The MPLS header is 4 bytes long, and located immediately before the IP header. The label is a 20 bit field within the MPLS header.
Label – identifies a portion of the label switched path
Experimental (EXP) – used for QoS marking, not actually used for experimental purposes
Bottom-of-Stack (S) – Flag, when set to a 1 means the IP headers are next
Time-To-Live (TTL) – Same meaning as IP header field