RFC9479: IS-IS Application-Specific Link Attributes
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Obsoletes:
RFC8919
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
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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