



Internet Engineering Task Force (IETF)                       L. Ginsberg
Request for Comments: 9479                                     P. Psenak
Obsoletes: 8919                                            Cisco Systems
Category: Standards Track                                     S. Previdi
ISSN: 2070-1721                                      Huawei Technologies
                                                           W. Henderickx
                                                                   Nokia
                                                                J. Drake
                                                        Juniper Networks
                                                            October 2023


               IS-IS 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 (e.g.,
   Segment Routing Policy and Loop-Free Alternates) that also make use
   of the link attribute advertisements have been defined.  In cases
   where multiple applications wish to make use of these link
   attributes, the current advertisements do not support application-
   specific values for a given attribute, nor do they support an
   indication of which applications are using the advertised value for a
   given link.  This document introduces link attribute advertisements
   that address both of these shortcomings.

   This document obsoletes RFC 8919.

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/rfc9479.

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.  Legacy Advertisements
     3.1.  Legacy Sub-TLVs
     3.2.  Legacy SRLG Advertisements
   4.  Advertising Application-Specific Link Attributes
     4.1.  Application Identifier Bit Mask
     4.2.  Application-Specific Link Attributes Sub-TLV
       4.2.1.  Special Considerations for Maximum Link Bandwidth
       4.2.2.  Special Considerations for Reservable/Unreserved
               Bandwidth
       4.2.3.  Considerations for Extended TE Metrics
     4.3.  Application-Specific SRLG TLV
   5.  Attribute Advertisements and Enablement
   6.  Deployment Considerations
     6.1.  Use of Legacy Advertisements
     6.2.  Use of Zero-Length Application Identifier Bit Masks
     6.3.  Interoperability, Backwards Compatibility, and Migration
           Concerns
       6.3.1.  Multiple Applications: Common Attributes with RSVP-TE
       6.3.2.  Multiple Applications: All Attributes Not Shared with
               RSVP-TE
       6.3.3.  Interoperability with Legacy Routers
       6.3.4.  Use of Application-Specific Advertisements for RSVP-TE
   7.  IANA Considerations
     7.1.  Application-Specific Link Attributes Sub-TLV
     7.2.  Application-Specific SRLG TLV
     7.3.  IS-IS Sub-Sub-TLV Codepoints for Application-Specific Link
           Attributes Registry
     7.4.  Link Attribute Application Identifiers Registry
     7.5.  IS-IS Sub-TLVs for Application-Specific SRLG TLV
   8.  Security Considerations
   9.  Changes to RFC 8919
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Advertisement of link attributes by the Intermediate System to
   Intermediate System (IS-IS) protocol in support of Traffic
   Engineering (TE) was introduced by [RFC5305] and extended by
   [RFC5307], [RFC6119], [RFC7308], and [RFC8570].  The 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 3.

   In recent years, new applications that have use cases for many of the
   link attributes historically used by RSVP-TE have been introduced.
   Such applications include Segment Routing (SR) Policy [RFC9256] and
   Loop-Free Alternates (LFAs) [RFC5286].  This has introduced ambiguity
   in that if a deployment includes a mix of RSVP-TE support and SR
   Policy support, for example, it is not possible to unambiguously
   indicate which advertisements are to be used by RSVP-TE and which
   advertisements are to be used by SR Policy.  If the topologies are
   fully congruent, this may not be an issue, but any incongruence leads
   to ambiguity.

   An example of where this ambiguity causes a problem is a network
   where RSVP-TE is enabled only on a subset of its links.  A link
   attribute is advertised for the purpose of another application (e.g.,
   SR Policy) for a link that is not enabled for RSVP-TE.  As soon as
   the router that is an RSVP-TE head end sees the link attribute being
   advertised for that link, it assumes that 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 to 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 IS-IS, 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.  Legacy Advertisements

   Existing advertisements used in support of RSVP-TE include sub-TLVs
   for TLVs Advertising Neighbor Information and TLVs for Shared Risk
   Link Group (SRLG) advertisements.

   Sub-TLV values are defined in the "IS-IS Sub-TLVs for TLVs
   Advertising Neighbor Information" registry.

   TLVs are defined in the "IS-IS TLV Codepoints" registry.

3.1.  Legacy Sub-TLVs

               +======+====================================+
               | Type | Description                        |
               +======+====================================+
               | 3    | Administrative group (color)       |
               +------+------------------------------------+
               | 9    | Maximum link bandwidth             |
               +------+------------------------------------+
               | 10   | Maximum reservable link bandwidth  |
               +------+------------------------------------+
               | 11   | Unreserved bandwidth               |
               +------+------------------------------------+
               | 14   | Extended Administrative Group      |
               +------+------------------------------------+
               | 18   | TE Default metric                  |
               +------+------------------------------------+
               | 33   | Unidirectional Link Delay          |
               +------+------------------------------------+
               | 34   | Min/Max Unidirectional Link Delay  |
               +------+------------------------------------+
               | 35   | Unidirectional Delay Variation     |
               +------+------------------------------------+
               | 36   | Unidirectional Link Loss           |
               +------+------------------------------------+
               | 37   | Unidirectional Residual Bandwidth  |
               +------+------------------------------------+
               | 38   | Unidirectional Available Bandwidth |
               +------+------------------------------------+
               | 39   | Unidirectional Utilized Bandwidth  |
               +------+------------------------------------+

                                  Table 1

3.2.  Legacy SRLG Advertisements

   TLV 138 (GMPLS-SRLG):
      Supports links identified by IPv4 addresses and unnumbered links.

   TLV 139 (IPv6 SRLG):
      Supports links identified by IPv6 addresses.

   Note that [RFC6119] prohibits the use of TLV 139 when it is possible
   to use TLV 138.

4.  Advertising Application-Specific Link Attributes

   Two codepoints are defined to support Application-Specific Link
   Attribute (ASLA) advertisements:

   1.  Application-Specific Link Attributes sub-TLV for TLVs Advertising
       Neighbor Information (defined in Section 4.2).

   2.  Application-Specific SRLG TLV (defined in Section 4.3).

   To support these advertisements, an application identifier bit mask
   is defined to identify the application(s) associated with a given
   advertisement (defined in Section 4.1).

   In addition to supporting the advertisement of link attributes used
   by standardized applications, link attributes can also be advertised
   for use by User-Defined Applications (UDAs).  Such applications are
   not subject to standardization and are outside the scope of this
   document.

   The following sections define the format of these advertisements.

4.1.  Application Identifier Bit Mask

   Identification of the set of applications associated with link
   attribute advertisements utilizes two bit masks.  One bit mask is for
   standard applications where the definition of each bit is defined in
   an IANA-controlled registry (see Section 7.4).  A second bit mask is
   for non-standard UDAs.

   The encoding defined below is used by both the Application-Specific
   Link Attributes sub-TLV and the Application-Specific SRLG TLV.

             0  1  2  3  4  5  6  7
            +--+--+--+--+--+--+--+--+
            | SABM Length + Flag    |  1 octet
            +--+--+--+--+--+--+--+--+
            | UDABM Length + Flag   |  1 octet
            +--+--+--+--+--+--+--+--+
            |   SABM         ...       0-8 octets
            +--+--+--+--+--+--+--+--+
            |   UDABM        ...       0-8 octets
            +--+--+--+--+--+--+--+--+

   SABM Length + Flag (1 octet):
      Standard Application Identifier Bit Mask Length + Flag

                0 1 2 3 4 5 6 7
               +-+-+-+-+-+-+-+-+
               |L| SABM Length |
               +-+-+-+-+-+-+-+-+

      L-flag:
         Legacy Flag.  See Section 4.2 for a description of how this
         flag is used.

      SABM Length:
         This field indicates the length in octets (0-8) of the Standard
         Application Identifier Bit Mask.  The length SHOULD be the
         minimum required to send all bits that are set.

   UDABM Length + Flag (1 octet):
      User-Defined Application Identifier Bit Mask Length + Flag

                0 1 2 3 4 5 6 7
               +-+-+-+-+-+-+-+-+
               |R| UDABM Length|
               +-+-+-+-+-+-+-+-+

      R:
         Reserved.  SHOULD be transmitted as 0 and MUST be ignored on
         receipt.

      UDABM Length:
         Indicates the length in octets (0-8) of the User-Defined
         Application Identifier Bit Mask.  The length SHOULD be the
         minimum required to send all bits that are set.

   SABM (variable length):
      Standard Application Identifier Bit Mask

      (SABM Length * 8) bits

      This field is omitted if SABM Length is 0.

                0 1 2 3 4 5 6 7 ...
               +-+-+-+-+-+-+-+-+...
               |R|S|F|          ...
               +-+-+-+-+-+-+-+-+...

