This section describes the background and the technical positioning of MPLS.
IP-based Internet prevailed in the mid 90s. The technology is simple and costs little to deploy. However, nowadays IP technology, which relies on the longest match algorithm, is not the most efficient choice for forwarding packets.
In comparison, asynchronous transfer mode (ATM) technology is much more efficient at forwarding packets. It uses labels (particularly, cells) of fixed length and maintains a label table that is much smaller than a routing table. ATM technology, however, is a complex protocol with a high deployment cost, which hinders its widespread popularity and growth.
Users wanted a technology that combines the best that IP and ATM technologies have to offer. This has sparked the emergence of MPLS technology.
MPLS is designed to increase forwarding rates. Unlike IP technology, MPLS analyzes packet headers only on the edge of a network, not at each hop. Therefore, packet processing time is shortened.
MPLS no longer has the high-speed forwarding advantages since application-specific integrated circuit (ASIC) technology has been developed to increase the routing rate. MPLS supports multi-layer labels, and its forwarding plane is connection-oriented. MPLS is widely used in virtual private network (VPN), and quality of service (QoS).
MPLS works between the data link layer and the network layer in the TCP/IP protocol stack. MPLS provides connections for the IP layer and obtains services from the data link layer. MPLS replaces IP forwarding with label switching. A label is a short connection identifier of fixed length that is meaningful to the local end. The label is similar to the ATM virtual path identifier (VPI)/virtual channel identifier (VCI) and the Frame Relay data link connection identifier (DLCI). The label is encapsulated between the data link layer and network layer.
MPLS can use any layer-2 media to transfer packets, but is not limited by any specific protocol on the data link layer.
The origin of MPLS is the Internet Protocol version 4 (IPv4). The core MPLS technology can be extended to multiple network protocols, such as the Internet Protocol version 6 (IPv6), Internet Packet Exchange (IPX), Appletalk, DECnet, and Connectionless Network Protocol (CLNP). MPLS supports label switching between multiple network protocols, as implied by its name.
The MPLS technology is a tunneling technology, but not a service or an application. It supports multiple protocols and services. Moreover, it improves data transmission security.