Geneve: Generic Network Virtualization Encapsulation

Networking has long featured a variety of tunneling, tagging, and other encapsulation mechanisms. However, the advent of network virtualization has caused a surge of renewed interest and a corresponding increase in the introduction of new protocols. The large number of protocols in this space, ranging all the way from VLANs and MPLS through the more recent VXLAN, NVGRE, and STT, often leads to questions about the need for new encapsulation formats and what it is about network virtualization in particular that leads to their proliferation. While many encapsulation protocols seek to simply partition the underlay network or bridge between two domains, network virtualization views the transit network as providing connectivity between multiple components of a distributed system. In many ways this system is similar to a chassis switch with the IP underlay network playing the role of the backplane and tunnel endpoints on the edge as line cards. When viewed in this light, the requirements placed on the tunnel protocol are significantly different in terms of the quantity of metadata necessary and the role of transit nodes. Current work such as and the NVO3 working group have described some of the properties that the data plane must have to support network virtualization. However, one additional defining requirement is the need to carry system state along with the packet data. The use of some metadata is certainly not a foreign concept - nearly all protocols used for virtualization have at least 24 bits of identifier space as a way to partition between tenants. This is often described as overcoming the limits of 12-bit VLANs, and when seen in that context, or any context where it is a true tenant identifier, 16 million possible entries is a large number. However, the reality is that the metadata is not exclusively used to identify tenants and encoding other information quickly starts to crowd the space. In fact, when compared to the tags used to exchange metadata between line cards on a chassis switch, 24-bit identifiers start to look quite small. There are nearly endless uses for this metadata, ranging from storing input ports for simple security policies to service based context for interposing advanced middleboxes. Existing tunnel protocols have each attempted to solve different aspects of these new requirements, only to be quickly rendered out of date by changing control plane implementations and advancements. Furthermore, software and hardware components and controllers all have different advantages and rates of evolution - a fact that should be viewed as a benefit, not a liability or limitation. This draft describes Geneve, a protocol which seeks to avoid these problems by providing a framework for tunneling for network virtualization rather than being prescriptive about the entire system.

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 . 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.

The NVO3 framework defines many of the concepts commonly used in network virtualization. In addition, the following terms are specifically meaningful in this document: Checksum offload. An optimization implemented by many NICs which enables computation and verification of upper layer protocol checksums in hardware on transmit and receive, respectively. This typically includes IP and TCP/UDP checksums which would otherwise be computed by the protocol stack in software. Clos network. A technique for composing network fabrics larger than a single switch while maintaining non-blocking bandwidth across connection points. ECMP is used to divide traffic across the multiple links and switches that constitute the fabric. Sometimes termed "leaf and spine" or "fat tree" topologies. ECMP. Equal Cost Multipath. A routing mechanism for selecting from among multiple best next hop paths by hashing packet headers in order to better utilize network bandwidth while avoiding reordering a single stream. Geneve. Generic Network Virtualization Encapsulation. The tunnel protocol described in this draft. LRO. Large Receive Offload. The receive-side equivalent function of LSO, in which multiple protocol segments (primarily TCP) are coalesced into larger data units. NIC. Network Interface Card. A NIC could be part of a tunnel endpoint or transit device and can either process Geneve packets or aid in the processing of Geneve packets. OAM. Operations, Administration, and Management. A suite of tools used to monitor and troubleshoot network problems. Transit device. A forwarding element along the path of the tunnel making up part of the Underlay Network. A transit device MAY be capable of understanding the Geneve packet format but does not originate or terminate Geneve packets. LSO. Large Segmentation Offload. A function provided by many commercial NICs that allows data units larger than the MTU to be passed to the NIC to improve performance, the NIC being responsible for creating smaller segments of size less than or equal to the MTU with correct protocol headers. When referring specifically to TCP/IP, this feature is often known as TSO (TCP Segmentation Offload). Tunnel endpoint. A component performing encapsulation and decapsulation of packets, such as Ethernet frames or IP datagrams, in Geneve headers. As the ultimate consumer of any tunnel metadata, endpoints have the highest level of requirements for parsing and interpreting tunnel headers. Tunnel endpoints may consist of either software or hardware implementations or a combination of the two. Endpoints are frequently a component of an NVE but may also be found in middleboxes or other elements making up an NVO3 Network. VM. Virtual Machine.

