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GMPLS (Generalized MPLS)

GMPLS (Generalized MPLS)

Generalized Multi-Protocol Label Switching (GMPLS) is a protocol suite extending MPLS to manage further classes of interfaces and switching technologies other than packet interfaces and switching, such as time division multiplexing, layer-2 switching, wavelength switching and fiber-switching.

GMPLS (Generalized MPLS)
GMPLS (Generalized MPLS)

GMPLS (Generalized Multiprotocol Label Switching), also known as Multiprotocol Lambda Switching. In particular, GMPLS will provide support for photonic networking, also known as optical communications.

As GMPLS evolves, it will require changes to existing protocols and will spur the evolution of new ones. The Link Management Protocol, for example, arose in part as a consequence of GMPLS evolution. GMPLS also involved changes to the Open Shortest Path First (OSPF) protocol and IS-IS intradomain routing protocol. GMPLS allows for a greatly increased number of parallel links between nodes in a network. This is important in photonic networking, where hundreds of parallel links (individual fibers in a bundled fiber optic cable, for example) may exist between a pair of nodes. GMPLS also facilitates rapid fault detection, fault isolation, and switchover to alternate channels, minimizing network downtime.

How Does GMPLS (Generalized MPLS) Work?

GMPLS is conceptually similar to MPLS, but instead of using an explicit label to distinguish an LSP at each LSR, some physical property of the received data stream is used to deduce which LSP it belongs to. The most commonly used schemes are:

  • using the timeslot to identify the LSP, on a Time Division Multiplexed (TDM) link.
  • using the wavelength to identify the LSP, on a Wavelength Division Multiplexed (WDM) link.
  • using the fiber or port on which a packet is received.

LSPs are therefore implicitly labeled in a GMPLS network.

GMPLS can be used to establish LSPs for circuit traffic (in addition to packet traffic). Using the TDM and WDM examples above, the LSP traffic is switched based on a continuous, constant property of the data stream – the data stream is not switched one packet at a time. This allows for a very efficient implementation in the data plane with zero per-packet lookups, making GMPLS a highly suitable protocol to run in high bandwidth networks.

Other than this, the forwarding operation of the LSRs in a GMPLS network is similar to the MPLS example discussed above. At each LSR, the implicit label on received data determines the outgoing interface and the implicit label with which to transmit onwards data.

Comparison of GMPLS Models

A comparison of the three GMPLS models in Figure 5 shows how Cisco S-GMPLS borrows the best features of the other models while engineering around one of the primary problems that have slowed GMPLS adoption.

Comparison of the three GMPLS Models
Comparison of the three GMPLS Models

Standards Framework Applicability

Table shows the protocol perspectives of the ASON framework. Today there are two applicable standards for UNI: Optical Internetworking Forum UNI (OIF-UNI) and GMPLS-UNI. In the context of S-GMPLS, when considering client layers with intra-service provider and inter-service provider networks, GMPLS-UNI is a preferred choice for UNI because the protocols are drawn from one standards organization, the IETF. Use of OIF-UNI introduces compatibility issues to interoperate with S-GMPLS because the original RSVP-TE signaling protocol in Overlay UNI (O-UNI) is modified and departs from the IETF RSVP-TE RFC.

ASON Framework Signaling Routing Service
OIF-UNI O-UNI No Inter service provider (wholesale), service provider to customer
Peer RSVP-TE OSPF-TE Intra service provider
S-GMPLS RSVP-TE OSPF-TE Intra service provider, inter service provider
IETF Overlay (GMPLS-UNI) RSVP-TE No Service provider to customer

GMPLS Control Plane Functions and Services

GMPLS focuses mainly on the control plane services that perform connection management for the data plane (the actual forwarding logic) for both packet-switched interfaces and non-packet-switched interfaces. The GMPLS control plane essentially facilitates four basic functions:

  1. Routing Control: Provides the routing capability, traffic engineering, and topology discovery.
  2. Resource Discovery: A mechanism to keep track of the system resource availability such as bandwidth, multiplexing capability, and ports.
  3. Connection Management: Provides end-to-end service provisioning for different services, including connection creation, modification, status query, and deletion.
  4. Connection Restoration: Implements an additional level of protection to the networks by establishing for each connection one or more presignaled backup paths and enabling very fast switching in case of failure between them.

The fundamental service offered by the GMPLS control plane is dynamic end-to-end connection provisioning. The operators need only to specify the connection parameters and send them to the ingress node. The network control plane then determines the optical paths across the network according to the parameters that the user provides and signals the corresponding nodes to establish the connection. The whole procedure can be done within seconds instead of hours. The other important service is bandwidth on demand, which extends the ease of provisioning even further by allowing the client devices that connect to the optical network to request the connection setup in real time as needed. In order to establish a connection that will be used to transfer data between a source–destination node pair, a light path needs to be established by allocating, in presence of the so-called continuity constraint, the same wavelength throughout the route of the transmitted data or selecting the proper wavelength conversion-capable nodes across the path. In fact, if the wavelength continuity constraint is not fully enforced, some wavelength conversion-capable nodes can be placed in the network to reduce the overall blocking probability in case of wavelength resource exhaustion on some nodes. Light paths can span more than one fiber link and remain entirely optical from end to end.

