PPP Architecture - Link Control Protocol Layer
The LCP is the real working part of PPP. The LCP sits on top of the physical layer and has a role in establishing, configuring, and testing the data-link connection. The LCP establishes the point-to-point link. The LCP also negotiates and sets up control options on the WAN data link, which are handled by the NCPs.
The LCP provides automatic configuration of the interfaces at each end, including:
Handling varying limits on packet size
Detecting common misconfiguration errors
Terminating the link
Determining when a link is functioning properly or when it is failing
PPP also uses the LCP to agree automatically on encapsulation formats (authentication, compression, error detection) as soon as the link is established.