Network Virtualization Overlays (NVO3) Encapsulation Considerations
draft-ietf-nvo3-encap-11

NVO3 Working Group                                       S. Boutros, Ed.
Internet-Draft                                         Ciena Corporation
Intended status: Informational                          D. Eastlake, Ed.
Expires: 1 June 2024                              Futurewei Technologies
                                                        29 November 2023

  Network Virtualization Overlays (NVO3) Encapsulation Considerations
                        draft-ietf-nvo3-encap-11

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.

Status of This Memo

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   This Internet-Draft will expire on 1 June 2024.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Design Team Goals . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Abbreviations and Acronyms  . . . . . . . . . . . . . . . . .   4
   5.  Encapsulation Issues and Background . . . . . . . . . . . . .   5
     5.1.  Geneve  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Generic UDP Encapsulation (GUE) . . . . . . . . . . . . .   6
     5.3.  Generic Protocol Extension (GPE) for VXLAN  . . . . . . .   7
   6.  Common Encapsulation Considerations . . . . . . . . . . . . .   8
     6.1.  Current Encapsulations  . . . . . . . . . . . . . . . . .   8
     6.2.  Useful Extensions Use Cases . . . . . . . . . . . . . . .   8
       6.2.1.  Telemetry Extensions  . . . . . . . . . . . . . . . .   8
       6.2.2.  Security/Integrity Extensions . . . . . . . . . . . .   9
       6.2.3.  Group Based Policy  . . . . . . . . . . . . . . . . .   9
     6.3.  Hardware Considerations . . . . . . . . . . . . . . . . .  10
     6.4.  Extension Size  . . . . . . . . . . . . . . . . . . . . .  10
     6.5.  Ordering of Extension Headers . . . . . . . . . . . . . .  11
     6.6.  TLV versus Bit Fields . . . . . . . . . . . . . . . . . .  11
     6.7.  Control Plane Considerations  . . . . . . . . . . . . . .  12
     6.8.  Split NVE . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.9.  Larger VNI Considerations . . . . . . . . . . . . . . . .  14
   7.  Design Team Recommendations . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   12. Informative References  . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Encapsulation Comparison . . . . . . . . . . . . . .  20
     A.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  20
     A.2.  Extensibility . . . . . . . . . . . . . . . . . . . . . .  20
       A.2.1.  Native Extensibility Support  . . . . . . . . . . . .  20
       A.2.2.  Extension Parsing . . . . . . . . . . . . . . . . . .  20

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       A.2.3.  Critical Extensions . . . . . . . . . . . . . . . . .  21
       A.2.4.  Maximal Header Length . . . . . . . . . . . . . . . .  21
     A.3.  Encapsulation Header  . . . . . . . . . . . . . . . . . .  21
       A.3.1.  Virtual Network Identifier (VNI)  . . . . . . . . . .  21
       A.3.2.  Next Protocol . . . . . . . . . . . . . . . . . . . .  21
       A.3.3.  Other Header Fields . . . . . . . . . . . . . . . . .  22
     A.4.  Comparison Summary  . . . . . . . . . . . . . . . . . . .  22
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   The NVO3 Working Group is chartered to gather requirements and
   develop solutions for network virtualization data planes based on
   encapsulation of virtual network traffic over an IP-based underlay
   data plane.  Requirements include due consideration for OAM and
   security.  Based on these requirements the WG was to select, extend,
   and/or develop one or more data plane encapsulation format(s).

   This led to WG drafts and an RFC describing three encapsulations as
   follows:

   *  [RFC8926] Geneve: Generic Network Virtualization Encapsulation

   *  [ietf_intarea_gue] Generic UDP Encapsulation

   *  [nvo3_vxlan_gpe] Generic Protocol Extension for VXLAN (VXLAN-GPE)

   Discussion on the list and in face-to-face meetings identified a
   number of technical problems with each of these encapsulations.
   Furthermore, there was clear consensus at the 96th IETF meeting in
   Berlin that, to maximize interoperability, the working group should
   progress only one data plane encapsulation.  In order to overcome a
   deadlock on the encapsulation decision, the WG consensus was to form
   a Design Team [RFC2418] to resolve this issue.

