Internet Engineering Task Force                          Marc Lasserre
    Internet Draft                                            Florin Balus
    Intended status: Informational                          Alcatel-Lucent
    Expires: Dec 2014
                                                              Thomas Morin
                                                     France Telecom Orange
                                                               Nabil Bitar
                                                             Yakov Rekhter
                                                              June 5, 2014
                      Framework for DC Network Virtualization
       This document provides a framework for Data Center (DC) Network
       Virtualization Overlays (NVO3) and it defines a reference model
       along with logical components required to design a solution.
    Status of this Memo
       This Internet-Draft is submitted in full conformance with the
       provisions of BCP 78 and BCP 79.
       Internet-Drafts are working documents of the Internet Engineering
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       Internet-Drafts are draft documents valid for a maximum of six
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       This Internet-Draft will expire on Dec 5, 2014.
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    Copyright Notice
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    Table of Contents
       1. Introduction.................................................3
          1.1. General terminology.....................................3
          1.2. DC network architecture.................................6
       2. Reference Models.............................................8
          2.1. Generic Reference Model.................................8
          2.2. NVE Reference Model....................................10
          2.3. NVE Service Types......................................10
             2.3.1. L2 NVE providing Ethernet LAN-like service........11
             2.3.2. L3 NVE providing IP/VRF-like service..............11
          2.4. Operational Management Considerations..................11
       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....................12
             3.1.4. Tunnel Overlays and Encapsulation options.........13
             3.1.5. Control Plane Components..........................14
    Distributed vs Centralized Control Plane........14
    Auto-provisioning/Service discovery.............14
    Address advertisement and tunnel mapping........15
    Overlay Tunneling...............................15
          3.2. Multi-homing...........................................16
          3.3. VM Mobility............................................17
       4. Key aspects of overlay networks.............................17
          4.1. Pros & Cons............................................17
          4.2. Overlay issues to consider.............................19
             4.2.1. Data plane vs Control plane driven................19
             4.2.2. Coordination between data plane and control plane.19
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             4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM)
             4.2.4. Path MTU..........................................20
             4.2.5. NVE location trade-offs...........................21
             4.2.6. Interaction between network overlays and underlays.22
       5. Security Considerations.....................................22
       6. IANA Considerations.........................................23
       7. References..................................................23
          7.1. Informative References.................................23
       8. Acknowledgments.............................................24
    1. Introduction
       This document provides a framework for Data Center (DC) Network
       Virtualization over Layer3 (L3) tunnels. This framework is intended
       to aid in standardizing protocols and mechanisms to support large-
       scale network virtualization 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.
       This document provides reference models and functional components of
       data center overlay networks as well as a discussion of technical
       issues that have to be addressed.
    1.1. General terminology
       This document uses the following terminology:
       NVO3 Network: An overlay network that provides a Layer2 (L2) or
       Layer3 (L3) service to Tenant Systems over an L3 underlay network
       using the architecture and protocols as defined by the NVO3 Working
       Network Virtualization Edge (NVE). An NVE is the network entity that
       sits at the edge of an underlay network and implements L2 and/or L3
       network virtualization functions. The network-facing side of the NVE
       uses the underlying L3 network to tunnel tenant frames to and from
       other NVEs. The tenant-facing side of the NVE sends and receives
       Ethernet frames to and from individual Tenant Systems.  An NVE could
       be implemented as part of a virtual switch within a hypervisor, a
       physical switch or router, a Network Service Appliance, or be split
       across multiple devices.
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       Virtual Network (VN): A VN is a logical abstraction of a physical
       network that provides L2 or L3 network services to a set of Tenant
       Systems. A VN is also known as a Closed User Group (CUG).
       Virtual Network Instance (VNI): A specific instance of a VN from the
       perspective of an NVE.
       Virtual Network Context (VN Context) Identifier: Field in overlay
       encapsulation header that identifies the specific VN the packet
       belongs to. The egress NVE uses the VN Context identifier to deliver
       the packet to the correct Tenant System. The VN Context identifier
       can be a locally significant identifier or a globally unique
       Underlay or Underlying Network: The network that provides the
       connectivity among NVEs and over which NVO3 packets are tunneled,
       where an NVO3 packet carries an NVO3 overlay header followed by a
       tenant packet. The Underlay Network does not need to be aware that
       it is carrying NVO3 packets. Addresses on the Underlay Network
       appear as "outer addresses" in encapsulated NVO3 packets. In
       general, the Underlay Network can use a completely different
       protocol (and address family) from that of the overlay. In the case
       of NVO3, the underlay network is IP.
