Callback and the Callback Control Protocol
After the authentication phase of the PPP connection process, CBCP negotiates the use of callback. If callback is negotiated, the answering PPP peer terminates the PPP connection, and then calls the original calling PPP peer at a specified phone number. CBCP messages use the PPP Protocol ID 0xC0-29 and have the same structure as LCP messages. However, only the first seven LCP message types are used, corresponding to LCP Codes 1 through 3. For the Callback-Request (Code set to 1), Callback-Response (Code set to 2), and Callback-Ack (Code set to 3) messages, the data portion of the CBCP message contains one or more CBCP options.
Table 4-4 lists the CBCP options used by Windows-based PPP peers.
Table 4-4. CBCP ...
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