<- RFC Index (8901..9000)
RFC 8919
Obsoleted by RFC 9479
Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 8919 P. Psenak
Category: Standards Track Cisco Systems
ISSN: 2070-1721 S. Previdi
Huawei Technologies
W. Henderickx
Nokia
J. Drake
Juniper Networks
October 2020
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 indication
of which applications are using the advertised value for a given
link. This document introduces new link attribute advertisements
that address both of these shortcomings.
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/rfc8919.
Copyright Notice
Copyright (c) 2020 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified 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. Sub-sub-TLV Codepoints for Application-Specific Link
Attributes Registry
7.4. Link Attribute Application Identifiers Registry
7.5. Sub-TLVs for TLV 238 Registry
8. Security Considerations
9. References
9.1. Normative References
9.2. 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]. 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 [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
[SEGMENT-ROUTING] 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 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.
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 Segment Routing
(SR). Included among these use cases is SR Policy, which is defined
in [SEGMENT-ROUTING]. 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.
3. 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".
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: Sub-TLVs for TLVs 22, 23, 25,
141, 222, and 223
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 new codepoints are defined to support Application-Specific Link
Attribute (ASLA) advertisements:
1) Application-Specific Link Attributes sub-TLV for TLVs 22, 23, 25,
141, 222, and 223 (defined in Section 4.2).
2) Application-Specific Shared Risk Link Group (SRLG) TLV (defined
in Section 4.3).
To support these new 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. 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.
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
a new IANA-controlled registry (see Section 7.4). 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.
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: 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.
| 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 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 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: 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 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 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 zero-
length SABM and UDABM so long as the constraints discussed in
Sections 4.2 and 6.2 are acceptable.
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 (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.
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
either by explicitly specifying the applications in the Application
Identifier Bit Mask or by using a zero-length Application Identifier
Bit Mask.
4.3. 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 [RFC5307]
and TLV 139 [RFC6119], 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 Section 4.1)
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 [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
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 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 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.
6. Deployment Considerations
This section discusses deployment considerations associated with the
use of application-specific link attribute 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 user-defined
applications 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, 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.
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 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:
1) Send ASLA advertisements while continuing to advertise using
legacy (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 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 updates made by IANA.
For the new registries defined under the "IS-IS TLV Codepoints"
registry with the "Expert Review" registration procedure (see
Sections 7.3 and 7.5), guidance for designated experts can be found
in [RFC7370].
7.1. Application-Specific Link Attributes Sub-TLV
IANA has registered the new sub-TLV defined in Section 4.2 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 | y | y | y(s) | y | y | y |
| | Link Attributes | | | | | | |
+------+----------------------+----+----+------+-----+-----+-----+
Table 3
7.2. Application-Specific SRLG TLV
IANA has registered the new TLV defined in Section 4.3 in the IS-IS
"TLV Codepoints Registry".
+=======+===========================+=====+=====+=====+=======+
| Value | Description | IIH | LSP | SNP | Purge |
+=======+===========================+=====+=====+=====+=======+
| 238 | Application-Specific SRLG | n | y | n | n |
+-------+---------------------------+-----+-----+-----+-------+
Table 4
7.3. 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 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 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.
7.4. 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 [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. 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 [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 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 [RFC8570]. As the advertisements defined
in this document limit the scope to specific applications, the impact
of tampering is similarly limited in scope.
9. References
9.1. Normative References
[ISO10589] International Organization for Standardization,
"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)", ISO/IEC 10589:2002, Second Edition, November 2002.
[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>.
9.2. 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>.
[SEGMENT-ROUTING]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-08, 8 July 2020,
<https://tools.ietf.org/html/draft-ietf-spring-segment-
routing-policy-08>.
Acknowledgements
The authors would like to thank Eric Rosen and Acee Lindem for their
careful review and content suggestions.
Authors' Addresses
Les Ginsberg
Cisco Systems
821 Alder Drive
Milpitas, CA 95035
United States of America
Email: ginsberg@cisco.com
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynske nivy 43
821 09 Bratislava
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