       Data Center (DC): A physical complex housing physical servers,
       network switches and routers, network service appliances and
       networked storage. The purpose of a Data Center is to provide
       application, compute and/or storage services. One such service is
       virtualized infrastructure data center services, also known as
       Infrastructure as a Service.
       Virtual Data Center (Virtual DC): A container for virtualized
       compute, storage and network services. A Virtual DC is associated
       with a single tenant, and can contain multiple VNs and Tenant
       Systems connected to one or more of these VNs.
       Virtual machine (VM): A software implementation of a physical
       machine that runs programs as if they were executing on a physical,
       non-virtualized machine.  Applications (generally) do not know they
       are running on a VM as opposed to running on a "bare metal" host or
       server, though some systems provide a para-virtualization
       environment that allows an operating system or application to be
       aware of the presence of virtualization for optimization purposes.
       Hypervisor: Software running on a server that allows multiple VMs to
       run on the same physical server. The hypervisor manages and provides
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       shared compute/memory/storage and network connectivity to the VMs
       that it hosts. Hypervisors often embed a Virtual Switch (see below).
       Server: A physical end host machine that runs user applications. A
       standalone (or "bare metal") server runs a conventional operating
       system hosting a single-tenant application. A virtualized server
       runs a hypervisor supporting one or more VMs.
       Virtual Switch (vSwitch): A function within a Hypervisor (typically
       implemented in software) that provides similar forwarding services
       to a physical Ethernet switch. A vSwitch forwards Ethernet frames
       between VMs running on the same server, or between a VM and a
       physical NIC card connecting the server to a physical Ethernet
       switch or router. A vSwitch also enforces network isolation between
       VMs that by policy are not permitted to communicate with each other
       (e.g., by honoring VLANs). A vSwitch may be bypassed when an NVE is
       enabled on the host server.
       Tenant: The customer using a virtual network and any associated
       resources (e.g., compute, storage and network).  A tenant could be
       an enterprise, or a department/organization within an enterprise.
       Tenant System: A physical or virtual system that can play the role
       of a host, or a forwarding element such as a router, switch,
       firewall, etc. It belongs to a single tenant and connects to one or
       more VNs of that tenant.
       Tenant Separation: Tenant Separation refers to isolating traffic of
       different tenants such that traffic from one tenant is not visible
       to or delivered to another tenant, except when allowed by policy.
       Tenant Separation also refers to address space separation, whereby
       different tenants can use the same address space without conflict.
       Virtual Access Points (VAPs): A logical connection point on the NVE
       for connecting a Tenant System to a virtual network. Tenant Systems
       connect to VNIs at an NVE through VAPs. VAPs can be physical ports
       or virtual ports identified through logical interface identifiers
       (e.g., VLAN ID, internal vSwitch Interface ID connected to a VM).
       End Device: A physical device that connects directly to the DC
       Underlay Network. This is in contrast to a Tenant System, which
       connects to a corresponding tenant VN. An End Device is administered
       by the DC operator rather than a tenant, and is part of the DC
       infrastructure. An End Device may implement NVO3 technology in
       support of NVO3 functions. Examples of an End Device include hosts
       (e.g., server or server blade), storage systems (e.g., file servers,
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       iSCSI storage systems), and network devices (e.g., firewall, load-
       balancer, IPSec gateway).
       Network Virtualization Authority (NVA): Entity that provides
       reachability and forwarding information to NVEs.
    1.2. DC network architecture
       A generic architecture for Data Centers is depicted in Figure 1:
                                  ,'           `.
                                 (  IP/MPLS WAN )
                                  `.           ,'
                                     \      /
                              +--------+   +--------+
                              |   DC   |+-+|   DC   |
                              |gateway |+-+|gateway |
                              +--------+   +--------+
                                    |       /
                                    .--. .--.
                                  (    '    '.--.
                                .-.' Intra-DC     '
                               (     network      )
                                (             .'-'
                                 '--'._.'.    )\ \
                                 / /     '--'  \ \
                                / /      | |    \ \
                       +--------+   +--------+   +--------+
                       | access |   | access |   | access |
                       | switch |   | switch |   | switch |
                       +--------+   +--------+   +--------+
                          /     \    /    \     /      \
                       __/_      \  /      \   /_      _\__
                 '--------'   '--------'   '--------'   '--------'
                 :  End   :   :  End   :   :  End   :   :  End   :
                 : Device :   : Device :   : Device :   : Device :
                 '--------'   '--------'   '--------'   '--------'
                 Figure 1 : A Generic Architecture for Data Centers
       An example of multi-tier DC network architecture is presented in
       Figure 1. It provides a view of physical components inside a DC.
