Uniquely identifies a set of tunnel instances
between a pair of ingress and egress LSRs.
When the MPLS signalling protocol is rsvp(2), this value
equals to the value signaled in the Tunnel ID
of the SESSION object. When the MPLS signalling protocol
is crldp(3), this value equals to the value
signaled in the LSP ID. Reference to MPLS-TE-STD-MIB.