Problem Statement: Overlays for Network Virtualization
draft-narten-nvo3-overlay-problem-statement-03

Internet Engineering Task Force                           T. Narten, Ed.
Internet-Draft                                                       IBM
Intended status: Informational                              M. Sridharan
Expires: January 18, 2013                                      Microsoft
                                                                 D. Dutt

                                                                D. Black
                                                                     EMC
                                                              L. Kreeger
                                                                   Cisco
                                                           July 17, 2012

         Problem Statement: Overlays for Network Virtualization
             draft-narten-nvo3-overlay-problem-statement-03

Abstract

   This document describes issues associated with providing multi-
   tenancy in large data center networks and an overlay-based network
   virtualization approach to addressing them.  A key multi-tenancy
   requirement is traffic isolation, so that a tenant's traffic is not
   visible to any other tenant.  This isolation can be achieved by
   assigning one or more virtual networks to each tenant such that
   traffic within a virtual network is isolated from traffic in other
   virtual networks.  The primary functionality required is provisioning
   virtual networks, associating a virtual machine's virtual network
   interface(s) with the appropriate virtual network, and maintaining
   that association as the virtual machine is activated, migrated and/or
   deactivated.  Use of an overlay-based approach enables scalable
   deployment on large network infrastructures.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 18, 2013.

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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
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Problem Details  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Dynamic Provisioning . . . . . . . . . . . . . . . . . . .  5
     2.2.  Virtual Machine Mobility Requirements  . . . . . . . . . .  5
     2.3.  Span of Virtual Networks . . . . . . . . . . . . . . . . .  6
     2.4.  Inadequate Forwarding Table Sizes in Switches  . . . . . .  6
     2.5.  Decoupling Logical and Physical Configuration  . . . . . .  6
     2.6.  Separating Tenant Addressing from Infrastructure
           Addressing . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.7.  Communication Between Virtual and Traditional Networks . .  7
     2.8.  Communication Between Virtual Networks . . . . . . . . . .  7
     2.9.  Overlay Design Characteristics . . . . . . . . . . . . . .  8
   3.  Network Overlays . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Limitations of Existing Virtual Network Models . . . . . .  9
     3.2.  Benefits of Network Overlays . . . . . . . . . . . . . . . 10
     3.3.  Overlay Networking Work Areas  . . . . . . . . . . . . . . 11
   4.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  IEEE 802.1aq - Shortest Path Bridging  . . . . . . . . . . 13
     4.2.  ARMD . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.3.  TRILL  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.4.  L2VPNs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.5.  Proxy Mobile IP  . . . . . . . . . . . . . . . . . . . . . 14
     4.6.  LISP . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.7.  Individual Submissions . . . . . . . . . . . . . . . . . . 14
   5.  Further Work . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Informative References . . . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 17
     A.1.  Changes from -01 . . . . . . . . . . . . . . . . . . . . . 17
     A.2.  Changes from -02 . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Introduction

   Server virtualization is increasingly becoming the norm in data
   centers.  With server virtualization, each physical server supports
   multiple virtual machines (VMs), each running its own operating
   system, middleware and applications.  Virtualization is a key enabler
   of workload agility, i.e., allowing any server to host any
   application and providing the flexibility of adding, shrinking, or
   moving services within the physical infrastructure.  Server
   virtualization provides numerous benefits, including higher
   utilization, increased security, reduced user downtime, reduced power
   usage, etc.

   Large scale multi-tenant data centers are taking advantage of the
   benefits of server virtualization to provide a new kind of hosting, a
   virtual hosted data center.  Multi-tenant data centers are ones where
   individual tenants could belong to a different company (in the case
   of a public provider) or a different department (in the case of an
   internal company data center).  Each tenant has the expectation of a
   level of security and privacy separating their resources from those
   of other tenants.  For example, one tenant's traffic must never be
   exposed to another tenant, except through carefully controlled
   interfaces, such as a security gateway.

