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Versions: 00 01 02 03 draft-ietf-nvo3-framework

Internet Engineering Task Force                           Marc Lasserre
Internet Draft                                             Florin Balus
Intended status: Informational                           Alcatel-Lucent
Expires: December 2012
                                                           Thomas Morin
                                                  France Telecom Orange

                                                            Nabil Bitar
                                                                Verizon

                                                           Yakov Rekhter
                                                                 Juniper

                                                          June 18, 2012




                  Framework for DC Network Virtualization
                   draft-lasserre-nvo3-framework-02.txt





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), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on December 18, 2012.






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Copyright Notice

   Copyright (c) 2012 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.



Abstract

   Several IETF drafts relate to the use of overlay networks to support
   large scale virtual data centers. This draft provides a framework
   for Network Virtualization over L3 (NVO3) and is intended to help
   plan a set of work items in order to provide a complete solution
   set. It defines a logical view of the main components with the
   intention of streamlining the terminology and focusing the solution
   set.



Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................4
      1.2. General terminology.......................................4
      1.3. DC network architecture...................................6
      1.4. Tenant networking view....................................8
   2. Reference Models...............................................9
      2.1. Generic Reference Model...................................9
      2.2. NVE Reference Model......................................11
      2.3. NVE Service Types........................................12
         2.3.1. L2 NVE providing Ethernet LAN-like service..........13
         2.3.2. L3 NVE providing IP/VRF-like service................13
   3. Functional components.........................................13
      3.1. Generic service virtualization components................13
         3.1.1. Virtual Access Points (VAPs)........................14
         3.1.2. Virtual Network Instance (VNI)......................14
         3.1.3. Overlay Modules and VN Context......................15
         3.1.4. Tunnel Overlays and Encapsulation options...........16
         3.1.5. Control Plane Components............................16



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         3.1.5.1. Auto-provisioning/Service discovery...............16
         3.1.5.2. Address advertisement and tunnel mapping..........17
         3.1.5.3. Tunnel management.................................17
      3.2. Service Overlay Topologies...............................17
   4. Key aspects of overlay networks...............................18
      4.1. Pros & 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. IANA Considerations...........................................22
   7. References....................................................23
      7.1. Normative References.....................................23
      7.2. Informative References...................................23
   8. Acknowledgments...............................................23

1. Introduction

   This document provides a framework for Data Center Network
   Virtualization over L3 tunnels. This framework is intended to aid in
   standardizing protocols and mechanisms to support large scale
   network virtualization for data centers.

   Several IETF drafts relate to the use of overlay networks for data
   centers.

   [NVOPS] defines the rationale for using overlay networks in order to
   build large data center networks. The use of virtualization leads to
   a very large number of communication domains and end systems to cope
   with. Existing virtual network models used for data center networks
   have known limitations, specifically in the context of multiple
   tenants. These issues can be summarized as:

     o Limited VLAN space

     o FIB explosion due to handling of large number of MACs/IP
        addresses

     o Spanning Tree limitations

     o Excessive ARP handling



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     o Broadcast storms

     o Inefficient Broadcast/Multicast handling

     o Limited mobility/portability support

     o Lack of service auto-discovery

   Overlay techniques have been used in the past to address some of
   these issues.

   [OVCPREQ] describes the requirements for a control plane protocol
   required by overlay border nodes to exchange overlay mappings.

   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 in the design of standards and
   mechanisms for large scale data centers.

1.1. Conventions used in this document

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

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

1.2. General terminology

   This document uses the following terminology:

   VN: Virtual Network. This is a virtual L2 or L3 domain that belongs
   a tenant.

   VNI: Virtual Network Instance. This is one instance of a virtual
   overlay network. Two Virtual Networks are isolated from one another
   and may use overlapping addresses.

