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Versions: 00 01 draft-ietf-nvo3-arch

Internet Engineering Task Force                                 D. Black
Internet-Draft                                                       EMC
Intended status: Informational                                 J. Hudson
Expires: January 09, 2014                                        Brocade
                                                              L. Kreeger
                                                                   Cisco
                                                             M. Lasserre
                                                          Alcatel-Lucent
                                                               T. Narten
                                                                     IBM
                                                           July 08, 2013


              An Architecture for Overlay Networks (NVO3)
                       draft-narten-nvo3-arch-00

Abstract

   This document presents a high-level overview of a possible
   architecture for building overlay networks in NVO3.  The architecture
   is given at a high-level, showing the major components of an overall
   system.  An important goal is to divide the space into individual
   smaller components that can be implemented independently and with
   clear interfaces and interactions with other components.  It should
   be possible to build and implement individual components in isolation
   and have them work with other components with no changes to other
   components.  That way implementers have flexibility in implementing
   individual components and can optimize and innovate within their
   respective components without requiring changes to other components.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 09, 2014.





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Copyright Notice

   Copyright (c) 2013 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  VN Service (L2 and L3)  . . . . . . . . . . . . . . . . .   5
     3.2.  Network Virtualization Edge (NVE) . . . . . . . . . . . .   6
     3.3.  Network Virtualization Authority (NVA)  . . . . . . . . .   8
     3.4.  VM Orchestration Systems  . . . . . . . . . . . . . . . .   8
   4.  Network Virtualization Edge (NVE) . . . . . . . . . . . . . .   9
     4.1.  NVE Co-located With Server Hypervisor . . . . . . . . . .  10
     4.2.  Split-NVE . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  NVE State . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Tenant Systems Types  . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Overlay-Aware Network Service Appliances  . . . . . . . .  12
     5.2.  Bare Metal Servers  . . . . . . . . . . . . . . . . . . .  12
     5.3.  Gateways  . . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  Network Virtualization Authority  . . . . . . . . . . . . . .  13
     6.1.  How an NVA Obtains Information  . . . . . . . . . . . . .  14
     6.2.  Internal NVA Architecture . . . . . . . . . . . . . . . .  14
     6.3.  NVA External Interface  . . . . . . . . . . . . . . . . .  15
   7.  NVE-to-NVA Protocol . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  NVE-NVA Interaction Models  . . . . . . . . . . . . . . .  15
     7.2.  Direct NVE-NVA Protocol . . . . . . . . . . . . . . . . .  16
     7.3.  Push vs. Pull Model . . . . . . . . . . . . . . . . . . .  17
   8.  Federated NVAs  . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Inter-NVA Peering . . . . . . . . . . . . . . . . . . . .  20
   9.  Control Protocol Work Areas . . . . . . . . . . . . . . . . .  20
   10. NVO3 Data Plane Encapsulation . . . . . . . . . . . . . . . .  20
   11. Operations and Management . . . . . . . . . . . . . . . . . .  21
   12. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22



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   15. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   16. Informative References  . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   This document presents a high-level overview of a possible
   architecture for building overlay networks in NVO3.  The architecture
   is given at a high-level, showing the major components of an overall
   system.  An important goal is to divide the space into smaller
   individual components that can be implemented independently and with
   clear interfaces and interactions with other components.  It should
   be possible to build and implement individual components in isolation
   and have them work with other components with no changes to other
   components.  That way implementers have flexibility in implementing
   individual components and can optimize and innovate within their
   respective components without necessarily requiring changes to other
   components.

   The motivation for overlay networks is given in
   [I-D.ietf-nvo3-overlay-problem-statement].  "Framework for DC Network
   Virtualization" [I-D.ietf-nvo3-framework] provides a framework for
   discussing overlay networks generally and the various components that
   must work together in building such systems.  This document differs
   from the framework document in that it doesn't attempt to cover all
   possible approaches within the general design space.  Rather, it
   describes one particular approach.

   This document is intended to be a concrete strawman that can be used
   for discussion within the IETF NVO3 WG on what the NVO3 architecture
   should look like.

2.  Terminology

   This document uses the same terminology as [I-D.ietf-nvo3-framework].
   In addition, the following terms are used:

   NV Domain  A Network Virtualization Domain is an administrative
      construct that defines a Network Virtualization Authority (NVA),
      the set of Network Virtualization Edges (NVEs) associated with
      that NVA, and the set of virtual networks the NVA manages and
      supports.  NVEs are associated with a (logically centralized) NVA,
      and an NVE supports communication for any of the virtual networks
      in the domain.

   NV Region  A set of two or more NV Domains that share information
      about part or all of a set of virtual networks that the individual
      NV Domains manage.  Two NVAs share information about particular



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      virtual networks for the purpose of supporting connectivity
      between tenants located in different NVA Domains.  NVAs can share
      information about an entire NV domain, or just individual virtual
      networks.

3.  Background

   Overlay networks are an approach for providing network virtualization
   services to a set of Tenant Systems (TSs) [I-D.ietf-nvo3-framework].
   With overlays, data traffic between tenants is tunneled across the
   underlying data center's IP network.  The use of tunnels provides a
   number of benefits by decoupling the network as viewed by tenants
   from the underlying physical network across which they communicate.

   Tenant Systems connect to Virtual Networks (VNs), with each VN having
   associated attributes defining properties of the network, such as the
   set of members that connect to it.  Tenant Systems connected to a
   virtual network typically communicate freely with other Tenant
   Systems on the same VN, but communication between Tenant Systems on
   one VN and those external to the VN (whether on another VN or
   connected to the Internet) is carefully controlled and governed by
   policy.