   To a tenant, virtual data centers are similar to their physical
   counterparts, consisting of end stations attached to a network,
   complete with services such as load balancers and firewalls.  But
   unlike a physical data center, end stations connect to a virtual
   network.  To end stations, a virtual network looks like a normal
   network (e.g., providing an ethernet service), except that the only
   end stations connected to the virtual network are those belonging to
   the tenant.

   A tenant is the administrative entity that is responsible for and
   manages a specific virtual network instance and its associated
   services (whether virtual or physical).  In a cloud environment, a
   tenant would correspond to the customer that has defined and is using
   a particular virtual network.  However, a tenant may also find it
   useful to create multiple different virtual network instances.
   Hence, there is a one-to-many mapping between tenants and virtual
   network instances.  A single tenant may operate multiple individual
   virtual network instances, each associated with a different service.

   How a virtual network is implemented does not matter to the tenant.
   It could be a pure routed network, a pure bridged network or a
   combination of bridged and routed networks.  The key requirement is
   that each individual virtual network instance be isolated from other
   virtual network instances.

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   This document outlines the problems encountered in scaling the number
   of isolated networks in a data center, as well as the problems of
   managing the creation/deletion, membership and span of these networks
   and makes the case that an overlay based approach, where individual
   networks are implemented within individual virtual networks that are
   dynamically controlled by a standardized control plane provides a
   number of advantages over current approaches.  The purpose of this
   document is to identify the set of problems that any solution has to
   address in building multi-tenant data centers.  With this approach,
   the goal is to allow the construction of standardized, interoperable
   implementations to allow the construction of multi-tenant data
   centers.

   Section 2 describes the problem space details.  Section 3 describes
   network overlays in more detail and the potential work areas.
   Sections 4 and 5 review related and further work, while Section 6
   closes with a summary.

2.  Problem Details

   The following subsections describe aspects of multi-tenant networking
   that pose problems for large scale network infrastructure.  Different
   problem aspects may arise based on the network architecture and
   scale.

2.1.  Dynamic Provisioning

   Cloud computing involves on-demand provisioning of resources for
   multi-tenant environments.  A common example of cloud computing is
   the public cloud, where a cloud service provider offers elastic
   services to multiple customers over the same infrastructure.  The on-
   demand nature of provisioning in conjunction with trusted hypervisors
   controlling network access by VMs can be achieved through resilient
   distributed network control mechanisms.

2.2.  Virtual Machine Mobility Requirements

   A key benefit of server virtualization is virtual machine (VM)
   mobility.  A VM can be migrated from one server to another, live,
   i.e., while continuing to run and without needing to shut it down and
   restart it at the new location.  A key requirement for live migration
   is that a VM retain critical network state at its new location,
   including its IP and MAC address(es).  Preservation of MAC addresses
   may be necessary, for example, when software licences are bound to
   MAC addresses.  More generally, any change in the VM's MAC addresses
   resulting from a move would be visible to the VM and thus potentially
   result in unexpected disruptions.  Retaining IP addresses after a

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   move is necessary to prevent existing transport connections (e.g.,
   TCP) from breaking and needing to be restarted.

   In traditional data centers, servers are assigned IP addresses based
   on their physical location, for example based on the Top of Rack
   (ToR) switch for the server rack or the VLAN configured to the
   server.  Servers can only move to other locations within the same IP
   subnet.  This constraint is not problematic for physical servers,
   which move infrequently, but it restricts the placement and movement
   of VMs within the data center.  Any solution for a scalable multi-
   tenant data center must allow a VM to be placed (or moved) anywhere
   within the data center, without being constrained by the subnet
   boundary concerns of the host servers.

2.3.  Span of Virtual Networks

   Another use case is cross pod expansion.  A pod typically consists of
   one or more racks of servers with its associated network and storage
   connectivity.  Tenants may start off on a pod and, due to expansion,
   require servers/VMs on other pods, especially the case when tenants
   on the other pods are not fully utilizing all their resources.  This
   use case requires that virtual networks span multiple pods in order
   to provide connectivity to all of the tenant's servers/VMs.

