Internet Engineering Task Force Marc Lasserre Internet Draft Florin Balus Intended status: Informational Alcatel-Lucent Expires: September 2012 Thomas Morin France Telecom Orange Nabil Bitar Verizon Yakov Rekhter Juniper Yuichi Ikejiri NTT Communications March 12, 2012 Framework for DC Network Virtualization draft-lasserre-nvo3-framework-01.txt Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on September 12, 2012. Lasserre, et al. Expires September 12, 2012 [Page 1] Internet-Draft Framework for DC Network Virtualization March 2012 Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract Several IETF drafts relate to the use of overlay networks to support large scale virtual data centers. This draft provides a framework for Network Virtualization over L3 (NVO3) and is intended to help plan a set of work items in order to provide a complete solution set. It defines a logical view of the main components with the intention of streamlining the terminology and focusing the solution set. Table of Contents 1. Introduction...................................................3 1.1. Conventions used in this document.........................4 1.2. General terminology.......................................4 1.3. DC network architecture...................................5 1.4. Tenant networking view....................................6 2. Reference Models...............................................7 2.1. Generic Reference Model...................................7 2.2. NVE Reference Model.......................................9 2.3. NVE Service Types........................................10 2.3.1. L2 NVE providing Ethernet LAN-like service..........11 2.3.2. L3 NVE providing IP/VRF-like service................11 3. Functional components.........................................11 3.1. Generic service virtualization components................11 3.1.1. Virtual Attachment Points (VAPs)....................12 3.1.2. Tenant Instance.....................................12 3.1.3. Overlay Modules and Tenant ID.......................13 3.1.4. Tunnel Overlays and Encapsulation options...........14 3.1.5. Control Plane Components............................14 Lasserre, et al. Expires September 12, 2012 [Page 2] Internet-Draft Framework for DC Network Virtualization March 2012 3.1.5.1. Auto-provisioning/Service discovery...............14 3.1.5.2. Address advertisement and tunnel mapping..........15 3.1.5.3. Tunnel management.................................15 3.2. Service Overlay Topologies...............................16 4. Key aspects of overlay networks...............................16 4.1. Pros & Cons..............................................16 4.2. Overlay issues to consider...............................17 4.2.1. Data plane vs Control plane driven..................17 4.2.2. Coordination between data plane and control plane...18 4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM) traffic....................................................18 4.2.4. Path MTU............................................19 4.2.5. NVE location trade-offs.............................19 4.2.6. Interaction between network overlays and underlays..20 5. Security Considerations.......................................21 6. IANA Considerations...........................................21 7. References....................................................21 7.1. Normative References.....................................21 7.2. Informative References...................................21 8. Acknowledgments...............................................22 1. Introduction This document provides a framework for Data Center Network Virtualization over L3 tunnels. This framework is intended to aid in standardizing protocols and mechanisms to support large scale network virtualization for data centers. Several IETF drafts relate to the use of overlay networks for data centers. [NVOPS] defines the rationale for using overlay networks in order to build large data center networks. The use of virtualization leads to a very large number of communication domains and end systems to cope with. Existing virtual network models used for data center networks have known limitations, specifically in the context of multiple tenants. These issues can be summarized as: o Limited VLAN space o FIB explosion due to handling of a large number of MACs/IP addresses o Spanning Tree limitations o Excessive ARP handling Lasserre, et al. Expires September 12, 2012 [Page 3] Internet-Draft Framework for DC Network Virtualization March 2012 o Broadcast storms o Inefficient Broadcast/Multicast handling o Limited mobility/portability support o Lack of service auto-discovery Overlay techniques have been used in the past to address some of these issues. [OVCPREQ] describes the requirements for a control plane protocol required by overlay border nodes to exchange overlay mappings. This document provides reference models that describe functional components of data center overlay networks. It also describes technical issues that have to be addressed in the design of protocols and mechanisms for large-scale data center networks. 