| Networking Working Group | L. Ginsberg |
| Internet-Draft | P. Psenak |
| Intended status: Standards Track | Cisco Systems |
| Expires: April 6, 2020 | S. Previdi |
| Huawei | |
| W. Henderickx | |
| Nokia | |
| J. Drake | |
| Juniper Networks | |
| October 4, 2019 |
IS-IS TE Attributes per application
draft-ietf-isis-te-app-07
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., SRTE, LFA) have been defined which also make use of the link attribute advertisements. 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", "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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 6, 2020.
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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 [RFC8570]. 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:
[I-D.ietf-spring-segment-routing-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.
[RFC7855] discusses use cases/requirements for SR. Included among these use cases is SRTE which is defined in
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, 25, 141, 222, and 223 and TLVs for SRLG advertisement.
Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 Code Point/Attribute Name -------------------------- 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
TLV 138 GMPLS-SRLG Supports links identified by IPv4 addresses and unnumbered links TLV 139 IPv6 SRLG Supports links identified by IPv6 addresses
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, 25, 141, 222, and 223 (defined in Section 4.2 ).
2)Application Specific Shared Risk Link Group (SRLG) TLV (defined in Section 4.3).
In support of these new advertisements, an application identifier bit mask is defined which identifies the application(s) associated with a given advertisement (defined in Section 4.1).
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.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| SABML+F | 1 octet
+-+-+-+-+-+-+-+-+
| UDABML+F | 1 octet
+-+-+-+-+-+-+-+-+
| SABM ... 0 - 127 octets
+-+-+-+-+-+-+-+-+
| UDABM ... 0 - 127 octets
+-+-+-+-+-+-+-+-+
SABML+F (1 octet)
Standard Application Identifier Bit Mask
Length/Flags
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L| SA-Length |
+-+-+-+-+-+-+-+-+
L-flag: When set, applications listed (both Standard
and User Defined) MUST use the legacy advertisements
for the corresponding link found in TLVs 22, 23,
25, 141, 222, and 223 or TLV 138 or TLV 139 as
appropriate.
SA-Length: Indicates the length in octets (0-127) of the Bit Mask
for Standard Applications.
UDABML+F (1 octet)
User Defined Application Identifier Bit Mask
Length/Flags
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R| UDA-Length |
+-+-+-+-+-+-+-+-+
R: Reserved. SHOULD be transmitted as 0 and
MUST be ignored on receipt
UDA-Length: Indicates the length in octets (0-127) of the Bit Mask
for User Defined Applications.
SABM (variable length)
Standard Application Identifier Bit Mask
(SA-Length * 8) bits
This is omitted if SA-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
Traffic Engineering
F-bit: Set to specify Loop Free Alternate
(includes all LFA types)
UDABM (variable length)
User Defined Application Identifier Bit Mask
(UDA-Length * 8) bits
0 1 2 3 4 5 6 7 ...
+-+-+-+-+-+-+-+-+...
| ...
+-+-+-+-+-+-+-+-+...
This is omitted if UDA-Length is 0.
NOTE: If both SA-length and UDA-Length are zero, then the
attributes associated with this Attribute Identifier Bit Mask
MAY be used by any Standard Application and any User Defined
Application.
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 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, 25, 141, 222, and 223 is defined which supports specification of the applications and application specific attribute values.
Type: 16 (temporarily assigned by IANA)
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] and [RFC8570]
When the L-flag is set in the Application Identifier Bit Mask, 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. 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 Identifier Bit Masks and all such advertisements MUST be ignored on receipt.
Multiple Application Specific Link Attribute 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. 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 is requested to assign the legacy sub-TLV identifer to the corresponding sub-sub-TLV.
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 which 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.
If different values for Maximum Link Bandwidth for a given link are advertised, all values MUST be ignored.
Unreserved bandwidth is an attribute specific to RSVP. 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 Unreserved Bandwidth is received with bits other than the RSVP-TE bit set, the advertisement MUST be ignored.
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.
