Generalized MPLS (GMPLS)

Introduction: GMPLS beyond packet switching

In the preceding discussion, MPLS was presented as a way to forward packets with labels through a provider core. It also provides mechanisms for traffic engineering and VPN services, while its classic model focuses on packet switching.

Classic MPLS primarily modeled the network as a collection of routers that forward . Beneath the router layer are transport technologies that operate according to different principles. The Internet therefore includes more than IP addresses: it also depends on fiber cables, TDM systems, and optical switches.

Historically, each layer was managed separately, with its own management system and control logic. Teams responsible for optical paths did not use the same mechanisms as teams handling MPLS packet traffic. This made automated service activation and joint resource planning more difficult.

Generalized MPLS (GMPLS) extends MPLS control-plane concepts beyond packet switching. One family of mechanisms can describe resources and establish paths in packet, TDM, WDM, and fiber-switching networks. GMPLS coordinates the control plane with different transport technologies.

Four GMPLS extensions

GMPLS generalizes the core MPLS concepts of labels, LSPs, and signaling protocols. They can therefore also be applied to technologies that do not forward traditional packets. The generalization covers four areas.

1. The Generalized Label

In classic MPLS, a label is a 20-bit value. In GMPLS, the concept of a label is expanded to include information that identifies a usable resource on a link, such as a time slot, wavelength, fiber, or port.

For Packet Switch Capable (PSC) Networks: A label is the familiar 20-bit number used in MPLS.
For Time-Division Multiplexing (TDM) Networks: A label represents a specific within a repeating TDM frame. Establishing a connection means reserving the same time slot across multiple devices.
For Wavelength Division Multiplexing (WDM) Networks: A label represents a specific (or color of light) on a fiber optic cable. In this case, the label identifies the selected wavelength.
For fiber-switching networks: A label can represent a single fiber in a bundle or a specific port on a fiber switch.

2. The Generalized Label Switched Path (LSP)

Since the label is generalized, the path it defines (the LSP) also takes on new meaning. A GMPLS LSP can be a virtual tunnel for packets or represent a physical, hard-wired circuit through the network. This brings packet switching and circuit switching under a common control model. The same control plane can be used to provision both.

3. Generalized Interface Types

GMPLS defines switching-capability types that nodes and their interfaces can advertise, allowing the network to account for each device's capabilities:

Packet-Switch Capable (PSC): Standard MPLS routers.
Layer-2 Switch Capable (L2SC): Devices like Ethernet switches.
Time-Division Multiplex Capable (TDM): Devices that can switch TDM circuits, like SONET/SDH cross-connects.
Lambda-Switch Capable (LSC): Optical devices that can switch individual wavelengths of light.
Fiber-Switch Capable (FSC): Optical switches that can direct entire physical fibers from one port to another.

4. New Link Bundling Capabilities

GMPLS introduces the ability to bundle multiple physical links into a single logical TE link. The object describes a resource made of component links, allowing routing and signaling to use one representation while retaining information about physical resources and shared failure risks.

The Control Plane: Extending the Protocols

To handle these resources, the MPLS control-plane protocols were extended. The changes covered routing protocols, which advertise new capabilities, and signaling protocols, which request and establish generalized paths.

IGP Extensions for GMPLS

As with MPLS-TE, GMPLS uses extensions to link-state IGPs like OSPF and IS-IS to distribute information. Routers use these protocols to flood new information throughout the network, populating everyone's Traffic Engineering Database (TED). These extensions add the ability to advertise:

GMPLS does not prescribe one medium or one signaling protocol. OSPF and IS-IS extensions describe switching capabilities and resources, while RSVP-TE carries generalized requests and labels. The data plane then performs the cross-connect operation, selects a time slot, or selects a wavelength according to the established path.

The meaning of a generalized label depends on the switching type and link context. Value 22 in the WDM example is not a globally unique identifier for the entire network.

The switching capability of each router interface (PSC, LSC, FSC, etc.).
Shared Risk Link Groups (SRLGs), to identify links that might fail together (e.g., multiple fibers within the same physical conduit).
The mapping between bundled TE Links and their component physical links.

