Internet Engineering Task Force                          Marc Lasserre
    Internet Draft                                            Florin Balus
    Intended status: Informational                          Alcatel-Lucent
    Expires: August 2013
                                                              Thomas Morin
                                                     France Telecom Orange
                                                               Nabil Bitar
                                                             Yakov Rekhter
                                                          February 4, 2013
                      Framework for DC Network Virtualization
    Status of this Memo
       This Internet-Draft is submitted in full conformance with the
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       This Internet-Draft will expire on August 4, 2013.
    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
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    Internet-Draft  Framework for DC Network Virtualization  February 2013
       ( 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.
       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
    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........12
             2.3.2. L3 NVE providing IP/VRF-like service..............12
       3. Functional components.......................................12
          3.1. Service Virtualization Components......................12
             3.1.1. Virtual Access Points (VAPs)......................12
             3.1.2. Virtual Network Instance (VNI)....................12
             3.1.3. Overlay Modules and VN Context....................13
             3.1.4. Tunnel Overlays and Encapsulation options.........14
             3.1.5. Control Plane Components..........................14
    Distributed vs Centralized Control Plane........14
    Auto-provisioning/Service discovery.............15
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    Address advertisement and tunnel mapping........15
    Overlay Tunneling...............................16
          3.2. Multi-homing...........................................16
          3.3. VM Mobility............................................17
          3.4. Service Overlay Topologies.............................18
       4. Key aspects of overlay networks.............................18
          4.1. Pros & Cons............................................18
          4.2. Overlay issues to consider.............................20
             4.2.1. Data plane vs Control plane driven................20
             4.2.2. Coordination between data plane and control plane.20
             4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM)
             4.2.4. Path MTU..........................................21
             4.2.5. NVE location trade-offs...........................22
             4.2.6. Interaction between network overlays and underlays.23
       5. Security Considerations.....................................23
       6. IANA Considerations.........................................24
       7. References..................................................24
          7.1. Normative References...................................24
          7.2. Informative References.................................24
       8. Acknowledgments.............................................24
    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 multi-tenant data center networks. Compute,
       storage and network virtualization are often used in these large
       data centers to support a large number of communication domains and
       end systems. [OVCPREQ] describes the requirements for a control
       plane protocol required by overlay border nodes to exchange overlay
       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, or a network
       service appliance.
       VN: Virtual Network. This is a virtual L2 or L3 domain that belongs
       to a tenant.
       VNI: Virtual Network Instance. This is one instance of a virtual
       overlay network. It refers to the state maintained for a given VN on
       a given NVE. 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
       VNID:  Virtual Network Identifier. In the case where the VN context
       identifier 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.
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       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 sppliances and
       networked storage. The purpose of a Data Center is to provide
       application, compute and/or storage services. One such service is
       virtualized infrastructure data center services, also known as
       Infrastructure as a Service.
       Virtual Data Center 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 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 or router. It also enforces network isolation between VMs
       that should not communicate with each other.
       Tenant: In a DC, a tenant refers to a customer that could be an
       organization within an enterprise, or an enterprise with a set of DC
       compute, storage and network resources associated with it.
       Tenant System: A physical or virtual system that can play the role
       of a host, or a forwarding element such as a router, switch,
       firewall, etc. It belongs to a single tenant and connects to one or
       more VNs of that tenant.
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       End device: A physical system to which networking service is
       provided. Examples include hosts (e.g. server or server blade),
       storage systems (e.g., file servers, iSCSI storage systems), and
       network devices (e.g., firewall, load-balancer, IPSec gateway). An
       end device may include internal networking functionality that
       interconnects the device's components (e.g. virtual switches that
       interconnect VMs running on the same server). NVE functionality may
       be implemented as part of that internal networking.
       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:
                                  ,'           `.
                                 (  IP/MPLS WAN )
                                  `.           ,'
                                 +--+--+   +-+---+
                                 |DC GW|+-+|DC GW|
                                 +-+---+   +-----+
                                    |       /
                                    .--. .--.
                                  (    '    '.--.