   A Network Virtualization Edge (NVE) [I-D.ietf-nvo3-framework] is the
   entity that implements the overlay functionality.  An NVE resides at
   the boundary between a Tenant System and the overlay network as shown
   in Figure 1.  An NVE creates and maintains local state about each
   Virtual Network for which it is providing service on behalf of a
   Tenant System.


        +--------+                                             +--------+
        | Tenant +--+                                     +----| Tenant |
        | System |  |                                    (')   | System |
        +--------+  |          ................         (   )  +--------+
                    |  +-+--+  .              .  +--+-+  (_)
                    |  | NVE|--.              .--| NVE|   |
                    +--|    |  .              .  |    |---+
                       +-+--+  .              .   +--+-+
                       /       .              .
                      /        .  L3 Overlay  .   +--+-++--------+
        +--------+   /         .    Network   .   | NVE|| Tenant |
        | Tenant +--+          .              .- -|    || System |
        | System |             .              .   +--+-++--------+
        +--------+             ................
                                      |
                                    +----+
                                    | NVE|



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                                    |    |
                                    +----+
                                      |
                                      |
                            =====================
                              |               |
                          +--------+      +--------+
                          | Tenant |      | Tenant |
                          | System |      | System |
                          +--------+      +--------+


   The dotted line indicates a network connection (i.e., IP).

                  Figure 1: NVO3 Generic Reference Model

   The following subsections describe key aspects of an overlay system
   in more detail.  Section 3.1 describes the service model (Ethernet
   vs. IP) provided to Tenant Systems.  Section 3.2 describes NVEs in
   more detail.  Section 3.3 introduces the Network Virtualization
   Authority, from which NVEs obtain information about virtual networks.
   Section 3.4 provides background on VM orchestration systems and their
   use of virtual networks.

3.1.  VN Service (L2 and L3)

   A Virtual Network provides either L2 or L3 service to connected
   tenants.  For L2 service, VNs transport Ethernet frames, and a Tenant
   System is provided with a service that is analogous to being
   connected to a specific L2 C-VLAN.  L2 broadcast frames are delivered
   to all (and multicast frames delivered to a subset of) the other
   Tenant Systems on the VN.  To a Tenant System, it appears as if they
   are connected to a regular L2 Ethernet link.  Within NVO3, tenant
   frames are tunneled to remote NVEs based on the MAC addresses of the
   frame headers as originated by the Tenant System.  On the underlay,
   NVO3 packets are forwarded between NVEs based on the outer addresses
   of tunneled packets.

   For L3 service, VNs transport IP datagrams, and a Tenant System is
   provided with a service that only supports IP traffic.  Within NVO3,
   tenant frames are tunneled to remote NVEs based on the IP addresses
   of the packet originated by the Tenant System; any L2 destination
   addresses provided by Tenant Systems are effectively ignored.

   L2 service is intended for systems that need native L2 Ethernet
   service and the ability to run protocols directly over Ethernet
   (i.e., not based on IP).  L3 service is intended for systems in which
   all the traffic can safely be assumed to be IP.  It is important to



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   note that whether NVO3 provides L2 or L3 service to a Tenant System,
   the Tenant System does not generally need to be aware of the
   distinction.  In both cases, the virtual network presents itself to
   the Tenant System as an L2 Ethernet interface.  An Ethernet interface
   is used in both cases simply as a widely supported interface type
   that essentially all Tenant Systems already support.  Consequently,
   no special software is needed on Tenant Systems to use an L3 vs. an
   L2 overlay service.

3.2.  Network Virtualization Edge (NVE)

   Tenant Systems connect to NVEs via a Tenant System Interface (TSI).
   The TSI logically connects to the NVE via a Virtual Access Point
   (VAP) as shown in Figure 2.  To the Tenant System, the TSI is like a
   NIC; the TSI presents itself to a Tenant System a normal network
   interface.  On the NVE side, a VAP is a logical network port (virtual
   or physical) into a specific virtual network.  Note that two
   different Tenant Systems (and TSIs) attached to a common NVE can
   share a VAP (e.g., TS1 and TS2 in Figure 2) so long as they connect
   to the same Virtual Network.































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                        |         Data Center Network (IP)        |
                        |                                         |
                        +-----------------------------------------+
                             |                           |
                             |       Tunnel Overlay      |
                +------------+---------+       +---------+------------+
                | +----------+-------+ |       | +-------+----------+ |
                | |  Overlay Module  | |       | |  Overlay Module  | |
                | +---------+--------+ |       | +---------+--------+ |
                |           |          |       |           |          |
         NVE1   |           |          |       |           |          | NVE2
                |  +--------+-------+  |       |  +--------+-------+  |
                |  | |VNI1|     |VNI2| |       |  | |VNI1|    |VNI2|  |
                |  +-+----------+---+  |       |  +-+-----------+--+  |
                |    | VAP1     | VAP2 |       |    | VAP1      | VAP2|
                +----+------------+----+       +----+-----------+ ----+
                     |          |                   |           |
                     |\         |                   |           |
                     | \        |                   |          /|
              -------+--\-------+-------------------+---------/-+-------
                     |   \      |     Tenant        |        /  |
                TSI1 |TSI2\     | TSI3            TSI1  TSI2/   TSI3
                    +---+ +---+ +---+             +---+ +---+   +---+
                    |TS1| |TS2| |TS3|             |TS4| |TS5|   |TS6|
                    +---+ +---+ +---+             +---+ +---+   +---+

                       Figure 2: NVE Reference Model

   The Overlay Module performs the actual encapsulation and
   decapsulation of tunneled packets.  The NVE maintains state about the
   virtual networks it is a part of so that it can provide the Overlay
   Module with such information as the destination address of the NVE to
   tunnel a packet to, or the Context ID that should be placed in the
   encapsulation header to identify the virtual network a tunneled
   packet belong to.