1.1. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying RFC-2119 significance. 1.2. General terminology Some general terminology is defined here. Terminology specific to this memo is introduced as needed in later sections. DC: Data Center ELAN: MEF ELAN, multipoint to multipoint Ethernet service Lasserre, et al. Expires September 12, 2012 [Page 4] Internet-Draft Framework for DC Network Virtualization March 2012 1.3. DC network architecture ,---------. ,' `. ( IP/MPLS WAN ) `. ,' `-+------+' +--+--+ +-+---+ |DC GW|+-+|DC GW| +-+---+ +-----+ | / .--. .--. ( ' '.--. .-.' Intra-DC ' ( network ) ( .'-' '--'._.'. )\ \ / / '--' \ \ / / | | \ \ +---+--+ +-`.+--+ +--+----+ | ToR | | ToR | | ToR | +-+--`.+ +-+-`.-+ +-+--+--+ .' \ .' \ .' `. __/_ _i./ i./_ _\__ '--------' '--------' '--------' '--------' : End : : End : : End : : End : : Device : : Device : : Device : : Device : '--------' '--------' '--------' '--------' Figure 1 : A Generic Architecture for Data Centers Figure 1 depicts a common and generic multi-tier DC network architecture. It provides a view of physical components inside a DC. A cloud network is composed of intra-Data Center (DC) networks and network services, and inter-DC network and network connectivity services. Depending upon the scale, DC distribution, operations model, Capex and Opex aspects, DC networking elements can act as strict L2 switches and/or provide IP routing capabilities, including service virtualization. In some DC architectures, it is possible that some tier layers are collapsed and/or provide L2 and/or L3 services, and that Internet connectivity, inter-DC connectivity and VPN support are handled by a smaller number of nodes. Nevertheless, one can assume that the functional blocks fit with the architecture depicted in Figure 1. Lasserre, et al. Expires September 12, 2012 [Page 5] Internet-Draft Framework for DC Network Virtualization March 2012 The following components can be present in a DC: o End Device: a DC resource to which the networking service is provided. End Device may be a compute resource (server or server blade), storage component or a network appliance (firewall, load-balancer, IPsec gateway). Alternatively, the End Device may include software based networking functions used to interconnect multiple hosts. An example of soft networking is the virtual switch in the server blades, used to interconnect multiple virtual machines (VMs). End Device may be single or multi-homed to the Top of Rack switches (ToRs). o Top of Rack (ToR): Hardware-based Ethernet switch aggregating all Ethernet links from the End Devices in a rack representing the entry point in the physical DC network for the hosts. ToRs may also provide routing functionality, virtual IP network connectivity, or Layer2 tunneling over IP for instance. ToRs are usually multi-homed to switches/routers in the Intra-DC network. Other deployment scenarios may use an intermediate Blade Switch before the ToR or an EoR (End of Row) switch to provide similar function as a ToR. o Intra-DC Network: High capacity network composed of core switches/routers aggregating multiple ToRs. Core network elements are usually Ethernet switches but can also support routing capabilities. o DC GW: Gateway to the outside world providing DC Interconnect and connectivity to Internet and VPN customers. In the current DC network model, this may be simply a Router connected to the Internet and/or an IPVPN/L2VPN PE. Some network implementations may dedicate DC GWs for different connectivity types (e.g., a DC GW for Internet, and another for VPN). We use throughout this document also the term "Tenant End System" to define an end system of a particular tenant, which can be for instance a virtual machine (VM), a non-virtualized server, or a physical appliance. One or more Tenant End Systems can be part of an End Device. 