Internet Engineering Task Force (IETF)                    P. Psenak, Ed.Request for Comments: 9492                                   L. GinsbergObsoletes: 8920                                            Cisco SystemsCategory: Standards Track                                  W. HenderickxISSN: 2070-1721                                                    Nokia                                                             J. Tantsura                                                                  Nvidia                                                                J. Drake                                                        Juniper Networks                                                            October 2023               OSPF Application-Specific Link AttributesAbstract   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 such   as Segment Routing (SR) Policy and Loop-Free Alternates (LFAs) 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 link attribute advertisements in   OSPFv2 and OSPFv3 that address both of these shortcomings.   This document obsoletes RFC 8920.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/rfc9492.Copyright Notice   Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the   Trust Legal Provisions and are provided without warranty as described   in the Revised BSD License.Table of Contents   1.  Introduction     1.1.  Requirements Language   2.  Requirements Discussion   3.  Existing Advertisement of Link Attributes   4.  Advertisement of Link Attributes     4.1.  OSPFv2 Extended Link Opaque LSA and OSPFv3 E-Router-LSA   5.  Advertisement of Application-Specific Values   6.  Reused TE Link Attributes     6.1.  Shared Risk Link Group (SRLG)     6.2.  Extended Metrics     6.3.  Administrative Group     6.4.  TE Metric   7.  Maximum Link Bandwidth   8.  Considerations for Extended TE Metrics   9.  Local Interface IPv6 Address Sub-TLV   10. Remote Interface IPv6 Address Sub-TLV   11. Attribute Advertisements and Enablement   12. Deployment Considerations     12.1.  Use of Legacy RSVP-TE LSA Advertisements     12.2.  Use of Zero-Length Application Identifier Bit Masks     12.3.  Interoperability, Backwards Compatibility, and Migration            Concerns       12.3.1.  Multiple Applications: Common Attributes with RSVP-TE       12.3.2.  Multiple Applications: Some Attributes Not Shared with               RSVP-TE       12.3.3.  Interoperability with Legacy Routers       12.3.4.  Use of Application-Specific Advertisements for RSVP-TE   13. Security Considerations   14. IANA Considerations     14.1.  OSPFv2     14.2.  OSPFv3   15. Changes to RFC 8920   16. References     16.1.  Normative References     16.2.  Informative References   Acknowledgments   Contributors   Authors' Addresses1.  Introduction   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 TE 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 SR Policy [RFC9256] and 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 a setup   failure for the path.   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.1.1.  Requirements Language   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.2.  Requirements Discussion   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:   1.  Support for indicating which applications are using the link       attribute advertisements on a link.   2.  Support for advertising application-specific values for the same       attribute on a link.   [RFC7855] discusses use cases and requirements for SR.  Included   among these use cases is SR Policy, which is defined in [RFC9256].   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 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.3.  Existing Advertisement of Link Attributes   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.4.  Advertisement of 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.4.1.  OSPFv2 Extended Link Opaque LSA and OSPFv3 E-Router-LSA   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:   1.  Advertisement of the link attributes does not make the link part       of the RSVP-TE topology.  It avoids any conflicts and is fully       compatible with [RFC3630] and [RFC5329].   2.  The OSPFv2 TE Opaque LSA and OSPFv3 Intra-Area-TE-LSA remain       truly opaque to OSPFv2 and OSPFv3 as originally defined in       [RFC3630] and [RFC5329], respectively.  Their contents are not       inspected by OSPF, which instead acts as a pure transport.   3.  There is a clear distinction between link attributes used by       RSVP-TE and link attributes used by other OSPFv2 or OSPFv3       applications.   4.  All link attributes that are used by other applications are       advertised in the Extended Link Opaque LSA in OSPFv2 [RFC7684] or       the OSPFv3 E-Router-LSA [RFC8362] in OSPFv3.   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 14.5.  Advertisement of Application-Specific Values   To allow advertisement of the application-specific values of the link   attribute, an 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-TLVs                  |   +-                                                             -+   |                            ...                                |   where:   Type:      10 (OSPFv2), 11 (OSPFv3)   Length:      Variable   SABM Length:      Standard Application Identifier Bit Mask Length in octets.  The      value MUST be 0, 4, or 8.  If the Standard Application Identifier      Bit Mask is not present, the SABM Length MUST be set to 0.   UDABM Length:      User-Defined Application Identifier Bit Mask Length in octets.      The value MUST be 0, 4, or 8.  If the User-Defined Application      Identifier Bit Mask is not present, the UDABM Length MUST be set      to 0.   Standard Application Identifier Bit Mask:      Optional set of bits, where each bit represents a single standard      application.  Bits are defined in the "Link Attribute Application      Identifiers" registry, which is defined in [RFC9479].  Current      assignments are repeated here for informational purposes:                              0 1 2 3 4 5 6 7 ...                             +-+-+-+-+-+-+-+-+...                             |R|S|F|          ...                             +-+-+-+-+-+-+-+-+...      Bit 0 (R-bit):  RSVP-TE.      