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RFC 7365

Informational
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Framework for Data Center (DC) Network Virtualization

 


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Internet Engineering Task Force (IETF)                       M. Lasserre
Request for Comments: 7365                                      F. Balus
Category: Informational                                   Alcatel-Lucent
ISSN: 2070-1721                                                 T. Morin
                                                                  Orange
                                                                N. Bitar
                                                                 Verizon
                                                              Y. Rekhter
                                                                 Juniper
                                                            October 2014


         Framework for Data Center (DC) Network Virtualization

Abstract

   This document provides a framework for Data Center (DC) Network
   Virtualization over Layer 3 (NVO3) and defines a reference model
   along with logical components required to design a solution.

Status of This Memo

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

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are 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/rfc7365.

Page 2 
Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................4
      1.1. General Terminology ........................................4
      1.2. DC Network Architecture ....................................7
   2. Reference Models ................................................8
      2.1. Generic Reference Model ....................................8
      2.2. NVE Reference Model .......................................10
      2.3. NVE Service Types .........................................11
           2.3.1. L2 NVE Providing Ethernet LAN-Like Service .........11
           2.3.2. L3 NVE Providing IP/VRF-Like Service ...............11
      2.4. Operational Management Considerations .....................12
   3. Functional Components ..........................................12
      3.1. Service Virtualization Components .........................12
           3.1.1. Virtual Access Points (VAPs) .......................12
           3.1.2. Virtual Network Instance (VNI) .....................12
           3.1.3. Overlay Modules and VN Context .....................14
           3.1.4. Tunnel Overlays and Encapsulation Options ..........14
           3.1.5. Control-Plane Components ...........................14
                  3.1.5.1. Distributed vs. Centralized
                           Control Plane .............................14
                  3.1.5.2. Auto-provisioning and Service Discovery ...15
                  3.1.5.3. Address Advertisement and Tunnel Mapping ..15
                  3.1.5.4. Overlay Tunneling .........................16
      3.2. Multihoming ...............................................16
      3.3. VM Mobility ...............................................17
   4. Key Aspects of Overlay Networks ................................17
      4.1. Pros and Cons .............................................18
      4.2. Overlay Issues to Consider ................................19
           4.2.1. Data Plane vs. Control Plane Driven ................19
           4.2.2. Coordination between Data Plane and Control Plane ..19
           4.2.3. Handling Broadcast, Unknown Unicast, and
                  Multicast (BUM) Traffic ............................20
           4.2.4. Path MTU ...........................................20
           4.2.5. NVE Location Trade-Offs ............................21
           4.2.6. Interaction between Network Overlays and
                  Underlays ..........................................22
   5. Security Considerations ........................................22
   6. Informative References .........................................24
   Acknowledgments ...................................................26
   Authors' Addresses ................................................26

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1.  Introduction

   This document provides a framework for Data Center (DC) Network
   Virtualization over Layer 3 (NVO3) tunnels.  This framework is
   intended to aid in standardizing protocols and mechanisms to support
   large-scale network virtualization for data centers.

   [RFC7364] defines the rationale for using overlay networks in order
   to build large multi-tenant data center networks.  Compute, storage
   and network virtualization are often used in these large data centers
   to support a large number of communication domains and end systems.

   This document provides reference models and functional components of
   data center overlay networks as well as a discussion of technical
   issues that have to be addressed.

1.1.  General Terminology

   This document uses the following terminology:

   NVO3 Network: An overlay network that provides a Layer 2 (L2) or
   Layer 3 (L3) service to Tenant Systems over an L3 underlay network
   using the architecture and protocols as defined by the NVO3 Working
   Group.

   Network Virtualization Edge (NVE): An NVE is the network entity that
   sits at the edge of an underlay network and implements L2 and/or L3
   network virtualization functions.  The network-facing side of the NVE
   uses the underlying L3 network to tunnel tenant frames to and from
   other NVEs.  The tenant-facing side of the NVE sends and receives
   Ethernet frames to and from individual Tenant Systems.  An NVE could
   be implemented as part of a virtual switch within a hypervisor, a
   physical switch or router, or a Network Service Appliance, or it
   could be split across multiple devices.

   Virtual Network (VN): A VN is a logical abstraction of a physical
   network that provides L2 or L3 network services to a set of Tenant
   Systems.  A VN is also known as a Closed User Group (CUG).

   Virtual Network Instance (VNI): A specific instance of a VN from the
   perspective of an NVE.

   Virtual Network Context (VN Context) Identifier: Field in an overlay
   encapsulation header that identifies the specific VN the packet
   belongs to.  The egress NVE uses the VN Context identifier to deliver
   the packet to the correct Tenant System.  The VN Context identifier
   can be a locally significant identifier or a globally unique
   identifier.

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   Underlay or Underlying Network: The network that provides the
   connectivity among NVEs and that NVO3 packets are tunneled over,
   where an NVO3 packet carries an NVO3 overlay header followed by a
   tenant packet.  The underlay network does not need to be aware that
   it is carrying NVO3 packets.  Addresses on the underlay network
   appear as "outer addresses" in encapsulated NVO3 packets.  In
   general, the underlay network can use a completely different protocol
   (and address family) from that of the overlay.  In the case of NVO3,
   the underlay network is IP.

   Data Center (DC): A physical complex housing physical servers,
   network switches and routers, network service appliances, and
   networked storage.  The purpose of a data center is to provide
   application, compute, and/or storage services.  One such service is
   virtualized infrastructure data center services, also known as
   "Infrastructure as a Service".

