TCP/IP Tutorial and Technical Overview - IBM Redbooks

A.1 MPLS: An introduction
The idea behind MPLS was to emulate some property of circuit-switching
network over a packet network, and to strike a happy middleground between
extreme connection-oriented switching and pure connectionless routing service.
The theory for the idea of “mixing types to meet in the middle” was based on the
observation that a sequence of correlated packets exist for stream and
multimedia applications. We wanted to process them in the same routing path in
a uniform fashion in order to guarantee quality of service (QoS). And we did not
want to repeatedly examine and process those packet headers. The idea is
feasible because we note that the headers in those related packets are the same
or similar because those related packets in a stream desire consistent and
similar processing treatment.
Multiprotocol Label Switching (MPLS) follows the same idea and comes up with
new techniques to make a pseudo (and short-term) connection in a path (or
subpath) for a sequence of correlated IP packets. The technology was proposed
in RFC 3031. The Multiprotocol Label Switching (MPLS) standard represents
the effort in the continued evolution of multilayer switching.
Generalized MPLS or GMPLS extends MPLS to encompass time-division (for
example, SONET/SDH), wavelength (lambdas), and spatial switching (for
example, incoming port or fiber to outgoing port or fiber). The focus of GMPLS is
on the control plane of these various layers to dynamically provision resources
and to provide network survivability using protection and restoration techniques.
A.1.1 Conventional routing versus MPLS forwarding mode
First, let us describe how the new paradigm shift might help in improving QoS
performance by comparing conventional routing to the MPLS forwarding mode.
In an MPLS environment, conventional layer-3 or network-layer routing (that is,
IP routing) is used to determine a path through the network. After the path is
determined, data packets are then switched through each node as they traverse
the network.
Conventional routing mode
In a traditional, connectionless network, every router runs a layer-3 routing
algorithm. As a packet traverses through the network, each router along the path
makes an independent forwarding decision for that packet. Using information
contained in the packet header, as well as information obtained from the routing
algorithm, the router chooses a “next hop” destination for the packet. In an IP
network, this process involves matching the destination address stored in the IP
header of each packet with the most specific route obtained from the IP routing
926 TCP/IP Tutorial and Technical Overview