RFC 8892 | ifType Guidelines | August 2020 |
Thaler & Romascanu | Standards Track | [Page] |
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.¶
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 (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.¶
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.¶
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.¶
This document addresses the following issues:¶
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.¶
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.¶
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.¶
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.¶
Someone wishing to register a new ifType or tunnelType value MUST:¶
Upon receipt of a registration request, the following steps MUST be followed:¶
[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:¶
The following template describes the fields that MUST be supplied in a registration request suitable for adding to the "Interface Types (ifType)" registry:¶
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:¶
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:¶
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:¶
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:¶
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.¶
IANA is to complete the following to clarify the relationship between MIB modules, YANG modules, and the relevant registries.¶
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."¶
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."¶
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."¶
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.)¶
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.¶
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].¶