1.4. Tenant networking view The DC network architecture is used to provide L2 and/or L3 service connectivity to each tenant. An example is depicted in Figure 2: Lasserre, et al. Expires September 12, 2012 [Page 6] Internet-Draft Framework for DC Network Virtualization March 2012 +----- L3 Infrastructure ----+ | | ,--+-'. ;--+--. ..... Rtr1 )...... . Rtr2 ) | '-----' | '-----' | Tenant1 |LAN12 Tenant1| |LAN11 ....|........ |LAN13 '':'''''''':' | | '':'''''''':' ,'. ,'. ,+. ,+. ,'. ,'. (VM )....(VM ) (VM )... (VM ) (VM )....(VM ) `-' `-' `-' `-' `-' `-' Figure 2 : Logical Service connectivity for a single tenant In this example one or more L3 contexts and one or more LANs (e.g., one per Application) running on DC switches are assigned for DC tenant 1. For a multi-tenant DC, a virtualized version of this type of service connectivity needs to be provided for each tenant by the Network Virtualization solution. 2. Reference Models 2.1. Generic Reference Model The following diagram shows a DC reference model for network virtualization using Layer3 overlays where edge devices provide a logical interconnect between Tenant End Systems that belong to specific tenant network. Lasserre, et al. Expires September 12, 2012 [Page 7] Internet-Draft Framework for DC Network Virtualization March 2012 +--------+ +--------+ | Tenant | | Tenant | | End +--+ +---| End | | System | | | | System | +--------+ | ................... | +--------+ | +-+--+ +--+-+ | | | NV | | NV | | +--|Edge| |Edge|--+ +-+--+ +--+-+ / . L3 Overlay . \ +--------+ / . Network . \ +--------+ | Tenant +--+ . . +----| Tenant | | End | . . | End | | System | . +----+ . | System | +--------+ .....| NV |........ +--------+ |Edge| +----+ | | +--------+ | Tenant | | End | | System | +--------+ Figure 3 : Generic reference model for DC network virtualization over a Layer3 infrastructure The functional components in Figure 3 do not necessarily map directly with the physical components described in Figure 1. For example, an End Device in Figure 1 can be a server blade with VMs and virtual switch, i.e. the VM is the Tenant End System and the NVE functions may be performed by the virtual switch and/or the hypervisor. Another example is the case where an End Device in Figure 1 can be a traditional physical server (no VMs, no virtual switch), i.e. the server is the Tenant End System and the NVE functions may be performed by the ToR. Other End Devices in this category are Physical Network Appliances or Storage Systems. Lasserre, et al. Expires September 12, 2012 [Page 8] Internet-Draft Framework for DC Network Virtualization March 2012 A Tenant End System attaches to a Network Virtualization Edge (NVE) node, either directly or via a switched network (typically Ethernet). The NVE implements network virtualization functions that allow for L2 and/or L3 tenant separation and for hiding tenant addressing information (MAC and IP addresses), tenant-related control plane activity and service contexts from the Routed Core nodes. Core nodes utilize L3 techniques to interconnect NVE nodes in support of the overlay network. Specifically, they perform forwarding based on outer L3 tunnel header, and generally do not maintain per tenant-service state albeit some applications (e.g., multicast) may require control plane or forwarding plane information that pertain to a tenant, group of tenants, tenant service or a set of services that belong to one or more tenants. When such tenant or tenant-service related information is maintained in the core, overlay virtualization provides knobs to control the magnitude of that information. 2.2. NVE Reference Model Figure 4 depicts the NVE reference model. An NVE contains one or more tenant service instances whereby a Tenant End Systems interfaces with its associated tenant service instance. The NVE also contains an overlay module that provides tunneling overlay functions (e.g. encapsulation/decapsulation of tenant traffic from/to the tenant forwarding instance, tenant identification and mapping, etc), as described in Figure 4. Lasserre, et al. Expires September 12, 2012 [Page 9] Internet-Draft Framework for DC Network Virtualization March 2012 +------- L3 Network ------+ | | | | +------------+--------+ +--------+------------+ | +----------+------+ | | +------+----------+ | | | Overlay Module | | | | Overlay Module | | | +--------+--------+ | | +--------+--------+ | | | | | | | | NVE1 | | | | NVE2 | | +-------+-------+ | | +-------+-------+ | | |Tenant Instance| | | |Tenant Instance| | | +-+-----------+-+ | | +-+-----------+-+ | | | | | | | | | +----+-----------+----+ +----+-----------+----+ | | | | -------+-----------+-----------------+-----------+------- | | Tenant | | | | Service IF | | Tenant End Systems Tenant End Systems Figure 5 : Generic reference model for NV Edge Note that some NVE functions may reside in one device or may be implemented separately in different devices: for example, data plane may reside in one device while the control plane components may be distributed between multiple devices. The NVE functionality could reside solely on the End Devices, on the ToRs or on both the End Devices and the ToRs. In the latter case we say that the End Device NVE component acts as the NVE Spoke, and ToRs act as NVE hubs. Tenant End Systems will interface with the tenant service instances maintained on the NVE spokes, and tenant service instances maintained on the NVE spokes will interface with the tenant service instances maintained on the NVE hubs. 2.3. NVE Service Types NVE components may be used to provide different types of virtualized service connectivity. This section defines the service types and associated attributes Lasserre, et al. Expires September 12, 2012 [Page 10] Internet-Draft Framework for DC Network Virtualization March 2012 2.3.1. L2 NVE providing Ethernet LAN-like service L2 NVE implements Ethernet LAN emulation (ELAN), an Ethernet based multipoint service where the Tenant End Systems appear to be interconnected by a LAN environment over a set of L3 tunnels. It provides per tenant virtual switching instance and associated MAC FIB, MAC address isolation across tenants, and L3 tunnel encapsulation across the core. 2.3.2. L3 NVE providing IP/VRF-like service Virtualized IP routing and forwarding is similar from a service definition perspective with IETF IP VPN (e.g., BGP/MPLS IPVPN and IPsec VPNs). It provides per tenant routing instance and associated IP FIB, IP address isolation across tenants, and L3 tunnel encapsulation across the core. 3. Functional components This section breaks down the Network Virtualization architecture into functional components to make it easier to discuss solution options for different modules. This version of the document gives an overview of generic functional components that are shared between L2 and L3 service types. Details specific for each service type will be added in future revisions. 3.1. Generic service virtualization components A Network Virtualization solution is built around a number of functional components as depicted in Figure 5: Lasserre, et al. Expires September 12, 2012 [Page 11] Internet-Draft Framework for DC Network Virtualization March 2012 +------- L3 Network ------+ | | | Tunnel Overlay | +------------+--------+ +--------+------------+ | +----------+------+ | | +------+----------+ | | | Overlay Module | | | | Overlay Module | | | +--------+--------+ | | +--------+--------+ | | |Tenant ID | | |Tenant ID | | | (TNI) | | | (TNI) | | +-------+-------+ | | +-------+-------+ | | |Tenant Instance| | | |Tenant Instance| | NVE2 | +-+-----------+-+ | | +-+-----------+-+ | NVE1 | | VAPs | | | | VAPs | | +----+-----------+----+ +----+-----------+----+ | | | | -------+-----------+-----------------+-----------+------- | | Tenant | | | | Service IF | | Tenant End Systems Tenant End Systems Figure 6 : Generic reference model for NV Edge 3.1.1. Virtual Attachment Points (VAPs) Tenant End Systems are connected to the Tenant Instance through Virtual Attachment Points (VAPs). The VAPs can be in reality physical ports on a ToR or virtual ports identified through logical interface identifiers (VLANs, internal VSwitch Interface ID leading to a VM). 3.1.2. Tenant Instance The Tenant Instance represents a set of configuration attributes defining access and tunnel policies and (L2 and/or L3) forwarding functions and possibly control plane functions. Per tenant FIB tables and control plane protocol instances are used to maintain separate private contexts across tenants. Hence tenants are free to use their own addressing schemes without concerns about address overlapping with other tenants. Lasserre, et al. Expires September 12, 2012 [Page 12] Internet-Draft Framework for DC Network Virtualization March 2012 3.1.3. Overlay Modules and Tenant ID Mechanisms for identifying each tenant service are required to allow the simultaneous overlay of