   On the data center network side, the NVE sends and receives native IP
   traffic.  When ingressing traffic from a Tenant System, the NVE
   identifies the egress NVE to which the packet should be sent, adds an
   overlay encapsulation header, and sends the packet on the underlay
   network.  When receiving traffic from a remote NVE, an NVE strips off
   the encapsulation header, and delivers the (original) packet to the
   appropriate Tenant System.

   Conceptually, the NVE is a single entity implementing the NVO3
   functionality.  In practice, there are a number of different
   implementation scenarios, as described in detail in Section 4.




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3.3.  Network Virtualization Authority (NVA)

   Address dissemination refers to the process of learning, building and
   distributing the mapping/forwarding information that NVEs need in
   order to tunnel traffic to each other on behalf of communicating
   Tenant Systems.  For example, in order to send traffic to a remote
   Tenant System, the sending NVE must know the destination NVE for that
   Tenant System.

   One way to build and maintain mapping tables is to use learning, as
   802.1 bridges do [IEEE-802.1Q].  When forwarding traffic to multicast
   or unknown unicast destinations, an NVE could simply flood traffic
   everywhere.  While flooding works, it can lead to traffic hot spots
   and can lead to problems in larger networks.

   Alternatively, NVEs can make use of a Network Virtualization
   Authority (NVA).  An NVA is the entity that provides address mapping
   and other information to NVEs.  NVEs interact with an NVA to obtain
   any required address mapping information they need in order to
   properly forward traffic on behalf of tenants.  The term NVA refers
   to the overall system, without regards to its scope or how it is
   implemented.  NVAs provide a service, and NVEs access that service
   via an NVE-to-NVA protocol.

   Even when an NVA is present, learning could be used as a fallback
   mechanism, should the NVA be unable to provide an answer or for other
   reasons.  This document does not consider flooding approaches in
   detail, as there are a number of benefits in using an approach that
   depends on the presence of an NVA.

   NVAs are discussed in more detail in Section 6.

3.4.  VM Orchestration Systems

   VM Orchestration systems manage server virtualization across a set of
   servers.  Although VM management is a separate topic from network
   virtualization, the two areas are closely related.  Managing the
   creation, placement, and movements of VMs also involves creating,
   attaching to and detaching from virtual networks.  A number of
   existing VM orchestration systems have incorporated aspects of
   virtual network management into their systems.

   When a new VM image is started, the VM Orchestration system
   determines where the VM should be placed, interacts with the
   hypervisor on the target server to load and start the server and
   controls when a VM should be shutdown or migrated elsewhere.  VM
   Orchestration systems also have knowledge about how a VM should
   connect to a network, possibly including the name of the virtual



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   network to which a VM is to connect.  The VM orchestration system can
   pass such information to the hypervisor when a VM is instantiated.
   VM orchestration systems have significant (and sometimes global)
   knowledge over the domain they manage.  They typically know on what
   servers a VM is running, and meta data associated with VM images can
   be useful from a network virtualization perspective.  For example,
   the meta data may include the addresses (MAC and IP) the VMs will use
   and the name(s) of the virtual network(s) they connect to.

   VM orchestration systems run a protocol with an agent running on the
   hypervisor of the servers they manage.  That protocol can also carry
   information about what virtual network a VM is associated with.  When
   the orchestrator instantiates a VM on a hypervisor, the hypervisor
   interacts with the NVE in order to attach the VM to the virtual
   networks it has access to.  In general, the hypervisor will need to
   communicate significant VM state changes to the NVE.  In the reverse
   direction, the NVE may need to communicate network connectivity
   information back to the hypervisor.  Example VM orchestration systems
   in use today include VMware's vCenter Server or Microsoft's System
   Center Virtual Machine Manager.  Both can pass information about what
   virtual networks a VM connects to down to the hypervisor.  The
   protocol used between the VM orchestration system and hypervisors is
   generally proprietary.

   It should be noted that VM orchestration systems may not have direct
   access to all networking related information a VM uses.  For example,
   a VM may make use of additional IP or MAC addresses that the VM
   management system is not aware of.

4.  Network Virtualization Edge (NVE)

   As introduced in Section 3.2 an NVE is the entity that implements the
   overlay functionality.  This section describes NVEs in more detail.
   An NVE will have two external interfaces:

   Tenant Facing:  On the tenant facing side, an NVE interacts with the
      with the hypervisor (or equivalent entity) to provide the NVO3
      service.  An NVE will need to be notified when a Tenant System
      "attaches" to a virtual network (so it can validate the request
      and set up any state needed to send and receive traffic on behalf
      of the Tenant System on that VN).  Likewise, an NVE will need to
      be informed when the Tenant System "detaches" from the virtual
      network so that it can reclaim state and resources appropriately.








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   DCN Facing:  On the data center network facing side, an NVE
      interfaces with the data center underlay network, sending and
      receiving tunneled IP packets to and from the underlay.  The NVE
      may also run a control protocol with other entities on the
      network, such as the Network Virtualization Authority.

4.1.  NVE Co-located With Server Hypervisor

   When server virtualization is used, the entire NVE functionality will
   typically be implemented as part of the hypervisor and/or virtual
   switch on the server.  In such cases, the Tenant System interacts
   with the hypervisor and the hypervisor interacts with the NVE.
   Because the hypervisor and NVE interaction is implemented entirely in
   software on the server, there is no "on-the-wire" protocol between
   Tenant Systems (or the hypervisor) and the NVE that needs to be
   standardized.  While there may be APIs between the NVE and hypervisor
   to support necessary interaction, the details of such an API are not
   in-scope for the IETF to work on.

