Network Virtualization Overlays (NVO3) Encapsulation Considerations

Internet-Draft	NVO3 Encapsulation Considerations	November 2023
Boutros & Eastlake	Expires 9 May 2024	[Page]

Abstract

The IETF Network Virtualization Overlays (NVO3) Working Group Chairs and Routing Area Director chartered a design team to take forward the encapsulation discussion and see if there was potential to design a common encapsulation that addresses the various technical concerns. This document provides a record, for the benefit of the IETF community, of the considerations arrived at by the NVO3 encapsulation design team, which may be helpful with future deliberations by working groups over the choice of encapsulation formats.¶

There are implications of having different encapsulations in real environments consisting of both software and hardware implementations and within and spanning multiple data centers. For example, OAM functions such as path MTU discovery become challenging with multiple encapsulations along the data path.¶

The design team recommended Geneve with a few modifications as the common encapsulation. This document provides more details, particularly in Section 7.¶

5. Encapsulation Issues and Background

The following subsections describe issues with current encapsulations as discussed by the NVO3 WG. Numerous extensions and options have been designed for GUE and Geneve but these have not yet been validated by the WG.¶

Also included are diagrams and information on the candidate encapsulations. These are mostly copied from other documents. Since each protocol is assumed to be sent over UDP, an initial UDP Header is shown which would be preceded by an IPv4 or IPv6 Header.¶

5.1. Geneve

The Geneve packet format, taken from [RFC8926], is shown in Figure 1 below.¶


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Outer UDP Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |    Dest Port = 6081 Geneve    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          UDP Length           |        UDP Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Geneve Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver|  Opt Len  |O|C|    Rsvd.  |          Protocol Type        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Virtual Network Identifier (VNI)       |    Reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                    Variable-Length Options                    ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Geneve Header

The type of payload being carried is indicated by an Ethertype [RFC7042] in the Protocol Type field in the Geneve Header; Ethernet itself is represented by Ethertype 0x6558. See [RFC8926] for details concerning UDP header fields. The O bit indicates an OAM packet. The C bit is the "Critical" bit which means that the options must be processed or the packet discarded.¶

Issues with Geneve [RFC8926] are as follows:¶

Can't be implemented cost-effectively in all use cases because variable length header and order of the TLVs makes it costly (in terms of number of gates) to implement in hardware.¶
Header doesn't fit into largest commonly available parse buffer (256 bytes in NIC). Cannot justify doubling buffer size unless it is mandatory for hardware to process additional option fields.¶

Selection of Geneve despite these issues may be the result of the Geneve design effort assuming that the Geneve header would typically be delivered to a server and parsed in software.¶

5.2. Generic UDP Encapsulation (GUE)


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

UDP Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Source port            |     Dest port = 6080 GUE      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        UDP Length             |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

GUE Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 0 |C|   Hlen  |  Proto/ctype  |             Flags             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                  Extensions Fields (optional)                 ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: GUE Header

The type of payload being carried is indicated by an IANA Internet protocol number in the Proto/ctype field. The C bit indicates a Control packet.¶

Issues with GUE [ietf_intarea_gue] are as follows:¶

There were a significant number of objections to GUE related to the complexity of implementation in hardware, similar to those noted for Geneve above.¶

5.3. Generic Protocol Extension (GPE) for VXLAN


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Outer UDP Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Source Port         |     Dest Port = 4790 GPE      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           UDP Length          |        UDP Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

VXLAN-GPE Header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|R|Ver|I|P|B|O|       Reserved                | Next Protocol |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                VXLAN Network Identifier (VNI) |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: GPE Header

The type of payload being carried is indicated by the Next Protocol field using a VXLAN-GPE-specific registry. The I bit indicates that the VNI is valid. The P bit indicates that the Next Protocol field is valid. The B bit indicates the packet is an ingress replicated Broadcase, Unknown Unicast, or Multicast packet. The O bit indicates an OAM packet.¶

