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Network Working Group                                            N. Zong
Internet-Draft                                                 L. Dunbar
Intended status: Informational                       Huawei Technologies
Expires: January 2, 2015                                        M. Shore
                                                    No Mountain Software
                                                                D. Lopez
                                                              Telefonica
                                                          G. Karagiannis
                                                    University of Twente
                                                            July 1, 2014


       Virtualized Network Function (VNF) Pool Problem Statement
                draft-zong-vnfpool-problem-statement-06

Abstract

   Network functions are traditionally implemented on specialized
   hardware rather than on general purpose servers, but there is a clear
   trend to implement a number of network functions, such as firewall or
   load balancer, as software on virtualized computing platforms.  These
   virtualized functions are called Virtualized Network Functions
   (VNFs), which can be used to build network services.  The use of VNFs
   to build network services introduces additional challenges on
   reliability, such as additional points of failure and the need to
   coordinate various VNFs.

   This document introduces a general idea of VNF Pool to support
   reliable function provision by the VNFs.  We then highlight the
   reliability challenges and issues when using the VNFs to build
   services.  Related IETF works are also briefly described.

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
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   This Internet-Draft will expire on January 2, 2015.



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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  From Specialized Hardware to Virtualized Network Function   4
     3.2.  Concept of VNF Set  . . . . . . . . . . . . . . . . . . .   5
     3.3.  Challenges to reliability . . . . . . . . . . . . . . . .   6
   4.  VNF Pool  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Challenges and Open Issues  . . . . . . . . . . . . . . . . .   8
     5.1.  Redundancy model inside VNF . . . . . . . . . . . . . . .   8
     5.2.  State synchronization inside VNF  . . . . . . . . . . . .   8
     5.3.  Interaction between VNF and Service Control Entity  . . .   8
     5.4.  Reliable transport  . . . . . . . . . . . . . . . . . . .   9
     5.5.  Scope Considerations  . . . . . . . . . . . . . . . . . .   9
   6.  Related Works . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Reliable Server Pooling (RSerPool)  . . . . . . . . . . .   9
     6.2.  Virtual Router Redundancy Protocol (VRRP) . . . . . . . .  10
     6.3.  Service Function Chaining (SFC) . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Network functions such as firewall, load balancer, WAN optimizer are
   conventionally deployed as specialized hardware servers in both
   network operators' networks and data center networks, as the building
   blocks of the network services.



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   A Virtualized Network Function (VNF) provides such network function
   through its implementation as software instances running on general
   purpose servers via a virtualization layer (i.e., hypervisor).  VNFs
   potentially offer benefits such as elastic service offering, reduced
   operational and equipment costs [NFV-WP].

   There is a trend to move network functions from specialized hardware
   servers to general purpose servers based on virtualized computing
   platforms, in order to build network services by using VNFs.  For
   example, in Service Function Chaining (SFC), a network service can be
   built using a set of sequentially connected VNF instances deployed at
   different points in the network [SFC].

   Nevertheless, the use of VNFs can pose additional challenges on the
   reliability of the provided services.  For a VNF instance, it
   typically would not have built-in reliability mechanisms on its host
   (i.e., a general purpose server).  Instead, there are more factors of
   risk such as software failure at various levels including hypervisors
   and virtual machines, hardware failure, and instance migration that
   may make a VNF instance unreliable.

   In order to achieve higher reliability, a VNF may adopt a pooling
   mechanism, where a number of VNF instances with the same function can
   be grouped as a pool to provide the function.  We call such a pool a
   VNF Pool.  Conceptually, a Pool Manager is used to manage a VNF Pool,
   e.g., selects active/standby VNF instances, and potentially interacts
   with a Service Control Entity.  A Service Control Entity is an entity
   that combines and orchestrates a set of network functions, e.g.,
   VNFs, to build network services.  The major benefit of using VNF Pool
   is that the reliability mechanisms such as redundancy management are
   achieved by the VNF Pool inside the VNF and thus transparent to the
   Service Control Entity.  A VNF Pool-enabled VNF still acts as a
   normal VNF when orchestrated by the Service Control Entity.

   We are specifically concerned with the reliability of an individual
   VNF based on the VNF Pool managed inside the VNF.  For example, how
   to manage the redundancy model, e.g., select active/standby for a VNF
   instance in a VNF Pool, considering the policy and the infrastructure
   conditions?  How are the service states of a VNF instance held and
   accessed for efficient synchronization with backup instances in a VNF
   Pool?  What pool states need to be maintained to support the pooling
   mechanism itself, and how are such states maintained?  We also
   consider the information exchanged between the VNF and Service
   Control Entity.  For example, how can a VNF Pool be addressed by the
   Service Control Entity?  After a VNF instance failover, how does the
   Pool Manager notify the Service Control Entity of some characteristic
   changes of the VNF, e.g., capacity change, but without disclosure of
   the pooling procedure?



