Network Working Group                                      M. Sridharan
Internet Draft                                             A. Greenberg
Intended Category: Informational                       N. Venkataramiah
Expires: August 25, 2013                                        Y. Wang
                                                                 K. Duda
                                                         Arista Networks
                                                                I. Ganga
                                                                  G. Lin
                                                              M. Pearson
                                                               P. Thaler
                                                             C. Tumuluri
                                                       February 25, 2013

     NVGRE: Network Virtualization using Generic Routing Encapsulation

Status of this Memo

   This memo provides information for the Internet Community. It does
   not specify an Internet standard of any kind; instead it relies on a
   proposed standard. Distribution of this memo is unlimited.

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   This document describes the usage of Generic Routing Encapsulation
   (GRE) header for Network Virtualization (NVGRE) in multi-tenant
   datacenters. Network Virtualization decouples virtual networks and
   addresses from physical network infrastructure, providing isolation
   and concurrency between multiple virtual networks on the same
   physical network infrastructure. This document also introduces a
   Network Virtualization framework to illustrate the use cases, but
   the focus is on specifying the data plane aspect of NVGRE.

Table of Contents

   1. Introduction...................................................3
      1.1. Terminology...............................................4
   2. Conventions used in this document..............................4
   3. Network Virtualization using GRE...............................4
      3.1. NVGRE Endpoint............................................5
      3.2. NVGRE frame format........................................5
   4. NVGRE Deployment Consideration.................................8
      4.1. Broadcast and Multicast Traffic...........................8
      4.2. Unicast Traffic...........................................9
      4.3. IP Fragmentation..........................................9
      4.4. Address/Policy Management & Routing.......................9
      4.5. Cross-subnet, Cross-premise Communication................10
      4.6. Internet Connectivity....................................12
      4.7. Management and Control Planes............................12
      4.8. NVGRE-Aware Devices......................................12
      4.9. Network Scalability with GRE.............................13
   5. Security Considerations.......................................14
   6. IANA Considerations...........................................14

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   7. References....................................................14
      7.1. Normative References.....................................14
      7.2. Informative References...................................14
   8. Acknowledgments...............................................15

1. Introduction

   Conventional data center network designs cater to largely static
   workloads and cause fragmentation of network and server capacity
   [VL2, COST-CCR]. There are several issues that limit dynamic
   allocation and consolidation of capacity. Layer-2 networks use Rapid
   Spanning Tree Protocol (RSTP) which is designed to eliminate loops
   by blocking redundant paths. These eliminated paths translate to
   wasted capacity and a highly oversubscribed network. There are
   alternative approaches such as TRILL that address this problem

   The network utilization inefficiencies are exacerbated by network
   fragmentation due to the use of VLANs for broadcast isolation. VLANs
   are used for traffic management and also as the mechanism for
   providing security and performance isolation among services
   belonging to different tenants. The Layer-2 network is carved into
   smaller sized subnets typically one subnet per VLAN, with VLAN tags
   configured on all the Layer-2 switches connected to server racks
   that run a given tenant's services. The current VLAN limits
   theoretically allow for 4K such subnets to be created. The size of
   the broadcast domain is typically restricted due to the overhead of
   broadcast traffic (e.g., ARP). The 4K VLAN limit is no longer
   sufficient in a shared infrastructure servicing multiple tenants.

   Data center operators must be able to achieve high utilization of
   server and network capacity. In order to achieve efficiency it
   should be possible to assign workloads that operate in a single
   Layer-2 network to any server in any rack in the network. It should
   also be possible to migrate workloads to any server anywhere in the
   network while retaining the workload's addresses. This can be
   achieved today by stretching VLANs however when workloads migrate
   the network needs to be reconfigured which is typically error prone.
   By decoupling the workload's location on the LAN from its network
   address, the network administrator configures the network once and
   not every time a service migrates. This decoupling enables any
   server to become part of any server resource pool.