2.4.  Inadequate Forwarding Table Sizes in Switches

   Today's virtualized environments place additional demands on the
   forwarding tables of switches.  Instead of just one link-layer
   address per server, the switching infrastructure has to learn
   addresses of the individual VMs (which could range in the 100s per
   server).  This is a requirement since traffic from/to the VMs to the
   rest of the physical network will traverse the physical network
   infrastructure.  This places a much larger demand on the switches'
   forwarding table capacity compared to non-virtualized environments,
   causing more traffic to be flooded or dropped when the addresses in
   use exceeds the forwarding table capacity.

2.5.  Decoupling Logical and Physical Configuration

   Data center operators must be able to achieve high utilization of
   server and network capacity.  For efficient and flexible allocation,
   operators should be able to spread a virtual network instance across
   servers in any rack in the data center.  It should also be possible
   to migrate compute workloads to any server anywhere in the network
   while retaining the workload's addresses.  This can be achieved today
   by stretching VLANs (e.g., by using TRILL or SPB).

   However, in order to limit the broadcast domain of each VLAN, multi-

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   destination frames within a VLAN should optimally flow only to those
   devices that have that VLAN configured.  When workloads migrate, the
   physical network (e.g., access lists) may need to be reconfigured
   which is typically time consuming and error prone.

2.6.  Separating Tenant Addressing from Infrastructure Addressing

   It is highly desirable to be able to number the data center underlay
   network using whatever addresses make sense for it, without having to
   worry about address collisions between addresses used by the underlay
   and those used by tenants.

2.7.  Communication Between Virtual and Traditional Networks

   Not all communication will be between devices connected to
   virtualized networks.  Devices using overlays will continue to access
   devices and make use of services on traditional, non-virtualized
   networks, whether in the data center, the public Internet, or at
   remote/branch campuses.  Any virtual network solution must be capable
   of interoperating with existing routers, VPN services, load
   balancers, intrusion detection services, firewalls, etc. on external
   networks.

   Communication between devices attached to a virtual network and
   devices connected to non-virtualized networks is handled
   architecturally by having specialized gateway devices that receive
   packets from a virtualized network, decapsulate them, process them as
   regular (i.e., non-virtualized) traffic, and finally forward them on
   to their appropriate destination (and vice versa).  Additional
   identification, such as VLAN tags, could be used on the non-
   virtualized side of such a gateway to enable forwarding of traffic
   for multiple virtual networks over a common non-virtualized link.

   A wide range of implementation approaches are possible.  Overlay
   gateway functionality could be combined with other network
   functionality into a network device that implements the overlay
   functionality, and then forwards traffic between other internal
   components that implement functionality such as full router service,
   load balancing, firewall support, VPN gateway, etc.

2.8.  Communication Between Virtual Networks

   Communication between devices on different virtual networks is
   handled architecturally by adding specialized interconnect
   functionality among the otherwise isolated virtual networks.  For a
   virtual network providing an Ethernet service, such interconnect
   functionality could be IP forwarding configured as part of the
   "default gateway" for each virtual network.  For a virtual network

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   providing IP service, the interconnect functionality could be IP
   forwarding configured as part of the IP addressing structure of each
   virtual network.  In both cases, the implementation of the
   interconnect functionality could be distributed across the NVEs, and
   could be combined with other network functionality (e.g., load
   balancing, firewall support) that is applied to traffic that is
   forwarded between virtual networks.

2.9.  Overlay Design Characteristics

   There are existing layer 2 overlay protocols in existence, but they
   were not necessarily designed to solve the problem in the environment
   of a highly virtualized data center.  Below are some of the
   characteristics of environments that must be taken into account by
   the overlay technology:

   1.  Highly distributed systems.  The overlay should work in an
       environment where there could be many thousands of access
       switches (e.g. residing within the hypervisors) and many more end
       systems (e.g.  VMs) connected to them.  This leads to a
       distributed mapping system that puts a low overhead on the
       overlay tunnel endpoints.

