In order to use a point-to-point link to carry IP packets, some
kind of data link protocol is needed. Originally
this was achieved using a very rudimentary "non-standard" protocol
called SLIP (for "Serial Line IP"). The current
recommended protocol for this purpose is PPP.
A PPP frame has the following structure:
The "FLAG" bytes delineate frame boundaries, and the data field
contains an IP datagram.
When a PPP link is started, the protocol specifies a moderately
complex set of "state transitions". Some of these states are:
Establishment
the PPP software exchanges Link Control
Protocol messages, negotiating basic parameters for the
link.
Authenticate
PPP includes facilities for verification of (eg)
username/password pairs (PAP) to ensure that the link initiator is
permitted to establish this data link.
Network
various essential network configuration variables are set
during this phase, eg IP addresses and netmasks.
