http://www.rfc-editor.org/rfc/rfc4447.txt
RFC 4447: Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP), L. Martini, E. Rosen, N. El-Aawar, T. Smith, G. Heron2006-04-01T23:00:00-00:00Layer 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]