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       A DC network is usually composed of intra-DC networks and network
       services, and inter-DC network and network connectivity services.
       DC networking elements can act as strict L2 switches and/or provide
       IP routing capabilities, including network service virtualization.
       In some DC architectures, some tier layers could provide L2 and/or
       L3 services. In addition, some tier layers may be collapsed, and
       Internet connectivity, inter-DC connectivity and VPN support may be
       handled by a smaller number of nodes. Nevertheless, one can assume
       that the network functional blocks in a DC fit in the architecture
       depicted in Figure 1.
       The following components can be present in a DC:
       - Access switch: Hardware-based Ethernet switch aggregating all
          Ethernet links from the End Devices in a rack representing the
          entry point in the physical DC network for the hosts. It may also
          provide routing functionality, virtual IP network connectivity, or
          Layer2 tunneling over IP for instance. Access switches are usually
          multi-homed to aggregation switches in the Intra-DC network. A
          typical example of an access switch is a Top of Rack (ToR) switch.
          Other deployment scenarios may use an intermediate Blade Switch
          before the ToR, or an EoR (End of Row) switch, to provide similar
          functions to a ToR.
       - Intra-DC Network: Network composed of high capacity core nodes
          (Ethernet switches/routers). Core nodes may provide virtual
          Ethernet bridging and/or IP routing services.
       - DC Gateway (DC GW): Gateway to the outside world providing DC
          Interconnect and connectivity to Internet and VPN customers. In
          the current DC network model, this may be simply a router
          connected to the Internet and/or an IP Virtual Private Network
          (VPN)/L2VPN PE. Some network implementations may dedicate DC GWs
          for different connectivity types (e.g., a DC GW for Internet, and
          another for VPN).
       Note that End Devices may be single or multi-homed to access
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    2. Reference Models
    2.1. Generic Reference Model
       Figure 2 depicts a DC reference model for network virtualization
       overlay where NVEs provide a logical interconnect between Tenant
       Systems that belong to a specific VN.
             +--------+                                    +--------+
             | Tenant +--+                            +----| Tenant |
             | System |  |                           (')   | System |
             +--------+  |    .................     (   )  +--------+
                         |  +---+           +---+    (_)
                         +--|NVE|---+   +---|NVE|-----+
                            +---+   |   |   +---+
                            / .    +-----+      .
                           /  . +--| NVA |--+   .
                          /   . |  +-----+   \  .
                         |    . |             \ .
                         |    . |   Overlay   +--+--++--------+
             +--------+  |    . |   Network   | NVE || Tenant |
             | Tenant +--+    . |             |     || System |
             | System |       .  \ +---+      +--+--++--------+
             +--------+       .....|NVE|.........
                             |               |
                         +--------+      +--------+
                         | Tenant |      | Tenant |
                         | System |      | System |
                         +--------+      +--------+
          Figure 2 : Generic reference model for DC network virtualization
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       In order to obtain reachability information, NVEs may exchange
       information directly between themselves via a control plane
       protocol. In this case, a control plane module resides in every NVE.
       It is also possible for NVEs to communicate with an external Network
       Virtualization Authority (NVA) to obtain reachability and forwarding
       information. In this case, a protocol is used between NVEs and
       NVA(s) to exchange information.
       It should be noted that NVAs may be organized in clusters for
       redundancy and scalability and can appear as one logically
       centralized controller. In this case, inter-NVA communication is
       necessary to synchronize state among nodes within a cluster or share
       information across clusters. The information exchanged between NVAs
       of the same cluster could be different from the information
       exchanged across clusters.
       A Tenant System can be attached to an NVE in several ways:
       - locally, by being co-located in the same End Device
       - remotely, via a point-to-point connection or a switched network
       When an NVE is co-located with a Tenant System, the state of the
       Tenant System can be determined without protocol assistance. For
       instance, the operational status of a VM can be communicated via a
       local API. When an NVE is remotely connected to a Tenant System, the
       state of the Tenant System or NVE needs to be exchanged directly or
       via a management entity, using a control plane protocol or API, or
       directly via a dataplane protocol.
