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Seamless Interconnect Underlay to Cloud Overlay Problem Statement

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Authors Linda Dunbar , Andrew G. Malis , Christian Jacquenet , Mehmet Toy
Last updated 2019-02-12
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Network Working Group                                         L. Dunbar
Internet Draft                                                 A. Malis
Intended status: Informational                                   Huawei
Expires: July  2019                                        C. Jacquenet
                                                                 M. Toy
                                                      February 12, 2019

     Seamless Interconnect Underlay to Cloud Overlay Problem Statement


   This document describes the problems that enterprises face today
   when connecting their branch offices to dynamic workloads in third
   party data centers (a.k.a. Cloud DCs).

   It examines some of the approaches interconnecting cloud DCs with
   enterprises' on-premises DCs & branch offices. This document also
   describes some of the (network) problems that many enterprises face
   when they have workloads & applications & data split among hybrid
   data centers, especially for those enterprises with multiple sites
   that are already interconnected by VPNs (e.g., MPLS L2VPN/L3VPN).

   Current operational problems are examined to determine whether there
   is a need to improve existing protocols or whether a new protocol is
   necessary to solve them.

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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Table of Contents

   1. Introduction...................................................3
   2. Definition of terms............................................4
   3. Current Practices in Interconnecting Enterprise Sites with Cloud
      3.1. Interconnect to Cloud DCs.................................5
      3.2. Interconnect to Hybrid Cloud DCs..........................7
      3.3. Connecting workloads among hybrid Cloud DCs...............7
   4. Desired Properties for Networks that interconnect Hybrid Clouds8
   5. Problems with MPLS-based VPNs extending to Hybrid Cloud DCs....9
   6. Problem with using IPsec tunnels to Cloud DCs.................10
      6.1. Complexity of multi-point any-to-any interconnection.....10
      6.2. Poor performance over long distance......................11
      6.3. Scaling Issues with IPsec Tunnels........................11
   7. Problems of Using SD-WAN to connect to Cloud DCs..............12
      7.1. SD-WAN among branch offices vs. interconnect to Cloud DCs12

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   8. End-to-End Security Concerns for Data Flows...................15
   9. Requirements for Dynamic Cloud Data Center VPNs...............15
   10. Security Considerations......................................16
   Solution drafts resulting from this work will address security
   concerns inherent to the solution(s), including both protocol
   aspects and the importance (for example) of securing workloads in
   cloud DCs and the use of secure interconnection mechanisms.......16
   IANA Considerations..............................................16
   11. References...................................................16
      11.1. Normative References....................................16
      11.2. Informative References..................................16
   12. Acknowledgments..............................................17

1. Introduction

   The ever-increasing use of cloud applications for communication
   services change the way corporate business works and shares
   information. Such cloud applications use resources hosted in third
   party DCs that also host services for other customers.

   With the advent of widely available third party cloud DCs in diverse
   geographic locations and the advancement of tools for monitoring and
   predicting application behaviors, it is technically feasible for
   enterprises to instantiate applications and workloads in locations
   that are geographically closest to their end-users. Such proximity
   improves end-to-end latency and overall user experience. Conversely,
   an enterprise can easily shutdown applications and workloads
   whenever end-users are in motion (thereby modifying the networking
   connection of subsequently relocated applications and workloads). In
   addition, an enterprise may wish to take advantage of more and more
   business applications offered by third party private cloud DCs.

   Most of those enterprise branch offices & on-premises data centers
   are already connected via VPNs, such as MPLS-based L2VPNs and
   L3VPNs. Then connecting to the cloud-hosted resources may not be
   straightforward if the provider of the VPN service does not have
   direct connections to the corresponding cloud DCs. Under those
   circumstances, the enterprise can upgrade the CPEs deployed in its
   various premises to utilize SD-WAN techniques to reach cloud
   resources (without any assistance from the VPN service provider), or
   wait for their VPN service provider to make new agreements with data

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   center providers to connect to the cloud resources. Either way has
   additional infrastructure and operational costs.

   In addition, it is an uptrend with more enterprises instantiating
   their apps & workloads in different cloud DCs to maximize the
   benefits of geographical proximity, elasticity and special features
   offered by different cloud DCs.

