Internet Engineering Task Force (IETF) P. Psenak, Ed.

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

OSPF Application-Specific Link Attributes

Abstract

Existing traffic-engineering-related link attribute advertisements
have been defined and are used in RSVP-TE deployments. Since the
original RSVP-TE use case was defined, additional applications such
as Segment Routing (SR) Policy and Loop-Free Alternates (LFAs) that
also make use of the link attribute advertisements have been defined.
In cases where multiple applications wish to make use of these link
attributes, the current advertisements do not support application-
specific values for a given attribute, nor do they support indication
of which applications are using the advertised value for a given
link. This document introduces link attribute advertisements in
OSPFv2 and OSPFv3 that address both of these shortcomings.

This document obsoletes RFC 8920.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9492.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Requirements Language
2. Requirements Discussion
3. Existing Advertisement of Link Attributes
4. Advertisement of Link Attributes
4.1. OSPFv2 Extended Link Opaque LSA and OSPFv3 E-Router-LSA
5. Advertisement of Application-Specific Values
6. Reused TE Link Attributes
6.1. Shared Risk Link Group (SRLG)
6.2. Extended Metrics
6.3. Administrative Group
6.4. TE Metric
7. Maximum Link Bandwidth
8. Considerations for Extended TE Metrics
9. Local Interface IPv6 Address Sub-TLV
10. Remote Interface IPv6 Address Sub-TLV
11. Attribute Advertisements and Enablement
12. Deployment Considerations
12.1. Use of Legacy RSVP-TE LSA Advertisements
12.2. Use of Zero-Length Application Identifier Bit Masks
12.3. Interoperability, Backwards Compatibility, and Migration
Concerns
12.3.1. Multiple Applications: Common Attributes with RSVP-TE
12.3.2. Multiple Applications: Some Attributes Not Shared with
RSVP-TE
12.3.3. Interoperability with Legacy Routers
12.3.4. Use of Application-Specific Advertisements for RSVP-TE
13. Security Considerations
14. IANA Considerations
14.1. OSPFv2
14.2. OSPFv3
15. Changes to RFC 8920
16. References
16.1. Normative References
16.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

Advertisement of link attributes by the OSPFv2 [RFC2328] and OSPFv3
[RFC5340] protocols in support of traffic engineering (TE) was
introduced by [RFC3630] and [RFC5329], respectively. It has been
extended by [RFC4203], [RFC7308], and [RFC7471]. Use of these
extensions has been associated with deployments supporting TE over
Multiprotocol Label Switching (MPLS) in the presence of the Resource
Reservation Protocol (RSVP), more succinctly referred to as RSVP-TE
[RFC3209].

For the purposes of this document, an application is a technology
that makes use of link attribute advertisements, examples of which
are listed in Section 5.

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

An example of where this ambiguity causes a problem is a network
where RSVP-TE is enabled only on a subset of its links. A link
attribute is advertised for the purpose of another application (e.g.,
SR Policy) for a link that is not enabled for RSVP-TE. As soon as
the router that is an RSVP-TE head end sees the link attribute being
advertised for that link, it assumes RSVP-TE is enabled on that link,
even though it is not. If such an RSVP-TE head-end router tries to
set up an RSVP-TE path via that link, it will result in a setup
failure for the path.

An additional issue arises in cases where both applications are
supported on a link but the link attribute values associated with
each application differ. Current advertisements do not support
advertising application-specific values for the same attribute on a
specific link.

This document defines extensions that address these issues. Also, as
evolution of use cases for link attributes can be expected to
continue in the years to come, this document defines a solution that
is easily extensible for the introduction of new applications and new
use cases.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Requirements Discussion

As stated previously, evolution of use cases for link attributes can
be expected to continue. Therefore, any discussion of existing use
cases is limited to requirements that are known at the time of this
writing. However, in order to determine the functionality required
beyond what already exists in OSPF, it is only necessary to discuss
use cases that justify the key points identified in the introduction,
which are:

1. Support for indicating which applications are using the link
attribute advertisements on a link.

2. Support for advertising application-specific values for the same
attribute on a link.

[RFC7855] discusses use cases and requirements for SR. Included
among these use cases is SR Policy, which is defined in [RFC9256].
If both RSVP-TE and SR Policy are deployed in a network, link
attribute advertisements can be used by one or both of these
applications. There is no requirement for the link attributes
advertised on a given link used by SR Policy to be identical to the
link attributes advertised on that same link used by RSVP-TE; thus,
there is a clear requirement to indicate independently which link
attribute advertisements are to be used by each application.

