IS-IS TE Attributes per
applicationCisco Systems821 Alder DriveMilpitas95035CAUSAginsberg@cisco.comCisco SystemsApollo Business Center Mlynske nivy 43Bratislava821 09Slovakiappsenak@cisco.comIndividualstefano@previdi.netNokiaCopernicuslaan 50Antwerp2018 94089Belgiumwim.henderickx@nokia.comJuniper Networksjdrake@juniper.net
Routing Area
Networking Working GroupExisting traffic engineering related link attribute advertisements
have been defined and are used in RSVP-TE deployments. 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 draft introduces new link attribute advertisements which address
both of these shortcomings. It also discusses backwards compatibility
issues and how to minimize duplicate advertisements in the presence of
routers which do not support the extensions defined in this
document.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].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], and [RFC7810]. Use of these extensions has been
associated with deployments supporting Traffic Engineering over
Multiprotocol Label Switching (MPLS) in the presence of Resource
Reservation Protocol (RSVP) - more succinctly referred to as
RSVP-TE.In recent years new applications have been introduced which have use
cases for many of the link attributes historically used by RSVP-TE. Such
applications include Segment Routing Traffic Engineering (SRTE) and Loop
Free Alternates (LFA). This has introduced ambiguity in that if a
deployment includes a mix of RSVP-TE support and SRTE 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 SRTE. If the topologies are fully congruent this may not be
an issue, but any incongruence leads to ambiguity.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 which 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 which is easily
extensible to the introduction of new applications and new use
cases.As stated previously, evolution of use cases for link attributes can
be expected to continue - so any discussion of existing use cases is
limited to requirements which 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 which
justify the key points identified in the introduction - which are:Support for indicating which applications are using the link
attribute advertisements on a linkSupport for advertising application specific values for the same
attribute on a link[RFC7855] discusses use cases/requirements for SR. Included
among these use cases is SRTE which is defined in [SR-POLICY]. If both
RSVP-TE and SRTE are deployed in a network, link attribute
advertisements can be used by one or both of these applications. As
there is no requirement for the link attributes advertised on a given
link used by SRTE to be identical to the link attributes advertised on
that same link used by RSVP-TE, there is a clear requirement to indicate
independently which link attribute advertisements are to be used by each
application.As the number of applications which 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.There are existing advertisements used in support of RSVP-TE. These
advertisements include sub-TLVs for TLVs 22, 23, 141, 222, and 223 and
TLVs for SRLG advertisement.Note that [RFC6119] prohibits the use of TLV 139 when it is
possible to use TLV 138.Two new code points are defined in support of Application Specific
Link Attribute Advertisements:1) Application Specific Link Attributes sub-TLV for TLVs 22, 23, 141,
222, and 2232)Application Specific Shared Risk Link Group (SRLG) TLVIn support of these new advertisements, an application bit mask is
defined which identifies the application(s) associated with a given
advertisement.The following sections define the format of these new
advertisements.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. 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.Standard Application Bits are defined/sent starting with
Bit 0. Additional bit definitions that may be defined in the future
SHOULD be assigned in ascending bit order so as to minimize the number
of octets that will need to be transmitted. Undefined bits 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.User Defined Application bits have no relationship to Standard
Application bits and are NOT managed by IANA or any other standards
body. It is recommended that bits are used starting with Bit 0 so as
to minimize the number of octets required to advertise all UDAs.A new sub-TLV for TLVs 22, 23, 141, 222, and 223 is defined which
supports specification of the applications and application specific
attribute values.When the L-flag is set in the Application Identifiers, all of the
applications specified in the bit mask MUST use the link attribute
sub-TLV advertisements listed in Section 3.1 for the corresponding
link. Application specific link attribute sub-sub-TLVs for the
corresponding link attributes MUST NOT be advertised for the set of
applications specified in the Standard/User Application Bit Masks and
all such advertisements MUST be ignored on receipt.Multiple 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 of the same link attribute for a given link. In cases where
conflicting values for the same application/attribute/link are
advertised all the conflicting values 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.A new registry of sub-sub-TLVs is to be created by IANA which
defines the link attribute sub-sub-TLV code points. A sub-sub-TLV is
defined for each of the existing sub-TLVs listed in Section 3.1.
