Internet Engineering Task Force (IETF)                    T. Narten, Ed.
Request for Comments: 7364                                           IBM
Category: Informational                                     E. Gray, Ed.
ISSN: 2070-1721                                                 Ericsson
                                                                D. Black
                                                                     EMC
                                                                 L. Fang
                                                               Microsoft
                                                              L. Kreeger
                                                                   Cisco
                                                            M. Napierala
                                                                    AT&T
                                                            October 2014

Problem Statement: Overlays for Network Virtualization

Abstract

   This document describes issues associated with providing multi-
   tenancy in large data center networks and how these issues may be
   addressed using an overlay-based network virtualization approach.  A
   key multi-tenancy requirement is traffic isolation so that one
   tenant's traffic is not visible to any other tenant.  Another
   requirement is address space isolation so that different tenants can
   use the same address space within different virtual networks.
   Traffic and address space isolation is achieved by assigning one or
   more virtual networks to each tenant, where traffic within a virtual
   network can only cross into another virtual network in a controlled
   fashion (e.g., via a configured router and/or a security gateway).
   Additional functionality is required to provision virtual networks,
   associating a virtual machine's 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7364.

Copyright Notice

   Copyright (c) 2014 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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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   1. Introduction ....................................................4
   2. Terminology .....................................................6
   3. Problem Areas ...................................................6
      3.1. Need for Dynamic Provisioning ..............................6
      3.2. Virtual Machine Mobility Limitations .......................7
      3.3. Inadequate Forwarding Table Sizes ..........................7
      3.4. Need to Decouple Logical and Physical Configuration ........7
      3.5. Need for Address Separation between Virtual Networks .......8
      3.6. Need for Address Separation between Virtual Networks and ...8
      3.7. Optimal Forwarding .........................................9
   4. Using Network Overlays to Provide Virtual Networks .............10
      4.1. Overview of Network Overlays ..............................10
      4.2. Communication between Virtual and Non-virtualized
           Networks ..................................................12
      4.3. Communication between Virtual Networks ....................12
      4.4. Overlay Design Characteristics ............................13
      4.5. Control-Plane Overlay Networking Work Areas ...............14
      4.6. Data-Plane Work Areas .....................................15
   5. Related IETF and IEEE Work .....................................15
      5.1. BGP/MPLS IP VPNs ..........................................16
      5.2. BGP/MPLS Ethernet VPNs ....................................16
      5.3. 802.1 VLANs ...............................................17
      5.4. IEEE 802.1aq -- Shortest Path Bridging ....................17
      5.5. VDP .......................................................17
      5.6. ARMD ......................................................18
      5.7. TRILL .....................................................18
      5.8. L2VPNs ....................................................18
      5.9. Proxy Mobile IP ...........................................19
      5.10. LISP .....................................................19
   6. Summary ........................................................19
   7. Security Considerations ........................................19
   8. References .....................................................20
      8.1. Normative Reference .......................................20
      8.2. Informative References ....................................20
   Acknowledgments ...................................................22
   Contributors ......................................................22
   Authors' Addresses ................................................23

1. Introduction

   Data centers are increasingly being consolidated and outsourced in an
   effort to improve the deployment time of applications and reduce
   operational costs.  This coincides with an increasing demand for
   compute, storage, and network resources from applications.  In order
   to scale compute, storage, and network resources, physical resources
   are being abstracted from their logical representation, in what is
   referred to as server, storage, and network virtualization.
   Virtualization can be implemented in various layers of computer
   systems or networks.

   The demand for server virtualization is increasing 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.

   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 (e.g., a firewall).

   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, Tenant Systems connect to a virtual
   network (VN).  To Tenant Systems, a virtual network looks like a
   normal network (e.g., providing an Ethernet or L3 service), except
   that the only end stations connected to the virtual network are those
   belonging to a tenant's specific virtual network.

   A tenant is the administrative entity on whose behalf one or more
   specific virtual network instances and their associated services
   (whether virtual or physical) are managed.  In a cloud environment, a
   tenant would correspond to the customer that 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 generally matter to the
   tenant; what matters is that the service provided (Layer 2 (L2) or
   Layer 3 (L3)) has the right semantics, performance, etc.  It could be
   implemented via a pure routed network, a pure bridged network, or a
   combination of bridged and routed networks.  A key requirement is
   that each individual virtual network instance be isolated from other
   virtual network instances, with traffic crossing from one virtual
   network to another only when allowed by policy.