Issues with VXLAN-GPE [nvo3_vxlan_gpe] are as follows:¶

GPE is not day-1 backwards compatible with VXLAN [RFC7348]. Although the frame format is similar, it uses a different UDP port, so would require changes to existing implementations even if the rest of the GPE frame were the same.¶
GPE is insufficiently extensible. It adds a Next Protocol field and some flag bits to the VXLAN header but is not otherwise extensible.¶
Security, e.g., of the VNI, has not been addressed by GPE. Although a shim header could be used for security and other extensions, this has not been defined yet and its implications on offloading in NICs are not understood.¶

6. Common Encapsulation Considerations

6.1. Current Encapsulations

Appendix A includes a detailed comparison between the three proposed encapsulations. The comparison indicates several common properties but also three major differences among the encapsulations:¶

Extensibility: Geneve and GUE were defined with built-in extensibility, while VXLAN-GPE is not inherently extensible. Note that any of the three encapsulations can be extended using the Network Service Header (NSH [RFC8300]).¶
Extension method: Geneve is extensible using Type/Length/Value (TLV) fields, while GUE uses a small set of possible extensions, and a set of flags that indicate which extensions are present.¶
Length field: Geneve and GUE include a Length field, indicating the length of the encapsulation header, while VXLAN-GPE does not include such a field.¶

6.2. Useful Extensions Use Cases

Non-vendor specific TLVs MUST follow the standardization process. The following use cases for extensions shows that there is a strong requirement to support variable length extensions with possible different subtypes.¶

6.2.1. Telemetry Extensions

In several scenarios it is beneficial to make information about the path a packet took through the network or through a network device as well as associated telemetry information available to the operator.¶

This includes not only tasks like debugging, troubleshooting, and network planning and optimization but also policy or service level agreement compliance checks.¶

Packet scheduling algorithms, especially for balancing traffic across equal cost paths or links, often leverage information contained within the packet, such as protocol number, IP address, or MAC address. Probe packets would thus either need to be sent between the exact same endpoints with the exact same parameters, or probe packets would need to be artificially constructed as "fake" packets and inserted along the path. Both approaches are often not feasible from an operational perspective because access to the end-system is not feasible or the diversity of parameters and associated probe packets to be created is simply too large. An extension providing an in-band telemetry mechanism [RFC9197] is an alternative in those cases.¶

6.2.2. Security/Integrity Extensions

Since the currently proposed NVO3 encapsulations do not protect their headers, a single bit corruption in the VNI field could deliver a packet to the wrong tenant. Extension headers are needed to use any sophisticated security.¶

The possibility of VNI spoofing with an NVO3 protocol is exacerbated by using UDP. Systems typically have no restrictions on applications being able to send to any UDP port so an unprivileged application can trivially spoof VXLAN [RFC7348] packets for instance, including using arbitrary VNIs.¶

One can envision support of an HMAC-like Message Authentication Code (MAC) [RFC2104] in an NVO3 extension to authenticate the header and the outer IP addresses, thereby preventing attackers from injecting packets with spoofed VNIs.¶

Another aspect of security is payload security. Essentially this makes packets that look like the following:¶

  IP|UDP|NVO3 Encap|DTLS/IPSEC-ESP Extension|payload.

This is desirable since we still have the UDP header for ECMP, the NVO3 header is in plain text so it can be read by network elements, and different security or other payload transforms can be supported on a single UDP port (we don't need a separate UDP port for DTLS/IPSEC [RFC9147]/[RFC6071]).¶

6.2.3. Group Based Policy

Another use case would be to carry the Group Based Policy (GBP) source group information within a NVO3 header extension in a similar manner as has been implemented for VXLAN [VXLANgroup]. This allows various forms of policy such as access control and QoS to be applied between abstract groups rather than coupled to specific endpoint addresses.¶

6.3. Hardware Considerations

Hardware restrictions should be taken into consideration along with future hardware enhancements that may provide more flexible metadata processing. However, the set of options that need to and will be implemented in hardware will be a subset of what is implemented in software, since software NVEs are likely to grow features, and hence option support, at a more rapid rate.¶

