In the Layer 2 circuit IETF drafts, the control word is optional for most Layer 2 protocols, except Frame Relay and ATM AAL5 where it is required, The control word is 32-bit that is inserted between the VC label and the transported layer 2 frame in case of AToM, it’s used to carries extra information such as protocol control information and sequence number, this information is needed to correctly and efficiently carry the layer 2 protocol across the MPLS network.
So how the control word is signaled? well, The targeted-LDP session is used for signaling the Pseudowire, the signaling protocol which is LDP set up the Pseudowire between PE routers, LDP has been extended with a new TLV to do this, the VC label advertised across the Pseudowire using label mapping message.
The Label mapping message that is advertised over the targeted-LDP contains some TLVs:
1-Pseudowire ID (PW-ID) FEC TLV
The PW-ID FEC TLV is used to identify the PW that the label bounded to, this PW-ID contain some others TLVs, one of these TLVs is C-bit, C-bit indicates whether the advertising PE expects the control word to be present for pseudowire packets, If the C-bit is set to 1, the advertising PE expects the control word to be present in every pseudowire packet on the pseudowire that is being signaled. If the C-bit is set to 0, no control word is expected to be present.
Mounir Mohamed, CCIE#19573