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   Note that we do not address the reliability related control or
   routing between adjacent VNFs that can form a network service, as
   such coordination could be done by the Service Control Entity.

   This document introduces a general idea of VNF Pool to support
   reliable functions provision by the VNFs.  We then highlight the
   reliability challenges and issues when using the VNFs to build
   services.  Related IETF works are also briefly described.

2.  Terminology

   Reliability: capability of a functional entity to consistently
   provide its function under various dynamic and even unexpected
   conditions such as fault, overload, etc.

   Service Control Entity: an entity of the service provider that
   decides how to combine and orchestrate the network functions to build
   network services.  Examples of Service Control Entity are
   orchestrator of DC services, SFC control plane, etc.

   Virtualized Network Function (VNF): a VNF provides the same
   functional behavior and interfaces as the equivalent network
   function, but is deployed as software instance(s) building on top of
   a virtualization layer [NFV-TERM].

   VNF Pool: a number of VNF instances providing the same network
   function.

   VNF Pool Element: a VNF instance inside a VNF pool.

   VNF Pool Manager: an entity that manages a VNF pool, and interacts
   with the Service Control Entity to provide the network function.

   VNF Set: a general set of VNF instances that can be grouped into
   multiple VNF Pools, where each pool corresponds to a specific VNF and
   different pools provide different functions.

3.  Background

3.1.  From Specialized Hardware to Virtualized Network Function

   Network functions are traditionally implemented on specialized
   hardware.  There is a trend to implement a number of network
   functions as software instances on general purpose servers, via
   virtualized computing platforms.  These virtualized functions are
   called Virtualized Network Functions (VNFs).  For example, in
   Figure 1, virtual firewall (vFW) can be deployed as software
   instances on general purpose servers, which could be located in Data



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   Center (DC) networks, network operators' networks, or end user
   premises.  Compared with traditional FW deployed as "standalone box"
   built by specialized hardware and software, vFW has potential
   advantages such as agility, scalability [NFV-WP].

          FW                  vFW           vFW           vFW
    +-------------+      +-----------+ +-----------+ +-----------+
    | Specialized |      |FW Software| |FW Software| |FW Software| ...
    | Hardware    |----\ +-----------+ +-----------+ +-----------+
    |  +          |----/ +------------------------------------------+
    | Software    |      |          Virtualization Platform         |
    +-------------+      +------------------------------------------+
                         +-----------------+  +-----------------+
                         | General Purpose |  | General Purpose |
                         | Server          |  | Server          | ...
                         +-----------------+  +-----------------+

                   Figure 1: Example of vFW.

3.2.  Concept of VNF Set

   We call a general set of VNF instances a VNF set.  A VNF set can
   include a single or multiple types of VNF, and each type of VNF may
   have a number of instances providing the same function.  The
   following examples are all valid VNF sets.

      1. n vFW instances: {vFW#1,vFW#2,...,vFW#n}.

      2. m vFW instances and k virtual load balancer (vLB) instances:
      {vFW#1,...,vFW#m,vLB#1,...,vLB#k}.

   To be more generic, we denote VNF-A#x the xth instance of a VNF of
   type A (e.g., vFW), VNF-B#y the yth instance of a VNF of type B
   (e.g., vLB), and so on.

   A VNF set can be used as part of a Service Function Chaining (SFC)
   [SFC], where the instances of various functions are sequentially
   connected to build a network service.  A simple example is shown in
   Figure 2.

                           Network Service
        +----------+           +----------+           +----------+
        |  VNF-A#x | data conn |  VNF-B#y | data conn |  VNF-C#z |
        |          |-----------|          |-----------|          |
        +----------+           +----------+           +----------+

                Figure 2: A VNF set used as part of a SFC.




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   Alternatively, a VNF set can be also used merely as a set of VNFs,
   where the instances provide network functions in a parallel way.  An
   example is shown in Figure 3.

             +----------+     +----------+     +----------+
             |  VNF-A#x |     |  VNF-B#y |     |  VNF-C#z |
             +----------+     +----------+     +----------+
                        \          |          /
               data conn \         |data     /data conn
                          \        |conn    /
                           \       |       /
                           +---------------+
                           |     Client    |
                           +---------------+

               Figure 3: A VNF set used as multiple VNFs.