   Virtual Data Center (Virtual DC): A container for virtualized
   compute, storage, and network services.  A virtual DC is associated
   with a single tenant and can contain multiple VNs and Tenant Systems
   connected to one or more of these VNs.

   Virtual Machine (VM): A software implementation of a physical machine
   that runs programs as if they were executing on a physical, non-
   virtualized machine.  Applications (generally) do not know they are
   running on a VM as opposed to running on a "bare metal" host or
   server, though some systems provide a para-virtualization environment
   that allows an operating system or application to be aware of the
   presence of virtualization for optimization purposes.

   Hypervisor: Software running on a server that allows multiple VMs to
   run on the same physical server.  The hypervisor manages and provides
   shared computation, memory, and storage services and network
   connectivity to the VMs that it hosts.  Hypervisors often embed a
   virtual switch (see below).

   Server: A physical end-host machine that runs user applications.  A
   standalone (or "bare metal") server runs a conventional operating
   system hosting a single-tenant application.  A virtualized server
   runs a hypervisor supporting one or more VMs.

   Virtual Switch (vSwitch): A function within a hypervisor (typically
   implemented in software) that provides similar forwarding services to
   a physical Ethernet switch.  A vSwitch forwards Ethernet frames
   between VMs running on the same server or between a VM and a physical
   Network Interface Card (NIC) connecting the server to a physical
   Ethernet switch or router.  A vSwitch also enforces network isolation
   between VMs that by policy are not permitted to communicate with each

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   other (e.g., by honoring VLANs).  A vSwitch may be bypassed when an
   NVE is enabled on the host server.

   Tenant: The customer using a virtual network and any associated
   resources (e.g., compute, storage, and network).  A tenant could be
   an enterprise or a department/organization within an enterprise.

   Tenant System: A physical or virtual system that can play the role of
   a host or a forwarding element such as a router, switch, firewall,
   etc.  It belongs to a single tenant and connects to one or more VNs
   of that tenant.

   Tenant Separation: Refers to isolating traffic of different tenants
   such that traffic from one tenant is not visible to or delivered to
   another tenant, except when allowed by policy.  Tenant separation
   also refers to address space separation, whereby different tenants
   can use the same address space without conflict.

   Virtual Access Points (VAPs): A logical connection point on the NVE
   for connecting a Tenant System to a virtual network.  Tenant Systems
   connect to VNIs at an NVE through VAPs.  VAPs can be physical ports
   or virtual ports identified through logical interface identifiers
   (e.g., VLAN ID or internal vSwitch Interface ID connected to a VM).

   End Device: A physical device that connects directly to the DC
   underlay network.  This is in contrast to a Tenant System, which
   connects to a corresponding tenant VN.  An End Device is administered
   by the DC operator rather than a tenant and is part of the DC
   infrastructure.  An End Device may implement NVO3 technology in
   support of NVO3 functions.  Examples of an End Device include hosts
   (e.g., server or server blade), storage systems (e.g., file servers
   and iSCSI storage systems), and network devices (e.g., firewall,
   load-balancer, and IPsec gateway).

   Network Virtualization Authority (NVA): Entity that provides
   reachability and forwarding information to NVEs.

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1.2.  DC Network Architecture

   A generic architecture for data centers is depicted in Figure 1:

                                ,---------.
                              ,'           `.
                             (  IP/MPLS WAN )
                              `.           ,'
                                `-+------+'
                                 \      /
                          +--------+   +--------+
                          |   DC   |+-+|   DC   |
                          |gateway |+-+|gateway |
                          +--------+   +--------+
                                |       /
                                .--. .--.
                              (    '    '.--.
                            .-.' Intra-DC     '
                           (     network      )
                            (             .'-'
                             '--'._.'.    )\ \
                             / /     '--'  \ \
                            / /      | |    \ \
                   +--------+   +--------+   +--------+
                   | access |   | access |   | access |
                   | switch |   | switch |   | switch |
                   +--------+   +--------+   +--------+
                      /     \    /    \     /      \
                   __/_      \  /      \   /_      _\__
             '--------'   '--------'   '--------'   '--------'
             :  End   :   :  End   :   :  End   :   :  End   :
             : Device :   : Device :   : Device :   : Device :
             '--------'   '--------'   '--------'   '--------'

             Figure 1: A Generic Architecture for Data Centers

   An example of multi-tier DC network architecture is presented in
   Figure 1.  It provides a view of the physical components inside a DC.

   A DC network is usually composed of intra-DC networks and network
   services, and inter-DC network and network connectivity services.

   DC networking elements can act as strict L2 switches and/or provide
   IP routing capabilities, including network service virtualization.

   In some DC architectures, some tier layers could provide L2 and/or L3
   services.  In addition, some tier layers may be collapsed, and
   Internet connectivity, inter-DC connectivity, and VPN support may be

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   handled by a smaller number of nodes.  Nevertheless, one can assume
   that the network functional blocks in a DC fit in the architecture
   depicted in Figure 1.

   The following components can be present in a DC:

   -  Access switch: Hardware-based Ethernet switch aggregating all
      Ethernet links from the End Devices in a rack representing the
      entry point in the physical DC network for the hosts.  It may also
      provide routing functionality, virtual IP network connectivity, or
      Layer 2 tunneling over IP, for instance.  Access switches are
      usually multihomed to aggregation switches in the Intra-DC
      network.  A typical example of an access switch is a Top-of-Rack
      (ToR) switch.  Other deployment scenarios may use an intermediate
      Blade Switch before the ToR, or an End-of-Row (EoR) switch, to
      provide similar functions to a ToR.

