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PPP Components and General Operation
(Page 2 of 2)
PPP General Operation
The fact that the PPP suite includes
literally dozens of protocols often makes it seem like it must be a
really complex technology. In fact, the general operation of PPP is
really quite straight-forward. The existence of all those PPP protocols
allows PPP to be flexible and extensible, supporting many higher layer
datagram types and various features.
The bottom line, however, is that
PPP operation involves just three basic steps. Beginning in a state
where there is no PPP link between the devices, these are the operations
that occur in PPP (also illustrated in Figure 24):
- Link Setup and Configuration:
Before the two devices can exchange information, they must make contact
and set up a link between them. During link setup, all the parameters
needed to manage the operation of the link are agreed upon by the two
devices. The LCP begins this process, and invokes the help of support
protocols as they are needed, for options like authentication. After
the link is set up in general terms, the appropriate NCP is called for
whatever layer three technology is being carried on the link to complete
- Link Operation: The devices
on the link use it to send datagrams. Each device transmits by taking
layer three datagrams, encapsulating them and sending them down to layer
one to be transmitted. Each device receives by taking PPP frames sent
up from its own physical layer, stripping off the PPP header and passing
the datagram up to layer three. Where appropriate, optional feature
protocols are used here, such as CCP for compression.
- Link Termination: When either
device decides it no longer wants to communicate, it terminates the
link. The link can of course be re-established if desired.
Figure 24: Overview of PPP Operation
In simplest terms, PPP consists of only three basic steps: link setup, link operation and link termination.
Link setup is by far the most complicated
of these general steps, as it involves several substeps used to negotiate
link parameters and options. The next topic describes the steps in link
setup, and discusses the phases that a link passes through as it is
set up, used, and eventually terminated.
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Version 3.0 - Version Date: September 20, 2005
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