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PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others)
(Page 3 of 3)
An Example NCP: The Internet Protocol Control Protocol (IPCP)
Let's look at the NCP for IP, IPCP.
When PPP is set up to carry IP datagrams, IPCP is invoked in the Network-Layer
Protocol phase (one of the PPP
phases) to set up an IP NCP link between
the two devices. The setup is carried out using the four Configure-
messages. For IP, there are two configuration options that can be specified
in an IPCP Configure-Request:
- IP-Compression-Protocol: Allows
devices to negotiate the use of something called Van Jacobson
TCP/IP header compression. This compresses the size of TCP and
IP headers to save bandwidth. Thus, this is similar in concept to the
Protocol-Field-Compression and Address-and-Control-Field-Compression
(ACFC) options in LCP.
- IP-Address: Allows the device sending
the Configure-Request to either specify an IP address it wants
to use for routing IP over the PPP link, or to request that the other
device supply it with one. This is most commonly used for dial-up networking
Again, the receiving device can send
back an IPCP Configure-Ack, an IPCP Configure-Nak, or
an IPCP Configure-Reject, just as they work in LCP. The other
NCPs are similar, but use different configuration options. After configuration
is complete, data can be sent for the layer three protocol corresponding
to the NCP negotiated. This is indicated by using the appropriate value
for the Protocol field in PPP data frames containing that layer
Note: As an aside, the full expanded name of the NCP for IP is The Point-to-Point Protocol Internet Protocol Control Protocol. Three protocols in one name. That has to be some sort of a record. J
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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