Internet Engineering Task Force (IETF) D. Black Request for Comments: 8014 Dell EMC Category: Informational J. Hudson ISSN: 2070-1721 L. Kreeger M. Lasserre Independent T. Narten IBM December 2016 An Architecture for Data-Center Network Virtualization over Layer 3 (NVO3) Abstract This document presents a high-level overview architecture for building data-center Network Virtualization over Layer 3 (NVO3) networks. The architecture is given at a high level, showing the major components of an overall system. An important goal is to divide the space into individual smaller components that can be implemented independently with clear inter-component interfaces and interactions. It should be possible to build and implement individual components in isolation and have them interoperate with other independently implemented components. That way, implementers have flexibility in implementing individual components and can optimize and innovate within their respective components without requiring changes to other components. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8014.
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Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. VN Service (L2 and L3) . . . . . . . . . . . . . . . . . 7 3.1.1. VLAN Tags in L2 Service . . . . . . . . . . . . . . . 8 3.1.2. Packet Lifetime Considerations . . . . . . . . . . . 8 3.2. Network Virtualization Edge (NVE) Background . . . . . . 9 3.3. Network Virtualization Authority (NVA) Background . . . . 10 3.4. VM Orchestration Systems . . . . . . . . . . . . . . . . 11 4. Network Virtualization Edge (NVE) . . . . . . . . . . . . . . 12 4.1. NVE Co-located with Server Hypervisor . . . . . . . . . . 12 4.2. Split-NVE . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2.1. Tenant VLAN Handling in Split-NVE Case . . . . . . . 14 4.3. NVE State . . . . . . . . . . . . . . . . . . . . . . . . 14 4.4. Multihoming of NVEs . . . . . . . . . . . . . . . . . . . 15 4.5. Virtual Access Point (VAP) . . . . . . . . . . . . . . . 16 5. Tenant System Types . . . . . . . . . . . . . . . . . . . . . 16 5.1. Overlay-Aware Network Service Appliances . . . . . . . . 16 5.2. Bare Metal Servers . . . . . . . . . . . . . . . . . . . 17 5.3. Gateways . . . . . . . . . . . . . . . . . . . . . . . . 17 5.3.1. Gateway Taxonomy . . . . . . . . . . . . . . . . . . 18 184.108.40.206. L2 Gateways (Bridging) . . . . . . . . . . . . . 18 220.127.116.11. L3 Gateways (Only IP Packets) . . . . . . . . . . 18 5.4. Distributed Inter-VN Gateways . . . . . . . . . . . . . . 19 5.5. ARP and Neighbor Discovery . . . . . . . . . . . . . . . 20 6. NVE-NVE Interaction . . . . . . . . . . . . . . . . . . . . . 20 7. Network Virtualization Authority (NVA) . . . . . . . . . . . 21 7.1. How an NVA Obtains Information . . . . . . . . . . . . . 21 7.2. Internal NVA Architecture . . . . . . . . . . . . . . . . 22 7.3. NVA External Interface . . . . . . . . . . . . . . . . . 22 8. NVE-NVA Protocol . . . . . . . . . . . . . . . . . . . . . . 24 8.1. NVE-NVA Interaction Models . . . . . . . . . . . . . . . 24 8.2. Direct NVE-NVA Protocol . . . . . . . . . . . . . . . . . 25 8.3. Propagating Information Between NVEs and NVAs . . . . . . 25 9. Federated NVAs . . . . . . . . . . . . . . . . . . . . . . . 26 9.1. Inter-NVA Peering . . . . . . . . . . . . . . . . . . . . 29 10. Control Protocol Work Areas . . . . . . . . . . . . . . . . . 29 11. NVO3 Data-Plane Encapsulation . . . . . . . . . . . . . . . . 29 12. Operations, Administration, and Maintenance (OAM) . . . . . . 30 13. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 14. Security Considerations . . . . . . . . . . . . . . . . . . . 31 15. Informative References . . . . . . . . . . . . . . . . . . . 32 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 34 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction This document presents a high-level architecture for building data- center Network Virtualization over Layer 3 (NVO3) networks. The architecture is given at a high level, which shows the major components of an overall system. An important goal is to divide the space into smaller individual components that can be implemented independently with clear inter-component interfaces and interactions. It should be possible to build and implement individual components in isolation and have them interoperate with other independently implemented components. That way, implementers have flexibility in implementing individual components and can optimize and innovate within their respective components without requiring changes