       The functional components in Figure 2 do not necessarily map
       directly to the physical components described in Figure 1. For
       example, an End Device can be a server blade with VMs and a virtual
       switch. A VM can be a Tenant System and the NVE functions may be
       performed by the host server. In this case, the Tenant System and
       NVE function are co-located. Another example is the case where the
       End Device is the Tenant System, and the NVE function can be
       implemented by the connected ToR. In this case, the Tenant System
       and NVE function are not co-located.
       Underlay nodes utilize L3 technologies to interconnect NVE nodes.
       These nodes perform forwarding based on outer L3 header information,
       and generally do not maintain 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
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       tenants, tenant service or a set of services that belong to one or
       more tenants. Mechanisms to control the amount of state maintained
       in the underlay may be needed.
    2.2. NVE Reference Model
       Figure 3 depicts the NVE reference model. One or more VNIs can be
       instantiated on an NVE. A Tenant System interfaces with a
       corresponding VNI via a VAP. An overlay module provides tunneling
       overlay functions (e.g., encapsulation and decapsulation of tenant
       traffic, tenant identification and mapping, etc.).
                         +-------- L3 Network -------+
                         |                           |
                         |        Tunnel Overlay     |
             +------------+---------+       +---------+------------+
             | +----------+-------+ |       | +---------+--------+ |
             | |  Overlay Module  | |       | |  Overlay Module  | |
             | +---------+--------+ |       | +---------+--------+ |
             |           |VN context|       | VN context|          |
             |           |          |       |           |          |
             |  +--------+-------+  |       |  +--------+-------+  |
             |  | |VNI|   .  |VNI|  |       |  | |VNI|   .  |VNI|  |
        NVE1 |  +-+------------+-+  |       |  +-+-----------+--+  | NVE2
             |    |   VAPs     |    |       |    |    VAPs   |     |
             +----+------------+----+       +----+-----------+-----+
                  |            |                 |           |
                  |            |                 |           |
                 Tenant Systems                 Tenant Systems
                      Figure 3 : Generic NVE reference model
       Note that some NVE functions (e.g., data plane and control plane
       functions) may reside in one device or may be implemented separately
       in different devices.
    2.3. NVE Service Types
       An NVE provides different types of virtualized network services to
       multiple tenants, i.e. an L2 service or an L3 service. Note that an
       NVE may be capable of providing both L2 and L3 services for a
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       tenant. This section defines the service types and associated
    2.3.1. L2 NVE providing Ethernet LAN-like service
       An L2 NVE implements Ethernet LAN emulation, an Ethernet based
       multipoint service similar to an IETF VPLS [RFC4761][RFC4762] or
       EVPN [EVPN] service, where the Tenant Systems appear to be
       interconnected by a LAN environment over an L3 overlay. As such, an
       L2 NVE provides per-tenant virtual switching instance (L2 VNI), and
       L3 (IP/MPLS) tunneling encapsulation of tenant MAC frames across the
       underlay. Note that the control plane for an L2 NVE could be
       implemented locally on the NVE or in a separate control entity.
    2.3.2. L3 NVE providing IP/VRF-like service
       An L3 NVE provides Virtualized IP forwarding service, similar to
       IETF IP VPN (e.g., BGP/MPLS IPVPN [RFC4364]) from a service
       definition perspective. That is, an L3 NVE provides per-tenant
       forwarding and routing instance (L3 VNI), and L3 (IP/MPLS) tunneling
       encapsulation of tenant IP packets across the underlay. Note that
       routing could be performed locally on the NVE or in a separate
       control entity.
    2.4. Operational Management Considerations
       NVO3 services are overlay services over an IP underlay.
       As far as the IP underlay is concerned, existing IP OAM facilities
       are used.
       With regards to the NVO3 overlay, both L2 and L3 services can be
       offered. it is expected that existing fault and performance OAM
       facilities will be used. Sections 4.1. and 4.2.6.  below provide
       further discussion of additional fault and performance management
       issues to consider.
       As far as configuration is concerned, the DC environment is driven
       by the need to bring new services up rapidly and is typically very
       dynamic specifically in the context of virtualized services. It is
       therefore critical to automate the configuration of NVO3 services.
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    3. Functional components
       This section decomposes the Network Virtualization architecture into
       functional components described in Figure 3 to make it easier to
       discuss solution options for these components.