As the number of applications that may wish to utilize link
attributes may grow in the future, an additional requirement is that
the extensions defined allow the association of additional
applications to link attributes without altering the format of the
advertisements or introducing backwards-compatibility issues.

Finally, there may still be many cases where a single attribute value
can be shared among multiple applications, so the solution must
minimize advertising duplicate link/attribute pairs whenever
possible.

3. Existing Advertisement of Link Attributes

There are existing advertisements used in support of RSVP-TE. These
advertisements are carried in the OSPFv2 TE Opaque Link State
Advertisement (LSA) [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329].
Additional RSVP-TE link attributes have been defined by [RFC4203],
[RFC7308], and [RFC7471].

Extended Link Opaque LSAs as defined in [RFC7684] for OSPFv2 and E-
Router-LSAs [RFC8362] for OSPFv3 are used to advertise link
attributes that are used by applications other than RSVP-TE or GMPLS
[RFC4203]. These LSAs were defined as generic containers for
distribution of the extended link attributes.

4. Advertisement of Link Attributes

This section outlines the solution for advertising link attributes
originally defined for RSVP-TE or GMPLS when they are used for other
applications.

4.1. OSPFv2 Extended Link Opaque LSA and OSPFv3 E-Router-LSA

The following are the advantages of Extended Link Opaque LSAs as
defined in [RFC7684] for OSPFv2 and E-Router-LSAs [RFC8362] for
OSPFv3 with respect to the advertisement of link attributes
originally defined for RSVP-TE when used in packet networks and in
GMPLS:

1. Advertisement of the link attributes does not make the link part
of the RSVP-TE topology. It avoids any conflicts and is fully
compatible with [RFC3630] and [RFC5329].

2. The OSPFv2 TE Opaque LSA and OSPFv3 Intra-Area-TE-LSA remain
truly opaque to OSPFv2 and OSPFv3 as originally defined in
[RFC3630] and [RFC5329], respectively. Their contents are not
inspected by OSPF, which instead acts as a pure transport.

3. There is a clear distinction between link attributes used by
RSVP-TE and link attributes used by other OSPFv2 or OSPFv3
applications.

4. All link attributes that are used by other applications are
advertised in the Extended Link Opaque LSA in OSPFv2 [RFC7684] or
the OSPFv3 E-Router-LSA [RFC8362] in OSPFv3.

The disadvantage of this approach is that in rare cases, the same
link attribute is advertised in both the TE Opaque and Extended Link
Attribute LSAs in OSPFv2 or the Intra-Area-TE-LSA and E-Router-LSA in
OSPFv3.

The Extended Link Opaque LSA [RFC7684] and E-Router-LSA [RFC8362] are
used to advertise any link attributes used for non-RSVP-TE
applications in OSPFv2 or OSPFv3, respectively, including those that
have been originally defined for RSVP-TE applications (see
Section 6).

TE link attributes used for RSVP-TE/GMPLS continue to use the OSPFv2
TE Opaque LSA [RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329].

The format of the link attribute TLVs that have been defined for
RSVP-TE applications will be kept unchanged even when they are used
for non-RSVP-TE applications. Unique codepoints are allocated for
these link attribute TLVs from the "OSPFv2 Extended Link TLV Sub-
TLVs" registry [RFC7684] and from the "OSPFv3 Extended-LSA Sub-TLVs"
registry [RFC8362], as specified in Section 14.