   Virtual Network Context or VN Context: Field that is part of the
   overlay encapsulation header which allows the encapsulated frame to
   be delivered to the appropriate virtual network endpoint by the
   egress NVE. The egress NVE uses this field to determine the
   appropriate virtual network context in which to process the packet.
   This field MAY be an explicit, unique (to the administrative domain)



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   virtual network identifier (VNID) or MAY express the necessary
   context information in other ways (e.g. a locally significant
   identifier).

   VNID:  Virtual Network Identifier. In the case where the VN context
   has global significance, this is the ID value that is carried in
   each data packet in the overlay encapsulation that identifies the
   Virtual Network the packet belongs to.

   NVE: Network Virtualization Edge. It is a network entity that sits
   on the edge of the NVO3 network. It implements network
   virtualization functions that allow for L2 and/or L3 tenant
   separation and for hiding tenant addressing information (MAC and IP
   addresses). An NVE could be implemented as part of a virtual switch
   within a hypervisor, a physical switch or router, a Network Service
   Appliance or even be embedded within an End Station.

   Underlay or Underlying Network: This is the network that provides
   the connectivity between NVEs. The Underlying Network can be
   completely unaware of the overlay packets. Addresses within the
   Underlying Network are also referred to as "outer addresses" because
   they exist in the outer encapsulation. The Underlying Network can
   use a completely different protocol (and address family) from that
   of the overlay.

   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 and/or compute and/or storage services. One such service
   is virtualized data center services, also known as Infrastructure as
   a Service.

   Virtual Data Center or Virtual DC: A container for virtualized
   compute, storage and network services. Managed by a single tenant, a
   Virtual DC can contain multiple VNs and multiple Tenant End Systems
   that are connected to one or more of these VNs.

   VM: Virtual Machine. Several Virtual Machines can share the
   resources of a single physical computer server using the services of
   a Hypervisor (see below definition).

   Hypervisor: Server virtualization software running on a physical
   compute server that hosts Virtual Machines. The hypervisor provides
   shared compute/memory/storage and network connectivity to the VMs
   that it hosts. Hypervisors often embed a Virtual Switch (see below).




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   Virtual Switch: A function within a Hypervisor (typically
   implemented in software) that provides similar services to a
   physical Ethernet switch.  It switches Ethernet frames between VMs'
   virtual NICs within the same physical server, or between a VM and a
   physical NIC card connecting the server to a physical Ethernet
   switch. It also enforces network isolation between VMs that should
   not communicate with each other.

   Tenant: A customer who consumes virtualized data center services
   offered by a cloud service provider. A single tenant may consume one
   or more Virtual Data Centers hosted by the same cloud service
   provider.

   Tenant End System: It defines an end system of a particular tenant,
   which can be for instance a virtual machine (VM), a non-virtualized
   server, or a physical appliance.

   ELAN: MEF ELAN, multipoint to multipoint Ethernet service

   EVPN: Ethernet VPN as defined in [EVPN]

1.3. DC network architecture

   A generic architecture for Data Centers is depicted in Figure 1:

























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                                ,---------.
                              ,'           `.
                             (  IP/MPLS WAN )
                              `.           ,'
                                `-+------+'
                             +--+--+   +-+---+
                             |DC GW|+-+|DC GW|
                             +-+---+   +-----+
                                 |       /
                                 .--. .--.
                               (    '    '.--.
                            .-.' Intra-DC     '
                           (     network      )
                            (             .'-'
                             '--'._.'.    )\ \
                              / /     '--'  \ \
                             / /      | |    \ \
                      +---+--+   +-`.+--+  +--+----+
                      | ToR  |   | ToR  |  |  ToR  |
                      +-+--`.+   +-+-`.-+  +-+--+--+
                      .'     \   .'    \   .'     `.
                   __/_      _i./       i./_       _\__
            '--------'    '--------'   '--------'   '--------'
            :  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
   this figure. It provides a view of physical components inside a DC.