   The following are key design objectives for next generation data
   centers: a) location independent addressing, b) the ability to a

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   scale the number of logical Layer-2/Layer-3 networks irrespective of
   the underlying physical topology or the number of concurrent VLANs,
   c) preserving Layer-2 semantics for services and allowing them to
   retain their addresses as they move within and across data centers,
   and d) providing broadcast isolation as workloads move around
   without burdening the network control plane.

1.1. Terminology

   Refer to [NVO3 Framework] or [NVO3 PA&S]

   NVE: Network Virtualization Endpoint

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

3. Network Virtualization using GRE

   This section describes Network Virtualization using GRE, NVGRE.
   Network virtualization involves creating virtual Layer 2 and/or
   Layer 3 topologies on top of an arbitrary physical Layer 2/Layer 3
   network. Connectivity in the virtual topology is provided by
   tunneling Ethernet frames in IP over the physical network. Virtual
   broadcast domains are realized as multicast distribution trees. The
   multicast distribution trees are analogous to the VLAN broadcast
   domains. A virtual Layer 2 network can span multiple physical
   subnets. Support for bi-directional IP unicast and multicast
   connectivity is the only requirement from the underlying physical
   network to support unicast communications within a virtual network.
   If the operator chooses to support broadcast and multicast traffic
   in the virtual topology the physical topology must support IP
   multicast. The physical network, for example, can be a conventional
   hierarchical 3-tier network, a full bisection bandwidth Clos
   network, or a large Layer 2 network with or without TRILL support.

   Every virtual Layer-2 network is associated with a 24 bit
   identifier, called Virtual Subnet Identifier (VSID). A 24 bit VSID
   allows up to 16 million virtual subnets in the same management
   domain in contrast to only 4K achievable with VLANs. Each VSID
   represents a virtual Layer-2 broadcast domain and routes can be

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   configured for communication between virtual subnets. The VSID can
   be crafted in such a way that it uniquely identifies a specific
   tenant's subnet. The VSID is carried in an outer header allowing
   unique identification of the tenant's virtual subnet to various
   devices in the network.

   GRE is a proposed IETF standard [RFC 2784, RFC 2890] and provides a
   way for encapsulating an arbitrary protocol over IP. NVGRE leverages
   the GRE header to carry VSID information in each packet. The VSID
   information in each packet can be used to build multi-tenanct-aware
   tools for traffic analysis, traffic inspection, and monitoring.

   The following sections detail the packet format for NVGRE, describe
   the functions of a NVGRE endpoint, illustrate typical traffic flow
   both within and across data centers, and discuss address, policy
   management and deployment considerations.

3.1. NVGRE Endpoint

   NVGRE endpoints are the ingress/egress points between the virtual
   and the physical networks. Any physical server or network device can
   be a NVGRE endpoint. One common deployment is for the endpoint to be
   part of a hypervisor. The primary function of this endpoint is to
   encapsulate/decapsulate Ethernet data frames to and from the GRE
   tunnel, ensure Layer-2 semantics, and apply isolation policy scoped
   on VSID. The endpoint can optionally participate in routing and
   function as a gateway in the virtual topology. To encapsulate an
   Ethernet frame, the endpoint needs to know the location information
   for the destination address in the frame. This information can be
   provisioned via a management plane, or obtained via a combination of
   control plane distribution or data plane learning approaches. This
   document assumes that the location information, including VSID, is
   available to the NVGRE endpoint.

3.2. NVGRE frame format

   GRE header format as specified in RFC 2784 and RFC 2890 is used for
   communication between NVGRE endpoints. NVGRE leverages the Key
   extension specified in RFC 2890 to carry the VSID. The packet format
   for Layer-2 encapsulation in GRE is shown in Figure 1.