to other components. The motivation for overlay networks is given in "Problem Statement: Overlays for Network Virtualization" [RFC7364]. "Framework for Data Center (DC) Network Virtualization" [RFC7365] provides a framework for discussing overlay networks generally and the various components that must work together in building such systems. This document differs from the framework document in that it doesn't attempt to cover all possible approaches within the general design space. Rather, it describes one particular approach that the NVO3 WG has focused on. 2. Terminology This document uses the same terminology as [RFC7365]. In addition, the following terms are used: NV Domain: A Network Virtualization Domain is an administrative construct that defines a Network Virtualization Authority (NVA), the set of Network Virtualization Edges (NVEs) associated with that NVA, and the set of virtual networks the NVA manages and supports. NVEs are associated with a (logically centralized) NVA, and an NVE supports communication for any of the virtual networks in the domain. NV Region: A region over which information about a set of virtual networks is shared. The degenerate case of a single NV Domain corresponds to an NV Region corresponding to that domain. The more interesting case occurs when two or more NV Domains share information about part or all of a set of virtual networks that they manage. Two NVAs share information about particular virtual networks for the purpose of supporting connectivity between tenants located in different NV Domains. NVAs can share information about an entire NV Domain, or just individual virtual networks.
Tenant System Interface (TSI): The interface to a Virtual Network (VN) as presented to a Tenant System (TS, see [RFC7365]). The TSI logically connects to the NVE via a Virtual Access Point (VAP). To the Tenant System, the TSI is like a Network Interface Card (NIC); the TSI presents itself to a Tenant System as a normal network interface. VLAN: Unless stated otherwise, the terms "VLAN" and "VLAN Tag" are used in this document to denote a Customer VLAN (C-VLAN) [IEEE.802.1Q]; the terms are used interchangeably to improve readability. 3. Background Overlay networks are an approach for providing network virtualization services to a set of Tenant Systems (TSs) [RFC7365]. With overlays, data traffic between tenants is tunneled across the underlying data center's IP network. The use of tunnels provides a number of benefits by decoupling the network as viewed by tenants from the underlying physical network across which they communicate. Additional discussion of some NVO3 use cases can be found in [USECASES]. Tenant Systems connect to Virtual Networks (VNs), with each VN having associated attributes defining properties of the network (such as the set of members that connect to it). Tenant Systems connected to a virtual network typically communicate freely with other Tenant Systems on the same VN, but communication between Tenant Systems on one VN and those external to the VN (whether on another VN or connected to the Internet) is carefully controlled and governed by policy. The NVO3 architecture does not impose any restrictions to the application of policy controls even within a VN. A Network Virtualization Edge (NVE) [RFC7365] is the entity that implements the overlay functionality. An NVE resides at the boundary between a Tenant System and the overlay network as shown in Figure 1. An NVE creates and maintains local state about each VN for which it is providing service on behalf of a Tenant System.
+--------+ +--------+ | Tenant +--+ +----| Tenant | | System | | (') | System | +--------+ | ................ ( ) +--------+ | +-+--+ . . +--+-+ (_) | | NVE|--. .--| NVE| | +--| | . . | |---+ +-+--+ . . +--+-+ / . . / . L3 Overlay . +--+-++--------+ +--------+ / . Network . | NVE|| Tenant | | Tenant +--+ . .- -| || System | | System | . . +--+-++--------+ +--------+ ................ | +----+ | NVE| | | +----+ | | ===================== | | +--------+ +--------+ | Tenant | | Tenant | | System | | System | +--------+ +--------+ Figure 1: NVO3 Generic Reference Model The following subsections describe key aspects of an overlay system in more detail. Section 3.1 describes the service model (Ethernet vs. IP) provided to Tenant Systems. Section 3.2 describes NVEs in more detail. Section 3.3 introduces the Network Virtualization Authority, from which NVEs obtain information about virtual networks. Section 3.4 provides background on Virtual Machine (VM) orchestration systems and their use of virtual networks.