      R-bit:
         Set to specify RSVP-TE.

      S-bit:
         Set to specify SR Policy (this is data plane independent).

      F-bit:
         Set to specify an LFA (includes all LFA types).

   UDABM (variable length):
      User-Defined Application Identifier Bit Mask

      (UDABM Length * 8) bits

                0 1 2 3 4 5 6 7 ...
               +-+-+-+-+-+-+-+-+...
               |                ...
               +-+-+-+-+-+-+-+-+...

      This field is omitted if UDABM Length is 0.

      |  Note: SABM/UDABM Length is arbitrarily limited to 8 octets in
      |  order to ensure that sufficient space is left to advertise link
      |  attributes without overrunning the maximum length of a sub-TLV.

   Standard Application Identifier Bits are defined and sent starting
   with bit 0.

   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 bits be used
   starting with bit 0 so as to minimize the number of octets required
   to advertise all UDAs.

   For both the SABM and UDABM, the following rules apply:

   *  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.

4.2.  Application-Specific Link Attributes Sub-TLV

   A sub-TLV for TLVs Advertising Neighbor Information is defined that
   supports specification of the applications and application-specific
   attribute values.

   Type:
      16

   Length:
      Variable (1 octet)

   Value:
      Application Identifier Bit Mask (as defined in Section 4.1)

      Link Attribute sub-sub-TLVs -- format matches the existing formats
      defined in [RFC5305], [RFC7308], and [RFC8570]

   If the SABM Length or UDABM Length in the Application Identifier Bit
   Mask is greater than 8, the entire sub-TLV MUST be ignored.

   When the SABM Length or UDABM Length is non-zero and the L-flag is
   NOT set, all applications specified in the bit mask MUST use the link
   attribute advertisements in the sub-TLV.

   When the L-flag is set in the Application Identifier Bit Mask, all of
   the applications specified in the bit mask MUST use the legacy
   advertisements for the corresponding link found in TLVs Advertising
   Neighbor Information.  Link attribute sub-sub-TLVs for the
   corresponding link attributes MUST NOT be advertised for the set of
   applications specified in the Standard Application Identifier Bit
   Mask or the User-Defined Application Identifier Bit Mask, and all
   such sub-sub-TLVs MUST be ignored on receipt.

   Multiple Application-Specific Link Attributes sub-TLVs for the same
   link MAY be advertised.  When multiple sub-TLVs for the same link are
   advertised, they SHOULD advertise non-conflicting application/
   attribute pairs.  A conflict exists when the same application is
   associated with two different values for the same link attribute for
   a given link.  In cases where conflicting values for the same
   application/attribute/link are advertised, the first advertisement
   received in the lowest-numbered Link State Protocol Data Unit (LSP)
   MUST be used, and subsequent advertisements of the same attribute
   MUST be ignored.

   For a given application, the setting of the L-flag MUST be the same
   in all sub-TLVs for a given link.  In cases where this constraint is
   violated, the L-flag MUST be considered set for this application.

   The end result of the set of rules defined above is that for a given
   application either the attribute values advertised in ASLA sub-sub-
   TLVs are used or the attribute values advertised in legacy sub-TLVs
   are used, but not both.

   Link attributes MAY be advertised associated with zero-length
   Application Identifier Bit Masks for both standard applications and
   UDAs.  Such link attribute advertisements MUST be used by standard
   applications and/or UDAs when no link attribute advertisements with a
   non-zero-length Application Identifier Bit Mask and a matching
   Application Identifier Bit set are present for a given link.
   Otherwise, such link attribute advertisements MUST NOT be used.

   IANA has created a registry of sub-sub-TLVs to define the link
   attribute sub-sub-TLV codepoints (see Section 7.3).  This document
   defines a sub-sub-TLV for each of the existing sub-TLVs listed in
   Section 3.1, except as noted below.  The format of the sub-sub-TLVs
   matches the format of the corresponding legacy sub-TLV, and IANA has
   assigned the legacy sub-TLV identifier to the corresponding sub-sub-
   TLV.

4.2.1.  Special Considerations for Maximum Link Bandwidth

   Maximum link bandwidth is an application-independent attribute of the
   link.  When advertised using the Application-Specific Link Attributes
   sub-TLV, multiple values for the same link MUST NOT be advertised.
   This can be accomplished most efficiently by having a single
   advertisement for a given link where the Application Identifier Bit
   Mask identifies all the applications that are making use of the value
   for that link.

