Guidelines and Registration Procedures for Interface Types and Tunnel Types
RFC 8892

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Internet Engineering Task Force (IETF)                         D. Thaler
Request for Comments: 8892                                     Microsoft
Updates: 2863                                               D. Romascanu
Category: Standards Track                                    Independent
ISSN: 2070-1721                                              August 2020

 Guidelines and Registration Procedures for Interface Types and Tunnel
                                 Types

Abstract

   This document provides guidelines and procedures for those who are
   defining, registering, or evaluating definitions of new interface
   types ("ifType" values) and tunnel types.  The original definition of
   the IANA interface type registry predated the use of IANA
   Considerations sections and YANG modules, so some confusion arose
   over time.  Tunnel types were added later, with the same requirements
   and allocation policy as interface types.  This document updates RFC
   2863 and provides updated guidance for these registries.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8892.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Problems
   4.  Interface Sub-layers and Sub-types
     4.1.  Alternate Values
   5.  Available Formats
   6.  Registration
     6.1.  Procedures
     6.2.  Media-Specific OID-Subtree Assignments
     6.3.  Registration Template
       6.3.1.  ifType
       6.3.2.  tunnelType
   7.  IANA Considerations
     7.1.  MIB and YANG Modules
     7.2.  Transmission Number Assignments
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

1.  Introduction

   The IANA IfType-MIB, which contains the list of interface type
   (ifType) values, was originally defined in [RFC1573] as a separate
   MIB module together with the Interfaces Group MIB (IF-MIB) module.
   The IF-MIB module was subsequently updated and is currently specified
   in [RFC2863], but the latest IF-MIB RFC no longer contains the IANA
   IfType-MIB.  Instead, the IANA IfType-MIB is maintained by IANA as a
   separate module.  Similarly, [RFC7224] created an initial IANA
   Interface Type YANG Module, and the current version is maintained by
   IANA.

   The current IANA IfType registry is at [ifType-registry], with the
   same values also appearing in both [yang-if-type] and the IANAifType
   textual convention at [IANAifType-MIB].

   Although the ifType registry was originally defined in a MIB module,
   the assignment and use of interface type values are not tied to MIB
   modules or any other management mechanism.  An interface type value
   can be used as the value of a data model object (MIB object, YANG
   object, etc.), as part of a unique identifier of a data model for a
   given interface type (e.g., in an OID), or simply as a value exposed
   by local APIs or UIs on a device.

   The TUNNEL-MIB was defined in [RFC2667] (now obsoleted by [RFC4087]),
   which created a tunnelType registry ([tunnelType-registry] and the
   IANAtunnelType textual convention at [IANAifType-MIB]), and it
   defined the assignment policy for tunnelType values to always be
   identical to the policy for assigning ifType values.

2.  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.

3.  Problems

   This document addresses the following issues:

   1.  As noted in Section 1, the original guidance was written with
       wording specific to MIB modules; accordingly, some confusion has
       resulted when using YANG modules.  This document clarifies that
       ifTypes and tunnelTypes are independent from the type of, or even
       existence of, a data model.

   2.  The use of and requirements around sub-layers and sub-types were
       not well understood, but good examples of both now exist.  This
       is discussed in Section 4.

   3.  Since the "Interface Types (ifType)" and "Tunnel Types
       (tunnelType)" registries were originally defined, and are still
       retrievable, in the format of MIB modules (in addition to other
       formats), confusion arose when adding YANG modules as another
       format as to whether each is a separate registry.  This is
       discussed in Section 5.

   4.  The registries are retrievable in the format of MIB and YANG
       modules, but there was previously no process guidance written to
       check that those formats were syntactically correct as updates
       were made, which led to the registry having syntax errors that
       broke tools.  Section 6.1 adds a validation step to the
       documented assignment procedure.

   5.  Various documents and registries previously said to submit
       requests via email, but a web form exists for submitting
       requests, which caused some confusion around which was to be
       used.  This is addressed in Section 6.1.

   6.  Transmission values [transmission-registry] have generally been
       allocated as part of ifType allocation, but no guidance existed
       as to whether a requester must ask for it or not, and the request
       form had no such required field.  As a result, IANA has asked the
       designated expert to decide for each allocation, but no relevant
       guidance for the designated expert has been documented.  This is
       remedied in Section 6.2.

