IS-IS Application-Specific Link Attributes

Introduction Advertisement of link attributes by the Intermediate System to Intermediate System (IS-IS) protocol in support of traffic engineering (TE) was introduced by and extended by , , , and . Use of these extensions has been associated with deployments supporting Traffic Engineering over Multiprotocol Label Switching (MPLS) in the presence of the Resource Reservation Protocol (RSVP), more succinctly referred to as RSVP-TE . For the purposes of this document, an application is a technology that makes use of link attribute advertisements, examples of which are listed in . 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 and Loop-Free Alternates (LFAs) . 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 path setup failure. An additional issue arises in cases where both applications are supported on a link but the link attribute values associated with each application differ. Current advertisements do not support advertising application-specific values for the same attribute on a specific link. This document defines extensions that address these issues. Also, as evolution of use cases for link attributes can be expected to continue in the years to come, this document defines a solution that is easily extensible to the introduction of new applications and new use cases.

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 when, and only when, they appear in all capitals, as shown here.

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:

Support for indicating which applications are using the link attribute advertisements on a link
Support for advertising application-specific values for the same attribute on a link

discusses use cases and requirements for Segment Routing (SR). Included among these use cases is SR Policy, which is defined in . If both RSVP-TE and SR Policy are deployed in a network, link attribute advertisements can be used by one or both of these applications. There is no requirement for the link attributes advertised on a given link used by SR Policy to be identical to the link attributes advertised on that same link used by RSVP-TE; thus, there is a clear requirement to indicate independently which link attribute advertisements are to be used by each application. As the number of applications that may wish to utilize link attributes may grow in the future, an additional requirement is that the extensions defined allow the association of additional applications to link attributes without altering the format of the advertisements or introducing new backwards-compatibility issues. Finally, there may still be many cases where a single attribute value can be shared among multiple applications, so the solution must minimize advertising duplicate link/attribute pairs whenever possible.

Legacy Advertisements Existing advertisements used in support of RSVP-TE include sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 and TLVs for Shared Risk Link Group (SRLG) advertisement. Sub-TLV values are defined in the "Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223" registry. TLVs are defined in the "TLV Codepoints Registry".

Legacy Sub-TLVs Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223

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

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 prohibits the use of TLV 139 when it is possible to use TLV 138.

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

Application-Specific Link Attributes sub-TLV for TLVs 22, 23, 25, 141, 222, and 223 (defined in ).
Application-Specific Shared Risk Link Group (SRLG) TLV (defined in ).

To support these new advertisements, an application identifier bit mask is defined to identify the application(s) associated with a given advertisement (defined in ). 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. Such applications are not subject to standardization and are outside the scope of this document. The following sections define the format of these new advertisements.

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 a new IANA-controlled registry (see ). A second bit mask is for non-standard user-defined applications (UDAs). The encoding defined below is used by both the Application-Specific Link Attributes sub-TLV and the Application-Specific SRLG TLV.

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 for a description of how this flag is used.
SABM Length:: 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 Segment Routing Policy.
F-bit:: Set to specify Loop-Free Alternate (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.

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 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.

Application-Specific Link Attributes Sub-TLV A new sub-TLV for TLVs 22, 23, 25, 141, 222, and 223 is defined that supports specification of the applications and application-specific attribute values.

Type:

Length:

Variable (1 octet)

Value:

Application Identifier Bit Mask (as defined in )
Link Attribute sub-sub-TLVs -- format matches the existing formats defined in , , and

If the SABM or UDABM Length in the Application Identifier Bit Mask is greater than 8, the entire sub-TLV MUST be ignored. 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 22, 23, 25, 141, 222, and 223, in TLV 138, or in TLV 139 as appropriate. Link attribute sub-sub-TLVs for the corresponding link attributes MUST NOT be advertised for the set of applications specified in the Standard or User-Defined Application Identifier Bit Masks, and all such advertisements 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 LSP SHOULD be used, and subsequent advertisements of the same attribute SHOULD 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. If link attributes are advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications, then any standard application and/or any user-defined application is permitted to use that set of link attributes so long as there is not another set of attributes advertised on that same link that is associated with a non-zero-length Application Identifier Bit Mask with a matching Application Identifier Bit set. IANA has created a new registry of sub-sub-TLVs to define the link attribute sub-sub-TLV codepoints (see ). This document defines a sub-sub-TLV for each of the existing sub-TLVs listed in , 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.

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 zero-length SABM and UDABM so long as the constraints discussed in Sections and are acceptable. If different values for maximum link bandwidth for a given link are advertised, all values MUST be ignored.

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 (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 RSVP-TE bit set, the advertisement MUST be ignored.

Considerations for Extended TE Metrics 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 either by explicitly specifying the applications in the Application Identifier Bit Mask or by using a zero-length Application Identifier Bit Mask.