   -  Intra-DC Network: Network composed of high-capacity core nodes
      (Ethernet switches/routers).  Core nodes may provide virtual
      Ethernet bridging and/or IP routing services.

   -  DC Gateway (DC GW): Gateway to the outside world providing DC
      interconnect and connectivity to Internet and VPN customers.  In
      the current DC network model, this may be simply a router
      connected to the Internet and/or an IP VPN/L2VPN PE.  Some network
      implementations may dedicate DC GWs for different connectivity
      types (e.g., a DC GW for Internet and another for VPN).

   Note that End Devices may be single-homed or multihomed to access
   switches.

2.  Reference Models

2.1.  Generic Reference Model

   Figure 2 depicts a DC reference model for network virtualization
   overlays where NVEs provide a logical interconnect between Tenant
   Systems that belong to a specific VN.

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         +--------+                                    +--------+
         | Tenant +--+                            +----| Tenant |
         | System |  |                           (')   | System |
         +--------+  |    .................     (   )  +--------+
                     |  +---+           +---+    (_)
                     +--|NVE|---+   +---|NVE|-----+
                        +---+   |   |   +---+
                        / .    +-----+      .
                       /  . +--| NVA |--+   .
                      /   . |  +-----+   \  .
                     |    . |             \ .
                     |    . |   Overlay   +--+--++--------+
         +--------+  |    . |   Network   | NVE || Tenant |
         | Tenant +--+    . |             |     || System |
         | System |       .  \ +---+      +--+--++--------+
         +--------+       .....|NVE|.........
                               +---+
                                 |
                                 |
                       =====================
                         |               |
                     +--------+      +--------+
                     | Tenant |      | Tenant |
                     | System |      | System |
                     +--------+      +--------+

      Figure 2: Generic Reference Model for DC Network Virtualization
                                 Overlays

   In order to obtain reachability information, NVEs may exchange
   information directly between themselves via a control-plane protocol.
   In this case, a control-plane module resides in every NVE.

   It is also possible for NVEs to communicate with an external Network
   Virtualization Authority (NVA) to obtain reachability and forwarding
   information.  In this case, a protocol is used between NVEs and
   NVA(s) to exchange information.

   It should be noted that NVAs may be organized in clusters for
   redundancy and scalability and can appear as one logically
   centralized controller.  In this case, inter-NVA communication is
   necessary to synchronize state among nodes within a cluster or share
   information across clusters.  The information exchanged between NVAs
   of the same cluster could be different from the information exchanged
   across clusters.

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   A Tenant System can be attached to an NVE in several ways:

   -  locally, by being co-located in the same End Device

   -  remotely, via a point-to-point connection or a switched network

   When an NVE is co-located with a Tenant System, the state of the
   Tenant System can be determined without protocol assistance.  For
   instance, the operational status of a VM can be communicated via a
   local API.  When an NVE is remotely connected to a Tenant System, the
   state of the Tenant System or NVE needs to be exchanged directly or
   via a management entity, using a control-plane protocol or API, or
   directly via a data-plane protocol.

   The functional components in Figure 2 do not necessarily map directly
   to the physical components described in Figure 1.  For example, an
   End Device can be a server blade with VMs and a virtual switch.  A VM
   can be a Tenant System, and the NVE functions may be performed by the
   host server.  In this case, the Tenant System and NVE function are
   co-located.  Another example is the case where the End Device is the
   Tenant System and the NVE function can be implemented by the
   connected ToR.  In this case, the Tenant System and NVE function are
   not co-located.

   Underlay nodes utilize L3 technologies to interconnect NVE nodes.
   These nodes perform forwarding based on outer L3 header information,
   and generally do not maintain state for each tenant service, albeit
   some applications (e.g., multicast) may require control-plane or
   forwarding-plane information that pertains to a tenant, group of
   tenants, tenant service, or a set of services that belong to one or
   more tenants.  Mechanisms to control the amount of state maintained
   in the underlay may be needed.

2.2.  NVE Reference Model

   Figure 3 depicts the NVE reference model.  One or more VNIs can be
   instantiated on an NVE.  A Tenant System interfaces with a
   corresponding VNI via a VAP.  An overlay module provides tunneling
   overlay functions (e.g., encapsulation and decapsulation of tenant
   traffic, tenant identification, and mapping, etc.).

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                     +-------- L3 Network -------+
                     |                           |
                     |        Tunnel Overlay     |
         +------------+---------+       +---------+------------+
         | +----------+-------+ |       | +---------+--------+ |
         | |  Overlay Module  | |       | |  Overlay Module  | |
         | +---------+--------+ |       | +---------+--------+ |
         |           |VN Context|       | VN Context|          |
         |           |          |       |           |          |
         |  +--------+-------+  |       |  +--------+-------+  |
         |  | |VNI|   .  |VNI|  |       |  | |VNI|   .  |VNI|  |
    NVE1 |  +-+------------+-+  |       |  +-+-----------+--+  | NVE2
         |    |   VAPs     |    |       |    |    VAPs   |     |
         +----+------------+----+       +----+-----------+-----+
              |            |                 |           |
              |            |                 |           |
             Tenant Systems                 Tenant Systems

                  Figure 3: Generic NVE Reference Model

   Note that some NVE functions (e.g., data-plane and control-plane
   functions) may reside in one device or may be implemented separately
   in different devices.

