What is the Link Control Protocol?

M. McGee

Link control protocol is a networking system that determines if a network connection is properly set up for information transmission. The link control protocol is part of the point-to-point protocol (PPP) that is the cornerstone of the modern networking interface. Before any information travels from the local system to a remote one, the link control protocol determines if everything is okay for transmission. When the line is set up properly, the information goes out. If there is a problem, then the protocol will terminate the connection.

A network router. The link control protocol makes sure that all parts of a network are set up properly.
A network router. The link control protocol makes sure that all parts of a network are set up properly.

When two network systems encounter each other, they establish a PPP connection. This connection allows the different systems to communicate, regardless of the hardware and software used to transmit information. Before any information will actually move across this connection, the link control protocol will determine if the connection is sound and if the remote terminal is transmitting properly.

This system sets up a commonly misunderstood situation. Many people believe that a PPP connection isn’t allowed until the link control protocol verifies the relationship. In reality, the link control protocol is part of the PPP connection. The PPP will set up the connection, and then the very first thing that goes across it is the link control protocol.

The link control protocol has four main jobs. Its first step when connecting to a new device is verifying that it is what it appears to be. It checks all the information coming from the remote source to verify that it matches up properly. If one signal says it’s a modem and one says it’s a network router, the protocol will disconnect rather than associate with a damaged or misleading device.

The protocol will figure out the proper-sized packets to send to the remote system. Data is sent across network systems in small chunks called packets. These packets break large information down into smaller pieces to prevent corruption and speed up transmission. By sizing the packets in a way that is preferable to the remote system, the data is more likely to transfer properly on the first transmission.

While it is querying the remote system, it also looks for errors in the system’s configuration. These errors may alter the method the local computer uses to transmit information, cause the link control protocol to disconnect or simply end up noted in the two systems' information logs. If two systems connect, it is likely that any errors encountered are minor—larger errors prevent connection entirely.

The protocol’s last job is monitoring the connection for changes in process. If the remote computer starts sending or requesting strange information, the link control protocol may shut the connection down. This is a safety measure to prevent hostile actions over the network connection.

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