With IP-based transport, routing (forwarding) of packets is based on the destination address in the IP header. Since IP is a connectionless protocol, packets in a data stream can be forwarded independently of each other. Connectionless protocols simplify network operation and are suitable for applications, such as file transfers, that do not require a guaranteed quality of service (QoS). However, connectionless protocols are not particularly suitable for continuous data streams or for time-sensitive applications, such as voice. By itself, IP-based transport is not robust enough to be the foundation for convergence.
Label switching (or tag switching) is a technique that can be used to incorporate connection-oriented features into IP networks (and other types of networks). With label switching, virtual connections are established and packets are switched (forwarded) through the network based on the labels rather than on the complete destination address.
With Multi-Protocol Label Switching (MPLS), label switched paths (LSPs) are established, and labels are attached to packets to identify particular LSPs. Forwarding of an IP packet is based on the 20 bit label in the MPLS header rather than on the 32 bit IP address (for IP version 4) in the IP header. With MPLS, network capacity can be reserved along the LSP so that the QoS for a traffic flow can be guaranteed. MPLS and the development of echo cancellation techniques enable voice to be transported in IP packets with an acceptable QoS.
The Label Distribution Protocol (LDP) and /or the Resource Reservation Protocol (RSVP) are employed in conjunction with MPLS to establish LSPs and reserve network capacity along these LSPs. The Internet Engineering Task Force (IETF) in developing a new extensible IP signaling protocol suite (NSIS), which will provide additional capabilities and more flexibility in establishing and controlling virtual connections.
MPLS labels can be stacked, and multiple LSPs can be nested in a higher level LSP. The signaling protocols can be used to control the network capacity assigned to an LSP containing multiple virtual connections. This gets us back to common channel signaling, with the control signaling separate from the data transport and with a signaling connection able to control multiple data connections, in this case virtual connections.
With the dominance of IP data and the capabilities of label switching techniques to support a guaranteed QoS, a strong case can be made for convergence based on IP/MPLS. A consensus seems to be forming in support of the IP/MPLS approach. However, this web site challenges this consensus. Click here to read a paper that compares the IP/MPLS approach with an alternative approach for convergence.