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The Point-to-Point Protocol (PPP)
In order to use a modem-based 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. The current
protocol for this purpose is PPP.
A PPP "frame" looks like:
The "FLAG" bytes delineate frame boundaries, and the data field
normally 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.
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