   It is also possible to advertise the same value for a given link
   multiple times with disjoint sets of applications specified in the
   Application Identifier Bit Mask.  This is less efficient but still
   valid.

   It is also possible to advertise a single advertisement with a zero-
   length SABM and UDABM so long as the constraints discussed in
   Sections 4.2 and 6.2 are satisfied.

   If different values for maximum link bandwidth for a given link are
   advertised, all values MUST be ignored.

4.2.2.  Special Considerations for Reservable/Unreserved Bandwidth

   Maximum reservable link bandwidth and unreserved bandwidth are
   attributes specific to RSVP-TE.  When advertised using the
   Application-Specific Link Attributes sub-TLV, bits other than the
   RSVP-TE bit (R-bit) MUST NOT be set in the Application Identifier Bit
   Mask.  If an advertisement of maximum reservable link bandwidth or
   unreserved bandwidth is received with bits other than the R-bit set,
   the advertisement MUST be ignored.

4.2.3.  Considerations for Extended TE Metrics

   [RFC8570] 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 will be associated
   with all of the applications utilizing that link.  This can be done
   by either explicitly specifying the applications in the Application
   Identifier Bit Mask or using a zero-length Application Identifier Bit
   Mask.  The use of zero-length Application Identifier Bit Masks is
   further discussed in Section 6.2.

4.3.  Application-Specific SRLG TLV

   A TLV is defined to advertise application-specific SRLGs for a given
   link.  Although similar in functionality to TLV 138 [RFC5307] and TLV
   139 [RFC6119], this single TLV provides support for IPv4, IPv6, and
   unnumbered identifiers for a link.  Unlike TLVs 138 and 139, it
   utilizes sub-TLVs to encode the link identifiers in order to provide
   the flexible formatting required to support multiple link identifier
   types.

   Type:
      238

   Length:
      Number of octets in the value field (1 octet)

   Value:
      Neighbor System-ID + pseudonode ID (7 octets)

      Application Identifier Bit Mask (as defined in Section 4.1)

      Length of sub-TLVs (1 octet)

      Link Identifier sub-TLVs (variable)

      0 or more SRLG values (each value is 4 octets)

   If the SABM Length or UDABM Length in the Application Identifier Bit
   Mask is greater than 8, the entire sub-TLV MUST be ignored.

   When the SABM Length or UDABM Length is non-zero and the L-flag is
   NOT set, all applications specified in the bit mask MUST use SRLG
   advertisements in the Application-Specific SRLG TLV.

   The following Link Identifier sub-TLVs are defined.  The values
   chosen intentionally match the equivalent sub-TLVs from [RFC5305],
   [RFC5307], and [RFC6119].

            +======+=========================================+
            | Type | Description                             |
            +======+=========================================+
            | 4    | Link Local/Remote Identifiers [RFC5307] |
            +------+-----------------------------------------+
            | 6    | IPv4 interface address [RFC5305]        |
            +------+-----------------------------------------+
            | 8    | IPv4 neighbor address [RFC5305]         |
            +------+-----------------------------------------+
            | 12   | IPv6 Interface Address [RFC6119]        |
            +------+-----------------------------------------+
            | 13   | IPv6 Neighbor Address [RFC6119]         |
            +------+-----------------------------------------+

                                 Table 2

   At least one set of link identifiers (IPv4, IPv6, or Link Local/
   Remote) MUST be present.  Multiple occurrences of the same identifier
   type MUST NOT be present.  TLVs that do not meet this requirement
   MUST be ignored.

   Multiple TLVs for the same link MAY be advertised.

   When the L-flag is set in the Application Identifier Bit Mask, SRLG
   values MUST NOT be included in the TLV.  Any SRLG values that are
   advertised MUST be ignored.  Based on the link identifiers
   advertised, the corresponding legacy TLV (see Section 3.2) can be
   identified, and the SRLG values advertised in the legacy TLV MUST be
   used by the set of applications specified in the Application
   Identifier Bit Mask.

   For a given application, the setting of the L-flag MUST be the same
   in all TLVs for a given link.  In cases where this constraint is
   violated, the L-flag MUST be considered set for this application.

5.  Attribute Advertisements and Enablement

   This document defines extensions to support the advertisement of
   ASLAs.