5. Advertisement of Application-Specific Values

To allow advertisement of the application-specific values of the link
attribute, an Application-Specific Link Attributes (ASLA) sub-TLV is
defined. The ASLA sub-TLV is a sub-TLV of the OSPFv2 Extended Link
TLV [RFC7684] and OSPFv3 Router-Link TLV [RFC8362].

In addition to advertising the link attributes for standardized
applications, link attributes can be advertised for the purpose of
applications that are not standardized. We call such an application
a "user-defined application" or "UDA". These applications are not
subject to standardization and are outside of the scope of this
specification.

The ASLA sub-TLV is an optional sub-TLV of the OSPFv2 Extended Link
TLV and OSPFv3 Router-Link TLV. Multiple ASLA sub-TLVs can be
present in a parent TLV when different applications want to control
different link attributes or when a different value of the same
attribute needs to be advertised by multiple applications. The ASLA
sub-TLV MUST be used for advertisement of the link attributes listed
at the end of this section if these are advertised inside the OSPFv2
Extended Link TLV and OSPFv3 Router-Link TLV. It has the following
format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SABM Length | UDABM Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Standard Application Identifier Bit Mask (SABM) |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User-Defined Application Identifier Bit Mask (UDABM) |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Attribute sub-TLVs |
+- -+
| ... |

where:

Type:
10 (OSPFv2), 11 (OSPFv3)

Length:
Variable

SABM Length:
Standard Application Identifier Bit Mask Length in octets. The
value MUST be 0, 4, or 8. If the Standard Application Identifier
Bit Mask is not present, the SABM Length MUST be set to 0.

UDABM Length:
User-Defined Application Identifier Bit Mask Length in octets.
The value MUST be 0, 4, or 8. If the User-Defined Application
Identifier Bit Mask is not present, the UDABM Length MUST be set
to 0.

Standard Application Identifier Bit Mask:
Optional set of bits, where each bit represents a single standard
application. Bits are defined in the "Link Attribute Application
Identifiers" registry, which is defined in [RFC9479]. Current
assignments are repeated here for informational purposes:

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

Bit 0 (R-bit): RSVP-TE.

Bit 1 (S-bit): SR Policy (this is data plane independent).

Bit 2 (F-bit): Loop-Free Alternate (includes all LFA types).

User-Defined Application Identifier Bit Mask:
Optional set of bits, where each bit represents a single user-
defined application.

If the SABM or UDABM Length is other than 0, 4, or 8, the ASLA sub-
TLV MUST be ignored by the receiver.

Standard Application Identifier Bits are defined and sent starting
with bit 0. Undefined bits that are transmitted MUST be transmitted
as 0 and MUST be ignored on receipt. Bits that are not transmitted
MUST be treated as if they are set to 0 on receipt. Bits that are
not supported by an implementation MUST be ignored on receipt.

User-Defined Application Identifier Bits have no relationship to
Standard Application Identifier Bits and are not managed by IANA or
any other standards body. It is recommended that these bits be used
starting with bit 0 so as to minimize the number of octets required
to advertise all UDAs. Undefined bits that are transmitted MUST be
transmitted as 0 and MUST be ignored on receipt. Bits that are not
transmitted MUST be treated as if they are set to 0 on receipt. Bits
that are not supported by an implementation MUST be ignored on
receipt.

If the link attribute advertisement is intended to be only used by a
specific set of applications, corresponding bit masks MUST be present
and one or more application-specific bits MUST be set for all
applications that use the link attributes advertised in the ASLA sub-
TLV.

Application Identifier Bit Masks apply to all link attributes that
support application-specific values and are advertised in the ASLA
sub-TLV.

The advantage of not making the Application Identifier Bit Masks part
of the attribute advertisement itself is that the format of any
previously defined link attributes can be kept and reused when
advertising them in the ASLA sub-TLV.

If the same attribute is advertised in more than one ASLA sub-TLV
with the application listed in the Application Identifier Bit Masks,
the application SHOULD use the first instance of advertisement and
ignore any subsequent advertisements of that attribute.

