| Internet-Draft | OSPF App-Specific Link Attributes | May 2023 |
| Psenak, et al. | Expires 20 November 2023 | [Page] |
Existing traffic-engineering-related link attribute advertisements have been defined and are used in RSVP-TE deployments. Since the original RSVP-TE use case was defined, additional applications (e.g., Segment Routing Policy and Loop-Free Alternates) that also make use of the link attribute advertisements have been defined. In cases where multiple applications wish to make use of these link attributes, the current advertisements do not support application-specific values for a given attribute, nor do they support indication of which applications are using the advertised value for a given link. This document introduces new link attribute advertisements in OSPFv2 and OSPFv3 that address both of these shortcomings.¶
This document obsoletes RFC 8920.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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This Internet-Draft will expire on 20 November 2023.¶
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NOTE: This document makes modest editorial changes to the content of RFC 8920 which it obsoletes. A detailed description of the changes is provided in Section 15.¶
Advertisement of link attributes by the OSPFv2 [RFC2328] and OSPFv3 [RFC5340] protocols in support of traffic engineering (TE) was introduced by [RFC3630] and [RFC5329], respectively. It has been extended by [RFC4203], [RFC7308], and [RFC7471]. Use of these extensions has been associated with deployments supporting Traffic Engineering over Multiprotocol Label Switching (MPLS) in the presence of the Resource Reservation Protocol (RSVP), more succinctly referred to as RSVP-TE [RFC3209].¶
For the purposes of this document, an application is a technology that makes use of link attribute advertisements, examples of which are listed in Section 5.¶
In recent years, new applications have been introduced that have use cases for many of the link attributes historically used by RSVP-TE. Such applications include Segment Routing (SR) Policy [SEGMENT-ROUTING] and Loop-Free Alternates (LFAs) [RFC5286]. This has introduced ambiguity in that if a deployment includes a mix of RSVP-TE support and SR Policy support, for example, it is not possible to unambiguously indicate which advertisements are to be used by RSVP-TE and which advertisements are to be used by SR Policy. If the topologies are fully congruent, this may not be an issue, but any incongruence leads to ambiguity.¶
An example of where this ambiguity causes a problem is a network where RSVP-TE is enabled only on a subset of its links. A link attribute is advertised for the purpose of another application (e.g., SR Policy) for a link that is not enabled for RSVP-TE. As soon as the router that is an RSVP-TE head end sees the link attribute being advertised for that link, it assumes RSVP-TE is enabled on that link, even though it is not. If such an RSVP-TE head-end router tries to set up an RSVP-TE path via that link, it will result in the path setup failure.¶
An additional issue arises in cases where both applications are supported on a link but the link attribute values associated with each application differ. Current advertisements do not support advertising application-specific values for the same attribute on a specific link.¶
This document defines extensions that address these issues. Also, as evolution of use cases for link attributes can be expected to continue in the years to come, this document defines a solution that is easily extensible for the introduction of new applications and new use cases.¶
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.¶
As stated previously, evolution of use cases for link attributes can be expected to continue. Therefore, any discussion of existing use cases is limited to requirements that are known at the time of this writing. However, in order to determine the functionality required beyond what already exists in OSPF, it is only necessary to discuss use cases that justify the key points identified in the introduction, which are:¶
[RFC7855] discusses use cases and requirements for Segment Routing (SR). Included among these use cases is SR Policy, which is defined in [SEGMENT-ROUTING]. If both RSVP-TE and SR Policy are deployed in a network, link attribute advertisements can be used by one or both of these applications. There is no requirement for the link attributes advertised on a given link used by SR Policy to be identical to the link attributes advertised on that same link used by RSVP-TE; thus, there is a clear requirement to indicate independently which link attribute advertisements are to be used by each application.¶