   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 ASLAs implies that RSVP
   is enabled on that link.  The absence of RSVP-TE ASLAs in combination
   with the absence of legacy advertisements implies that RSVP is not
   enabled on that link.

   In the case of SR Policy, the advertisement of ASLAs does not
   indicate enablement of SR Policy on that link.  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 ASLAs 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 ASLA advertisements and enablement for those
   applications.

   This document allows the advertisement of ASLAs with no application
   identifiers, i.e., neither the Standard Application Identifier Bit
   Mask nor the User-Defined Application Identifier Bit Mask is present
   (see Section 4.1).  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.

6.  Deployment Considerations

   This section discusses deployment considerations associated with the
   use of ASLA advertisements.

6.1.  Use of Legacy Advertisements

   Bit identifiers for standard applications are defined in Section 4.1.
   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 legacy advertisements.  The standard applications
   defined in this document may continue to use legacy 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 legacy
   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 6.3.

   New applications that future documents define to make use of the
   advertisements defined in this document MUST NOT make use of legacy
   advertisements.  This simplifies deployment of new applications by
   eliminating the need to support multiple ways to advertise attributes
   for the new applications.

6.2.  Use of Zero-Length Application Identifier Bit Masks

   Link attribute advertisements associated with zero-length Application
   Identifier Bit Masks for both standard applications and UDAs are
   usable by any application, subject to the restrictions specified in
   Section 4.2.  If support for a new application is introduced on any
   node in a network in the presence of such advertisements, the new
   application will use these advertisements, when the aforementioned
   restrictions are met.  If this is not what is intended, then existing
   link attribute advertisements MUST be readvertised with an explicit
   set of applications specified before a new application is introduced.

6.3.  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.

6.3.1.  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 and
   sending application-specific advertisements with the L-flag set and
   no link attribute values.  This avoids duplication of link attribute
   advertisements.

6.3.2.  Multiple Applications: All 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, it is necessary to use application-specific
   advertisements as defined in this document.  Attributes for
   applications other than RSVP-TE MUST be advertised using application-
   specific advertisements that have the L-flag clear.  In cases where
   some link attributes are shared with RSVP-TE, this requires duplicate
   advertisements for those attributes.

   These guidelines apply to cases where RSVP-TE is not using any
   advertised attributes on a link and to cases where RSVP-TE is using
   some link attribute advertisements on the link but some link
   attributes cannot be shared with RSVP-TE.

6.3.3.  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 ASLA advertisements while continuing to advertise legacy
       advertisements (all advertisements are then duplicated).
       Receiving routers continue to use legacy advertisements.

   2.  Enable the use of the ASLA advertisements on all routers.

   3.  Remove legacy advertisements.

   When the migration is complete, it then becomes possible to advertise
   incongruent values per application on a given link.

   Note that the use of the L-flag is of no value in the migration.

   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.

6.3.4.  Use of Application-Specific Advertisements for RSVP-TE

   The extensions defined in this document include RSVP-TE as one of the
   applications.  It is therefore possible, in the future, for
   implementations to migrate to the use of application-specific
   advertisements in support of RSVP-TE.  This could be done in the
   following stepwise manner:

   1.  Upgrade all routers to support the extensions in this document.

   2.  Advertise all legacy link attributes using ASLA advertisements
       with the L-flag clear and the R-bit set.  At this point, both
       legacy and application-specific advertisements are being sent.

   3.  Remove legacy advertisements.

7.  IANA Considerations

   This section lists the protocol codepoint changes introduced by this
   document and the related IANA updates.

   For the registries defined under the "IS-IS TLV Codepoints" group of
   registries with a registration procedure of "Expert Review" (see
   Sections 7.3 and 7.5), guidance for designated experts can be found
   in [RFC7370].

   Note that in all cases where the registry reference was to RFC 8919,
   the registry has been updated to refer to this document.

7.1.  Application-Specific Link Attributes Sub-TLV

   IANA has registered the sub-TLV defined in Section 4.2 in the "IS-IS
   Sub-TLVs for TLVs Advertising Neighbor Information" registry.

