6 Chapter 1 An Introduction to MPLS

Traffic Engineering

The ATM world has a rich feature set that is used for traffic engineering. Traffic engineering is simply a process by which traffic is optimized to follow certain paths based on specified requirements. The IP world also has features, although not nearly as extensive as ATM, to provide for traffic engineering. The problem experienced by service providers is how to combine the traffic engineering of IP with the traffic engineering of ATM. Since ATM and IP are totally separate technologies, it is difficult to implement combined end-to-end traffic engineering.

Quality of Service

Both IP and ATM have Quality of Service (QoS) capabilities. The difference between the two has to do with their operation. IP is connectionless and ATM is connection-oriented. Again, the problem experienced by a service provider is how to combine these two different ways of implementing QoS into a firm end-to-end solution.

MPLS Label Stack

Now that you have seen some of the challenges of merging the IP and ATM worlds, it’s time to talk about MPLS. MPLS, as a technology, evolved from early attempts to glue the IP world and ATM world together. What we know as MPLS today is, for the most part, a standardized version of Cisco’s proprietary tag switching.

The best place to get started with a discussion of MPLS is with the label itself. The MPLS label, or more specifically the MPLS label stack, is composed of four octets (32 bits) and is illustrated in Figure 1.5.

F I G U R E 1 . 5 The MPLS label stack

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

The label is the magic of MPLS, so it is important for you to be familiar with the fields in the MPLS label stack, henceforth referred to as the

MPLS Label Stack 7

MPLS label, or simply the label. The fields in the label are as follows:

Label This field is the label itself, and it is 20 bits in length. With 20 bits, there can be over one million labels.

Experimental (EXP) The Experimental (EXP) field is three bits in length and is used to map the standard IP packet ToS (type of service) into the Experimental field for MPLS CoS (class of service).

S MPLS labels can be stacked one on top of the other. The S, or stack bit, is used to indicate the bottom of the stack. A value of 1 in this field indicates the bottom, or last label, of the stack.

TTL The TTL (Time-to-Live) field from the IP TTL (or Ipv6 Hop Limit field) is decremented by 1 and then copied into the MPLS label TTL field. Upon exiting the MPLS network, the MPLS label TTL value is copied back into the IP TTL field. If this field is set to 0, the packet will be discarded. The TTL field is 8 bits in length.

Shim Header

Now that you’re familiar with the label, you need to know where it’s located. Figure 1.6 shows the placement of the MPLS label.

F I G U R E 1 . 6 MPLS label stack placement

IP Packet

Take a look at Figure 1.6 and find the frame that contains the MPLS label stack. Where is the label? Smack dab between the Layer 2 header and the Layer 3 header. The MPLS label stack is sometimes referred to as a shim header because of how it is placed between the Layer 2 header and the Layer 3 payload.

In Figure 1.7, the placement of the MPLS label is shown with a variety of frame-mode encapsulations. Notice in all the frame-mode encapsulations that the placement of the MPLS label remains the same.