   2.  Many highly distributed virtual networks with sparse membership.
       Each virtual network could be highly dispersed inside the data
       center.  Also, along with expectation of many virtual networks,
       the number of end systems connected to any one virtual network is
       expected to be relatively low; Therefore, the percentage of
       access switches participating in any given virtual network would
       also be expected to be low.  For this reason, efficient pruning
       of multi-destination traffic should be taken into consideration.

   3.  Highly dynamic end systems.  End systems connected to virtual
       networks can be very dynamic, both in terms of creation/deletion/
       power-on/off and in terms of mobility across the access switches.

   4.  Work with existing, widely deployed network Ethernet switches and
       IP routers without requiring wholesale replacement.  The first
       hop switch that adds and removes the overlay header will require
       new equipment and/or new software.

   5.  Network infrastructure administered by a single administrative
       domain.  This is consistent with operation within a data center,
       and not across the Internet.

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3.  Network Overlays

   Virtual Networks are used to isolate a tenant's traffic from that of
   other tenants (or even traffic within the same tenant that requires
   isolation).  There are two main characteristics of virtual networks:

   1.  Providing network address space that is isolated from other
       virtual networks.  The same network addresses may be used in
       different virtual networks on the same underlying network
       infrastructure.

   2.  Limiting the scope of frames sent on the virtual network.  Frames
       sent by end systems attached to a virtual network are delivered
       as expected to other end systems on that virtual network and may
       exit a virtual network only through controlled exit points such
       as a security gateway.  Likewise, frames sourced outside of the
       virtual network may enter the virtual network only through
       controlled entry points, such as a security gateway.

3.1.  Limitations of Existing Virtual Network Models

   Virtual networks are not new to networking.  For example, VLANs are a
   well known construct in the networking industry.  A VLAN is an L2
   bridging construct that provides some of the semantics of virtual
   networks mentioned above: a MAC address is unique within a VLAN, but
   not necessarily across VLANs.  Traffic sourced within a VLAN
   (including broadcast and multicast traffic) remains within the VLAN
   it originates from.  Traffic forwarded from one VLAN to another
   typically involves router (L3) processing.  The forwarding table look
   up operation is keyed on {VLAN, MAC address} tuples.

   But there are problems and limitations with L2 VLANs.  VLANs are a
   pure L2 bridging construct and VLAN identifiers are carried along
   with data frames to allow each forwarding point to know what VLAN the
   frame belongs to.  A VLAN today is defined as a 12 bit number,
   limiting the total number of VLANs to 4096 (though typically, this
   number is 4094 since 0 and 4095 are reserved).  Due to the large
   number of tenants that a cloud provider might service, the 4094 VLAN
   limit is often inadequate.  In addition, there is often a need for
   multiple VLANs per tenant, which exacerbates the issue.  The use of a
   sufficiently large VNID, present in the overlay control plane and
   possibly also in the dataplane would eliminate current VLAN size
   limitations associated with single 12-bit VLAN tags.

   For IP/MPLS networks, Ethernet Virtual Private Network (E-VPN)
   [I-D.ietf-l2vpn-evpn] provides an emulated Ethernet service in which
   each tenant has its own Ethernet network over a common IP or MPLS
   infrastructure and a BGP/MPLS control plane is used to distribute the

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   tenant MAC addresses and the MPLS labels that identify the tenants
   and tenant MAC addresses.  Within the BGP/MPLS control plane a thirty
   two bit Ethernet Tag is used to identify the broadcast domains
   (VLANs) associated with a given L2 VLAN service instance and these
   Ethernet tags are mapped to VLAN IDs understood by the tenant at the
   service edges.  This means that the limit of 4096 VLANs is associated
   with an individual tenant service edge, enabling a much higher level
   of scalability.  Interconnectivity between tenants is also allowed in
   a controlled fashion.

   IP/MPLS networks also provide an IP VPN service (L3 VPN) [RFC4364] in
   which each tenant has its own IP network over a common IP or MPLS
   infrastructure and a BGP/MPLS control plane is used to distribute the
   tenant IP routes and the MPLS labels that identify the tenants and
   tenant IP routes.  As with E-VPNs, interconnectivity between tenants
   is also allowed in a controlled fashion.