2. Definition of terms

   Cloud DC:   Third party Data Centers that usually host applications
               and workload owned by different organizations or

   Controller: Used interchangeably with SD-WAN controller to manage
               SD-WAN overlay path creation/deletion and monitoring the
               path conditions between two or more sites.

   DSVPN:      Dynamic Smart Virtual Private Network. DSVPN is a secure
               network that exchanges data between sites without
               needing to pass traffic through an organization's
               headquarter virtual private network (VPN) server or

   Heterogeneous Cloud: applications & workloads split among Cloud DCs
               owned & managed by different operators.

   Hybrid Clouds: Hybrid Clouds (usually plural) refer to enterprises
               using their own premises DCs in addition to Cloud
               services provided by multiple cloud operators.  For
               example, an enterprise not only have applications
               running in their own DCs, but also have applications
               hosted in multiple third party cloud DCs ((AWS, Azure,
               Google, Salesforces, SAP, etc).  . ONUG also has a
               notion of heterogeneous cloud, refers to enterprises
               does not have its own DC, only uses services by 3rd
               party cloud operators.

   SD-WAN:     Software Defined Wide Area Network. In this document,
               "SD-WAN" refers to the solutions specified by ONUG (Open
               Network User Group),

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               defined-wide-area-network-sd-wan/, which is about
               pooling WAN bandwidth from multiple underlay networks to
               get better WAN bandwidth management, visibility &
               control. When the underlay networks are private
               networks, traffic can traverse without additional
               encryption; when the underlay networks are public, such
               as Internet, some traffic needs to be encrypted when
               traversing through (depending on user provided

   VPC:        Virtual Private Cloud. A service offered by Cloud DC
               operators to allocate logically-isolated cloud
               resources, including compute, networking and storage.

3. Current Practices in Interconnecting Enterprise Sites with Cloud DCs

3.1. Interconnect to Cloud DCs

   Most Cloud operators offer some type of network gateway through
   which an enterprise can reach their workloads hosted in the Cloud
   DCs. For example, AWS (Amazon Web Services) offers the following
   options to reach workloads in AWS Cloud DCs:

     - Internet gateway for any external entities to reach the
        workloads hosted in AWS Cloud DC via the Internet.
     - Virtual gateway (vGW) where IPsec tunnels [RFC6071] are
        established between an enterprise's own gateway and AWS vGW, so
        that the communications between those gateways can be secured
        from the underlay (which might be the public Internet).
     - Direct Connect, which allows enterprises to purchase direct
        connect from network service providers to get a private leased
        line interconnecting the enterprises gateway(s) and the AWS
        Direct Connect routers. Via Direct Connect, an AWS Transit
        Gateway can be used to interconnect multiple VPCs in different
        Availability Zones.

   CPEs at one Enterprise branch office are connected to the Internet
   to reach AWS's vGW via IPsec tunnels. Other ports of such CPEs are
   connected to AWS DirectConnect via a private network (without any

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     |    ,---.         ,---. |
     |   (TN-1 )       ( TN-2)|
     |    `-+-'  +--+   `-+-' |
     |      +----|vR|-----+   |
     |           ++-+         |
     |            |         +-+----+
     |            |        /Internet\ For External
     |            +-------+ Gateway  +----------------------
     |                     \        / to reach via Internet
     |                      +-+----+
     |                        |

     |    ,---.         ,---. |
     |   (TN-1 )       ( TN-2)|
     |    `-+-'  +--+   `-+-' |
     |      +----|vR|-----+   |
     |           ++-+         |
     |            |         +-+----+
     |            |        / virtual\ For IPsec Tunnel
     |            +-------+ Gateway  +----------------------
     |                     \        /  termination
     |                      +-+----+
     |                        |

     |    ,---.         ,---. |
     |   (TN-1 )       ( TN-2)|
     |    `-+-'  +--+   `-+-' |
     |      +----|vR|-----+   |
     |           ++-+         |
     |            |         +-+----+              +------+
     |            |        /        \ For Direct /customer\
     |            +-------+ Gateway  +----------+ gateway  |
     |                     \        /  Connect   \        /
     |                      +-+----+              +------+
     |                        |

     Figure 1: Examples of Cloud DC connections.