As the number of applications that may wish to utilize link attributes may grow in the future, an additional requirement is that the extensions defined allow the association of additional applications to link attributes without altering the format of the advertisements or introducing new backwards-compatibility issues.¶
Finally, there may still be many cases where a single attribute value can be shared among multiple applications, so the solution must minimize advertising duplicate link/attribute pairs whenever possible.¶
There are existing advertisements used in support of RSVP-TE. These advertisements are carried in the OSPFv2 TE Opaque Link State Advertisement (LSA) [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329]. Additional RSVP-TE link attributes have been defined by [RFC4203], [RFC7308], and [RFC7471].¶
Extended Link Opaque LSAs as defined in [RFC7684] for OSPFv2 and E-Router-LSAs [RFC8362] for OSPFv3 are used to advertise link attributes that are used by applications other than RSVP-TE or GMPLS [RFC4203]. These LSAs were defined as generic containers for distribution of the extended link attributes.¶
This section outlines the solution for advertising link attributes originally defined for RSVP-TE or GMPLS when they are used for other applications.¶
The following are the advantages of Extended Link Opaque LSAs as defined in [RFC7684] for OSPFv2 and E-Router-LSAs [RFC8362] for OSPFv3 with respect to the advertisement of link attributes originally defined for RSVP-TE when used in packet networks and in GMPLS:¶
The disadvantage of this approach is that in rare cases, the same link attribute is advertised in both the TE Opaque and Extended Link Attribute LSAs in OSPFv2 or the Intra-Area-TE-LSA and E-Router-LSA in OSPFv3.¶
The Extended Link Opaque LSA [RFC7684] and E-Router-LSA [RFC8362] are used to advertise any link attributes used for non-RSVP-TE applications in OSPFv2 or OSPFv3, respectively, including those that have been originally defined for RSVP-TE applications (see Section 6).¶
TE link attributes used for RSVP-TE/GMPLS continue to use the OSPFv2 TE Opaque LSA [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329].¶
The format of the link attribute TLVs that have been defined for RSVP-TE applications will be kept unchanged even when they are used for non-RSVP-TE applications. Unique codepoints are allocated for these link attribute TLVs from the "OSPFv2 Extended Link TLV Sub-TLVs" registry [RFC7684] and from the "OSPFv3 Extended-LSA Sub-TLVs" registry [RFC8362], as specified in Section 14.¶
To allow advertisement of the application-specific values of the link attribute, a new Application-Specific Link Attributes (ASLA) sub-TLV is defined. The ASLA sub-TLV is a sub-TLV of the OSPFv2 Extended Link TLV [RFC7684] and OSPFv3 Router-Link TLV [RFC8362].¶
In addition to advertising the link attributes for standardized applications, link attributes can be advertised for the purpose of applications that are not standardized. We call such an application a "user-defined application" or "UDA". These applications are not subject to standardization and are outside of the scope of this specification.¶
The ASLA sub-TLV is an optional sub-TLV of the OSPFv2 Extended Link TLV and OSPFv3 Router-Link TLV. Multiple ASLA sub-TLVs can be present in a parent TLV when different applications want to control different link attributes or when a different value of the same attribute needs to be advertised by multiple applications. The ASLA sub-TLV MUST be used for advertisement of the link attributes listed at the end of this section if these are advertised inside the OSPFv2 Extended Link TLV and OSPFv3 Router-Link TLV. It has the following format:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SABM Length | UDABM Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Standard Application Identifier Bit Mask(SABM) | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | User-Defined Application Identifier Bit Mask(UDABM) | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Attribute sub-sub-TLVs | +- -+ | ... |¶
where:¶
Optional set of bits, where each bit represents a single standard application. Bits are defined in the "Link Attribute Applications" registry, which is defined in [RFC8919]. Current assignments are repeated here for informational purposes:¶
0 1 2 3 4 5 6 7 ...
+-+-+-+-+-+-+-+-+...
|R|S|F| ...
+-+-+-+-+-+-+-+-+...