2.3.  NVE Service Types

   An NVE provides different types of virtualized network services to
   multiple tenants, i.e., an L2 service or an L3 service.  Note that an
   NVE may be capable of providing both L2 and L3 services for a tenant.
   This section defines the service types and associated attributes.

2.3.1.  L2 NVE Providing Ethernet LAN-Like Service

   An L2 NVE implements Ethernet LAN emulation, an Ethernet-based
   multipoint service similar to an IETF Virtual Private LAN Service
   (VPLS) [RFC4761][RFC4762] or Ethernet VPN [EVPN] service, where the
   Tenant Systems appear to be interconnected by a LAN environment over
   an L3 overlay.  As such, an L2 NVE provides per-tenant virtual
   switching instance (L2 VNI) and L3 (IP/MPLS) tunneling encapsulation
   of tenant Media Access Control (MAC) frames across the underlay.
   Note that the control plane for an L2 NVE could be implemented
   locally on the NVE or in a separate control entity.

2.3.2.  L3 NVE Providing IP/VRF-Like Service

   An L3 NVE provides virtualized IP forwarding service, similar to IETF
   IP VPN (e.g., BGP/MPLS IP VPN [RFC4364]) from a service definition
   perspective.  That is, an L3 NVE provides per-tenant forwarding and

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   routing instance (L3 VNI) and L3 (IP/MPLS) tunneling encapsulation of
   tenant IP packets across the underlay.  Note that routing could be
   performed locally on the NVE or in a separate control entity.

2.4.  Operational Management Considerations

   NVO3 services are overlay services over an IP underlay.

   As far as the IP underlay is concerned, existing IP Operations,
   Administration, and Maintenance (OAM) facilities are used.

   With regard to the NVO3 overlay, both L2 and L3 services can be
   offered.  It is expected that existing fault and performance OAM
   facilities will be used.  Sections 4.1 and 4.2.6 provide further
   discussion of additional fault and performance management issues to
   consider.

   As far as configuration is concerned, the DC environment is driven by
   the need to bring new services up rapidly and is typically very
   dynamic, specifically in the context of virtualized services.  It is
   therefore critical to automate the configuration of NVO3 services.

3. Functional Components

   This section decomposes the network virtualization architecture into
   the functional components described in Figure 3 to make it easier to
   discuss solution options for these components.

3.1.  Service Virtualization Components

3.1.1.  Virtual Access Points (VAPs)

   Tenant Systems are connected to VNIs through Virtual Access Points
   (VAPs).

   VAPs can be physical ports or virtual ports identified through
   logical interface identifiers (e.g., VLAN ID and internal vSwitch
   Interface ID connected to a VM).

3.1.2.  Virtual Network Instance (VNI)

   A VNI is a specific VN instance on an NVE.  Each VNI defines a
   forwarding context that contains reachability information and
   policies.

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3.1.3.  Overlay Modules and VN Context

   Mechanisms for identifying each tenant service are required to allow
   the simultaneous overlay of multiple tenant services over the same
   underlay L3 network topology.  In the data plane, each NVE, upon
   sending a tenant packet, must be able to encode the VN Context for
   the destination NVE in addition to the L3 tunneling information
   (e.g., source IP address identifying the source NVE and the
   destination IP address identifying the destination NVE, or MPLS
   label).  This allows the destination NVE to identify the tenant
   service instance and therefore appropriately process and forward the
   tenant packet.

   The overlay module provides tunneling overlay functions: tunnel
   initiation/termination as in the case of stateful tunnels (see
   Section 3.1.4) and/or encapsulation/decapsulation of frames from the
   VAPs/L3 underlay.

   In a multi-tenant context, tunneling aggregates frames from/to
   different VNIs.  Tenant identification and traffic demultiplexing are
   based on the VN Context identifier.

   The following approaches can be considered:

   -  VN Context identifier per Tenant: This is a globally unique (on a
      per-DC administrative domain) VN identifier used to identify the
      corresponding VNI.  Examples of such identifiers in existing
      technologies are IEEE VLAN IDs and Service Instance IDs (I-SIDs)
      that identify virtual L2 domains when using IEEE 802.1Q and IEEE
      802.1ah, respectively.  Note that multiple VN identifiers can
      belong to a tenant.

   -  One VN Context identifier per VNI: Each VNI value is automatically
      generated by the egress NVE, or a control plane associated with
      that NVE, and usually distributed by a control-plane protocol to
      all the related NVEs.  An example of this approach is the use of
      per-VRF MPLS labels in IP VPN [RFC4364].  The VNI value is
      therefore locally significant to the egress NVE.

   -  One VN Context identifier per VAP: A value locally significant to
      an NVE is assigned and usually distributed by a control-plane
      protocol to identify a VAP.  An example of this approach is the
      use of per-CE MPLS labels in IP VPN [RFC4364].

   Note that when using one VN Context per VNI or per VAP, an additional
   global identifier (e.g., a VN identifier or name) may be used by the
   control plane to identify the tenant context.

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3.1.4.  Tunnel Overlays and Encapsulation Options

   Once the VN Context identifier is added to the frame, an L3 tunnel
   encapsulation is used to transport the frame to the destination NVE.