   A cloud network is composed of intra-Data Center (DC) networks and
   network services, and, inter-DC network and network connectivity
   services. Depending upon the scale, DC distribution, operations
   model, Capex and Opex aspects, DC networking elements can act as
   strict L2 switches and/or provide IP routing capabilities, including
   also service virtualization.

   In some DC architectures, it is possible that some tier layers
   provide L2 and/or L3 services, are collapsed, and that Internet
   connectivity, inter-DC connectivity and VPN support are handled by a
   smaller number of nodes. Nevertheless, one can assume that the
   functional blocks fit with the architecture above.

   The following components can be present in a DC:



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     o End Device: a DC resource to which the networking service is
        provided. End Device may be a compute resource (server or
        server blade), storage component or a network appliance
        (firewall, load-balancer, IPsec gateway). Alternatively, the
        End Device may include software based networking functions used
        to interconnect multiple hosts. An example of soft networking
        is the virtual switch in the server blades, used to
        interconnect multiple virtual machines (VMs). End Device may be
        single or multi-homed to the Top of Rack switches (ToRs).

     o Top of Rack (ToR): 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. ToRs
        may also provide routing functionality, virtual IP network
        connectivity, or Layer2 tunneling over IP for instance. ToRs
        are usually multi-homed to switches in the Intra-DC network.
        Other deployment scenarios may use an intermediate Blade Switch
        before the ToR or an EoR (End of Row) switch to provide similar
        function as a ToR.

     o Intra-DC Network: High capacity network composed of core
        switches aggregating multiple ToRs. Core switches are usually
        Ethernet switches but can also support routing capabilities.

     o 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 IPVPN/L2VPN PE. Some network implementations
        may dedicate DC GWs for different connectivity types (e.g., a
        DC GW for Internet, and another for VPN).

1.4. Tenant networking view

   The DC network architecture is used to provide L2 and/or L3 service
   connectivity to each tenant. An example is depicted in Figure 2:














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                         +----- L3 Infrastructure ----+
                         |                            |
                      ,--+-'.                      ;--+--.
                 .....  Rtr1 )......              .  Rtr2 )
                 |    '-----'      |               '-----'
                 |     Tenant1     |LAN12      Tenant1|
                 |LAN11        ....|........          |LAN13
             '':'''''''':'       |        |     '':'''''''':'
              ,'.      ,'.      ,+.      ,+.     ,'.      ,'.
             (VM ) .. (VM )    (VM ) .. (VM )   (VM ) .. (VM )
              `-'      `-'      `-'      `-'     `-'      `-'

        Figure 2 : Logical Service connectivity for a single tenant

   In this example one or more L3 contexts and one or more LANs (e.g.,
   one per Application) running on DC switches are assigned for DC
   tenant 1.

   For a multi-tenant DC, a virtualized version of this type of service
   connectivity needs to be provided for each tenant by the Network
   Virtualization solution.

2. Reference Models

2.1. Generic Reference Model

   The following diagram shows a DC reference model for network
   virtualization using Layer3 overlays where edge devices provide a
   logical interconnect between Tenant End Systems that belong to
   specific tenant network.



















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         +--------+                                  +--------+
         | Tenant |                                  | Tenant |
         |  End   +--+                           +---|  End   |
         | System |  |                           |   | System |
         +--------+  |    ...................    |   +--------+
                     |  +-+--+           +--+-+  |
                     |  | NV |           | NV |  |
                     +--|Edge|           |Edge|--+
                        +-+--+           +--+-+
                       /  .    L3 Overlay   .  \
         +--------+   /   .     Network     .   \     +--------+
         | Tenant +--+    .                 .    +----| Tenant |
         |  End   |       .                 .         |  End   |
         | System |       .    +----+       .         | System |
         +--------+       .....| NV |........         +--------+
                               |Edge|
                               +----+
                                 |
                                 |
                              +--------+
                              | Tenant |
                              |  End   |
                              | System |
                              +--------+

     Figure 3 : Generic reference model for DC network virtualization
                       over a Layer3 infrastructure

   The functional components in this picture do not necessarily map
   directly with the physical components described in Figure 1.

