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NVGRE: Network Virtualization Using Generic Routing Encapsulation

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7637.
Authors Pankaj Garg , Yu-Shun Wang
Last updated 2015-10-14 (Latest revision 2015-04-13)
RFC stream Independent Submission
Intended RFC status Informational
IETF conflict review conflict-review-sridharan-virtualization-nvgre
Stream ISE state Published RFC
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Document shepherd Eliot Lear
Shepherd write-up Show Last changed 2015-04-21
IESG IESG state Became RFC 7637 (Informational)
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IANA IANA review state IANA OK - No Actions Needed
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Network Working Group                                       P. Garg Ed.
Internet Draft                                              Y. Wang Ed.
Intended Category: Informational                              Microsoft
Expires: October 12, 2015                                April 13, 2015

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

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   This Internet-Draft will expire on October 12, 2015.

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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...................................................2
      1.1. Terminology...............................................4
   2. Conventions used in this document..............................4
   3. NVGRE: Network Virtualization using GRE........................5
      3.1. NVGRE Endpoint............................................5
      3.2. NVGRE frame format........................................6
      3.3. Inner 802.1Q Tag..........................................9
      3.4. Reserved VSID.............................................9
   4. NVGRE Deployment Consideration................................10
      4.1. ECMP Support.............................................10
      4.2. Broadcast and Multicast Traffic..........................10
      4.3. Unicast Traffic..........................................10
      4.4. IP Fragmentation.........................................11
      4.5. Address/Policy Management & Routing......................11
      4.6. Cross-subnet, Cross-premise Communication................11
      4.7. Internet Connectivity....................................13
      4.8. Management and Control Planes............................13
      4.9. NVGRE-Aware Devices......................................13
      4.10. Network Scalability with NVGRE..........................14
   5. Security Considerations.......................................15
   6. IANA Considerations...........................................15
   7. References....................................................15
      7.1. Normative References.....................................15
      7.2. Informative References...................................16
   8. Authors and Contributors......................................16
   9. Acknowledgments...............................................17

1. Introduction

   Conventional data center network designs cater to largely static
   workloads and cause fragmentation of network and server capacity

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   [6][7]. 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 [13].

   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 host 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. 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 workloads' 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

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

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   This document describes use of the Generic Routing Encapsulation
   (GRE, [3][4]) header for network virtualization. Network
   virtualization decouples a virtual network from the underlying
   physical network infrastructure by virtualizing network addresses.
   Combined with a management and control plane for the virtual-to-
   physical mapping, network virtualization can enable flexible virtual
   machine placement and movement, and provide network isolation for a
   multi-tenant datacenter.

   Network virtualization enables customers to bring their own address
   spaces into a multi-tenant datacenter while the datacenter
   administrators can place the customer virtual machines anywhere in
   the datacenter without reconfiguring their network switches or
   routers, irrespective of the customer address spaces.

1.1. Terminology

   Please refer to [9][11] for more formal definition of terminology.
   The following terms were used in this document.

   Customer Address (CA): These are the virtual IP addresses assigned
   and configured on the virtual NIC within each VM. These are the only
   addresses visible to VMs and applications running within VMs.

   Network Virtualization Edge (NVE): An entity that performs the
   network virtualization encapsulation and decapsulation.

   Provider Address (PA): These are the IP addresses used in the
   physical network. PA's are associated with VM CA's through the
   network virtualization mapping policy.

   Virtual Machine (VM): These are instances of OS's running on top of
   hypervisor over a physical machine or server. Multiple VMs can share
   the same physical server via the hypervisor, yet are completely
   isolated from each other in terms of compute, storage, and other OS

   Virtual Subnet Identifier (VSID): a 24-bit ID that uniquely
   identifies a virtual subnet or virtual layer 2 broadcast domain.

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].

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   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 (NVGRE)

   This section describes Network Virtualization using GRE, NVGRE.
   Network virtualization involves creating virtual Layer 2 topologies
   on top of a physical Layer 3 network. Connectivity in the virtual
   topology is provided by tunneling Ethernet frames in GRE over IP
   over the physical network.

   In NVGRE, every virtual Layer 2 network is associated with a 24-bit
   identifier, called a Virtual Subnet Identifier (VSID). A VSID is
   carried in an outer header as defined in Section 3.2. , allowing
   unique identification of a tenant's virtual subnet to various
   devices in the network. A 24-bit VSID supports 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, which can be used to identify a virtual subnet of
   a given tenant. To support multi-subnet virtual topology, datacenter
   administrators can configure routes to facilitate communication
   between virtual subnets of the same tenant.

