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

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

                                                            Nabil Bitar
                                                                Verizon

                                                           Yakov Rekhter
                                                                 Juniper

                                                         Yuichi Ikejiri
                                                     NTT Communications

                                                         March 12, 2012




                  Framework for DC Network Virtualization
                   draft-lasserre-nvo3-framework-01.txt


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

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   http://www.ietf.org/shadow.html

   This Internet-Draft will expire on September 12, 2012.






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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document.



Abstract

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



Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................4
      1.2. General terminology.......................................4
      1.3. DC network architecture...................................5
      1.4. Tenant networking view....................................6
   2. Reference Models...............................................7
      2.1. Generic Reference Model...................................7
      2.2. NVE Reference Model.......................................9
      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
   3. Functional components.........................................11
      3.1. Generic service virtualization components................11
         3.1.1. Virtual Attachment Points (VAPs)....................12
         3.1.2. Tenant Instance.....................................12
         3.1.3. Overlay Modules and Tenant ID.......................13
         3.1.4. Tunnel Overlays and Encapsulation options...........14
         3.1.5. Control Plane Components............................14



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         3.1.5.1. Auto-provisioning/Service discovery...............14
         3.1.5.2. Address advertisement and tunnel mapping..........15
         3.1.5.3. Tunnel management.................................15
      3.2. Service Overlay Topologies...............................16
   4. Key aspects of overlay networks...............................16
      4.1. Pros & Cons..............................................16
      4.2. Overlay issues to consider...............................17
         4.2.1. Data plane vs Control plane driven..................17
         4.2.2. Coordination between data plane and control plane...18
         4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM)
         traffic....................................................18
         4.2.4. Path MTU............................................19
         4.2.5. NVE location trade-offs.............................19
         4.2.6. Interaction between network overlays and underlays..20
   5. Security Considerations.......................................21
   6. IANA Considerations...........................................21
   7. References....................................................21
      7.1. Normative References.....................................21
      7.2. Informative References...................................21
   8. Acknowledgments...............................................22

1. Introduction

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

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

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

     o Limited VLAN space

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

     o Spanning Tree limitations

     o Excessive ARP handling



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     o Broadcast storms

     o Inefficient Broadcast/Multicast handling

     o Limited mobility/portability support

     o Lack of service auto-discovery

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

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

   This document provides reference models that describe functional
   components of data center overlay networks. It also describes
   technical issues that have to be addressed in the design of
   protocols and mechanisms for large-scale data center networks.

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

   Some general terminology is defined here. Terminology specific to
   this memo is introduced as needed in later sections.

   DC: Data Center

   ELAN: MEF ELAN, multipoint to multipoint Ethernet service












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1.3. DC network architecture

                                ,---------.
                              ,'           `.
                             (  IP/MPLS WAN )
                              `.           ,'
                                `-+------+'
                             +--+--+   +-+---+
                             |DC GW|+-+|DC GW|
                             +-+---+   +-----+
                                 |       /
                                 .--. .--.
                               (    '    '.--.
                            .-.' Intra-DC     '
                           (     network      )
                            (             .'-'
                             '--'._.'.    )\ \
                              / /     '--'  \ \
                             / /      | |    \ \
                      +---+--+   +-`.+--+  +--+----+
                      | ToR  |   | ToR  |  |  ToR  |
                      +-+--`.+   +-+-`.-+  +-+--+--+
                      .'     \   .'    \   .'     `.
                   __/_      _i./       i./_       _\__
            '--------'    '--------'   '--------'   '--------'
            :  End   :    :  End   :   :  End   :   :  End   :
            : Device :    : Device :   : Device :   : Device :
            '--------'    '--------'   '--------'   '--------'

            Figure 1 : A Generic Architecture for Data Centers

   Figure 1 depicts a common and generic multi-tier DC network
   architecture. 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 are
   collapsed and/or provide L2 and/or L3 services, and that Internet
   connectivity, inter-DC connectivity and VPN support are handled by a
   smaller number of nodes. Nevertheless, one can assume that the
   functional blocks fit with the architecture depicted in Figure 1.



