Layer 2 services (such as Frame Relay, Asynchronous Transfer Mode, and Ethernet) can be "emulated" over an MPLS backbone by encapsulating the Layer 2 Protocol Data Units (PDU) and transmitting them over "pseudowires". It is also possible to use pseudowires to provide low-rate Time Division Multiplexed and a Synchronous Optical NETworking circuit emulation over an MPLS-enabled network. This document specifies a protocol for establishing and maintaining the pseudowires, using extensions to Label Distribution Protocol (LDP). Procedures for encapsulating Layer 2 PDUs are specified in a set of companion documents. [STANDARDS-TRACK]