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Versions: 00 01 02 03 draft-ietf-ecrit-psap-callback

ECRIT                                                     H. Schulzrinne
Internet-Draft                                       Columbia University
Intended status: Informational                             H. Tschofenig
Expires: September 9, 2010                        Nokia Siemens Networks
                                                                M. Patel
                                             InterDigital Communications
                                                           March 8, 2010


             Public Safety Answering Point (PSAP) Callbacks
              draft-schulzrinne-ecrit-psap-callback-03.txt

Abstract

   After an emergency call is completed (either prematurely terminated
   by the emergency caller or normally by the call-taker) it is possible
   that the call-taker feels the need for further communication or for a
   clarification.  For example, the call may have been dropped by
   accident without the call-taker having sufficient information about
   the current situation of a wounded person.  A call-taker may trigger
   a callback towards the emergency caller using the contact information
   provided with the initial emergency call.  This callback could, under
   certain circumstances, then be treated like any other call and as a
   consequence, it may get blocked by authorization policies or may get
   forwarded to an answering machine.

   The IETF emergency services architecture addresses callbacks in a
   limited fashion and thereby covers a couple of scenarios.  This
   document discusses some shortcomings and raises the question whether
   additional solution techniques are needed.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.



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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on September 9, 2010.

Copyright Notice

   Copyright (c) 2010 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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   described in the BSD License.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Routing Asymmetry  . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Multi-Stage Resolution . . . . . . . . . . . . . . . . . .  4
     1.3.  Call Forwarding  . . . . . . . . . . . . . . . . . . . . .  5
     1.4.  PSTN Interworking  . . . . . . . . . . . . . . . . . . . .  7
     1.5.  Network-based Service URN Resolution . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  Design Approaches  . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Topics for Investigation . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Informative References . . . . . . . . . . . . . . . . . . 16
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18


































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1.  Introduction

   Summoning police, the fire department or an ambulance in emergencies
   is one of the fundamental and most-valued functions of the telephone.
   As telephone functionality moves from circuit-switched telephony to
   Internet telephony, its users rightfully expect that this core
   functionality will continue to work at least as well as it has for
   the legacy technology.  New devices and services are being made
   available that could be used to make a request for help, which are
   not traditional telephones, and users are increasingly expecting them
   to be used to place emergency calls.

   Regulatory requirements demand that the emergency call itself
   provides enough information to allow the call-taker to initiate a
   call back to the emergency caller in case the call dropped or to
   interact with the emergency caller in case of further questions.
   Such a call, referred as PSAP callback subsequently in this document,
   may, however, be blocked or forwarded to an answering machine as SIP
   entities (SIP proxies as well as the SIP UA itself) cannot associate
   the potential importantance of the call based on the SIP signaling.

      Note that the authors are, however, not aware of regulatory
      requirements for providing preferential treatment of callbacks
      initiated by the call-taker at the PSAP towards the emergency
      caller.

   Section 10 of [I-D.ietf-ecrit-framework] discusses the identifiers
   required for callbacks, namely AOR URI and a globally routable URI in
   a Contact: header.  Section 13 of [I-D.ietf-ecrit-framework] provides
   the following guidance regarding callback handling:

      A UA may be able to determine a PSAP call back by examining the
      domain of incoming calls after placing an emergency call and
      comparing that to the domain of the answering PSAP from the
      emergency call.  Any call from the same domain and directed to the
      supplied Contact header or AoR after an emergency call should be
      accepted as a call-back from the PSAP if it occurs within a
      reasonable time after an emergency call was placed.

   This approach mimics a stateful packet filtering firewall and is
   indeed helpful in a number of cases.  It is also relatively simple to
   implement.  Below, we discuss a few cases where this approach fails.

1.1.  Routing Asymmetry

   In some deployment environments it is common to have incoming and
   outgoing SIP messaging to use different routes.




