One MPLS label is a field of 32 bits with a certain structure. Figure 2-1 shows the syntax of one MPLS label.
The first 20 bits are the label value. This value can be between 0 and 220–1, or 1,048,575. However, the first 16 values are exempted from normal use; that is, they have a special meaning. The bits 20 to 22 are the three experimental (EXP) bits. These bits are used solely for quality of service (QoS).
These bits are named “experimental” for historical reasons. At one time, nobody knew what they were going to be used for. Chapter 12, “MPLS and Quality ...