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   The following components can be present in a DC:

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

     o Top of Rack (ToR): Hardware-based Ethernet switch aggregating
        all Ethernet links from the End Devices in a rack representing
        the entry point in the physical DC network for the hosts. ToRs
        may also provide routing functionality, virtual IP network
        connectivity, or Layer2 tunneling over IP for instance. ToRs
        are usually multi-homed to switches/routers 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/routers aggregating multiple ToRs. Core network
        elements 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).

   We use throughout this document also the term "Tenant End System" to
   define an end system of a particular tenant, which can be for
   instance a virtual machine (VM), a non-virtualized server, or a
   physical appliance. One or more Tenant End Systems can be part of an
   End Device.

1.4. Tenant networking view

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





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

        Figure 2 : Logical Service connectivity for a single tenant

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

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



2. Reference Models

2.1. Generic Reference Model

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















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

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

   The functional components in Figure 3 do not necessarily map
   directly with the physical components described in Figure 1.

   For example, an End Device in Figure 1 can be a server blade with
   VMs and virtual switch, i.e. the VM is the Tenant End System and the
   NVE functions may be performed by the virtual switch and/or the
   hypervisor.

   Another example is the case where an End Device in Figure 1 can be a
   traditional physical server (no VMs, no virtual switch), i.e. the
   server is the Tenant End System and the NVE functions may be
   performed by the ToR. Other End Devices in this category are
   Physical Network Appliances or Storage Systems.



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

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

   Core nodes utilize L3 techniques to interconnect NVE nodes in
   support of the overlay network. Specifically, they 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 core,
   overlay virtualization provides knobs to control the magnitude of
   that information.

2.2. NVE Reference Model

   Figure 4 depicts the NVE reference model. An NVE contains one or
   more tenant service instances whereby a Tenant End Systems
   interfaces with its associated tenant service instance. The NVE also
   contains an overlay module that provides tunneling overlay functions
   (e.g. encapsulation/decapsulation of tenant traffic from/to the
   tenant forwarding instance, tenant identification and mapping, etc),
   as described in Figure 4.




















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                      +------- L3 Network ------+
                      |                         |
                      |                         |
         +------------+--------+       +--------+------------+
         | +----------+------+ |       | +------+----------+ |
         | | Overlay Module  | |       | | Overlay Module  | |
         | +--------+--------+ |       | +--------+--------+ |
         |          |          |       |          |          |
         |   NVE1   |          |        |         |   NVE2   |
         |  +-------+-------+  |       |  +-------+-------+  |
         |  |Tenant Instance|  |       |  |Tenant Instance|  |
         |  +-+-----------+-+  |       |  +-+-----------+-+  |
         |    |           |    |       |    |           |    |
         +----+-----------+----+       +----+-----------+----+
              |           |                 |           |
       -------+-----------+-----------------+-----------+-------
              |           |     Tenant      |           |
              |           |   Service IF    |           |
           Tenant End Systems             Tenant End Systems

              Figure 5 : Generic reference model for NV Edge

   Note that some NVE functions may reside in one device or may be
   implemented separately in different devices: for example, data plane
   may reside in one device while the control plane components may be
   distributed between multiple devices.

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

2.3. NVE Service Types

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





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2.3.1. L2 NVE providing Ethernet LAN-like service

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

2.3.2. L3 NVE providing IP/VRF-like service

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

3. Functional components

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

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

3.1. Generic service virtualization components

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


















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                      +------- L3 Network ------+
                      |                         |
                      |       Tunnel Overlay    |
         +------------+--------+       +--------+------------+
         | +----------+------+ |       | +------+----------+ |
         | | Overlay Module  | |       | | Overlay Module  | |
         | +--------+--------+ |       | +--------+--------+ |
         |          |Tenant ID |       |          |Tenant ID |
         |          | (TNI)    |       |          | (TNI)    |
         |  +-------+-------+  |       |  +-------+-------+  |
         |  |Tenant Instance|  |       |  |Tenant Instance|  |
    NVE2 |  +-+-----------+-+  |       |  +-+-----------+-+  | NVE1
         |    |   VAPs    |    |       |    |   VAPs    |    |
         +----+-----------+----+       +----+-----------+----+
              |           |                 |           |
       -------+-----------+-----------------+-----------+-------
              |           |     Tenant      |           |
              |           |   Service IF    |           |
            Tenant End Systems            Tenant End Systems

              Figure 6 : Generic reference model for NV Edge

3.1.1. Virtual Attachment Points (VAPs)

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

3.1.2. Tenant Instance

   The Tenant Instance represents a set of configuration attributes
   defining access and tunnel policies and (L2 and/or L3) forwarding
   functions and possibly control plane functions.

