"This document describes Time To Live (TTL) processing in hierarchical Multi-Protocol Label Switching (MPLS) networks and is motivated by the need to formalize a TTL-transparent mode of operation for an MPLS label-switched path. It updates RFC 3032@ ""MPLS Label Stack Encoding"". TTL processing in both Pipe and Uniform Model hierarchical tunnels are specified with examples for both ""push"" and ""pop"" cases. The document also complements RFC 3270@ ""MPLS Support of Differentiated Services"" and ties together the terminology introduced in that document with TTL processing in hierarchical MPLS networks."