   For example, an End Device can be a server blade with VMs and
   virtual switch, i.e. the VM is the Tenant End System and the NVE
   functions may be performed by the virtual switch and/or the
   hypervisor.

   Another example is the case where an End Device can be a traditional
   physical server (no VMs, no virtual switch), i.e. the server is the
   Tenant End System and the NVE functions may be performed by the ToR.
   Other End Devices in this category are Physical Network Appliances
   or Storage Systems.



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   A Tenant End System attaches to a Network Virtualization Edge (NVE)
   node, either directly or via a switched network (typically
   Ethernet).

   The NVE implements network virtualization functions that allow for
   L2 and/or L3 tenant separation and for hiding tenant addressing
   information (MAC and IP addresses), tenant-related control plane
   activity and service contexts from the Routed Backbone nodes.

   Core nodes utilize L3 techniques to interconnect NVE nodes in
   support of the overlay network. These devices perform forwarding
   based on outer L3 tunnel header, and generally do not maintain per
   tenant-service state albeit some applications (e.g., multicast) may
   require control plane or forwarding plane information that pertain
   to a tenant, group of tenants, tenant service or a set of services
   that belong to one or more tunnels. When such tenant or tenant-
   service related information is maintained in the core, overlay
   virtualization provides knobs to control the magnitude of that
   information.

2.2. NVE Reference Model

   The NVE is composed of a tenant service instance that Tenant End
   Systems interface with and an overlay module that provides tunneling
   overlay functions (e.g. encapsulation/decapsulation of tenant
   traffic from/to the tenant forwarding instance, tenant
   identification and mapping, etc), as described in figure 4:






















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                      +------- L3 Network ------+
                      |                         |
                      |       Tunnel Overlay    |
        +------------+---------+       +---------+------------+
        | +----------+-------+ |       | +---------+--------+ |
        | |  Overlay Module  | |       | |  Overlay Module  | |
        | +---------+--------+ |       | +---------+--------+ |
        |           |VN context|       | VN context|          |
        |           |          |       |           |          |
        |  +--------+-------+  |       |  +--------+-------+  |
        |  |     VNI        |  |       |  |       VNI      |  |
   NVE1 |  +-+------------+-+  |       |  +-+-----------+--+  | NVE2
        |    |   VAPs     |    |       |    |    VAPs   |     |
        +----+------------+----+       +----+------------+----+
             |            |                 |            |
      -------+------------+-----------------+------------+-------
             |            |     Tenant      |            |
             |            |   Service IF    |            |
            Tenant End Systems            Tenant End Systems

              Figure 4 : Generic reference model for NV Edge

   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.

   For example, the NVE functionality could reside solely on the End
   Devices, on the ToRs or on both the End Devices and the ToRs. In the
   latter case we say that the the End Device NVE component acts as the
   NVE Spoke, and ToRs act as NVE hubs. Tenant End Systems will
   interface with the tenant service instances maintained on the NVE
   spokes, and tenant service instances maintained on the NVE spokes
   will interface with the tenant service instances maintained on the
   NVE hubs.

2.3. NVE Service Types

   NVE components may be used to provide different types of virtualized
   service connectivity. This section defines the service types and
   associated attributes





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2.3.1. L2 NVE providing Ethernet LAN-like service

   L2 NVE implements Ethernet LAN emulation (ELAN), an Ethernet based
   multipoint service where the Tenant End Systems appear to be
   interconnected by a LAN environment over a set of L3 tunnels. It
   provides per tenant virtual switching instance with MAC addressing
   isolation and L3 tunnel encapsulation across the core.

2.3.2. L3 NVE providing IP/VRF-like service

   Virtualized IP routing and forwarding is similar from a service
   definition perspective with IETF IP VPN (e.g., BGP/MPLS IPVPN and
   IPsec VPNs). It provides per tenant routing instance with addressing
   isolation and L3 tunnel encapsulation across the core.