   For data center virtualization, two key issues must be addressed.
   First, address space separation between tenants must be supported.
   Second, it must be possible to place (and migrate) VMs anywhere in
   the data center, without restricting VM addressing to match the
   subnet boundaries of the underlying data center network.

   This document outlines problems encountered in scaling the number of
   isolated virtual networks in a data center.  Furthermore, the
   document presents issues associated with managing those virtual
   networks in relation to operations, such as virtual network creation/
   deletion and end-node membership change.  Finally, this document
   makes the case that an overlay-based approach has a number of
   advantages over traditional, non-overlay approaches.  The purpose of
   this document is to identify the set of issues 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.

   This document is the problem statement for the "Network
   Virtualization over Layer 3" (NVO3) Working Group.  NVO3 is focused
   on the construction of overlay networks that operate over an IP (L3)
   underlay transport network.  NVO3 expects to provide both L2 service
   and IP service to Tenant Systems (though perhaps as two different
   solutions).  Some deployments require an L2 service, others an L3
   service, and some may require both.

   Section 2 gives terminology.  Section 3 describes the problem space
   details.  Section 4 describes overlay networks in more detail.
   Section 5 reviews related and further work, and Section 6 closes with
   a summary.

2. Terminology

   This document uses the same terminology as [RFC7365].  In addition,
   this document use the following terms.

   Overlay Network:  A virtual network in which the separation of
      tenants is hidden from the underlying physical infrastructure.
      That is, the underlying transport network does not need to know
      about tenancy separation to correctly forward traffic.  IEEE 802.1
      Provider Backbone Bridging (PBB) [IEEE-802.1Q] is an example of an
      L2 overlay network.  PBB uses MAC-in-MAC encapsulation (where
      "MAC" refers to "Media Access Control"), and the underlying
      transport network forwards traffic using only the Backbone MAC
      (B-MAC) and Backbone VLAN Identifier (B-VID) in the outer header.
      The underlay transport network is unaware of the tenancy
      separation provided by, for example, a 24-bit Backbone Service
      Instance Identifier (I-SID).

   C-VLAN:  This document refers to Customer VLANs (C-VLANs) as
      implemented by many routers, i.e., an L2 virtual network
      identified by a Customer VLAN Identifier (C-VID).  An end station
      (e.g., a VM) in this context that is part of an L2 virtual network
      will effectively belong to a C-VLAN.  Within an IEEE 802.1Q-2011
      network, other tags may be used as well, but such usage is
      generally not visible to the end station.  Section 5.3 provides
      more details on VLANs defined by [IEEE-802.1Q].

   This document uses the phrase "virtual network instance" with its
   ordinary meaning to represent an instance of a virtual network.  Its
   usage may differ from the "VNI" acronym defined in the framework
   document [RFC7365].  The "VNI" acronym is not used in this document.

3. Problem Areas

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

3.1. Need for Dynamic Provisioning

   Some service providers offer services to multiple customers whereby
   services are dynamic and the resources assigned to support them must
   be able to change quickly as demand changes.  In current systems, it
   can be difficult to provision resources for individual tenants (e.g.,
   QoS) in such a way that provisioned properties migrate automatically
   when services are dynamically moved around within the data center to
   optimize workloads.

3.2. Virtual Machine Mobility Limitations

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

   In data center networks, servers are typically assigned IP addresses
   based on their physical location, for example, based on the Top-of-
   Rack (ToR) switch for the server rack or the C-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.

3.3. Inadequate Forwarding Table Sizes

   Today's virtualized environments place additional demands on the
   forwarding tables of forwarding nodes in the physical infrastructure.
   The core problem is that location independence results in specific
   end state information being propagated into the forwarding system
   (e.g., /32 host routes in IPv4 networks or MAC addresses in IEEE
   802.3 Ethernet networks).  In L2 networks, for instance, instead of
   just one address per server, the network infrastructure may have to
   learn addresses of the individual VMs (which could range in the
   hundreds per server).  This increases the demand on a forwarding
   node's table capacity compared to non-virtualized environments.