   Some more detailed use cases of VNFs are documented in other drafts
   [VNFPOOL-UC1] [VNFPOOL-UC2] [VNFPOOL-UC3].

3.3.  Challenges to reliability

   The use of VNFs introduces additional challenges to the reliability
   of the provided network services.  For a VNF instance, it typically
   would not have built-in reliability mechanisms on its host (i.e., a
   general purpose server).  Instead, there are more factors of risk
   that may make VNF instance unreliable.

      1.  Instance failure due to hardware failure or status change such
      as server overload.

      2.  Instance failure due to software failure at various levels
      including hypervisor, Virtual Machine (VM), VNF.

      3.  Instance migration caused by instance performance downgrade
      caused by load (e.g., CPU, memory, disk I/O), server consolidation
      or other service requirement changes.  This is distinct from a
      hard failure, although it may give the appearance of one.

4.  VNF Pool

   There are a number of existing technologies for providing reliable
   functions, such as Reliable Server Pooling (RSerPool) [RFC5351],
   Virtual Router Redundancy Protocol (VRRP) [RFC5798], amongst many
   others.  Both technologies provide the service with an abstract
   object (e.g., pool handle in RSerPool, virtual router ID in VRRP)
   representing a group of identical functional instances.  The dynamic
   mapping of such abstract object to the actual serving instance is



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   managed internally in the group to cover the failover procedure.  The
   advantage is to provide reliable functions in a transparent manner
   for both end-hosts and service control entities.

   We adopt the similar idea of VNF Pool to provide reliable network
   functions, as shown in figure 4.

                         +------------------------+
                         | Service Control Entity |
                         +------------------------+
                                ^        ^
                                |        |
                    +-----------+        +------------+
                    |                                 |
                    v                                 v
   + - - - - - - - - - - - - - - - +  + - - - - - - - - - - - - - - - +
   |  VNF-A  +--------------+      |  |  VNF-B  +--------------+      |
   |         | Pool Manager |      |  |         | Pool Manager |      |
   |         +--------------+      |  |         +--------------+      |
   | + - - - - - - - - - - - - - + |  | + - - - - - - - - - - - - - + |
   | |+---------+     +---------+| |  | |+---------+     +---------+| |
   | || VNF-A#1 | ... | VNF-A#n || |  | || VNF-B#1 | ... | VNF-B#m || |
   | |+---------+     +---------+| |  | |+---------+     +---------+| |
   | |         VNF-A Pool        | |  | |         VNF-B Pool        | |
   | + - - - - - - - - - - - - - + |  | + - - - - - - - - - - - - - + |
   + - - - - - - - - - - - - - - - +  + - - - - - - - - - - - - - - - +

                        Figure 4: VNF Pool Architecture.

   In VNF Pool architecture, each VNF has a VNF Pool containing a number
   of VNF instances (or VNF Pool Elements) providing the same function.
   In this sense, a VNF set can be grouped into multiple VNF Pools,
   where each pool corresponds to a specific VNF, thus different pools
   provide different functions.  Each VNF also has a Pool Manager that
   manages the VNF instances in the VNF Pool.  Pool Manager interacts
   with the Service Control Entity to provide the network function.

   The main benefit of using VNF Pool is that the pooling mechanisms
   such as redundancy management are achieved by the VNF Pool inside the
   VNF and thus transparent to the Service Control Entity.  The Service
   Control Entity simply interacts with the Pool Manager in each VNF to
   request and orchestrate the network functions with desired
   reliability level.  In another word, a VNF Pool-enabled VNF still
   acts as a normal VNF when orchestrated by the Service Control Entity.







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5.  Challenges and Open Issues

5.1.  Redundancy model inside VNF

   Before a live VNF instance fails, one or more backup instances in the
   same VNF Pool need to be selected.  How to select such backup
   instances?  Moreover, there are policies influencing the appropriate
   selection of backup instance.  For example, it should be avoided that
   a live VNF instance and its backup instances are placed in a single
   physical server, or locations with shared risks in the network.  On
   the other hand, it would be desirable to place the live and backup
   instances in geographically closed locations.  Information from the
   underlying network may need to be collected via - e.g., the interface
   with Application Layer Traffic Optimization (ALTO) [ALTO], or
   Interface to Routing System (I2RS) [I2RS].  Various infrastructure
   conditions may also need to be considered for appropriate placement
   of instances.

5.2.  State synchronization inside VNF

   Service states related to the specific function performed by a VNF
   instance, e.g., NAT translation table, TCP connection states, should
   be synchronized between a live VNF instance and its backup instances
   for stateful failover.  Who is responsible for and how to collect,
   hold, and access such service states to achieve efficient
   synchronization?  A VNF instance should provide negotiated level of
   state sharing with the necessary performance to fulfill the service
   requirements - e.g., state synchronization method, format of state
   data, location and mechanism to access state data.

