ICE agent is
   the controlling agent using regular nomination and this candidate
   pair is nominated for media flow.  The firewall then sets the session
   time of the dynamic mapping equal to the Lifetime field in FW-
   FLOWDATA attribute.

   If the initial ICE connectivity check includes the ICE-CONTROLLING
   attribute and the USE-CANDIDATE attribute, firewall can determine
   that the ICE agent is the controlling agent using aggressive using
   nomination.  If the ICE connectivity check is successful It then



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   waits for the media traffic to flow before setting the session time
   of the dynamic mapping equal to Lifetime field in FW-FLOWDATA
   attribute.

   DISCUSSION: If WebRTC implementations of RTP support multiplexing of
   multiple media sessions onto a single RTP session, FW-FLOWDATA
   attribute can be enhanced to carry a flag indicating the same so that
   firewall can immediately close the dynamic mapping created for other
   pairs in the ICE checklist once media starts flowing on one the
   candidate pairs.  In case of multi-homing firewalls can track
   multiple host IP addresses using authentication supplicant or, for
   hosts lacking the supplicant, use address-based authentication
   method.


6.  STUN Attribute Format


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Lifetime                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Nonce                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Timestamp                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | LCA  Count    |  RCA  Count   |    Reserved                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                 Candidate Address Info                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                 Authentication Tag                            |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: FW-FLOWDATA Attribute

   Lifetime:  32-bit unsigned integer.  The length of time in seconds
      that the STUN attribute is valid for the purpose of firewall
      creating dynamic mapping.  The lifetime of the firewall dynamic
      mapping is set to this value.  After the lifetime expires the
      mapping is deleted, unless the lifetime is extended using a
      another FW-FLOWDATA attribute.





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   Nonce:  96-bit unsigned integer.  Random value chosen by the WebRTC
      Server that uniquely identifies the STUN attribute.

   Timestamp:  64-bit unsigned integer field containing a timestamp.
      The value indicates the number of seconds since January 1, 1970,
      00:00 UTC, by using a fixed point format.  In this format, the
      integer number of seconds is contained in the first 48 bits of the
      field, and the remaining 16 bits indicate the number of 1/64K
      fractions of a second.

   LCA Count:  8-bit unsigned integer.  Number of local candidate
      addresses.

   RCA Count:  8-bit unsigned integer.  Number of remote candidate
      addresses.

   Reserved:  16-bit unsigned integer.  An unused field.  It MUST be
      initialized to zero by the sender and MUST be ignored by the
      receiver.

   Candidate Address Info:
        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Family         | Protocol        |         Port                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                 Address (32 bits or 128 bits)                 |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      It consists of an 8-bit address family, L4 protocol (for example
      17 for UDP, 6 for TCP) and a 16-bit port, followed by a fixed-
      length value representing the IP address.  If the 16-bit port
      value is 0 it indicates "all ports".  The address family can take
      on the following values: 0x01:IPv4 and 0x02:IPv6.  An endpoint may
      send Zero or more CAI in its FLOWDATA

   Authentication Tag:  A 96-bit field that carries the Message
      Authentication Code for the FW-FLOWDATA STUN attribute.


7.  Key Provisioning

   Static keys are preconfigured, either manually or through a network
   management system.  The simplest way to implement FW-FLOWDATA
   validation is to use static keys.  The provisioning of static keys
   requires either manual operator intervention on the WebRTC Server and
   each firewall in the enterprise or a network management system



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   performing the same task.

   Alternatively using Dynamic Group Key Distribution, group keys are
   dynamically distributed among the WebRTC server and enterprise
   firewalls using GDOI [RFC6407].  In this way, each firewall requests
   a group key from a key server as part of an encrypted and integrity-
   protected key agreement protocol.  Once the key server has
   authenticated and authorized the firewalls, it distributes a group
   key to the group member.  The authentication in this model can be
   based on public key mechanisms, thereby avoiding the need for static
   key provisioning.


8.  Security Considerations

   Hosts using WebRTC calls will see lot of FW-FLOWDATA attributes.
   They determine the key by trying a number of candidate keys and
   seeing if one of them is correct.  The attack works when the keys
   have low entropy, such as a word from the dictionary.  This attack
   can be mitigated by using strong keys with large entropy.  In
   situations where even stronger mitigation is required, the keys can
   be dynamically changed using GDOI.  The WebRTC server controls how
   long a firewall session is kept open via the Lifetime value and
   WebRTC server could use different Lifetime values depending on the
   anticipated level of trust of the device (e.g. company provided
   laptop might be trusted more than a Bring Your Own Device (BYOD));
   the device with more trust need to obtain its authentication
   attribute less often).  Firewalls in addition to timestamp checking
   can also maintain a cache of used Nonces, IP source addresses
   associated with used Nonces as an effective countermeasure against
   replay attacks.

   All the security considerations applicable to STUN [RFC5389] and ICE
   [RFC5245] are applicable to this document as well.


9.  IANA Considerations

   Allocate new STUN attribute value for FW-FLOWDATA from the
   [STUN-ATTR] registry.


10.  Acknowledgements

   The authors would like to thank Prashanth Patil and Ramesh Nethi for
   review comments.





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11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, October 2011.

11.2.  Informative References

   [I-D.ietf-rtcweb-use-cases-and-requirements]
              Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use-cases and Requirements",
              draft-ietf-rtcweb-use-cases-and-requirements-09 (work in
              progress), June 2012.

   [STUN-ATTR]
              IANA, "IANA: STUN Attributes", December 2011, <http://
              www.iana.org/assignments/stun-parameters/
              stun-parameters.xml#stun-parameters-3>.


Authors' Addresses

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: tireddy@cisco.com








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   Muthu Arul Mozhi Perumal
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: mperumal@cisco.com


   Ram Mohan Ravindranath
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: rmohanr@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Email: dwing@cisco.com
























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