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                                                   ,-------.
                                                 ,'         `.
                      ,-------.                 /  Emergency  \
                    ,'         `.              |   Services    |
                   /  VoIP       \      I      |   Network     |
                  |   Provider    |     n      |               |
                  |               |     t      |               |
                  |               |     e      |               |
                  |   +-------+   |     r      |               |
               +--+---|Inbound|<--+-----m      |               |
               |  |   |Proxy  |   |     e      |   +------+    |
               |  |   +-------+   |     d      |   |PSAP  |    |
               |  |               |     i      |   +--+---+    |
     +----+    |  |               |     a-+    |      |        |
     | UA |<---+  |               |     t |    |      |        |
     |    |----+  |               |     e |    |      |        |
     +----+    |  |               |       |    |      |        |
               |  |               |     P  |   |      |        |
               |  |               |     r  |   |      |        |
               |  |   +--------+  |     o   |  |      |        |
               +--+-->|Outbound|--+---->v   |  |   +--+---+    |
                  |   |Proxy   |  |     i    | | +-+ESRP  |    |
                  |   +--------+  |     d    | | | +------+    |
                  |               |     e     || |             |
                  |               |     r     |+-+             |
                   \             /             |               |
                    `.         ,'               \             /
                      '-------'                  `.         ,'
                                                   '-------'

                  Figure 1: Example for Routing Asymmetry

1.2.  Multi-Stage Resolution

   Consider the following emergency call routing scenario shown in
   Figure 2 where routing towards the PSAP occurs in several stages.  An
   emergency call uses a SIP UA that does not run LoST on the end point.
   Hence, the call is marked with the 'urn:service:sos' Service URN
   [RFC5031].  The user's VoIP provider receives the emergency call and
   determines where to route it.  Local configuration or a LoST lookup
   might, in our example, reveal that emergency calls are routed via a
   dedicated provider FooBar and targeted to a specific entity, referred
   as esrp1@foobar.com.  FooBar does not handle emergency calls itself
   but performs another resolution step to let calls enter the emergency
   services network and in this case another resolution step takes place
   and esrp-a@esinet.org is determined as the recipient, pointing to an
   edge device at the IP-based emergency services network.  Inside the
   emergency services there might be more sophisticated routing taking



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   place somewhat depending on the existing structure of the emergency
   services infrastructure.


                                      ,-------.
    +----+                          ,'         `.
    | UA |--- urn:service:sos      /  Emergency  \
    +----+   \                    |   Services    |
              \  ,-------.        |   Network     |
               ,'         `.      |               |
              /   VoIP      \     |               |
             (    Provider   )    |               |
              \             /     |               |
               `.         ,'      |               |
                 '---+---'        |   +------+    |
                     |            |   |PSAP  |    |
             esrp1@foobar.com     |   +--+---+    |
                     |            |      |        |
                     |            |      |        |
                 ,---+---.        |      |        |
               ,'         `.      |      |        |
              /   Provider  \     |      |        |
             +    FooBar     )    |      |        |
              \             /     |      |        |
               `.         ,'      |   +--+---+    |
                 '---+---'        | +-+ESRP  |    |
                     |            | | +------+    |
                     |            | |             |
                     +------------+-+             |
                esrp-a@esinet.org |               |
                                   \             /
                                    `.         ,'
                                      '-------'

               Figure 2: Example for Multi-Stage Resolution

1.3.  Call Forwarding

   Imagine the following case where an emergency call enters an
   emergency network (state.org) via an ERSP but then gets forwarded to
   a different emergency services network (in our example to police-
   town.org, fire-town.org or medic-town.org).  The same considerations
   apply when the the police, fire and ambulance networks are part of
   the state.org sub-domains (e.g., police.state.org).







