LCP is responsible for establishing, maintaining, and tearing
down a connection between two endpoints. LCP also tests the link and
determines whether it is active. LCP establishes a point-to-point
connection as follows:
LCP must first detect a clocking signal on
each endpoint. However, because the clocking signal can be generated
by a network clock and shared with devices on the network, the presence
of a clocking signal is only a preliminary indication that the link
might be functioning.
When a clocking signal is detected, a PPP host
begins transmitting PPP Configure-Request packets.
If the remote endpoint on the point-to-point link
receives the Configure-Request packet, it transmits a Configure-Acknowledgement
packet to the source of the request.
After receiving the acknowledgement, the initiating
endpoint identifies the link as established. At the same time, the
remote endpoint sends its own request packets and processes the acknowledgement
packets. In a functioning network, both endpoints treat the connection
as established.
During connection establishment, LCP also negotiates connection
parameters such as FCS and HDLC framing. By default, PPP uses a 16-bit
FCS, but you can configure PPP to use either a 32-bit FCS or a 0-bit
FCS (no FCS). Alternatively, you can enable HDLC encapsulation across
the PPP connection.
After a connection is established, PPP hosts generate Echo-Request
and Echo-Response packets to maintain a PPP link.
