What is Multiprotocol Label Switching ?

Label switching is a technique of network relaying to overcome the problems perceived by traditional IP-table switching (also known as traditional layer 3 hop-by-hop routing. Here, the switching of network packets occurs at a lower level, namely the data link layer rather than the traditional network layer. Each packet is assigned a label number and the switching takes place after examination of the label assigned to each packet. The switching is much faster than IP-routing. New technologies such as Multiprotocol Label Switching (MPLS) use label switching. The established ATM protocol also uses label switching at its core.

According to RFC 2475, An Architecture for Differentiated Services, December 1998 Examples of the label switching (or virtual circuit) model includes Frame Relay, ATM, and MPLS. In this model path forwarding state and traffic management or Quality of Service (QoS) state is established for traffic streams on each hop along a network path. Traffic aggregates of varying granularity are associated with a label switched path at an ingress node, and packets/cells within each label switched path are marked with a forwarding label that is used to look up the next-hop node, the per-hop forwarding behavior, and the replacement label at each hop. This model permits finer granularity resource allocation to traffic streams since label values are not globally significant but are only significant on a single link; therefore resources can be reserved for the aggregate of packets/cells received on a link with a particular label, and the label switching semantics govern the next-hop selection, allowing a traffic stream to follow a specially engineered path through the network. A related topic is "Multilayer Switching," which discusses silicon-based wire-speed routing devices that examine not only layer 3 packet information, but also layer 4 (transport) and layer 7 (application) information).

MPLS is scalable and protocol-independent. In an MPLS network, data packets are assigned labels. Packet-forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself. This allows one to create end-to-end circuits across any type of transport medium, using any protocol. The primary benefit is to eliminate dependence on a particular OSI model data link layer (layer 2) technology, such as Asynchronous Transfer Mode (ATM), Frame Relay, Synchronous Optical Networking (SONET) or Ethernet, and eliminate the need for multiple layer-2 networks to satisfy different types of traffic. Multiprotocol label switching belongs to the family of packet-switched networks.MPLS operates at a layer that is generally considered to lie between traditional definitions of OSI Layer 2 (data link layer) and Layer 3 (network layer) and thus is often referred to as a layer 2.5 protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames.

A number of different technologies were previously deployed with essentially identical goals, such as Frame Relay and ATM. Frame Relay and ATM use "labels" to move frames or cells throughout a network. The header of the Frame Relay frame and the ATM cell refers to the virtual circuit that the frame or cell resides on. The similarity between Frame Relay, ATM, and MPLS is that at each hop throughout the network, the “label” value in the header is changed. This is different from the forwarding of IP packets.MPLS technologies have evolved with the strengths and weaknesses of ATM in mind. MPLS is designed to have lower overhead than ATM while providing connection-oriented services for variable-length frames and has replaced much use of ATM in the market.

In particular, MPLS dispenses with the cell-switching and signaling-protocol baggage of ATM. MPLS recognizes that small ATM cells are not needed in the core of modern networks since modern optical networks are so fast (as of 2017, at 200 Gbit/s and beyond) that even full-length 1500 byte packets do not incur significant real-time queuing delays (the need to reduce such delays e.g., to support voice traffic was the motivation for the cell nature of ATM). At the same time, MPLS attempts to preserve the traffic engineering (TE) and out-of-band control that made Frame Relay and ATM attractive for deploying large-scale networks.

A telecommunications network is a group of nodes interconnected by links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, message switching, or packet switching, to pass messages and signals. For each message, multiple nodes may cooperate to pass the message from an originating node to the destination node, via multiple network hops. For this routing function, each node in the network is assigned a network address for identification and locating it on the network. The collection of addresses in the network is called the address space of the network.