Format of the sub-sub-TLVs matches the format of the corresponding
legacy sub-TLV and IANA is requested to assign the legacy sub-TLV
identifer to the corresponding sub-sub-TLV.A new TLV is defined to advertise application specific SRLGs for a
given link. Although similar in functionality to TLV 138 (defined by
[RFC5307]) and TLV 139 (defined by [RFC6119], a single TLV provides
support for IPv4, IPv6, and unnumbered identifiers for a link. Unlike
TLVs 138/139, it utilizes sub-TLVs to encode the link identifiers in
order to provide the flexible formatting required to support multiple
link identifier types.At least one set of link identifiers (IPv4, IPv6, or
unnumbered) MUST be present. TLVs which 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 Identifiers, SRLG values
MUST NOT be included in the TLV. Any SRLG values which 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 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.If link attributes are advertised associated with zero length
application bit masks for both standard applications and user defined
applications, then that set of link attributes MAY be used by any
application. If support for a new application is introduced on any node
in a network in the presence of such advertisements, these
advertisements MAY 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.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.The advertisement of link attributes to be used by RSVP-TE implies
that RSVP is enabled on that link.In the case of SRTE, advertisement of application specific link
attributes does NOT indicate enablement of SRTE. The advertisements are
only used to support constraints which may be applied when specifying an
explicit path. SRTE is implicitly enabled on all links which are part of
the Segment Routing enabled topology independent of the existence of
link attribute advertisementsIn the case of LFA, advertisement of link attributes does NOT
indicate enablement of that application on that link. Enablement is
controlled by local configuration and/or the use of administrative
tags.If, in the future, 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.Existing deployments of RSVP-TE utilize the legacy advertisements
listed in Section 3. Routers which 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 must be able
to co-exist with use of the legacy advertisements by routers which do
not support the extensions defined in this document. The following
sub-sections discuss interoperability and backwards compatibility
concerns for a number of deployment scenarios.Note that in all cases the defined strategy can be employed on a per
link basis.In deployments where RSVP-TE is the only application utilizing link
attribute advertisements, use of the the legacy advertisements can
continue without change.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 L-bit set and no link
attribute values. This avoids duplication of link attribute
advertisements.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 which have the L-bit clear. In cases where
some link attributes are shared with RSVP-TE, this requires duplicate
advertisements for those attributes.The discussion in this section applies 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.The extensions defined in this document support RSVP-TE as one of
the supported applications - so a long term goal for deployments would
be to deprecate use of the legacy advertisements in support of
RSVP-TE. This can be done in the following step-wise manner:1)Upgrade all routers to support extensions in this document2)Readvertise all legacy link attributes using application specific
advertisements with L-bit clear and R-bit set.3)Remove legacy advertisementsThis document defines a new sub-TLV for TLVs 22, 23, 141, 222, and
223.This document defines one new TLV:This document requests a new IANA registry be created to control the
assignment of sub-sub-TLV codepoints for the Application Specific Link
Attributes sub-TLV. The suggested name of the new registry is
"sub-sub-TLV code points for application link attributes". The
registration procedure is "Expert Review" as defined in [RFC8126]. The
following assignments are made by this document:This document requests a new IANA registry be created to control the
assignment of application bit identifiers. The suggested name of the new
registry is "Link Attribute Applications". The registration procedure is
"Expert Review" as defined in [RFC8126]. The following assignments are
made by this document:This document requests a new IANA registry be created to control the
assignment of sub-TLV types for the application specific SRLG TLV. The
suggested name of the new registry is "Sub-TLVs for TLV 238". The
registration procedure is "Expert Review" as defined in [RFC8126]. The
following assignments are made by this document:Security concerns for IS-IS are addressed in [ISO10589, [RFC5304],
and [RFC5310].The authors would like to thank Eric Rosen and Acee Lindem for their
careful review and content suggestions.Segment Routing Policy for Traffic Engineering,
draft-filsfils-spring-segment-routing-policy-01(work in
progress)