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PPP Link Control Protocol (LCP)
(Page 3 of 3)
LCP Link Maintenance
Once the link has been negotiated,
LCP passes control to the appropriate authentication and/or NCP protocols
as discussed in the
PPP Link Setup and Phases topic. Eventually
the link setup will complete and go into the open state. LCP messages
can then be used by either device to manage or debug the link:
LCP Link Termination
- Code-Reject and Protocol-Reject:
These frame types are used to provide feedback when one device receives
an invalid frame due to either an unrecognized LCP Code (LCP
frame type) or a bad Protocol identifier.
- Echo-Request, Echo-Reply and
Discard-Request: These frames can be used for testing the
Finally, when the link is ready to
be shut down, LCP terminates it. The device initiating the shutdown
(which may not be the one that initiated the link in the first place)
sends a Terminate-Request message. The other device replies back
with a Terminate-Ack. A termination request indicates that the
device sending it needs to close the link. Like a four-year-old who
tells you he needs to go now, bad!, this is
a request that cannot be denied. J
Other LCP Messages
The standard RFC 1570, PPP
LCP Extensions, also defines two new LCP message types. The
Identification message is used to allow a device to identify
itself to its peer on the link. The Time-Remaining message lets
one device tell the other how much time remains in the current session.
Relationship Between LCP and Other PPP Protocols
Note that many of the other protocols
used in PPP are modeled after LCP. They use the same basic techniques
for establishing protocol connections, and send and receive a subset
of LCP message types. They also exchange configuration options in a
similar manner. The next topic shows how the Network Control Protocols
(NCPs) are based on LCP. You will see the same thing in looking at feature
protocols such as CCP, ECP and others.
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Version 3.0 - Version Date: September 20, 2005
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