INTERNET-DRAFT                                         Sami Boutros(Ed.)
Intended Status: Informational                                    VMware

                                                            Ilango Ganga

                                                             Pankaj Garg

                                                            Rajeev Manur

                                                             Tal Mizrahi

                                                             David Mozes

                                                           Erik Nordmark

                                                           Michael Smith

Expires: December 9, 2017                                   June 7, 2017

                   NVO3 Encapsulation Considerations


   As communicated by WG Chairs, the IETF NVO3 chairs and Routing Area
   director have chartered a design team to take forward the
   encapsulation discussion and see if there is potential to design a
   common encapsulation that addresses the various technical concerns.

   There are implications of different encapsulations in real
   environments consisting of both software and hardware implementations
   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 recommend Geneve with few modifications as the common
   encapsulation, more details are described in section 7.

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Status of this Memo

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Table of Contents

   1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . .  4
   2. Design Team Goals . . . . . . . . . . . . . . . . . . . . . . .  4
   3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4. Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5. Issues with current Encapsulations  . . . . . . . . . . . . . .  5
     5.1 Geneve . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.2 GUE  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5

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     5.3 VXLAN-GPE  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6. Common Encapsulation Considerations . . . . . . . . . . . . . .  6
     6.1 Current Encapsulations . . . . . . . . . . . . . . . . . . .  6
     6.2 Useful Extensions Use cases  . . . . . . . . . . . . . . . .  6
       6.2.1. Telemetry extensions. . . . . . . . . . . . . . . . . .  6
       6.2.2. Security/Integrity extensions . . . . . . . . . . . . .  7
       6.2.3. Group Base Policy . . . . . . . . . . . . . . . . . . .  7
     6.3 Hardware Considerations  . . . . . . . . . . . . . . . . . .  7
     6.4 Extension Size . . . . . . . . . . . . . . . . . . . . . . .  8
     6.5 Extension Ordering . . . . . . . . . . . . . . . . . . . . .  9
     6.6 TLV vs Bit Fields  . . . . . . . . . . . . . . . . . . . . .  9
     6.7 Control Plane Considerations . . . . . . . . . . . . . . . . 10
     6.8 Split NVE  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.9 Larger VNI Considerations  . . . . . . . . . . . . . . . . . 11
   7. Design team recommendations . . . . . . . . . . . . . . . . . . 11
   8. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 14
   9. Security Considerations . . . . . . . . . . . . . . . . . . . . 14
   10.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2  Informative References . . . . . . . . . . . . . . . . . . 15
   11. Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.2.  Extensibility . . . . . . . . . . . . . . . . . . . . . . 15
       11.2.1.  Native Extensibility Support  . . . . . . . . . . . . 15
       11.2.2.  Extension Parsing . . . . . . . . . . . . . . . . . . 15
       11.2.3.  Critical Extensions . . . . . . . . . . . . . . . . . 16
       11.2.4.  Maximal Header Length . . . . . . . . . . . . . . . . 16
     11.3.  Encapsulation Header  . . . . . . . . . . . . . . . . . . 16
       11.3.1.  Virtual Network Identifier (VNI)  . . . . . . . . . . 16
       11.3.2.  Next Protocol . . . . . . . . . . . . . . . . . . . . 16
       11.3.3.  Other Header Fields . . . . . . . . . . . . . . . . . 17
     11.4.  Comparison Summary  . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses (In alphabetical order) . . . . . . . . . . . . 18

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1. Problem Statement

   As communicated by WG Chairs, the NVO3 WG charter states that it may
   produce requirements for network virtualization data planes based on
   encapsulation of virtual network traffic over an IP-based underlay
   data plane. Such requirements should consider OAM and security. Based
   on these requirements the WG will select, extend, and/or develop one
   or more data plane encapsulation format(s).