2.  Design Team Goals

   The Design Team (DT) formed as described above was to take one of the
   proposed encapsulations and enhance it to address the technical
   concerns.  The simple evolution of deployed networks as well as
   applicability to all locations in the NVO3 architecture are goals.
   The DT was to specifically avoid a design that is burdensome on
   hardware implementations but should allow future extensibility.  The
   chosen design also needs to operate well with ICMP and in Equal Cost
   Multi-Path (ECMP) environments.  If further extensibility is
   required, then it should be done in such a manner that it does not
   require the consent of an entity outside of the IETF.

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3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Abbreviations and Acronyms

   The following abbreviations and acronyms are used in this document:

   ACL  - Access Control List

   DT  - NVO3 encapsulation Design Team

   ECMP  - Equal Cost Multi-Path

   EVPN  - Ethernet VPN [RFC8365]

   Geneve  - Generic Network Virtualization Encapsulation [RFC8926]

   GPE  - Generic Protocol Extension

   GUE  - Generic UDP Encapsulation [ietf_intarea_gue]

   HMAC  - Hash based keyed Message Authentication Code [RFC2104]

   IEEE  - Institute for Electrical and Electronic Engineers
      (www.ieee.org)

   NIC  - Network Interface Card (refers to network interface hardware
      which is not necessarily a discrete "card")

   NSH  - Network Service Header [RFC8300]

   NVA  - Network Virtualization Authority

   NVE  - Network Virtual Edge (device)

   NVO3  - Network Virtualization Overlays over Layer 3

   OAM  - Operations, Administration, and Maintenance [RFC6291]

   PWE3  - Pseudowire Emulation Edge to Edge

   TCAM  - Ternary Content-Addressable Memory

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   TLV  - Type, Length, and Value

   Transit device  - Underlay network devices between NVE(s).

   UUID  - Universally Unique Identifier

   VNI  - Virtual Network Identifier

   VXLAN  - Virtual eXtensible LAN [RFC7348]

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                    ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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                          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

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   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
   Broadcast, 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.

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   *  Security, e.g., of the VNI, as discussed in Section 6.2.2, has not
      been addressed by GPE.  Although a shim header could be added for
      security and to support other extensions, this has not been
      defined yet.  More study would be needed to understand the
      implication of such a shim on offloading in NICs.

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

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   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.

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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

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   header length which would then become the de facto maximum total
   header length.

   The recommended minimum total available 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

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   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.

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   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.

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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, perhaps for a
   telemetry case, 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:

   1.  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 (see Section 6.9).  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.

   2.  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.

   3.  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

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       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.

   4.  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.

   5.  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.

   6.  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.

   7.  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 size using TLV extensions
       for telemetry purpose 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.

   8.  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

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       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.

   9.  The DT has addressed the usage requirements (see Section 6) 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.

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   *  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.

8.  Acknowledgements

   The authors would like to thank Tom Herbert for providing the
   motivation for the Security/Integrity extension, and for his valuable
   comments, T.  Sridhar for his valuable comments and feedback, Anoop
   Ghanwani for his extensive comments, and Ignas Bagdonas.

9.  Security Considerations

   This document does not introduce any additional security constraints;
   however, Section 6.2.2 discusess security/integrity extensions and
   this document suggests, in Section 7, that the the nvo3 WG work on
   security options for Geneve.

10.  IANA Considerations

   This document requires no IANA actions.

11.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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.

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   [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>.

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   [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>.

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   [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.

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   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.

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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.

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   +----------------+----------------+----------------+----------------+
   |                |     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

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Contributors

   The following co-authors have contributed to this document:.

   Ilango Ganga
   Intel
   Email: ilango.s.ganga@intel.com

   Pankaj Garg
   Microsoft
   Email: pankajg@microsoft.com

   Rajeev Manur
   Broadcom
   Email: rajeev.manur@broadcom.com

   Tal Mizrahi
   Huawei
   Email: tal.mizrahi.phd@gmail.com

   David Mozes
   Email: mosesster@gmail.com

   Erik Nordmark
   ZEDEDA
   Email: nordmark@sonic.net

   Michael Smith
   Cisco
   Email: michsmit@cisco.com

   Sam Aldrin
   Google
   Email: aldrin.ietf@gmail.com

Authors' Addresses

   Sami Boutros (editor)
   Ciena Corporation
   United States of America

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   Email: sboutros@ciena.com

   Donald E. Eastlake 3rd (editor)
   Futurewei Technologies
   2386 Panoramic Circle
   Apopka, Florida 32703
   United States of America
   Phone: +1-508-333-2270
   Email: d3e3e3@gmail.com

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