   VM Mobility [I-D.raggarwa-data-center-mobility] introduces the
   concept of a combined L2/L3 VPN service in order to support the
   mobility of individual Virtual Machines (VMs) between Data Centers
   connected over a common IP or MPLS infrastructure.

   There are a number of VPN approaches that provide some if not all of
   the desired semantics of virtual networks.  A gap analysis will be
   needed to assess how well existing approaches satisfy the
   requirements.

3.2.  Benefits of Network Overlays

   To address the problems described earlier, a network overlay model
   can be used.

   The idea behind an overlay is quite straightforward.  Each virtual
   network instance is implemented as an overlay.  The original frame is
   encapsulated by the first hop network device.  The encapsulation
   identifies the destination of the device that will perform the
   decapsulation before delivering the frame to the endpoint.  The rest
   of the network forwards the frame based on the encapsulation header
   and can be oblivious to the payload that is carried inside.  To avoid
   belaboring the point each time, the first hop network device can be a
   traditional switch or router or the virtual switch residing inside a
   hypervisor.  Furthermore, the endpoint can be a VM or it can be a
   physical server.  Examples of architectures based on network overlays
   include BGP/MPLS VPNs [RFC4364], TRILL [RFC6325], LISP
   [I-D.ietf-lisp], and Shortest Path Bridging [SPB].

   With the overlay, a virtual network identifier (or VNID) can be
   carried as part of the overlay header so that every data frame

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   explicitly identifies the specific virtual network the frame belongs
   to.  Since both routed and bridged semantics can be supported by a
   virtual data center, the original frame carried within the overlay
   header can be an Ethernet frame complete with MAC addresses or just
   the IP packet.

   The use of a sufficiently large VNID would address current VLAN
   limitations associated with single 12-bit VLAN tags.  This VNID can
   be carried in the control plane.  In the data plane, an overlay
   header provides a place to carry either the VNID, or a locally-
   significant identifier.  In both cases, the identifier in the overlay
   header specifies which virtual network the data packet belongs to.

   A key aspect of overlays is the decoupling of the "virtual" MAC and
   IP addresses used by VMs from the physical network infrastructure and
   the infrastructure IP addresses used by the data center.  If a VM
   changes location, the switches at the edge of the overlay simply
   update their mapping tables to reflect the new location of the VM
   within the data center's infrastructure space.  Because an overlay
   network is used, a VM can now be located anywhere in the data center
   that the overlay reaches without regards to traditional constraints
   implied by L2 properties such as VLAN numbering, or the span of an L2
   broadcast domain scoped to a single pod or access switch.

   Multi-tenancy is supported by isolating the traffic of one virtual
   network instance from traffic of another.  Traffic from one virtual
   network instance cannot be delivered to another instance without
   (conceptually) exiting the instance and entering the other instance
   via an entity that has connectivity to both virtual network
   instances.  Without the existence of this entity, tenant traffic
   remains isolated within each individual virtual network instance.

   Overlays are designed to allow a set of VMs to be placed within a
   single virtual network instance, whether that virtual network
   provides a bridged network or a routed network.

3.3.  Overlay Networking Work Areas

   There are three specific and separate potential work areas needed to
   realize an overlay solution.  The areas correspond to different
   possible "on-the-wire" protocols, where distinct entities interact
   with each other.

   One area of work concerns the address dissemination protocol an NVE
   uses to build and maintain the mapping tables it uses to deliver
   encapsulated frames to their proper destination.  One approach is to
   build mapping tables entirely via learning (as is done in 802.1
   networks).  But to provide better scaling properties, a more

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   sophisticated approach is needed, i.e., the use of a specialized
   control plane protocol.  While there are some advantages to using or
   leveraging an existing protocol for maintaining mapping tables, the
   fact that large numbers of NVE's will likely reside in hypervisors
   places constraints on the resources (cpu and memory) that can be
   dedicated to such functions.  For example, routing protocols (e.g.,
   IS-IS, BGP) may have scaling difficulties if implemented directly in
   all NVEs, based on both flooding and convergence time concerns.  An
   alternative approach would be to use a standard query protocol
   between NVEs and the set of network nodes that maintain address
   mappings used across the data center for the entire overlay system.