4.  Interface Sub-layers and Sub-types

   When multiple sub-layers exist below the network layer, each sub-
   layer can be represented by its own row in the ifTable with its own
   ifType, with the ifStackTable being used to identify the upward and
   downward multiplexing relationships between rows.  Section 3.1.1 of
   [RFC2863] provides more discussion, and 3.1.2 provides guidance for
   defining interface sub-layers.  More recent experience shows that
   those guidelines were phrased in a way that is now too restrictive,
   since at the time [RFC2863] was written, MIB modules were the
   dominant data model.

   This document clarifies that the same guidance also applies to YANG
   modules.

   Some ifTypes may define sub-types.  For example, the tunnel(131)
   ifType defines sub-types known as "tunnelTypes", where each
   tunnelType can have its own MIB and/or YANG module with protocol-
   specific information, but there is enough in common that some
   information is exposed in a generic IP Tunnel MIB corresponding to
   the tunnel(131) ifType.

   For requests that involve multiple sub-layers below the network
   layer, requests MUST include (or reference) a discussion of the
   multiplexing relationships between sub-layers, ideally with a
   diagram.  Various well-written examples exist of such definitions,
   including Section 3.4.1 of [RFC3637], Section 3.1.1 of [RFC4087], and
   Section 3.1.1 of [RFC5066].

   Definers of sub-layers and sub-types should consider which model is
   more appropriate for their needs.  A sub-layer is generally used
   whenever either a dynamic relationship exists (i.e., when the set of
   instances above or below a given instance can change over time) or a
   multiplexing relationship exists with another sub-layer.  A sub-type
   can be used when neither of these is true but where one interface
   type is conceptually a subclass of another interface type, as far as
   a management data model is concerned.

   In general, the intent of an interface type or sub-type is that its
   definition should be sufficient to identify an interoperable
   protocol.  In some cases, however, a protocol might be defined in a
   way that is not sufficient to provide interoperability with other
   compliant implementations of that protocol.  In such a case, an
   ifType definition should discuss whether specific instantiations (or
   profiles) of behavior should use a sub-layer model (e.g., each vendor
   might layer the protocol over its own sub-layer that provides the
   missing details) or a sub-type model (i.e., each vendor might
   subclass the protocol without any layering relationship).  If a sub-
   type model is more appropriate, then the data model for the protocol
   might include a sub-type identifier so that management software can
   discover objects specific to the sub-type.  In either case, such
   discussion is important to guide definers of a data model for the
   more specific information (i.e., a lower sub-layer or a sub-type), as
   well as the designated expert, who must review requests for any such
   ifTypes or sub-types.

4.1.  Alternate Values

   Another design decision is whether to reuse an existing ifType or
   tunnelType value, possibly using a sub-type or sub-layer model for
   refinements, or to use a different value for a new mechanism.

   If there is already an entry that could easily satisfy the modeling
   and functional requirements for the requested entry, it should be
   reused so that applications and tools that use the existing value can
   be used without changes.  If, however, the modeling and functional
   requirements are significantly different enough such that having
   existing applications and tools use the existing value would be seen
   as a problem, a new value should be used.

   For example, originally different ifType values were used for
   different flavors of Ethernet (ethernetCsmacd(6), iso88023Csmacd(7),
   fastEther(62), etc.), typically because they were registered by
   different vendors.  Using different values was, however, seen as
   problematic because all were functionally similar, so [RFC3635] then
   deprecated all but ethernetCsmacd(6).

   As another example, a udp(8) tunnelType value was defined in
   [RFC2667] with the description "The value UDP indicates that the
   payload packet is encapsulated within a normal UDP packet (e.g., RFC
   1234)."  The Teredo tunnel protocol [RFC4380] was later defined and
   encapsulates packets over UDP, but the protocol model is quite
   different between [RFC1234] and Teredo.  For example, [RFC1234]
   supports encapsulation of multicast/broadcast traffic, whereas Teredo
   does not.  As such, it would be more confusing to applications and
   tools to represent them using the same tunnel type, and so [RFC4087]
   defined a new value for Teredo.