¶
If the SABM or UDABM Length is other than 0, 4, or 8, the ASLA sub-TLV MUST be ignored by the receiver.¶
Standard Application Identifier Bits are defined and sent starting with bit 0. Undefined bits that are transmitted MUST be transmitted as 0 and MUST be ignored on receipt. Bits that are not transmitted MUST be treated as if they are set to 0 on receipt. Bits that are not supported by an implementation MUST be ignored on receipt.¶
User-Defined Application Identifier Bits have no relationship to Standard Application Identifier Bits and are not managed by IANA or any other standards body. It is recommended that these bits be used starting with bit 0 so as to minimize the number of octets required to advertise all UDAs. Undefined bits that are transmitted MUST be transmitted as 0 and MUST be ignored on receipt. Bits that are not transmitted MUST be treated as if they are set to 0 on receipt. Bits that are not supported by an implementation MUST be ignored on receipt.¶
If the link attribute advertisement is intended to be only used by a specific set of applications, corresponding bit masks MUST be present, and application-specific bit(s) MUST be set for all applications that use the link attributes advertised in the ASLA sub-TLV.¶
Application Identifier Bit Masks apply to all link attributes that support application-specific values and are advertised in the ASLA sub-TLV.¶
The advantage of not making the Application Identifier Bit Masks part of the attribute advertisement itself is that the format of any previously defined link attributes can be kept and reused when advertising them in the ASLA sub-TLV.¶
If the same attribute is advertised in more than one ASLA sub-TLVs with the application listed in the Application Identifier Bit Masks, the application SHOULD use the first instance of advertisement and ignore any subsequent advertisements of that attribute.¶
Link attributes MAY be advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications. Such link attribute advertisements MUST be used by standard applications and/or user defined applications when no link attribute advertisements with a non-zero-length Application Identifier Bit Mask and a matching Application Identifier Bit set are present. Otherwise, such link attribute advertisements MUST NOT be used.¶
This document defines the initial set of link attributes that MUST use the ASLA sub-TLV if advertised in the OSPFv2 Extended Link TLV or in the OSPFv3 Router-Link TLV. Documents that define new link attributes MUST state whether the new attributes support application-specific values and, as such, are advertised in an ASLA sub-TLV. The standard link attributes that are advertised in ASLA sub-TLVs are:¶
This section defines the use case and indicates the codepoints (Section 14) from the "OSPFv2 Extended Link TLV Sub-TLVs" registry and "OSPFv3 Extended-LSA Sub-TLVs" registry for some of the link attributes that have been originally defined for RSVP-TE or GMPLS.¶
The SRLG of a link can be used in OSPF-calculated IPFRR (IP Fast Reroute) [RFC5714] to compute a backup path that does not share any SRLG group with the protected link.¶
To advertise the SRLG of the link in the OSPFv2 Extended Link TLV, the same format for the sub-TLV defined in Section 1.3 of [RFC4203] is used with TLV type 11. Similarly, for OSPFv3 to advertise the SRLG in the OSPFv3 Router-Link TLV, TLV type 12 is used.¶
[RFC3630] defines several link bandwidth types. [RFC7471] defines extended link metrics that are based on link bandwidth, delay, and loss characteristics. All of these can be used to compute primary and backup paths within an OSPF area to satisfy requirements for bandwidth, delay (nominal or worst case), or loss.¶
To advertise extended link metrics in the OSPFv2 Extended Link TLV, the same format for the sub-TLVs defined in [RFC7471] is used with the following TLV types:¶
To advertise extended link metrics in the Router-Link TLV inside the OSPFv3 E-Router-LSA, the same format for the sub-TLVs defined in [RFC7471] is used with the following TLV types:¶
[RFC3630] and [RFC7308] define the Administrative Group and Extended Administrative Group sub-TLVs, respectively.¶
To advertise the Administrative Group and Extended Administrative Group in the OSPFv2 Extended Link TLV, the same format for the sub-TLVs defined in [RFC3630] and [RFC7308] is used with the following TLV types:¶
To advertise the Administrative Group and Extended Administrative Group in the OSPFv3 Router-Link TLV, the same format for the sub-TLVs defined in [RFC3630] and [RFC7308] is used with the following TLV types:¶
[RFC3630] defines the Traffic Engineering Metric.¶