   Implementing NVE functionality entirely on a server has the
   disadvantage that server CPU resources must be spent implementing the
   NVO3 functionality.  Experimentation with overlay approaches and
   previous experience with TCP and checksum adapter offloads suggests
   that offloading some portions of the encapsulation and decapsulation
   operations an NVE performs onto the physical network adaptor can
   produce performance improvements.  As has been done with checksum and
   /or TCP server offload and other optimization approaches, there may
   be benefits to offloading common operations onto adaptors where
   possible.  Just as important, the addition of an overlay header can
   disable existing adaptor offload capabilities that are generally not
   prepared to handle the addition of a new header.  In any case, how to
   distribute the implementation of specific functionality between a
   server and network adaptors is a matter between server and adaptor
   vendors and does not require any IETF standardization.

4.2.  Split-NVE

   Another possible scenario leads to the need for a split NVE
   implementation.  A hypervisor running on a server could be aware that
   NVO3 is in use, but have some of the actual NVO3 functionality
   implemented on an adjacent switch to which the server is attached.
   While one could imagine a number of link types between a server and
   the NVE, the simplest deployment scenario would involve a server and
   NVE separated by a simple L2 Ethernet link, across which LLDP runs.
   A more complicated scenario would have the server and NVE separated
   by a bridged access network, such as when the NVE resides on a ToR,
   with an embedded switch residing between servers and the ToR.




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   While the above talks about a scenario involving a hypervisor, it
   should be noted that the same scenario can apply to Network Service
   Appliances as discussed in Section 5.1.  In general, when this
   document discusses the interaction between a hypervisor and NVE, the
   discussion applies to Network Service Appliances as well.

   For the split NVE case, protocols will be needed that allow the
   hypervisor and NVE to negotiate and setup the necessary state so that
   traffic sent across the access link between a server and the NVE can
   be associated with the correct virtual network instance.
   Specifically, on the access link, traffic belonging to a specific
   Tenant System would be tagged with a specific VLAN C-TAG that
   identifies which specific NVO3 virtual network instance it belongs
   to.  The hypervisor-NVE protocol would negotiate which VLAN C-TAG to
   use for a particular virtual network instance.  More details of the
   protocol requirements for functionality between hypervisors and NVEs
   can be found in [I-D.kreeger-nvo3-hypervisor-nve-cp].

4.3.  NVE State

   NVEs maintain internal data structures and state to support the
   sending and receiving of tenant traffic.  An NVE may need some or all
   of the following information:

   1.  An NVE keeps track of which attached Tenant Systems are connected
       to which virtual networks.  When a Tenant System attaches to a
       virtual network, the NVE will need to create or update local
       state for that virtual network.  When the last Tenant System
       detaches from a given VN, the NVE can reclaim state associated
       with that VN.

   2.  For tenant unicast traffic, an NVE maintains a per-VN table of
       mappings from Tenant System (inner) addresses to remote NVE
       (outer) addresses.

   3.  For tenant multicast (or broadcast) traffic, an NVE maintains a
       per-VN table of mappings and other information on how to deliver
       multicast (or broadcast) traffic.  If the underlying network
       supports IP multicast, the NVE could use IP multicast to deliver
       tenant traffic.  Alternatively, if the underlying network does
       not support multicast, an NVE could use serial unicast to deliver
       traffic.  In such a case, an NVE would need to know which
       destinations are subscribers to the tenant multicast group.  An
       NVE could use both approaches, switching from one mode to the
       other depending on such factors as bandwidth efficiency and group
       membership sparseness.





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   4.  An NVE maintains necessary information to encapsulate outgoing
       traffic, including what type of encapsulation and what value to
       use for a Context ID within the encapsulation header.

   5.  In order to deliver incoming encapsulated packets to the correct
       Tenant Systems, an NVE maintains the necessary information to map
       incoming traffic to the appropriate VAP and Tenant System.

   6.  An NVE may find it convenient to maintain additional per-VN
       information such as QoS settings, Path MTU information, ACLs,
       etc.

5.  Tenant Systems Types

   This section describes a number of special Tenant System types, and
   how they fit into an NVO3 system.

5.1.  Overlay-Aware Network Service Appliances

   Some Network Service Appliances [I-D.kreeger-nvo3-overlay-cp]
   (virtual or physical) provide tenant-aware services . That is, the
   specific service they provide depends on the identity of the tenant
   making use of the service.  For example, firewalls are now becoming
   available that support multi-tenancy where a single firewall provides
   virtual firewall service on a per-tenant basis, using per-tenant
   configuration rules and maintaining per-tenant state.  Such
   appliances will be aware of the VN an activity corresponds to while
   processing requests.  Unlike server virtualization, which shields VMs
   from needing to know about multi-tenancy, a Network Service
   Appliances explicitly supports multi-tenancy.  In such cases, the
   Network Service Appliance itself will be aware of network
   virtualization and either embed an NVE directly, or implement a split
   NVE as described in Section 4.2.  Unlike server virtualization,
   however, the Network Service Appliance will not be running a
   traditional hypervisor and the VM Orchestration system may not
   interact with the Network Service Appliance.  The NVE on such
   appliances will need to support a control plane to obtain the
   necessary information needed to fully participate in an NVO3 Domain.

5.2.  Bare Metal Servers

   Many data centers will continue to have at least some servers
   operating as non-virtualized (or "bare metal") machines running a
   traditional operating system and workload.  In such systems, there
   will be no NVE functionality on the server, and the server will have
   no knowledge of NVO3 (including whether overlays are even in use).
   In such environments, the NVE functionality can reside on the first-
   hop physical switch.  In such a case, the network administrator would



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   (manually) configure the switch to enable the appropriate NVO3
   functionality on the switch port connecting the server and associate
   that port with a specific virtual network.  Such configuration would
   typically be static, since the server is not virtualized, and once
   configured, is unlikely to change frequently.  Consequently, this
   scenario does not require any protocol or standards work.