   GRE is a proposed IETF standard [3][4] 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-tenant-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. The NVGRE endpoints are the NVEs as
   defined in the NVO Framework document [9]. Any physical server or
   network device can be an 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

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   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

   The GRE header format as specified in RFC 2784 and RFC 2890 [3][4]
   is used for communication between NVGRE endpoints. NVGRE leverages
   the Key extension specified in RFC 2890 [4] 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)        |    FlowID     |
   Inner Ethernet Header
   |                (Inner) Destination MAC Address                |
   |(Inner)Destination MAC Address |  (Inner)Source MAC Address    |
   |                  (Inner) Source MAC Address                   |
   |       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                      |
   |                                                               |
   |                                                               |

                    Figure 1 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 endpoint may or may not be on the same
   physical subnet. The destination Ethernet address is set to the MAC
   address of the nexthop IP address for the destination NVE. The outer
   VLAN tag information is optional and can be used for traffic
   management and broadcast scalability on the physical network.

   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 Customer Address (CA) for a customer

   The GRE header:

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

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

       - Virtual Subnet ID (VSID): This is a 24-bit value that is used
         to identify the NVGRE based Virtual Layer 2 Network.
       - FlowID: This is an 8-bit value that is used to provide per-
         flow entropy for flows in the same VSID. The FlowID MUST NOT
         be modified by transit devices. The encapsulating NVE SHOULD
         provide as much entropy as possible in the FlowId. If a FlowID
         is not generated, it MUST be set to all zero.

   o The protocol type field in the GRE header is set to 0x6558
   (transparent Ethernet bridging)[2].

   The inner headers (headers of the GRE payload):

   o The inner Ethernet frame comprises of an inner Ethernet header
   followed by optional 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 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

3.3. Inner 802.1Q Tag

   The inner Ethernet header of NVGRE MUST NOT contain 802.1Q tag. The
   encapsulating NVE MUST remove any existing 802.1Q Tag before
   encapsulation of the frame in NVGRE. A decapsulating NVE MUST drop
   the frame if the inner Ethernet frame contains an 802.1Q tag.

3.4. Reserved VSID

   The VSID range from 0-0xFFF is reserved for future use.

   The VSID 0xFFFFFF is reserved for vendor specific NVE-NVE
   communication. The sender NVE SHOULD verify receiver NVE's vendor
   before sending a packet using this VSID, however such verification
   mechanism is out of scope of this document. Implementations SHOULD
   choose a mechanism that meets their requirements.

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4. NVGRE Deployment Considerations

4.1. ECMP Support

   ECMP may be used to provide load balancing. If ECMP is used, it is
   RECOMMENDED that ECMP hash is calculated either using the outer IP
   frame fields and entire Key field (32-bit) or inner IP and transport
   frame fields.

4.2. Broadcast and Multicast Traffic

   To support broadcast and multicast traffic inside a virtual subnet,
   one or more administratively scoped multicast addresses [8][10] can
   be assigned for the VSID. All multicast or broadcast traffic
   originating from within a VSID is encapsulated and sent to the
   assigned multicast address. From an administrative standpoint it is
   possible for network operators to configure a PA multicast address
   for each multicast address that is used inside a VSID, to facilitate
   optimal multicast handling. Depending on the hardware capabilities
   of the physical network devices and the physical network
   architecture, multiple virtual subnet may re-use the same physical
   IP multicast address.

   Alternatively, based upon the configuration at NVE, the broadcast
   and multicast in the virtual subnet can be supported using N-Way
   unicast. In N-Way unicast, the sender NVE would send one
   encapsulated packet to every NVE in the virtual subnet. The sender
   NVE can encapsulate and send the packet as described in the Unicast
   Traffic Section 4.3. This alleviates the need for multicast support
   in the physical network.

4.3. 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 PA. Bridging
   uses the outer Ethernet encapsulation for scope on the LAN. The only
   requirement 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.

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4.4. IP Fragmentation

   RFC 2003 [12] 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 sender NVE MUST NOT fragment NVGRE packets. A receiver NVE MAY
   discard fragmented NVGRE packets. It is RECOMMENDED that MTU of
   physical network accommodates the larger frame size due to
   encapsulation. Path MTU or configuration via control plane can be
   used to meet this requirement.