To advertise the Traffic Engineering Metric in the OSPFv2 Extended Link TLV, the same format for the sub-TLV defined in Section 2.5.5 of [RFC3630] is used with TLV type 22. Similarly, for OSPFv3 to advertise the Traffic Engineering Metric in the OSPFv3 Router-Link TLV, TLV type 22 is used.¶
Maximum link bandwidth is an application-independent attribute of the link that is defined in [RFC3630]. Because it is an application-independent attribute, it MUST NOT be advertised in the ASLA sub-TLV. Instead, it MAY be advertised as a sub-TLV of the Extended Link TLV in the Extended Link Opaque LSA in OSPFv2 [RFC7684] or as a sub-TLV of the Router-Link TLV in the E-Router-LSA Router-Link TLV in OSPFv3 [RFC8362].¶
To advertise the maximum link bandwidth in the OSPFv2 Extended Link TLV, the same format for the sub-TLV defined in [RFC3630] is used with TLV type 23.¶
To advertise the maximum link bandwidth in the OSPFv3 Router-Link TLV, the same format for the sub-TLV defined in [RFC3630] is used with TLV type 23.¶
[RFC7471] defines a number of dynamic performance metrics associated with a link. It is conceivable that such metrics could be measured specific to traffic associated with a specific application. Therefore, this document includes support for advertising these link attributes specific to a given application. However, in practice, it may well be more practical to have these metrics reflect the performance of all traffic on the link regardless of application. In such cases, advertisements for these attributes can be associated with all of the applications utilizing that link. This can be done either by explicitly specifying the applications in the Application Identifier Bit Mask or by using a zero-length Application Identifier Bit Mask. The use of zero-length Application Identifier Bit Mask is further discussed in Section 12.2.¶
The Local Interface IPv6 Address sub-TLV is an application-independent attribute of the link that is defined in [RFC5329]. Because it is an application-independent attribute, it MUST NOT be advertised in the ASLA sub-TLV. Instead, it MAY be advertised as a sub-TLV of the Router-Link TLV inside the OSPFv3 E-Router-LSA [RFC8362].¶
To advertise the Local Interface IPv6 Address sub-TLV in the OSPFv3 Router-Link TLV, the same format for the sub-TLV defined in [RFC5329] is used with TLV type 24.¶
The Remote Interface IPv6 Address sub-TLV is an application-independent attribute of the link that is defined in [RFC5329]. Because it is an application-independent attribute, it MUST NOT be advertised in the ASLA sub-TLV. Instead, it MAY be advertised as a sub-TLV of the Router-Link TLV inside the OSPFv3 E-Router-LSA [RFC8362].¶
To advertise the Remote Interface IPv6 Address sub-TLV in the OSPFv3 Router-Link TLV, the same format for the sub-TLV defined in [RFC5329] is used with TLV type 25.¶
This document defines extensions to support the advertisement of application-specific link attributes.¶
There are applications where the application enablement on the link is relevant; for example, with RSVP-TE, one needs to make sure that RSVP is enabled on the link before sending an RSVP-TE signaling message over it.¶
There are applications where the enablement of the application on the link is irrelevant and has nothing to do with the fact that some link attributes are advertised for the purpose of such application. An example of this is LFA.¶
Whether the presence of link attribute advertisements for a given application indicates that the application is enabled on that link depends upon the application. Similarly, whether the absence of link attribute advertisements indicates that the application is not enabled depends upon the application.¶
In the case of RSVP-TE, the advertisement of application-specific link attributes has no implication of RSVP-TE being enabled on that link. The RSVP-TE enablement is solely derived from the information carried in the OSPFv2 TE Opaque LSA [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329].¶
In the case of SR Policy, advertisement of application-specific link attributes does not indicate enablement of SR Policy. The advertisements are only used to support constraints that may be applied when specifying an explicit path. SR Policy is implicitly enabled on all links that are part of the SR-enabled topology independent of the existence of link attribute advertisements.¶
In the case of LFA, the advertisement of application-specific link attributes does not indicate enablement of LFA on that link. Enablement is controlled by local configuration.¶
In the future, if additional standard applications are defined to use this mechanism, the specification defining this use MUST define the relationship between application-specific link attribute advertisements and enablement for that application.¶
