Adding a Label
MPLS is a method that takes the best of both worlds and creates a concept that allows IP packets to travel through the network as if IP were a connection-oriented protocol (which it isn't). Using special routers called Label Switching Routers (LSRs) does this. These routers connect a traditional IP network to an MPLS network. A packet enters the MPLS network through an ingress LSR, which attaches a label to the packet, and exits the MPLS switched network through an egress LSR. The ingress LSR is the router that performs the necessary processing to determine the path a packet will need to take through the switched network. This can be done using traditional routing protocols such as OSPF. The path is identified by the label that the ingress router attaches to the packet. As you can see, the ingress router must perform the traditional role that a router fills. It must perform a lookup in the routing table and decide to which network the packet needs to be sent for eventual delivery to the host computer.
However, as the packet passes through the switched network, it is only necessary for the switch to take a quick look at the label to make a decision on which port to output the packet. A table called the Label Information Base (LIB) is used in a manner similar to a routing table to determine the correct port based on the packet's label information. The switch doesn't perform IP header processing, looking at the IP address, the TTL value, and so on. It just spends a small amount of time doing a lookup of the label in the table and outputting the packet on the correct port.
When the packet reaches the egress LSR, the label is removed by the router, and then the IP packet is processed in the normal manner by traditional routers on the destination network.
If this sounds like a simple concept, that's because it is. MPLS still is in the development stages, so you'll find that different vendors implement it in different ways. Several Internet draft documents attempt to create a standard for MPLS. Other features, such as Quality of Service (QoS) and traffic management techniques, are being developed to make MPLS a long-term solution.
Using Frame Relay and ATM with MPLS
One of the best features about the current design of MPLS is that it separates the label-switching concept from the underlying technology. That is, you don't have to build special switches that are meant for just MPLS networks. MPLS doesn't care what the underlying transport is. It is concerned only with setting up a path and reducing the amount of processing a packet takes as it travels through the circuit.
Because of this, it's a simple matter for an ATM or Frame Relay switch vendor to reprogram or upgrade its product line to use MPLS. For ATM switches, the VPI (virtual path identifier) and VCI (virtual channel identifier) fields in the ATM cell are used for the Label field. In Frame Relay switches, an extra field is added to the IP header to store the label. However, don't get confused and think that an MPLS network is an ATM network or a Frame Relay network. These switches must be reprogrammed to understand the label concept. It's even possible, for example, for an ATM switch to switch both ATM and MPLS traffic at the same time. By allowing for the continued use of existing equipment (and
these switches are not inexpensive items), large ISPs or network providers can leverage their current investment, while preparing to install newer MPLS equipment when the standards evolve to a stage that makes it a good investment.
For the long term it's most likely that MPLS will be implemented using technology similar to Frame Relay instead of ATM. This is because of the small cell size of the ATM cell (53 bytes) combined with a high overhead (the 5-byte cell header). In a small network with little traffic, this 5-byte cell header seems insignificant. However, when you scale this to large bandwidth network pipes, this amount of overhead consumes a large amount of bandwidth given the small amount of data carried in the 53- byte cell. Thus, variable-length frames are most likely to become the basis for MPLS networks in the next few years.