   In summary, definers of new interface or tunnel mechanisms should use
   a new ifType or tunnelType value rather than reuse an existing value
   when key aspects such as the header format or the link model (point-
   to-point, non-broadcast multi-access, broadcast-capable multi-access,
   unidirectional broadcast, etc.) are significantly different from
   existing values, but they should reuse the same value when the
   differences can be expressed in terms of differing values of existing
   objects other than ifType/tunnelType in the standard YANG or MIB
   module.

5.  Available Formats

   Many registries are available in multiple formats.  For example, XML,
   HTML, CSV, and plain text are common formats in which many registries
   are available.  This document clarifies that the [IANAifType-MIB],
   [yang-if-type], and [yang-tunnel-type] MIB and YANG modules are
   merely additional formats in which the "Interface Types (ifType)" and
   "Tunnel Types (tunnelType)" registries are available.  The MIB and
   YANG modules are not separate registries, and the same values are
   always present in all formats of the same registry.

   The confusion stemmed in part from the fact that the IANA "Protocol
   Registries" [protocol-registries] listed the YANG and MIB module
   formats separately, as if they were separate registries.  However,
   the entries for the yang-if-type and iana-tunnel-type YANG modules
   said "See ifType definitions registry" and "See tunnelType registry
   (mib-2.interface.ifTable.ifEntry.ifType.tunnelType)" respectively,
   although the entry for the IANAifType-MIB had no such note.
   Section 7.1 addresses this.

6.  Registration

   The IANA policy (using terms defined in [RFC8126]) for registration
   is Expert Review for both interface types and tunnel types.  The role
   of the designated expert in the procedure is to raise possible
   concerns about wider implications of proposals for use and deployment
   of interface types.  While it is recommended that the responsible
   Area Director and the IESG take into consideration the designated
   expert opinions, nothing in the procedure empowers the designated
   expert to override properly arrived-at IETF or working group
   consensus.

6.1.  Procedures

   Someone wishing to register a new ifType or tunnelType value MUST:

   1.  Check the IANA registry to see whether there is already an entry
       that could easily satisfy the modeling and functional
       requirements for the requested entry.  If there is already such
       an entry, use it or update the existing specification.  Text in
       an Internet-Draft or part of some permanently available, stable
       specification may be written to clarify the usage of an existing
       entry or entries for the desired purpose.

   2.  Check the IANA registry to see whether there is already some
       other entry with the desired name.  If there is already an
       unrelated entry under the desired name, choose a different name.

   3.  Prepare a registration request using the template specified in
       Section 6.3.  The registration request can be contained in an
       Internet-Draft, submitted alone, or submitted as part of some
       permanently available, stable specification.  The registration
       request can also be submitted in some other form (as part of
       another document or as a stand-alone document), but the
       registration request will be treated as an "IETF Contribution"
       under the guidelines of [RFC5378].

   4.  Submit the registration request (or pointer to a document
       containing it) to IANA at iana@iana.org or (if requesting an
       ifType) via the web form at <https://www.iana.org/form/iftype>.

   Upon receipt of a registration request, the following steps MUST be
   followed:

   1.  IANA checks the submission for completeness; if required
       information is missing or any citations are not correct, IANA
       will reject the registration request.  A registrant can resubmit
       a corrected request if desired.

   2.  IANA requests Expert Review of the registration request against
       the corresponding guidelines from this document.

   3.  The designated expert will evaluate the request against the
       criteria.

   4.  Once the designated expert approves a registration, IANA updates
       [ifType-registry], [IANAifType-MIB], and [yang-if-type] to show
       the registration for an interface type, or [tunnelType-registry],
       [IANAifType-MIB], and [yang-tunnel-type] to show the registration
       for a tunnel type.  When adding values, IANA should verify that
       the updated MIB module and YANG module formats are syntactically
       correct before publishing the update.  There are various existing
       tools or websites that can be used to do this verification.

   5.  If instead the designated expert does not approve registration
       (e.g., for any of the reasons in [RFC8126], Section 5), a
       registrant can resubmit a corrected request if desired, or the
       IESG can override the designated expert and approve it per the
       process in Section 3.3 of [RFC8126].