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                                   ,-------.
                                 ,'         `.
                                /  Emergency  \
                               |   Services    |
                               |   Network     |
                               |   (state.org) |
                               |               |
                               |               |
                               |   +------+    |
                               |   |PSAP  +--+ |
                               |   +--+---+  | |
                               |      |      | |
                               |      |      | |
                               |      |      | |
                               |      |      | |
                               |      |      | |
                               |   +--+---+  | |
             ------------------+---+ESRP  |  | |
             esrp-a@state.org  |   +------+  | |
                               |             | |
                               |    Call Fwd | |
                               |     +-+-+---+ |
                                \    | | |    /
                                 `.  | | |  ,'
                                   '-|-|-|-'           ,-------.
                            Police   | | | Fire      ,'         `.
                        +------------+ | +----+     /  Emergency  \
         ,-------.      |              |      |    |   Services    |
       ,'         `.    |              |      |    |   Network     |
      /  Emergency  \   |          Ambulance  |    | fire-town.org |
     |   Services    |  |              |      |    |               |
     |   Network     |  |              +----+ |    |   +------+    |
     |police-town.org|  |     ,-------.     | +----+---+PSAP  |    |
     |               |  |   ,'         `.   |      |   +------+    |
     |   +------+    |  |  /  Emergency  \  |      |               |
     |   |PSAP  +----+--+ |   Services    | |      |               ,
     |   +------+    |    |   Network     | |      `~~~~~~~~~~~~~~~
     |               |    |medic-town.org | |
     |               ,    |               | |
     `~~~~~~~~~~~~~~~     |   +------+    | |
                          |   |PSAP  +----+ +
                          |   +------+    |
                          |               |
                          |               ,
                          `~~~~~~~~~~~~~~~

                   Figure 3: Example for Call Forwarding




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1.4.  PSTN Interworking

   In case an emergency call enters the PSTN, as shown in Figure 4,
   there is no guarantee that the callback some time later does leave
   the same PSTN/VoIP gateway or that the same end point identifier is
   used in the forward as well as in the backward direction making it
   difficult to reliably detect PSAP callbacks.


     +-----------+
     | PSTN      |-------------+
     | Calltaker |             |
     | Bob       |<--------+   |
     +-----------+         |   v
                -------------------
            ////                   \\\\      +------------+
           |                           |     |PSTN / VoIP |
           |             PSTN          |---->|Gateway     |
            \\\\                   ////      |            |
                -------------------          +----+-------+
                           ^                      |
                           |                      |
                     +-------------+              |  +--------+
                     |             |              |  |VoIP    |
                     | PSTN / VoIP |              +->|Service |
                     | Gateway     |                 |Provider|
                     |             |<------Invite----|   Y    |
                     +-------------+                 +--------+
                                                      |     ^
                                                      |     |
                                                    Invite Invite
                                                      |     |
                                                      V     |
                                                     +-------+
                                                     | SIP   |
                                                     | UA    |
                                                     | Alice |
                                                     +-------+

                  Figure 4: Example for PSTN Interworking

1.5.  Network-based Service URN Resolution

   The mechanism described in [I-D.ietf-ecrit-framework] assumes that
   all devices at the call signaling path store information about the
   domain of the communication recipient.  This is necessary to match
   the stored domain name against the domain of the sender when an
   incoming call arrives.



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   However, the IETF emergency services architecture also considers
   those cases where the resolution from the Service URN to the PSAP URI
   happens somewhere in the network rather than immediately at the end
   point itself.  In such a case, the end device is therefore not able
   to match the domain of the sender with any information from the
   outgoing emergency call.

   Figure 5 shows this message exchange graphically.


        ,-------.
      ,'         `.
     /  Emergency  \
    |   Services    |
    |   Network     |
    |police-town.org|
    |               |
    |   +------+    |    Invite to police.example.com
    |   |PSAP  +<---+------------------------+
    |   |      +----+------------------+     ^
    |   +------+    |Invite from       |     |
    |               ,police.example.com|     |
    `~~~~~~~~~~~~~~~                   v     |
    +--------+                        ++-----+-+
    |        |            query       |VoIP    |
    | LoST   |<-----------------------|Service |
    | Server |   police.example.com   |Provider|
    |        |----------------------->|        |
    +--------+                        +--------+
                                       |     ^
                                 Invite|     | Invite
                                   from|     | to
                     police.example.com|     | urn:service:sos
                                       V     |
                                      +-------+
                                      | SIP   |
                                      | UA    |
                                      | Alice |
                                      +-------+

        Figure 5: Example for Network-based Service URN Resolution










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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].

   Emergency services related terminology is borrowed from [RFC5012].












































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3.  Design Approaches

   The starting point of the investigations is the currently provided
   functionality in Section 13 of [I-D.ietf-ecrit-framework].  It
   focuses on identifying a response to a previously made emergency
   call.  As described in the introduction this approach is quite coarse
   grained since any call from the PSAP's domain is given preferential
   treatment.  This approach is, however, likely going to be practical.
   Still there are a couple of limitations, as discussed in this
   document.