Bit 1 (S-bit):  SR Policy (this is data plane independent).      Bit 2 (F-bit):  Loop-Free Alternate (includes all LFA types).   User-Defined Application Identifier Bit Mask:      Optional set of bits, where each bit represents a single user-      defined application.   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 one or more application-specific bits 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-TLV   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:   *  Shared Risk Link Group [RFC4203]   *  Unidirectional Link Delay [RFC7471]   *  Min/Max Unidirectional Link Delay [RFC7471]   *  Unidirectional Delay Variation [RFC7471]   *  Unidirectional Link Loss [RFC7471]   *  Unidirectional Residual Bandwidth [RFC7471]   *  Unidirectional Available Bandwidth [RFC7471]   *  Unidirectional Utilized Bandwidth [RFC7471]   *  Administrative Group [RFC3630]   *  Extended Administrative Group [RFC7308]   *  TE Metric [RFC3630]6.  Reused TE Link Attributes   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.6.1.  Shared Risk Link Group (SRLG)   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 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.6.2.  Extended Metrics   [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:   12:  Unidirectional Link Delay   13:  Min/Max Unidirectional Link Delay   14:  Unidirectional Delay Variation   15:  Unidirectional Link Loss   16:  Unidirectional Residual Bandwidth   17:  Unidirectional Available Bandwidth   18:  Unidirectional Utilized Bandwidth   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:   13:  Unidirectional Link Delay   14:  Min/Max Unidirectional Link Delay   15:  Unidirectional Delay Variation   16:  Unidirectional Link Loss   17:  Unidirectional Residual Bandwidth   18:  Unidirectional Available Bandwidth   19:  Unidirectional Utilized Bandwidth6.3.  Administrative Group   [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:   19:  Administrative Group   20:  Extended Administrative Group   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:   20:  Administrative Group   21:  Extended Administrative Group6.4.  TE Metric   [RFC3630] defines the TE Metric.   To advertise the TE 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 TE Metric   in the OSPFv3 Router-Link TLV, TLV type 22 is used.7.  Maximum Link Bandwidth   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 23.8.  Considerations for Extended TE Metrics   [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 12.2.9.  Local Interface IPv6 Address Sub-TLV   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 24.10.  Remote Interface IPv6 Address Sub-TLV   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 25.11.  Attribute Advertisements and Enablement   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 SABM and   the UDABM 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.12.  Deployment Considerations12.1.  Use of Legacy RSVP-TE LSA Advertisements   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:   *  The application is RSVP-TE.   *  The application is SR Policy or LFA, and RSVP-TE is not deployed      anywhere in the network.   *  The application is SR Policy or LFA, RSVP-TE is deployed in the      network, and both the set of links on which SR Policy and/or LFA      advertisements are required and the attribute values used by SR      Policy and/or LFA on all such links are fully congruent with the      links and attribute values used by RSVP-TE.   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.12.2.  Use of Zero-Length Application Identifier Bit Masks   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 5.  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.12.3.  Interoperability, Backwards Compatibility, and Migration Concerns   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.12.3.1.  Multiple Applications: Common Attributes with RSVP-TE   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.12.3.2.  Multiple Applications: Some Attributes Not Shared with RSVP-TE   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.12.3.3.  Interoperability with Legacy Routers   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:   1)  Send new application-specific advertisements while continuing to       advertise using the legacy advertisement (all advertisements are       then duplicated).  Receiving routers continue to use legacy       advertisements.   2)  Enable the use of the application-specific advertisements on all       routers.   3)  Keep legacy advertisements if needed for RSVP-TE purposes.   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.12.3.4.  Use of Application-Specific Advertisements for RSVP-TE   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.13.  Security Considerations   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 or otherwise   compromise 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 an improved way to advertise link attributes.   Tampering with the information defined in this document may have an   effect on applications using it, including impacting TE, 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.14.  IANA Considerations   This specification updates two existing registries:   *  the "OSPFv2 Extended Link TLV Sub-TLVs" registry   *  the "OSPFv3 Extended-LSA Sub-TLVs" registry   The values defined in this document have been allocated using the   IETF Review procedure as described in [RFC8126].14.1.  OSPFv2   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 in the "OSPFv2 Extended Link   TLV Sub-TLVs" registry:   10:  Application-Specific Link Attributes   11:  Shared Risk Link Group   12:  Unidirectional Link Delay   13:  Min/Max Unidirectional Link Delay   14:  Unidirectional Delay Variation   15:  Unidirectional Link Loss   16:  Unidirectional Residual Bandwidth   17:  Unidirectional Available Bandwidth   18:  Unidirectional Utilized Bandwidth   19:  Administrative Group   20:  Extended Administrative Group   22:  TE Metric   23:  Maximum link bandwidth14.2.  