3.1. VN Service (L2 and L3) A VN provides either Layer 2 (L2) or Layer 3 (L3) service to connected tenants. For L2 service, VNs transport Ethernet frames, and a Tenant System is provided with a service that is analogous to being connected to a specific L2 C-VLAN. L2 broadcast frames are generally delivered to all (and multicast frames delivered to a subset of) the other Tenant Systems on the VN. To a Tenant System, it appears as if they are connected to a regular L2 Ethernet link. Within the NVO3 architecture, tenant frames are tunneled to remote NVEs based on the Media Access Control (MAC) addresses of the frame headers as originated by the Tenant System. On the underlay, NVO3 packets are forwarded between NVEs based on the outer addresses of tunneled packets. For L3 service, VNs are routed networks that transport IP datagrams, and a Tenant System is provided with a service that supports only IP traffic. Within the NVO3 architecture, tenant frames are tunneled to remote NVEs based on the IP addresses of the packet originated by the Tenant System; any L2 destination addresses provided by Tenant Systems are effectively ignored by the NVEs and overlay network. For L3 service, the Tenant System will be configured with an IP subnet that is effectively a point-to-point link, i.e., having only the Tenant System and a next-hop router address on it. L2 service is intended for systems that need native L2 Ethernet service and the ability to run protocols directly over Ethernet (i.e., not based on IP). L3 service is intended for systems in which all the traffic can safely be assumed to be IP. It is important to note that whether or not an NVO3 network provides L2 or L3 service to a Tenant System, the Tenant System does not generally need to be aware of the distinction. In both cases, the virtual network presents itself to the Tenant System as an L2 Ethernet interface. An Ethernet interface is used in both cases simply as a widely supported interface type that essentially all Tenant Systems already support. Consequently, no special software is needed on Tenant Systems to use an L3 vs. an L2 overlay service. NVO3 can also provide a combined L2 and L3 service to tenants. A combined service provides L2 service for intra-VN communication but also provides L3 service for L3 traffic entering or leaving the VN. Architecturally, the handling of a combined L2/L3 service within the NVO3 architecture is intended to match what is commonly done today in non-overlay environments by devices providing a combined bridge/ router service. With combined service, the virtual network itself retains the semantics of L2 service, and all traffic is processed according to its L2 semantics. In addition, however, traffic requiring IP processing is also processed at the IP level.
The IP processing for a combined service can be implemented on a standalone device attached to the virtual network (e.g., an IP router) or implemented locally on the NVE (see Section 5.4 on Distributed Inter-VN Gateways). For unicast traffic, NVE implementation of a combined service may result in a packet being delivered to another Tenant System attached to the same NVE (on either the same or a different VN), tunneled to a remote NVE, or even forwarded outside the NV Domain. For multicast or broadcast packets, the combination of NVE L2 and L3 processing may result in copies of the packet receiving both L2 and L3 treatments to realize delivery to all of the destinations involved. This distributed NVE implementation of IP routing results in the same network delivery behavior as if the L2 processing of the packet included delivery of the packet to an IP router attached to the L2 VN as a Tenant System, with the router having additional network attachments to other networks, either virtual or not. 3.1.1. VLAN Tags in L2 Service An NVO3 L2 virtual network service may include encapsulated L2 VLAN tags provided by a Tenant System but does not use encapsulated tags in deciding where and how to forward traffic. Such VLAN tags can be passed through so that Tenant Systems that send or expect to receive them can be supported as appropriate. The processing of VLAN tags that an NVE receives from a TS is controlled by settings associated with the VAP. Just as in the case with ports on Ethernet switches, a number of settings are possible. For example, Customer VLAN Tags (C-TAGs) can be passed through transparently, could always be stripped upon receipt from a Tenant System, could be compared against a list of explicitly configured tags, etc. Note that there are additional considerations when VLAN tags are used to identify both the VN and a Tenant System VLAN within that VN, as described in Section 4.2.1. 3.1.2. Packet Lifetime Considerations For L3 service, Tenant Systems should expect the IPv4 Time to Live (TTL) or IPv6 Hop Limit in the packets they send to be decremented by at least 1. For L2 service, neither the TTL nor the Hop Limit (when the packet is IP) is modified. The underlay network manages TTLs and Hop Limits in the outer IP encapsulation -- the values in these fields could be independent from or related to the values in the same fields of tenant IP packets.