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   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
   Outer Ethernet Header:             |
   |                (Outer) Destination MAC Address                |
   |(Outer)Destination MAC Address |  (Outer)Source MAC Address    |
   |                  (Outer) Source MAC Address                   |
   |Optional Ethertype=C-Tag 802.1Q| Outer VLAN Tag Information    |
   |       Ethertype 0x0800        |
   Outer IPv4 Header:
   |Version|  IHL  |Type of Service|          Total Length         |
   |         Identification        |Flags|      Fragment Offset    |
   |  Time to Live | Protocol 0x2F |         Header Checksum       |
   |                      (Outer) Source Address                   |
   |                  (Outer) Destination Address                  |
   GRE Header:
   |0| |1|0| Reserved0       | Ver |   Protocol Type 0x6558        |
   |               Virtual Subnet ID (VSID)        |   Reserved    |
   Inner Ethernet Header
   |                (Inner) Destination MAC Address                |
   |(Inner)Destination MAC Address |  (Inner)Source MAC Address    |
   |                  (Inner) Source MAC Address                   |
   |Optional Ethertype=C-Tag 802.1Q| PCP |0| VID set to 0          |
   |       Ethertype 0x0800        |

   (Continued on the next page)

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   Inner IPv4 Header:
   |Version|  IHL  |Type of Service|          Total Length         |
   |         Identification        |Flags|      Fragment Offset    |
   |  Time to Live |    Protocol   |         Header Checksum       |
   |                       Source Address                          |
   |                    Destination Address                        |
   |                    Options                    |    Padding    |
   |                      Original IP Payload                      |
   |                                                               |
   |                                                               |

                           GRE Encapsulation Frame Format

   The outer/delivery headers include the outer Ethernet header and the
   outer IP header:

   o The outer Ethernet header: The source Ethernet address in the
   outer frame is set to the MAC address associated with the NVGRE
   endpoint. The destination Ethernet address is set to the MAC address
   of the nexthop IP address for the destination NVE. The destination
   endpoint may or may not be on the same physical subnet. The outer
   VLAN tag information is optional and can be used for traffic
   management and broadcast scalability.

   o The outer IP header: Both IPv4 and IPv6 can be used as the
   delivery protocol for GRE. The IPv4 header is shown for illustrative
   purposes. Henceforth the IP address in the outer frame is referred
   to as the Provider Address (PA). There can be one or more PA address
   associated with an NVGRE endpoint, with policy controlling the
   choice of PA to use for a given CA.

   The GRE header:

   o The C (Checksum Present) and S (Sequence Number Present) bits in
   the GRE header MUST be zero.

   o The K bit (Key Present) in the GRE header MUST be one. The 32-bit
   Key field in the GRE header is used to carry the Virtual Subnet ID
   (VSID) and the optional FlowID:

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       - Virtual Subnet ID (VSID): The first 24 bits are used for VSID
         as shown in Figure 1.
       - FlowID: The last 8 bits of the Key field are (optional)
         FlowID, which can be used to add per-flow entropy within the
         same VSID, where the entire Key field (32-bit) is used for
         ECMP purposes by switches or routers in the physical network
         infrastructure. If a FlowID is not generated, the FlowID field
         MUST be set to all zero.

   o The protocol type field in the GRE header is set to 0x6558
   (transparent Ethernet bridging) [ETHTYPES].

   The inner headers (headers of the GRE payload):

   o The inner Ethernet frame comprises of an inner Ethernet header
   followed by the inner IP header, followed by the IP payload. The
   inner frame could be any Ethernet data frame not just IP. Note that
   the inner Ethernet frame's FCS is not encapsulated.

   o Inner VLAN tag: The inner Ethernet header of NVGRE SHOULD NOT
   contain inner VLAN Tag. When an NVE performs NVGRE encapsulation, it
   SHOULD remove any existing VLAN Tag before encapsulating NVGRE
   headers. If a VLAN-tagged frame arrives encapsulated in NVGRE, then
   the decapsulating NVE SHOULD drop the frame.