It is hard to predict which options will be implemented in which piece of hardware and when. That depends on whether the hardware will be in the form of a NIC providing increasing offload capabilities to software NVEs, or a switch chip being used as an NVE gateway towards non-NVO3 parts of the network, or even a transit device that participates in the NVO3 dataplane, e.g., for OAM purposes.¶

A result of this is that it doesn't look useful to prescribe some order of the option so that the ones that are likely to be implemented in hardware come first; we can't decide such an order when we define the options, however a control plane can enforce such an order for some hardware implementation.¶

We do know that hardware needs to initially be able to efficiently skip over the NVO3 header to find the inner payload. That is needed both for NICs implementing various TCP offload mechanisms and for transit devices and NVEs applying policy or ACLs to the inner payload.¶

6.4. Extension Size

Extension header length has a significant impact on hardware and software implementations. A maximum total header length that is too small will unnecessarily constrain software flexibility. A maximum total header length that is too large will place a nontrivial cost on hardware implementations. Thus, the DT recommends that there be a minimum and maximum total available extension header length specified. The maximum total header length is determined by the size of the bit field allocated for the total extension header length field. The risk with this approach is that it may be difficult to extend the total header size in the future. The minimum total header length is determined by a requirement in the specifications that all implementations must meet. The risk with this approach is that all implementations will only implement support for the minimum total header length which would then become the de facto maximum total header length.¶

The recommended minimum total svailable header length is 64 bytes.¶

The size of an extension header should always be 4 byte aligned.¶

The maximum length of a single option should be large enough to meet the different extension use case requirements, e.g., in-band telemetry and future use.¶

6.5. Ordering of Extension Headers

To support hardware nodes at the target NVE or at a transit device that can process one or a few extension headers in TCAM, a control plane in such a deployment can signal a capability to ensure a specific extension header will always appear in a specific order, for example the first one in the packet.¶

The order of the extension headers should be hardware friendly for both the sender and the receiver and possibly some transit devices also.¶

Transit devices don't participate in control plane communication between the end points and are not required to process the extension headers; however, if they do, they may need to process only a small subset of the extension headers that will be consumed by target NVEs.¶

6.6. TLV versus Bit Fields

If there is a well-known initial set of options that are likely to be implemented in software and in hardware, it can be efficient to use the bit fields approach to indicate the presence of extensions as in GUE. However, as described in section 6.3, if options are added over time and different subsets of options are likely to be implemented in different pieces of hardware, then it would be hard for the IETF to specify which options should get the early bit fields. TLVs are a lot more flexible, which avoids the need to determine the relative importance different options. However, general TLVs of arbitrary order, size, and repetition are difficult to implement in hardware. A middle ground is to use TLVs with restrictions on their size and alignment, observing that individual TLVs can have a fixed length, and support via the control plane a method such that an NVE will only receive options that it needs and implements. The control plane approach can potentially be used to control the order of the TLVs sent to a particular NVE. Note that transit devices are not likely to participate in the control plane; hence, to the extent that they need to participate in option processing, some other method must be used. Transit devices would have issues with future GUE bit fields being defined for future options as well.¶

A benefit of TLVs from a hardware perspective is that they are self describing, i.e., all the information is in the TLV. In a bit field approach, the hardware needs to look up the bit to determine the length of the data associated with the bit through some separate table, which would add hardware complexity.¶

There are use cases where multiple modules of software are running on an NVE. These can be modules such as a diagnostic module by one vendor that does packet sampling and another module from a different vendor that implements a firewall. Using a TLV format, it is easier to have different software modules process different TLVs, which could be standard extensions or vendor specific extensions defined by the different vendors, without conflicting with each other. This can help with hardware modularity as well. There are some implementations with options that allows different software modules, like MAC learning and security, to process different options.¶

6.7. Control Plane Considerations

Given that we want to allow considerable flexibility and extensibility, e.g., for software NVEs, yet be able to support important extensions in less flexible contexts such as hardware NVEs, it is useful to consider the control plane. By control plane in this section we mean both protocols, such as EVPN [RFC8365] and others, and deployment specific configuration.¶

If each NVE can express in the control plane that it only supports certain extensions (which could be a single extension, or a few), and the source NVEs only include supported extensions in the NVO3 packets, then the target NVE can both use a simpler parser (e.g., a TCAM might be usable to look for a single NVO3 extension) and the depth of the inner payload in the NVO3 packet will be minimized. Furthermore, if the target NVE cares about a few extensions and can express in the control plane the desired order of those extensions in the NVO3 packets, then the deployment can provide useful functionality with simplified hardware requirements for the target NVE.¶

Transit devices that are not aware of the NVO3 extensions somewhat benefit from such an approach, since the inner payload is less deep in the packet if no extraneous extension headers are included in the packet. In general, a transit device is not likely to participate in the NVO3 control plane. However, configuration mechanisms can take into account limitations of the transit devices used in particular deployments.¶

Note that with this approach different NVEs could desire different extensions or sets of extensions, which means that the source NVE needs to be able to place different sets of extensions in different NVO3 packets, and perhaps in different order. It also assumes that underlay multicast or replication servers are not used together with NVO3 extension headers.¶

There is a need to consider mandatory extensions versus optional extensions. Mandatory extensions require the receiver to drop the packet if the extension is unknown. A control plane mechanism can prevent the need for dropping unknown extensions, since they would not be included to target NVEs that do not support them.¶

The control planes defined today need to add the ability to describe the different encapsulations. Thus, perhaps EVPN [RFC8365] and any other control plane protocol that the IETF defines should have a way to indicate the supported NVO3 extensions and their order, for each of the encapsulations supported.¶

The WG should consider developing a separate draft on guidance for option processing and control plane participation. This should provide examples/guidance on range of usage models and deployments scenarios for specific options and ordering that are relevant for that specific deployment. This includes end points and middle boxes using the options. Having the control plane negotiate the constraints is the most appropriate and flexible way to address these requirements.¶

6.8. Split NVE

If the working group sees a need for having the hosts send and receive options in a split NVE case [RFC8394], this is possible using any of the existing extensible encapsulations (Geneve, GUE, GPE+NSH) by defining a way to carry those over other transports. NSH can already be used over different transports.¶

If we need to do this with other encapsulations it can be done by defining an Ethertype so that it can be carried over Ethernet and [IEEE802.1Q].¶

If we need to carry other encapsulations over MPLS, it would require an EVPN control plane to signal that other encapsulation header + options will be present in front of the L2 packet. The VNI can be ignored in the header, and the MPLS label will be the one used to identify the EVPN L2 instance.¶

6.9. Larger VNI Considerations

The DT discussed whether we should make the VNI 32-bits or larger. The benefit of a 24-bit VNI would be to avoid unnecessary changes with existing proposals and implementations that are almost all, if not all, using 24-bit VNI. If we need a larger VNI, an extension can be used to support that.¶

7. Design Team Recommendations

The Design Team (DT) concluded that Geneve is most suitable as a starting point for a proposed standard for network virtualization, for the following reasons:¶