3.4. Need to Decouple 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.

   In networks of many types (e.g., IP subnets, MPLS VPNs, VLANs, etc.),
   moving servers elsewhere in the network may require expanding the
   scope of a portion of the network (e.g., subnet, VPN, VLAN, etc.)
   beyond its original boundaries.  While this can be done, it requires
   potentially complex network configuration changes and may, in some
   cases (e.g., a VLAN or L2VPN), conflict with the desire to bound the
   size of broadcast domains.  In addition, when VMs migrate, the
   physical network (e.g., access lists) may need to be reconfigured,
   which can be time consuming and error prone.

   An important use case is cross-pod expansion.  A pod typically
   consists of one or more racks of servers with associated network and
   storage connectivity.  A tenant's virtual network may start off on a
   pod and, due to expansion, require servers/VMs on other pods,
   especially the case when 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 tenants'
   servers/VMs.  Such expansion can be difficult to achieve when tenant
   addressing is tied to the addressing used by the underlay network or
   when the expansion requires that the scope of the underlying C-VLAN
   expand beyond its original pod boundary.

3.5. Need for Address Separation between Virtual Networks

   Individual tenants need control over the addresses they use within a
   virtual network.  But it can be problematic when different tenants
   want to use the same addresses or even if the same tenant wants to
   reuse the same addresses in different virtual networks.
   Consequently, virtual networks must allow tenants to use whatever
   addresses they want without concern for what addresses are being used
   by other tenants or other virtual networks.

3.6. Need for Address Separation between Virtual Networks and
Infrastructure

   As in the previous case, a tenant needs to be able to use whatever
   addresses it wants in a virtual network independent of what addresses
   the underlying data center network is using.  Tenants (and the
   underlay infrastructure provider) should be able use whatever
   addresses make sense for them without having to worry about address
   collisions between addresses used by tenants and those used by the
   underlay data center network.

3.7. Optimal Forwarding

   Another problem area relates to the optimal forwarding of traffic
   between peers that are not connected to the same virtual network.
   Such forwarding happens when a host on a virtual network communicates
   with a host not on any virtual network (e.g., an Internet host) as
   well as when a host on a virtual network communicates with a host on
   a different virtual network.  A virtual network may have two (or
   more) gateways for forwarding traffic onto and off of the virtual
   network, and the optimal choice of which gateway to use may depend on
   the set of available paths between the communicating peers.  The set
   of available gateways may not be equally "close" to a given
   destination.  The issue appears both when a VM is initially
   instantiated on a virtual network or when a VM migrates or is moved
   to a different location.  After a migration, for instance, a VM's
   best-choice gateway for such traffic may change, i.e., the VM may get
   better service by switching to the "closer" gateway, and this may
   improve the utilization of network resources.

   IP implementations in network endpoints typically do not distinguish
   between multiple routers on the same subnet -- there may only be a
   single default gateway in use, and any use of multiple routers
   usually considers all of them to be one hop away.  Routing protocol
   functionality is constrained by the requirement to cope with these
   endpoint limitations -- for example, the Virtual Router Redundancy
   Protocol (VRRP) has one router serve as the master to handle all
   outbound traffic.  This problem can be particularly acute when the
   virtual network spans multiple data centers, as a VM is likely to
   receive significantly better service when forwarding external traffic
   through a local router compared to using a router at a remote data
   center.

   The optimal forwarding problem applies to both outbound and inbound
   traffic.  For outbound traffic, the choice of outbound router
   determines the path of outgoing traffic from the VM, which may be
   sub-optimal after a VM move.  For inbound traffic, the location of
   the VM within the IP subnet for the VM is not visible to the routers
   beyond the virtual network.  Thus, the routing infrastructure will
   have no information as to which of the two externally visible
   gateways leading into the virtual network would be the better choice
   for reaching a particular VM.

   The issue is further complicated when middleboxes (e.g., load
   balancers, firewalls, etc.) must be traversed.  Middleboxes may have
   session state that must be preserved for ongoing communication, and
   traffic must continue to flow through the middlebox, regardless of
   which router is "closest".