5.3.  Gateways

   Gateways on VNs relay traffic onto and off of a virtual network.
   Tenant Systems use gateways to reach destinations outside of the
   local VN.  Gateways receive encapsulated traffic from one VN, remove
   the encapsulation header, and send the native packet out onto the
   data center network for delivery.  Outside traffic enters a VN in a
   reverse manner.

   Gateways can be either virtual (i.e., implemented as a VM) or
   physical (i.e., as a standalone physical device).  For performance
   reasons, standalone hardware gateways may be desirable in some cases.
   Such gateways could consist of a simple switch forwarding traffic
   from a VN onto the local data center network, or could embed router
   functionality.  On such gateways, network interfaces connecting to
   virtual networks will (at least conceptually) embed NVE (or split-
   NVE) functionality within them.  As in the case with Network Service
   Appliances, gateways will not support a hypervisor and will need an
   appropriate control plane protocol to obtain the information needed
   to provide NVO3 service.

   Gateways handle several different use cases.  For example, a virtual
   network could consist of systems supporting overlays together with
   legacy Tenant Systems that do not.  Gateways could be used to connect
   legacy systems supporting, e.g., L2 VLANs, to specific virtual
   networks, effectively making them part of the same virtual network.
   Gateways could also forward traffic between a virtual network and
   other hosts on the data center network or relay traffic between
   different VNs.  Finally, gateways can provide external connectivity
   such as Internet or VPN access.

6.  Network Virtualization Authority

   Before sending to and receiving traffic from a virtual network, an
   NVE must obtain the information needed to build its internal
   forwarding tables and state as listed in Section 4.3.  An NVE obtains
   such information from a Network Virtualization Authority.

   The Network Virtualization Authority (NVA) is the entity that
   provides address mapping and other information to NVEs.  NVEs
   interact with an NVA to obtain any required information they need in



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   order to properly forward traffic on behalf of tenants.  The term NVA
   refers to the overall system, without regards to its scope or how it
   is implemented.

6.1.  How an NVA Obtains Information

   There are two primary ways in which an NVA can obtain the address
   dissemination information it manages.

   On virtualized systems, the NVA may be able to obtain the address
   mapping information associated with VMs from the VM orchestration
   system itself.  If the VM orchestration system contains a master
   database for all the virtualization information, having the NVA
   obtain information directly to the orchestration system would be a
   natural approach.  Indeed, the NVA could effectively be co-located
   with the VM orchestration system itself.

   However, as described in Section 4 not all NVEs are associated with
   hypervisors.  In such cases, NVAs cannot leverage VM orchestration
   protocols to interact with an NVE and will instead need to peer
   directly with them.  By peering directly with an NVE, NVAs can obtain
   information about the TSes connected to that NVE and can distribute
   information to the NVE about the VNs those TSes are associated with.
   For example, whenever a Tenant System attaches to an NVE, that NVE
   would notify the NVA that the TS is now associated with that NVE.
   Likewise when a TS detaches from an NVE, that NVE would inform the
   NVA.  By communicating directly with NVEs, both the NVA and the NVE
   are able to maintain up-to-date information about all active tenants
   and the NVEs to which they are attached.

6.2.  Internal NVA Architecture

   For reliability and fault tolerance reasons, an NVA would be
   implemented in a distributed or replicated manner without single
   points of failure.  How the NVA is implemented, however, is not
   important to an NVE so long as the NVA provides a consistent and
   well-defined interface to the NVE.  For example, an NVA could be
   implemented via database techniques whereby a server stores address
   mapping information in a traditional (possibly replicated) database.
   Alternatively, an NVA could be implemented in a distributed fashion
   using an existing (or modified) routing protocol to maintain and
   distribute mappings.  So long as there is a clear interface between
   the NVE and NVA, how an NVA is architected and implemented is not
   important to an NVE.

   A number of architectural approaches could be used to implement NVAs
   themselves.  NVAs manage address bindings and distribute them to
   where they need to go.  One approach would be to use BGP (possibly



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   with extensions) and route reflectors.  Another approach could use a
   transaction-based database model with replicated servers.  Because
   the implementation details are local to an NVA, there is no need to
   pick exactly one solution technology, so long as the external
   interfaces to the NVEs (and remote NVAs) are sufficiently well
   defined to achieve interoperability.

6.3.  NVA External Interface

   [note: the following section discusses various options that the WG
   has not yet expressed an opinion on.  Discussion is encouraged. ]

   Conceptually, an NVA is a single entity.  An NVE interacts with the
   NVA, and it is the NVA's responsibility for ensuring that
   interactions between the NVE and NVA result in consistent behavior
   across the NVA and all other NVEs using the same NVA.  Because an NVA
   is built from multiple internal components, an NVA will have to
   ensure that information flows to all internal NVA components
   appropriately.

   One architectural question is whether interactions between an NVE and
   NVA all use a single NVA IP address.  If NVEs only have one IP
   address to interact with, it would be the responsibility of the NVA
   to handle NVA component failures, e.g., by using a "floating IP
   address" that migrates among NVA components to ensure that the NVA
   can always be reached via the one address.

   Alternatively, an NVA could export multiple IP addresses, making it
   the responsibility of the NVE to failover to alternate addresses
   should one fail.  The NVA would then also have to ensure that the
   information provided through all addresses is consistent, so that it
   would not matter to the NVE which address it used.

7.  NVE-to-NVA Protocol

   [Note: this and later sections are a bit sketchy and need work.
   Discussion is encouraged.]

   As outlined in Section 4.3, an NVE needs certain information in order
   to perform its functions.  To obtain such information from an NVA, an
   NVE-to-NVA protocol is needed.  While having a direct NVE-to-NVA
   protocol might seem straightforward, the existence of existing VM
   orchestration systems complicates the choices an NVE has for
   interacting with the NVA.

