MPLS, GMPLS, MPLS-TP

What is MPLS ?

 MPLS stands for "Multiprotocol Label Switching".   In an MPLS network, incoming packets are assigned a "label" by a "label edge router (LER)".  Packets are forwarded along a "label switch path (LSP)" where each "label switch router (LSR)" makes forwarding decisions based solely on the contents of the label.  At each hop, the LSR strips off the existing label and applies a new label which tells the next hop how to forward the packet.

Label Switch Paths (LSPs) are established by network operators for a variety of purposes, such as to guarantee a certain level of performance, to route around network congestion, or to create IP tunnels for network-based virtual private networks.  In many ways, LSPs are no different than circuit-switched paths in ATM or Frame Relay networks, except that they are not dependent on a particular Layer 2 technology.
 
An LSP can be established that crosses multiple Layer 2 transports such as ATM, Frame Relay or Ethernet.  Thus, one of the true promises of MPLS is the ability to create end-to-end circuits, with specific performance characteristics, across any type of transport medium, eliminating the need for overlay networks or Layer 2 only control mechanisms.

 MPLS evolved from numerous prior technologies including Cisco's "Tag Switching", IBM's "ARIS", and Toshiba's "Cell-Switched Router".

What is GMPLS ?

Generalized MPLS extends MPLS to encompass time-division (e.g. SONET ADMs), wavelength (optical lambdas) and spatial switching (e.g. incoming port or fiber to outgoing port or fiber)." GMPLS represents a natural extension of MPLS to allow MPLS to be used as the control mechanism for configuring not only packet-based paths, but also paths in non-packet based devices such as optical switches, TDM muxes, and SONET/ADMs.

GMPLS (Generalized Multiprotocol Label Switching), also known as Multiprotocol Lambda Switching.
Components of GMPLS

GMPLS introduces a new protocol called the "Link Management Protocol" or LMP.  LMP runs between adjacent nodes and is responsible for establishing control channel connectivity as well as failure detection.  LMP also verifies connectivity between channels.

Additionally, the IETF's "Common Control and Measurement Plane" working group (ccamp) is working on defining extensions to interior gateway routing protocols such as OSPF and IS-IS to enable them to support GMPLS operation.

Features of GMPLS

GMPLS supports several features including:

• Link Bundling - the grouping of multiple, independent physical links into a single logical link
• Link Hierarchy - the issuing of a suite of labels to support the various requirements of physical and logical devices across a given path
• Unnumbered Links - the ability to configure paths without requiring an IP address on every physical or logical interface
• Constraint Based Routing - the ability to automatically provision additional bandwidth, or change forwarding behavior based on network conditions such as congestion or demands for additional bandwidth

What is MPLS-TP ?

MPLS-TP is a set of MPLS protocols that are being defined in IETF. It is a simplified version of MPLS for transport networks with some of the MPLS functions turned off, such as Penultimate Hop Popping (PHP), Label-Switched Paths (LSPs) merge, and Equal Cost Multi Path (ECMP). MPLS-TP does not require MPLS control plane capabilities and enables the management plane to set up LSPs manually. Its OAM may operate without any IP layer functionalities.

 Pseudowires and LSPs

The essential features of MPLS-TP defined by IETF and ITU-T are:

• MPLS forwarding plane with restrictions

• PWE3 Pseudowire architecture

• Control Plane: static or dynamic Generalized MPLS (G-MPLS)

• Enhanced OAM functionality

• OAM monitors and drives protection switching

• Use of Generic Associated Channel (G-ACh) to support fault, configuration, accounting, performance, and security (FCAPS) functions

• Multicasting is under further study

MPLS-TP is a subset of the MPLS technology. Therefore, it's possible to build parallel IP+MPLS and MPLS-TP networks on the same physical infrastructure, one offering IP and MPLS-based VPN transport and the other one offering traditional circuit-based services. Don't expect to see many providers using this approach. Those that are embracing IP+MPLS wholeheartedly are already offering legacy services across their new MPLS networks.

More conservative service providers might opt to upgrade their existing SONET/SDH/DWDM transport network to MPLS-TP. Those providers will probably retain the separation between tightly-managed transport network (where every action is triggered by the central management software) and the more dynamic IP+MPLS network, which will be just one of many clients of the MPLS-TP network. I suspect that these service providers will be left with a single client of their MPLS-TP network in the long run, as most end-user traffic (including voice) will be migrated to IP anyway … unless, of course, they choose to focus on the fixed-bandwidth-reselling business, where the static nature of MPLS-TP will definitely be beneficial.

Breaking down the differences between MPLS and MPLS-TP


When it comes to the major differences between MPLS and MPLS-TP, here's what you need to know.

Bidirectional Label Switched Paths (LSPs). MPLS is based on the traditional IP routing paradigm -- traffic from A to B can flow over different paths than traffic from B to A. But transport networks commonly use bidirectional circuits, and MPLS-TP also mandates the support of bidirectional LSPs (a path through an MPLS network). In addition, MPLS-TP must support point-to-multipoint paths.

Management plane LSP setup. Paths across MPLS networks are set up with control-plane protocols (IP routing protocols or Resource Reservation Protocol (RSVP) for MPLS Traffic Engineering (MPLS-TE). MPLS-TP could use the same path setup mechanisms as MPLS (control plane-based LSP setup) or the traditional transport network approach where the paths are configured from the central network management system (management plane LSP setup).

Control plane is not mandatory. Going a step farther, MPLS-TP nodes should be able to work with no control plane, with paths across the network computed solely by the network management system and downloaded into the network elements.

Out-of-band management. MPLS nodes usually use in-band management or at least in-band exchange of control-plane messages. MPLS-TP network elements have to support out-of-band management over a dedicated management network (similar to the way some transport networks are managed today).

Total separation of management/control and data plane. Data forwarding within an MPLS-TP network element must continue even if its management or control plane fails. High-end routers provide similar functionality with non-stop forwarding, but this kind of functionality was never mandatory in traditional MPLS.

No IP in the forwarding plane. MPLS nodes usually run IP on all interfaces because they have to support the in-band exchange of control-plane messages. MPLS-TP network elements must be able to run without IP in the forwarding plane.

Explicit support of ring topologies. Many transport networks use ring topologies to reduce complexity. MPLS-TP thus includes mandatory support for numerous ring-specific mechanisms.