4. Using Network Overlays to Provide Virtual Networks

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

   1.  Virtual networks isolate the address space used in one virtual
       network from the address space used by another virtual network.
       The same network addresses may be used in different virtual
       networks at the same time.  In addition, the address space used
       by a virtual network is independent from that used by the
       underlying physical network.

   2.  Virtual networks limit the scope of packets sent on the virtual
       network.  Packets sent by Tenant Systems attached to a virtual
       network are delivered as expected to other Tenant Systems on that
       virtual network and may exit a virtual network only through
       controlled exit points, such as a security gateway.  Likewise,
       packets sourced from outside of the virtual network may enter the
       virtual network only through controlled entry points, such as a
       security gateway.

4.1. Overview of Network Overlays

   To address the problems described in Section 3, a network overlay
   approach can be used.

   The idea behind an overlay is quite straightforward.  Each virtual
   network instance is implemented as an overlay.  The original packet
   is encapsulated by the first-hop network device, called a Network
   Virtualization Edge (NVE), and tunneled to a remote NVE.  The
   encapsulation identifies the destination of the device that will
   perform the decapsulation (i.e., the egress NVE for the tunneled
   packet) before delivering the original packet to the endpoint.  The
   rest of the network forwards the packet based on the encapsulation
   header and can be oblivious to the payload that is carried inside.

   Overlays are based on what is commonly known as a "map-and-encap"
   architecture.  When processing and forwarding packets, three distinct
   and logically separable steps take place:

   1.  The first-hop overlay device implements a mapping operation that
       determines where the encapsulated packet should be sent to reach
       its intended destination VM.  Specifically, the mapping function
       maps the destination address (either L2 or L3) of a packet
       received from a VM into the corresponding destination address of

       the egress NVE device.  The destination address will be the
       underlay address of the NVE device doing the decapsulation and is
       an IP address.

   2.  Once the mapping has been determined, the ingress overlay NVE
       device encapsulates the received packet within an overlay header.

   3.  The final step is to actually forward the (now encapsulated)
       packet to its destination.  The packet is forwarded by the
       underlay (i.e., the IP network) based entirely on its outer
       address.  Upon receipt at the destination, the egress overlay NVE
       device decapsulates the original packet and delivers it to the
       intended recipient VM.

   Each of the above steps is logically distinct, though an
   implementation might combine them for efficiency or other reasons.
   It should be noted that in L3 BGP/VPN terminology, the above steps
   are commonly known as "forwarding" or "virtual forwarding".

   The first-hop NVE 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 IP VPNs
   [RFC4364], Transparent Interconnection of Lots of Links (TRILL)
   [RFC6325], the Locator/ID Separation Protocol (LISP) [RFC6830], and
   Shortest Path Bridging (SPB) [IEEE-802.1aq].

   In the data plane, an overlay header provides a place to carry either
   the virtual network identifier or an identifier that is locally
   significant to the edge device.  In both cases, the identifier in the
   overlay header specifies which specific virtual network the data
   packet belongs to.  Since both routed and bridged semantics can be
   supported by a virtual data center, the original packet carried
   within the overlay header can be an Ethernet frame or just the IP
   packet.

   A key aspect of overlays is the decoupling of the "virtual" MAC and/
   or 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 overlay edge devices 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 regard to traditional constraints imposed by the
   underlay network, such as the C-VLAN scope or the IP subnet scope.

   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 (e.g., a gateway) that has connectivity to both virtual
   network instances.  Without the existence of a gateway 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.

4.2. Communication between Virtual and Non-virtualized 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 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).

   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.

4.3. 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 L2 service, such interconnect
   functionality could be IP forwarding configured as part of the
   "default gateway" for each virtual network.  For a virtual network
   providing L3 service, the interconnect functionality could be IP
   forwarding configured as part of routing between IP subnets, or it
   could be based on configured inter-virtual-network traffic policies.

   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 and firewall support)
   that is applied to traffic forwarded between virtual networks.

4.4. Overlay Design Characteristics

   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
       Tenant 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 Tenant Systems connected to any one virtual network
       is expected to be relatively low; therefore, the percentage of
       NVEs participating in any given virtual network would also be
       expected to be low.  For this reason, efficient delivery of
       multi-destination traffic within a virtual network instance
       should be taken into consideration.