   This has led to drafts describing three encapsulations being adopted
   by the working group:

   - draft-ietf-nvo3-geneve-03

   - draft-ietf-nvo3-gue-04

   - draft-ietf-nvo3-vxlan-gpe-02

   Discussion on the list and in face-to-face meetings has identified a
   number of technical problems with each of these encapsulations.
   Furthermore, there was clear consensus at the IETF meeting in Berlin
   that it is undesirable for the working group to progress more than
   one data plane encapsulation. Although consensus could not be reached
   on the list, the overall consensus was for a single encapsulation
   (RFC2418, Section 3.3). Nonetheless there has been resistance to
   converging on a single encapsulation format.

2. Design Team Goals

   As communicated by WG Chairs, the design team should 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 should specifically avoid a design that is burdensome on
   hardware implementations, but should allow future extensibility. The
   chosen design should also operate well with ICMP and in 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.

3. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

4. Abbreviations

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   NVO3 Network Virtualization Overlays over Layer 3

   OAM Operations, Administration, and Maintenance

   TLV Type, Length, and Value

   VNI Virtual Network Identifier

   NVE Network Virtualization Edge

   NVA Network Virtualization Authority

   NIC Network interface card

   Transit device Underlay network devices between NVE(s).

5. Issues with current Encapsulations

   As summarized by WG Chairs.

5.1 Geneve

   - Can't be implemented cost-effectively in all use cases because
   variable length header and order of the TLVs makes is 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.

5.2 GUE

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


   - GPE is not day-1 backwards compatible with VXLAN. 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 is the same.

   - GPE is insufficiently extensible. Numerous extensions and options
   have been designed for GUE and Geneve. Note that these have not yet
   been validated by the WG.

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   - 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).

   - 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 extension is 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 TLV 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, as well
   as network planning and network 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 from 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, be it that access to the end-system is
   not feasible, or that the diversity of parameters and associated
   probe packets to be created is simply too large. An in-band telemetry
   mechanism in extensions 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. Extensions are needed to use any
   sophisticated security.

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

   One can envision HMAC-like support in some NVO3 extension to
   authenticate the header and the outer IP addresses, thereby
   preventing attackers from injecting packets with spoofed VNIs.

   An other aspect of security is payload security. Essentially this is
   to make packets that look like IP|UDP|NVO3 Encap|DTLS/IPSEC-ESP
   Extension|payload. This is nice since we still have the UDP header
   for ECMP, the NVO3 header is in plain text so it can by 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
   for DTLS/IPSEC).

6.2.3. Group Base 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 [VXLAN-GBP]. This allows
   various forms of policy such as access control and QoS to be applied
   between abstract groups rather than coupled to specific endpoint

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

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   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.

   We note that 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 an transit
   devices that participates in the NVO3 dataplane e.g. for OAM

   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
   order for some hardware implementations.

   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
   for both NICs doing e.g. TCP offload and transit devices and NVEs
   applying policy/ACLs to the inner payload.

6.4 Extension Size

   Extension header length has a significant impact to hardware and
   software implementations. A total header length that is too small
   will unnecessarily constrained software flexibility. A total header
   length that is too large will place a nontrivial cost on hardware
   implementations. Thus, the design team recommends that there be a
   minimum and maximum total extension header length selected. The
   maximum total header length is determined by the bits 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 the minimum total header length which would then become the
   de facto maximum total header length. The recommended minimum total
   header length is 64 bytes.

   Single Extension size should always be 4 bytes 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.

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6.5 Extension Ordering

   In order to support hardware nodes at the tunnel endpoint or at the
   transit that can process one or few extensions TLVs in TCAM. A
   control plane in such a deployment can signal a capability to ensure
   a specific TLV will always appear in a specific order for example the
   first one in the packet.

   The order of the TLVs should be HW friendly for both the sender and
   the receiver and possibly the transit node too.

   Transit nodes doesn't participate in control plane communication
   between the end points and are not required to process the options
   however, if they do, they need to process only a small subset of
   options that will be consumed by tunnel endpoints.

