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]