However, according to the mandatory clash constraint, two light paths traversing the same fiber link cannot share the same wavelength on that link. That is, each wavelength on a given fiber is not a sharable resource between light paths.

GMPLS Interfaces

GMPLS encompasses control plane signaling for multiple interface types. The diversity of controlling not only switched packets and cells but also TDM network traffic and optical network components makes GMPLS flexible enough to position itself in the direct migration path from electronic to all-optical network switching. The five main interface types supported by GMPLS follow:

  1. Packet Switching Capable (PSC): These interfaces recognize packet boundaries and can forward packets based on the IP header or a standard MPLS “shim” header.
  2. Layer 2 Switch-Capable (L2SC): These interfaces recognize frame and cell headers and can forward data based on the content of the frame or cell header (for example, an ATM LSR that forwards data based on its Virtual Path Identifier/Virtual Circuit Identifier (VPI/VCI) value, or Ethernet bridges that forward the data based on the MAC header).
  3. Time-Division Multiplexing-Capable (TDMC): These interfaces forward the data based on the time slot in a repeating cycle (for example, SDH cross-connect or ADM, interfaces implementing the Digital Wrapper G.709, and Plesichronous Digital Hierarchy [PDH] interfaces).
  4. Lambda Switch-Capable (LSC): These interfaces are for wavelength-based MPLS control of optical devices and wavelength switching devices, such as optical ADMs (OADMs) and OXCs, operating at the granularity of the single wavelength or group of wavelengths (waveband). These interfaces forward the optical signal from an incoming optical wavelength to an outgoing optical wavelength. Traffic is forwarded based upon wavelength or waveband.
  5. Fiber-Switch-Capable (FSC): These interfaces forward the signal from one or more incoming fibers to one or more outgoing fibers for spatial control of interface selection, automated patch panels, and physical fiber switching systems. Traffic is forwarded based on port, fiber, or interface.

These supported interfaces are hierarchal in structure and controlled simultaneously by GMPLS.


  • Routing challenges
    1. Limited number of labels
    2. Very large number of links
      1. Link identification will be a big problem
      2. Scalability of the Link state protocol
      3. Port connection detection
  • Signaling challenges
    1. Long label setup time
    2. Bi-directional LSPs setup
  • Management challenges
    1. Failure detection
    2. Failure protection and restoration

MPLS and GMPLS Protocols

MPLS defines only the forwarding mechanism; it uses other protocols to establish the LSPs. Two separate protocols are needed to perform this task: a routing protocol and a signaling protocol. These are described below.

It is also possible to establish MPLS LSPs with static provisioning. This involves configuring each network element along the LSP route with the appropriate ingress / transit / egress information. Static provisioning has not been very widely deployed to date, but it can have a role in the access network. It is also likely to be one of the operating modes for MPLS Transport Profile (MPLS-TP).

MPLS and GMPLS Routing Protocols

The routing protocol distributes network topology information through the network so that the route of an LSP can be calculated automatically. An interior gateway protocol, such as OSPF or IS-IS, is normally used, as MPLS networks typically cover a single administrative domain.

However, these routing protocols only distribute network topology. When traffic engineering is required to establish LSPs with guaranteed QoS characteristics and backup LSPs that avoid any single point of failure, the traffic engineering (TE) extensions to these protocols are used. These extensions distribute QoS and Shared Risk Link Group (SRLG) information on each link in the network. This information enables the route calculator to determine routes through the network with guaranteed QoS parameters, and backup LSPs that traverse different links and/or network elements from the primary path.

Various mechanisms to extend this traffic engineering to inter-area and inter-carrier routing have been proposed, but none is yet universally accepted. Our White Paper on “Inter-Area Routing, Path Selection and Traffic Engineering” provides a detailed discussion of this topic.

MPLS and GMPLS Signaling Protocols

The signaling protocol informs the switches along the route which labels and links to use for each LSP. This information is used to program the switching fabric. For MPLS, one of three main signaling protocols is used, depending on the application.

  • LDP is used for
    1. MPLS transport where traffic engineering is not required.
    2. Certain MPLS services, for example pseudowires.
  • RSVP-TE is used for
    1. MPLS transport where traffic engineering is required.
    2. All GMPLS transport.
  • BGP is used (as a signaling protocol) for certain MPLS services, for example BGP/MPLS Layer 3 VPNs.

Differences between MPLS and GMPLS

Generalized MPLS differs from traditional MPLS in that it extends support to multiple types of switching such as TDM, wavelength and fiber (port) switching. For instance, GMPLS is the de facto control plane of wavelength switched optical network (WSON). The support for the additional types of switching has driven GMPLS to extend certain base functions of traditional MPLS and, in some cases, to add functionality.

These changes and additions impact basic label-switched path (LSP) properties: how labels are requested and communicated, the unidirectional nature of LSPs, how errors are propagated, and information provided for synchronizing the ingress and egress LSRs.

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