This document allows the advertisement of application-specific link attributes with no application identifiers, i.e., both the Standard Application Identifier Bit Mask and the User-Defined Application Identifier Bit Mask are not present (see Section 5). This supports the use of the link attribute by any application. In the presence of an application where the advertisement of link attributes is used to infer the enablement of an application on that link (e.g., RSVP-TE), the absence of the application identifier leaves ambiguous whether that application is enabled on such a link. This needs to be considered when making use of the "any application" encoding.¶
Bit identifiers for standard applications are defined in Section 5. All of the identifiers defined in this document are associated with applications that were already deployed in some networks prior to the writing of this document. Therefore, such applications have been deployed using the RSVP-TE LSA advertisements. The standard applications defined in this document may continue to use RSVP-TE LSA advertisements for a given link so long as at least one of the following conditions is true:¶
Under the conditions defined above, implementations that support the extensions defined in this document have the choice of using RSVP-TE LSA advertisements or application-specific advertisements in support of SR Policy and/or LFA. This will require implementations to provide controls specifying which types of advertisements are to be sent and processed on receipt for these applications. Further discussion of the associated issues can be found in Section 12.3.¶
New applications that future documents define to make use of the advertisements defined in this document MUST NOT make use of RSVP-TE LSA advertisements. This simplifies deployment of new applications by eliminating the need to support multiple ways to advertise attributes for the new applications.¶
Link attribute advertisements associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications are usable by any application, subject to the restrictions specified in Section 4.2. If support for a new application is introduced on any node in a network in the presence of such advertisements, the new application will use these advertisements, when the aforementioned restrictions are met. If this is not what is intended, then existing link attribute advertisements MUST be readvertised with an explicit set of applications specified before a new application is introduced.¶
Existing deployments of RSVP-TE, SR Policy, and/or LFA utilize the legacy advertisements listed in Section 3. Routers that do not support the extensions defined in this document will only process legacy advertisements and are likely to infer that RSVP-TE is enabled on the links for which legacy advertisements exist. It is expected that deployments using the legacy advertisements will persist for a significant period of time. Therefore, deployments using the extensions defined in this document in the presence of routers that do not support these extensions need to be able to interoperate with the use of legacy advertisements by the legacy routers. The following subsections discuss interoperability and backwards-compatibility concerns for a number of deployment scenarios.¶
In cases where multiple applications are utilizing a given link, one of the applications is RSVP-TE, and all link attributes for a given link are common to the set of applications utilizing that link, interoperability is achieved by using legacy advertisements for RSVP-TE. Attributes for applications other than RSVP-TE MUST be advertised using application-specific advertisements. This results in duplicate advertisements for those attributes.¶
In cases where one or more applications other than RSVP-TE are utilizing a given link and one or more link attribute values are not shared with RSVP-TE, interoperability is achieved by using legacy advertisements for RSVP-TE. Attributes for applications other than RSVP-TE MUST be advertised using application-specific advertisements. In cases where some link attributes are shared with RSVP-TE, this requires duplicate advertisements for those attributes.¶
For the standard applications defined in this document, routers that do not support the extensions defined in this document will send and receive only legacy link attribute advertisements. In addition, the link attribute values associated with these applications are always shared since legacy routers have no way of advertising or processing application-specific values. So long as there is any legacy router in the network that has any of the standard applications defined in this document enabled, all routers MUST continue to advertise link attributes for these applications using only legacy advertisements. ASLA advertisements for these applications MUST NOT be sent. Once all legacy routers have been upgraded, migration from legacy advertisements to ASLA advertisements can be achieved via the following steps:¶