3. Functional components

   This section breaks down the Network Virtualization architecture
   into functional components to make it easier to discuss solution
   options for different modules.

   This version of the document gives an overview of generic functional
   components that are shared between L2 and L3 service types. Details
   specific for each service type will be added in future revisions.

3.1. Generic service virtualization components

   A Network Virtualization solution is built around a number of
   functional components as depicted in Figure 5:




















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

              Figure 5 : Generic reference model for NV Edge

3.1.1. Virtual Access Points (VAPs)

   Tenant End Systems are connected to the Tenant Instance through
   Virtual Access Points (VAPs). The VAPs can be in reality physical
   ports on a ToR or virtual ports identified through logical interface
   identifiers (VLANs, internal VSwitch Interface ID leading to a VM).

3.1.2. Virtual Network Instance (VNI)

   The VNI represents a set of configuration attributes defining access
   and tunnel policies and (L2 and/or L3) forwarding functions.

   Per tenant FIB tables and control plane protocol instances are used
   to maintain separate private contexts between tenants. Hence tenants
   are free to use their own addressing schemes without concerns about
   address overlapping with other tenants.







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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 tunnel source address
   identifying the source NVE and the tunnel destination L3 address
   identifying the destination NVE. 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, encapsulation/decapsulation of frames from
   VAPs/L3 Backbone and may provide for transit forwarding of IP
   traffic (e.g., transparent tunnel forwarding).

   In a multi-tenant context, the tunnel aggregates frames from/to
   different VNIs. Tenant identification and traffic demultiplexing are
   based on the VN Context (e.g. VNID).

   The following approaches can been considered:

     o One VN Context per Tenant: A globally unique (on a per-DC
        administrative domain) VNID is used to identify the related
        Tenant instances. An example of this approach is the use of
        IEEE VLAN or ISID tags to provide virtual L2 domains.

     o One VN Context per VNI: A per-tenant local value is
        automatically generated by the egress 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].

     o One VN Context per VAP: A per-VAP local value is assigned and
        usually distributed by a control plane protocol. An example of
        this approach is the use of per CE-PE MPLS labels in IP VPN
        [RFC4364].

   Note that when using one VN Context per VNI or per VAP, an
   additional global identifier 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 is added to the frame, a L3 Tunnel encapsulation
   is used to transport the frame to the destination NVE. The backbone
   devices do not usually keep any per service state, simply forwarding
   the frames based on the outer tunnel header.

   Different IP tunneling options (GRE/L2TP/IPSec) and tunneling
   options (BGP VPN, PW, VPLS) are available for both Ethernet and IP
   formats.

3.1.5. Control Plane Components

   Control plane components may be used to provide the following
   capabilities:

     . Auto-provisioning/Service discovery

     . Address advertisement and tunnel mapping

     . Tunnel management

   A control plane component can be an on-net control protocol or a
   management control entity.

3.1.5.1. Auto-provisioning/Service discovery

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

   A mechanism for communicating this information between Tenant End
   Systems and the local NVE is required. As a result the VAPs are
   created and mapped to the appropriate Tenant Instance.

   Depending upon the implementation, this control interface can be
   implemented using an auto-discovery protocol between Tenant End
   Systems and their local NVE or through management entities.

   When a protocol is used, appropriate security and authentication
   mechanisms to verify that Tenant End System information is not
   spoofed or altered are required. This is one critical aspect for
   providing integrity and tenant isolation in the system.



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   Another control plane protocol can also be used to advertize NVE
   tenant service instance (tenant and service type provided to the
   tenant) to other NVEs. Alternatively, management control entities
   can also be used to perform these functions.