                                .-.' Intra-DC     '
                               (     network      )
                                (             .'-'
                                 '--'._.'.    )\ \
                                 / /     '--'  \ \
                                / /      | |    \ \
                          +---+--+   +-`.+--+  +--+----+
                          | ToR  |   | ToR  |  |  ToR  |
                          +-+--`.+   +-+-`.-+  +-+--+--+
                           /     \    /    \   /       \
                        __/_      \  /      \ /_       _\__
                 '--------'   '--------'   '--------'   '--------'
                 :  End   :   :  End   :   :  End   :   :  End   :
                 : Device :   : Device :   : Device :   : Device :
                 '--------'   '--------'   '--------'   '--------'
                 Figure 1 : A Generic Architecture for Data Centers
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       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
       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 in the architecture above.
       The following components can be present in a DC:
          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).
       Note that End Devices may be single or multi-homed to ToRs.
    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) are assigned for DC tenant1.
       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 L3 (IP/MPLS) overlays where NVEs provide a
       logical interconnect between Tenant Systems that belong to a
       specific tenant network.
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             +--------+                                    +--------+
             | Tenant +--+                            +----| Tenant |
             | System |  |                           (')   | System |
             +--------+  |    ...................   (   )  +--------+
                         |  +-+--+           +--+-+  (_)
                         |  | NV |           | NV |   |
                         +--|Edge|           |Edge|---+
                            +-+--+           +--+-+
                            / .                 .
                           /  .   L3 Overlay +--+-++--------+
             +--------+   /   .    Network   | NV || Tenant |
             | Tenant +--+    .              |Edge|| System |
             | System |       .    +----+    +--+-++--------+
             +--------+       .....| NV |........
                             |               |
                         +--------+      +--------+
                         | Tenant |      | Tenant |
                         | System |      | System |
                         +--------+      +--------+
          Figure 3 : Generic reference model for DC network virtualization
                           over a Layer3 infrastructure
       A Tenant System can be attached to a Network Virtualization Edge
       (NVE) node in several ways:
         - locally, by being co-located in the same device
         - remotely, via a point-to-point connection or a switched network
         (e.g., Ethernet)
       When an NVE is local, the state of Tenant Systems can be provided
       without protocol assistance. For instance, the operational status of
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       a VM can be communicated via a local API. When an NVE is remote, the
       state of Tenant Systems needs to be exchanged via a data or control
       plane protocol, or via a management entity.
       The functional components in Figure 3 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 System and the NVE
       functions may be performed by the virtual switch and/or the
       hypervisor. In this case, the Tenant System and NVE function are co-
       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 System and the NVE function may be performed by the ToR.
       Other End Devices in this category are physical network appliances
       or storage systems.
       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 underlay nodes.
       Underlay 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 tenants. When such tenant or tenant-
       service related information is maintained in the underlay, overlay
       virtualization provides knobs to control the magnitude of that
    2.2. NVE Reference Model
       One or more VNIs can be instantiated on an NVE. Tenant Systems
       interface with a corresponding VNI via a Virtual Access Point (VAP).
       An overlay module that provides tunneling overlay functions (e.g.,
       encapsulation and 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|  |       |  | |VNI|   .  |VNI|  |
        NVE1 |  +-+------------+-+  |       |  +-+-----------+--+  | NVE2
             |    |   VAPs     |    |       |    |    VAPs   |     |
             +----+------------+----+       +----+-----------+-----+
                  |            |                 |           |
                  |            |     Tenant      |           |
                  |            |   Service IF    |           |
                 Tenant Systems                 Tenant 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, or be distributed between the End
       Devices and the ToRs. In the latter case we say that the End Device
       NVE component acts as the NVE Spoke, and ToRs act as NVE hubs.
       Tenant Systems will interface with VNIs maintained on the NVE
       spokes, and VNIs maintained on the NVE spokes will interface with
       VNIs maintained on the NVE hubs.
    2.3. NVE Service Types
       NVE components may be used to provide different types of virtualized
       network services. This section defines the service types and
       associated attributes. Note that an NVE may be capable of providing
       both L2 and L3 services.
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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 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 (IP/MPLS) tunnel encapsulation across the underlay.
    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
       [RFC4364] and IPsec VPNs). It provides per tenant routing instance
       with addressing isolation and L3 (IP/MPLS) tunnel encapsulation
       across the underlay.
    3. Functional components
       This section decomposes the Network Virtualization architecture into
       functional components described in Figure 4 to make it easier to
       discuss solution options for these components.
    3.1. Service Virtualization Components
    3.1.1. Virtual Access Points (VAPs)
       Tenant Systems are connected to the VNI Instance through Virtual
       Access Points (VAPs).