OSPFv3   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 in the "OSPFv3 Extended-LSA Sub-   TLVs" registry:   11:  Application-Specific Link Attributes   12:  Shared Risk Link Group   13:  Unidirectional Link Delay   14:  Min/Max Unidirectional Link Delay   15:  Unidirectional Delay Variation   16:  Unidirectional Link Loss   17:  Unidirectional Residual Bandwidth   18:  Unidirectional Available Bandwidth   19:  Unidirectional Utilized Bandwidth   20:  Administrative Group   21:  Extended Administrative Group   22:  TE Metric   23:  Maximum link bandwidth   24:  Local Interface IPv6 Address   25:  Remote Interface IPv6 Address15.  Changes to RFC 8920   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.  [RFC8920] defines   advertising link attributes with zero-length SABM and zero-length   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 subsection discussing the use of zero-length Application Identifier   Bit Masks has been added for greater consistency with [RFC9479].  See   Section 12.2.16.  References16.1.  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>.   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,              DOI 10.17487/RFC2328, April 1998,              <https://www.rfc-editor.org/info/rfc2328>.   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering              (TE) Extensions to OSPF Version 2", RFC 3630,              DOI 10.17487/RFC3630, September 2003,              <https://www.rfc-editor.org/info/rfc3630>.   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in              Support of Generalized Multi-Protocol Label Switching              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,              <https://www.rfc-editor.org/info/rfc4203>.   [RFC5329]  Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,              "Traffic Engineering Extensions to OSPF Version 3",              RFC 5329, DOI 10.17487/RFC5329, September 2008,              <https://www.rfc-editor.org/info/rfc5329>.   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,              <https://www.rfc-editor.org/info/rfc5340>.   [RFC7308]  Osborne, E., "Extended Administrative Groups in MPLS              Traffic Engineering (MPLS-TE)", RFC 7308,              DOI 10.17487/RFC7308, July 2014,              <https://www.rfc-editor.org/info/rfc7308>.   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.              Previdi, "OSPF Traffic Engineering (TE) Metric              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,              <https://www.rfc-editor.org/info/rfc7471>.   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November              2015, <https://www.rfc-editor.org/info/rfc7684>.   [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>.   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and              F. Baker, "OSPFv3 Link State Advertisement (LSA)              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April              2018, <https://www.rfc-editor.org/info/rfc8362>.   [RFC9479]  Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and              J. Drake, "IS-IS Application-Specific Link Attributes",              RFC 9479, DOI 10.17487/RFC9479, October 2023,              <https://www.rfc-editor.org/info/rfc9479>.16.2.  Informative References   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,              <https://www.rfc-editor.org/info/rfc3209>.   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,              <https://www.rfc-editor.org/info/rfc4552>.   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for              IP Fast Reroute: Loop-Free Alternates", RFC 5286,              DOI 10.17487/RFC5286, September 2008,              <https://www.rfc-editor.org/info/rfc5286>.   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic              Authentication", RFC 5709, DOI 10.17487/RFC5709, October              2009, <https://www.rfc-editor.org/info/rfc5709>.   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",              RFC 5714, DOI 10.17487/RFC5714, January 2010,              <https://www.rfc-editor.org/info/rfc5714>.   [RFC7166]  Bhatia, M., Manral, V., and A. Lindem, "Supporting              Authentication Trailer for OSPFv3", RFC 7166,              DOI 10.17487/RFC7166, March 2014,              <https://www.rfc-editor.org/info/rfc7166>.   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,              "Security Extension for OSPFv2 When Using Manual Key              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,              <https://www.rfc-editor.org/info/rfc7474>.   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,              Litkowski, S., Horneffer, M., and R. Shakir, "Source              Packet Routing in Networking (SPRING) Problem Statement              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May              2016, <https://www.rfc-editor.org/info/rfc7855>.   [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>.   [RFC8920]  Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,              J., and J. Drake, "OSPF Application-Specific Link              Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020,              <https://www.rfc-editor.org/info/rfc8920>.   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,              A., and P. Mattes, "Segment Routing Policy Architecture",              RFC 9256, DOI 10.17487/RFC9256, July 2022,              <https://www.rfc-editor.org/info/rfc9256>.Acknowledgments   The following acknowledgments are included in [RFC8920]:   Thanks to Chris Bowers for his review and comments.   Thanks to Alvaro Retana for his detailed review and comments.   For this document, the authors would like to thank Bruno Decraene.Contributors   The following people contributed to the content of this document and   should be considered as coauthors:   Acee Lindem   LabN Consulting, L.L.C.   United States of America   Email: acee.ietf@gmail.com   Ketan Talaulikar   Cisco Systems   India   Email: ketant.ietf@gmail.com   Hannes Gredler   RtBrick Inc.   Email: hannes@rtbrick.comAuthors' Addresses   Peter Psenak (editor)   Cisco Systems   Slovakia   Email: ppsenak@cisco.com   Les Ginsberg   Cisco Systems   United States of America   Email: ginsberg@cisco.com   Wim Henderickx   Nokia   Copernicuslaan 50   2018 94089 Antwerp   Belgium   Email: wim.henderickx@nokia.com   Jeff Tantsura   Nvidia   United States of America   Email: jefftant.ietf@gmail.com   John Drake   Juniper Networks   United States of America   Email: jdrake@juniper.net