   From an architectural perspective, one can view the address mapping
   dissemination problem as having two distinct and separable
   components.  The first component consists of a back-end "oracle" that
   is responsible for distributing and maintaining the mapping
   information for the entire overlay system.  The second component
   consists of the on-the-wire protocols an NVE uses when interacting
   with the oracle.

   The back-end oracle could provide high performance, high resiliency,
   failover, etc. and could be implemented in significantly different
   ways.  For example, one model uses a traditional, centralized
   "directory-based" database, using replicated instances for
   reliability and failover.  A second model involves using and possibly
   extending an existing routing protocol (e.g., BGP, IS-IS, etc.).  To
   support different architectural models, it is useful to have one
   standard protocol for the NVE-oracle interaction while allowing
   different protocols and architectural approaches for the oracle
   itself.  Separating the two allows NVEs to transparently interact
   with different types of oracles, i.e., either of the two
   architectural models described above.  Having separate protocols
   could also allow for a simplified NVE that only interacts with the
   oracle for the mapping table entries it needs and allows the oracle
   (and its associated protocols) to evolve independently over time with
   minimal impact to the NVEs.

   A third work area considers the attachment and detachment of VMs (or
   Tenant End Systems [I-D.lasserre-nvo3-framework] more generally) from
   a specific virtual network instance.  When a VM attaches, the Network
   Virtualization Edge (NVE) [I-D.lasserre-nvo3-framework] associates
   the VM with a specific overlay for the purposes of tunneling traffic
   sourced from or destined to the VM.  When a VM disconnects, it is
   removed from the overlay and the NVE effectively terminates any
   tunnels associated with the VM.  To achieve this functionality, a
   standardized interaction between the NVE and hypervisor may be
   needed, for example in the case where the NVE resides on a separate
   device from the VM.

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   In summary, there are three areas of potential work.  The first area
   concerns the oracle itself and any on-the-wire protocols it needs.  A
   second area concerns the interaction between the oracle and NVEs.
   The third work area concerns protocols associated with attaching and
   detaching a VM from a particular virtual network instance.  All three
   work areas are important to the development of a scalable,
   interoperable solution.

4.  Related Work

4.1.  IEEE 802.1aq - Shortest Path Bridging

   Shortest Path Bridging (SPB) is an IS-IS based overlay for L2
   Ethernets.  SPB supports multi-pathing and addresses a number of
   shortcoming in the original Ethernet Spanning Tree Protocol.  SPB-M
   uses IEEE 802.1ah MAC-in-MAC encapsulation and supports a 24-bit
   I-SID, which can be used to identify virtual network instances.  SPB
   is entirely L2 based, extending the L2 Ethernet bridging model.

4.2.  ARMD

   ARMD is chartered to look at data center scaling issues with a focus
   on address resolution.  ARMD is currently chartered to develop a
   problem statement and is not currently developing solutions.  While
   an overlay-based approach may address some of the "pain points" that
   have been raised in ARMD (e.g., better support for multi-tenancy), an
   overlay approach may also push some of the L2 scaling concerns (e.g.,
   excessive flooding) to the IP level (flooding via IP multicast).
   Analysis will be needed to understand the scaling tradeoffs of an
   overlay based approach compared with existing approaches.  On the
   other hand, existing IP-based approaches such as proxy ARP may help
   mitigate some concerns.