    +======+======================+====+====+======+=====+=====+=====+
    | Type | Description          | 22 | 23 | 25   | 141 | 222 | 223 |
    +======+======================+====+====+======+=====+=====+=====+
    | 16   | Application-Specific | y  | y  | y(s) | y   | y   | y   |
    |      | Link Attributes      |    |    |      |     |     |     |
    +------+----------------------+----+----+------+-----+-----+-----+

                                 Table 3

7.2.  Application-Specific SRLG TLV

   IANA has registered the TLV defined in Section 4.3 in the "IS-IS Top-
   Level TLV Codepoints" registry.

      +=======+===========================+=====+=====+=====+=======+
      | Value | Description               | IIH | LSP | SNP | Purge |
      +=======+===========================+=====+=====+=====+=======+
      | 238   | Application-Specific SRLG | n   | y   | n   | n     |
      +-------+---------------------------+-----+-----+-----+-------+

                                  Table 4

7.3.  IS-IS Sub-Sub-TLV Codepoints for Application-Specific Link
      Attributes Registry

   IANA has created a registry titled "IS-IS Sub-Sub-TLV Codepoints for
   Application-Specific Link Attributes" under the "IS-IS TLV
   Codepoints" registry to control the assignment of sub-sub-TLV
   codepoints for the Application-Specific Link Attributes sub-TLV
   defined in Section 7.1.  The registration procedure is "Expert
   Review" as defined in [RFC8126].  The initial contents of this
   registry are as follows:

        +========+====================================+===========+
        | Type   | Description                        | Reference |
        +========+====================================+===========+
        | 0-2    | Unassigned                         |           |
        +--------+------------------------------------+-----------+
        | 3      | Administrative group (color)       | [RFC5305] |
        +--------+------------------------------------+-----------+
        | 4-8    | Unassigned                         |           |
        +--------+------------------------------------+-----------+
        | 9      | Maximum link bandwidth             | [RFC5305] |
        +--------+------------------------------------+-----------+
        | 10     | Maximum reservable link bandwidth  | [RFC5305] |
        +--------+------------------------------------+-----------+
        | 11     | Unreserved bandwidth               | [RFC5305] |
        +--------+------------------------------------+-----------+
        | 12-13  | Unassigned                         |           |
        +--------+------------------------------------+-----------+
        | 14     | Extended Administrative Group      | [RFC7308] |
        +--------+------------------------------------+-----------+
        | 15-17  | Unassigned                         |           |
        +--------+------------------------------------+-----------+
        | 18     | TE Default metric                  | [RFC5305] |
        +--------+------------------------------------+-----------+
        | 19-32  | Unassigned                         |           |
        +--------+------------------------------------+-----------+
        | 33     | Unidirectional Link Delay          | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 34     | Min/Max Unidirectional Link Delay  | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 35     | Unidirectional Delay Variation     | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 36     | Unidirectional Link Loss           | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 37     | Unidirectional Residual Bandwidth  | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 38     | Unidirectional Available Bandwidth | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 39     | Unidirectional Utilized Bandwidth  | [RFC8570] |
        +--------+------------------------------------+-----------+
        | 40-255 | Unassigned                         |           |
        +--------+------------------------------------+-----------+

                                  Table 5

   IANA has also added the following notes to this registry:

      Note: For future codepoints, in cases where the document that
      defines the encoding is different from the document that assigns
      the codepoint, the encoding reference MUST be to the document that
      defines the encoding.

      Note: If a link attribute can be advertised both as a sub-TLV of
      TLVs advertising neighbor information and as a sub-sub-TLV of the
      Application-Specific Link Attributes sub-TLV defined in RFC 9479,
      then the same numerical code should be assigned to the link
      attribute whenever possible.

7.4.  Link Attribute Application Identifiers Registry

   IANA has created a registry titled "Link Attribute Application
   Identifiers" within the "Interior Gateway Protocol (IGP) Parameters"
   group of registries to control the assignment of Application
   Identifier Bits.  The registration policy for this registry is
   "Expert Review" as defined in [RFC8126].  Bit definitions SHOULD be
   assigned such that all bits in the lowest available octet are
   allocated before assigning bits in the next octet.  This minimizes
   the number of octets that will need to be transmitted.  The initial
   contents of this registry are as follows:

                 +======+================================+
                 | Bit  | Name                           |
                 +======+================================+
                 | 0    | RSVP-TE (R-bit)                |
                 +------+--------------------------------+
                 | 1    | Segment Routing Policy (S-bit) |
                 +------+--------------------------------+
                 | 2    | Loop-Free Alternate (F-bit)    |
                 +------+--------------------------------+
                 | 3-63 | Unassigned                     |
                 +------+--------------------------------+