   3.  Highly dynamic Tenant Systems: Tenant Systems connected to
       virtual networks can be very dynamic, both in terms of
       creation/deletion/power-on/power-off and in terms of mobility
       from one access device to another.

   4.  Be incrementally deployable, without necessarily requiring major
       upgrade of the entire network: The first-hop device (or end
       system) that adds and removes the overlay header may require new
       software and may require new hardware (e.g., for improved
       performance).  The rest of the network should not need to change
       just to enable the use of overlays.

   5.  Work with existing data center network deployments without
       requiring major changes in operational or other practices: For
       example, some data centers have not enabled multicast beyond
       link-local scope.  Overlays should be capable of leveraging
       underlay multicast support where appropriate, but not require its
       enablement in order to use an overlay solution.

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

4.5. Control-Plane Overlay Networking Work Areas

   There are three specific and separate potential work areas in the
   area of control-plane protocols 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 packets to their proper destination.  One approach is to
   build mapping tables entirely via learning (as is done in 802.1
   networks).  Another approach is to use 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 NVEs will likely reside in hypervisors places constraints
   on the resources (CPU and memory) that can be dedicated to such
   functions.

   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 Network
   Virtualization Authority (NVA) that is responsible for distributing
   and maintaining the mapping information for the entire overlay
   system.  For this document, we use the term "NVA" to refer to an
   entity that supplies answers, without regard to how it knows the
   answers it is providing.  The second component consists of the on-
   the-wire protocols an NVE uses when interacting with the NVA.

   The first two areas of work are thus: describing the NVA function and
   defining NVA-NVE interactions.

   The back-end NVA 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-NVA interaction while allowing
   different protocols and architectural approaches for the NVA itself.
   Separating the two allows NVEs to transparently interact with
   different types of NVAs, 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 NVA for the mapping table
   entries it needs and allows the NVA (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 Systems [RFC7365], more generally) from a specific virtual
   network instance.  When a VM attaches, the NVE associates the VM with
   a specific overlay for the purposes of tunneling traffic sourced from
   or destined to the VM.  When a VM disconnects, the NVE should notify
   the NVA that the Tenant System to NVE address mapping is no longer
   valid.  In addition, if this VM was the last remaining member of the
   virtual network, then the NVE can also terminate any tunnels used to
   deliver tenant multi-destination packets within the VN to the NVE.
   In the case where an NVE and hypervisor are on separate physical
   devices separated by an access network, a standardized protocol may
   be needed.

   In summary, there are three areas of potential work.  The first area
   concerns the implementation of the NVA function itself and any
   protocols it needs (e.g., if implemented in a distributed fashion).
   A second area concerns the interaction between the NVA 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 scalable,
   interoperable solutions.

4.6. Data-Plane Work Areas

   The data plane carries encapsulated packets for Tenant Systems.  The
   data-plane encapsulation header carries a VN Context identifier
   [RFC7365] for the virtual network to which the data packet belongs.
   Numerous encapsulation or tunneling protocols already exist that can
   be leveraged.  In the absence of strong and compelling justification,
   it would not seem necessary or helpful to develop yet another
   encapsulation format just for NVO3.

5. Related IETF and IEEE Work

   The following subsections discuss related IETF and IEEE work.  These
   subsections are not meant to provide complete coverage of all IETF
   and IEEE work related to data centers, and the descriptions should
   not be considered comprehensive.  Each area aims to address
   particular limitations of today's data center networks.  In all
   areas, scaling is a common theme as are multi-tenancy and VM
   mobility.  Comparing and evaluating the work result and progress of
   each work area listed is out of the scope of this document.  The

   intent of this section is to provide a reference to the interested
   readers.  Note that NVO3 is scoped to running over an IP/L3 underlay
   network.