3.2. Network Virtualization Edge (NVE) Background Tenant Systems connect to NVEs via a Tenant System Interface (TSI). The TSI logically connects to the NVE via a Virtual Access Point (VAP), and each VAP is associated with one VN as shown in Figure 2. To the Tenant System, the TSI is like a NIC; the TSI presents itself to a Tenant System as a normal network interface. On the NVE side, a VAP is a logical network port (virtual or physical) into a specific virtual network. Note that two different Tenant Systems (and TSIs) attached to a common NVE can share a VAP (e.g., TS1 and TS2 in Figure 2) so long as they connect to the same VN. | Data-Center Network (IP) | | | +-----------------------------------------+ | | | Tunnel Overlay | +------------+---------+ +---------+------------+ | +----------+-------+ | | +-------+----------+ | | | Overlay Module | | | | Overlay Module | | | +---------+--------+ | | +---------+--------+ | | | | | | | NVE1 | | | | | | NVE2 | +--------+-------+ | | +--------+-------+ | | | VNI1 VNI2 | | | | VNI1 VNI2 | | | +-+----------+---+ | | +-+-----------+--+ | | | VAP1 | VAP2 | | | VAP1 | VAP2| +----+----------+------+ +----+-----------+-----+ | | | | |\ | | | | \ | | /| -------+--\-------+-------------------+---------/-+------- | \ | Tenant | / | TSI1 |TSI2\ | TSI3 TSI1 TSI2/ TSI3 +---+ +---+ +---+ +---+ +---+ +---+ |TS1| |TS2| |TS3| |TS4| |TS5| |TS6| +---+ +---+ +---+ +---+ +---+ +---+ Figure 2: NVE Reference Model The Overlay Module performs the actual encapsulation and decapsulation of tunneled packets. The NVE maintains state about the virtual networks it is a part of so that it can provide the Overlay Module with information such as the destination address of the NVE to tunnel a packet to and the Context ID that should be placed in the encapsulation header to identify the virtual network that a tunneled packet belongs to.