    3.1. Service Virtualization Components
    3.1.1. Virtual Access Points (VAPs)
       Tenant Systems are connected to VNIs through Virtual Access Points
       VAPs can be physical ports or virtual ports identified through
       logical interface identifiers (e.g., VLAN ID, internal vSwitch
       Interface ID connected to a VM).
    3.1.2. Virtual Network Instance (VNI)
       A VNI is a specific VN instance on an NVE. Each VNI defines a
       forwarding context that contains reachability information and
    3.1.3. Overlay Modules and VN Context
       Mechanisms for identifying each tenant service are required to allow
       the simultaneous overlay of multiple tenant services over the same
       underlay L3 network topology. In the data plane, each NVE, upon
       sending a tenant packet, must be able to encode the VN Context for
       the destination NVE in addition to the L3 tunneling information
       (e.g., source IP address identifying the source NVE and the
       destination IP address identifying the destination NVE, or MPLS
       label). This allows the destination NVE to identify the tenant
       service instance and therefore appropriately process and forward the
       tenant packet.
       The Overlay module provides tunneling overlay functions: tunnel
       initiation/termination as in the case of stateful tunnels (see
       Section 3.1.4), and/or simply encapsulation/decapsulation of frames
       from VAPs/L3 underlay.
       In a multi-tenant context, tunneling aggregates frames from/to
       different VNIs. Tenant identification and traffic demultiplexing are
       based on the VN Context identifier.
       The following approaches can be considered:
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       - VN Context identifier per Tenant: Globally unique (on a per-DC
          administrative domain) VN identifier used to identify the
          corresponding VNI. Examples of such identifiers in existing
          technologies are IEEE VLAN IDs and ISID tags that identify virtual
          L2 domains when using IEEE 802.1aq and IEEE 802.1ah, respectively.
          Note that multiple VN identifiers can belong to a tenant.
       - One VN Context identifier per VNI: Each VNI value is automatically
          generated by the egress NVE, or a control plane associated with
          that NVE, and usually distributed by a control plane protocol to
          all the related NVEs. An example of this approach is the use of
          per VRF MPLS labels in IP VPN [RFC4364]. The VNI value is
          therefore locally significant to the egress NVE.
       - One VN Context identifier per VAP: A value locally significant to
          an NVE is assigned and usually distributed by a control plane
          protocol to identify a VAP. An example of this approach is the use
          of per CE-PE MPLS labels in IP VPN [RFC4364].
       Note that when using one VN Context per VNI or per VAP, an
       additional global identifier (e.g., a VN identifier or name) may be
       used by the control plane to identify the Tenant context.
    3.1.4. Tunnel Overlays and Encapsulation options
       Once the VN context identifier is added to the frame, an L3 Tunnel
       encapsulation is used to transport the frame to the destination NVE.
       Different IP tunneling options (e.g., GRE, L2TP, IPSec) and MPLS
       tunneling can be used. Tunneling could be stateless or stateful.
       Stateless tunneling simply entails the encapsulation of a tenant
       packet with another header necessary for forwarding the packet
       across the underlay (e.g., IP tunneling over an IP underlay).
       Stateful tunneling on the other hand entails maintaining tunneling
       state at the tunnel endpoints (i.e., NVEs). Tenant packets on an
       ingress NVE can then be transmitted over such tunnels to a
       destination (egress) NVE by encapsulating the packets with a
       corresponding tunneling header. The tunneling state at the endpoints
       may be configured or dynamically established. Solutions should
       specify the tunneling technology used, whether it is stateful or
       stateless. In this document, however, tunneling and tunneling
       encapsulation are used interchangeably to simply mean the
       encapsulation of a tenant packet with a tunneling header necessary
       to carry the packet between an ingress NVE and an egress NVE across
       the underlay. It should be noted that stateful tunneling, especially
       when configuration is involved, does impose management overhead and
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       scale constraints. When confidentiality is required, the use of
       opportunistic encryption can be used as a stateless tunneling
    3.1.5. Control Plane Components 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 models provide a facility to maintain global
       state, and distribute that state to the network. When used in
       combination with distributed protocols, greater network
       efficiencies, improved reliability and robustness can be achieved.
       Domain and/or deployment specific constraints define the balance
       between centralized and distributed approaches. 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 VM orchestration
       systems, since these are the only entities that have visibility of
       which VM belongs to which tenant.
       A mechanism for communicating this information to the NVE is
       required. VAPs have to be created and mapped to the appropriate VNI.