When the migration is complete, it then becomes possible to advertise incongruent values per application on a given link.¶
Documents defining new applications that make use of the application-specific advertisements defined in this document MUST discuss interoperability and backwards-compatibility issues that could occur in the presence of routers that do not support the new application.¶
The extensions defined in this document support RSVP-TE as one of the supported applications. It is, however, RECOMMENDED to advertise all link attributes for RSVP-TE in the existing OSPFv2 TE Opaque LSA [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329] to maintain backwards compatibility. RSVP-TE can eventually utilize the application-specific advertisements for newly defined link attributes that are defined as application specific.¶
Link attributes that are not allowed to be advertised in the ASLA sub-TLV, such as maximum reservable link bandwidth and unreserved bandwidth, MUST use the OSPFv2 TE Opaque LSA [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329] and MUST NOT be advertised in the ASLA sub-TLV.¶
Existing security extensions as described in [RFC2328], [RFC5340], and [RFC8362] apply to extensions defined in this document. While OSPF is under a single administrative domain, there can be deployments where potential attackers have access to one or more networks in the OSPF routing domain. In these deployments, stronger authentication mechanisms such as those specified in [RFC5709], [RFC7474], [RFC4552], or [RFC7166] SHOULD be used.¶
Implementations must ensure that if any of the TLVs and sub-TLVs defined in this document are malformed, they are detected and do not facilitate a vulnerability for attackers to crash the OSPF router or routing process. Reception of a malformed TLV or sub-TLV SHOULD be counted and/or logged for further analysis. Logging of malformed TLVs and sub-TLVs SHOULD be rate-limited to prevent a denial-of-service (DoS) attack (distributed or otherwise) from overloading the OSPF control plane.¶
This document defines a new way to advertise link attributes. Tampering with the information defined in this document may have an effect on applications using it, including impacting traffic engineering, which uses various link attributes for its path computation. This is similar in nature to the impacts associated with, for example, [RFC3630]. As the advertisements defined in this document limit the scope to specific applications, the impact of tampering is similarly limited in scope.¶
This specification updates two existing registries:¶
The new values defined in this document have been allocated using the IETF Review procedure as described in [RFC8126].¶
The "OSPFv2 Extended Link TLV Sub-TLVs" registry [RFC7684] defines sub-TLVs at any level of nesting for OSPFv2 Extended Link TLVs. IANA has assigned the following sub-TLV types from the "OSPFv2 Extended Link TLV Sub-TLVs" registry:¶
The "OSPFv3 Extended-LSA Sub-TLVs" registry [RFC8362] defines sub-TLVs at any level of nesting for OSPFv3 Extended LSAs. IANA has assigned the following sub-TLV types from the "OSPFv3 Extended-LSA Sub-TLVs" registry:¶
Discussion within the LSR WG indicated that there was confusion regarding the use of ASLA advertisements that had a zero length SABM/UDABM. The discussion can be seen by searching the LSR WG mailing list archives for the thread "Proposed Errata for RFCs 8919/8920" starting on 15 June 2021.¶
Changes to Section 5 have been introduced to clarify normative behavior in the presence of such advertisements. RFC 8920 defines advertising link attributes with zero length Standard Application Bit Mask (SABM) and zero length User Defined ApplicationBit Mask (UDABM) as a means of advertising link attributes that can be used by any application. However, the text uses the word "permitted", suggesting that the use of such advertisements is "optional". Such an interpretation could lead to interoperability issues and is not what was intended.¶
The replacement text makes explicit the specific conditions when such advertisements MUST be used and the specific conditions under which they MUST NOT be used.¶
A new sub-section discussing the use of zero-length Application Identifier Bit Masks has been added for greater consistency with [RFC8919]. See Section 12.2.¶
RFC 8920 included the following acknowledgements:¶
Thanks to Chris Bowers for his review and comments.¶
Thanks to Alvaro Retana for his detailed review and comments.¶
For the new version, the authors would like to thank Bruno Decraene.¶
The following people contributed to the content of this document and should be considered as coauthors:¶