**5. MPLS and traffic engineering**

MPLS is strategically significant for Traffic Engineering because it can potentially provide most of the functionality available from the overlay model, in an integrated manner, and at a lower cost than the currently competing alternatives. Equally importantly, MPLS offers the possibility to automate aspects of the Traffic Engineering function.

The concept of MPLS traffic trunks is used, according to Li and Rekhter [7], a traffic trunk is an aggregation of traffic flows of the same class which are placed inside a Label Switched Path. Essentially, a traffic trunk is an abstract representation of traffic to which specific characteristics can be associated. It is useful to view traffic trunks as objects that can be routed; that is, the path through which a traffic trunk traverses can be changed. In this respect, traffic trunks are similar to virtual circuits in ATM and Frame Relay networks. It is important, however, to emphasize that there is a fundamental distinction between a traffic trunk and the path, and indeed the LSP, through which it traverses. An LSP is a specification of the label switched path through which the traffic traverses. In practice, the terms LSP and traffic trunk are often used synonymously.

The attractiveness of MPLS for Traffic Engineering can be attributed to the following factors:

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The LSPs in MPLS networks are usually established based on the contents of IP routing tables in core routers. However, there is nothing that would prevent LSPs being established

2. The router where the LSP starts (head-end router) and the router where the LSP ends

The other routers along the LSP do not inspect the packets traversing the LSP and are thus oblivious to their content; they just need to understand the signaling protocol that is used to

With the necessary infrastructure in place, it was only a matter of time before someone would get the idea to use LSPs to implement MPLS-based traffic engineering. The MPLS traffic engineering technology has evolved and matured significantly since then, but the

1. The network operator configures an MPLS traffic engineering path on the head-end router. (The configuration mechanism involves a tunnel interface that represents the

2. The head-end router computes the best hop-by-hop path across the network, based on resource availability advertised by other routers. Extensions to link-state routing

**NOTE:** The first MPLS TE implementations supported only static hop-by-hop definitions. These can still be used in situations where you need a very tight hop-by-hop control over the path the MPLS TE LSP will take or in networks using a routing protocol that does not

protocols (OSPF or IS-IS) are used to advertise resource availability.

1. All the routers along the path agree on a common signaling protocol.

(tail-end router) agree on what's traveling across the LSP.

concepts have not changed much since its introduction:

unidirectional MPLS TE LSP.)

have MPLS TE extensions.

Fig. 2. The MPLS domain

establish the LSP.

and used through other means, provided that:

