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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: January 2013
                                                                Thomas Morin
                                                       France Telecom Orange
     
                                                                 Nabil Bitar
                                                                     Verizon
     
                                                               Yakov Rekhter
                                                                     Juniper
     
                                                                July 9, 2012
     
     
     
     
                       Framework for DC Network Virtualization
                        draft-lasserre-nvo3-framework-03.txt
     
     
     
     
     
     Status of this Memo
     
        This Internet-Draft is submitted in full conformance with the
        provisions of BCP 78 and BCP 79.
     
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        This Internet-Draft will expire on January 9, 2013.
     
     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
     
     
     
     
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        (http://trustee.ietf.org/license-info) in effect on the date of
        publication of this document. Please review these documents
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     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....................................7
        2. Reference Models...............................................8
           2.1. Generic Reference Model...................................8
           2.2. NVE Reference Model......................................10
           2.3. NVE Service Types........................................11
              2.3.1. L2 NVE providing Ethernet LAN-like service..........11
              2.3.2. L3 NVE providing IP/VRF-like service................11
        3. Functional components.........................................11
           3.1. Generic service virtualization components................12
              3.1.1. Virtual Access Points (VAPs)........................12
              3.1.2. Virtual Network Instance (VNI)......................12
              3.1.3. Overlay Modules and VN Context......................13
              3.1.4. Tunnel Overlays and Encapsulation options...........14
              3.1.5. Control Plane Components............................14
              3.1.5.1. Auto-provisioning/Service discovery...............14
              3.1.5.2. Address advertisement and tunnel mapping..........15
     
     
     
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              3.1.5.3. Tunnel management.................................15
           3.2. Service Overlay Topologies...............................16
        4. Key aspects of overlay networks...............................16
           4.1. Pros & Cons..............................................16
           4.2. Overlay issues to consider...............................17
              4.2.1. Data plane vs Control plane driven..................17
              4.2.2. Coordination between data plane and control plane...18
              4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM)
              traffic....................................................18
              4.2.4. Path MTU............................................19
              4.2.5. NVE location trade-offs.............................19
              4.2.6. Interaction between network overlays and underlays..20
        5. Security Considerations.......................................21
        6. IANA Considerations...........................................21
        7. References....................................................21
           7.1. Normative References.....................................21
           7.2. Informative References...................................21
        8. Acknowledgments...............................................22
     
     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.
     
        [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.
     
     
     
     
     
     
     
     
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     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:
     
        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.
     
        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)
        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.
     
        Underlay or Underlying Network: This is the network that provides
        the connectivity between NVEs. The Underlying Network can be
     
     
     
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        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).
     
        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
     
     
     
     
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        EVPN: Ethernet VPN as defined in [EVPN]
     
     1.3. DC network architecture
     
        A generic architecture for Data Centers is depicted in Figure 1:
     
                                     ,---------.
                                   ,'           `.
                                  (  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.
     
     
     
     
     
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        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:
     
          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 type) 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 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:
     
                           +------- L3 Network ------+
                           |                         |
                           |       Tunnel Overlay    |
             +------------+---------+       +---------+------------+
             | +----------+-------+ |       | +---------+--------+ |
             | |  Overlay Module  | |       | |  Overlay Module  | |
             | +---------+--------+ |       | +---------+--------+ |
             |           |VN context|       | VN context|          |
             |           |          |       |           |          |
             |  +--------+-------+  |       |  +--------+-------+  |
             |  | |VNI|   .  |VNI|  |       |  | |VNI|   .  |VNI|  |
        NVE1 |  +-+------------+-+  |       |  +-+-----------+--+  | NVE2
             |    |   VAPs     |    |       |    |    VAPs   |     |
             +----+------------+----+       +----+------------+----+
                  |            |                 |            |
           -------+------------+-----------------+------------+-------
     
     
     
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                  |            |     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
     
     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.
     
     
     
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        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:
     
                          +------- L3 Network ------+
                          |                         |
                          |       Tunnel Overlay    |
             +------------+--------+       +--------+------------+
             | +----------+------+ |       | +------+----------+ |
             | | Overlay Module  | |       | | Overlay Module  | |
             | +--------+--------+ |       | +--------+--------+ |
             |          |VN Context|       |          |VN Context|
             |          |          |       |          |          |
             |  +-------+-------+  |       |  +-------+-------+  |
             |  ||VNI| ... |VNI||  |       |  ||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 VNI 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.
     
     
     
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        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.
     
     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].
     
     
     
     
     
     
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        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.
     
     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.
     
     
     
     
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        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.
     
        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.
     
     
     
     
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     3.2. Service Overlay Topologies
     
        A number of service topologies may be used to optimize the service
        connectivity and to address NVE performance limitations.
     
        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
     
     
     
     
     
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               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.
     
          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 Hash-based 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
     
        In the case of an L2NVE, it is possible to dynamically learn MAC
        addresses against VAPs. It is also possible that such addresses be
        known and controlled via management or a control protocol for both
        L2NVEs and L3NVEs.
     
        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 may lead to significant scalability limitations. Specific
        forwarding rules must be enforced to prevent loops from happening.
        This can be achieved using a spanning tree, a shortest path tree, or
        a split-horizon mesh.
     
        It should be noted that the amount of state to be distributed is
        dependent upon network topology and the number of virtual machines.
     
     
     
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        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
     
        For an L2 NVE, the NVE needs to be able to determine MAC addresses
        of the end systems present on a VAP (for instance, dataplane
        learning may be relied upon for this purpose). For an L3 NVE, the
        NVE needs to be able to determine IP addresses of the end systems
        present on a VAP.
     
        In both cases, coordination with the NVE control protocol is needed
        such that when the NVE determines that the set of addresses behind a
        VAP has changed, it triggers the local NVE control plane to
        distribute this information to its peers.
     
     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.
     
     
     
     
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     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:
     
          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.
     
     
     
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        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
     
               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
     
     
     
     
     
     
     
     
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     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.
     
     
     
     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
     
     
     
     
     
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        [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.
     
     
     
     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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