7.1.  NVE-NVA Interaction Models

   An NVE interacts with an NVA in at least two (quite different) ways:



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   o  NVEs supporting VMs and hypervisors can obtain necessary
      information entirely through the hypervisor-facing side of the
      NVE.  Such an approach is a natural extension to existing VM
      orchestration systems supporting server virtualization because an
      existing protocol between the hypervisor and VM Orchestration
      system already exists and can be leveraged to obtain any needed
      information.  Specifically, VM orchestration systems used to
      create, terminate and migrate VMs already use well-defined (though
      typically proprietary) protocols to handle the interactions
      between the hypervisor and VM orchestration system.  For such
      systems, it is a natural extension to leverage the existing
      orchestration protocol as a sort of proxy protocol for handling
      the interactions between an NVE and the NVA.  Indeed, existing
      implementation already do this.

   o  Alternatively, an NVE can obtain needed information by interacting
      directly with an NVA via a protocol operating over the data center
      underlay network.  Such an approach is needed to support NVEs that
      are not associated with systems performing server virtualization
      (e.g., as in the case of a standalone gateway) or where the NVE
      needs to communicate directly with the NVA for other reasons.

   [Note: The following paragraph is included to stimulate discussion,
   and the WG will need to decide what direction it wants to take.]

   The WG The NVO3 architecture should support both of the above models
   and indeed, it is possible that both models could be used
   simultaneously.  Existing virtualization environments are already
   using the first model.  But they are not sufficient to cover the case
   of standalone gateways -- such gateways do not support virtualization
   and do not interface with existing VM orchestration systems.  Also, A
   hybrid approach might be desirable in some cases where the first
   model is used to obtain the information, but the latter approach is
   used to validate and further authenticate the information before
   using it.

7.2.  Direct NVE-NVA Protocol

   An NVE can interact directly with an NVA via an NVE-to-NVA protocol.
   Such a protocol can be either independent of the NVA internal
   protocol, or an extension of it.  Using a dedicated protocol provides
   architectural separation and independence between the NVE and NVA.
   The NVE and NVA interact in a well-defined way, and changes in the
   NVA (or NVE) do not need to impact each other.  Using a dedicated
   protocol also ensures that both NVE and NVA implementations can
   evolve independently and without dependencies on each other.  Such
   independence is important because the upgrade path for NVEs and NVAs
   is quite different.  Upgrading all the NVEs at a site will likely be



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   more difficult in practice than upgrading NVAs because of their large
   number - one on each end device.  In practice, it is assumed that an
   NVE will be implemented once, and then (hopefully) not again, whereas
   an NVA (and its associated protocols) are more likely to evolve over
   time as experience is gained from usage.

   Requirements for a direct NVE-NVA protocol can be found in
   [I-D.kreeger-nvo3-overlay-cp]

7.3.  Push vs. Pull Model

   [Note: This section is included to stimulate discussion, as the WG
   has had a number of discussions on this point.  Depending on how WG
   discussion goes, this section may not even be needed in future
   versions of the document.]

   There has been discussion within NVO3 about a "push vs. pull"
   approach for NVE-to-NVA interaction.  In the push model, the NVA
   would push address binding information to the NVE.  Since the NVA has
   current knowledge of which NVE each Tenant System is connected to,
   the NVA can proactively push updates out to the NVEs when they occur.
   With a push model, the NVE can be more passive, relying on the NVA to
   ensure that an NVE always has most current information.  The push
   model has the benefit that NVEs will always have the mapping
   information they need, and do not need to query the NVA on a cache
   miss.  Note that in the push model, it is not required that an NVE
   maintain information about all virtual networks in the entire NV
   Domain; an NVE only needs to maintain information about the VNs
   associated with TSs associated with the NVE.

   In the pull model, an NVE may not have all the mappings it needs when
   it attempts to forward tenant traffic.  If an NVE attempts to send
   traffic to a destination for which it has no forwarding entry, the
   NVE queries the NVA to get the needed information or to definitively
   determine that no such entry exists.  While the pull model has the
   advantage that an NVE doesn't need table entries for destinations it
   is not forwarding traffic to, it has the disadvantage of delaying the
   sending of traffic on a cache miss.

   Rather than pick exactly one approach, the NVO3 architecture will
   likely support flavors of both the push and pull model.  In the case
   that the NVA has updated information to push to the NVEs, there is no
   reason to prohibit such a model.  Likewise, when the NVA is willing
   to generate queries for missing information on demand, there is no
   reason to have the architecture prevent such a model.

8.  Federated NVAs




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   An NVA provides service to the set of NVEs in its NV Domain.  Each
   NVA manages network virtualization information for the virtual
   networks within its NV Domain.  An NV domain is administered by a
   single entity.

   In some cases, it will be necessary to expand the scope of a specific
   VN or even an entire NV domain beyond a single NVA.  For example,
   multiple data centers managed by the same administrator may wish to
   operate all of its data centers as a single NV region.  Such cases
   are handled by having different NVAs peer with each other to exchange
   mapping information about specific VNs.  NVAs operate in a federated
   manner with a set of NVAs operating as a loosely-coupled federation
   of individual NVAs.  If a virtual network spans multiple NVAs (e.g.,
   located at different data centers), and an NVE needs to deliver
   tenant traffic to an NVE at a remote NVA, it still interacts only
   with its NVA, even when obtaining mappings for NVEs associated with
   domains at a remote NVA.