3.1.5.2. Address advertisement and tunnel mapping

   As traffic reaches an ingress NVE, a lookup is performed to
   determine which tunnel the packet needs to be sent to. It is then
   encapsulated with a tunnel header containing the destination address
   of the egress overlay node. Intermediate nodes (between the ingress
   and egress NVEs) switch or route traffic based upon the outer
   destination address.

   One key step in this process consists of mapping a final destination
   address to the proper tunnel. NVEs are responsible for maintaining
   such mappings in their lookup tables. Several ways of populating
   these lookup tables are possible: control plane driven, management
   plane driven, or data plane driven.

   When a control plane protocol is used to distribute address
   advertisement and tunneling information, the auto-
   provisioning/Service discovery could be accomplished by the same
   protocol. In this scenario, the auto-provisioning/Service discovery
   could be combined with (be inferred from) the address advertisement
   and tunnel mapping. Furthermore, a control plane protocol that
   carries both MAC and IP addresses eliminates the need for ARP, and
   hence addresses one of the issues with explosive ARP handling.

3.1.5.3. Tunnel management

   A control plane protocol may be required to exchange tunnel state
   information. This may include setting up tunnels and/or providing
   tunnel state information.

   This applies to both unicast and multicast tunnels.

   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.

3.2. Service Overlay Topologies

   A number of service topologies may be used to optimize the service
   connectivity and to address NVE performance limitations.




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   The topology described in Figure 3 suggests the use of a tunnel mesh
   between the NVEs where each tenant instance is one hop away from a
   service processing perspective. Partial mesh topologies and an NVE
   hierarchy may be used where certain NVEs may act as service transit
   points.

4. Key aspects of overlay networks

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

4.1. Pros & Cons

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

   Overlay networks offer the following key advantages:

     o Unicast tunneling state management is handled at the edge of
        the network. Intermediate transport nodes are unaware of such
        state. Note that this is not the case when multicast is enabled
        in the core network.

     o Tunnels are used to aggregate traffic and hence offer the
        advantage of minimizing the amount of forwarding state required
        within the underlay network

     o Decoupling of the overlay addresses (MAC and IP) used by VMs
        from the underlay network. This offers a clear separation
        between addresses used within the overlay and the underlay
        networks and it enables the use of overlapping addresses spaces
        by Tenant End Systems

     o Support of a large number of virtual network identifiers

   Overlay networks also create several challenges:

     o Overlay networks have no controls of underlay networks and lack
        critical network information

          o Overlays typically probe the network to measure link
             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.



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

     o Fairness of resource sharing and collaboration among end-nodes
        in overlay networks are two critical issues

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

     o Overlaid traffic may not traverse firewalls and NAT devices.

     o Multicast service scalability. Multicast support may be
        required in the overlay network to address for each tenant
        flood containment or efficient multicast handling.

     o Load balancing may not be optimal as the hash algorithm may not
        work well due to the limited number of combinations of tunnel
        source and destination addresses

4.2. Overlay issues to consider

4.2.1. Data plane vs Control plane driven

   Dynamic (data plane) learning implies that flooding of unknown
   destinations be supported and hence implies that broadcast and/or
   multicast be supported. Multicasting in the core network for dynamic
   learning can lead to significant scalability limitations. Specific
   forwarding rules must be enforced to prevent loops from happening.
   This can be achieved using a spanning tree protocol or a shortest
   path tree, or using a split-horizon mesh.

   It should be noted that the amount of state to be distributed is a
   function of the number of virtual machines. Different forms of
   caching can also be utilized to minimize state distribution between
   the various elements.

4.2.2. Coordination between data plane and control plane

   Often a combination of data plane and control based learning is
   necessary. Learning is applied towards end-user facing ports whereas
   distribution is used on the tunnel ports. Coordination between the
   learning engine and the control protocol is needed such that when a
   new address gets learned or an old address is removed, it triggers
   the local control plane to distribute this information to its peers.




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

   There are two techniques to support packet replication needed for
   broadcast, unknown unicast and multicast:

     o Ingress replication

     o Use of core 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 is of consideration.