Application-Specific SRLG TLV A new TLV is defined to advertise application-specific SRLGs for a given link. Although similar in functionality to TLV 138 and TLV 139 , a 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 )
Length of sub-TLVs (1 octet)
Link Identifier sub-TLVs (variable)
0 or more SRLG values (each value is 4 octets)

The following Link Identifier sub-TLVs are defined. The values chosen intentionally match the equivalent sub-TLVs from , , and .

Type	Description
4	Link Local/Remote Identifiers
6	IPv4 interface address
8	IPv4 neighbor address
12	IPv6 Interface Address
13	IPv6 Neighbor Address

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 ) 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.

Attribute Advertisements and Enablement This document defines extensions to support the advertisement of application-specific link attributes. 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 implies that RSVP is enabled on that link. The absence of RSVP-TE application-specific link attributes 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 application-specific link attributes 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 application-specific link attributes does not indicate enablement of LFA on that link. Enablement is controlled by local configuration. In the future, if additional standard applications are defined to use this mechanism, the specification defining this use MUST define the relationship between application-specific link attribute advertisements and enablement for that application. This document allows the advertisement of application-specific link attributes with no application identifiers, i.e., both the Standard Application Identifier Bit Mask and the User-Defined Application Identifier Bit Mask are not present (see ). This supports the use of the link attribute by any application. In the presence of an application where the advertisement of link attribute advertisements 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.

Deployment Considerations This section discusses deployment considerations associated with the use of application-specific link attribute advertisements.

Use of Legacy Advertisements Bit identifiers for standard applications are defined in . 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 . 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.

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 . If support for a new application is introduced on any node in a network in the presence of such advertisements, these advertisements are permitted to be used by the new application. If this is not what is intended, then existing advertisements MUST be readvertised with an explicit set of applications specified before a new application is introduced.

Interoperability, Backwards Compatibility, and Migration Concerns Existing deployments of RSVP-TE, SR Policy, and/or LFA utilize the legacy advertisements listed in . 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.

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.

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.

Interoperability with Legacy Routers For the 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. So long as there is any legacy router in the network that has any of the applications enabled, all routers MUST continue to advertise link attributes using legacy advertisements. In addition, the link attribute values associated with the set of applications supported by legacy routers (RSVP-TE, SR Policy, and/or LFA) are always shared since legacy routers have no way of advertising or processing application-specific values. Once all legacy routers have been upgraded, migration from legacy advertisements to ASLA advertisements can be achieved via the following steps:

Send ASLA advertisements while continuing to advertise using legacy (all advertisements are then duplicated). Receiving routers continue to use legacy advertisements.
Enable the use of the ASLA advertisements on all routers.
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.

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:

Upgrade all routers to support the extensions in this document.
Advertise all legacy link attributes using ASLA advertisements with the L-flag clear and R-bit set. At this point, both legacy and application-specific advertisements are being sent.
Remove legacy advertisements.

IANA Considerations This section lists the protocol codepoint changes introduced by this document and the related updates made by IANA. For the new registries defined under the "IS-IS TLV Codepoints" registry with the "Expert Review" registration procedure (see Sections and ), guidance for designated experts can be found in .

Application-Specific Link Attributes Sub-TLV IANA has registered the new sub-TLV defined in in the "Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223" registry.

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

Application-Specific SRLG TLV IANA has registered the new TLV defined in in the IS-IS "TLV Codepoints Registry".

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

Sub-sub-TLV Codepoints for Application-Specific Link Attributes Registry IANA has created a new registry titled "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 . The registration procedure is "Expert Review" as defined in . The initial contents of this registry are as follows:

Type	Description	Reference
0-2	Unassigned
3	Administrative group (color)
4-8	Unassigned
9	Maximum link bandwidth
10	Maximum reservable link bandwidth
11	Unreserved bandwidth
12-13	Unassigned
14	Extended Administrative Group
15-17	Unassigned
18	TE Default Metric
19-32	Unassigned
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
40-255	Unassigned

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 22, 23, 25, 141, 222, and 223 and as a sub-sub-TLV of the Application-Specific Link Attributes sub-TLV defined in RFC 8919, then the same numerical code should be assigned to the link attribute whenever possible.

Link Attribute Application Identifiers Registry IANA has created a new registry titled "Link Attribute Application Identifiers" under the "Interior Gateway Protocol (IGP) Parameters" registry to control the assignment of Application Identifier Bits. The registration policy for this registry is "Expert Review" as defined in . 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

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

Value	Description	Reference
0-3	Unassigned
4	Link Local/Remote Identifiers
5	Unassigned
6	IPv4 interface address
7	Unassigned
8	IPv4 neighbor address
9-11	Unassigned
12	IPv6 Interface Address
13	IPv6 Neighbor Address
14-255	Unassigned

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.

Security Considerations Security concerns for IS-IS are addressed in , , and . 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 a new way to advertise link attributes. Tampering with the information defined in this document may have an effect on applications using it, including impacting traffic engineering as discussed in . As the advertisements defined in this document limit the scope to specific applications, the impact of tampering is similarly limited in scope.

References Normative References 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) International Organization for Standardization Informative References

Acknowledgements The authors would like to thank and for their careful review and content suggestions.