       Depending upon the implementation, this control interface can be
       implemented using an auto-discovery protocol between Tenant Systems
       and their local NVE or through management entities. In either case,
       appropriate security and authentication mechanisms to verify that
       Tenant System information is not spoofed or altered are required.
       This is one critical aspect for providing integrity and tenant
       isolation in the system.
       NVEs may learn reachability information to VNIs on other NVEs via a
       control protocol that exchanges such information among NVEs, or via
       a management control entity.
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     Address advertisement and tunnel mapping
       As traffic reaches an ingress NVE on a VAP, a lookup is performed to
       determine which NVE or local VAP the packet needs to be sent to. If
       the packet is to be sent to another NVE, the packet is encapsulated
       with a tunnel header containing the destination information
       (destination IP address or MPLS label) of the egress NVE.
       Intermediate nodes (between the ingress and egress NVEs) switch or
       route traffic based upon the tunnel destination information.
       A key step in the above process consists of identifying the
       destination NVE the packet is to be tunneled to. NVEs are
       responsible for maintaining a set of forwarding or mapping tables
       that hold the bindings between destination VM and egress NVE
       addresses. Several ways of populating these tables are possible:
       control plane driven, management plane driven, or data plane driven.
       When a control plane protocol is used to distribute address
       reachability and tunneling information, the auto-
       provisioning/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 as
       discussed in [RFC6820]. Overlay Tunneling
       For overlay tunneling, and dependent upon the tunneling technology
       used for encapsulating the Tenant System packets, it may be
       sufficient to have one or more local NVE addresses assigned and used
       in the source and destination fields of a tunneling encapsulation
       header. Other information that is part of the
       tunneling encapsulation header may also need to be configured. In
       certain cases, local NVE configuration may be sufficient while in
       other cases, some tunneling related information may need to
       be shared among NVEs. The information that needs to be shared will
       be technology dependent. For instance, potential information could
       include tunnel identity, encapsulation type, and/or tunnel
       resources. In certain cases, such as when using IP multicast in the
       underlay, tunnels which interconnect NVEs may need to be
       established. When tunneling information needs to be exchanged or
       shared among NVEs, a control plane protocol may be required. For
       instance, it may be necessary to provide active/standby status
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       information between NVEs, up/down status information,
       pruning/grafting information for multicast tunnels, etc.
       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.
       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 techniques or
       on MPLS re-rerouting capabilities.
       When a Tenant System is co-located with the NVE, the Tenant System
       is effectively single homed to the NVE via a virtual port. When the
       Tenant System and the NVE are separated, the Tenant System is
       connected to the NVE via a logical Layer2 (L2) construct such as a
       VLAN and it can be multi-homed to various NVEs. An NVE may provide
       an L2 service to the end system or an 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, loop avoidance techniques must be used. 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 from a DC can be handled by two or more DC
       gateways. Each gateway provides access to external networks such as
       VPNs or the Internet. A gateway may be connected to two or more edge
       nodes in the external network for redundancy. When a connection to
       an upstream node is lost, the alternative connection is used and the
       failed route withdrawn.
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    3.3. VM Mobility
       In DC environments utilizing VM technologies, an important feature
       is that VMs can move from one server to another server in the same
       or different L2 physical domains (within or across DCs) in a
       seamless manner.
       A VM can be moved from one server to another in stopped or suspended
       state ("cold" VM mobility) or in running/active state ("hot" VM
       mobility). With "hot" mobility, VM L2 and L3 addresses need to be
       preserved. With "cold" mobility, it may be desired to preserve at
       least VM L3 addresses.
       Solutions to maintain connectivity while a VM is moved are necessary
       in the case of "hot" mobility. This implies that connectivity among
       VMs is preserved. For instance, for L2 VNs, ARP caches are updated
       Upon VM mobility, NVE policies that define connectivity among VMs
       must be maintained.
       During VM mobility, it is expected that the path to the VM's default
       gateway assures adequate QoS to VM applications, i.e. QoS that
       matches the expected service level agreement for these applications.
    4. Key aspects of overlay networks
       The intent of this section is to highlight specific issues that
       proposed overlay solutions need to address.
    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:
          - Unicast tunneling state management and association of Tenant
            Systems reachability are handled at the edge of the network (at
            the NVE). Intermediate transport nodes are unaware of such
            state. Note that when multicast is enabled in the underlay
            network to build multicast trees for tenant VNs, there would be
            more state related to tenants in the underlay core network.
