Revised Definition of the GMPLS Switching Capability and Type Fields
updates
- rfc-3471 — Generalized Multi-Protocol Label Switching (GMPLS) Signaling Functional Description
- rfc-4202 — Routing Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)
- rfc-4203 — OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)
- rfc-5307 — IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)
Extracted elements (15)
The document narrows the scope of Switching Type values to serve solely as identifiers for per-technology SCSI field formats, rather than as indicators of hierarchy levels within a switching technology. This resolves inconsistency introduced by ad hoc additions like EVPL and 802_1 PBB-TE.
The introduction of separate Switching Types for EVPL (value 30) and 802_1 PBB-TE (value 40) for Ethernet technologies, rather than reusing L2SC, was driven by the need to disambiguate the SCSI field format for different Ethernet switching technologies operating within the same provider network.
The original approach of using Switching Types to indicate hierarchy levels within a technology (e.g., PSC-1 through PSC-4) proved problematic because different technologies of the same switching type can require different SCSI field formats, making implicit type-to-format mapping impractical for multi-technology implementations.
At the time of publication, there are no known deployments or implementations using PSC-2, PSC-3, or PSC-4, so deprecating these values raises no compatibility issues for existing routing and signaling implementations.
A different Switching Type value MUST be used to differentiate between different Switching Capability-specific information field formats, since the SCSI field format is dependent on the Switching Cap field value.
A different Switching Type value SHOULD be used for each data-plane technology, even when those technologies share the same type of multiplexing or switching. For example, SDH and OTN should use two different Switching Types despite both being TDM technologies.
Any time a new switching technology might use a different Switching Capability-specific information field format, a new Switching Type SHOULD be used.
PSC-2 (value 2), PSC-3 (value 3), and PSC-4 (value 4) Switching Types are deprecated and SHOULD be treated as unsupported types. In the case of signaling, they should be processed according to Section 2.1.1 of RFC 3473.
The revised definition of the Switching Type field in signaling (RFC 3471/3473) is: it indicates the type of switching to be performed on a particular link via GMPLS Switching Type values, and maps to one of the values advertised for the corresponding link in the routing Switching Capability Descriptor. This is unchanged in practice from the prior definition.
The Switching Capability (Switching Cap) field in the Interface Switching Capability Descriptor is an 8-bit field carried in OSPF and IS-IS routing protocols to indicate the type of switching being advertised. Its value determines the format of the variable-length Switching Capability-specific information (SCSI) field that follows.
IANA also updated the IANA-GMPLS-TC-MIB IANAGmplsSwitchingTypeTC definitions to mark psc2(2), psc3(3), and psc4(4) as deprecated, with a reference to this document added as item 3.
IANA updated the Switching Types portion of the 'Generalized Multi-Protocol Label Switching (GMPLS) Signaling Parameters' registry to deprecate values 2, 3, and 4 (PSC-2, PSC-3, PSC-4) and add a reference to this document. Registration procedure is Standards Action.
This document only impacts the values carried in a single Switching Type field in signaling and routing protocols. As no new protocol formats or mechanisms are defined, there are no particular security implications. General MPLS/GMPLS security issues are addressed in RFC 5920.
The GMPLS signaling generalized label request object contains an 8-bit LSP Encoding Type, an 8-bit Switching Type, and a 16-bit G-PID field. The Switching Type maps to one of the values advertised for the corresponding link in the routing Switching Capability Descriptor.
The Interface Switching Capability Descriptor used in OSPF and IS-IS contains an 8-bit Switching Cap field, an 8-bit Encoding field, 16 reserved bits, followed by a variable-length Switching Capability-specific information field whose format depends on the Switching Cap value.