Signaling with RSVP-TE Extensions

The Resource Reservation Protocol - Traffic Engineering (RSVP-TE) is one of the signaling protocols used with GMPLS. It was extended with objects that handle generalized requirements:

Generalized Label Request:When an ingress router sends an RSVP PATH message, it uses a "Generalized Label Request" object. The object specifies the LSP encoding, switching type, and payload protocol identifier, while accompanying signaling objects carry resource requirements. An example is a full-duplex SONET OC-48 circuit.
Generalized Label Object:The RSVP RESV message, which travels back from the egress to the ingress, carries the assigned "Generalized Label." For a WDM network, this object would contain the specific wavelength number that was reserved on that link.
Explicit Route Object (ERO): Just like in MPLS-TE, this object carries the exact, hop-by-hop path that the LSP should take, as calculated by the CSPF algorithm.

Teaching example: provisioning a wavelength

The following teaching example uses illustrative values. A bank in London needs a dedicated 10 Gbps connection to its data center in Frankfurt for disaster recovery; additional mechanisms provide the required security. Instead of a virtual packet tunnel, the bank requests a dedicated circuit of light, a lightpath. In this scenario, manual configuration would require settings at every node, while GMPLS can carry the request and signal the path end to end.

Here is how the automated provisioning process works:

The request:An administrator on a Network Management System (NMS) issues a command to the London edge node: "Create a 10 Gbps lightpath LSP from me to the Frankfurt edge node."
Path calculation (CSPF):The London node acts as the ingress node. It consults its TED, populated by extended OSPF-TE, and runs CSPF to find the shortest physical fiber path that satisfies the constraint of one common, unused wavelength on every segment. Suppose CSPF selects London → Paris → Brussels → Frankfurt and wavelength #22 is free on all three links.
Signaling - The PATH Message: The London router generates an RSVP-TE PATH message. This message contains the explicit route (ERO: Paris, Brussels, Frankfurt) and a Generalized Label Request specifying the need for a 10 Gbps wavelength. It sends the message to the Paris router. The Paris router forwards it to Brussels, and Brussels forwards it to Frankfurt.
Signaling - the RESV message: The Frankfurt node receives the PATH message and recognizes the destination. It selects an available wavelength for the final link (Brussels → Frankfurt): wavelength #22, as required by the path calculation. It creates an RSVP-TE RESV message containing a Generalized Label object with value 22 on that link. It configures its optical switch to connect the input port from Brussels to the customer port, and sends the RESV message back to Brussels.
Path setup:The Brussels node receives the RESV message. It reads value 22 and reserves wavelength #22 on its link to Frankfurt. It configures its internal optical cross-connect to connect the fiber from Paris to the fiber going to Frankfurt on that wavelength. It allocates its own local value for the Paris → Brussels link (suppose it is also wavelength #22), includes it in the RESV message, and sends it back to Paris.
Service established: This process continues back to London. When the London node receives the final RESV message, the end-to-end LSP is established. Assuming the required resources are available, it provides a dedicated physical 10 Gbps light circuit to Frankfurt. The control plane has automatically provisioned a Layer 1 service.

GMPLS operational functions and deployment considerations

GMPLS provides common control-plane mechanisms for multiple network layers. This supports the following tasks:

Automated Provisioning:It replaces slow, error-prone manual configuration with rapid, automated service setup, reducing operational costs and allowing for new "bandwidth-on-demand" services.
Multi-Layer Optimization: A unified control plane can make more intelligent decisions, optimizing resources across both the IP/packet layer and the underlying optical transport layer simultaneously.
Improved Resilience: It allows for sophisticated, multi-layer protection and restoration schemes, automatically routing around failures at both the packet and optical levels.

GMPLS protocols can provide control-plane mechanisms used in multi-layer architectures. The Automatically Switched Optical Network (ASON) architecture is described by ITU-T Recommendation G.8080. Common control can automate service setup, resource optimization, and protection or restoration after failures.