          - Tunneling is used to aggregate traffic and hide tenant
            addresses from the underlay network, and hence offer the
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            advantage of minimizing the amount of forwarding state required
            within the underlay network
          - Decoupling of the overlay addresses (MAC and IP) used by VMs
            from the underlay network for tenant separation and separation
            of the tenant address spaces from the underlay address space.
          - Support of a large number of virtual network identifiers
       Overlay networks also create several challenges:
          - Overlay networks have typically no control of underlay networks
            and lack underlay network information (e.g. underlay
            - Overlay networks and/or their associated management entities
               typically probe the network to measure link or path
               properties, such as available bandwidth or packet loss rate.
               It is difficult to accurately evaluate network properties. It
               might be preferable for the underlay network to expose usage
               and performance information.
            - Miscommunication or lack of coordination between overlay and
               underlay networks can lead to an inefficient usage of network
            - When multiple overlays co-exist on top of a common underlay
               network, the lack of coordination between overlays can lead
               to performance issues and/or resource usage inefficiencies.
          - Traffic carried over an overlay might fail to traverse
            firewalls and NAT devices.
          - Multicast service scalability: Multicast support may be
            required in the underlay network to address tenant flood
            containment or efficient multicast handling. The underlay may
            also be required to maintain multicast state on a per-tenant
            basis, or even on a per-individual multicast flow of a given
            tenant. Ingress replication at the NVE eliminates that
            additional multicast state in the underlay core, but depending
            on the multicast traffic volume, it may cause inefficient use
            of bandwidth.
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    4.2. Overlay issues to consider
    4.2.1. Data plane vs Control plane driven
       In the case of an L2 NVE, it is possible to dynamically learn MAC
       addresses against VAPs. It is also possible that such addresses be
       known and controlled via management or a control protocol for both
       L2 NVEs and L3 NVEs. Dynamic data plane learning implies that
       flooding of unknown destinations be supported and hence implies that
       broadcast and/or multicast be supported or that ingress replication
       be used as described in section 4.2.3. Multicasting in the underlay
       network for dynamic learning may lead to significant scalability
       limitations. Specific forwarding rules must be enforced to prevent
       loops from happening. This can be achieved using a spanning tree, a
       shortest path tree, or a split-horizon mesh.
       It should be noted that the amount of state to be distributed is
       dependent upon network topology and the number of virtual machines.
       Different forms of caching can also be utilized to minimize state
       distribution between the various elements. The control plane should
       not require an NVE to maintain the locations of all the Tenant
       Systems whose VNs are not present on the NVE. The use of a control
       plane does not imply that the data plane on NVEs has to maintain all
       the forwarding state in the control plane.
    4.2.2. Coordination between data plane and control plane
       For an L2 NVE, the NVE needs to be able to determine MAC addresses
       of the Tenant Systems connected via a VAP. This can be achieved via
       dataplane learning or a control plane. For an L3 NVE, the NVE needs
       to be able to determine IP addresses of the Tenant 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 NVE control plane to distribute
       this information to its peers.
    4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM) traffic
       There are several options to support packet replication needed for
       broadcast, unknown unicast and multicast. Typical methods include:
       - Ingress replication
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       - Use of underlay multicast trees
       There is a bandwidth vs state trade-off between the two approaches.
       Depending upon the degree of replication required (i.e. the number
       of hosts per group) and the amount of multicast state to maintain,
       trading bandwidth for state should be considered.
       When the number of hosts per group is large, the use of underlay
       multicast trees may be more appropriate. When the number of hosts is
       small (e.g. 2-3) and/or the amount of multicast traffic is small,
       ingress replication may not be an issue.
       Depending upon the size of the data center network and hence the
       number of (S,G) entries, and also the duration of multicast flows,
       the use of underlay multicast trees can be a challenge.
       When flows are well known, it is possible to pre-provision such
       multicast trees. However, it is often difficult to predict
       application flows ahead of time, 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.
       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.
       It is possible for the MTU to be configured manually or to be
       discovered dynamically. Various Path MTU discovery techniques exist
       in order to determine the proper MTU size to use:
       - Classical ICMP-based 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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       - Extended MTU Path Discovery techniques such as defined in
         - Tenant Systems send probe packets of different sizes, and rely
            on confirmation of receipt or lack thereof from receivers to
            allow a sender to discover the MTU of the end-to-end paths.