4. o For illustrative purposes IPv4 headers are shown as the inner IP
   headers but IPv6 headers may be used. Henceforth the IP address
   contained in the inner frame is referred to as the Customer Address
   (CA).NVGRE Deployment Consideration

4.1. Broadcast and Multicast Traffic

   The following discussion applies if the network operator chooses to
   support broadcast and multicast traffic. Each virtual subnet is
   assigned an administratively scoped multicast address to carry
   broadcast and multicast traffic. All traffic originating from within
   a VSID is encapsulated and sent to the assigned multicast address.
   As an example, the addresses can be derived from a administratively
   scoped multicast address as specified in RFC 2365 for IPv4
   (organization Local Scope, or an Organization-Local
   scope multicast address for IPv6 as specified in RFC 4291. This
   provides a wide range of address choices. Purely from an efficiency
   standpoint for every multicast address that a tenant uses the
   network operator may configure a corresponding multicast address in
   the PA space. To support broadcast and multicast traffic in the
   virtual topology the physical topology must support IP multicast.
   Depending on the hardware capabilities of the physical network

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   devices multiple virtual broadcast domains may be assigned the same
   physical IP multicast address. For interoperability reasons, a
   future version of this draft will specify a standard way to map VSID
   to IP multicast address.

4.2. Unicast Traffic

   The NVGRE endpoint encapsulates a Layer-2 packet in GRE using the
   source PA associated with the endpoint with the destination PA
   corresponding to the location of the destination endpoint. As
   outlined earlier there can be one or more PAs associated with an
   endpoint and policy will control which ones get used for
   communication. The encapsulated GRE packet is bridged and routed
   normally by the physical network to the destination. Bridging uses
   the outer Ethernet encapsulation for scope on the LAN. The only
   assumption is bi-directional IP connectivity from the underlying
   physical network. On the destination the NVGRE endpoint decapsulates
   the GRE packet to recover the original Layer-2 frame. Traffic flows
   similarly on the reverse path.

4.3. IP Fragmentation

   RFC 2003 section 5.1 specifies mechanisms for handling fragmentation
   when encapsulating IP within IP. The subset of mechanisms NVGRE
   selects are intended to ensure that NVGRE encapsulated frames are
   not fragmented after encapsulation en-route to the destination NVGRE
   endpoint, and that traffic sources can leverage Path MTU discovery.
   A future version of this draft will clarify the details around
   setting the DF bit on the outer IP header as well as maintaining per
   destination NVGRE endpoint MTU soft state so that ICMP Datagram Too
   Big messages can be exploited. Fragmentation behavior when tunneling
   non-IP Ethernet frames in GRE will also be specified in a future

4.4. Address/Policy Management & Routing

   Address acquisition is beyond the scope of this document and can be
   obtained statically, dynamically or using stateless address auto-
   configuration. CA and PA space can be either IPv4 or IPv6. In fact
   the address families don't have to match, for example, CA can be
   IPv4 while PA is IPv6 and vice versa. The isolation policies MUST be
   explicitly configured in the NVGRE endpoint. A typical policy table
   entry consists of CA, MAC address, VSID and optionally, the specific
   PA if more than one PA is associated with the NVGRE endpoint. If
   there are multiple virtual subnets, explicit routing information
   MUST be configured along with a default gateway for cross-subnet
   communication. Routing between virtual subnets can be optionally

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   handled by the NVGRE endpoint acting as a gateway. If
   broadcast/multicast support is required the NVGRE endpoints MUST
   participate in IGMP/MLD for all subscribed multicast groups.

4.5. Cross-subnet, Cross-premise Communication

   One application of this framework is that it provides a seamless
   path for enterprises looking to expand their virtual machine hosting
   capabilities into public clouds. Enterprises can bring their entire
   IP subnet(s) and isolation policies, thus making the transition to
   or from the cloud simpler. It is possible to move portions of a IP
   subnet to the cloud however that requires additional configuration
   on the enterprise network and is not discussed in this document.
   Enterprises can continue to use existing communications models like
   site-to-site VPN to secure their traffic.

   A VPN gateway is used to establish a secure site-to-site tunnel over
   the Internet and all the enterprise services running in virtual
   machines in the cloud use the VPN gateway to communicate back to the
   enterprise. For simplicity we use a VPN GW configured as a VM shown
   in Figure 2 to illustrate cross-subnet, cross-premise communication.