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3.2. Interconnect to Hybrid Cloud DCs

   According to Gartner, by 2020 "hybrid will be the most common usage
   of the cloud" as more enterprises see the benefits of integrating
   public and private cloud infrastructures. However, enabling the
   growth of hybrid cloud deployments in the enterprise requires fast
   and safe interconnection between public and private cloud services.
   For an enterprise to connect to applications & workloads hosted in
   multiple Cloud DCs, the enterprise can use IPsec tunnels established
   over the Internet or a (virtualized) leased line service to connect
   its on-premises gateways to each of the Cloud DC's gateways, virtual
   routers instantiated in the Cloud DCs, or any other suitable design
   (including a combination thereof).

   Some enterprises prefer to instantiate their own virtual
   CPEs/routers inside the Cloud DC to connect the workloads within the
   Cloud DC. Then an overlay path is established between customer
   gateways to the virtual CPEs/routers for reaching the workloads
   inside the cloud DC.

3.3. Connecting workloads among hybrid Cloud DCs

   There are multiple approaches to interconnect workloads among
   different Cloud DCs:

     - Utilize Cloud DC provided transit gateways, which usually does
        not work if Cloud DCs are owned and managed by different Cloud
     - Hairpin all the traffic through the customer gateway, which
        creates additional transmission delay & incurs cost when
        exiting Cloud DCs, or
     - Establish direct tunnels among different VPCs (Virtual Private
        Clouds) via client's own virtual routers instantiated within
        Cloud DCs. DMVPN (Dynamic Multipoint Virtual Private Network)
        or DSVPN (Dynamic Smart VPN) techniques can be used to
        establish direct Multi-point-to-Point or multi-point-to multi-
        point tunnels among those client's own virtual routers.

   DMVPN & DSVPN use NHRP (Next Hop Resolution Protocol) [RFC2735] so
   that spoke nodes can register their IP addresses & WAN ports with
   the hub node. The IETF ION (Internetworking over NBMA (non-broadcast

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   multiple access) WG standardized NHRP for connection-oriented NBMA
   network (such as ATM) network address resolution more than two
   decades ago.

   There are many differences between virtual routers in Public Cloud
   DCs and the nodes in an NBMA network. NHRP & DSVPN are not cannot be
   used for registering virtual routers in Cloud DCs unless an
   extension of such protocols is developed for that purpose.  Other
   protocols such as BGP can be used, as described in [BGP-SDWAN].

4. Desired Properties for Networks that interconnect Hybrid Clouds
   The networks that interconnect hybrid cloud DCs must address the
   following requirements:
     - High availability at any time, whatever the duration of the
        connection to the cloud DC.
        Many enterprises include cloud infrastructures in their
        disaster recovery strategy, e.g., by enforcing periodic backup
        policies within the cloud, or by running backup applications in
        the Cloud, etc. Therefore, the connection to the cloud DCs may
        not be permanent, but rather needs to be on-demand.

     - Global reachability from different geographical zones, thereby
        facilitating the proximity of applications as a function of the
        end users' location, to improve latency.
     - Elasticity and mobility, to instantiate additional applications
        at Cloud DCs when end-users' usages increase and shut down
        applications at locations when there are fewer end-users.
        Some enterprises have front-end web portals running in cloud
        DCs and database servers in their on-premises DCs. Those Front-
        end web portals need to be reachable from the public Internet.
        The backend connection to the sensitive data in database
        servers hosted in the on-premises DCs might need secure

     - Scalable security management. IPsec is commonly used to
        interconnect cloud gateways with CPEs deployed in the
        enterprise premises. For enterprises with a large number or
        branch offices, managing the IPsec's Security Associations
        among many nodes can be very difficult.

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5. Problems with MPLS-based VPNs extending to Hybrid Cloud DCs

   Traditional MPLS-based VPNs have been widely deployed as an
   effective way to support businesses and organizations that require
   network performance and reliability. MPLS shifted the burden of
   managing a VPN service from enterprises to service providers. The
   CPEs attached to MPLS VPNs are also simpler and less expensive,
   since they do not need to manage routes to remote sites; they simply
   pass all outbound traffic to the MPLS VPN PEs to which the CPEs are
   attached (albeit multi-homing scenarios require more processing
   logic on CPEs).  MPLS has addressed the problems of scale,
   availability, and fast recovery from network faults, and
   incorporated traffic-engineering capabilities.

   However, traditional MPLS-based VPN solutions are sub-optimized for
   connecting end-users to dynamic workloads/applications in cloud DCs

     - The Provider Edge (PE) nodes of the enterprise's VPNs might not
        have direct connections to third party cloud DCs that are used
        for hosting workloads with the goal of providing an easy access
        to enterprises' end-users.