Geneve is designed to support network virtualization use cases, where tunnels are typically established to act as a backplane between the virtual switches residing in hypervisors, physical switches, or middleboxes or other appliances. An arbitrary IP network can be used as an underlay although Clos networks composed using ECMP links are a common choice to provide consistent bisectional bandwidth across all connection points. shows an example of a hypervisor, top of rack switch for connectivity to physical servers, and a WAN uplink connected using Geneve tunnels over a simplified Clos network. These tunnels are used to encapsulate and forward frames from the attached components such as VMs or physical links.

+---------------------+ +-------+ +------+ Hypervisor ()===================================() Switch-Switch Geneve Tunnels]]> To support the needs of network virtualization, the tunnel protocol should be able to take advantage of the differing (and evolving) capabilities of each type of device in both the underlay and overlay networks. This results in the following requirements being placed on the data plane tunneling protocol: The data plane is generic and extensible enough to support current and future control planes. Tunnel components are efficiently implementable in both hardware and software without restricting capabilities to the lowest common denominator. High performance over existing IP fabrics. These requirements are described further in the following subsections.

Although some protocols for network virtualization have included a control plane as part of the tunnel format specification (most notably, the original VXLAN spec prescribed a multicast learning-based control plane), these specifications have largely been treated as describing only the data format. The VXLAN packet format has actually seen a wide variety of control planes built on top of it. There is a clear advantage in settling on a data format: most of the protocols are only superficially different and there is little advantage in duplicating effort. However, the same cannot be said of control planes, which are diverse in very fundamental ways. The case for standardization is also less clear given the wide variety in requirements, goals, and deployment scenarios. As a result of this reality, Geneve aims to be a pure tunnel format specification that is capable of fulfilling the needs of many control planes by explicitly not selecting any one of them. This simultaneously promotes a shared data format and increases the chances that it will not be obsoleted by future control plane enhancements.

Achieving the level of flexibility needed to support current and future control planes effectively requires an options infrastructure to allow new metadata types to be defined, deployed, and either finalized or retired. Options also allow for differentiation of products by encouraging independent development in each vendor's core specialty, leading to an overall faster pace of advancement. By far the most common mechanism for implementing options is Type-Length-Value (TLV) format. It should be noted that while options can be used to support non-wirespeed control packets, they are equally important on data packets as well to segregate and direct forwarding (for instance, the examples given before of input port based security policies and service interposition both require tags to be placed on data packets). Therefore, while it would be desirable to limit the extensibility to only control packets for the purposes of simplifying the datapath, that would not satisfy the design requirements.

There is often a conflict between software flexibility and hardware performance that is difficult to resolve. For a given set of functionality, it is obviously desirable to maximize performance. However, that does not mean new features that cannot be run at that speed today should be disallowed. Therefore, for a protocol to be efficiently implementable means that a set of common capabilities can be reasonably handled across platforms along with a graceful mechanism to handle more advanced features in the appropriate situations. The use of a variable length header and options in a protocol often raises questions about whether it is truly efficiently implementable in hardware. To answer this question in the context of Geneve, it is important to first divide "hardware" into two categories: tunnel endpoints and transit devices. Endpoints must be able to parse the variable header, including any options, and take action. Since these devices are actively participating in the protocol, they are the most affected by Geneve. However, as endpoints are the ultimate consumers of the data, transmitters can tailor their output to the capabilities of the recipient. As new functionality becomes sufficiently well defined to add to endpoints, supporting options can be designed using ordering restrictions and other techniques to ease parsing. Transit devices MAY be able to interpret the options and participate in Geneve packet processing. However, as non-terminating devices, they do not originate or terminate the Geneve packet. The participation of transit devices in Geneve packet processing is OPTIONAL. Further, either tunnel endpoints or transit devices MAY use offload capabilities of NICs such as checksum offload to improve the performance of Geneve packet processing. The presence of a Geneve variable length header SHOULD NOT prevent the tunnel endpoints and transit devices from using such offload capabilities.