multiple tenant services over the same underlay L3 network topology. In the data plane, each NVE, upon sending a tenant packet, must be able to encode the TNI for the destination NVE in addition to the L3 tunnel source address identifying the source NVE and the tunnel destination L3 address identifying the destination NVE. This allows the destination NVE to identify the tenant service instance and therefore appropriately process and forward the tenant packet. The Overlay module provides tunneling overlay functions: tunnel initiation/termination, encapsulation/decapsulation of frames from VAPs/L3 Backbone and may provide for transit forwarding of IP traffic (e.g., transparent forwarding of tunnel packets). In a multi-tenant context, the tunnel aggregates frames from/to different Tenant Instances. Tenant identification and traffic demultiplexing are based on the Tenant Identifier (TNI). Historically the following approaches have been considered: o One ID per Tenant: A globally unique (on a per-DC administrative domain) Tenant ID is used to identify the related Tenant instances. An example of this approach is the use of IEEE VLAN or ISID tags to provide virtual L2 domains. o One ID per Tenant Instance: A per-tenant local ID is automatically generated by the egress NVE and usually distributed by a control plane protocol to all the related NVEs. An example of this approach is the use of per VRF MPLS labels in IP VPN [RFC4364]. o One ID per VAP: A per-VAP local ID is assigned and usually distributed by a control plane protocol. An example of this approach is the use of per CE-PE MPLS labels in IP VPN [RFC4364]. Note that when using one ID per Tenant Instance or per VAP, an additional global identifier may be used by the control plane to identify the Tenant context (e.g., historically equivalent to the route target community attribute in [RFC4364]). Lasserre, et al. Expires September 12, 2012 [Page 13] Internet-Draft Framework for DC Network Virtualization March 2012 3.1.4. Tunnel Overlays and Encapsulation options Once the TNI is added to the tenant data frame, L3 Tunnel encapsulation is used to transport the resulting frame to the destination NVE. The backbone devices do not usually keep any per service state, simply forwarding the frames based on the outer tunnel header. Different IP tunneling options (e.g., GRE/L2TPv3/IPSec) and MPLS- based tunneling options (e.g., BGP VPN, PW, VPLS) can be used for tunneling Ethernet and IP packets. 3.1.5. Control Plane Components Control plane components may be used to provide the following capabilities: . Service Auto-provisioning/Auto-discovery . Address advertisement and tunnel mapping . Tunnel establishment/tear-down and routing A control plane component can be an on-net control protocol or a management control entity. 3.1.5.1. Auto-provisioning/Service discovery NVEs must be able to select the appropriate Tenant Instance for each Tenant End System. This is based on state information that is often provided by external entities. For example, in a VM environment, this information is provided by compute management systems, since these are the only entities that have visibility of which VM belongs to which tenant. A mechanism for communicating this information between Tenant End Systems and the local NVE is required. As a result the VAPs are created and mapped to the appropriate Tenant Instance. Depending upon the implementation, this control interface can be implemented using an auto-discovery protocol between Tenant End Systems and their local NVE or through management entities. When a protocol is used, appropriate security and authentication mechanisms to verify that Tenant End System information is not Lasserre, et al. Expires September 12, 2012 [Page 14] Internet-Draft Framework for DC Network Virtualization March 2012 spoofed or altered are required. This is one critical aspect for providing integrity and tenant isolation in the system. Another control plane protocol can also be used to advertize NVE tenant service instance (tenant and service type provided to the tenant) to other NVEs. Alternatively, management control entities can also be used to perform these functions. 