The DT studied whether VNI should be in the base header or in an extension header and whether it should be a 24-bit or 32-bit field. The Design Team agreed that VNI is critical information for network virtualization and MUST be present in all packets. The DT also agreed that a 24-bit VNI, which is supported by Geneve, matches the existing widely used encapsulation formats, i.e., VXLAN [RFC7348] and NVGRE [RFC7637], and hence is more suitable to use going forward.¶
The Geneve header has the total options length which allows skipping over the options for NIC offload operations and will allow transit devices to view flow information in the inner payload.¶
The DT considered the option of using NSH [RFC8300] with VXLAN-GPE but given that NSH is targeted at service chaining and contains service chaining information, it is less suitable for the network virtualization use case. The other downside for VXLAN-GPE was lack of a header length in VXLAN-GPE which makes skipping over the headers to process inner payload more difficult. Total Option Length is present in Geneve. It is not possible to skip any options in the middle with VXLAN-GPE. In principle a split between a base header and a header with options is interesting (whether that options header is NSH or some new header without ties to a service path). We explored whether it would make sense to either use NSH for this, or define a new NVO3 options header. However, we observed that this makes it slightly harder to find the inner payload since the length field is not in the NVO3 header itself. Thus, one more field would have to be extracted to compute the start of the inner payload. Also, if the experience with IPv6 extension headers is a guide, there would be a risk that key pieces of hardware might not implement the options header, resulting in future calls to deprecate its use. Making the options part of the base NVO3 header has less of those issues. Even though the implementation of any particular option can not be predicted ahead of time, the option mechanism and ability to skip the options is likely to be broadly implemented.¶
The DT compared the TLV vs bit fields style extension. It was deemed that parsing both TLV and bit fields is expensive and, while bit fields may be simpler to parse, it is also more restrictive and requires guessing which extensions will be widely implemented so they can get early bit assignments. Given that half the bits are already assigned in GUE, a widely deployed extension may appear in a flag extension, and this will require extra processing, to dig the flag from the flag extension and then look for the extension itself. Also bit fields are not flexible enough to address the requirements from OAM, Telemetry, and security extensions, for variable length option and different subtypes of the same option. While TLVs are more flexible, a control plane can restrict the number of option TLVs as well as the order and size of the TLVs to make it simpler for a dataplane implementation to handle.¶
The DT briefly discussed the multi-vendor NVE case, and the need to allow vendors to put their own extensions in the NVE header. This is possible with TLVs.¶
The DT also agreed that the C (Critical) bit in Geneve is helpful. It indicates that the header includes options which must be parsed or the packet discarded. It allows a receiver NVE to easily decide whether to process options or not, for example a UUID based packet trace, and how an optional extension such as that can be ignored by a receiver NVE and thus make it easy for NVE to skip over the options. Thus, the C bit remains as defined in Geneve.¶
There are already some extensions that are being discussed (see section 6.2) of varying sizes. By using Geneve options it is possible to get in band parameters like switch id, ingress port, egress port, internal delay, and queue in telemetry defined extension TLV from switches. It is also possible to add security extension TLVs like HMAC [RFC2104] and DTLS/IPSEC [RFC9147]/[RFC6071] to authenticate the Geneve packet header and secure the Geneve packet payload by software or hardware tunnel endpoints. A Group Based Policy extension TLV can be carried as well.¶
There are already implementations of Geneve options deployed in production networks as of this writing. There is as well new hardware supporting Geneve TLV parsing. In addition, an In-band Telemetry [INT] specification is being developed by P4.org that illustrates the option of INT meta data carried over Geneve. OVN/OVS [OVN] have also defined some option TLV(s) for Geneve.¶
The DT has addressed the usage models while considering the requirements and implementations in general including software and hardware.¶

There seems to be interest in standardizing some well-known secure option TLVs to secure the header and payload to guarantee encapsulation header integrity and tenant data privacy. The Design Team recommends that the working group consider standardizing such option(s).¶

The DT recommends the following enhancements to Geneve to make it more suitable to hardware and yet provide the flexibility for software:¶

The DT proposes a text such as, while TLVs are more flexible, a control plane can restrict the number of option TLVs as well the order and size of the TLVs to make it simpler for a data plane implementation in software or hardware to handle. For example, there may be some critical information such as a secure hash that must be processed in a certain order at lowest latency.¶
A control plane can negotiate a subset of option TLVs and certain TLV ordering, as well as limiting the total number of option TLVs present in the packet, for example, to allow for hardware capable of processing fewer options. Hence, the control plane needs to have the ability to describe the supported TLVs subset and their order.¶
The Geneve documents should specify that the subset and order of option TLVs SHOULD be configurable for each remote NVE in the absence of a protocol control plane.¶
The DT recommends that Geneve follow fragmentation recommendations in overlay services like PWE3 and the L2/L3 VPN recommendations to guarantee larger MTU for the tunnel overhead ([RFC3985] Section 5.3).¶
The DT requests that Geneve provide a recommendation for critical bit processing - text could specify how critical bits can be used with control plane specifying the critical options.¶
Given that there is a telemetry option use case for a length of 256 bytes, we recommend that Geneve increase the Single TLV option length to 256.¶
The DT requests that Geneve address Requirements for OAM considerations for alternate marking and for performance measurements that need a 2 bit field in the header and clarify the need for the current OAM bit in the Geneve Header.¶
The DT recommends that the WG work on security options for Geneve.¶

