





Independent Submission                                      P. Garg, Ed.
Request for Comments: 7637                                  Y. Wang, Ed.
Category: Informational                                        Microsoft
ISSN: 2070-1721                                           September 2015


   NVGRE: Network Virtualization Using Generic Routing Encapsulation

Abstract

   This document describes the usage of the Generic Routing
   Encapsulation (GRE) header for Network Virtualization (NVGRE) in
   multi-tenant data centers.  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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7637.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.





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Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................4
   2. Conventions Used in This Document ...............................4
   3. Network Virtualization Using GRE (NVGRE) ........................4
      3.1. NVGRE Endpoint .............................................5
      3.2. NVGRE Frame Format .........................................5
      3.3. Inner Tag as Defined by IEEE 802.1Q ........................8
      3.4. Reserved VSID ..............................................8
   4. NVGRE Deployment Considerations .................................9
      4.1. ECMP Support ...............................................9
      4.2. Broadcast and Multicast Traffic ............................9
      4.3. Unicast Traffic ............................................9
      4.4. IP Fragmentation ..........................................10
      4.5. Address/Policy Management and Routing .....................10
      4.6. Cross-Subnet, Cross-Premise Communication .................10
      4.7. Internet Connectivity .....................................12
      4.8. Management and Control Planes .............................12
      4.9. NVGRE-Aware Devices .......................................12
      4.10. Network Scalability with NVGRE ...........................13
   5. Security Considerations ........................................14
   6. Normative References ...........................................14
   Contributors ......................................................16
   Authors' Addresses ................................................17

1.  Introduction

   Conventional data center network designs cater to largely static
   workloads and cause fragmentation of network and server capacity [6]
   [7].  There are several issues that limit dynamic allocation and
   consolidation of capacity.  Layer 2 networks use the 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 the Transparent Interconnection of Lots of Links
   (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 4,000 such subnets to be created.  The size




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   of the broadcast domain is typically restricted due to the overhead
   of broadcast traffic.  The 4,000-subnet limit on VLANs 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 and that is typically error
   prone.  By decoupling the workload's location on the LAN from its
   network address, the network administrator configures the network
   once, 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 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
         centers

      d) providing broadcast isolation as workloads move around without
         burdening the network control plane

   This document describes use of the Generic Routing Encapsulation
   (GRE) header [3] [4] 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 data center.

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




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1.1.  Terminology

   Please refer to RFCs 7364 [10] and 7365 [11] for more formal
   definitions of terminology.  The following terms are used in this
   document.

   Customer Address (CA): This is the virtual IP address assigned and
   configured on the virtual Network Interface Controller (NIC) within
   each VM.  This is the only address visible to VMs and applications
   running within VMs.

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

   Provider Address (PA): This is the IP address used in the physical
   network.  PAs are associated with VM CAs through the network
   virtualization mapping policy.

   Virtual Machine (VM): This is an instance of an OS running on top of
   the 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 CPU usage, storage, and other OS
   resources.

   Virtual Subnet Identifier (VSID): This is 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

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

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.  This allows



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   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 4,000 that
   is 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, data center
   administrators can configure routes to facilitate communication
   between virtual subnets of the same tenant.

   GRE is a Proposed Standard from the IETF [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 an 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 Network Virtualization over Layer 3 (NVO3) Framework document
   [11].  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 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 RFCs 2784 [3] and 2890 [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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   Outer Ethernet Header:
    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) 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|  HL   |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        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   Inner IPv4 Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version|  HL   |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

   Note: HL stands for Header Length.

   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 next-hop 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 associated with an NVGRE endpoint, with policy
      controlling the choice of which PA to use for a given Customer
      Address (CA) for a customer VM.

   In 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 (Key Present) bit 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 zeros.

   o  The Protocol Type field in the GRE header is set to 0x6558
      (Transparent Ethernet Bridging) [2].

   In the inner headers (headers of the GRE payload):

   o  The inner Ethernet frame comprises 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 Frame Check Sequence (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 (CA).

3.3.  Inner Tag as Defined by IEEE 802.1Q

   The inner Ethernet header of NVGRE MUST NOT contain the tag as
   defined by IEEE 802.1Q [5].  The encapsulating NVE MUST remove any
   existing IEEE 802.1Q tag before encapsulation of the frame in NVGRE.
   A decapsulating NVE MUST drop the frame if the inner Ethernet frame
   contains an IEEE 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-to-NVE
   communication.  The sender NVE SHOULD verify the receiver NVE's
   vendor before sending a packet using this VSID; however, such a
   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

   Equal-Cost Multipath (ECMP) may be used to provide load balancing.
   If ECMP is used, it is RECOMMENDED that the ECMP hash is calculated
   either using the outer IP frame fields and entire Key field (32 bits)
   or the 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] [9] 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; this
   facilitates optimal multicast handling.  Depending on the hardware
   capabilities of the physical network devices and the physical network
   architecture, multiple virtual subnets may use the same physical IP
   multicast address.