6.2.  Media-Specific OID-Subtree Assignments

   [IANAifType-MIB] notes:

   |  The relationship between the assignment of ifType values and of
   |  OIDs to particular media-specific MIBs is solely the purview of
   |  IANA and is subject to change without notice.  Quite often, a
   |  media-specific MIB's OID-subtree assignment within MIB-II's
   |  'transmission' subtree will be the same as its ifType value.
   |  However, in some circumstances this will not be the case, and
   |  implementors must not pre-assume any specific relationship between
   |  ifType values and transmission subtree OIDs.

   The advice above remains unchanged, but this document changes the
   allocation procedure to streamline the process, so that an ifType
   value and a transmission number value with the same value will be
   assigned at the same time.

   Rationale:

   (1)  This saves future effort if a transmission number is later
        deemed necessary, since no IANA request is needed that would
        then require another Expert Review.

   (2)  The transmission numbering space is not scarce, so there seems
        to be little need to reserve the number for a different purpose
        than what the ifType is for.

   (3)  The designated expert need not review whether a transmission
        number value should be registered when processing each ifType
        request, thus reducing the possibility of delaying assignment of
        ifType values.

   (4)  There is no case on record where allocating the same value could
        have caused any problems.

6.3.  Registration Template

6.3.1.  ifType

   The following template describes the fields that MUST be supplied in
   a registration request suitable for adding to the "Interface Types
   (ifType)" registry:

   Label for IANA ifType requested:  As explained in Section 7.1.1 of
      [RFC2578], a label for a named-number enumeration must consist of
      one or more letters or digits, up to a maximum of 64 characters,
      and the initial character must be a lowercase letter.  (However,
      labels longer than 32 characters are not recommended.)  Note that
      hyphens are not allowed.

   Name of IANA ifType requested:  A short description (e.g., a protocol
      name) suitable to appear in a comment in the registry.

   Description of the proposed use of the IANA ifType:  Requesters MUST
      include answers, either directly or via a link to a document with
      the answers, to the following questions in the explanation of the
      proposed use of the IANA IfType:

      *  How would IP run over your ifType?

      *  Would there be another interface sub-layer between your ifType
         and IP?

      *  Would your ifType be vendor specific / proprietary?  (If so,
         the label MUST start with a string that shows that.  For
         example, if your company's name or acronym is xxx, then the
         ifType label would be something like xxxSomeIfTypeLabel.)

      *  Would your ifType require or allow vendor-specific extensions?
         If so, would the vendor use their own ifType in a sub-layer
         below the requested ifType, a sub-type of the ifType, or some
         other mechanism?

   Reference, Internet-Draft, or Specification:  A link to a document is
      required.

   Additional information or comments:  Optional; any additional
      comments for IANA or the designated expert.

6.3.2.  tunnelType

   Prior to this document, no form existed for tunnelType (and new
   tunnelType requests did not need to use the ifType form that did
   exist).  This document therefore specifies a tunnelType form.

   The following template describes the fields that MUST be supplied in
   a registration request suitable for adding to the "Tunnel Types
   (tunnelType)" registry:

   Label for IANA tunnelType requested:  As explained in Section 7.1.1
      of [RFC2578], a label for a named-number enumeration must consist
      of one or more letters or digits, up to a maximum of 64
      characters, and the initial character must be a lowercase letter.
      (However, labels longer than 32 characters are not recommended.)
      Note that hyphens are not allowed.

   Name of IANA tunnelType requested:  A short description (e.g., a
      protocol name) suitable to appear in a comment in the registry.

   Description of the proposed use of the IANA tunnelType:  Requesters
      MUST include answers, either directly or via a link to a document
      with the answers, to the following questions in the explanation of
      the proposed use of the IANA tunnelType:

      *  How would IP run over your tunnelType?

      *  Would there be another interface sub-layer between your
         tunnelType and IP?

      *  Would your tunnelType be vendor-specific or proprietary?  (If
         so, the label MUST start with a string that shows that.  For
         example, if your company's name or acronym is xxx, then the
         tunnelType label would be something like
         xxxSomeTunnelTypeLabel.)