IP has clearly cemented its place as the dominant transport mechanism and many techniques have evolved over time to make it robust, efficient, and inexpensive. As a result, it is natural to use IP fabrics as a transit network for Geneve. Fortunately, the use of IP encapsulation and addressing is enough to achieve the primary goal of delivering packets to the correct point in the network through standard switching and routing. In addition, nearly all underlay fabrics are designed to exploit parallelism in traffic to spread load across multiple links without introducing reordering in individual flows. These equal cost multipathing (ECMP) techniques typically involve parsing and hashing the addresses and port numbers from the packet to select an outgoing link. However, the use of tunnels often results in poor ECMP performance without additional knowledge of the protocol as the encapsulated traffic is hidden from the fabric by design and only endpoint addresses are available for hashing. Since it is desirable for Geneve to perform well on these existing fabrics, it is necessary for entropy from encapsulated packets to be exposed in the tunnel header. The most common technique for this is to use the UDP source port, which is discussed further in .

The Geneve packet format consists of a compact tunnel header encapsulated in UDP over either IPv4 or IPv6. A small fixed tunnel header provides control information plus a base level of functionality and interoperability with a focus on simplicity. This header is then followed by a set of variable options to allow for future innovation. Finally, the payload consists of a protocol data unit of the indicated type, such as an Ethernet frame. and illustrate the Geneve packet format transported (for example) over Ethernet along with an Ethernet payload.

The use of an encapsulating UDP header follows the connectionless semantics of Ethernet and IP in addition to providing entropy to routers performing ECMP. The header fields are therefore interpreted as follows: A source port selected by the originating tunnel endpoint. This source port SHOULD be the same for all packets belonging to a single encapsulated flow to prevent reordering due to the use of different paths. To encourage an even distribution of flows across multiple links, the source port SHOULD be calculated using a hash of the encapsulated packet headers using, for example, a traditional 5-tuple. Since the port represents a flow identifier rather than a true UDP connection, the entire 16-bit range MAY be used to maximize entropy. IANA has assigned port 6081 as the fixed well-known destination port for Geneve. Although the well-known value should be used by default, it is RECOMMENDED that implementations make this configurable. The chosen port is used for identification of Geneve packets and MUST NOT be reversed for different ends of a connection as is done with TCP. The length of the UDP packet including the UDP header. The checksum MAY be set to zero on transmit for packets encapsulated in both IPv4 and IPv6 . When a packet is received with a UDP checksum of zero it MUST be accepted and decapsulated. If the originating tunnel endpoint optionally encapsulates a packet with a non-zero checksum, it MUST be a correctly computed UDP checksum. Upon receiving such a packet, the egress endpoint MUST validate the checksum. If the checksum is not correct, the packet MUST be dropped, otherwise the packet MUST be accepted for decapsulation. It is RECOMMENDED that the UDP checksum be computed to protect the Geneve header and options in situations where the network reliability is not high and the packet is not protected by another checksum or CRC.

The current version number is 0. Packets received by an endpoint with an unknown version MUST be dropped. Non-terminating devices processing Geneve packets with an unknown version number MUST treat them as UDP packets with an unknown payload. The length of the options fields, expressed in four byte multiples, not including the eight byte fixed tunnel header. This results in a minimum total Geneve header size of 8 bytes and a maximum of 260 bytes. The start of the payload headers can be found using this offset from the end of the base Geneve header. OAM packet. This packet contains a control message instead of a data payload. Endpoints MUST NOT forward the payload and transit devices MUST NOT attempt to interpret or process it. Since these are infrequent control messages, it is RECOMMENDED that endpoints direct these packets to a high priority control queue (for example, to direct the packet to a general purpose CPU from a forwarding ASIC or to separate out control traffic on a NIC). Transit devices MUST NOT alter forwarding behavior on the basis of this bit, such as ECMP link selection. Critical options present. One or more options has the critical bit set (see ). If this bit is set then tunnel endpoints MUST parse the options list to interpret any critical options. On endpoints where option parsing is not supported the packet MUST be dropped on the basis of the 'C' bit in the base header. If the bit is not set tunnel endpoints MAY strip all options using 'Opt Len' and forward the decapsulated packet. Transit devices MUST NOT drop or modify packets on the basis of this bit. Reserved field which MUST be zero on transmission and ignored on receipt. The type of the protocol data unit appearing after the Geneve header. This follows the EtherType convention with Ethernet itself being represented by the value 0x6558. An identifier for a unique element of a virtual network. In many situations this may represent an L2 segment, however, the control plane defines the forwarding semantics of decapsulated packets. The VNI MAY be used as part of ECMP forwarding decisions or MAY be used as a mechanism to distinguish between overlapping address spaces contained in the encapsulated packet when load balancing across CPUs. Reserved field which MUST be zero on transmission and ignored on receipt. Transit devices MUST maintain consistent forwarding behavior irrespective of the value of 'Opt Len', including ECMP link selection. These devices SHOULD be able to forward packets containing options without resorting to a slow path.