       The VAPs can be physical ports or virtual ports identified through
       logical interface identifiers (e.g., VLAN ID, internal vSwitch
       Interface ID coonected 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 information (e.g.,
       source IP address identifying the source NVE and the destination IP
       address identifying the destination NVE, or MPLS label). This allows
       the destination NVE to identify the tenant service instance and
       therefore appropriately process and forward the tenant packet.
       The Overlay module provides tunneling overlay functions: tunnel
       initiation/termination, 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 identifier (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
       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 identifier 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 (e.g., GRE, L2TP, IPSec) and MPLS
       tunneling options (e.g., BGP VPN, VPLS) can be used.
    3.1.5. Control Plane Components
       Control plane components may be used to provide the following
          . Auto-provisioning/Service discovery
          . Address advertisement and tunnel mapping
          . Tunnel management
       A control plane component can be an on-net control protocol
       implemented on the NVE or a management control entity. Distributed vs Centralized Control Plane
       A control/management plane entity can be centralized or distributed.
       Both approaches have been used extensively in the past. The routing
       model of the Internet is a good example of a distributed approach.
       Transport networks have usually used a centralized approach to
       manage transport paths.
       It is also possible to combine the two approaches i.e. using a
       hybrid model. A global view of network state can have many benefits
       but it does not preclude the use of distributed protocols within the
       network. Centralized controllers provide a facility to maintain
       global state, and distribute that state to the network which in
       combination with distributed protocols can aid in achieving greater
       network efficiencies, and improve reliability and robustness. Domain
       and/or deployment specific constraints define the balance between
       centralized and distributed approaches.
       On one hand, a control plane module can reside in every NVE. This is
       how routing control plane modules are implemented in routers. At the
       same time, an external controller can manage a group of NVEs via an
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       agent in each NVE. This is how an SDN controller could communicate
       with the nodes it controls, via OpenFlow [OF] for instance.
       In the case where a logically centralized control plane is
       preferred, the controller will need to be distributed to more than
       one node for redundancy and scalability in order to manage a large
       number of NVEs. Hence, inter-controller communication is necessary
       to synchronize state among controllers. It should be noted that
       controllers may be organized in clusters. The information exchanged
       between controllers of the same cluster could be different from the
       information exchanged across clusters. Auto-provisioning/Service discovery
       NVEs must be able to identify the appropriate VNI for each Tenant
       System. This is based on state information that is often provided by
       external entities. For example, in an environment where a VM is a
       Tenant System, this information is provided by compute management
       systems, since these are the only entities that have visibility of
       which VM belongs to which tenant.
       A mechanism for communicating this information between Tenant
       Systems and the corresponding NVE is required. As a result the VAPs
       are created and mapped to the appropriate VNI. Depending upon the
       implementation, this control interface can be implemented using an
       auto-discovery protocol between Tenant Systems and their local NVE
       or through management entities. In either case, appropriate security
       and authentication mechanisms to verify that Tenant System
       information is not spoofed or altered are required. This is one
       critical aspect for providing integrity and tenant isolation in the
       A control plane protocol can also be used to advertize supported VNs
       to other NVEs. Alternatively, management control entities can also
       be used to perform these functions. 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
       information (destination IP address or MPLS label) of the egress
       overlay node. Intermediate nodes (between the ingress and egress
       NVEs) switch or route traffic based upon the outer destination
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       One key step in this process consists of mapping a final destination
       information to the proper tunnel. NVEs are responsible for
       maintaining such mappings in their forwarding tables. Several ways
       of populating these tables are possible: control plane driven,
       management plane driven, or data plane driven.
       When a control plane protocol is used to distribute address
       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 associated 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. Overlay Tunneling
       For overlay tunneling, and dependent upon the tunneling technology
       used for encapsulating the tenant system packets, it may be
       sufficient to have one or more local NVE addresses assigned and used
       in the source and destination fields of a tunneling encapsulating
       header. Other information that is part of the
       tunneling encapsulation header may also need to be configured. In
       certain cases, local NVE configuration may be sufficient while in
       other cases, some tunneling related information may need to
       be shared among NVEs. The information that needs to be shared will
       be technology dependent. This includes the discovery and
       announcement of the tunneling technology used. In certain cases,
       such as when using IP multicast in the underlay, tunnels may need to
       be established, interconnecting NVEs. When tunneling information
       needs to be exchanged or shared among NVEs, a control plane protocol
       may be required. For instance, it may be necessary to provide
       active/standby status information between NVEs, up/down status
       information, pruning/grafting information for multicast tunnels,
       In addition, a control plane may be required to setup the tunnel
       path for some tunneling technologies. This applies to both unicast
       and multicast tunneling.