   To expand on the initially provided solution the following
   description starts with attempt to identify the caller as a PSAP.
   There are two approaches for accomplishing this functionality.


                    +----------+
                    | List of  |+
                    | valid    ||
                    | PSAP ids ||
                    +----------+|
                     +----------+
                         *
                         * whitelist
                         *
                         V
      Incoming      +----------+    Normal
      SIP Msg       | SIP      |+   Treatment
     -------------->| Entity   ||=============>
      + Identity    |          ||(if not in whitelist)
                    +----------+|
                    +----------+
                         ||
                         ||
                         || Preferential
                         || Treatment
                         ++=============>
                           (in whitelist)

                  Figure 6: Identity-based Authorization

   In Figure 6 an interaction is presented that allows a SIP entity to
   make a policy decision whether to bypass installed authorization
   policies and thereby providing preferential treatment.  To make this
   decision the sender's identity is compared with a whitelist of valid
   PSAPs.  The identity assurances in SIP can come in different forms,
   such as SIP Identity [RFC4474] or with P-Asserted-Identity [RFC3325].
   The former technique relies on a cryptographic assurance and the



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   latter on a chain of trust.

   The establishment of a whitelist with PSAP identities is
   operationally complex and does not easily scale world wide.  When
   there is a local relationship between the VSP/ASP and the PSAP then
   populating the whitelist is far simpler.

   An alternative approach to an identity based authorization model is
   outlined in Figure 7.  In fact, RFC 4484 [RFC4484] already
   illustrated the basic requirements for this technique.


                  +----------+
                  | List of  |+
                  | trust    ||
                  | anchor   ||
                  +----------+|
                   +----------+
                       *
                       *
                       *
                       V
    Incoming      +----------+    Normal
    SIP Msg       | SIP      |+   Treatment
   -------------->| Entity   ||=============>
    + trait       |          ||(no indication
                  +----------+| of PSAP)
                  +----------+
                       ||
                       ||
                       || Preferential
                       || Treatment
                       ++=============>
                         (indicated as
                          PSAP)

                    Figure 7: Trait-based Authorization

   In a trait-based authorization scenario an incoming SIP message
   contains a form of trait, i.e. some form of assertion.  The assertion
   contains an indication that the sending party has the role of a PSAP
   (or similar emergency services entity).  The assertion is either
   cryptographically protected to enable end-to-end verification or an
   chain of trust security model has to be assumed.  In Figure 7 we
   assume an end-to-end security model where trust anchors are
   provisioned to ensure the ability for a SIP entity to verify the
   received assertion.




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   From a solution point of view various approaches are feasible, such
   as SIP SAML (see [I-D.ietf-sip-saml]) or URI Parameters for
   indicating the Calling Party's Category and Originating Line
   Information (see [I-D.patel-dispatch-cpc-oli-parameter]).

   Still, a drawback of the outlined approaches above is that it does
   not allow any mechanism to distinguish different types of calls
   initiated by PSAPs.  Not every call from a PSAP is indeed a response
   to an emergency call.

   This leads us to another mechanism on top of the previously presented
   onces, namely the indication is that the communication attempt is of
   emergency nature.  As such, it is a slight modification of the one
   presented previously.  In addition to the indication that the calling
   party is a PSAP there is an expression that the specific call is of
   emergency services nature.  This indication cannot be verified by
   external parties, similarly to the emergency call marking for a
   citizen-to-authority emergency call using a Service URN, because it
   heavily depends on the intention of the call taker itself.
































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4.  Topics for Investigation

   When you make an IP-based emergency call to an IP-based PSAP then the
   PSAP will get two pieces of identity information about the emergency
   caller:

   o  Contact-URI: Information that uniquely identifies the device the
      call came from.

   o  Address of Record: Long-term contact information

   Should the callback functionality be tied to a previous emergency
   call setup and as such enabled only for a specific time?  For
   example, preferential treatment for callbacks could be provided only
   within one hour after the initial emergency call was made.