   Figure Figure 3 shows a scenario where two separate NV Domains (1 and
   2) share information about Virtual Network "1217".  VM1 and VM1 both
   connect to the same Virtual Network (1217), even though the two VMs
   are in separate NV Domains.  There are two cases to consider.  In the
   first case, NV Domain B (NVB) does not allow NVE-A to tunnel traffic
   directly to NVE-B. There could be a number of reasons for this.  For
   example, NV Domains 1 and 2 may not share a common address space
   (i.e., require traversal through a NAT device), or for policy
   reasons, a domain might require that all traffic between separate NV
   Domains be funneled through a particular device (e.g., a firewall).
   In such cases, NVA-2 will advertise to NVA-1 that VM1 on virtual
   network 1217 is available, and direct that traffic between the two
   nodes go through IP-G. IP-G would then decapsulate received traffic
   from one NV Domain, translate it appropriately for the other domain
   and re-encapsulate the packet for delivery.

                      xxxxxx                          xxxxxx        +-----+
   +-----+     xxxxxxxx    xxxxxx               xxxxxxx     xxxxx   | VM2 |
   | VM1 |    xx                xx            xxx               xx  |-----|
   |-----|   xx      +           x          xx                   x  |NVE-B|
   |NVE-A|   x                   x  +----+  x                     x +-----+
   +--+--+   x     NV Domain 1   x  |IP-G|--x                      x    |
      +-------x                 xx--+    | x                       xx   |
              x                x    +----+ x      NV Domain 2       x   |
           +---x             xx            xx                       x---+
           |    xxxx        xx           +->xx                     xx
           |       xxxxxxxxxx            |   xx                   xx
       +---+-+                           |     xx                xx
       |NVA-1|                        +--+--+    xx           xxx
       +-----+                        |NVA-2|     xxxx     xxxx



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                                      +-----+        xxxxxxx

            Figure 3: VM1 and VM2 are in different NV Domains.

   NVAs at one site share information and interact with NVAs at other
   sites, but only in a controlled manner.  It is expected that policy
   and access control will be applied at the boundaries between
   different sites (and NVAs) so as to minimize dependencies on external
   NVAs that could negatively impact the operation within a site.  It is
   an architectural principle that operations involving NVAs at one site
   not be immediately impacted by failures or errors at another site.
   (Of course, communication between NVEs in different NVO3 domains may
   be impacted by such failures or errors.)  It is a strong requirement
   that an NVA continue to operate properly for local NVEs even if
   external communication is interrupted (e.g., should communication
   between a local and remote NVA fail).

   At a high level, a federation of interconnected NVAs has some
   analogies to BGP and Autonomous Systems.  Like an Autonomous System,
   NVAs at one site are managed by a single administrative entity and do
   not interact with external NVAs except as allowed by policy.
   Likewise, the interface between NVAs at different sites is well
   defined, so that the internal details of operations at one site are
   largely hidden to other sites.  Finally, an NVA only peers with other
   NVAs that it has a trusted relationship with, i.e., where a virtual
   network is intended to span multiple NVAs.

   [Note: the following are motivations for having a federated NVA model
   and are intended for discussion.  Depending on discussion, these may
   be removed from future versions of this document. ] Reasons for using
   a federated model include:

   o  Provide isolation between NVAs operating at different sites at
      different geographic locations.

   o  Control the quantity and rate of information updates that flow
      (and must be processed) between different NVAs in different data
      centers.

   o  Control the set of external NVAs (and external sites) a site peers
      with.  A site will only peer with other sites that are cooperating
      in providing an overlay service.

   o  Allow policy to be applied between sites.  A site will want to
      carefully control what information it exports (and to whom) as
      well as what information it is willing to import (and from whom).





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   o  Allow different protocols and architectures to be used to for
      intra- vs. inter-NVA communication.  For example, within a single
      data center, a replicated transaction server using database
      techniques might be an attractive implementation option for an
      NVA, and protocols optimized for intra-NVA communication would
      likely be different from protocols involving inter-NVA
      communication between different sites.

   o  Allow for optimized protocols, rather than using a one-size-fits
      all approach.  Within a data center, networks tend to have lower-
      latency, higher-speed and higher redundancy when compared with WAN
      links interconnecting data centers.  The design constraints and
      tradeoffs for a protocol operating within a data center network
      are different from those operating over WAN links.  While a single
      protocol could be used for both cases, there could be advantages
      to using different and more specialized protocols for the intra-
      and inter-NVA case.

8.1.  Inter-NVA Peering

   To support peering between different NVAs, an inter-NVA protocol is
   needed.  The inter-NVA protocol defines what information is exchanged
   between NVAs.  It is assumed that the protocol will be used to share
   addressing information between data centers and must scale well over
   WAN links.

9.  Control Protocol Work Areas

   The NVO3 architecture consists of two major distinct entities: NVEs
   and NVAs.  In order to provide isolation and independence between
   these two entities, the NVO3 architecture calls for well defined
   protocols for interfacing between them.  For an individual NVA, the
   architecture calls for a single conceptual entity, that could be
   implemented in a distributed or replicated fashion.  While the IETF
   may choose to define one or more specific architectural approaches to
   building individual NVAs, there is little need for it to pick exactly
   one approach to the exclusion of others.  An NVA for a single domain
   will likely be deployed as a single vendor product and thus their is
   little benefit in standardizing the internal structure of an NVA.

   Individual NVAs peer with each other in a federated manner.  The NVO3
   architecture calls for a well-defined interface between NVAs.

   Finally, a hypervisor-to-NVE protocol is needed to cover the split-
   NVE scenario described in Section 4.2.