The base Geneve header is followed by zero or more options in Type-Length-Value format. Each option consists of a four byte option header and a variable amount of option data interpreted according to the type. Namespace for the 'Type' field. IANA will be requested to create a "Geneve Option Class" registry to allocate identifiers for organizations, technologies, and vendors that have an interest in creating types for options. Each organization may allocate types independently to allow experimentation and rapid innovation. It is expected that over time certain options will become well known and a given implementation may use option types from a variety of sources. In addition, IANA will be requested to reserve specific ranges for standardized and experimental options. Type indicating the format of the data contained in this option. Options are primarily designed to encourage future extensibility and innovation and so standardized forms of these options will be defined in a separate document. The high order bit of the option type indicates that this is a critical option. If the receiving endpoint does not recognize this option and this bit is set then the packet MUST be dropped. If the critical bit is set in any option then the 'C' bit in the Geneve base header MUST also be set. Transit devices MUST NOT drop packets on the basis of this bit. The following figure shows the location of the 'C' bit in the 'Type' field:

The requirement to drop a packet with an unknown critical option applies to the entire tunnel endpoint system and not a particular component of the implementation. For example, in a system comprised of a forwarding ASIC and a general purpose CPU, this does not mean that the packet must be dropped in the ASIC. An implementation may send the packet to the CPU using a rate-limited control channel for slow-path exception handling. Option control flags reserved for future use. MUST be zero on transmission and ignored on receipt. Length of the option, expressed in four byte multiples excluding the option header. The total length of each option may be between 4 and 128 bytes. Packets in which the total length of all options is not equal to the 'Opt Len' in the base header are invalid and MUST be silently dropped if received by an endpoint. Option data interpreted according to 'Type'.

Geneve options are primarily intended to be originated and processed by tunnel endpoints. However, options MAY be processed by transit devices along the tunnel path as well. Transit devices not processing Geneve headers SHOULD process Geneve packets as any other UDP packet and maintain consistent forwarding behavior. In tunnel endpoints, the generation and interpretation of options is determined by the control plane, which is out of the scope of this document. However, to ensure interoperability between heterogeneous devices some requirements are imposed on options and the devices that process them: Receiving endpoints MUST drop packets containing unknown options with the 'C' bit set in the option type. Conversely, transit devices MUST NOT drop packets as a result of encountering unknown options, including those with the 'C' bit set. Some options may be defined in such a way that the position in the option list is significant. Therefore, options MUST NOT be reordered by transit devices. An option MUST NOT affect the parsing or interpretation of any other option. When designing a Geneve option, it is important to consider how the option will evolve in the future. Once an option is defined it is reasonable to expect that implementations may come to depend on a specific behavior. As a result, the scope of any future changes must be carefully described upfront. Unexpectedly significant interoperability issues may result from changing the length of an option that was defined to be a certain size. A particular option is specified to have either a fixed length, which is constant, or a variable length, which may change over time or for different use cases. This property is part of the definition of the option and conveyed by the ‘Type’. For fixed length options, some implementations may choose to ignore the length field in the option header and instead parse based on the well known length associated with the type. In this case, redefining the length will impact not only parsing of the option in question but also any options that follow. Therefore, options that are defined to be fixed length in size MUST NOT be redefined to a different length. Instead, a new 'Type' should be allocated.

As an IP-based tunnel protocol, Geneve shares many properties and techniques with existing protocols. The application of some of these are described in further detail, although in general most concepts applicable to the IP layer or to IP tunnels generally also function in the context of Geneve.