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   +-----------------------+        +-----------------------+
   |       Server 1        |        |       Server 2        |
   | +--------+ +--------+ |        | +-------------------+ |
   | | VM1    | | VM2    | |        | |    VPN Gateway    | |
   | | IP=CA1 | | IP=CA2 | |        | | Internal  External| |
   | |        | |        | |        | |  IP=CAg   IP=GAdc | |
   | +--------+ +--------+ |        | +-------------------+ |
   |       Hypervisor      |        |     | Hypervisor| ^   |
   +-----------------------+        +-------------------:---+
               | IP=PA1                   | IP=PA4    | :
               |                          |           | :
               |     +-------------------------+      | : VPN
               +-----|     Layer 3 Network     |------+ : Tunnel
                     +-------------------------+        :
                                  |                     :
        |                                               :  |
        |                     Internet                  :  |
        |                                               :  |
                                  |                     v
                                  |   +-------------------+
                                  |   |    VPN Gateway    |
                                  |---|                   |
                             IP=GAcorp| External IP=GAcorp|
                                    |  Corp Layer 3 Network |
                                    |      (In CA Space)    |
                                   |       Server X            |
                                   | +----------+ +----------+ |
                                   | | Corp VMe | | Corp VM2 | |
                                   | |  IP=CAe  | | IP=CAE2  | |
                                   | +----------+ +----------+ |
                                   |         Hypervisor        |
                     Cross-Subnet, Cross-Premise Communication

   The flow here is similar to the unicast traffic flow between VMs,
   the key difference in this case the packet needs to be sent to a VPN
   gateway before it gets forwarded to the destination. As part of
   routing configuration in the CA space, a VPN gateway is provisioned
   per-tenant for communication back to the enterprise. The example

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   illustrates an outbound connection between VM1 inside the datacenter
   and VMe inside the enterprise network. The outbound packet from CA1
   to CAe when it hits the hypervisor on Server 1 matches the default
   gateway rule as CAe is not part of the tenant virtual network in the
   datacenter. The packet is encapsulated and sent to the PA of tenant
   VPN gateway (PA4) running as a VM on Server 2. The packet is
   decapsulated on Server 2 and delivered to the VM gateway. The
   gateway in turn validates and sends the packet on the site-to-site
   tunnel back to the enterprise network. As the communication here is
   external to the datacenter the PA address for the VPN tunnel is
   globally routable. The outer header of this packet is sourced from
   GAdc destined to GAcorp. This packet is routed through the internet
   to the enterprise VPN gateway which is the other end of the site-to-
   site tunnel at which point the VPN decapsulates the packet and sends
   it inside the enterprise where the CAe is routable on the network.
   The reverse path is similar once the packet hits the enterprise VPN

4.6. Internet Connectivity

   To enable connectivity to the Internet, an Internet gateway is
   needed that bridges the virtualized CA space to the public Internet
   address space. The gateway performs translation between the
   virtualized world and the Internet, for example, the NVGRE endpoint
   can be part of a load balancer or a NAT. Section 4 has more
   discussions around building GRE gateways.

4.7. Management and Control Planes

   There are several protocols that can manage and distribute policy;
   however this document does not recommend any one mechanism.
   Implementations SHOULD choose a mechanism that meets their scale

4.8. NVGRE-Aware Devices

   One example of a typical deployment consists of virtualized servers
   deployed across multiple racks connected by one or more layers of
   Layer-2 switches which in turn may be connected to a layer 3 routing
   domain. Even though routing in the physical infrastructure will work
   without any modification with GRE, devices that perform specialized
   processing in the network need to be able to parse GRE to get access
   to tenant specific information. Devices that understand and parse
   the VSID can provide rich multi-tenancy aware services inside the
   data center. As outlined earlier it is imperative to exploit
   multiple paths inside the network through techniques such as Equal
   Cost Multipath (ECMP). The Key field may provide additional entropy

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   to the switches to exploit path diversity inside the network. One
   such example could be to use the upper 8 bits of the Key field to
   add flow based entropy and tag all the packets from a flow with an
   entropy label. A diverse ecosystem play is expected to emerge as
   more and more devices become multitenancy aware. In the interim,
   without requiring any hardware upgrades, there are alternatives to
   exploit path diversity with GRE by associating multiple PAs with
   NVGRE endpoints with policy controlling the choice of PA to be used.