5.1. BGP/MPLS IP VPNs

   BGP/MPLS IP VPNs [RFC4364] support multi-tenancy, VPN traffic
   isolation, address overlapping, and address separation between
   tenants and network infrastructure.  The BGP/MPLS control plane is
   used to distribute the VPN labels and the tenant IP addresses that
   identify the tenants (or to be more specific, the particular VPN/
   virtual network) and tenant IP addresses.  Deployment of enterprise
   L3 VPNs has been shown to scale to thousands of VPNs and millions of
   VPN prefixes.  BGP/MPLS IP VPNs are currently deployed in some large
   enterprise data centers.  The potential limitation for deploying BGP/
   MPLS IP VPNs in data center environments is the practicality of using
   BGP in the data center, especially reaching into the servers or
   hypervisors.  There may be computing workforce skill set issues,
   equipment support issues, and potential new scaling challenges.  A
   combination of BGP and lighter-weight IP signaling protocols, e.g.,
   the Extensible Messaging and Presence Protocol (XMPP), has been
   proposed to extend the solutions into the data center environment
   [END-SYSTEM] while taking advantage of built-in VPN features with its
   rich policy support; it is especially useful for inter-tenant
   connectivity.

5.2. BGP/MPLS Ethernet VPNs

   Ethernet Virtual Private Networks (E-VPNs) [EVPN] provide an emulated
   L2 service in which each tenant has its own Ethernet network over a
   common IP or MPLS infrastructure.  A BGP/MPLS control plane is used
   to distribute the tenant MAC addresses and the MPLS labels that
   identify the tenants and tenant MAC addresses.  Within the BGP/MPLS
   control plane, a 32-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 any VLAN-based
   limitation on the customer site is associated with an individual
   tenant service edge, enabling a much higher level of scalability.
   Interconnection between tenants is also allowed in a controlled
   fashion.

   VM mobility [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.

5.3. 802.1 VLANs

   VLANs are a well-understood construct in the networking industry,
   providing an L2 service via a physical network in which tenant
   forwarding information is part of the physical network
   infrastructure.  A VLAN is an L2 bridging construct that provides the
   semantics of virtual networks mentioned above: a MAC address can be
   kept unique within a VLAN, but it is not necessarily unique across
   VLANs.  Traffic scoped within a VLAN (including broadcast and
   multicast traffic) can be kept within the VLAN it originates from.
   Traffic forwarded from one VLAN to another typically involves router
   (L3) processing.  The forwarding table lookup operation may be keyed
   on {VLAN, MAC address} tuples.

   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.  Various types of VLANs are available
   today and can be used for network virtualization, even together.  The
   C-VLAN, Service VLAN (S-VLAN), and Backbone VLAN (B-VLAN) IDs
   [IEEE-802.1Q] are 12 bits.  The 24-bit I-SID [IEEE-802.1aq] allows
   the support of more than 16 million virtual networks.

5.4. IEEE 802.1aq -- Shortest Path Bridging

   Shortest Path Bridging (SPB) [IEEE-802.1aq] is an overlay based on
   IS-IS that operates over L2 Ethernets.  SPB supports multipathing and
   addresses a number of shortcomings in the original Ethernet Spanning
   Tree Protocol.  Shortest Path Bridging Mac (SPBM) uses IEEE 802.1ah
   PBB (MAC-in-MAC) encapsulation and supports a 24-bit I-SID, which can
   be used to identify virtual network instances.  SPBM provides multi-
   pathing and supports easy virtual network creation or update.

   SPBM extends IS-IS in order to perform link-state routing among core
   SPBM nodes, obviating the need for bridge learning for communication
   among core SPBM nodes.  Learning is still used to build and maintain
   the mapping tables of edge nodes to encapsulate Tenant System traffic
   for transport across the SPBM core.

   SPB is compatible with all other 802.1 standards and thus allows
   leveraging of other features, e.g., VSI Discovery Protocol (VDP),
   Operations, Administration, and Maintenance (OAM), or scalability
   solutions.

5.5. VDP

   VDP is the Virtual Station Interface (VSI) Discovery and
   Configuration Protocol specified by IEEE P802.1Qbg [IEEE-802.1Qbg].
   VDP is a protocol that supports the association of a VSI with a port.

   VDP is run between the end station (e.g., a server running a
   hypervisor) and its adjacent switch (i.e., the device on the edge of
   the network).  VDP is used, for example, to communicate to the switch
   that a virtual machine (virtual station) is moving, i.e., designed
   for VM migration.