6.6 TLV vs 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-field approach 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 TLV of arbitrary order, size, and repetition of the same
   order is difficult to implement in hardware. A middle ground is to
   use TLV with restrictions on the size and alignment, observing that
   individual TLVs can have a fixed length, and support in the control
   plane such that an NVE will only receive options that to 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 they need more effort, But transit devices would have
   issues with future GUE bits being defined for future options as well.

   A benefit of TLVs  from a HW perspective is that they are self
   describing i.e., all the information is in the TLV. In a Bit fields
   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
   NVE. This can be modules such as a diagnostic module by one vendor
   that does packet sampling and another module from a different vendor

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   that does 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 like mac learning and security
   handle different options.

6.7 Control Plane Considerations

   Given that we want to allow large flexibility and extensibility for
   e.g. software NVEs, yet be able to support key extensions in less
   flexible e.g. hardware NVEs, it is useful to consider the control
   plane. By control plane in this context we mean both protocols such
   as EVPN and others, and also deployment specific configuration.

   If each NVE can express in the control plane that they only care
   about particular extensions (could be a single extension, or a few),
   and the source NVEs only include requested 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 it can provide useful functionality with
   minimal hardware requirements.

   Note that 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 extensions are included in
   the packet. However, 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 in this approach different NVEs could desire different
   (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

   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 targets that do not support them.

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   The control planes defined today need to add the ability to describe
   the different encapsulations. Thus perhaps EVPN, and any other
   control plane protocol that the IETF defines, should have a way to
   enumerate the supported NVO3 extensions and their order.

   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. So, having the control plane negotiate the
   constraints is most appropriate and flexible way to address these

6.8 Split NVE

   If the working group sees a need for having the hosts send and
   receive options in a split NVE case, 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 Ether type for other encapsulations so that it can be
   carried over Ethernet and 802.1Q.

   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

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

7. Design team recommendations

   We concluded that Geneve is most suitable as a starting point for
   proposed standard for network virtualization, for the following

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   1. We studied whether VNI should be in base header or in extensions
   and whether it should be 24-bit or 32-bit. The design team agreed
   that VNI is critical information for network virtualization and MUST
   be present in all packets. Design team also agreed that 24-bit VNI
   matches the existing widely used encapsulation format i.e. VxLAN and
   NVGRE and hence more suitable to use going forward.

   2. Geneve has the total options length that allow skipping over the
   options for NIC offload operations, and will allow transit devices to
   view flow information in the inner payload.

   3. We considered the option of using NSH 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
   header length in VxLAN-GPE and hence 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 guidance, 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.

   4. We compared the TLV vs Bit-fields style extension and 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. As
   well 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 TLV 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
   dataplane implementation to handle.

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   5. We briefly discussed 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. We also agreed that the C bit in Geneve is helpful to allow
   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 receiver NVE and thus make it easy for NVE
   to skip over the options. Thus the C-bit remains as defined in

   7. There are already some extensions that are being discussed (see
   section 6.2) of varying sizes, by using Geneve option 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 and DTLS/IPSEC to authenticate the Geneve packet header and
   secure the Geneve packet payload by software or hardware tunnel
   endpoints. As well, a Group Based Policy extension TLV can be

   8. There are implemented Geneve options today in production. There
   are as well new HW supporting Geneve TLV parsing. In addition In-band
   Telemetry (INT) specification being developed by illustrates
   the option of INT meta data carried over Geneve. OVN/OVS have also
   defined some option TLV(s) for Geneve.

   9. The DT has addressed the usage models while considering the
   requirements and implementations in general that includes software
   and hardware.

   There seems to be interest to standardize 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

   We recommend the following enhancements to Geneve to make it more
   suitable to hardware and yet provide the flexibility for software:

   We would propose a text such as, while TLV 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 secure hash that must be
   processed in certain order at lowest latency.

