1.4 Relationship to Other Protocols

The CBCP runs after the authentication phase of the PPP. The CBCP message is designated through the protocol field identifier of 0xC029 in the PPP frame, as specified in [IANAPPPNUM]. The CBCP will be run as a part of the PPP negotiation only when the callback configuration option (option 0x0D) is negotiated in the LCP phase, with the operation value of 0x06 as specified in [IANAPPPNUM].

Figure 2: Protocol stack diagram

As specified in the preceding figure, the PPP is responsible for enabling the transportation of a multi-protocol datagram over the data link layer that provides point-to-point connectivity. Each of the PPP-based protocols like LCP, CBCP, and IPCP have a designated protocol identifier, as specified in PPP Data Link Layer Protocol Numbers [IANAPPPNUM]. Based on the protocol identifier, the PPP will be able to associate the messages to the appropriate protocol.

PPP initializes the CBCP based on the LCP Callback option negotiation result. After the CBCP is done with the negotiation and has the result, the result is used by PPP to determine the course of the PPP session; that is, either to continue to the NCP phase or to terminate the session and prepare for callback.