A fundamental concept of MPLS, or multiprotocol label switching, is that of labeling packets that travel to different nodes or “switches”(hence the name). Typically, in a routed IP network, the routers designate an independent forwarding path for these packet based solely on the packet’s network-layer label. Then, whenever the packet arrives at a router, the router then processes which node to send the packet to next.
An important thing to know is that MPLS is a technique, not a service — so it can be used to deliver a variety of network services, such as IP VPNs and Ethernet. And even though providers create the MPLS structure, the actual MPLS services may go by several different names, including “IP VPN” and “metro Ethernet”.
With MPLS, when the packet enters into any network, the packet gets assigned a forwarding equivalence class (FEC), which is indicated by attaching a short bit sequence, the label, to the packet. Each router in the network has a router table specifying how to transport these packets of a specific FEC type. Then, once the labeled packets has enters the network, the routers do not need to analyze each packets header. Alternatively, each subsequent router uses the label as a sort of directory for the router table that provides them with the new designated FEC for the packet.
This allows the MPLS network to handle the labeled packets with particular characteristics, designated by the FEC, in a consistent manner. Packets containing real-time traffic, such as video or voice data, can be simply mapped to faster routes across the network — something that’s much more difficult with conventional routing. This ensures that the packets will be delivered quickly and efficiently. The key structural point within this MPLS network is that the labels specify the method for “attaching” additional information to each packet.
Is MPLS a Layer 2 or Layer 3 service?
It is not well known whether MPLS is a Layer 2 or Layer 3 service. MPLS doesn’t fit exactly into the OSI seven-layer hierarchy. In fact, one of MPLS’s most important benefits is that it that it utilizes multiple paths that separate it from the original data-link service. MPLS can create router tables for ATM or frame relay switches (using existing headers and labels) or for older IP routers by attaching MPLS tags to IP packets.
Ultimately, the network providers can best utilize MPLS for a wide assortment of services. The two of the most popular implementations for MPLS networks are the Layer 3, also known as a virtual private routed network (VPRN) and the Layer 2, otherwise known as virtual private LAN service (VPLS), VPNs.
An important component of Layer 3 is RFC 2547 VPNs, which have been utilized by most of the major network service providers, including AT&T, Verizon, BT, and several others. One of the very important characteristics of a RFC 2547 is that traffic becomes isolated within MPLS VPNs as it enters the network.
Although there are various kinds of layer 2 MPLS services, one of the major components that they all share is that the Layer 2 packet (a.k.a. frame relay frame or ATM cell) is enclosed in an MPLS header (the label) and then redirected through the MPLS core. Once it reaches its destination point, this packet’s labels are removed, and the packet can arrives at the final destination within the MPLS network. Therefore, Layer 2 MPLS services effectively apply services such as frame relay or Ethernet within an IP WAN.
The various kinds of MPLS
Pseudowire Edge-to-Edge Emulation (PWE3) is the type of MPLS that’s commonly used to condense connection-oriented frame relay and ATM services. PWE3 maps point-to-point tunnels across the MPLS backbone, and therefore works nicely for circuit-oriented networking procedures. Also, PWE3 can be used to maintain connectionless LAN protocols, although it’s not the ideal solution.
For connectionless protocols, primarily Ethernet, there is a different requirement called virtual private LAN service, or VPLS. VPLS points out some of the particular challenges with extending Ethernet throughout the metropolitan area or WAN, largely being accessibility and scalability. Numerous providers, including Extreme, Siemens, and Nortel, are endorsing an alternate method called Provider Backbone Transport, or PBT for short, for metropolitan area Ethernet. PBT is based on using current IEEE 802.1 VLAN labels to transport Ethernet services throughout the provider network. PBT and T-MPLS are head-to-head competitors in this realm and it is not yet known which one will emerge victorious in the industry.
Another more recent requirement is the ITU’s transport-MPLS (T-MPLS), which is intended to simplify the distribution of Ethernet services. Finally, a form of MPLS called Generalized Multiprotocol Label Switching (GMPLS) grants routers the ability to logically signal the optical layer, allowing providers to establish, change, or eliminate optical links in real time. Therefore, the service providers can deliver “optical wavelength” services created from MPLS.