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Internet Engineering Task Force (IETF)                    T. Narten, Ed.Request for Comments: 7364                                           IBMCategory: Informational                                     E. Gray, Ed.ISSN: 2070-1721                                                 Ericsson                                                                D. Black                                                                     EMC                                                                 L. Fang                                                               Microsoft                                                              L. Kreeger                                                                   Cisco                                                            M. Napierala                                                                    AT&T                                                            October 2014Problem Statement: Overlays for Network VirtualizationAbstract   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.Narten, et al.                Informational                     [Page 1]

RFC 7364           Overlays for Network Virtualization      October 2014Status 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Narten, et al.                Informational                     [Page 2]

RFC 7364           Overlays for Network Virtualization      October 2014Table of Contents1. Introduction ....................................................42. Terminology .....................................................63. Problem Areas ...................................................63.1. Need for Dynamic Provisioning ..............................63.2. Virtual Machine Mobility Limitations .......................73.3. Inadequate Forwarding Table Sizes ..........................73.4. Need to Decouple Logical and Physical Configuration ........73.5. Need for Address Separation between Virtual Networks .......83.6. Need for Address Separation between Virtual Networks and ...83.7. Optimal Forwarding .........................................94. Using Network Overlays to Provide Virtual Networks .............104.1. Overview of Network Overlays ..............................10      4.2. Communication between Virtual and Non-virtualized           Networks ..................................................124.3. Communication between Virtual Networks ....................124.4. Overlay Design Characteristics ............................134.5. Control-Plane Overlay Networking Work Areas ...............144.6. Data-Plane Work Areas .....................................155. Related IETF and IEEE Work .....................................155.1. BGP/MPLS IP VPNs ..........................................165.2. BGP/MPLS Ethernet VPNs ....................................165.3. 802.1 VLANs ...............................................175.4. IEEE 802.1aq -- Shortest Path Bridging ....................175.5. VDP .......................................................175.6. ARMD ......................................................185.7. TRILL .....................................................185.8. L2VPNs ....................................................185.9. Proxy Mobile IP ...........................................195.10. LISP .....................................................196. Summary ........................................................197. Security Considerations ........................................198. References .....................................................208.1. Normative Reference .......................................208.2. Informative References ....................................20   Acknowledgments ...................................................22   Contributors ......................................................22   Authors' Addresses ................................................23Narten, et al.                Informational                     [Page 3]

RFC 7364           Overlays for Network Virtualization      October 20141.  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-Narten, et al.                Informational                     [Page 4]

RFC 7364           Overlays for Network Virtualization      October 2014   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, andSection 6 closes with   a summary.Narten, et al.                Informational                     [Page 5]

RFC 7364           Overlays for Network Virtualization      October 20142.  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.Narten, et al.                Informational                     [Page 6]

RFC 7364           Overlays for Network Virtualization      October 20143.2.  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.Narten, et al.                Informational                     [Page 7]

RFC 7364           Overlays for Network Virtualization      October 2014   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.Narten, et al.                Informational                     [Page 8]

RFC 7364           Overlays for Network Virtualization      October 20143.7.  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".Narten, et al.                Informational                     [Page 9]

RFC 7364           Overlays for Network Virtualization      October 20144.  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 inSection 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 ofNarten, et al.                Informational                    [Page 10]

RFC 7364           Overlays for Network Virtualization      October 2014       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.Narten, et al.                Informational                    [Page 11]

RFC 7364           Overlays for Network Virtualization      October 2014   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.Narten, et al.                Informational                    [Page 12]

RFC 7364           Overlays for Network Virtualization      October 2014   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.Narten, et al.                Informational                    [Page 13]

RFC 7364           Overlays for Network Virtualization      October 2014   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 aNarten, et al.                Informational                    [Page 14]

RFC 7364           Overlays for Network Virtualization      October 2014   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.  TheNarten, et al.                Informational                    [Page 15]

RFC 7364           Overlays for Network Virtualization      October 2014   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.Narten, et al.                Informational                    [Page 16]

RFC 7364           Overlays for Network Virtualization      October 20145.3.  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.Narten, et al.                Informational                    [Page 17]

RFC 7364           Overlays for Network Virtualization      October 2014   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 smallerNarten, et al.                Informational                    [Page 18]

RFC 7364           Overlays for Network Virtualization      October 2014   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.Narten, et al.                Informational                    [Page 19]

RFC 7364           Overlays for Network Virtualization      October 2014   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 inSection 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.  References8.1.  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.Narten, et al.                Informational                    [Page 20]

RFC 7364           Overlays for Network Virtualization      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>.Narten, et al.                Informational                    [Page 21]

RFC 7364           Overlays for Network Virtualization      October 2014   [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.Narten, et al.                Informational                    [Page 22]

RFC 7364           Overlays for Network Virtualization      October 2014Authors' 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.comNarten, et al.                Informational                    [Page 23]