                                  Table 6

7.5.  IS-IS Sub-TLVs for Application-Specific SRLG TLV

   IANA has created a registry titled "IS-IS Sub-TLVs for Application-
   Specific SRLG TLV" under the "IS-IS TLV Codepoints" registry to
   control the assignment of sub-TLV types for the Application-Specific
   SRLG TLV (TLV 238).  The registration procedure is "Expert Review" as
   defined in [RFC8126].  The initial contents of this registry are as
   follows:

          +========+===============================+===========+
          | Value  | Description                   | Reference |
          +========+===============================+===========+
          | 0-3    | Unassigned                    |           |
          +--------+-------------------------------+-----------+
          | 4      | Link Local/Remote Identifiers | [RFC5307] |
          +--------+-------------------------------+-----------+
          | 5      | Unassigned                    |           |
          +--------+-------------------------------+-----------+
          | 6      | IPv4 interface address        | [RFC5305] |
          +--------+-------------------------------+-----------+
          | 7      | Unassigned                    |           |
          +--------+-------------------------------+-----------+
          | 8      | IPv4 neighbor address         | [RFC5305] |
          +--------+-------------------------------+-----------+
          | 9-11   | Unassigned                    |           |
          +--------+-------------------------------+-----------+
          | 12     | IPv6 Interface Address        | [RFC6119] |
          +--------+-------------------------------+-----------+
          | 13     | IPv6 Neighbor Address         | [RFC6119] |
          +--------+-------------------------------+-----------+
          | 14-255 | Unassigned                    |           |
          +--------+-------------------------------+-----------+

                                 Table 7

   IANA has also added the following note to this registry:

      Note: For future codepoints, in cases where the document that
      defines the encoding is different from the document that assigns
      the codepoint, the encoding reference MUST be to the document that
      defines the encoding.

8.  Security Considerations

   Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
   and [RFC5310].  While IS-IS is deployed under a single administrative
   domain, there can be deployments where potential attackers have
   access to one or more networks in the IS-IS routing domain.  In these
   deployments, the stronger authentication mechanisms defined in the
   aforementioned documents SHOULD be used.

   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 as discussed
   in [RFC8570].  As the advertisements defined in this document limit
   the scope to specific applications, the impact of tampering is
   similarly limited in scope.

9.  Changes to RFC 8919

   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 Sections 4.2, 4.3, and 6.2 have been introduced to clarify
   normative behavior in the presence of such advertisements.  In
   particular, the text in [RFC8919] used 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.

10.  References

10.1.  Normative References

   [ISO10589] ISO, "Information technology - Telecommunications and
              information exchange between systems - Intermediate System
              to Intermediate System intra-domain routing information
              exchange protocol for use in conjunction with the protocol
              for providing the connectionless-mode network service (ISO
              8473)", Second Edition, ISO/IEC 10589:2002, November 2002,
              <https://www.iso.org/standard/30932.html>.

   [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>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <https://www.rfc-editor.org/info/rfc5307>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
              February 2011, <https://www.rfc-editor.org/info/rfc6119>.

   [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>.

   [RFC7370]  Ginsberg, L., "Updates to the IS-IS TLV Codepoints
              Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
              <https://www.rfc-editor.org/info/rfc7370>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

10.2.  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>.

   [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>.

   [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>.

   [RFC8919]  Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
              J. Drake, "IS-IS Application-Specific Link Attributes",
              RFC 8919, DOI 10.17487/RFC8919, October 2020,
              <https://www.rfc-editor.org/info/rfc8919>.

   [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>.

Acknowledgements

   RFC 8919 included the following acknowledgements:

   |  Eric Rosen and Acee Lindem for their careful review and content
   |  suggestions.

   For the new version (this document), the authors would like to thank
   Bruno Decraene.

Authors' Addresses

   Les Ginsberg
   Cisco Systems
   United States of America
   Email: ginsberg@cisco.com


   Peter Psenak
   Cisco Systems
   Slovakia
   Email: ppsenak@cisco.com


   Stefano Previdi
   Huawei Technologies
   Email: stefano@previdi.net


   Wim Henderickx
   Nokia
   Copernicuslaan 50
   2018 94089 Antwerp
   Belgium
   Email: wim.henderickx@nokia.com


   John Drake
   Juniper Networks
   Email: jdrake@juniper.net