   Other than service states, pool states could be operational
   information of VNF pool itself, e.g. redundancy settings, backup
   location/status, etc.  What pool states need to be maintained to
   support the pooling mechanism itself, and how are such states
   maintained?

5.3.  Interaction between VNF and Service Control Entity

   Some information needs to be exchanged between a VNF and the Service
   Control Entity when the Service Control Entity orchestrates a VNF
   Pool-enable VNF.  For example, how can a VNF Pool be addressed by the
   Service Control Entity?  A Pool Manager can advertise the locator
   (e.g., IP address) of the active instance - subject to dynamic due to
   failover.  It is also possible to use a virtual address for the whole
   VNF Pool (similar to RSerPool or VRRP), and map between virtual and
   actual addresses.  Moreover, after a VNF instance failover, how does
   the Pool Manager notify the Service Control Entity of some




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   characteristic changes of the VNF, e.g., capacity change, but without
   disclosure of the pooling procedure?

5.4.  Reliable transport

   The transport mechanism used to carry the pool control messages,
   e.g., redundancy management, should provide reliable message
   delivery.  Transport redundancy mechanisms such as Multipath TCP
   (MPTCP) [MPTCP] and the Stream Control Transmission Protocol (SCTP)
   [RFC3286] will need to be evaluated for applicability.  Latency
   requirements for pool control message delivery must also be
   evaluated.

5.5.  Scope Considerations

   Ideally, the reliability goal is that the network service provided by
   the VNFs will continue throughout an interruption within the VNFs ,
   and VNF instances failure or migration will not be visible to the
   external entities.  Our work of VNF Pool initially focuses on several
   reliability mechanisms that are mainly associated with a redundancy
   model based on a VNF Pool.  Additional mechanisms may include pool
   state maintenance only for pooling purpose.  Service state
   synchronization is out of scope for this phase.

   We currently assume that a VNF Pool contains the instances of same
   functional type, e.g., FW, LB, etc.  Different types of VNFs are
   envisioned to be held in separate VNF Pools.  VNF Pool composed of
   both virtualized and non-virtualized functional instances may be
   included after further use case and requirements study.

   We are specifically concerned with the reliability of an individual
   VNF based on the VNF Pool managed inside the VNF.  We do not address
   the reliability related control or routing between adjacent VNFs that
   can form a network service, as such coordination could be done by the
   Service Control Entity.

   We do not intend to resolve the service availability that usually
   involves more factors including the interruptions in various OSI
   layers, and even user perception on service performance.

6.  Related Works

6.1.  Reliable Server Pooling (RSerPool)

   RSerPool supports high availability and scalability of the
   applications through the use of pools of servers [RFC5351].  The main
   functions of RSerPool involve server pool management, as well as
   receiving requests from a client to bind to a desired server.  The



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   applicability and gaps of RSerPool to our work of VNF Pool are
   described in another draft [VNFPOOL-RSP].

6.2.  Virtual Router Redundancy Protocol (VRRP)

   VRRP specifies an election protocol that dynamically assigns
   responsibility of a virtual router to one of the VRRP routers called
   master on a LAN [RFC5798].  The election process provides dynamic
   failover in the forwarding responsibility should the Master become
   unavailable.  The advantage of VRRP is a higher availability default
   path without requiring configuration of dynamic routing or router
   discovery protocols on every end-host.

6.3.  Service Function Chaining (SFC)

   A service chain defines an ordered set of service functions that must
   be applied to packets [SFC].  Although the VNFs can be used as part
   of a SFC, SFC and our work of VNF Pool have different focus.

   As mentioned in the section of scope consideration, we mostly
   consider the reliability of an individual VNF based on the VNF Pool
   inside the VNF.  We do not address the reliability related control or
   routing between adjacent VNFs in the forwarding graph.  Moreover,
   according to VNF Pool architecture and principles, the VNF Pools will
   be orthogonal to and invisible to the SFC.  A VNF Pool-enabled VNF
   still acts as a normal VNF when orchestrated by the SFC.  Just like
   the communication between any pool users and VNF Pool, the
   information exchanged between the VNF Pool and the SFC may include
   some operational information of the VNF Pool.