    3.2. Multi-homing
       Multi-homing techniques can be used to increase the reliability of
       an nvo3 network. It is also important to ensure that physical
       diversity in an nvo3 network is taken into account to avoid single
       points of failure.
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       Multi-homing can be enabled in various nodes, from tenant systems
       into TORs, TORs into core switches/routers, and core nodes into DC
       The nvo3 underlay nodes (i.e. from NVEs to DC GWs) rely on IP
       routing as the means to re-route traffic upon failures and/or ECMP
       techniques or on MPLS re-rerouting capabilities.
       When a tenant system is co-located with the NVE on the same end-
       system, the tenant system is single homed to the NVE via a vport
       that is virtual NIC (vNIC). When the end system and the NVEs are
       separated, the end system is connected to the NVE via a logical
       Layer2 (L2) construct such as a VLAN. In this latter case, an end
       device or vSwitch on that device could be multi-homed to various
       NVEs. An NVE may provide an L2 service to the end system or a l3
       service. An NVE may be multi-homed to a next layer in the DC at
       Layer2 (L2) or Layer3 (L3). When an NVE provides an L2 service and
       is not co-located with the end system, techniques such as Ethernet
       Link Aggregation Group (LAG) or Spanning Tree Protocol (STP) can be
       used to switch traffic between an end system and connected
       NVEs without creating loops. Similarly, when the NVE provides L3
       service, similar dual-homing techniques can be used. When the NVE
       provides a L3 service to the end system, it is possible that no
       dynamic routing protocol is enabled between the end system and the
       NVE. The end system can be multi-homed to multiple physically-
       separated L3 NVEs over multiple interfaces. When one of the
       links connected to an NVE fails, the other interfaces can be used to
       reach the end system.
       External connectivity out of an nvo3 domain can be handled by two or
       more nvo3 gateways. Each gateway is connected to a different domain
       (e.g. ISP), providing access to external networks such as VPNs or
       the Internet. A gateway may be connected to two nodes. When a
       connection to an upstream node is lost, the alternative connection
       is used and the failed route withdrawn.
    3.3. VM Mobility
       In DC environments utilizing VM technologies, an important feature
       is that VMs can move from one server to another server in the same
       or different L2 physical domains (within or across DCs) in a
       seamless manner.
       A VM can be moved from one server to another in stopped or suspended
       state ("cold" VM mobility) or in running/active state ("hot" VM
       mobility). With "hot" mobility, VM L2 and L3 addresses need to be
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       preserved. With "cold" mobility, it may be desired to preserve VM L3
       Solutions to maintain connectivity while a VM is moved are necessary
       in the case of "hot" mobility. This implies that transport
       connections among VMs are preserved and that ARP caches are updated
       Upon VM mobility, NVE policies that define connectivity among VMs
       must be maintained.
       Optimal routing during VM mobility is also an important aspect to
       address. It is expected that the VM's default gateway be as close as
       possible to the server hosting the VM and triangular routing be
    3.4. 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
    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 and association with tenant
            systems reachability are 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
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          o Tunneling is used to aggregate traffic and hide tenant
            addresses from the underkay network, 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 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 or
                 path properties, such as available bandwidth or packet
                 loss rate. It is difficult to accurately evaluate network
                 properties. It might be preferable for the underlay
                 network to expose usage and performance information.
          o Miscommunication or lack of coordination between overlay and
            underlay networks can lead to an inefficient usage of network
          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 underlay network to address for each tenant
            flood containment or efficient multicast handling. The underlay
            may be also be required to maintain multicast state on a per-
            tenant basis, or even on a per-individual multicast flow of a
            given tenant.
          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. Other
            NVO3 mechanisms may use additional entropy information than
            source and destination addresses.
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    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 or that ingress replication be used as
       described in section 4.2.3. Multicasting in the underlay network for
       dynamic learning may lead to significant scalability limitations.
       Specific forwarding rules must be enforced to prevent loops from
       happening. This can be achieved using a spanning tree, a shortest
       path tree, or a split-horizon mesh.
       It should be noted that the amount of state to be distributed is
       dependent upon network topology and the number of virtual machines.