     - It usually takes some time to deploy provider edge (PE) routers
        at new locations. When enterprise's workloads are changed from
        one cloud DC to another (i.e., removed from one DC and re-
        instantiated to another location when demand changes), the
        enterprise branch offices need to be connected to the new cloud
        DC, but the network service provider might not have PEs located
        at the new location.

        One of the main drivers for moving workloads into the cloud is
        the widely available cloud DCs at geographically diverse
        locations, where apps can be instantiated so that they can be
        as close to their end-users as possible. When the user base
        changes, the applications may be migrated to a new cloud DC
        location closest to the new user base.

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     - Most of the cloud DCs do not expose their internal networks, so
        the MPLS-based VPNs can only reach Cloud DC's Gateways, not to
        the workloads hosted inside.

     - Many cloud DCs use an overlay to connect their gateways to the
        workloads located inside the DC. There has not been any
        standard to address the interworking between the Cloud Overlay
        and the enterprise' existing underlay networks.

   Another roadblock is the lack of a standard way to express and
   enforce consistent security policies for workloads that not only use
   virtual addresses, but in which are also very likely hosted in
   different locations within the Cloud DC [RFC8192]. The current VPN
   path computation and bandwidth allocation schemes may not be
   flexible enough to address the need for enterprises to rapidly
   connect to dynamically instantiated (or removed) workloads and
   applications regardless of their location/nature (i.e., third party
   cloud DCs).

6. Problem with using IPsec tunnels to Cloud DCs
   As described in the previous section, many Cloud operators expose
   their gateways for external entities (which can be enterprises
   themselves) to directly establish IPsec tunnels. Enterprises can
   also instantiate virtual routers within Cloud DCs to connect to
   their on-premises devices via IPsec tunnels. If there is only one
   enterprise location that needs to reach the Cloud DC, an IPsec
   tunnel is a very convenient solution.

   However, many medium-to-large enterprises usually have multiple
   sites and multiple data centers. For workloads and apps hosted in
   cloud DCs, multiple sites need to communicate securely with those
   cloud workloads and apps. This section documents some of the issues
   associated with using IPsec tunnels to connect enterprise premises
   with cloud gateways.

6.1. Complexity of multi-point any-to-any interconnection

   The dynamic workload instantiated in cloud DC needs to communicate
   with multiple branch offices and on-premises data centers. Most
   enterprises need multi-point interconnection among multiple
   locations, which can be provided by means of MPLS L2/L3 VPNs.

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   Using IPsec overlay paths to connect all branches & on-premises data
   centers to cloud DCs requires CPEs to manage routing among Cloud DCs
   gateways and the CPEs located at other branch locations, which can
   dramatically increase the complexity of the design, possibly at the
   cost of jeopardizing the CPE performance.

   The complexity of requiring CPEs to maintain routing among other
   CPEs is one of the reasons why enterprises migrated from Frame Relay
   based services to MPLS-based VPN services.

   MPLS-based VPNs have their PEs directly connected to the CPEs.
   Therefore, CPEs only need to forward all traffic to the directly
   attached PEs, which are therefore responsible for enforcing the
   routing policy within the corresponding VPNs. Even for multi-homed
   CPEs, the CPEs only need to forward traffic among the directly
   connected PEs. However, when using IPsec tunnels between CPEs and
   Cloud DCs, the CPEs need to compute, select, establish and maintain
   routes for traffic to be forwarded to Cloud DCs, to remote CPEs via
   VPN, or directly.

6.2. Poor performance over long distance

   When enterprise CPEs or gateways are far away from cloud DC gateways
   or across country/continent boundaries, performance of IPsec tunnels
   over the public Internet can be problematic and unpredictable. Even
   though there are many monitoring tools available to measure delay
   and various performance characteristics of the network, the
   measurement for paths over the Internet is passive and past
   measurements may not represent future performance.

   Many cloud providers can replicate workloads in different available
   zones. An App instantiated in a cloud DC closest to clients may have
   to cooperate with another App (or its mirror image) in another
   region or database server(s) in the on-premises DC. This kind of
   coordination requires predicable networking behavior/performance
   among those locations.