3.1.5.2. Address advertisement and tunnel mapping As traffic reaches an ingress NVE, a lookup is performed to determine which tunnel the packet needs to be sent to. It is then encapsulated with a tunnel header containing the destination address of the egress NVE. Intermediate nodes (between the ingress and egress NVEs) switch or route traffic based upon the outer destination address. It should be noted that an NVE may be implemented on a gateway to provide traffic forwarding between two different types of overlay networks, and may not be directly connected to a tenant End System. One key step in this process consists of mapping a final destination address to the proper tunnel. NVEs are responsible for maintaining such mappings in their lookup tables. Several ways of populating these lookup tables are possible: control plane driven, management plane driven, or data plane driven. When a control plane protocol is used to distribute address advertisement and tunneling information, the service auto- provisioning/auto-discovery could be accomplished by the same protocol. In this scenario, the auto-provisioning/Service discovery could be combined with (be inferred from) the address advertisement and tunnel mapping. Furthermore, a control plane protocol that carries both IP addresses and associated MACs eliminates the need for ARP and hence addresses one of the issues with explosive ARP handling. 3.1.5.3. Tunnel management A control plane protocol may be required to setup/teardown tunnels, exchange tunnel state information, and/or provide for tunnel endpoint routing. This applies to both unicast and multicast tunnels. For instance, it may be necessary to provide active/standby tunnel status information between NVEs, up/down status information, pruning/grafting information for multicast tunnels, etc. Lasserre, et al. Expires September 12, 2012 [Page 15] Internet-Draft Framework for DC Network Virtualization March 2012 3.2. Service Overlay Topologies A number of service topologies may be used to optimize the service connectivity and to address NVE performance limitations. The topology described in Figure 3 suggests the use of a tunnel mesh between the NVEs where each tenant instance is one hop away from a service processing perspective. This should not be construed to imply that a tunnel mesh must be configured as tunneling can simply be encapsulation/decapsulation with a tunnel header. Partial mesh topologies and a NVE hierarchy may be used where certain NVEs may act as service transit points. 4. Key aspects of overlay networks The intent of this section is to highlight specific issues that proposed overlay solutions need to address. 4.1. Pros & Cons An overlay network is a layer of virtual network topology on top of the physical network. Overlay networks offer the following key advantages: o Unicast tunneling state management is handled at the edge of the network. Intermediate transport nodes are unaware of such state. Note that this is not often the case when multicast is enabled in the core network. o Tunnels are used to aggregate traffic and hence offer the advantage of minimizing the amount of forwarding state required within the underlay network. o Decoupling of the overlay addresses (MAC and IP) used by VMs or Tenant End Systems in general from the underlay network. This offers a clear separation between addresses used within the overlay and the underlay networks and it enables the use of overlapping addresses spaces by Tenant End Systems. o Support of a large number of virtual network identifiers. Overlay networks also create several challenges: o Overlay networks have no controls of underlay networks and lack critical network information Lasserre, et al. Expires September 12, 2012 [Page 16] Internet-Draft Framework for DC Network Virtualization March 2012 o Overlays may probe the network to measure link properties, such as available bandwidth or packet loss rate. It is difficult to accurately evaluate network properties. It might be preferable for the underlay network to expose usage and performance information for itself or the overlay networks. o Miscommunication between overlay and underlay networks can lead to an inefficient usage of network resources. o Fairness of resource sharing and co-ordination among edge-nodes in overlay networks are two critical issues. When multiple overlays co-exist on top of a common underlay network, the lack of coordination between overlays can lead to performance issues. o Overlaid traffic may not traverse firewalls and NAT devices. o Multicast service scalability. Multicast support may be required in the overlay network to address for each tenant flood containment or efficient multicast handling. o Load balancing may not be optimal as the hash