       Different forms of caching can also be utilized to minimize state
       distribution between the various elements. The control plane should
       not require an NVE to maintain the locations of all the tenant
       systems whose VNs are not present on the NVE. The use of a control
       plane does not imply that the data plane on NVEs has to maintain all
       the forwarding state in the control plane.
    4.2.2. Coordination between data plane and control plane
       For an L2 NVE, the NVE needs to be able to determine MAC addresses
       of the end systems connected via a VAP. This can be achieved via
       dataplane learning or a control plane. For an L3 NVE, the NVE needs
       to be able to determine IP addresses of the end systems connected
       via 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
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          o Use of underlay multicast trees
       There is a bandwidth vs state trade-off between the two approaches.
       Depending upon the degree of replication required (i.e. the number
       of hosts per group) and the amount of multicast state to maintain,
       trading bandwidth for state should be considered.
       When the number of hosts per group is large, the use of underlay
       multicast trees may be more appropriate. When the number of hosts is
       small (e.g. 2-3), 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 underlay multicast trees can be a challenge.
       When flows are well known, it is possible to pre-provision such
       multicast trees. However, it is often difficult to predict
       application flows ahead of time, and hence programming of (S,G)
       entries for short-lived flows could be impractical.
       A possible trade-off is to use in the underlay shared multicast
       trees as opposed to dedicated multicast trees.
    4.2.4. Path MTU
       When using overlay tunneling, an outer header is added to the
       original frame. This can cause the MTU of the path to the egress
       tunnel endpoint to be exceeded.
       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
       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]
                Tenant Systems rely on ICMP messages to discover the MTU of
                 the end-to-end path to its destination. This method is not
                 always possible, such as when traversing middle boxes
                 (e.g. firewalls) which disable ICMP for security reasons
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          o Extended MTU Path Discovery techniques such as defined in
       It is also possible to rely on the overlay layer to perform
       segmentation and reassembly operations without relying on the Tenant
       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 tunneling and possibly additional nvo3
       header encapsulation overhead.
    4.2.5. NVE location trade-offs
       In the case of DC traffic, traffic originated from a VM is native
       Ethernet traffic. This traffic can be switched by a local virtual
       switch or ToR switch and then by a DC gateway. The NVE function can
       be embedded within any of these elements.
       There are several criteria to consider when deciding where the NVE
       function should happen:
          o Processing and memory requirements
              o Datapath (e.g. lookups, filtering,
              o Control plane processing (e.g. routing, signaling, OAM) and
                 where specific control plane functions should be enabled
          o FIB/RIB size
          o Multicast support
              o Routing/signaling protocols
              o Packet replication capability
              o Multicast FIB
          o Fragmentation support
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          o QoS support (e.g. marking, policing, queuing)
          o Resiliency
    4.2.6. Interaction between network overlays and underlays
       When multiple overlays co-exist on top of a common underlay network,
       resources (e.g., bandwidth) should be provisioned to ensure that
       traffic from overlays can be accommodated and QoS objectives can be
       met. Overlays can have partially overlapping paths (nodes and
       Each overlay is selfish by nature. It sends traffic so as to
       optimize its own performance without considering the impact on other
       overlays, unless the underlay paths are traffic engineered on a per
       overlay basis to avoid congestion of underlay resources.
       Better visibility between overlays and underlays, or generally
       coordination in placing overlay demand on an underlay network, can
       be achieved by providing mechanisms to exchange performance and
       liveliness information between the underlay and overlay(s) or the
       use of such information by a coordination system. Such information
       may include:
          o Performance metrics (throughput, delay, loss, jitter)
          o Cost metrics
    5. Security Considerations
       Nvo3 solutions must at least consider and address the following:
          . Secure and authenticated communication between an NVE and an
            NVE management system.
          . Isolation between tenant overlay networks. The use of per-
            tenant FIB tables (VNIs) on an NVE is essential.
          . Security of any protocol used to carry overlay network
          . Avoiding packets from reaching the wrong NVI, especially during
            VM moves.
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    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
       [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
       Lucy Yong, Huawei
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       This document was prepared using
    Authors' Addresses
       Marc Lasserre
       Florin Balus
       777 E. Middlefield Road
       Mountain View, CA, USA 94043
       Thomas Morin
       France Telecom Orange
       Nabil Bitar
       40 Sylvan Road
       Waltham, MA 02145
       Yakov Rekhter
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