6.3. Scaling Issues with IPsec Tunnels

   IPsec can achieve secure overlay connections between two locations
   over any underlay network, e.g., between CPEs and Cloud DC Gateways.

   If there is only one enterprise location connected to the cloud
   gateway, a small number of IPsec tunnels can be configured on-demand

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   between the on-premises DC and the Cloud DC, which is an easy and
   flexible solution.

   However, for multiple enterprise locations to reach workloads hosted
   in cloud DCs, the cloud DC gateway needs to maintain multiple IPsec
   tunnels to all those locations (e.g., as a hub & spoke topology).
   For a company with hundreds or thousands of locations, there could
   be hundreds (or even thousands) of IPsec tunnels terminating at the
   cloud DC gateway, which is not only very expensive (because Cloud
   Operators usually charge their customers based on connections), but
   can be very processing intensive for the gateway. Many cloud
   operators only allow a limited number of (IPsec) tunnels & bandwidth
   to each customer.  Alternatively, you could use a solution like
   group encryption where a single IPsec SA is necessary at the GW but
   the drawback here is key distribution and maintenance of a key
   server, etc.

7. Problems of Using SD-WAN to connect to Cloud DCs
   SD-WAN can establish parallel paths over multiple underlay networks
   between two locations on-demand, for example, to support the
   connections established between two CPEs interconnected by a
   traditional MPLS VPN ([RFC4364] or [RFC4664]) or by IPsec [RFC6071]

   SD-WAN lets enterprises augment their current VPN network with cost-
   effective, readily available Broadband Internet connectivity,
   enabling some traffic offloading to paths over the Internet
   according to differentiated, possibly application-based traffic
   forwarding policies, or when the MPLS VPN connection between the two
   locations is congested, or otherwise undesirable or unavailable.

7.1. SD-WAN among branch offices vs. interconnect to Cloud DCs

   SD-WAN interconnection of branch offices is not as simple as it
   appears. For an enterprise with multiple sites, using SD-WAN overlay
   paths among sites requires each CPE to manage all the addresses that
   local hosts have the potential to reach, i.e., map internal VPN
   addresses to appropriate SD-WAN paths. This is similar to the
   complexity of Frame Relay based VPNs, where each CPE needed to
   maintain mesh routing for all destinations if they were to avoid an
   extra hop through a hub router. Even though SD-WAN CPEs can get
   assistance from a central controller (instead of running a routing
   protocol) to resolve the mapping between destinations and SD-WAN
   paths, SD-WAN CPEs are still responsible for routing table

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   maintenance as remote destinations change their attachments, e.g.,
   the dynamic workload in other DCs are de-commissioned or added.

   Even though originally envisioned for interconnecting branch
   offices, SD-WAN offers a very attractive way for enterprises to
   connect to Cloud DCs.

   The SD-WAN for interconnecting branch offices and the SD-WAN for
   interconnecting to Cloud DCs have some differences:

     - SD-WAN for interconnecting branch offices usually have two end-
        points (e.g., CPEs) controlled by one entity (e.g., a
        controller or management system operated by the enterprise).
     - SD-WAN for Cloud DC interconnects may consider CPEs owned or
        managed by the enterprise, while remote end-points are being
        managed or controlled by Cloud DCs (For the ease of
        description, let's call such CPEs asymmetrically-managed CPEs).

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     - Cloud DCs may have different entry points (or devices) with one
        entry point that terminates a private direct connection (based
        upon a leased line for example) and other entry points being
        devices terminating the IPsec tunnels, as shown in Figure 2.

     Therefore, the SD-WAN design becomes asymmetric.
     |    ,---.         ,---. |
     |   (TN-1 )       ( TN-2)|
     |    `-+-'  +---+  `-+-' |
     |      +----|vR1|----+   |
     |           ++--+        |
     |            |         +-+----+
     |            |        /Internet\ One path via
     |            +-------+ Gateway  +---------------------+
     |                     \        /   Internet            \
     |                      +-+----+                         \
     +------------------------+                               \
     +------------------------+                 native traffic  \
     |    ,---.         ,---. |                without encryption|
     |   (TN-3 )       ( TN-4)|             |
     |    `-+-'  +--+   `-+-' |                                  |      +------+
     |      +----|vR|-----+   |                                  +------+ CPE  |
     |           ++-+         |                                  |      +------+
     |            |         +-+----+                             |
     |            |        / virtual\ One path via IPsec Tunnel  |
     |            +-------+ Gateway  +-------------------------- +
     |                     \        /      Encrypted traffic over|
     |                      +-+----+          public network     |
     +------------------------+                                  |
     +------------------------+                                  |
     |    ,---.         ,---. |                   Native traffic |
     |   (TN-5 )       ( TN-6)|               without encryption |
     |    `-+-'  +--+   `-+-' |               over secure network|
     |      +----|vR|-----+   |                                  |
     |           ++-+         |                                  |
     |            |         +-+----+              +------+       |
     |            |        /        \ Via Direct /customer\      |
     |            +-------+ Gateway  +----------+ gateway  |-----+
     |                     \        /  Connect   \        /
     |                      +-+----+              +------+