On the side facing the data-center network, the NVE sends and receives native IP traffic. When ingressing traffic from a Tenant System, the NVE identifies the egress NVE to which the packet should be sent, adds an overlay encapsulation header, and sends the packet on the underlay network. When receiving traffic from a remote NVE, an NVE strips off the encapsulation header and delivers the (original) packet to the appropriate Tenant System. When the source and destination Tenant System are on the same NVE, no encapsulation is needed and the NVE forwards traffic directly. Conceptually, the NVE is a single entity implementing the NVO3 functionality. In practice, there are a number of different implementation scenarios, as described in detail in Section 4. 3.3. Network Virtualization Authority (NVA) Background Address dissemination refers to the process of learning, building, and distributing the mapping/forwarding information that NVEs need in order to tunnel traffic to each other on behalf of communicating Tenant Systems. For example, in order to send traffic to a remote Tenant System, the sending NVE must know the destination NVE for that Tenant System. One way to build and maintain mapping tables is to use learning, as 802.1 bridges do [IEEE.802.1Q]. When forwarding traffic to multicast or unknown unicast destinations, an NVE could simply flood traffic. While flooding works, it can lead to traffic hot spots and to problems in larger networks (e.g., excessive amounts of flooded traffic). Alternatively, to reduce the scope of where flooding must take place, or to eliminate it all together, NVEs can make use of a Network Virtualization Authority (NVA). An NVA is the entity that provides address mapping and other information to NVEs. NVEs interact with an NVA to obtain any required address-mapping information they need in order to properly forward traffic on behalf of tenants. The term "NVA" refers to the overall system, without regard to its scope or how it is implemented. NVAs provide a service, and NVEs access that service via an NVE-NVA protocol as discussed in Section 8. Even when an NVA is present, Ethernet bridge MAC address learning could be used as a fallback mechanism, should the NVA be unable to provide an answer or for other reasons. This document does not consider flooding approaches in detail, as there are a number of benefits in using an approach that depends on the presence of an NVA. For the rest of this document, it is assumed that an NVA exists and will be used. NVAs are discussed in more detail in Section 7.
3.4. VM Orchestration Systems VM orchestration systems manage server virtualization across a set of servers. Although VM management is a separate topic from network virtualization, the two areas are closely related. Managing the creation, placement, and movement of VMs also involves creating, attaching to, and detaching from virtual networks. A number of existing VM orchestration systems have incorporated aspects of virtual network management into their systems. Note also that although this section uses the terms "VM" and "hypervisor" throughout, the same issues apply to other virtualization approaches, including Linux Containers (LXC), BSD Jails, Network Service Appliances as discussed in Section 5.1, etc. From an NVO3 perspective, it should be assumed that where the document uses the term "VM" and "hypervisor", the intention is that the discussion also applies to other systems, where, e.g., the host operating system plays the role of the hypervisor in supporting virtualization, and a container plays the equivalent role as a VM. When a new VM image is started, the VM orchestration system determines where the VM should be placed, interacts with the hypervisor on the target server to load and start the VM, and controls when a VM should be shut down or migrated elsewhere. VM orchestration systems also have knowledge about how a VM should connect to a network, possibly including the name of the virtual network to which a VM is to connect. The VM orchestration system can pass such information to the hypervisor when a VM is instantiated. VM orchestration systems have significant (and sometimes global) knowledge over the domain they manage. They typically know on what servers a VM is running, and metadata associated with VM images can be useful from a network virtualization perspective. For example, the metadata may include the addresses (MAC and IP) the VMs will use and the name(s) of the virtual network(s) they connect to. VM orchestration systems run a protocol with an agent running on the hypervisor of the servers they manage. That protocol can also carry information about what virtual network a VM is associated with. When the orchestrator instantiates a VM on a hypervisor, the hypervisor interacts with the NVE in order to attach the VM to the virtual networks it has access to. In general, the hypervisor will need to communicate significant VM state changes to the NVE. In the reverse direction, the NVE may need to communicate network connectivity information back to the hypervisor. Examples of deployed VM orchestration systems include VMware's vCenter Server, Microsoft's System Center Virtual Machine Manager, and systems based on OpenStack and its associated plugins (e.g., Nova and Neutron). Each can pass information about what virtual networks a VM connects to down to the