   Per tenant FIB tables and control plane protocol instances are used
   to maintain separate private contexts across 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 Tenant ID

   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 TNI for the
   destination NVE in addition to the L3 tunnel source address
   identifying the source NVE and the tunnel destination L3 address
   identifying the destination NVE. This allows the destination NVE to
   identify the tenant service instance and therefore appropriately
   process and forward the tenant packet.

   The Overlay module provides tunneling overlay functions: tunnel
   initiation/termination, encapsulation/decapsulation of frames from
   VAPs/L3 Backbone and may provide for transit forwarding of IP
   traffic (e.g., transparent forwarding of tunnel packets).

   In a multi-tenant context, the tunnel aggregates frames from/to
   different Tenant Instances. Tenant identification and traffic
   demultiplexing are based on the Tenant Identifier (TNI).

   Historically the following approaches have been considered:

     o One ID per Tenant: A globally unique (on a per-DC
        administrative domain) Tenant ID 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 ID per Tenant Instance: A per-tenant local ID 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 ID per VAP: A per-VAP local ID is assigned and usually
        distributed by a control plane protocol. An example of this
        approach is the use of per CE-PE MPLS labels in IP VPN
        [RFC4364].

   Note that when using one ID per Tenant Instance or per VAP, an
   additional global identifier may be used by the control plane to
   identify the Tenant context (e.g., historically equivalent to the
   route target community attribute in [RFC4364]).






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3.1.4. Tunnel Overlays and Encapsulation options

   Once the TNI is added to the tenant data frame, L3 Tunnel
   encapsulation is used to transport the resulting 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/L2TPv3/IPSec) and MPLS-
   based tunneling options (e.g., BGP VPN, PW, VPLS) can be used for
   tunneling Ethernet and IP packets.

3.1.5. Control Plane Components

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

     . Service Auto-provisioning/Auto-discovery

     . Address advertisement and tunnel mapping

     . Tunnel establishment/tear-down and routing

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

3.1.5.1. Auto-provisioning/Service discovery

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

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

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

   When a protocol is used, appropriate security and authentication
   mechanisms to verify that Tenant End System information is not




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   spoofed or altered are required. This is one critical aspect for
   providing integrity and tenant isolation in the system.

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

3.1.5.2. Address advertisement and tunnel mapping

   As traffic reaches an ingress NVE, a lookup is performed to
   determine which tunnel the packet needs to be sent to. It is then
   encapsulated with a tunnel header containing the destination address
   of the egress NVE. Intermediate nodes (between the ingress and
   egress NVEs) switch or route traffic based upon the outer
   destination address. It should be noted that an NVE may be
   implemented on a gateway to provide traffic forwarding between two
   different types of overlay networks, and may not be directly
   connected to a tenant End System.

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

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

3.1.5.3. Tunnel management

   A control plane protocol may be required to setup/teardown tunnels,
   exchange tunnel state information, and/or provide for tunnel
   endpoint routing. This applies to both unicast and multicast
   tunnels.

   For instance, it may be necessary to provide active/standby tunnel
   status information between NVEs, up/down status information,
   pruning/grafting information for multicast tunnels, etc.



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3.2. Service Overlay Topologies

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

   The topology described in Figure 3 suggests the use of a tunnel mesh
   between the NVEs where each tenant instance is one hop away from a
   service processing perspective. This should not be construed to
   imply that a tunnel mesh must be configured as tunneling can simply
   be encapsulation/decapsulation with a tunnel header. Partial mesh
   topologies and a NVE hierarchy may be used where certain NVEs may
   act as service transit points.

4. Key aspects of overlay networks

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

4.1. Pros & Cons

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

   Overlay networks offer the following key advantages:

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

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

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

     o Support of a large number of virtual network identifiers.

   Overlay networks also create several challenges:

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



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          o Overlays may probe the network to measure link properties,
             such as available bandwidth or packet loss rate. It is
             difficult to accurately evaluate network properties. It
             might be preferable for the underlay network to expose
             usage and performance information for itself or the
             overlay networks.