12. Informative References

[ietf_gue_extensions]: Herbert, T., Yong, L., and F. Templin, "Extensions for Generic UDP Encapsulation", work in progress, 8 March 2019, <https://datatracker.ietf.org/doc/draft-ietf-intarea-gue-extensions/>.
[ietf_intarea_gue]: Herbert, T., Yong, L., and O. Zia, "Generic UDP Encapsulation", work in progress, 26 October 2019.
[IEEE802.1Q]: 802.1 WG, IEEE., "Bridges and Bridged Networks", IEEE Std 802.1Q-2014, 3 November 2014.
[INT]: P4.org, "In-band Network Telemetry (INT) Dataplane Specification", November 2020, <https://p4.org/p4-spec/docs/INT_v2_1.pdf>.
[nvo3_vxlan_gpe]: Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol Extension for VXLAN (VXLAN-GPE)", work in progress, 4 November 2023, <https://datatracker.ietf.org/doc/draft-ietf-nvo3-vxlan-gpe/>.
[OVN]: Network, O. V., "", <https://www.openvswitch.org/>.
[RFC2104]: Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>.
[RFC2418]: Bradner, S., "IETF Working Group Guidelines and Procedures", BCP 25, RFC 2418, DOI 10.17487/RFC2418, September 1998, <https://www.rfc-editor.org/info/rfc2418>.
[RFC3985]: Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, DOI 10.17487/RFC3985, March 2005, <https://www.rfc-editor.org/info/rfc3985>.
[RFC6071]: Frankel, S. and S. Krishnan, "IP Security (IPsec) and Internet Key Exchange (IKE) Document Roadmap", RFC 6071, DOI 10.17487/RFC6071, February 2011, <https://www.rfc-editor.org/info/rfc6071>.
[RFC6291]: Andersson, L., van Helvoort, H., Bonica, R., Romascanu, D., and S. Mansfield, "Guidelines for the Use of the "OAM" Acronym in the IETF", BCP 161, RFC 6291, DOI 10.17487/RFC6291, June 2011, <https://www.rfc-editor.org/info/rfc6291>.
[RFC7042]: Eastlake 3rd, D. and J. Abley, "IANA Considerations and IETF Protocol and Documentation Usage for IEEE 802 Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042, October 2013, <https://www.rfc-editor.org/info/rfc7042>.
[RFC7348]: Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, L., Sridhar, T., Bursell, M., and C. Wright, "Virtual eXtensible Local Area Network (VXLAN): A Framework for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014, <https://www.rfc-editor.org/info/rfc7348>.
[RFC7637]: Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network Virtualization Using Generic Routing Encapsulation", RFC 7637, DOI 10.17487/RFC7637, September 2015, <https://www.rfc-editor.org/info/rfc7637>.
[RFC8300]: Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed., "Network Service Header (NSH)", RFC 8300, DOI 10.17487/RFC8300, January 2018, <https://www.rfc-editor.org/info/rfc8300>.
[RFC8365]: Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R., Uttaro, J., and W. Henderickx, "A Network Virtualization Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365, DOI 10.17487/RFC8365, March 2018, <https://www.rfc-editor.org/info/rfc8365>.
[RFC8394]: Li, Y., Eastlake 3rd, D., Kreeger, L., Narten, T., and D. Black, "Split Network Virtualization Edge (Split-NVE) Control-Plane Requirements", RFC 8394, DOI 10.17487/RFC8394, May 2018, <https://www.rfc-editor.org/info/rfc8394>.
[RFC8926]: Gross, J., Ed., Ganga, I., Ed., and T. Sridhar, Ed., "Geneve: Generic Network Virtualization Encapsulation", RFC 8926, DOI 10.17487/RFC8926, November 2020, <https://www.rfc-editor.org/info/rfc8926>.
[RFC9147]: Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.
[RFC9197]: Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi, Ed., "Data Fields for In Situ Operations, Administration, and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197, May 2022, <https://www.rfc-editor.org/info/rfc9197>.
[VXLANgroup]: Smith, M. and L. Kreeger, "VXLAN Group Policy Option", work in progress, 22 October 2018, <https://datatracker.ietf.org/doc/html/draft-smith-vxlan-group-policy-05>.