   Different IP tunneling options (e.g., Generic Routing Encapsulation
   (GRE), the Layer 2 Tunneling Protocol (L2TP), and IPsec) and MPLS
   tunneling can be used.  Tunneling could be stateless or stateful.
   Stateless tunneling simply entails the encapsulation of a tenant
   packet with another header necessary for forwarding the packet across
   the underlay (e.g., IP tunneling over an IP underlay).  Stateful
   tunneling, on the other hand, entails maintaining tunneling state at
   the tunnel endpoints (i.e., NVEs).  Tenant packets on an ingress NVE
   can then be transmitted over such tunnels to a destination (egress)
   NVE by encapsulating the packets with a corresponding tunneling
   header.  The tunneling state at the endpoints may be configured or
   dynamically established.  Solutions should specify the tunneling
   technology used and whether it is stateful or stateless.  In this
   document, however, tunneling and tunneling encapsulation are used
   interchangeably to simply mean the encapsulation of a tenant packet
   with a tunneling header necessary to carry the packet between an
   ingress NVE and an egress NVE across the underlay.  It should be
   noted that stateful tunneling, especially when configuration is
   involved, does impose management overhead and scale constraints.
   When confidentiality is required, the use of opportunistic security
   [OPPSEC] can be used as a stateless tunneling solution.

3.1.5.  Control-Plane Components

3.1.5.1.  Distributed vs. Centralized Control Plane

   Control- and management-plane entities can be centralized or
   distributed.  Both approaches have been used extensively in the past.
   The routing model of the Internet is a good example of a distributed
   approach.  Transport networks have usually used a centralized
   approach to manage transport paths.

   It is also possible to combine the two approaches, i.e., using a
   hybrid model.  A global view of network state can have many benefits,
   but it does not preclude the use of distributed protocols within the
   network.  Centralized models provide a facility to maintain global
   state and distribute that state to the network.  When used in
   combination with distributed protocols, greater network efficiencies,
   improved reliability, and robustness can be achieved.  Domain- and/or
   deployment-specific constraints define the balance between
   centralized and distributed approaches.

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3.1.5.2.  Auto-provisioning and Service Discovery

   NVEs must be able to identify the appropriate VNI for each Tenant
   System.  This is based on state information that is often provided by
   external entities.  For example, in an environment where a VM is a
   Tenant System, this information is provided by VM orchestration
   systems, since these are the only entities that have visibility of
   which VM belongs to which tenant.

   A mechanism for communicating this information to the NVE is
   required.  VAPs have to be created and mapped to the appropriate VNI.
   Depending upon the implementation, this control interface can be
   implemented using an auto-discovery protocol between Tenant Systems
   and their local NVE or through management entities.  In either case,
   appropriate security and authentication mechanisms to verify that
   Tenant System information is not spoofed or altered are required.
   This is one critical aspect for providing integrity and tenant
   isolation in the system.

   NVEs may learn reachability information for VNIs on other NVEs via a
   control protocol that exchanges such information among NVEs or via a
   management-control entity.

3.1.5.3.  Address Advertisement and Tunnel Mapping

   As traffic reaches an ingress NVE on a VAP, a lookup is performed to
   determine which NVE or local VAP the packet needs to be sent to.  If
   the packet is to be sent to another NVE, the packet is encapsulated
   with a tunnel header containing the destination information
   (destination IP address or MPLS label) of the egress NVE.
   Intermediate nodes (between the ingress and egress NVEs) switch or
   route traffic based upon the tunnel destination information.

   A key step in the above process consists of identifying the
   destination NVE the packet is to be tunneled to.  NVEs are
   responsible for maintaining a set of forwarding or mapping tables
   that hold the bindings between destination VM and egress NVE
   addresses.  Several ways of populating these tables are possible:
   control plane driven, management plane driven, or data plane driven.

   When a control-plane protocol is used to distribute address
   reachability and tunneling information, the auto-provisioning and
   service discovery could be accomplished by the same protocol.  In
   this scenario, the auto-provisioning and service discovery could be
   combined with (be inferred from) the address advertisement and
   associated tunnel mapping.  Furthermore, a control-plane protocol

Top      ToC       Page 16 
   that carries both MAC and IP addresses eliminates the need for the
   Address Resolution Protocol (ARP) and hence addresses one of the
   issues with explosive ARP handling as discussed in [RFC6820].

3.1.5.4.  Overlay Tunneling

   For overlay tunneling, and dependent upon the tunneling technology
   used for encapsulating the Tenant System packets, it may be
   sufficient to have one or more local NVE addresses assigned and used
   in the source and destination fields of a tunneling encapsulation
   header.  Other information that is part of the tunneling
   encapsulation header may also need to be configured.  In certain
   cases, local NVE configuration may be sufficient while in other
   cases, some tunneling-related information may need to be shared among
   NVEs.  The information that needs to be shared will be technology
   dependent.  For instance, potential information could include tunnel
   identity, encapsulation type, and/or tunnel resources.  In certain
   cases, such as when using IP multicast in the underlay, tunnels that
   interconnect NVEs may need to be established.  When tunneling
   information needs to be exchanged or shared among NVEs, a control-
   plane protocol may be required.  For instance, it may be necessary to
   provide active/standby status information between NVEs, up/down
   status information, pruning/grafting information for multicast
   tunnels, etc.

   In addition, a control plane may be required to set up the tunnel
   path for some tunneling technologies.  This applies to both unicast
   and multicast tunneling.