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

     o Fairness of resource sharing and co-ordination among edge-nodes
        in overlay networks are two critical issues. When multiple
        overlays co-exist on top of a common underlay network, the lack
        of coordination between overlays can lead to performance
        issues.

     o Overlaid traffic may not traverse firewalls and NAT devices.

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

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

4.2. Overlay issues to consider

4.2.1. Data plane vs Control plane driven

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

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






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4.2.2. Coordination between data plane and control plane

   Often a combination of dynamic data plane and control based learning
   is necessary. MAC Data-plane learning or IP data plane learning can
   be applied on tenant VAPs at the NVE whereas control plane-based MAC
   and IP reachability distribution can be performed across the overlay
   network among the NVEs, possibly with the help of a control plane
   mediation device (e.g., BGP route reflector if BGP is used to
   distribute such information). Coordination between the data-plane
   learning process and the control plane reachability distribution
   process is needed such that when a new address gets learned or an
   old address is removed, it triggers the local control plane to
   advertise this information to its peers.

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

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

     o Ingress replication

     o Use of core multicast trees

   There is a bandwidth vs state trade-off between the two approaches.
   Depending upon the degree of replication required (i.e. the number
   of hosts per group) and the amount of multicast state to maintain,
   trading bandwidth for state is of consideration.

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

   Depending upon the size of the data center network and hence the
   number of (S,G) entries, but also the duration of multicast flows,
   the use of core multicast trees can be a challenge.

   When flows are well known, it is possible to pre-provision such
   multicast trees. However, it is often difficult to predict
   application flows ahead of time, and hence programming of (S,G)
   entries for short-lived flows could be impractical.

   A possible trade-off is to use shared multicast trees in the core as
   opposed to dedicated multicast trees.





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4.2.4. Path MTU

   When using overlay tunneling, an outer header is added to the
   original tenant frame. This can cause the MTU of the path to the
   egress tunnel endpoint to be exceeded.

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

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

   It is possible for the MTU to be configured manually or to be
   discovered dynamically. Various Path MTU discovery techniques exist
   in order to determine the proper MTU size to use:

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

          o Tenant End Systems rely on ICMP messages to discover the
             MTU of the end-to-end path to its destination. This method
             is not always possible, such as when traversing middle
             boxes (e.g. firewalls) which disable ICMP for security
             reasons

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

   It is also possible to rely on the overlay layer to perform
   segmentation and reassembly operations without relying on the Tenant
   End Systems to know about the end-to-end MTU. The assumption is that
   some hardware assist is available on the NVE node to perform such
   fragmentation and reassembly 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 the underlay network [FLOYD].

   Finally, the underlay network may be designed in such a way that the
   MTU can accommodate the extra tunnel overhead.

4.2.5. NVE location trade-offs

   In the case of DC traffic, traffic originated from a VM is native
   Ethernet traffic. This traffic may be receiving ELAN service or IP
   service. In the case of ELAN service, it can be switched by a local




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   VM switch or ToR switch and then by a DC gateway. The NVE function
   can be embedded within any of these elements.

   There are several criteria to consider when deciding where the NVE
   processing boundary happens:

     o Processing and memory requirements

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

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

     o Multicast support

          o Routing protocols

          o Packet replication capability

     o Fragmentation support

     o QoS transparency

     o Resiliency

4.2.6. Interaction between network overlays and underlays

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

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

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

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

     o Cost metrics






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   This information may then be used to traffic engineer the underlay
   network and/or traffic engineer the overlay networks in a
   coordinated fashion over the overlay.

5. Security Considerations

   The tenant to overlay mapping function can introduce significant
   security risks if appropriate protocols/mechanisms used to establish
   that mapping are not trusted, do not support mutual authentication
   and/or cannot be established over trusted interfaces and/or mutually
   authenticated connections.

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



6. IANA Considerations

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



7. References

7.1. Normative References

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

7.2. Informative References

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

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

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

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




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   [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

   Javier Benitez, Colt

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





























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Authors' Addresses

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

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

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

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

   Yakov Rekhter
   Juniper
   Email: yakov@juniper.net

   Yuichi Ikejiri
   NTT Communications
   1-1-6, Uchisaiwai-cho, Chiyoda-ku
   Tokyo, 100-8019 Japan
   Email: y.ikejiri@ntt.com


















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