Link Control Protocol
In computing, the Link Control Protocol (LCP) forms part of the Point-to-Point Protocol. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine the standards of the ensuing data transmission. LCP is logically a Transport Layer protocol according to the OSI model, however occurs as part of the Data link layer in terms of the actual implemented Network stack
The LCP protocol:
- checks the identity of the linked device and either accepts or rejects the peer device
- determines the acceptable packet size for transmission
- searches for errors in configuration
- can terminate the link if requirements exceed the parameters
Devices cannot use PPP to transmit data over a network until the LCP packet determines the acceptability of the link, but LCP packets are embedded into PPP packets and therefore a basic PPP connection has to be established before LCP can reconfigure it. The LCP over PPP packets have control code 0xC021 and their info field contains the LCP packet, which has four fields. (Code, Id, Length, Data)
- Code: Operation requested: configure link, terminate link, ... and acknowledge and deny codes.
- data: Parameters for the operation
LCP was created to overcome the noisy and unreliable physical lines of the time (e.g. phone lines on dial up modems), in a way which didn't lock the PPP protocol into specific proprietary vendor protocols and physical transmission media.