     Figure 2: Different Underlays to Reach Cloud DC

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8. End-to-End Security Concerns for Data Flows

     When IPsec tunnels established from enterprise on-premises CPEs
     are terminated at the Cloud DC gateway where the workloads or
     applications are hosted, some enterprises have concerns regarding
     traffic to/from their workload being exposed to others behind the
     data center gateway (e.g., exposed to other organizations that
     have workloads in the same data center).
     To ensure that traffic to/from workloads is not exposed to
     unwanted entities, IPsec tunnels may go all the way to the
     workload (servers, or VMs) within the DC.

9. Requirements for Dynamic Cloud Data Center VPNs

   In order to address the aforementioned issues, any solution for
   enterprise VPNs that includes connectivity to dynamic workloads or
   applications in cloud data centers should satisfy a set of

     - The solution should allow enterprises to take advantage of the
        current state-of-the-art in VPN technology, in both traditional
        MPLS-based VPNs and IPsec-based VPNs (or any combination
        thereof) that run over the public Internet.
     - The solution should not require an enterprise to upgrade all
        their existing CPEs.
     - The solution should support scalable IPsec key management among
        all nodes involved in DC interconnect schemes.
     - The solution needs to support easy and fast, on-the-fly, VPN
        connections to dynamic workloads and applications in third
        party data centers, and easily allow these workloads to migrate
        both within a data center and between data centers.
     - Allow VPNs to provide bandwidth and other performance
     - Be a cost-effective solution for enterprises to incorporate
        dynamic cloud-based applications and workloads into their
        existing VPN environment.

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10. Security Considerations

   The draft discusses security requirements as a part of the problem
   space, particularly in sections 4, 5, and 8.

   Solution drafts resulting from this work will address security
   concerns inherent to the solution(s), including both protocol
   aspects and the importance (for example) of securing workloads in
   cloud DCs and the use of secure interconnection mechanisms.

   IANA Considerations

   This document requires no IANA actions. RFC Editor: Please remove
   this section before publication.

11. References

    11.1. Normative References

    11.2. Informative References

   [RFC2735]   B. Fox, et al "NHRP Support for Virtual Private
   networks". Dec. 1999.

   [RFC8192] S. Hares, et al "Interface to Network Security Functions
             (I2NSF) Problem Statement and Use Cases", July 2017

    [ITU-T-X1036] ITU-T Recommendation X.1036, "Framework for creation,
             storage, distribution and enforcement of policies for
             network security", Nov 2007.

    [RFC6071] S. Frankel and S. Krishnan, "IP Security (IPsec) and
             Internet Key Exchange (IKE) Document Roadmap", Feb 2011.

   [RFC4364] E. Rosen and Y. Rekhter, "BGP/MPLS IP Virtual Private
             Networks (VPNs)", Feb 2006

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   [RFC4664] L. Andersson and E. Rosen, "Framework for Layer 2 Virtual
             Private Networks (L2VPNs)", Sept 2006.

   [BGP-SDWAN] L. Dunbar, et al. "BGP Extension for SDWAN Overlay
             Networks", draft-dunbar-idr-bgp-sdwan-overlay-ext-03,
             work-in-progress, Nov 2018.

12. Acknowledgments

   Many thanks to Ignas Bagdonas, Michael Huang, Liu Yuan Jiao,
   Katherine Zhao, and Jim Guichard for the discussion and

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Authors' Addresses

   Linda Dunbar

   Andrew G. Malis

   Christian Jacquenet
   Rennes, 35000

   Mehmet Toy
   One Verizon Way
   Basking Ridge, NJ 07920

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