      *  Would your tunnelType require or allow vendor-specific
         extensions?  If so, would the vendor use their own tunnelType
         in a sub-layer below the requested tunnelType, or some sort of
         sub-type of the tunnelType, or some other mechanism?

   Reference, Internet-Draft, or Specification:  A link to a document is
      required.

   Additional information or comments:  Optionally any additional
      comments for IANA or the designated expert.

7.  IANA Considerations

   This entire document is about IANA considerations, but this section
   discusses actions taken by and to be taken by IANA.  There are three
   registries affected:

   1.  Interface Types (ifType) [ifType-registry]: The registration
       process is updated in Section 6.1, and the template is updated in
       Section 6.3.1.

   2.  Tunnel Types (tunnelType) [tunnelType-registry]: The registration
       process is updated in Section 6.1, and the template is updated in
       Section 6.3.2.

   3.  Transmission Number Values [transmission-registry]: The
       assignment process is updated in Section 6.2.

   At the time of publication of this document, IANA is unable to
   perform some of the actions requested below due to limitations of
   their current platform and toolset.  In such cases, IANA is requested
   to perform these actions as and when the migration to a new platform
   that would enable these actions is complete.

7.1.  MIB and YANG Modules

   IANA is to complete the following to clarify the relationship between
   MIB modules, YANG modules, and the relevant registries.

   1.   The following note has been added to the IANAifType-MIB at
        [protocol-registries]: "This is one of the available formats of
        the Interface Types (ifType) and Tunnel Types (tunnelType)
        registries."

   2.   The note for the iana-if-type YANG module at
        [protocol-registries] has been updated to read: "This is one of
        the available formats of the Interface Types (ifType) registry."

   3.   The note for the iana-tunnel-type YANG module at
        [protocol-registries] has been updated to read: "This is one of
        the available formats of the Tunnel Types (tunnelType)
        registry."

   4.   The new "Interface Parameters" category at [protocol-registries]
        includes entries for "Interface Types (ifType)"
        [ifType-registry], "Tunnel Types (tunnelType)"
        [tunnelType-registry], and "Transmission Number Values"
        [transmission-registry].

   5.   Update the "Interface Types (ifType)" registry [ifType-registry]
        to list MIB [IANAifType-MIB] and YANG [yang-if-type] as
        Available Formats.

   6.   Update the "Tunnel Types (tunnelType)" registry
        [tunnelType-registry] to list MIB [IANAifType-MIB] and YANG
        [yang-tunnel-type] as Available Formats.

   7.   Replace the [yang-if-type] page with the YANG module content
        rather than having a page that claims to have multiple Available
        Formats.

   8.   Replace the [yang-tunnel-type] page with the YANG module content
        rather than having a page that claims to have multiple Available
        Formats.

   9.   In addition, [IANAifType-MIB] is to be updated as follows:

        OLD:

        |  Requests for new values should be made to IANA via email
        |  (iana@iana.org).

        NEW:

        |  Interface types must not be directly added to the IANAifType-
        |  MIB MIB module.  They must instead be added to the "Interface
        |  Types (ifType)" registry at [ifType-registry].

        (Note that [yang-if-type] was previously updated with this
        language.)

   10.  IANA has added this document as a reference in the "Interface
        Types (ifType)", "Tunnel Types (tunnelType)", and "Transmission
        Number Values" registries, as well as the iana-if-type YANG
        Module, iana-tunnel-type YANG Module, and IANAifType-MIB.

7.2.  Transmission Number Assignments

   Per the discussion in Section 6.2, [ifType-registry] has been updated
   as follows:

   OLD:

   |  For every ifType registration, the corresponding transmission
   |  number value should be registered or marked "Reserved".

   NEW:

   |  For future ifType assignments, an OID-subtree assignment MIB-II's
   |  'transmission' subtree will be made with the same value.

   Similarly, the following change has been made to
   [transmission-registry]:

   OLD:

   |  For every transmission number registration, the corresponding
   |  ifType value should be registered or marked "Reserved".

   NEW:

   |  For future transmission number assignments, an 'ifType' will be
   |  made with the same value.

8.  Security Considerations

   Since this document does not introduce any technology or protocol,
   there are no security issues to be considered for this document
   itself.