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

This document defines the initial set of link attributes that MUST
use the ASLA sub-TLV if advertised in the OSPFv2 Extended Link TLV or
in the OSPFv3 Router-Link TLV. Documents that define new link
attributes MUST state whether the new attributes support application-
specific values and, as such, are advertised in an ASLA sub-TLV. The
standard link attributes that are advertised in ASLA sub-TLVs are:

* Shared Risk Link Group [RFC4203]

* Unidirectional Link Delay [RFC7471]

* Min/Max Unidirectional Link Delay [RFC7471]

* Unidirectional Delay Variation [RFC7471]

* Unidirectional Link Loss [RFC7471]

* Unidirectional Residual Bandwidth [RFC7471]

* Unidirectional Available Bandwidth [RFC7471]

* Unidirectional Utilized Bandwidth [RFC7471]

* Administrative Group [RFC3630]

* Extended Administrative Group [RFC7308]

* TE Metric [RFC3630]

6. Reused TE Link Attributes

This section defines the use case and indicates the codepoints
(Section 14) from the "OSPFv2 Extended Link TLV Sub-TLVs" registry
and "OSPFv3 Extended-LSA Sub-TLVs" registry for some of the link
attributes that have been originally defined for RSVP-TE or GMPLS.

6.1. Shared Risk Link Group (SRLG)

The SRLG of a link can be used in OSPF-calculated IPFRR (IP Fast
Reroute) [RFC5714] to compute a backup path that does not share any
SRLG with the protected link.

To advertise the SRLG of the link in the OSPFv2 Extended Link TLV,
the same format for the sub-TLV defined in Section 1.3 of [RFC4203]
is used with TLV type 11. Similarly, for OSPFv3 to advertise the
SRLG in the OSPFv3 Router-Link TLV, TLV type 12 is used.

6.2. Extended Metrics

[RFC3630] defines several link bandwidth types. [RFC7471] defines
extended link metrics that are based on link bandwidth, delay, and
loss characteristics. All of these can be used to compute primary
and backup paths within an OSPF area to satisfy requirements for
bandwidth, delay (nominal or worst case), or loss.

To advertise extended link metrics in the OSPFv2 Extended Link TLV,
the same format for the sub-TLVs defined in [RFC7471] is used with
the following TLV types:

12: Unidirectional Link Delay

13: Min/Max Unidirectional Link Delay

14: Unidirectional Delay Variation

15: Unidirectional Link Loss

16: Unidirectional Residual Bandwidth

17: Unidirectional Available Bandwidth

18: Unidirectional Utilized Bandwidth

To advertise extended link metrics in the Router-Link TLV inside the
OSPFv3 E-Router-LSA, the same format for the sub-TLVs defined in
[RFC7471] is used with the following TLV types:

13: Unidirectional Link Delay

14: Min/Max Unidirectional Link Delay

15: Unidirectional Delay Variation

16: Unidirectional Link Loss

17: Unidirectional Residual Bandwidth

18: Unidirectional Available Bandwidth

19: Unidirectional Utilized Bandwidth

6.3. Administrative Group

[RFC3630] and [RFC7308] define the Administrative Group and Extended
Administrative Group sub-TLVs, respectively.

To advertise the Administrative Group and Extended Administrative
Group in the OSPFv2 Extended Link TLV, the same format for the sub-
TLVs defined in [RFC3630] and [RFC7308] is used with the following
TLV types:

19: Administrative Group

20: Extended Administrative Group

To advertise the Administrative Group and Extended Administrative
Group in the OSPFv3 Router-Link TLV, the same format for the sub-TLVs
defined in [RFC3630] and [RFC7308] is used with the following TLV
types:

20: Administrative Group

21: Extended Administrative Group

6.4. TE Metric

[RFC3630] defines the TE Metric.

To advertise the TE Metric in the OSPFv2 Extended Link TLV, the same
format for the sub-TLV defined in Section 2.5.5 of [RFC3630] is used
with TLV type 22. Similarly, for OSPFv3 to advertise the TE Metric
in the OSPFv3 Router-Link TLV, TLV type 22 is used.