3.2.  Multihoming

   Multihoming techniques can be used to increase the reliability of an
   NVO3 network.  It is also important to ensure that the physical
   diversity in an NVO3 network is taken into account to avoid single
   points of failure.

   Multihoming can be enabled in various nodes, from Tenant Systems into
   ToRs, ToRs into core switches/routers, and core nodes into DC GWs.

   The NVO3 underlay nodes (i.e., from NVEs to DC GWs) rely on IP
   routing techniques or MPLS re-rerouting capabilities as the means to
   re-route traffic upon failures.

   When a Tenant System is co-located with the NVE, the Tenant System is
   effectively single-homed to the NVE via a virtual port.  When the
   Tenant System and the NVE are separated, the Tenant System is
   connected to the NVE via a logical L2 construct such as a VLAN, and
   it can be multihomed to various NVEs.  An NVE may provide an L2

Top      ToC       Page 17 
   service to the end system or an l3 service.  An NVE may be multihomed
   to a next layer in the DC at L2 or L3.  When an NVE provides an L2
   service and is not co-located with the end system, loop-avoidance
   techniques must be used.  Similarly, when the NVE provides L3
   service, similar dual-homing techniques can be used.  When the NVE
   provides an L3 service to the end system, it is possible that no
   dynamic routing protocol is enabled between the end system and the
   NVE.  The end system can be multihomed to multiple physically
   separated L3 NVEs over multiple interfaces.  When one of the links
   connected to an NVE fails, the other interfaces can be used to reach
   the end system.

   External connectivity from a DC can be handled by two or more DC
   gateways.  Each gateway provides access to external networks such as
   VPNs or the Internet.  A gateway may be connected to two or more edge
   nodes in the external network for redundancy.  When a connection to
   an upstream node is lost, the alternative connection is used, and the
   failed route withdrawn.

3.3.  VM Mobility

   In DC environments utilizing VM technologies, an important feature is
   that VMs can move from one server to another server in the same or
   different L2 physical domains (within or across DCs) in a seamless
   manner.

   A VM can be moved from one server to another in stopped or suspended
   state ("cold" VM mobility) or in running/active state ("hot" VM
   mobility).  With "hot" mobility, VM L2 and L3 addresses need to be
   preserved.  With "cold" mobility, it may be desired to preserve at
   least VM L3 addresses.

   Solutions to maintain connectivity while a VM is moved are necessary
   in the case of "hot" mobility.  This implies that connectivity among
   VMs is preserved.  For instance, for L2 VNs, ARP caches are updated
   accordingly.

   Upon VM mobility, NVE policies that define connectivity among VMs
   must be maintained.

   During VM mobility, it is expected that the path to the VM's default
   gateway assures adequate QoS to VM applications, i.e., QoS that
   matches the expected service-level agreement for these applications.

4.  Key Aspects of Overlay Networks

   The intent of this section is to highlight specific issues that
   proposed overlay solutions need to address.

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4.1.  Pros and Cons

   An overlay network is a layer of virtual network topology on top of
   the physical network.

   Overlay networks offer the following key advantages:

   -  Unicast tunneling state management and association of Tenant
      Systems reachability are handled at the edge of the network (at
      the NVE).  Intermediate transport nodes are unaware of such state.
      Note that when multicast is enabled in the underlay network to
      build multicast trees for tenant VNs, there would be more state
      related to tenants in the underlay core network.

   -  Tunneling is used to aggregate traffic and hide tenant addresses
      from the underlay network and hence offers the advantage of
      minimizing the amount of forwarding state required within the
      underlay network.

   -  Decoupling of the overlay addresses (MAC and IP) used by VMs from
      the underlay network provides tenant separation and separation of
      the tenant address spaces from the underlay address space.


   -  Overlay networks support of a large number of virtual network
      identifiers.

   Overlay networks also create several challenges:

   -  Overlay networks typically have no control of underlay networks
      and lack underlay network information (e.g., underlay
      utilization):

      o  Overlay networks and/or their associated management entities
         typically probe the network to measure link or path properties,
         such as available bandwidth or packet loss rate.  It is
         difficult to accurately evaluate network properties.  It might
         be preferable for the underlay network to expose usage and
         performance information.

      o  Miscommunication or lack of coordination between overlay and
         underlay networks can lead to an inefficient usage of network
         resources.

      o  When multiple overlays co-exist on top of a common underlay
         network, the lack of coordination between overlays can lead to
         performance issues and/or resource usage inefficiencies.

Top      ToC       Page 19 
   -  Traffic carried over an overlay might fail to traverse firewalls
      and NAT devices.

   -  Multicast service scalability: Multicast support may be required
      in the underlay network to address tenant flood containment or
      efficient multicast handling.  The underlay may also be required
      to maintain multicast state on a per-tenant basis or even on a
      per-individual multicast flow of a given tenant.  Ingress
      replication at the NVE eliminates that additional multicast state
      in the underlay core, but depending on the multicast traffic
      volume, it may cause inefficient use of bandwidth.

4.2.  Overlay Issues to Consider

4.2.1.  Data Plane vs. Control Plane Driven

   In the case of an L2 NVE, it is possible to dynamically learn MAC
   addresses against VAPs.  It is also possible that such addresses be
   known and controlled via management or a control protocol for both L2
   NVEs and L3 NVEs.  Dynamic data-plane learning implies that flooding
   of unknown destinations be supported and hence implies that broadcast
   and/or multicast be supported or that ingress replication be used as
   described in Section 4.2.3.  Multicasting in the underlay network for
   dynamic learning may lead to significant scalability limitations.
   Specific forwarding rules must be enforced to prevent loops from
   happening.  This can be achieved using a spanning tree, a shortest
   path tree, or a split-horizon mesh.