To prevent fragmentation and maximize performance, the best practice when using Geneve is to ensure that the MTU of the physical network is greater than or equal to the MTU of the encapsulated network plus tunnel headers. Manual or upper layer (such as TCP MSS clamping) configuration can be used to ensure that fragmentation never takes place, however, in some situations this may not be feasible. It is strongly RECOMMENDED that Path MTU Discovery (, ) be used by setting the DF bit in the IP header when Geneve packets are transmitted over IPv4 (this is the default with IPv6). The use of Path MTU Discovery on the transit network provides the encapsulating endpoint with soft-state about the link that it may use to prevent or minimize fragmentation depending on its role in the virtualized network. Note that some implementations may not be capable of supporting fragmentation or other less common features of the IP header, such as options and extension headers.

When encapsulating IP (including over Ethernet) packets in Geneve, there are several considerations for propagating DSCP and ECN bits from the inner header to the tunnel on transmission and the reverse on reception. provides guidance for mapping DSCP between inner and outer IP headers. Network virtualization is typically more closely aligned with the Pipe model described, where the DSCP value on the tunnel header is set based on a policy (which may be a fixed value, one based on the inner traffic class, or some other mechanism for grouping traffic). Aspects of the Uniform model (which treats the inner and outer DSCP value as a single field by copying on ingress and egress) may also apply, such as the ability to remark the inner header on tunnel egress based on transit marking. However, the Uniform model is not conceptually consistent with network virtualization, which seeks to provide strong isolation between encapsulated traffic and the physical network. describes the mechanism for exposing ECN capabilities on IP tunnels and propagating congestion markers to the inner packets. This behavior MUST be followed for IP packets encapsulated in Geneve.

Geneve tunnels may either be point-to-point unicast between two endpoints or may utilize broadcast or multicast addressing. It is not required that inner and outer addressing match in this respect. For example, in physical networks that do not support multicast, encapsulated multicast traffic may be replicated into multiple unicast tunnels or forwarded by policy to a unicast location (possibly to be replicated there). With physical networks that do support multicast it may be desirable to use this capability to take advantage of hardware replication for encapsulated packets. In this case, multicast addresses may be allocated in the physical network corresponding to tenants, encapsulated multicast groups, or some other factor. The allocation of these groups is a component of the control plane and therefore outside of the scope of this document. When physical multicast is in use, the 'C' bit in the Geneve header may be used with groups of devices with heterogeneous capabilities as each device can interpret only the options that are significant to it if they are not critical.

Generally speaking, a Geneve tunnel is a unidirectional concept. IP is not a connection oriented protocol and it is possible for two endpoints to communicate with each other using different paths or to have one side not transmit anything at all. As Geneve is an IP-based protocol, the tunnel layer inherits these same characteristics. It is possible for a tunnel to encapsulate a protocol, such as TCP, which is connection oriented and maintains session state at that layer. In addition, implementations MAY model Geneve tunnels as connected, bidirectional links, such as to provide the abstraction of a virtual port. In both of these cases, bidirectionality of the tunnel is handled at a higher layer and does not affect the operation of Geneve itself.

Geneve is intended to be flexible to a wide range of current and future applications. As a result, certain constraints may be placed on the use of metadata or other aspects of the protocol in order to optimize for a particular use case. For example, some applications may limit the types of options which are supported or enforce a maximum number or length of options. Other applications may only handle certain encapsulated payload types, such as Ethernet or IP. This could be either globally throughout the system or, for example, restricted to certain classes of devices or network paths. These constraints may be communicated to tunnel endpoints either explicitly through a control plane or implicitly by the nature of the application. As Geneve is defined as a data plane protocol that is control plane agnostic, the exact mechanism is not defined in this document.

Modern NICs currently provide a variety of offloads to enable the efficient processing of packets. The implementation of many of these offloads requires only that the encapsulated packet be easily parsed (for example, checksum offload). However, optimizations such as LSO and LRO involve some processing of the options themselves since they must be replicated/merged across multiple packets. In these situations, it is desirable to not require changes to the offload logic to handle the introduction of new options. To enable this, some constraints are placed on the definitions of options to allow for simple processing rules: When performing LSO, a NIC MUST replicate the entire Geneve header and all options, including those unknown to the device, onto each resulting segment. However, a given option definition may override this rule and specify different behavior in supporting devices. Conversely, when performing LRO, a NIC MAY assume that a binary comparison of the options (including unknown options) is sufficient to ensure equality and MAY merge packets with equal Geneve headers. Options MUST NOT be reordered during the course of offload processing, including when merging packets for the purpose of LRO. NICs performing offloads MUST NOT drop packets with unknown options, including those marked as critical. There is no requirement that a given implementation of Geneve employ the offloads listed as examples above. However, as these offloads are currently widely deployed in commercially available NICs, the rules described here are intended to enable efficient handling of current and future options across a variety of devices.