hypervisor. The protocol used between the VM orchestration system and hypervisors is generally proprietary. It should be noted that VM orchestration systems may not have direct access to all networking-related information a VM uses. For example, a VM may make use of additional IP or MAC addresses that the VM management system is not aware of. 4. Network Virtualization Edge (NVE) As introduced in Section 3.2, an NVE is the entity that implements the overlay functionality. This section describes NVEs in more detail. An NVE will have two external interfaces: Facing the Tenant System: On the side facing the Tenant System, an NVE interacts with the hypervisor (or equivalent entity) to provide the NVO3 service. An NVE will need to be notified when a Tenant System "attaches" to a virtual network (so it can validate the request and set up any state needed to send and receive traffic on behalf of the Tenant System on that VN). Likewise, an NVE will need to be informed when the Tenant System "detaches" from the virtual network so that it can reclaim state and resources appropriately. Facing the Data-Center Network: On the side facing the data-center network, an NVE interfaces with the data-center underlay network, sending and receiving tunneled packets to and from the underlay. The NVE may also run a control protocol with other entities on the network, such as the Network Virtualization Authority. 4.1. NVE Co-located with Server Hypervisor When server virtualization is used, the entire NVE functionality will typically be implemented as part of the hypervisor and/or virtual switch on the server. In such cases, the Tenant System interacts with the hypervisor, and the hypervisor interacts with the NVE. Because the interaction between the hypervisor and NVE is implemented entirely in software on the server, there is no "on-the-wire" protocol between Tenant Systems (or the hypervisor) and the NVE that needs to be standardized. While there may be APIs between the NVE and hypervisor to support necessary interaction, the details of such APIs are not in scope for the NVO3 WG at the time of publication of this memo. Implementing NVE functionality entirely on a server has the disadvantage that server CPU resources must be spent implementing the NVO3 functionality. Experimentation with overlay approaches and previous experience with TCP and checksum adapter offloads suggest
that offloading certain NVE operations (e.g., encapsulation and decapsulation operations) onto the physical network adapter can produce performance advantages. As has been done with checksum and/ or TCP server offload and other optimization approaches, there may be benefits to offloading common operations onto adapters where possible. Just as important, the addition of an overlay header can disable existing adapter offload capabilities that are generally not prepared to handle the addition of a new header or other operations associated with an NVE. While the exact details of how to split the implementation of specific NVE functionality between a server and its network adapters are an implementation matter and outside the scope of IETF standardization, the NVO3 architecture should be cognizant of and support such separation. Ideally, it may even be possible to bypass the hypervisor completely on critical data-path operations so that packets between a Tenant System and its VN can be sent and received without having the hypervisor involved in each individual packet operation. 4.2. Split-NVE Another possible scenario leads to the need for a split-NVE implementation. An NVE running on a server (e.g., within a hypervisor) could support NVO3 service towards the tenant but not perform all NVE functions (e.g., encapsulation) directly on the server; some of the actual NVO3 functionality could be implemented on (i.e., offloaded to) an adjacent switch to which the server is attached. While one could imagine a number of link types between a server and the NVE, one simple deployment scenario would involve a server and NVE separated by a simple L2 Ethernet link. A more complicated scenario would have the server and NVE separated by a bridged access network, such as when the NVE resides on a Top of Rack (ToR) switch, with an embedded switch residing between servers and the ToR switch. For the split-NVE case, protocols will be needed that allow the hypervisor and NVE to negotiate and set up the necessary state so that traffic sent across the access link between a server and the NVE can be associated with the correct virtual network instance. Specifically, on the access link, traffic belonging to a specific Tenant System would be tagged with a specific VLAN C-TAG that identifies which specific NVO3 virtual network instance it connects to. The hypervisor-NVE protocol would negotiate which VLAN C-TAG to use for a particular virtual network instance. More details of the protocol requirements for functionality between hypervisors and NVEs can be found in [NVE-NVA].