   It should be noted that the amount of state to be distributed is
   dependent upon network topology and the number of virtual machines.
   Different forms of caching can also be utilized to minimize state
   distribution between the various elements.  The control plane should
   not require an NVE to maintain the locations of all the Tenant
   Systems whose VNs are not present on the NVE.  The use of a control
   plane does not imply that the data plane on NVEs has to maintain all
   the forwarding state in the control plane.

4.2.2.  Coordination between Data Plane and Control Plane

   For an L2 NVE, the NVE needs to be able to determine MAC addresses of
   the Tenant Systems connected via a VAP.  This can be achieved via
   data-plane learning or a control plane.  For an L3 NVE, the NVE needs
   to be able to determine the IP addresses of the Tenant Systems
   connected via a VAP.

Top      ToC       Page 20 
   In both cases, coordination with the NVE control protocol is needed
   such that when the NVE determines that the set of addresses behind a
   VAP has changed, it triggers the NVE control plane to distribute this
   information to its peers.

4.2.3.  Handling Broadcast, Unknown Unicast, and Multicast (BUM) Traffic

   There are several options to support packet replication needed for
   broadcast, unknown unicast, and multicast.  Typical methods include:

   - Ingress replication

   - Use of underlay multicast trees

   There is a bandwidth vs. state trade-off between the two approaches.
   Depending upon the degree of replication required (i.e., the number
   of hosts per group) and the amount of multicast state to maintain,
   trading bandwidth for state should be considered.

   When the number of hosts per group is large, the use of underlay
   multicast trees may be more appropriate.  When the number of hosts is
   small (e.g., 2-3) and/or the amount of multicast traffic is small,
   ingress replication may not be an issue.

   Depending upon the size of the data center network and hence the
   number of (S,G) entries, and also the duration of multicast flows,
   the use of underlay multicast trees can be a challenge.

   When flows are well known, it is possible to pre-provision such
   multicast trees.  However, it is often difficult to predict
   application flows ahead of time; hence, programming of (S,G) entries
   for short-lived flows could be impractical.

   A possible trade-off is to use in the underlay shared multicast trees
   as opposed to dedicated multicast trees.

4.2.4. Path MTU

   When using overlay tunneling, an outer header is added to the
   original frame.  This can cause the MTU of the path to the egress
   tunnel endpoint to be exceeded.

   It is usually not desirable to rely on IP fragmentation for
   performance reasons.  Ideally, the interface MTU as seen by a Tenant
   System is adjusted such that no fragmentation is needed.

Top      ToC       Page 21 
   It is possible for the MTU to be configured manually or to be
   discovered dynamically.  Various Path MTU discovery techniques exist
   in order to determine the proper MTU size to use:

   -  Classical ICMP-based Path MTU Discovery [RFC1191] [RFC1981]

      Tenant Systems rely on ICMP messages to discover the MTU of the
      end-to-end path to its destination.  This method is not always
      possible, such as when traversing middleboxes (e.g., firewalls)
      that disable ICMP for security reasons.

   -  Extended Path MTU Discovery techniques such as those defined in
      [RFC4821]

      Tenant Systems send probe packets of different sizes and rely on
      confirmation of receipt or lack thereof from receivers to allow a
      sender to discover the MTU of the end-to-end paths.

   While it could also be possible to rely on the NVE to perform
   segmentation and reassembly operations without relying on the Tenant
   Systems to know about the end-to-end MTU, this would lead to
   undesired performance and congestion issues as well as significantly
   increase the complexity of hardware NVEs required for buffering and
   reassembly logic.

   Preferably, the underlay network should be designed in such a way
   that the MTU can accommodate the extra tunneling and possibly
   additional NVO3 header encapsulation overhead.

4.2.5.  NVE Location Trade-Offs

   In the case of DC traffic, traffic originated from a VM is native
   Ethernet traffic.  This traffic can be switched by a local virtual
   switch or ToR switch and then by a DC gateway.  The NVE function can
   be embedded within any of these elements.

   There are several criteria to consider when deciding where the NVE
   function should happen:

   -  Processing and memory requirements

      o  Datapath (e.g., lookups, filtering, and
         encapsulation/decapsulation)

      o  Control-plane processing (e.g., routing, signaling, and OAM)
         and where specific control-plane functions should be enabled

   -  FIB/RIB size

Top      ToC       Page 22 
   -  Multicast support

      o  Routing/signaling protocols

      o  Packet replication capability

      o  Multicast FIB

   -  Fragmentation support

   -  QoS support (e.g., marking, policing, and queuing)

   -  Resiliency

4.2.6.  Interaction between Network Overlays and Underlays

   When multiple overlays co-exist on top of a common underlay network,
   resources (e.g., bandwidth) should be provisioned to ensure that
   traffic from overlays can be accommodated and QoS objectives can be
   met.  Overlays can have partially overlapping paths (nodes and
   links).

   Each overlay is selfish by nature.  It sends traffic so as to
   optimize its own performance without considering the impact on other
   overlays, unless the underlay paths are traffic engineered on a per-
   overlay basis to avoid congestion of underlay resources.