10.  NVO3 Data Plane Encapsulation




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   When tunneling tenant traffic, NVEs add encapsulation header to the
   original tenant packet.  The exact encapsulation to use for NVO3 does
   not seem to be critical.  The main requirement is that the
   encapsulation support a Context ID of sufficient size
   [I-D.ietf-nvo3-dataplane-requirements].  A number of encapsulations
   already exist that provide a VN Context of sufficient size for NVO3.
   For example, VXLAN [I-D.mahalingam-dutt-dcops-vxlan] has a 24-bit
   VXLAN Network Identifier (VNI).  NVGRE
   [I-D.sridharan-virtualization-nvgre] has a 24-bit Tenant Network ID
   (TNI).  MPLS-over-GRE provides a 20-bit label field.  While there is
   widespread recognition that a 12-bit VN Context would be too small
   (only 4096 distinct values), it is generally agreed that 20 bits (1
   million distinct values) and 24 bits (16.8 million distinct values)
   are sufficient for a wide variety of deployment scenarios.

   [Note: the following paragraph is included for WG discussion.  Future
   versions of this document may omit this text.]

   While one might argue that a new encapsulation should be defined just
   for NVO3, no compelling requirements for doing so have been
   identified yet.  Moreover, optimized implementations for existing
   encapsulations are already starting to become available on the market
   (i.e., in silicon).  If the IETF were to define a new encapsulation
   format, it would take at least 2 (and likely more) years before
   optimized implementations of the new format would become available in
   products.  In addition, a new encapsulation format would not likely
   displace existing formats, at least not for years.  Thus, there seems
   little reason to define a new encapsulation.  However, it does make
   sense for NVO3 to support multiple encapsulation formats, so as to
   allow NVEs to use their preferred encapsulations when possible.  This
   implies that the address dissemination protocols must also include an
   indication of supported encapsulations along with the address mapping
   details.

11.  Operations and Management

   The simplicity of operating and debugging overlay networks will be
   critical for successful deployment.  Some architectural choices can
   facilitate or hinder OAM.  Related OAM drafts include
   [I-D.ashwood-nvo3-operational-requirement].

12.  Summary

   This document provides a start at a general architecture for overlays
   in NVO3.  The architecture calls for three main areas of protocol
   work:





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   1.  A hypervisor-to-NVE protocol to support Split NVEs as discussed
       in Section 4.2.

   2.  An NVE to NVA protocol for address dissemination.

   3.  An NVA-to-NVA protocol for exchange of information about specific
       virtual networks between NVAs.

   It should be noted that existing protocols or extensions of existing
   protocols are applicable.

13.  Acknowledgments

   Helpful comments and improvements to this document have come from
   Dennis (Xiaohong) Qin.

14.  IANA Considerations

   This memo includes no request to IANA.

15.  Security Considerations

   Yep, kind of sparse.  But we'll get there eventually. :-)

16.  Informative References

   [I-D.ashwood-nvo3-operational-requirement]
              Ashwood-Smith, P., Iyengar, R., Tsou, T., Sajassi, A.,
              Boucadair, M., Jacquenet, C., and M. Daikoku, "NVO3
              Operational Requirements", draft-ashwood-nvo3-operational-
              requirement-02 (work in progress), January 2013.

   [I-D.ietf-nvo3-dataplane-requirements]
              Bitar, N., Lasserre, M., Balus, F., Morin, T., Jin, L.,
              and B. Khasnabish, "NVO3 Data Plane Requirements", draft-
              ietf-nvo3-dataplane-requirements-01 (work in progress),
              July 2013.

   [I-D.ietf-nvo3-framework]
              Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
              Rekhter, "Framework for DC Network Virtualization", draft-
              ietf-nvo3-framework-03 (work in progress), July 2013.

   [I-D.ietf-nvo3-overlay-problem-statement]
              Narten, T., Gray, E., Black, D., Fang, L., Kreeger, L.,
              and M. Napierala, "Problem Statement: Overlays for Network
              Virtualization", draft-ietf-nvo3-overlay-problem-
              statement-03 (work in progress), May 2013.



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   [I-D.kreeger-nvo3-hypervisor-nve-cp]
              Kreeger, L., Narten, T., and D. Black, "Network
              Virtualization Hypervisor-to-NVE Overlay Control Protocol
              Requirements", draft-kreeger-nvo3-hypervisor-nve-cp-01
              (work in progress), February 2013.

   [I-D.kreeger-nvo3-overlay-cp]
              Kreeger, L., Dutt, D., Narten, T., Black, D., and M.
              Sridharan, "Network Virtualization Overlay Control
              Protocol Requirements", draft-kreeger-nvo3-overlay-cp-04
              (work in progress), June 2013.

   [I-D.mahalingam-dutt-dcops-vxlan]
              Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "VXLAN: A
              Framework for Overlaying Virtualized Layer 2 Networks over
              Layer 3 Networks", draft-mahalingam-dutt-dcops-vxlan-04
              (work in progress), May 2013.

   [I-D.sridharan-virtualization-nvgre]
              Sridharan, M., Greenberg, A., Venkataramaiah, N., Wang,
              Y., Duda, K., Ganga, I., Lin, G., Pearson, M., Thaler, P.,
              and C. Tumuluri, "NVGRE: Network Virtualization using
              Generic Routing Encapsulation", draft-sridharan-
              virtualization-nvgre-02 (work in progress), February 2013.

   [IEEE-802.1Q]
              IEEE 802.1Q-2011, ., "IEEE standard for local and
              metropolitan area networks: Media access control (MAC)
              bridges and virtual bridged local area networks, ", August
              2011.

Authors' Addresses

   David Black
   EMC

   Email: david.black@emc.com


   Jon Hudson
   Brocade
   120 Holger Way
   San Jose, CA  95134
   USA

   Email: jon.hudson@gmail.com




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   Lawrence Kreeger
   Cisco

   Email: kreeger@cisco.com


   Marc Lasserre
   Alcatel-Lucent

   Email: marc.lasserre@alcatel-lucent.com


   Thomas Narten
   IBM

   Email: narten@us.ibm.com



































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