Routing Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)
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Extracted elements (24)
GMPLS decouples TE links from routing adjacencies, treating a TE link as a 'logical' link that groups physical resources for use by Constrained SPF and GMPLS signaling. This is necessary because non-packet-switched links (e.g., OXC-to-OXC) cannot carry routing hellos, LSPs can be advertised as point-to-point TE links, and multiple physical links may be bundled into one TE link.
Layer-specific TE attributes must follow five principles: separation of attributes per layer, support for inter-layer relationships (e.g., '4 client links of type X can be supported by this server layer link'), support for inheritable attributes (e.g., protection), layer extensibility for future layers, and explicit attribute scope. The present document captures only single-layer general attributes; inter-layer relationships are deferred to a future document.
Maximum LSP Bandwidth supersedes the static Maximum Link Bandwidth of prior MPLS TE because it is expressed per priority level and varies dynamically as LSPs are set up and torn down, enabling path computation to accurately account for preemption and bandwidth availability at each priority.
To prevent data traffic from exhausting control channel capacity in GMPLS optical networks, a router can restrict BGP nexthop resolution: if the nexthop resolves via an interface that is not packet-switch capable, a discard route is installed for the BGP destination rather than forwarding over the control channel.
An interface on an OXC with internal DWDM that handles multiple wavelengths individually is classified as LSC (not FSC), because individual lambdas can be identified and switched. Multiple ISCDs are advertised when the internal DWDM supports discrete bandwidths at different data rates.
An opaque OXC interface is always advertised as LSC (not FSC), even when external DWDM is present, because it handles a single wavelength transparently; the DWDM framing must match the OXC framing. A PXC interface with no external DWDM is advertised as FSC because it is not limited to one wavelength.
ISCD values may change dynamically as LSPs are allocated or released, requiring the advertising LSR to re-flood modified descriptors. For example, allocating a VC-3 LSP on an STM-64 TDM interface may reduce the advertised Maximum LSP Bandwidth for non-preemptable priorities from STM-64 to STM-16.
RFC 4202 defines abstract GMPLS TE link properties only; the actual TLV encoding of these properties into IS-IS is specified in RFC 4205 (GMPLS-ISIS) and into OSPF in RFC 4203 (GMPLS-OSPF). Implementations must refer to those documents for wire-format details.
A Link State Advertisement carries the ISCD(s) only of the near-end (the advertising LSR's side). For a unidirectional link, the far-end ISCD is assumed identical to the near-end; thus a unidirectional link is required to have the same interface switching capabilities at both ends.
Both ends of a TE link MUST use the same method for carrying label information. The label type is determined by the switching capability pair: [PSC,PSC] uses shim header labels, [TDM,TDM] uses TDM time slots, [LSC,LSC] uses lambda labels, [FSC,FSC] uses port labels; mixed pairs such as [PSC,TDM] use TDM time slots and [PSC,LSC] or [TDM,LSC] use lambda labels.
For backward compatibility with implementations that consume Maximum Link Bandwidth, an implementation MAY set the Maximum Link Bandwidth to the Maximum LSP Bandwidth at priority 7.
When advertising an unnumbered TE link, the advertisement MUST carry both the local and the remote identifiers. If the LSR does not know the remote identifier, it SHOULD use a value of 0.
When an LSR must establish multiple diversely routed LSPs to another LSR, path computation SHOULD attempt to route them so they share no common links and have disjoint path SRLGs.
An interface may carry multiple ISCDs to represent hybrid switching capabilities (e.g., LSC+TDM for an interface that can either cross-connect a lambda or demultiplex its TDM channels). Multiple ISCDs are also used for interfaces supporting multiple SDH multiplexing hierarchy branches or discrete bandwidth levels.
An unnumbered link uses a non-zero 32-bit identifier assigned independently by the LSR at each end, unique within the scope of that LSR. The advertising LSR carries both the 'link local identifier' (its own assignment) and the 'link remote identifier' (the far-end's assignment) in the TE link advertisement.
FSC (Fiber-Switch Capable) interfaces switch the entire contents of an interface to another interface without distinguishing individual lambdas, channels, or packets. An FSC interface cannot restrict itself to switching only a single lambda; it operates at the granularity of the whole interface.
Link Protection Type describes the protection capability of a TE link using six values in ascending order of protection: Extra Traffic, Unprotected, Shared, Dedicated 1:1, Dedicated 1+1, and Enhanced (e.g., 4-fiber BLSR/MS-SPRING). If a Link State Advertisement does not carry this field, the Link Protection Type is considered unknown.
LSC (Lambda-Switch Capable) interfaces carry Reservable Bandwidth per priority as additional ISCD information. Multiple ISCDs may be advertised per interface to handle multiple data rates or encodings within a single TE link (e.g., STM-16 and STM-64 lambdas on the same interface).
PSC (Packet-Switch Capable) interfaces (PSC-1 through PSC-4) carry additional ISCD fields: Maximum LSP Bandwidth per priority level, Minimum LSP Bandwidth, and Interface MTU. Maximum LSP Bandwidth is the smaller of the unreserved bandwidth at a given priority and a locally configured Maximum LSP Size, and replaces the static Maximum Link Bandwidth from prior MPLS TE extensions.
Shared Risk Link Group (SRLG) Information is an unordered list of 32-bit SRLG identifiers (unique within an IGP domain) indicating which shared-risk groups a link belongs to. The SRLG of an LSP is the union of the SRLGs of all its member links; for a bundled link it is the union of all component link SRLGs.
TDM (Time-Division Multiplex Capable) interfaces carry additional ISCD fields: Maximum LSP Bandwidth per priority, Minimum LSP Bandwidth, and a flag indicating Standard or Arbitrary SDH multiplexing. Standard SDH constrains LSP bandwidth to a branch of the SDH hierarchy; Arbitrary SDH allows any bandwidth that is a multiple of Minimum LSP Bandwidth.
The Interface Switching Capability Descriptor (ISCD) describes the switching capability and encoding type of an interface. Defined switching capabilities are PSC-1 through PSC-4, L2SC, TDM, LSC, and FSC. If no ISCD is present for an interface, it is assumed to be PSC-1. A single interface may carry multiple ISCDs.
GMPLS routing extensions will potentially be used to control underlying transport infrastructure, making it vital that there be secure and/or authenticated means of transferring TE link state information among network entities. This document defers specification of security mechanisms to the protocol-specific encoding documents (GMPLS-OSPF, GMPLS-ISIS), which must state how the advertised information is to be secured.
Bandwidth values — Unreserved Bandwidth, Maximum LSP Bandwidth, and Minimum LSP Bandwidth — are encoded as IEEE 754 32-bit binary floating-point numbers, as specified in Section 3.1.2 of RFC 3471 (GMPLS Signaling Functional Description).