algorithm may not work well due to the limited number of combinations of tunnel source and destination addresses 4.2. Overlay issues to consider 4.2.1. Data plane vs Control plane driven Dynamic (data plane) learning implies that flooding of unknown destinations be supported and hence implies that broadcast and/or multicast be supported. Multicasting in the core network for dynamic learning can lead to significant scalability limitations. Specific forwarding rules must be enforced to prevent loops from happening. This can be achieved using a spanning tree protocol or a shortest path tree, or using split-horizon forwarding rules. It should be noted that the amount of state to be distributed is a function of the number of virtual machines. Different forms of caching can also be utilized to minimize state distribution among the various elements. Lasserre, et al. Expires September 12, 2012 [Page 17] Internet-Draft Framework for DC Network Virtualization March 2012 4.2.2. Coordination between data plane and control plane Often a combination of dynamic data plane and control based learning is necessary. MAC Data-plane learning or IP data plane learning can be applied on tenant VAPs at the NVE whereas control plane-based MAC and IP reachability distribution can be performed across the overlay network among the NVEs, possibly with the help of a control plane mediation device (e.g., BGP route reflector if BGP is used to distribute such information). Coordination between the data-plane learning process and the control plane reachability distribution process is needed such that when a new address gets learned or an old address is removed, it triggers the local control plane to advertise this information to its peers. 4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM) traffic There are two techniques to support packet replication needed for broadcast, unknown unicast and multicast: o Ingress replication o Use of core multicast trees There is a bandwidth vs state trade-off between the two approaches. Depending upon the degree of replication required (i.e. the number of hosts per group) and the amount of multicast state to maintain, trading bandwidth for state is of consideration. When the number of hosts per group is large, the use of core multicast trees may be more appropriate. When the number of hosts is small (e.g. 2-3), ingress replication may not be an issue depending on multicast stream bandwidth. Depending upon the size of the data center network and hence the number of (S,G) entries, but also the duration of multicast flows, the use of core multicast trees can be a challenge. When flows are well known, it is possible to pre-provision such multicast trees. However, it is often difficult to predict application flows ahead of time, and hence programming of (S,G) entries for short-lived flows could be impractical. A possible trade-off is to use shared multicast trees in the core as opposed to dedicated multicast trees. Lasserre, et al. Expires September 12, 2012 [Page 18] Internet-Draft Framework for DC Network Virtualization March 2012 4.2.4. Path MTU When using overlay tunneling, an outer header is added to the original tenant frame. This can cause the MTU of the path to the egress tunnel endpoint to be exceeded. In this section, we will only consider the case of an IP overlay. It is usually not desirable to rely on IP fragmentation for performance reasons. Ideally, the interface MTU as seen by a Tenant End System is adjusted such that no fragmentation is needed. TCP will adjust its maximum segment size accordingly. It is possible for the MTU to be configured manually or to be discovered dynamically. Various Path MTU discovery techniques exist in order to determine the proper MTU size to use: o Classical ICMP-based MTU Path Discovery [RFC1191] [RFC1981] o Tenant End Systems rely on ICMP messages to discover the MTU of the end-to-end path to its destination. This method is not always possible, such as when traversing middle boxes (e.g. firewalls) which disable ICMP for security reasons o Extended MTU Path Discovery techniques such as defined in [RFC4821] It is also possible to rely on the overlay layer to perform segmentation and reassembly operations without relying on the Tenant End Systems to know about the end-to-end MTU. The assumption is that some hardware assist is available on the NVE node to perform such fragmentation and reassembly operations. However, fragmentation by the overlay layer can lead to performance and congestion issues due to TCP dynamics and might require new congestion avoidance mechanisms from the underlay network [FLOYD]. Finally, the underlay network may be designed in such a way that the MTU can accommodate the extra tunnel overhead. 