Type: 238 (Temporarily assigned by IANA)
Length: Number of octets in the value field (1 octet)
Value:
Neighbor System-ID + pseudo-node 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 type
values are suggested and will be assigned by IANA - but as
the formats are identical to existing sub-TLVs defined for
TLVs 22, 23, 25, 141, 222, and 223 the use of the suggested
sub-TLV types is strongly encouraged.
Type Description
4 Link Local/Remote Identifiers (see [RFC5307])
6 IPv4 interface address (see [RFC5305])
8 IPv4 neighbor address (see [RFC5305])
12 IPv6 Interface Address (see [RFC6119])
13 IPv6 Neighbor Address (see [RFC6119])
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 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 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.
If link attributes are advertised associated with zero length Application Identifier 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.
In the case of RSVP-TE, the advertisement of application specific link attributes 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 advertisements
In the case of LFA, advertisement of application specific link attributes does NOT indicate enablement of LFA on that link. Enablement is controlled by local configuration.
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.
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.
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. This allows that RSVP-TE could eventually utilize the application specific advertisements. This can be done in the following step-wise manner:
1)Upgrade all routers to support extensions in this document
2)Readvertise all legacy link attributes using application specific advertisements with L-bit clear and R-bit set.
3)Remove legacy advertisements
Migrating RSVP-TE away from legacy advertisements could result in some implementation simplification as it allows the removal of code which encodes/decodes the legacy advertisements. Whether this is seen as desirable is something for the marketplace to determine.
This document defines a new sub-TLV for TLVs 22, 23, 25, 141, 222, and 223.
Type Description 22 23 25 141 222 223
---- --------------------- ---- ---- ---- ---- ---- ----
16 Application Specific y y y(s) y y y
Link Attributes
This document defines one new TLV:
Type Description IIH LSP SNP Purge
---- --------------------- --- --- --- -----
238 Application Specific n y n n
SRLG
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 specific link attributes". The registration procedure is "Expert Review" as defined in [RFC8126]. The following assignments are made by this document:
Type Description --------------------------------------------------------- 0-2 Unassigned 3 Administrative group (color) 4-8 Unassigned 9 Maximum link bandwidth 10 Maximum reservable link bandwidth 11 Unreserved bandwidth 12-13 Unassigned 14 Extended Administrative Group 15-17 Unassigned 18 TE Default Metric 19-32 Unassigned 33 Unidirectional Link Delay 34 Min/Max Unidirectional Link Delay 35 Unidirectional Delay Variation 36 Unidirectional Link Loss 37 Unidirectional Residual Bandwidth 38 Unidirectional Available Bandwidth 39 Unidirectional Utilized Bandwidth 40-255 Unassigned
This document requests a new IANA registry be created, under the category of "Interior Gateway Protocol (IGP) Parameters", to control the assignment of Application Identifier Bits. The suggested name of the new registry is "Link Attribute Applications". The registration policy for this registry is "Standards Action" ([RFC8126] and [RFC7120]). The following assignments are made by this document:
Bit # Name --------------------------------------------------------- 0 RSVP-TE (R-bit) 1 Segment Routing Traffic Engineering (S-bit) 2 Loop Free Alternate (F-bit)
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:
Value Description --------------------------------------------------------- 0-3 Unassigned 4 Link Local/Remote Identifiers (see [RFC5307]) 5 Unassigned 6 IPv4 interface address (see [RFC5305]) 7 Unassigned 8 IPv4 neighbor address (see [RFC5305]) 9-11 Unassigned 12 IPv6 Interface Address (see [RFC6119]) 13 IPv6 Neighbor Address (see [RFC6119]) 14-255 Unassigned
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.
| [I-D.ietf-spring-segment-routing-policy] | Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d., bogdanov@google.com, b. and P. Mattes, "Segment Routing Policy Architecture", Internet-Draft draft-ietf-spring-segment-routing-policy-03, May 2019. |
| [RFC7855] | Previdi, S., Filsfils, C., 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. |