   A control plane can negotiate a subset of option TLVs and certain TLV

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   ordering, as well can limit the total number of option TLVs present
   in the packet, for example, to allow hardware capable of processing
   fewer options. Hence, the control planes need to have the ability to
   describe the supported TLVs subset and their order.

   The Geneve draft could specify that the subset and order of option
   TLVs should be configurable for each remote NVE in the absence of a
   protocol control plane.

   We recommend Geneve to follow fragmentation recommendations in
   overlay services like PWE3, and L2/L3 VPN recommendation to guarantee
   larger MTU for the tunnel overhead

   We request Geneve to provide a recommendation for critical bit
   processing - text could look like 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 Geneve to increase the Single TLV option length
   to 256.

   We request Geneve to address Requirements for OAM considerations for
   alternate marking and for performance measurements that need 2 bits
   in the header. And clarify the need of the current OAM bit in the
   Geneve Header.

   We recommend the WG to 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, and would like to thank T. Sridhar for his valuable
   comments and feedback.

9. Security Considerations

   This document does not introduce any additional security constraints.

10.  References

10.1  Normative References

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

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10.2  Informative References

   [Geneve] Generic Network Virtualization Encapsulation [I-D.ietf-nvo3-
   [GUE] Generic UDP Encapsulation [I-D.ietf-nvo3-gue]
   [NSH] Network Service Header [I-D.ietf-sfc-nsh]
   [VXLAN-GPE] Virtual eXtensible Local Area Network - Generic Protocol
   Extension [I-D.ietf-nvo3-vxlan-gpe]

   [VXLAN-GBP] VXLAN Group Policy Option - [I-D.draft-smith-vxlan-group-

11. Appendix A

11.1.  Overview

   This section presents a comparison of the three NVO3 encapsulation
   proposals, Geneve, GUE, and 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 3.2 below.

11.2.  Extensibility

11.2.1.  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 [I-D.ietf-nvo3-vxlan-gpe], extensibility can
   be attained to some extent if the Network Service Header (NSH) [I-
   D.ietf-sfc-nsh] 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).  It should be noted
   that NSH-over-VXLAN-GPE implies an additional overhead of the 8-
   octets NSH header, in addition to the VXLAN-GPE header.

11.2.2.  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 [I-D.herbert-

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   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

   The Geneve and GUE headers both include a length field, defining the
   total length of the encapsulation, including the optional extensions.

   The length field simplifies the parsing of transit devices that skip
   the encapsulation header without parsing its extensions.

11.2.3.  Critical Extensions

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

11.2.4.  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.

11.3.  Encapsulation Header

11.3.1.  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 [I-D.herbert-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 [I-D.herbert-gue-extensions].

11.3.2.  Next Protocol

   The three encapsulation headers include a field that specifies the

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   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 [I-D.ietf-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

11.3.3.  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 [I-D.ietf-nvo3-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.

11.4.  Comparison Summary

   The following table summarizes the comparison between the three NVO3

     |                |     Geneve     |      GUE       |   VXLAN-GPE    |
     | Outer transport|     UDP/IP     |     UDP/IP     |     UDP/IP     |
     | Base header    |    8 octets    |    4 octets    |    8 octets    |
     | length         |                |                |  (16 octets    |
     |                |                |                |   using NSH)   |
     | Extensibility  |Variable length |Extension fields| No native ext- |
     |                |    options     |                | ensibility.    |
     |                |                |                | Extensible     |
     |                |                |                | using NSH.     |

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     | 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     |       -        |    27 bits     |

                     Figure 1: NVO3 Encapsulation Comparison

Authors' Addresses (In alphabetical order)

   Sami Boutros

   Ilango Ganga

   Pankaj Garg

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   Rajeev Manur

   Tal Mizrahi

   David Mozes

   Erik Nordmark
   Arista Networks

   Michael Smith

   Sam Aldrin

   Ignas Bagdonas

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