5.6. ARMD

   The Address Resolution for Massive numbers of hosts in the Data
   center (ARMD) WG examined data center scaling issues with a focus on
   address resolution and developed a problem statement document
   [RFC6820].  While an overlay-based approach may address some of the
   "pain points" that were raised in ARMD (e.g., better support for
   multi-tenancy), analysis will be needed to understand the scaling
   trade-offs 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.

5.7. TRILL

   TRILL is a network protocol that provides an Ethernet L2 service to
   end systems and is designed to operate over any L2 link type.  TRILL
   establishes forwarding paths using IS-IS routing and encapsulates
   traffic within its own TRILL header.  TRILL, as originally defined,
   supports only the standard (and limited) 12-bit C-VID identifier.
   Work to extend TRILL to support more than 4094 VLANs has recently
   completed and is defined in [RFC7172]

5.8. 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 as discussed in this document 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 the Layer 2 Tunneling Protocol (L2TP)
   [RFC3931] require significant tunnel state at the encapsulating and
   decapsulating endpoints.  Overlays require less tunnel state than
   other approaches, which is important to allow overlays to scale to
   hundreds of thousands of endpoints.  It is assumed that smaller

   switches (i.e., virtual switches in hypervisors or the adjacent
   devices to which VMs connect) will be part of the overlay network and
   be responsible for encapsulating and decapsulating packets.

5.9. Proxy Mobile IP

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

5.10. LISP

   LISP [RFC6830] 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.

6. Summary

   This document has argued that network virtualization using overlays
   addresses a number of issues being faced as data centers scale in
   size.  In addition, careful study of current data center problems is
   needed for development of proper requirements and standard solutions.

   This document identifies three potential control protocol work areas.
   The first involves a back-end NVA and how it learns and distributes
   the mapping information NVEs use when processing tenant traffic.  A
   second involves the protocol an NVE would use to communicate with the
   back-end NVA to obtain the mapping information.  The third potential
   work concerns the interactions that take place when a VM attaches or
   detaches from a specific virtual network instance.

   There are a number of approaches that provide some, if not all, of
   the desired semantics of virtual networks.  Each approach needs to be
   analyzed in detail to assess how well it satisfies the requirements.

7. Security Considerations

   Because this document describes the problem space associated with the
   need for virtualization of networks in complex, large-scale, data-
   center networks, it does not itself introduce any security risks.
   However, it is clear that security concerns need to be a
   consideration of any solutions proposed to address this problem
   space.

   Solutions will need to address both data-plane and control-plane
   security concerns.

   In the data plane, isolation of virtual network traffic from other
   virtual networks is a primary concern -- for NVO3, this isolation may
   be based on VN identifiers that are not involved in underlay network
   packet forwarding between overlay edges (NVEs).  Use of a VN
   identifier in the overlay reduces the underlay network's role in
   isolating virtual networks by comparison to approaches where VN
   identifiers are involved in packet forwarding (e.g., 802.1 VLANs as
   described in Section 5.3).

   In addition to isolation, assurances against spoofing, snooping,
   transit modification and denial of service are examples of other
   important data-plane considerations.  Some limited environments may
   even require confidentiality.

   In the control plane, the primary security concern is ensuring that
   an unauthorized party does not compromise the control-plane protocol
   in ways that improperly impact the data plane.  Some environments may
   also be concerned about confidentiality of the control plane.

   More generally, denial-of-service concerns may also be a
   consideration.  For example, a tenant on one virtual network could
   consume excessive network resources in a way that degrades services
   for other tenants on other virtual networks.

8. References

8.1. Normative Reference

   [RFC7365]  Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
              Rekhter, "Framework for Data Center (DC) Network
              Virtualization", RFC 7365, October 2014,
              <http://www.rfc-editor.org/info/rfc7365>.

8.2. Informative References

   [END-SYSTEM]
              Marques, P., Fang, L., Sheth, N., Napierala, M., and N.
              Bitar, "BGP-signaled end-system IP/VPNs", Work in
              Progress, draft-ietf-l3vpn-end-system-04, October 2014.

   [EVPN]     Sajassi, A., Aggarwal, R., Bitar, N., Isaac, A., and J.
              Uttaro, "BGP MPLS Based Ethernet VPN", Work in Progress,
              draft-ietf-l2vpn-evpn-10, October 2014.

