When establishing a connection via PPP, the participating devices negotiate a common behavior for the data transfer. For example, they first decide whether they should connect at all using the security method, names and passwords specified.
Once the connection is established, the reliability of the line can be constantly monitored using the LCP. This is achieved within the protocol by the LCP echo request and its associated LCP echo reply. The LCP echo request is a query in the form of a data packet which is transferred to the remote site along with the data. A valid response (LCP echo reply) to this request shows that the connection is reliable and stable. This request is repeated at specified intervals so that the connection can be continually monitored.
What happens when there is no reply? First a few retries will be initiated in case the disturbance is temporary. If all the retries remain unanswered, the line will be dropped and an alternative route sought. If, for example, the high-speed connection refuses to work, an available ISDN port can open the way to the Internet as a backup.