       While it could also be possible to rely on the NVE to perform
       segmentation and reassembly operations without relying on the Tenant
       Systems to know about the end-to-end MTU, this would lead to
       undesired performance and congestion issues as well as significantly
       increase the complexity of hardware NVEs required for buffering and
       reassembly logic.
       Preferably, the underlay network should be designed in such a way
       that the MTU can accommodate the extra tunneling and possibly
       additional NVO3 header encapsulation overhead.
    4.2.5. NVE location trade-offs
       In the case of DC traffic, traffic originated from a VM is native
       Ethernet traffic. This traffic can be switched by a local virtual
       switch or ToR switch and then by a DC gateway. The NVE function can
       be embedded within any of these elements.
       There are several criteria to consider when deciding where the NVE
       function should happen:
       - Processing and memory requirements
         - Datapath (e.g. lookups, filtering, encapsulation/decapsulation)
         - Control plane processing (e.g. routing, signaling, OAM) and
            where specific control plane functions should be enabled
       - FIB/RIB size
       - Multicast support
         - Routing/signaling protocols
         - Packet replication capability
         - Multicast FIB
       - Fragmentation support
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       - QoS support (e.g. marking, policing, queuing)
       - Resiliency
    4.2.6. Interaction between network overlays and underlays
       When multiple overlays co-exist on top of a common underlay network,
       resources (e.g., bandwidth) should be provisioned to ensure that
       traffic from overlays can be accommodated and QoS objectives can be
       met. Overlays can have partially overlapping paths (nodes and
       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, may
       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:
       - Performance metrics (throughput, delay, loss, jitter)
       - Cost metrics
       such as defined in [RFC2330].
    5. Security Considerations
       Since NVEs and NVAs play a central role in NVO3, it is critical that
       a secure access to NVEs and NVAs be ensured such that no
       unauthorized access is possible.
       As discussed in section , Tenant Systems identification is
       based upon state that is often provided by management systems (e.g.
       a VM orchestration system in a virtualized environment). Secure
       access to such management systems must also be ensured.
       When an NVE receives data from a Tenant System, the tenant identity
       needs to be verified in order to guarantee that it is authorized to
       access the corresponding VN. This can be achieved by identifying
       incoming packets against specific VAPs in some cases. In other
       circumstances, authentication may be necessary.
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       Data integrity can be assured if authorized access to NVEs, NVAs,
       and intermediate underlay nodes is ensured. Otherwise, encryption
       must be used.
       NVO3 provides data confidentiality through data separation. The use
       of both VNIs and tunneling of tenant traffic by NVEs ensures that
       NVO3 data is kept in a separate context and thus separated from
       other tenant traffic. When NVO3 data traverses untrusted networks,
       data encryption may be needed.
       Not only tenant data but also NVO3 control data must be secured
       (e.g. control traffic between NVAs and NVEs, between NVAs and
       between NVEs).
       It may also be desirable to restrict the types of information that
       can be exchanged between overlays and underlays (e.g. topology
    6. IANA Considerations
       IANA does not need to take any action for this draft.
    7. References
    7.1. Informative References
       [NVOPS]  Narten, T. et al, "Problem Statement : Overlays for
                 Network Virtualization", draft-ietf-nvo3-overlay-problem-
                 statement (work in progress)
       [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
                 Networks (VPNs)", RFC 4364, February 2006.
       [RFC4761] Kompella, K. et al, "Virtual Private LAN Service (VPLS)
                 Using BGP for auto-discovery and Signaling", RFC4761,
                 January 2007
       [RFC4762] Lasserre, M. et al, "Virtual Private LAN Service (VPLS)
                 Using Label Distribution Protocol (LDP) Signaling",
                 RFC4762, January 2007
       [EVPN]  Sajassi, A. et al, "BGP MPLS Based Ethernet VPN", draft-
                 ietf-l2vpn-evpn (work in progress)
       [RFC1191] Mogul, J. "Path MTU Discovery", RFC1191, November 1990
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       [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
       [RFC6820] Narten, T. et al, "Address Resolution Problems in Large
                 Data Center Networks", RFC6820, January 2013
       [RFC2330] Paxson, V. et al, "Framework for IP Performance Metrics",
                 RFC2330, May 1998
    8. Acknowledgments
       In addition to the authors the following people have contributed to
       this document:
       Dimitrios Stiliadis, Rotem Salomonovitch, Lucy Yong, Thomas Narten,
       Larry Kreeger, David Black.
       This document was prepared using
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    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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