7. Maximum Link Bandwidth

Maximum link bandwidth is an application-independent attribute of the
link that is defined in [RFC3630]. Because it is an application-
independent attribute, it MUST NOT be advertised in the ASLA sub-TLV.
Instead, it MAY be advertised as a sub-TLV of the Extended Link TLV
in the Extended Link Opaque LSA in OSPFv2 [RFC7684] or as a sub-TLV
of the Router-Link TLV in the E-Router-LSA Router-Link TLV in OSPFv3
[RFC8362].

To advertise the maximum link bandwidth in the OSPFv2 Extended Link
TLV, the same format for the sub-TLV defined in [RFC3630] is used
with TLV type 23.

To advertise the maximum link bandwidth in the OSPFv3 Router-Link
TLV, the same format for the sub-TLV defined in [RFC3630] is used
with TLV type 23.

8. Considerations for Extended TE Metrics

[RFC7471] defines a number of dynamic performance metrics associated
with a link. It is conceivable that such metrics could be measured
specific to traffic associated with a specific application.
Therefore, this document includes support for advertising these link
attributes specific to a given application. However, in practice, it
may well be more practical to have these metrics reflect the
performance of all traffic on the link regardless of application. In
such cases, advertisements for these attributes can be associated
with all of the applications utilizing that link. This can be done
either by explicitly specifying the applications in the Application
Identifier Bit Mask or by using a zero-length Application Identifier
Bit Mask. The use of zero-length Application Identifier Bit Mask is
further discussed in Section 12.2.

9. Local Interface IPv6 Address Sub-TLV

The Local Interface IPv6 Address sub-TLV is an application-
independent attribute of the link that is defined in [RFC5329].
Because it is an application-independent attribute, it MUST NOT be
advertised in the ASLA sub-TLV. Instead, it MAY be advertised as a
sub-TLV of the Router-Link TLV inside the OSPFv3 E-Router-LSA
[RFC8362].

To advertise the Local Interface IPv6 Address sub-TLV in the OSPFv3
Router-Link TLV, the same format for the sub-TLV defined in [RFC5329]
is used with TLV type 24.

10. Remote Interface IPv6 Address Sub-TLV

The Remote Interface IPv6 Address sub-TLV is an application-
independent attribute of the link that is defined in [RFC5329].
Because it is an application-independent attribute, it MUST NOT be
advertised in the ASLA sub-TLV. Instead, it MAY be advertised as a
sub-TLV of the Router-Link TLV inside the OSPFv3 E-Router-LSA
[RFC8362].

To advertise the Remote Interface IPv6 Address sub-TLV in the OSPFv3
Router-Link TLV, the same format for the sub-TLV defined in [RFC5329]
is used with TLV type 25.

11. Attribute Advertisements and Enablement

This document defines extensions to support the advertisement of
application-specific link attributes.

There are applications where the application enablement on the link
is relevant; for example, with RSVP-TE, one needs to make sure that
RSVP is enabled on the link before sending an RSVP-TE signaling
message over it.

There are applications where the enablement of the application on the
link is irrelevant and has nothing to do with the fact that some link
attributes are advertised for the purpose of such application. An
example of this is LFA.

Whether the presence of link attribute advertisements for a given
application indicates that the application is enabled on that link
depends upon the application. Similarly, whether the absence of link
attribute advertisements indicates that the application is not
enabled depends upon the application.

In the case of RSVP-TE, the advertisement of application-specific
link attributes has no implication of RSVP-TE being enabled on that
link. The RSVP-TE enablement is solely derived from the information
carried in the OSPFv2 TE Opaque LSA [RFC3630] and OSPFv3 Intra-Area-
TE-LSA [RFC5329].

In the case of SR Policy, advertisement of application-specific link
attributes does not indicate enablement of SR Policy. The
advertisements are only used to support constraints that may be
applied when specifying an explicit path. SR Policy is implicitly
enabled on all links that are part of the SR-enabled topology
independent of the existence of link attribute advertisements.

