path: root/doc/rfc/rfc9492.txt

Internet Engineering Task Force (IETF)                    P. Psenak, Ed.
Request for Comments: 9492                                   L. Ginsberg
Obsoletes: 8920                                            Cisco Systems
Category: Standards Track                                  W. Henderickx
ISSN: 2070-1721                                                    Nokia
                                                             J. Tantsura
                                                                  Nvidia
                                                                J. Drake
                                                        Juniper Networks
                                                            October 2023


               OSPF Application-Specific Link Attributes

Abstract

   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' Addresses

1.  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 Bandwidth

6.3.  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 Group

6.4.  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 Considerations

12.1.  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 bandwidth

14.2.  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 Address

15.  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.  References

16.1.  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.com


Authors' 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