Geneve is capable of encapsulating a wide range of protocols and therefore a given implementation is likely to support only a small subset of the possibilities. However, as Ethernet is expected to be widely deployed, it is useful to describe the behavior of VLANs inside encapsulated Ethernet frames. As with any protocol, support for inner VLAN headers is OPTIONAL. In many cases, the use of encapsulated VLANs may be disallowed due to security or implementation considerations. However, in other cases trunking of VLAN frames across a Geneve tunnel can prove useful. As a result, the processing of inner VLAN tags upon ingress or egress from a tunnel endpoint is based upon the configuration of the endpoint and/or control plane and not explicitly defined as part of the data format.

Viewed exclusively from the data plane, Geneve does not introduce any interoperability issues as it appears to most devices as UDP packets. However, as there are already a number of tunnel protocols deployed in network virtualization environments, there is a practical question of transition and coexistence. Since Geneve is a superset of the functionality of the three most common protocols used for network virtualization (VXLAN, NVGRE, and STT) it should be straightforward to port an existing control plane to run on top of it with minimal effort. With both the old and new packet formats supporting the same set of capabilities, there is no need for a hard transition - endpoints directly communicating with each other use any common protocol, which may be different even within a single overall system. As transit devices are primarily forwarding packets on the basis of the IP header, all protocols appear similar and these devices do not introduce additional interoperability concerns. To assist with this transition, it is strongly suggested that implementations support simultaneous operation of both Geneve and existing tunnel protocols as it is expected to be common for a single node to communicate with a mixture of other nodes. Eventually, older protocols may be phased out as they are no longer in use.

As UDP/IP packets, Geneve does not have any inherent security mechanisms. As a result, an attacker with access to the underlay network transporting the IP packets has the ability to snoop or inject packets. Legitimate but malicious tunnel endpoints may also spoof identifiers in the tunnel header to gain access to networks owned by other tenants. Within a particular security domain, such as a data center operated by a single provider, the most common and highest performing security mechanism is isolation of trusted components. Tunnel traffic can be carried over a separate VLAN and filtered at any untrusted boundaries. In addition, tunnel endpoints should only be operated in environments controlled by the service provider, such as the hypervisor itself rather than within a customer VM. When crossing an untrusted link, such as the public Internet, IPsec may be used to provide authentication and/or encryption of the IP packets formed as part of Geneve encapsulation. If the remote tunnel endpoint is not completely trusted, for example it resides on a customer premises, then it may also be necessary to sanitize any tunnel metadata to prevent tenant-hopping attacks. Geneve does not otherwise affect the security of the encapsulated packets.

IANA has allocated UDP port 6081 as the well-known destination port for Geneve. Upon publication, the registry should be updated to cite this document. The original request was:

Contact: Jesse Gross Description: Generic Network Virtualization Encapsulation (Geneve) Reference: This document Port Number: 6081]]> In addition, IANA is requested to create a "Geneve Option Class" registry to allocate Option Classes. This shall be a registry of 16-bit hexadecimal values along with descriptive strings. The identifiers 0x0-0xFF are to be reserved for standardized options for allocation by IETF Review and 0xFFF0-0xFFFF for Experimental Use. Otherwise, identifiers are to be assigned to any organization with an interest in creating Geneve options on a First Come First Served basis. The registry is to be populated with the following initial values: Option Class Description 0x0000..0x00FF Unassigned - IETF Review 0x0100 Linux 0x0101 Open vSwitch 0x0102 Open Virtual Networking (OVN) 0x0103 In-band Network Telemetry (INT) 0x0104 VMware 0x0105..0xFFEF Unassigned - First Come First Served 0xFFF0..FFFF Experimental

The following individuals were authors of an earlier version of this document and made significant contributions:

The authors wish to thank Martin Casado, Bruce Davie and Dave Thaler for their input, feedback, and helpful suggestions.