Appendix A. Encapsulation Comparison

A.1. Overview

This section presents a comparison of the three NVO3 encapsulation proposals, Geneve [RFC8926], GUE [ietf_intarea_gue], and VXLAN-GPE [nvo3_vxlan_gpe]. The three encapsulations use an outer UDP/IP transport. Geneve and VXLAN-GPE use an 8-octet header, while GUE uses a 4-octet header. In addition to the base header, optional extensions may be included in the encapsulation, as discussed in Section A.2 below.¶

A.2. Extensibility

A.2.1. Native Extensibility Support

The Geneve and GUE encapsulations both enable optional headers to be incorporated at the end of the base encapsulation header.¶

VXLAN-GPE does not provide native support for header extensions. However, as discussed in [nvo3_vxlan_gpe], extensibility can be attained to some extent if the Network Service Header (NSH) [RFC8300] is used immediately following the VXLAN-GPE header. NSH supports either a fixed-size extension (MD Type 1), or a variable-size TLV-based extension (MD Type 2). Note that NSH-over-VXLAN-GPE implies an additional overhead of the 8-octets NSH header, in addition to the VXLAN-GPE header.¶

A.2.2. Extension Parsing

The Geneve Variable Length Options are defined as Type/Length/Value (TLV) extensions. Similarly, VXLAN-GPE, when using NSH, can include NSH TLV-based extensions. In contrast, GUE defines a small set of possible extension fields (proposed in [ietf_gue_extensions]), and a set of flags in the GUE header that indicate for each extension type whether it is present or not.¶

TLV-based extensions, as defined in Geneve, provide the flexibility for a large number of possible extension types. Similar behavior can be supported in NSH-over-VXLAN-GPE when using MD Type 2. The flag-based approach taken in GUE strives to simplify implementations by defining a small number of possible extensions used in a fixed order.¶

The Geneve and GUE headers both include a length field, defining the total length of the encapsulation, including the optional extensions. This length field simplifies the parsing by transit devices that skip the encapsulation header without parsing its extensions.¶

A.2.3. Critical Extensions

The Geneve encapsulation header includes the 'C' field, which indicates whether the current Geneve header includes critical options, that is to say, options which must be parsed by the target NVE. If the endpoint is not able to process a critical option, the packet is discarded.¶

A.2.4. Maximal Header Length

The maximal header length in Geneve, including options, is 260 octets. GUE defines the maximal header to be 128 octets. VXLAN-GPE uses a fixed-length header of 8 octets, unless NSH-over-VXLAN-GPE is used, yielding an encapsulation header of up to 264 octets.¶

A.3. Encapsulation Header

A.3.1. Virtual Network Identifier (VNI)

The Geneve and VXLAN-GPE headers both include a 24-bit VNI field. GUE, on the other hand, enables the use of a 32-bit field called VNID; this field is not included in the GUE header, but was defined as an optional extension in [ietf_gue_extensions].¶

The VXLAN-GPE header includes the 'I' bit, indicating that the VNI field is valid in the current header. A similar indicator is defined as a flag in the GUE header [ietf_gue_extensions].¶