   It is expected that communication can span multiple data centers and
   also cross the virtual to physical boundary. Typical scenarios that
   require virtual-to-physical communication includes access to storage
   and databases. Scenarios demanding lossless Ethernet functionality
   may not be amenable to NVGRE as traffic is carried over an IP
   network. NVGRE endpoints mediate between the network virtualized and
   non-network virtualized environments. This functionality can be
   incorporated into Top of Rack switches, storage appliances, load
   balancers, routers etc. or built as a stand-alone appliance.

   It is imperative to consider the impact of any solution on host
   performance. Today's server operating systems employ sophisticated
   acceleration techniques such as checksum offload, Large Send Offload
   (LSO), Receive Segment Coalescing (RSC), Receive Side Scaling (RSS),
   Virtual Machine Queue (VMQ) etc. These technologies should become
   GRE aware. IPsec Security Associations (SA) can be offloaded to the
   NIC so that computationally expensive cryptographic operations are
   performed at line rate in the NIC hardware. These SAs are based on
   the IP addresses of the endpoints. As each packet on the wire gets
   translated, the NVGRE endpoint SHOULD intercept the offload requests
   and do the appropriate address translation. This will ensure that
   IPsec continues to be usable with network virtualization while
   taking advantage of hardware offload capabilities for improved

4.9. Network Scalability with GRE

   One of the key benefits of using GRE is the IP address scalability
   and in turn MAC address table scalability that can be achieved.
   NVGRE endpoint can use one PA to represent multiple CAs. This lowers
   the burden on the MAC address table sizes at the Top of Rack
   switches. One obvious benefit is in the context of server
   virtualization which has increased the demands on the network
   infrastructure. By embedding a NVGRE endpoint in a hypervisor it is
   possible to scale significantly. This framework allows for location
   information to be preconfigured inside a NVGRE endpoint allowing
   broadcast ARP traffic to be proxied locally. This approach can scale
   to large sized virtual subnets. These virtual subnets can be spread

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   across multiple layer-3 physical subnets. It allows workloads to be
   moved around without imposing a huge burden on the network control
   plane. By eliminating most broadcast traffic and converting others
   to multicast the routers and switches can function more efficiently
   by building efficient multicast trees. By using server and network
   capacity efficiently it is possible to drive down the cost of
   building and managing data centers.

5. Security Considerations

   This proposal extends the Layer-2 subnet across the data center and
   increases the scope for spoofing attacks. Mitigations of such
   attacks are possible with authentication/encryption using IPsec or
   any other IP based mechanism. The control plane for policy
   distribution is expected to be secured by using any of the existing
   security protocols. Further management traffic can be isolated in a
   separate subnet/VLAN.

6. IANA Considerations


7. References

7.1. Normative References

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


7.2. Informative References

   [VL2] A. Greenberg et al, "VL2: A Scalable and Flexible Data Center
         Network", Proc. SIGCOMM 2009.

   [COST-CCR] A. Greenberg et al, "The Cost of a Cloud: Research
               Problems in the Data Center", ACM SIGCOMM Computer
               Communication Review.

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8. Acknowledgments

   This document was prepared using

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Authors' Addresses

   Murari Sridharan
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

   Kenneth Duda
   Arista Networks, Inc.
   5470 Great America Pkwy
   Santa Clara, CA 95054

   Ilango Ganga
   Intel Corporation
   2200 Mission College Blvd.
   M/S: SC12-325
   Santa Clara, CA - 95054

   Albert Greenberg
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

   Geng Lin
   One Dell Way
   Round Rock, TX 78682

   Mark Pearson
   Hewlett-Packard Co.
   8000 Foothills Blvd.
   Roseville, CA 95747

   Patricia Thaler
   Broadcom Corporation
   3151 Zanker Road

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   San Jose, CA 95134

   Chait Tumuluri
   Emulex Corporation
   3333 Susan Street
   Costa Mesa, CA 92626

   Narasimhan Venkataramiah
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

   Yu-Shun Wang
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

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