This chapter first gave an overview of LDP, including LDP components and operations that are related to pseudowire emulation over MPLS. Then the chapter explained the control signaling and data switching details of AToM.
Despite multiple possible combinations of label distribution and management modes for pseudowire signaling, AToM implements the combination that uses LDP downstream unso-licited advertisement, independent control, and liberal retention modes.
Pseudowire emulation over MPLS introduces new protocol extensions and signaling proce-dures, such as the Pseudowire ID FEC element in the FEC TLV that is defined to represent a pseudowire that connects two attachment circuits on different PE routers, the negotiation of the control word presence, and sequence number resynchronization.
Pseudowire emulation over MPLS also specifies new encapsulation methods and data switch-ing procedures, such as the control word that is customized for carrying transport-specific information, Layer 2 payload encapsulations, ingress/egress processing optimized for transporting over pseudowires, and the sequence number for detecting out-of-order packets.