   For security considerations related to MIB and YANG modules that
   expose these values, see Section 9 of [RFC2863], Section 6 of
   [RFC4087], and Section 3 of [RFC8675].

9.  References

9.1.  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>.

   [RFC2578]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Structure of Management Information
              Version 2 (SMIv2)", STD 58, RFC 2578,
              DOI 10.17487/RFC2578, April 1999,
              <https://www.rfc-editor.org/info/rfc2578>.

   [RFC2863]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group
              MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000,
              <https://www.rfc-editor.org/info/rfc2863>.

   [RFC5378]  Bradner, S., Ed. and J. Contreras, Ed., "Rights
              Contributors Provide to the IETF Trust", BCP 78, RFC 5378,
              DOI 10.17487/RFC5378, November 2008,
              <https://www.rfc-editor.org/info/rfc5378>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

9.2.  Informative References

   [IANAifType-MIB]
              IANA, "IANAifType-MIB Definitions",
              <http://www.iana.org/assignments/ianaiftype-mib>.

   [ifType-registry]
              IANA, "Interface Types (ifType)",
              <https://www.iana.org/assignments/smi-numbers>.

   [protocol-registries]
              IANA, "Protocol Registries",
              <https://www.iana.org/protocols>.

   [RFC1234]  Provan, D., "Tunneling IPX traffic through IP networks",
              RFC 1234, DOI 10.17487/RFC1234, June 1991,
              <https://www.rfc-editor.org/info/rfc1234>.

   [RFC1573]  McCloghrie, K. and F. Kastenholz, "Evolution of the
              Interfaces Group of MIB-II", RFC 1573,
              DOI 10.17487/RFC1573, January 1994,
              <https://www.rfc-editor.org/info/rfc1573>.

   [RFC2667]  Thaler, D., "IP Tunnel MIB", RFC 2667,
              DOI 10.17487/RFC2667, August 1999,
              <https://www.rfc-editor.org/info/rfc2667>.

   [RFC3635]  Flick, J., "Definitions of Managed Objects for the
              Ethernet-like Interface Types", RFC 3635,
              DOI 10.17487/RFC3635, September 2003,
              <https://www.rfc-editor.org/info/rfc3635>.

   [RFC3637]  Heard, C.M., Ed., "Definitions of Managed Objects for the
              Ethernet WAN Interface Sublayer", RFC 3637,
              DOI 10.17487/RFC3637, September 2003,
              <https://www.rfc-editor.org/info/rfc3637>.

   [RFC4087]  Thaler, D., "IP Tunnel MIB", RFC 4087,
              DOI 10.17487/RFC4087, June 2005,
              <https://www.rfc-editor.org/info/rfc4087>.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,
              <https://www.rfc-editor.org/info/rfc4380>.

   [RFC5066]  Beili, E., "Ethernet in the First Mile Copper (EFMCu)
              Interfaces MIB", RFC 5066, DOI 10.17487/RFC5066, November
              2007, <https://www.rfc-editor.org/info/rfc5066>.

   [RFC7224]  Bjorklund, M., "IANA Interface Type YANG Module",
              RFC 7224, DOI 10.17487/RFC7224, May 2014,
              <https://www.rfc-editor.org/info/rfc7224>.

   [RFC8675]  Boucadair, M., Farrer, I., and R. Asati, "A YANG Data
              Model for Tunnel Interface Types", RFC 8675,
              DOI 10.17487/RFC8675, November 2019,
              <https://www.rfc-editor.org/info/rfc8675>.

   [transmission-registry]
              IANA, "Transmission Number Values",
              <https://www.iana.org/assignments/smi-numbers>.

   [tunnelType-registry]
              IANA, "Tunnel Types (tunnelType)",
              <https://www.iana.org/assignments/smi-numbers>.

   [yang-if-type]
              IANA, "iana-if-type YANG Module",
              <http://www.iana.org/assignments/iana-if-type>.

   [yang-tunnel-type]
              IANA, "iana-tunnel-type YANG Module",
              <https://www.iana.org/assignments/iana-tunnel-type>.

Authors' Addresses

   Dave Thaler
   Microsoft

   Email: dthaler@microsoft.com

   Dan Romascanu
   Independent

   Email: dromasca@gmail.com