4.3.  TRILL

   TRILL is an L2-based approach aimed at improving deficiencies and
   limitations with current Ethernet networks and STP in particular.
   Although it differs from Shortest Path Bridging in many architectural
   and implementation details, it is similar in that is provides an L2-
   based service to end systems.  TRILL as defined today, supports only
   the standard (and limited) 12-bit VLAN model.  Approaches to extend
   TRILL to support more than 4094 VLANs are currently under
   investigation [I-D.ietf-trill-fine-labeling]

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4.4.  L2VPNs

   The IETF has specified a number of approaches for connecting L2
   domains together as part of the L2VPN Working Group.  That group,
   however has historically been focused on Provider-provisioned L2
   VPNs, where the service provider participates in management and
   provisioning of the VPN.  In addition, much of the target environment
   for such deployments involves carrying L2 traffic over WANs.  Overlay
   approaches are intended be used within data centers where the overlay
   network is managed by the data center operator, rather than by an
   outside party.  While overlays can run across the Internet as well,
   they will extend well into the data center itself (e.g., up to and
   including hypervisors) and include large numbers of machines within
   the data center itself.

   Other L2VPN approaches, such as L2TP [RFC2661] require significant
   tunnel state at the encapsulating and decapsulating end points.
   Overlays require less tunnel state than other approaches, which is
   important to allow overlays to scale to hundreds of thousands of end
   points.  It is assumed that smaller switches (i.e., virtual switches
   in hypervisors or the physical switches to which VMs connect) will be
   part of the overlay network and be responsible for encapsulating and
   decapsulating packets.

4.5.  Proxy Mobile IP

   Proxy Mobile IP [RFC5213] [RFC5844] makes use of the GRE Key Field
   [RFC5845] [RFC6245], but not in a way that supports multi-tenancy.

4.6.  LISP

   LISP[I-D.ietf-lisp] essentially provides an IP over IP overlay where
   the internal addresses are end station Identifiers and the outer IP
   addresses represent the location of the end station within the core
   IP network topology.  The LISP overlay header uses a 24-bit Instance
   ID used to support overlapping inner IP addresses.

4.7.  Individual Submissions

   Many individual submissions also look to addressing some or all of
   the issues addressed in this draft.  Examples of such drafts are
   VXLAN [I-D.mahalingam-dutt-dcops-vxlan], NVGRE
   [I-D.sridharan-virtualization-nvgre] and Virtual Machine Mobility in
   L3 networks[I-D.wkumari-dcops-l3-vmmobility].

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5.  Further Work

   It is believed that overlay-based approaches may be able to reduce
   the overall amount of flooding and other multicast and broadcast
   related traffic (e.g, ARP and ND) currently experienced within
   current data centers with a large flat L2 network.  Further analysis
   is needed to characterize expected improvements.

6.  Summary

   This document has argued that network virtualization using L3
   overlays addresses a number of issues being faced as data centers
   scale in size.  In addition, careful consideration of a number of
   issues would lead to the development of interoperable implementation
   of virtualization overlays.

   Three potential work were identified.  The first involves the
   interaction that take place when a VM attaches or detaches from an
   overlay.  A second involves the protocol an NVE would use to
   communicate with a backend "oracle" to learn and disseminate mapping
   information about the VMs the NVE communicates with.  The third
   potential work area involves the backend oracle itself, i.e., how it
   provides failover and how it interacts with oracles in other domains.

7.  Acknowledgments

   Helpful comments and improvements to this document have come from
   Ariel Hendel, Vinit Jain, and Benson Schliesser.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   TBD

10.  Informative References

   [I-D.ietf-l2vpn-evpn]
              Sajassi, A., Aggarwal, R., Henderickx, W., Balus, F.,
              Isaac, A., and J. Uttaro, "BGP MPLS Based Ethernet VPN",
              draft-ietf-l2vpn-evpn-01 (work in progress), July 2012.

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   [I-D.ietf-lisp]
              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-23 (work in progress), May 2012.

   [I-D.ietf-trill-fine-labeling]
              Eastlake, D., Zhang, M., Agarwal, P., Perlman, R., and D.
              Dutt, "TRILL: Fine-Grained Labeling",
              draft-ietf-trill-fine-labeling-01 (work in progress),
              June 2012.

   [I-D.kreeger-nvo3-overlay-cp]
              Black, D., Dutt, D., Kreeger, L., Sridhavan, M., and T.
              Narten, "Network Virtualization Overlay Control Protocol
              Requirements", draft-kreeger-nvo3-overlay-cp-00 (work in
              progress), January 2012.