   Alternatively, based upon the configuration at the NVE, 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 Section 4.3 ("Unicast Traffic").
   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 bidirectional 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

   Section 5.1 of RFC 2003 [12] 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 the MTU of
   the 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 and Routing

   Address acquisition is beyond the scope of this document and can be
   obtained statically, dynamically, or using stateless address
   autoconfiguration.  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 an 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 gateway 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 is that 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



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   example illustrates an outbound connection between VM1 inside the
   data center 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 the 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 data
   center.  The virtualization policy will indicate the packet to be
   encapsulated and sent to the PA of the 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
   data center, 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
   that 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 needs 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 that 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 outside the 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-tenant-aware services inside the data
   center.  As outlined earlier, it is imperative to exploit multiple
   paths inside the network through techniques such as ECMP.  The Key



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   field (a 32-bit field, including both the 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 which PA to use.

   It is expected that communication can span multiple data centers and
   also cross the virtual/physical boundary.  Typical scenarios that
   require virtual-to-physical communication include 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 (SAs) 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
   performance.

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.  An
   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 an NVGRE endpoint in a hypervisor, it
   is possible to scale significantly.  This framework enables location
   information to be preconfigured inside an NVGRE endpoint, thus
   allowing broadcast ARP traffic to be proxied locally.  This approach
   can scale to large-sized virtual subnets.  These virtual subnets can
   be spread across multiple Layer 3 physical subnets.  It allows



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RFC 7637                          NVGRE                   September 2015


   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 optimally 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 an NVGRE-based solution in a network that provides
   error detection along the NVGRE packet path, for example, using
   Ethernet Cyclic Redundancy Check (CRC) or IPsec or any other error
   detection mechanism.

6.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
        <http://www.rfc-editor.org/info/rfc2119>.

   [2]  IANA, "IEEE 802 Numbers",
        <http://www.iana.org/assignments/ieee-802-numbers>.

   [3]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
        "Generic Routing Encapsulation (GRE)", RFC 2784,
        DOI 10.17487/RFC2784, March 2000,
        <http://www.rfc-editor.org/info/rfc2784>.

   [4]  Dommety, G., "Key and Sequence Number Extensions to GRE",
        RFC 2890, DOI 10.17487/RFC2890, September 2000,
        <http://www.rfc-editor.org/info/rfc2890>.

   [5]  IEEE, "IEEE Standard for Local and metropolitan area
        networks--Media Access Control (MAC) Bridges and Virtual Bridged
        Local Area Networks", IEEE Std 802.1Q.

   [6]  Greenberg, A., et al., "VL2: A Scalable and Flexible Data Center
        Network", Communications of the ACM,
        DOI 10.1145/1897852.1897877, 2011.



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RFC 7637                          NVGRE                   September 2015


   [7]  Greenberg, A., et al., "The Cost of a Cloud: Research Problems
        in Data Center Networks", ACM SIGCOMM Computer Communication
        Review, DOI 10.1145/1496091.1496103, 2009.

   [8]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006,
        <http://www.rfc-editor.org/info/rfc4291>.

   [9]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
        RFC 2365, DOI 10.17487/RFC2365, July 1998,
        <http://www.rfc-editor.org/info/rfc2365>.

   [10] Narten, T., Ed., Gray, E., Ed., Black, D., Fang, L., Kreeger,
        L., and M. Napierala, "Problem Statement: Overlays for Network
        Virtualization", RFC 7364, DOI 10.17487/RFC7364, October 2014,
        <http://www.rfc-editor.org/info/rfc7364>.

   [11] Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y. Rekhter,
        "Framework for Data Center (DC) Network Virtualization",
        RFC 7365, DOI 10.17487/RFC7365, October 2014,
        <http://www.rfc-editor.org/info/rfc7365>.

   [12] Perkins, C., "IP Encapsulation within IP", RFC 2003,
        DOI 10.17487/RFC2003, October 1996,
        <http://www.rfc-editor.org/info/rfc2003>.

   [13] Touch, J. and R. Perlman, "Transparent Interconnection of Lots
        of Links (TRILL): Problem and Applicability Statement",
        RFC 5556, DOI 10.17487/RFC5556, May 2009,
        <http://www.rfc-editor.org/info/rfc5556>.





















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RFC 7637                          NVGRE                   September 2015


Contributors

   Murari Sridharan
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052
   United States
   Email: muraris@microsoft.com

   Albert Greenberg
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052
   United States
   Email: albert@microsoft.com

   Narasimhan Venkataramiah
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052
   United States
   Email: navenkat@microsoft.com

   Kenneth Duda
   Arista Networks, Inc.
   5470 Great America Pkwy
   Santa Clara, CA 95054
   United States
   Email: kduda@aristanetworks.com

   Ilango Ganga
   Intel Corporation
   2200 Mission College Blvd.
   M/S: SC12-325
   Santa Clara, CA 95054
   United States
   Email: ilango.s.ganga@intel.com

   Geng Lin
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States
   Email: genglin@google.com







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RFC 7637                          NVGRE                   September 2015


   Mark Pearson
   Hewlett-Packard Co.
   8000 Foothills Blvd.
   Roseville, CA 95747
   United States
   Email: mark.pearson@hp.com

   Patricia Thaler
   Broadcom Corporation
   3151 Zanker Road
   San Jose, CA 95134
   United States
   Email: pthaler@broadcom.com

   Chait Tumuluri
   Emulex Corporation
   3333 Susan Street
   Costa Mesa, CA 92626
   United States
   Email: chait@emulex.com

Authors' Addresses

   Pankaj Garg (editor)
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052
   United States
   Email: pankajg@microsoft.com

   Yu-Shun Wang (editor)
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052
   United States
   Email: yushwang@microsoft.com















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