4.2.1. Tenant VLAN Handling in Split-NVE Case Preserving tenant VLAN tags across an NVO3 VN, as described in Section 3.1.1, poses additional complications in the split-NVE case. The portion of the NVE that performs the encapsulation function needs access to the specific VLAN tags that the Tenant System is using in order to include them in the encapsulated packet. When an NVE is implemented entirely within the hypervisor, the NVE has access to the complete original packet (including any VLAN tags) sent by the tenant. In the split-NVE case, however, the VLAN tag used between the hypervisor and offloaded portions of the NVE normally only identifies the specific VN that traffic belongs to. In order to allow a tenant to preserve VLAN information from end to end between Tenant Systems in the split-NVE case, additional mechanisms would be needed (e.g., carry an additional VLAN tag by carrying both a C-TAG and a Service VLAN Tag (S-TAG) as specified in [IEEE.802.1Q] where the C-TAG identifies the tenant VLAN end to end and the S-TAG identifies the VN locally between each Tenant System and the corresponding NVE). 4.3. NVE State NVEs maintain internal data structures and state to support the sending and receiving of tenant traffic. An NVE may need some or all of the following information: 1. An NVE keeps track of which attached Tenant Systems are connected to which virtual networks. When a Tenant System attaches to a virtual network, the NVE will need to create or update the local state for that virtual network. When the last Tenant System detaches from a given VN, the NVE can reclaim state associated with that VN. 2. For tenant unicast traffic, an NVE maintains a per-VN table of mappings from Tenant System (inner) addresses to remote NVE (outer) addresses. 3. For tenant multicast (or broadcast) traffic, an NVE maintains a per-VN table of mappings and other information on how to deliver tenant multicast (or broadcast) traffic. If the underlying network supports IP multicast, the NVE could use IP multicast to deliver tenant traffic. In such a case, the NVE would need to know what IP underlay multicast address to use for a given VN. Alternatively, if the underlying network does not support multicast, a source NVE could use unicast replication to deliver traffic. In such a case, an NVE would need to know which remote NVEs are participating in the VN. An NVE could use both approaches, switching from one mode to the other depending on
factors such as bandwidth efficiency and group membership sparseness. [FRAMEWORK-MCAST] discusses the subject of multicast handling in NVO3 in further detail. 4. An NVE maintains necessary information to encapsulate outgoing traffic, including what type of encapsulation and what value to use for a Context ID to identify the VN within the encapsulation header. 5. In order to deliver incoming encapsulated packets to the correct Tenant Systems, an NVE maintains the necessary information to map incoming traffic to the appropriate VAP (i.e., TSI). 6. An NVE may find it convenient to maintain additional per-VN information such as QoS settings, Path MTU information, Access Control Lists (ACLs), etc. 4.4. Multihoming of NVEs NVEs may be multihomed. That is, an NVE may have more than one IP address associated with it on the underlay network. Multihoming happens in two different scenarios. First, an NVE may have multiple interfaces connecting it to the underlay. Each of those interfaces will typically have a different IP address, resulting in a specific Tenant Address (on a specific VN) being reachable through the same NVE but through more than one underlay IP address. Second, a specific Tenant System may be reachable through more than one NVE, each having one or more underlay addresses. In both cases, NVE address-mapping functionality needs to support one-to-many mappings and enable a sending NVE to (at a minimum) be able to fail over from one IP address to another, e.g., should a specific NVE underlay address become unreachable. Finally, multihomed NVEs introduce complexities when source unicast replication is used to implement tenant multicast as described in Section 4.3. Specifically, an NVE should only receive one copy of a replicated packet. Multihoming is needed to support important use cases. First, a bare metal server may have multiple uplink connections to either the same or different NVEs. Having only a single physical path to an upstream NVE, or indeed, having all traffic flow through a single NVE would be considered unacceptable in highly resilient deployment scenarios that seek to avoid single points of failure. Moreover, in today's networks, the availability of multiple paths would require that they be usable in an active-active fashion (e.g., for load balancing).
4.5. Virtual Access Point (VAP) The VAP is the NVE side of the interface between the NVE and the TS. Traffic to and from the tenant flows through the VAP. If an NVE runs into difficulties sending traffic received on the VAP, it may need to signal such errors back to the VAP. Because the VAP is an emulation of a physical port, its ability to signal NVE errors is limited and lacks sufficient granularity to reflect all possible errors an NVE may encounter (e.g., inability to reach a particular destination). Some errors, such as an NVE losing all of its connections to the underlay, could be reflected back to the VAP by effectively disabling it. This state change would reflect itself on the TS as an interface going down, allowing the TS to implement interface error handling (e.g., failover) in the same manner as when a physical interface becomes disabled.