   [I-D.lasserre-nvo3-framework]
              Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
              Rekhter, "Framework for DC Network Virtualization",
              draft-lasserre-nvo3-framework-03 (work in progress),
              July 2012.

   [I-D.mahalingam-dutt-dcops-vxlan]
              Sridhar, T., Bursell, M., Kreeger, L., Dutt, D., Wright,
              C., Mahalingam, M., Duda, K., and P. Agarwal, "VXLAN: A
              Framework for Overlaying Virtualized Layer 2 Networks over
              Layer 3 Networks", draft-mahalingam-dutt-dcops-vxlan-01
              (work in progress), February 2012.

   [I-D.raggarwa-data-center-mobility]
              Aggarwal, R., Rekhter, Y., Henderickx, W., Shekhar, R.,
              and L. Fang, "Data Center Mobility based on BGP/MPLS, IP
              Routing and NHRP", draft-raggarwa-data-center-mobility-03
              (work in progress), June 2012.

   [I-D.sridharan-virtualization-nvgre]
              Sridhavan, M., Greenberg, A., Venkataramaiah, N., Wang,
              Y., Duda, K., Ganga, I., Lin, G., Pearson, M., Thaler, P.,
              and C. Tumuluri, "NVGRE: Network Virtualization using
              Generic Routing Encapsulation",
              draft-sridharan-virtualization-nvgre-01 (work in
              progress), July 2012.

   [I-D.wkumari-dcops-l3-vmmobility]
              Kumari, W. and J. Halpern, "Virtual Machine mobility in L3
              Networks.", draft-wkumari-dcops-l3-vmmobility-00 (work in
              progress), August 2011.

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   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, August 1999.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
              MPLS in IP or Generic Routing Encapsulation (GRE)",
              RFC 4023, March 2005.

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

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5213]  Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.

   [RFC5844]  Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
              Mobile IPv6", RFC 5844, May 2010.

   [RFC5845]  Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
              "Generic Routing Encapsulation (GRE) Key Option for Proxy
              Mobile IPv6", RFC 5845, June 2010.

   [RFC6245]  Yegani, P., Leung, K., Lior, A., Chowdhury, K., and J.
              Navali, "Generic Routing Encapsulation (GRE) Key Extension
              for Mobile IPv4", RFC 6245, May 2011.

   [RFC6325]  Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011.

   [SPB]      "IEEE P802.1aq/D4.5 Draft Standard for Local and
              Metropolitan Area Networks -- Media Access Control (MAC)
              Bridges and Virtual Bridged Local Area Networks,
              Amendment 8: Shortest Path Bridging", February 2012.

Appendix A.  Change Log

A.1.  Changes from -01

   1.  Removed Section 4.2 (Standardization Issues) and Section 5
       (Control Plane) as those are more appropriately covered in and
       overlap with material in [I-D.lasserre-nvo3-framework] and
       [I-D.kreeger-nvo3-overlay-cp].

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   2.  Expanded introduction and better explained terms such as tenant
       and virtual network instance.  These had been covered in a
       section that has since been removed.

   3.  Added Section 3.3 "Overlay Networking Work Areas" to better
       articulate the three separable work components (or "on-the-wire
       protocols") where work is needed.

   4.  Added section on Shortest Path Bridging in Related Work section.

   5.  Revised some of the terminology to be consistent with
       [I-D.lasserre-nvo3-framework] and [I-D.kreeger-nvo3-overlay-cp].

A.2.  Changes from -02

   1.  Numerous changes in response to discussions on the nvo3 mailing
       list, with majority of changes in Section 2 (Problem Details) and
       Section 3 (Network Overlays).  Best to see diffs for specific
       text changes.

Authors' Addresses

   Thomas Narten (editor)
   IBM

   Email: narten@us.ibm.com

   Murari Sridharan
   Microsoft

   Email: muraris@microsoft.com

   Dinesh Dutt

   Email: ddutt.ietf@hobbesdutt.com

   David Black
   EMC

   Email: david.black@emc.com

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   Lawrence Kreeger
   Cisco

   Email: kreeger@cisco.com

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