4.2.5. NVE location trade-offs In the case of DC traffic, traffic originated from a VM is native Ethernet traffic. This traffic may be receiving ELAN service or IP service. In the case of ELAN service, it can be switched by a local Lasserre, et al. Expires September 12, 2012 [Page 19] Internet-Draft Framework for DC Network Virtualization March 2012 VM switch or ToR switch and then by a DC gateway. The NVE function can be embedded within any of these elements. There are several criteria to consider when deciding where the NVE processing boundary happens: o Processing and memory requirements o Datapath (e.g. FIB size, lookups, filtering, encapsulation/decapsulation) o Control plane (e.g. RIB size, routing, signaling, OAM) o Multicast support o Routing protocols o Packet replication capability o Fragmentation support o QoS transparency o Resiliency 4.2.6. Interaction between network overlays and underlays When multiple overlays co-exist on top of a common underlay network, this can cause some performance issues. These overlays have partially overlapping paths and nodes. Each overlay is selfish by nature in that it sends traffic so as to optimize its own performance without considering the impact on other overlays, unless the underlay tunnels are traffic engineered on a per overlay basis so as to avoid oversubscribing underlay resources. Better visibility between overlays and underlays or their controllers can be achieved by providing mechanisms to exchange information about: o Performance metrics (throughput, delay, loss, jitter) o Cost metrics Lasserre, et al. Expires September 12, 2012 [Page 20] Internet-Draft Framework for DC Network Virtualization March 2012 This information may then be used to traffic engineer the underlay network and/or traffic engineer the overlay networks in a coordinated fashion over the overlay. 5. Security Considerations The tenant to overlay mapping function can introduce significant security risks if appropriate protocols/mechanisms used to establish that mapping are not trusted, do not support mutual authentication and/or cannot be established over trusted interfaces and/or mutually authenticated connections. No other new security issues are introduced beyond those described already in the related L2VPN and L3VPN RFCs. 6. IANA Considerations IANA does not need to take any action for this draft. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 7.2. Informative References [NVOPS] Narten, T. et al, "Problem Statement : Overlays for Network Virtualization", draft-narten-nvo3-overlay-problem- statement (work in progress) [OVCPREQ] Kreeger, L. et al, "Network Virtualization Overlay Control Protocol Requirements", draft-kreeger-nvo3-overlay-cp (work in progress) [FLOYD] Sally Floyd, Allyn Romanow, "Dynamics of TCP Traffic over ATM Networks", IEEE JSAC, V. 13 N. 4, May 1995 [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, February 2006. Lasserre, et al. Expires September 12, 2012 [Page 21] Internet-Draft Framework for DC Network Virtualization March 2012 [RFC1191] Mogul, J. "Path MTU Discovery", RFC1191, November 1990 [RFC1981] McCann, J. et al, "Path MTU Discovery for IPv6", RFC1981, August 1996 [RFC4821] Mathis, M. et al, "Packetization Layer Path MTU Discovery", RFC4821, March 2007 8. Acknowledgments In addition to the authors the following people have contributed to this document: Dimitrios Stiliadis, Rotem Salomonovitch, Alcatel-Lucent Javier Benitez, Colt This document was prepared using 2-Word-v2.0.template.dot. Lasserre, et al. Expires September 12, 2012 [Page 22] Internet-Draft Framework for DC Network Virtualization March 2012 Authors' Addresses Marc Lasserre Alcatel-Lucent Email: marc.lasserre@alcatel-lucent.com Florin Balus Alcatel-Lucent 777 E. Middlefield Road Mountain View, CA, USA 94043 Email: florin.balus@alcatel-lucent.com Thomas Morin France Telecom Orange Email: thomas.morin@orange.com Nabil Bitar Verizon 60 Sylvan Road Waltham, MA 02145 Email: nabil.n.bitar@verizon.com Yakov Rekhter Juniper Email: yakov@juniper.net Yuichi Ikejiri NTT Communications 1-1-6, Uchisaiwai-cho, Chiyoda-ku Tokyo, 100-8019 Japan Email: y.ikejiri@ntt.com Lasserre, et al. Expires September 12, 2012 [Page 23]