A.3.2. Next Protocol

All three encapsulation headers include a field that specifies the type of the next protocol header, which resides after the NVO3 encapsulation header. The Geneve header includes a 16-bit field that uses the IEEE Ethertype convention. GUE uses an 8-bit field, which uses the IANA Internet protocol numbering. The VXLAN-GPE header incorporates an 8-bit Next Protocol field, using a VXLAN-GPE-specific registry, defined in [nvo3_vxlan_gpe].¶

The VXLAN-GPE header also includes the 'P' bit, which explicitly indicates whether the Next Protocol field is present in the current header.¶

A.3.3. Other Header Fields

The OAM bit, which is defined in Geneve and in VXLAN-GPE, indicates whether the current packet is an OAM packet. The GUE header includes a similar field, but uses different terminology; the GUE 'C-bit' specifies whether the current packet is a control packet. Note that the GUE control bit can potentially be used in a large set of protocols that are not OAM protocols. However, the control packet examples discussed in [ietf_intarea_gue] are OAM-related.¶

Each of the three NVO3 encapsulation headers includes a 2-bit Version field, which is currently defined to be zero.¶

The Geneve and VXLAN-GPE headers include reserved fields; 14 bits in the Geneve header, and 27 bits in the VXLAN-GPE header are reserved.¶

A.4. Comparison Summary

The following table summarizes the comparison between the three NVO3 encapsulations. In some cases a plus sign ("+") or minus sign ("-") is used to indicate that the header is stronger or weaker in an area respectively.¶


+----------------+----------------+----------------+----------------+
|                |     Geneve     |      GUE       |   VXLAN-GPE    |
+----------------+----------------+----------------+----------------+
| Outer transport|     UDP/IP     |     UDP/IP     |     UDP/IP     |
| UDP Port Number|     6081       |     6080       |     4790       |
+----------------+----------------+----------------+----------------+
| Base header    |    8 octets    |    4 octets    |    8 octets    |
| length         |                |                |  (16 octets    |
|                |                |                |   using NSH)   |
+----------------+----------------+----------------+----------------+
| Extensibility  |Variable length |Extension fields| No native ext- |
|                |    options     |                | ensibility.    |
|                |                |                | Might use NSH. |
+----------------+----------------+----------------+----------------+
| Extension      |   TLV-based    |   Flag-based   |   TLV-based    |
| parsing method |                |                |(using NSH with |
|                |                |                |   MD Type 2)   |
+----------------+----------------+----------------+----------------+
| Extension      |    Variable    |     Fixed      |    Variable    |
| order          |                |                |  (using NSH)   |
+----------------+----------------+----------------+----------------+
| Length field   |       +        |       +        |       -        |
+----------------+----------------+----------------+----------------+
| Max Header     |   260 octets   |   128 octets   |    8 octets    |
| Length         |                |                |(264 using NSH) |
+----------------+----------------+----------------+----------------+
| Critical exte- |       +        |       -        |       -        |
| nsion bit      |                |                |                |
+----------------+----------------+----------------+----------------+
| VNI field size |    24 bits     |    32 bits     |    24 bits     |
|                |                |  (extension)   |                |
+----------------+----------------+----------------+----------------+
| Next protocol  |    16 bits     |     8 bits     |     8 bits     |
| field          |   Ethertype    | Internet prot- |  New registry  |
|                |   registry     | ocol registry  |                |
+----------------+----------------+----------------+----------------+
| Next protocol  |       -        |       -        |       +        |
| indicator      |                |                |                |
+----------------+----------------+----------------+----------------+
| OAM / control  |    OAM bit     |  Control bit   |    OAM bit     |
| field          |                |                |                |
+----------------+----------------+----------------+----------------+
| Version field  |    2 bits      |    2 bits      |    2 bits      |
+----------------+----------------+----------------+----------------+
| Reserved bits  |   14 bits      |     none       |   27 bits      |
+----------------+----------------+----------------+----------------+

Figure 4: NVO3 Encapsulations Comparison