The MPLS shim header contains the following fields:
3-bit experimental (EXP) field
The EXP field can be used to identify different traffic classes to support the DiffServ QoS model.
1-bit bottom-of-stack flag
The bottom-of-stack bit is set to 1 to indicate the last stack entry.
8-bit Time-To-Live (TTL) field.
The TTL field is used for loop mitigation, similar to the TTL field carried in IP headers.