In the case of LFA, the advertisement of application-specific link
attributes does not indicate enablement of LFA on that link.
Enablement is controlled by local configuration.

In the future, if additional standard applications are defined to use
this mechanism, the specification defining this use MUST define the
relationship between application-specific link attribute
advertisements and enablement for that application.

This document allows the advertisement of application-specific link
attributes with no application identifiers, i.e., both the SABM and
the UDABM are not present (see Section 5). This supports the use of
the link attribute by any application. In the presence of an
application where the advertisement of link attributes is used to
infer the enablement of an application on that link (e.g., RSVP-TE),
the absence of the application identifier leaves ambiguous whether
that application is enabled on such a link. This needs to be
considered when making use of the "any application" encoding.

12. Deployment Considerations

12.1. Use of Legacy RSVP-TE LSA Advertisements

Bit identifiers for standard applications are defined in Section 5.
All of the identifiers defined in this document are associated with
applications that were already deployed in some networks prior to the
writing of this document. Therefore, such applications have been
deployed using the RSVP-TE LSA advertisements. The standard
applications defined in this document may continue to use RSVP-TE LSA
advertisements for a given link so long as at least one of the
following conditions is true:

* The application is RSVP-TE.

* The application is SR Policy or LFA, and RSVP-TE is not deployed
anywhere in the network.

* The application is SR Policy or LFA, RSVP-TE is deployed in the
network, and both the set of links on which SR Policy and/or LFA
advertisements are required and the attribute values used by SR
Policy and/or LFA on all such links are fully congruent with the
links and attribute values used by RSVP-TE.

Under the conditions defined above, implementations that support the
extensions defined in this document have the choice of using RSVP-TE
LSA advertisements or application-specific advertisements in support
of SR Policy and/or LFA. This will require implementations to
provide controls specifying which types of advertisements are to be
sent and processed on receipt for these applications. Further
discussion of the associated issues can be found in Section 12.3.

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

12.2. Use of Zero-Length Application Identifier Bit Masks

Link attribute advertisements associated with zero-length Application
Identifier Bit Masks for both standard applications and user-defined
applications are usable by any application, subject to the
restrictions specified in Section 5. If support for a new
application is introduced on any node in a network in the presence of
such advertisements, the new application will use these
advertisements when the aforementioned restrictions are met. If this
is not what is intended, then existing link attribute advertisements
MUST be readvertised with an explicit set of applications specified
before a new application is introduced.

12.3. Interoperability, Backwards Compatibility, and Migration Concerns

Existing deployments of RSVP-TE, SR Policy, and/or LFA utilize the
legacy advertisements listed in Section 3. Routers that do not
support the extensions defined in this document will only process
legacy advertisements and are likely to infer that RSVP-TE is enabled
on the links for which legacy advertisements exist. It is expected
that deployments using the legacy advertisements will persist for a
significant period of time. Therefore, deployments using the
extensions defined in this document in the presence of routers that
do not support these extensions need to be able to interoperate with
the use of legacy advertisements by the legacy routers. The
following subsections discuss interoperability and backwards-
compatibility concerns for a number of deployment scenarios.

12.3.1. Multiple Applications: Common Attributes with RSVP-TE

In cases where multiple applications are utilizing a given link, one
of the applications is RSVP-TE, and all link attributes for a given
link are common to the set of applications utilizing that link,
interoperability is achieved by using legacy advertisements for RSVP-
TE. Attributes for applications other than RSVP-TE MUST be
advertised using application-specific advertisements. This results
in duplicate advertisements for those attributes.

12.3.2. Multiple Applications: Some Attributes Not Shared with RSVP-TE

In cases where one or more applications other than RSVP-TE are
utilizing a given link and one or more link attribute values are not
shared with RSVP-TE, interoperability is achieved by using legacy
advertisements for RSVP-TE. Attributes for applications other than
RSVP-TE MUST be advertised using application-specific advertisements.
In cases where some link attributes are shared with RSVP-TE, this
requires duplicate advertisements for those attributes.