7.  Security Considerations

   Any technology which allows the insertion, deletion, reordering, or
   manipulation of network functions has the potential to be subverted
   by an attacker, with serious consequences.  Distributed VNFs
   introduce an additional attack vector, in which bad actors join
   several VNFs of a service.  Replay attacks have the potential to
   create denials of service, reordering, adding, or removing VNFs.  VNF
   reliability technologies must provide cryptographic protections
   against spoofing and insertion attacks as well as replay attacks, in
   the form of client authentication, origin authentication on VNF
   reliability management (control plane) traffic, and replay
   protections.  There may be circumstances under which an attacker
   masquerading as a VNF manager can introduce data leakage or similar
   attacks, and consequently server authentication would be required, as
   well.





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   Failing over a VNF or otherwise transferring service state raises
   issues related to the transfer of security state, including VNF
   element identity and credentials, session-associated cryptographic
   state, and so on.  Where possible, transfer of security state should
   be avoided as a matter of good practice, and this will require
   particular attention as solutions are drafted.

8.  IANA Considerations

   This document has no actions for IANA.

9.  Acknowledgements

   The authors would like to thank Chidung Lac from Orange, Daniel King
   from Lancaster University, Lingli Deng, Zhen Cao from China Mobile,
   Richard Yang from Yale University, Hidetoshi Yokota from KDDI,
   Mukhtiar Shaikh from Brocade, Qiang Zu from Ericsson, Marco Liebsch
   from NEC, Kapil Sood from Intel, Adrian Farrel, and Susan Hares for
   their valuable comments.

10.  References

10.1.  Normative References

   TBD.

10.2.  Informative References

   [NFV-WP] NFV Whitepaper: "Network Function Virtualization", issue 1,
   2012, http://portal.etsi.org/NFV/NFV_White_Paper.pdf.

   [SFC] "Service Function Chaining (SFC)",
   <http://datatracker.ietf.org/wg/sfc/>.

   [NFV-TERM] ETSI GS NFV 003: "Terminology for Main Conceptional
   Entities in NFV", Version 0.0.4, 2013.

   [VNFPOOL-UC1] L.  Xia, Q.  Wu, D.  King, H.  Yokota, and N.  Khan,
   "Requirements and Use Cases for Virtual Network Functions", draft-
   xia-vnfpool-use-cases-00, February 2014.

   [VNFPOOL-UC2] D.  King, M.  Liebsch, P.  Willis and J.  Ryoo,
   "Virtualization of Mobile Core Network Use Case", draft-king-vnfpool-
   mobile-use-case-00, February 2014.

   [VNFPOOL-UC3] S.  Hares and K.  Subramaniam, "Use Cases for Resource
   Pools with Virtual Network Functions (VNFs)", draft-hares-vnf-pool-
   use-case-00, January 2014.



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   [ALTO] "Application-Layer Traffic Optimization (alto)",
   <http://datatracker.ietf.org/wg/alto/>.

   [I2RS] "Interface to the Routing System (i2rs)",
   <http://datatracker.ietf.org/wg/i2rs/>.

   [MPTCP] "Multipath TCP (mptcp)", <http://datatracker.ietf.org/wg/
   mptcp/>.

   [RFC3286] L.  Ong and J.  Yoakum, "An Introduction to the Stream
   Control Transmission Protocol (SCTP)", RFC3286, May 2002.

   [NFV-REL] ETSI GS NFV REL 001: "Network Function Virtualization;
   Resiliency Requirements", Version 0.0.7, 2014.

   [NFV-SWA] ETSI GS NFV SWA 001: "Network Function Virtualization; SW
   Architecture; Virtual Network Functions Architecture", Version 0.1.0,
   2014.

   [RFC5351] P.  Lei, L.  Ong, M.  Tuexen and T.  Dreibholz, "An
   Overview of Reliable Server Pooling Protocols", RFC5351, September
   2008.

   [RFC5798] S.  Nadas, "Virtual Router Redundancy Protocol (VRRP)
   Version 3 for IPv4 and IPv6", RFC5798, March 2010.

   [VNFPOOL-RSP] T.  Dreibholz, M.  Tuexen, M.  Shore and N.  Zong, "The
   Applicability of Reliable Server Pooling (RSerPool) for Virtual
   Network Function Resource Pooling (VNFPOOL)", draft-dreibholz-
   vnfpool-rserpool-applic-00, October 2013.

Authors' Addresses

   Ning Zong
   Huawei Technologies

   Email: zongning@huawei.com


   Linda Dunbar
   Huawei Technologies

   Email: linda.dunbar@huawei.com








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   Melinda Shore
   No Mountain Software

   Email: melinda.shore@nomountain.net


   Diego Lopez
   Telefonica

   Email: diego@tid.es


   Georgios Karagiannis
   University of Twente

   Email: g.karagiannis@utwente.nl



































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