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Multi-protocol label switching (MPLS) is a telecommunication transport mechanism that directs data from one network node to the next by using labels instead of long network Internet Protocol (IP) addresses, avoiding complex lookups in a routing table. So, what is MPLS all about? Let us find out.

In order to improve throughput and delay performance of IP, some sort of combination of standard routing protocols like open shortest path first (OSPF) and asynchronous transfer mode (ATM) switch networks were proposed in the mid-1990s. But these developments were not on a common platform. So, in 1997, Internet Engineering Task Force set up MPLS working group to develop a common standard that is free from any proprietary issues.

Fig. 1: MPLS architecture
Fig. 1: MPLS architecture

In 2001, MPLS working group issued its first proposal that clarified the kind of role MPLS could play in the world of the Internet. Before turning to the details of MPLS, let us briefly examine some basics.

Basically, switching is the process by which two circuits are interconnected for exchanging information and is basically classified as circuit switching and packet switching. Circuit switching is a connection-oriented routing technique that provides a permanent/dedicated path for the entire duration of a conversation.

Contrary to this, packet switching is a connection-less routing technique in which the entire message is divided into packets and then addressed and numbered. Packet switch sends the addressed and numbered packets one by one to the destination, in different routes, by using the entire available spectrum.

In this technique, no dedicated path is used between the source and destination. At the destination, packets arrive randomly at different times; the first packet may arrive last. The receiver has to wait until all packets are received. Finally, all packets are arranged sequentially and then converted into a message. Since the packets are routed through different routes, this routing becomes connection-less.

Although packet switching offers some advantages over circuit switching, it also imposes some limitations. Limitations of packet switching can be overcome by using label switching technique.

In label switching, connection-less IP routing is converted into connection-oriented routing by superimposing the network layer function with data link layer function. The label is attached to the data packet according to class and type of services in a similar way that is followed for categories and priorities in circuit switching. For further routing of the destined IP packet, intermediate routers use appropriate labels only.

Label switching is used in MPLS. The MPLS frame uses various data link frames like ATM, frame relay, point-to point IP/Ethernet (T1/E1) and synchronous optical network, among others. Since MPLS uses label switching and is a protocol-independent transport mechanism, it is called multi-protocol label switching.

MPLS architecture
A typical MPLS architecture is shown in Fig. 1. Components of an MPLS IP network are customer edge (CE), label edge router (LER) and label switching router (LSR).

CE works at IP level. LER is the entry point of MPLS domain and is known as the provider edge. LSR works as a transit switch between LERs. Label switched path (LSP) is the data path between two routers through which packets travel. Lines shown between CE and LER carry IP packets bi-directionally.

Fig. 2: Illustration of traffic flow in MPLS domain
Fig. 2: Illustration of traffic flow in MPLS domain
Fig. 3: Generic MPLS label format
Fig. 3: Generic MPLS label format

Customer edge (CE). It structures the customer message into IP packets and sends it to the entry node of MPLS domain. While receiving IP packets from the egress node of MPLS domain, CE sends packets to the network layer of its own, after removing the IP address.

Label edge router (LER). This works as the gateway of MPLS domain and sits at the edge of the MPLS domain. Ingress LER receives IP packet from CE and assigns the appropriate label. After wrapping the label, it sends the labelled packet towards the next hop through LSP, which is assigned for the specific forward equivalence class (FEC). Assigning the label is termed as label binding. LER also acts as the egress router. Egress LER receives labelled IP packets from the previous transit router, pops up the label (removes the label) and routes the IP packets towards the destined CE.

Label switching router (LSR). This functions as a transit switch in MPLS cloud. It receives labelled IP packets through the appropriate LSP. It analyses the label bound over the packet, consults the forwarding information table [label information base (LIB)] and routes the packet through the appropriate LSP.

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