   Better visibility between overlays and underlays, or general
   coordination in placing overlay demands on an underlay network, may
   be achieved by providing mechanisms to exchange performance and
   liveliness information between the underlay and overlay(s) or by the
   use of such information by a coordination system.  Such information
   may include:

   -  Performance metrics (throughput, delay, loss, jitter) such as
      defined in [RFC3148], [RFC2679], [RFC2680], and [RFC3393].

   -  Cost metrics

5.  Security Considerations

   There are three points of view when considering security for NVO3.
   First, the service offered by a service provider via NVO3 technology
   to a tenant must meet the mutually agreed security requirements.
   Second, a network implementing NVO3 must be able to trust the virtual
   network identity associated with packets received from a tenant.
   Third, an NVO3 network must consider the security associated with
   running as an overlay across the underlay network.

Top      ToC       Page 23 
   To meet a tenant's security requirements, the NVO3 service must
   deliver packets from the tenant to the indicated destination(s) in
   the overlay network and external networks.  The NVO3 service provides
   data confidentiality through data separation.  The use of both VNIs
   and tunneling of tenant traffic by NVEs ensures that NVO3 data is
   kept in a separate context and thus separated from other tenant
   traffic.  The infrastructure supporting an NVO3 service (e.g.,
   management systems, NVEs, NVAs, and intermediate underlay networks)
   should be limited to authorized access so that data integrity can be
   expected.  If a tenant requires that its data be confidential, then
   the Tenant System may choose to encrypt its data before transmission
   into the NVO3 service.

   An NVO3 service must be able to verify the VNI received on a packet
   from the tenant.  To ensure this, not only tenant data but also NVO3
   control data must be secured (e.g., control traffic between NVAs and
   NVEs, between NVAs, and between NVEs).  Since NVEs and NVAs play a
   central role in NVO3, it is critical that secure access to NVEs and
   NVAs be ensured such that no unauthorized access is possible.  As
   discussed in Section 3.1.5.2, identification of Tenant Systems is
   based upon state that is often provided by management systems (e.g.,
   a VM orchestration system in a virtualized environment).  Secure
   access to such management systems must also be ensured.  When an NVE
   receives data from a Tenant System, the tenant identity needs to be
   verified in order to guarantee that it is authorized to access the
   corresponding VN.  This can be achieved by identifying incoming
   packets against specific VAPs in some cases.  In other circumstances,
   authentication may be necessary.  Once this verification is done, the
   packet is allowed into the NVO3 overlay, and no integrity protection
   is provided on the overlay packet encapsulation (e.g., the VNI,
   destination NVE, etc.).

   Since an NVO3 service can run across diverse underlay networks, when
   the underlay network is not trusted to provide at least data
   integrity, data encryption is needed to assure correct packet
   delivery.

   It is also desirable to restrict the types of information (e.g.,
   topology information as discussed in Section 4.2.6) that can be
   exchanged between an NVO3 service and underlay networks based upon
   their agreed security requirements.

Top      ToC       Page 24 
6.  Informative References

   [EVPN]     Sajassi, A., Aggarwal, R., Bitar, N., Isaac, A., and J.
              Uttaro, "BGP MPLS Based Ethernet VPN", Work in Progress,
              draft-ietf-l2vpn-evpn-10, October 2014.

   [OPPSEC]   Dukhovni, V. "Opportunistic Security: Some Protection Most
              of the Time", Work in Progress, draft-dukhovni-
              opportunistic-security-04, August 2014.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990, <http://www.rfc-editor.org/info/rfc1191>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996,
              <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2679]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Delay Metric for IPPM", RFC 2679, September 1999,
              <http://www.rfc-editor.org/info/rfc2679>.

   [RFC2680]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Packet Loss Metric for IPPM", RFC 2680, September 1999,
              <http://www.rfc-editor.org/info/rfc2680>.

   [RFC3148]  Mathis, M. and M. Allman, "A Framework for Defining
              Empirical Bulk Transfer Capacity Metrics", RFC 3148, July
              2001, <http://www.rfc-editor.org/info/rfc3148>.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              November 2002, <http://www.rfc-editor.org/info/rfc3393>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006,
              <http://www.rfc-editor.org/info/rfc4364>.

   [RFC4761]  Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, January 2007,
              <http://www.rfc-editor.org/info/rfc4761>.

   [RFC4762]  Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, January 2007,
              <http://www.rfc-editor.org/info/rfc4762>.

Top      ToC       Page 25 
   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC6820]  Narten, T., Karir, M., and I. Foo, "Address Resolution
              Problems in Large Data Center Networks", RFC 6820, January
              2013, <http://www.rfc-editor.org/info/rfc6820>.

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

Top      ToC       Page 26 
Acknowledgments

   In addition to the authors, the following people contributed to this
   document: Dimitrios Stiliadis, Rotem Salomonovitch, Lucy Yong, Thomas
   Narten, Larry Kreeger, and David Black.

Authors' Addresses

   Marc Lasserre
   Alcatel-Lucent
   EMail: marc.lasserre@alcatel-lucent.com


   Florin Balus
   Alcatel-Lucent
   777 E. Middlefield Road
   Mountain View, CA 94043
   United States
   EMail: florin.balus@alcatel-lucent.com


   Thomas Morin
   Orange
   EMail: thomas.morin@orange.com


   Nabil Bitar
   Verizon
   50 Sylvan Road
   Waltham, MA 02145
   United States
   EMail: nabil.n.bitar@verizon.com


   Yakov Rekhter
   Juniper
   EMail: yakov@juniper.net