   [IEEE-802.1Q]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Media Access Control (MAC) Bridges and Virtual
              Bridged Local Area Networks", IEEE 802.1Q-2011, August
              2011, <http://standards.ieee.org/getieee802/
              download/802.1Q-2011.pdf>.

   [IEEE-802.1Qbg]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Media Access Control (MAC) Bridges and Virtual
              Bridged Local Area Networks -- Amendment 21: Edge Virtual
              Bridging", IEEE 802.1Qbg-2012, July 2012,
              <http://standards.ieee.org/getieee802/
              download/802.1Qbg-2012.pdf>.

   [IEEE-802.1aq]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Media Access Control (MAC) Bridges and Virtual
              Bridged Local Area Networks -- Amendment 20: Shortest Path
              Bridging", IEEE 802.1aq, June 2012,
              <http://standards.ieee.org/getieee802/
              download/802.1aq-2012.pdf>.

   [MOBILITY] Aggarwal, R., Rekhter, Y., Henderickx, W., Shekhar, R.,
              Fang, L., and A. Sajassi, "Data Center Mobility based on
              E-VPN, BGP/MPLS IP VPN, IP Routing and NHRP", Work in
              Progress, draft-raggarwa-data-center-mobility-07, June
              2014.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005,
              <http://www.rfc-editor.org/info/rfc3931>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006,
              <http://www.rfc-editor.org/info/rfc4364>.

   [RFC5213]  Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008,
              <http://www.rfc-editor.org/info/rfc5213>.

   [RFC5844]  Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
              Mobile IPv6", RFC 5844, May 2010,
              <http://www.rfc-editor.org/info/rfc5844>.

   [RFC5845]  Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
              "Generic Routing Encapsulation (GRE) Key Option for Proxy
              Mobile IPv6", RFC 5845, June 2010,
              <http://www.rfc-editor.org/info/rfc5845>.

   [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,
              <http://www.rfc-editor.org/info/rfc6245>.

   [RFC6325]  Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011,
              <http://www.rfc-editor.org/info/6325>.

   [RFC6820]  Narten, T., Karir, M., and I. Foo, "Address Resolution
              Problems in Large Data Center Networks", RFC 6820, January
              2013, <http://www.rfc-editor.org/info/rfc6820>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830, January
              2013, <http://www.rfc-editor.org/info/rfc6830>.

   [RFC7172]  Eastlake, D., Zhang, M., Agarwal, P., Perlman, R., and D.
              Dutt, "Transparent Interconnection of Lots of Links
              (TRILL): Fine-Grained Labeling", RFC 7172, May 2014,
              <http://www.rfc-editor.org/info/rfc7172>.

Acknowledgments

   Helpful comments and improvements to this document have come from Lou
   Berger, John Drake, Ilango Ganga, Ariel Hendel, Vinit Jain, Petr
   Lapukhov, Thomas Morin, Benson Schliesser, Qin Wu, Xiaohu Xu, Lucy
   Yong, and many others on the NVO3 mailing list.

   Special thanks to Janos Farkas for his persistence and numerous
   detailed comments related to the lack of precision in the text
   relating to IEEE 802.1 technologies.

Contributors

   Dinesh Dutt and Murari Sridharin were original co-authors of the
   Internet-Draft that led to the BoF that formed the NVO3 WG.  That
   original draft eventually became the basis for this document.

Authors' Addresses

   Thomas Narten (editor)
   IBM
   Research Triangle Park, NC
   United States
   EMail: narten@us.ibm.com

   Eric Gray (editor)
   Ericsson
   EMail: eric.gray@ericsson.com

   David Black
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748
   United States
   EMail: david.black@emc.com

   Luyuan Fang
   Microsoft
   5600 148th Ave NE
   Redmond, WA  98052
   United States
   EMail: lufang@microsoft.com

   Lawrence Kreeger
   Cisco
   170 W. Tasman Avenue
   San Jose, CA  95134
   United States
   EMail: kreeger@cisco.com

   Maria Napierala
   AT&T
   200 S. Laurel Avenue
   Middletown, NJ  07748
   United States
   EMail: mnapierala@att.com