   Is it expected that the callback reaches primarily the device that
   initiated the emergency call?  In some cases the device that was used
   to originally initiate the call does not respond anymore to a
   callback (e.g. imagine a fixed line phone that was used to report a
   fire in a house and is out of order soon afterwards).  Since the
   initial emergency call provided a second contact mechanism (namely
   the address of record) it could be used by the call taker as well.
   Should this communication also experience the same type of override
   privilege as the initially transmitted callback to the emergency
   caller's device?

   Should any restrictions be made regarding the media being used for
   callback?  Is it acceptable to return an instant message when the
   caller started the conversation with audio?





















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5.  Security Considerations

   This document provides discussions problems of PSAP callbacks and
   explores the design space.

   An important aspect from a security point of view is the relationship
   between the emergency services network and the VSP (assuming that the
   emergency call travels via the VSP and not directly between the SIP
   UA and the PSAP).  If there is some form of relationship between the
   emergency services operator and the VSP then the identification of a
   PSAP call back is less problematic than in the case where the two
   entities have not entered in some form of relationship that would
   allow the VSP to verify whether the marked callback message indeed
   came from a legitimate source.

   The main attack surface can be seen in the usage of PSAP callback
   marking to bypass blacklists, ignore call forwarding procedures and
   similar features to interact with users and to get their attention.
   For example, using PSAP callback marking devices would be able to
   recognize these types of incoming messages leading to the device
   overriding user interface configurations, such as vibrate-only mode.
   As such, the requirement is to ensure that the mechanisms described
   in this document can not be used for malicious purposes, including
   SPIT.

   It is important that PSAP callback marked SIP messages, which cannot
   be verified adequately, are treated like a call that does not have
   any marking attached instead of failing the call processing
   procedure.






















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6.  Acknowledgements

   We would like to thank members from the ECRIT working group, in
   particular Brian Rosen, for their discussions around PSAP callbacks.
   The working group discussed the topic of callbacks at their virtual
   interim meeting in February 2010 and the following persons provided
   valuable input: John Elwell, Bernard Aboba, Cullen Jennings, Keith
   Drage, Marc Linsner, Roger Marshall, Dan Romascanu, Geoff Thompson,
   Milan Patel, Janet Gunn.










































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7.  References

7.1.  Informative References

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

7.2.  Informative References

   [I-D.ietf-ecrit-framework]
              Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
              "Framework for Emergency Calling using Internet
              Multimedia", draft-ietf-ecrit-framework-10 (work in
              progress), July 2009.

   [I-D.ietf-sip-saml]
              Tschofenig, H., Hodges, J., Peterson, J., Polk, J., and D.
              Sicker, "SIP SAML Profile and Binding",
              draft-ietf-sip-saml-06 (work in progress), March 2009.

   [I-D.patel-dispatch-cpc-oli-parameter]
              Patel, M., Jesske, R., and M. Dolly, "Uniform Resource
              Identifier (URI) Parameters for indicating the Calling
              Party's Category and Originating Line Information",
              draft-patel-dispatch-cpc-oli-parameter-02 (work in
              progress), November 2009.

   [I-D.patel-ecrit-sos-parameter]
              Patel, M., "SOS Uniform Resource Identifier (URI)
              Parameter for Marking of Session Initiation Protocol (SIP)
              Requests related to Emergency Services",
              draft-patel-ecrit-sos-parameter-08 (work in progress),
              February 2010.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4484]  Peterson, J., Polk, J., Sicker, D., and H. Tschofenig,
              "Trait-Based Authorization Requirements for the Session
              Initiation Protocol (SIP)", RFC 4484, August 2006.

   [RFC5012]  Schulzrinne, H. and R. Marshall, "Requirements for



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              Emergency Context Resolution with Internet Technologies",
              RFC 5012, January 2008.

   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              January 2008.













































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Authors' Addresses

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   US

   Phone: +1 212 939 7004
   Email: hgs+ecrit@cs.columbia.edu
   URI:   http://www.cs.columbia.edu


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Milan Patel
   InterDigital Communications


   Email: Milan.Patel@interdigital.com





















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