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

   Depending upon the size of the data center network and hence the
   number of (S,G) entries, but also the duration of multicast flows,
   the use of core 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, and hence programming of (S,G)
   entries for short-lived flows could be impractical.

   A possible trade-off is to use in the core 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.

   In this section, we will only consider the case of an IP overlay.

   It is usually not desirable to rely on IP fragmentation for
   performance reasons. Ideally, the interface MTU as seen by a Tenant
   End System is adjusted such that no fragmentation is needed. TCP
   will adjust its maximum segment size accordingly.

   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:



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

          o Tenant End 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 middle
             boxes (e.g. firewalls) which disable ICMP for security
             reasons

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

   It is also possible to rely on the overlay layer to perform
   segmentation and reassembly operations without relying on the Tenant
   End Systems to know about the end-to-end MTU. The assumption is that
   some hardware assist is available on the NVE node to perform such
   SAR operations. However, fragmentation by the overlay layer can lead
   to performance and congestion issues due to TCP dynamics and might
   require new congestion avoidance mechanisms from then underlay
   network [FLOYD].

   Finally, the underlay network may be designed in such a way that the
   MTU can accommodate the extra tunnel 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 VM 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
   processing boundary happens:

     o Processing and memory requirements

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

          o Control plane processing (e.g. routing, signaling, OAM)

     o FIB/RIB size

     o Multicast support

          o Routing protocols




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          o Packet replication capability

     o Fragmentation support

     o QoS transparency

     o Resiliency

4.2.6. Interaction between network overlays and underlays

   When multiple overlays co-exist on top of a common underlay network,
   this can cause some performance issues. These overlays have
   partially overlapping paths and nodes.

   Each overlay is selfish by nature in that it sends traffic so as to
   optimize its own performance without considering the impact on other
   overlays, unless the underlay tunnels are traffic engineered on a
   per overlay basis so as to avoid sharing underlay resources.

   Better visibility between overlays and underlays can be achieved by
   providing mechanisms to exchange information about:

     o Performance metrics (throughput, delay, loss, jitter)

     o Cost metrics



5. Security Considerations

   The tenant to overlay mapping function can introduce significant
   security risks if appropriate protocols are not used that can
   support mutual authentication.

   No other new security issues are introduced beyond those described
   already in the related L2VPN and L3VPN RFCs.



6. IANA Considerations

   IANA does not need to take any action for this draft.







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7. References

7.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2. Informative References

   [NVOPS]  Narten, T. et al, "Problem Statement : Overlays for Network
             Virtualization", draft-narten-nvo3-overlay-problem-
             statement (work in progress)

   [OVCPREQ] Kreeger, L. et al, "Network Virtualization Overlay Control
             Protocol Requirements", draft-kreeger-nvo3-overlay-cp
             (work in progress)

   [FLOYD]  Sally Floyd, Allyn Romanow, "Dynamics of TCP Traffic over
             ATM Networks", IEEE JSAC, V. 13 N. 4, May 1995

   [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
             Networks (VPNs)", RFC 4364, February 2006.

   [RFC1191] Mogul, J. "Path MTU Discovery", RFC1191, November 1990

   [RFC1981] McCann, J. et al, "Path MTU Discovery for IPv6", RFC1981,
             August 1996

   [RFC4821] Mathis, M. et al, "Packetization Layer Path MTU
             Discovery", RFC4821, March 2007



8. Acknowledgments

   In addition to the authors the following people have contributed to
   this document:

   Dimitrios Stiliadis, Rotem Salomonovitch, Alcatel-Lucent

   This document was prepared using 2-Word-v2.0.template.dot.








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Authors' Addresses

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

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

   Thomas Morin
   France Telecom Orange
   Email: thomas.morin@orange.com

   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA 02145
   Email: nabil.bitar@verizon.com

   Yakov Rekhter
   Juniper
   Email: yakov@juniper.net






















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