12.3.3. Interoperability with Legacy Routers

For the standard applications defined in this document, routers that
do not support the extensions defined in this document will send and
receive only legacy link attribute advertisements. In addition, the
link attribute values associated with these applications are always
shared since legacy routers have no way of advertising or processing
application-specific values. So long as there is any legacy router
in the network that has any of the standard applications defined in
this document enabled, all routers MUST continue to advertise link
attributes for these applications using only legacy advertisements.
ASLA advertisements for these applications MUST NOT be sent. Once
all legacy routers have been upgraded, migration from legacy
advertisements to ASLA advertisements can be achieved via the
following steps:

1) Send new application-specific advertisements while continuing to
advertise using the legacy advertisement (all advertisements are
then duplicated). Receiving routers continue to use legacy
advertisements.

2) Enable the use of the application-specific advertisements on all
routers.

3) Keep legacy advertisements if needed for RSVP-TE purposes.

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

Documents defining new applications that make use of the application-
specific advertisements defined in this document MUST discuss
interoperability and backwards-compatibility issues that could occur
in the presence of routers that do not support the new application.

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

The extensions defined in this document support RSVP-TE as one of the
supported applications. It is, however, RECOMMENDED to advertise all
link attributes for RSVP-TE in the existing OSPFv2 TE Opaque LSA
[RFC3630] and OSPFv3 Intra-Area-TE-LSA [RFC5329] to maintain
backwards compatibility. RSVP-TE can eventually utilize the
application-specific advertisements for newly defined link attributes
that are defined as application specific.

Link attributes that are not allowed to be advertised in the ASLA
sub-TLV, such as maximum reservable link bandwidth and unreserved
bandwidth, MUST use the OSPFv2 TE Opaque LSA [RFC3630] and OSPFv3
Intra-Area-TE-LSA [RFC5329] and MUST NOT be advertised in the ASLA
sub-TLV.

13. Security Considerations

Existing security extensions as described in [RFC2328], [RFC5340],
and [RFC8362] apply to extensions defined in this document. While
OSPF is under a single administrative domain, there can be
deployments where potential attackers have access to one or more
networks in the OSPF routing domain. In these deployments, stronger
authentication mechanisms such as those specified in [RFC5709],
[RFC7474], [RFC4552], or [RFC7166] SHOULD be used.

Implementations must ensure that if any of the TLVs and sub-TLVs
defined in this document are malformed, they are detected and do not
facilitate a vulnerability for attackers to crash or otherwise
compromise the OSPF router or routing process. Reception of a
malformed TLV or sub-TLV SHOULD be counted and/or logged for further
analysis. Logging of malformed TLVs and sub-TLVs SHOULD be rate-
limited to prevent a denial-of-service (DoS) attack (distributed or
otherwise) from overloading the OSPF control plane.

This document defines an improved way to advertise link attributes.
Tampering with the information defined in this document may have an
effect on applications using it, including impacting TE, which uses
various link attributes for its path computation. This is similar in
nature to the impacts associated with, for example, [RFC3630]. As
the advertisements defined in this document limit the scope to
specific applications, the impact of tampering is similarly limited
in scope.

14. IANA Considerations

This specification updates two existing registries:

* the "OSPFv2 Extended Link TLV Sub-TLVs" registry

* the "OSPFv3 Extended-LSA Sub-TLVs" registry

The values defined in this document have been allocated using the
IETF Review procedure as described in [RFC8126].

14.1. OSPFv2

The "OSPFv2 Extended Link TLV Sub-TLVs" registry [RFC7684] defines
sub-TLVs at any level of nesting for OSPFv2 Extended Link TLVs. IANA
has assigned the following sub-TLV types in the "OSPFv2 Extended Link
TLV Sub-TLVs" registry:

10: Application-Specific Link Attributes

11: Shared Risk Link Group

12: Unidirectional Link Delay

13: Min/Max Unidirectional Link Delay

14: Unidirectional Delay Variation

15: Unidirectional Link Loss

16: Unidirectional Residual Bandwidth

17: Unidirectional Available Bandwidth

18: Unidirectional Utilized Bandwidth

19: Administrative Group

20: Extended Administrative Group

22: TE Metric

23: Maximum link bandwidth

14.2. OSPFv3

The "OSPFv3 Extended-LSA Sub-TLVs" registry [RFC8362] defines sub-
TLVs at any level of nesting for OSPFv3 Extended LSAs. IANA has
assigned the following sub-TLV types in the "OSPFv3 Extended-LSA Sub-
TLVs" registry:

11: Application-Specific Link Attributes

12: Shared Risk Link Group

13: Unidirectional Link Delay

14: Min/Max Unidirectional Link Delay

15: Unidirectional Delay Variation

16: Unidirectional Link Loss

17: Unidirectional Residual Bandwidth

18: Unidirectional Available Bandwidth

19: Unidirectional Utilized Bandwidth

20: Administrative Group

21: Extended Administrative Group

22: TE Metric

23: Maximum link bandwidth

24: Local Interface IPv6 Address

25: Remote Interface IPv6 Address

15. Changes to RFC 8920

Discussion within the LSR WG indicated that there was confusion
regarding the use of ASLA advertisements that had a zero-length SABM/
UDABM. The discussion can be seen by searching the LSR WG mailing
list archives for the thread "Proposed Errata for RFCs 8919/8920"
starting on 15 June 2021.

Changes to Section 5 have been introduced to clarify normative
behavior in the presence of such advertisements. [RFC8920] defines
advertising link attributes with zero-length SABM and zero-length
UDABM as a means of advertising link attributes that can be used by
any application. However, the text uses the word "permitted",
suggesting that the use of such advertisements is "optional". Such
an interpretation could lead to interoperability issues and is not
what was intended.

The replacement text makes explicit the specific conditions when such
advertisements MUST be used and the specific conditions under which
they MUST NOT be used.

A subsection discussing the use of zero-length Application Identifier
Bit Masks has been added for greater consistency with [RFC9479]. See
Section 12.2.

16. References

16.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.

[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.

[RFC5329] Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
"Traffic Engineering Extensions to OSPF Version 3",
RFC 5329, DOI 10.17487/RFC5329, September 2008,
<https://www.rfc-editor.org/info/rfc5329>.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.

[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
Traffic Engineering (MPLS-TE)", RFC 7308,
DOI 10.17487/RFC7308, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.

[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.

[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.

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

[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.

[RFC9479] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
RFC 9479, DOI 10.17487/RFC9479, October 2023,
<https://www.rfc-editor.org/info/rfc9479>.

16.2. Informative References

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.

[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.

[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.

[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, DOI 10.17487/RFC5709, October
2009, <https://www.rfc-editor.org/info/rfc5709>.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.

[RFC7166] Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 7166,
DOI 10.17487/RFC7166, March 2014,
<https://www.rfc-editor.org/info/rfc7166>.

[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.

[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.

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

[RFC8920] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020,
<https://www.rfc-editor.org/info/rfc8920>.

[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.

Acknowledgments

The following acknowledgments are included in [RFC8920]:

Thanks to Chris Bowers for his review and comments.

Thanks to Alvaro Retana for his detailed review and comments.

For this document, the authors would like to thank Bruno Decraene.

Contributors

The following people contributed to the content of this document and
should be considered as coauthors:

Acee Lindem
LabN Consulting, L.L.C.
United States of America
Email: [email protected]

Ketan Talaulikar
Cisco Systems
India
Email: [email protected]

Hannes Gredler
RtBrick Inc.
Email: [email protected]

Authors' Addresses

Peter Psenak (editor)
Cisco Systems
Slovakia
Email: [email protected]

Les Ginsberg
Cisco Systems
United States of America
Email: [email protected]

Wim Henderickx
Nokia
Copernicuslaan 50
2018 94089 Antwerp
Belgium
Email: [email protected]

Jeff Tantsura
Nvidia
United States of America
Email: [email protected]

John Drake
Juniper Networks
United States of America
Email: [email protected]