4.5. 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, a CA can be
   IPv4 while the PA is IPv6 and vice versa.

4.6. 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 VMe1| | Corp VMe2| |
                                   | |  IP=CAe1 | |  IP=CAe2 | |
                                   | +----------+ +----------+ |
                                   |         Hypervisor        |
            Figure 2 Cross-Subnet, Cross-Premise Communication

   The packet flow 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 per-tenant VPN gateway is
   provisioned for communication back to the enterprise. The example

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   illustrates an outbound connection between VM1 inside the datacenter
   and VMe1 inside the enterprise network. When the outbound packet
   from CA1 to CAe1 reaches the hypervisor on Server 1, the NVE in
   Server 1 can perform an equivalent of a route lookup on the packet.
   The cross premise packet will match the default gateway rule as CAe1
   is not part of the tenant virtual network in the datacenter. The
   virtualization policy will indicate the packet to be 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 VPN 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
   gateway decapsulates the packet and sends it inside the enterprise
   where the CAe1 is routable on the network. The reverse path is
   similar once the packet reaches the enterprise VPN gateway.

4.7. 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 need to perform translation between the
   virtualized world and the Internet. For example, the NVGRE endpoint
   can be part of a load balancer or a NAT, which replaces the VPN
   Gateway on Server 2 shown in Figure 2.

4.8. Management and Control Planes

   There are several protocols that can manage and distribute policy;
   however, it is out of scope of this document. Implementations SHOULD
   choose a mechanism that meets their scale requirements.

4.9. 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 NVGRE, 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

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   techniques such as Equal Cost Multipath (ECMP). The Key field (32-
   bit field, including both VSID and the optional FlowID) can provide
   additional entropy to the switches to exploit path diversity inside
   the network. A diverse ecosystem is expected to emerge as more and
   more devices become multi-tenant 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
   NVGRE 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

4.10. Network Scalability with NVGRE

   One of the key benefits of using NVGRE 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.

   The checksum in the GRE header is not supported. The mitigation of
   this is to deploy NVGRE based solution in a network that provides
   error detection along the NVGRE packet path, for example, using
   Ethernet CRC or IPsec or any other error detection mechanism.

6. IANA Considerations

   This document has no IANA actions.

7. References

7.1. Normative References

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

   [2]   Ethertypes,

   [3]   D. Farinacci et al, "Generic Routing Encapsulation (GRE)", RFC
         2784, March, 2000.

   [4]   G. Dommety, "Key and Sequence Number Extensions to GRE", RFC
         2890, September 2000.

   [5]   Institute of Electrical and Electronics Engineers, "Virtual
         Bridged Local Area Networks", IEEE Standard 802.1Q, 2005
         Edition, May 2006.

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7.2. Informative References

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

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

   [8]   B. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
         RFC 4291, February 2006.

   [9]   M. Lasserre et al, "Framework for DC Network Virtualization",
         RFC 7365, October 2014.

   [10]  D. Meyer, "Administratively Scoped IP Multicast", BCP 23, RFC
         2365, July 1998.

   [11]  T. Narten et al, "Problem Statement: Overlays for Network
         Virtualization", RFC 7364, October 2014.

   [12]  C. Perkins, "IP Encapsulation within IP", RFC 2003, October

   [13]  J. Touch, R. Perlman, "Transparent Interconnection of Lots of
         Links (TRILL): Problem and Applicability Statement", RFC 5556,
         May 2009.

8. Authors and Contributors

   M. Sridharan
   A. Greenberg
   Y. Wang
   P. Garg
   N. Venkataramiah

   K. Duda
   Arista Networks

   I. Ganga

   G. Lin

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   M. Pearson

   P. Thaler

   C. Tumuluri

9. Acknowledgments

   This document was prepared using

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Authors' Addresses

   Murari Sridharan
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

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

   Albert Greenberg
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

   Pankaj Garg
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

   Narasimhan Venkataramiah
   Facebook Inc
   1730 Minor Ave.
   Seattle, WA 98101

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

   Ilango Ganga
   Intel Corporation
   2200 Mission College Blvd.

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   M/S: SC12-325
   Santa Clara, CA - 95054

   Geng Lin
   1600 Amphitheatre Parkway
   Mountain View, California 94043

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

   Patricia Thaler
   Broadcom Corporation
   3151 Zanker Road
   San Jose, CA 95134

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

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