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PPP Compression Control Protocol (CCP) and Compression Algorithms
(Page 2 of 4)
CCP Operation: Compression Setup
When most people talk about compression
in PPP they mention CCP, which is considered the compression
protocol for PPP. However, CCP is actually used only to configure and
control the use of compression; it is the algorithms that do the real
work of compressing and decompressing. This separation of powers
provides flexibility, since it allows each implementation to choose
what type of compression they wish to use.
CCP is analogous to the Network
Control Protocols (NCPs) that negotiate
parameters specific to a network layer protocol sent on the link. An
NCP lets two devices decide how they will carry layer three traffic,
such as how IPCP lets the devices determine how to carry IP. CCP lets
two devices decide how they will compress data, in the same basic way.
Similarly, just as each NCP is like
a lite version of LCP,
CCP is also like a lite version of LCP. It is used to set
up a compression connection called a CCP link within an LCP link
between two devices. Once established, compressed frames can be sent
between the two devices. CCP also provides messaging capabilities for
managing and eventually terminating a CCP link, again very similar to
how each network layer protocol sets up a NCP link within LCP. A CCP
link is maintained independently of any NCP links.
CCP uses the same subset of seven
LCP message types that the NCPs use, and adds two additional ones. The
use of these messages for each of the life stages of a CCP
link is as follows, which should look very familiar if you've already
read about how the NCPs and LCP itself work:
- Link Configuration: Like the NCPs, compression
configuration is done once CCP reaches the Network-Layer Protocol
phase. The process of setting up compression and negotiating parameters
is accomplished using Configure-Request, Configure-Ack,
Configure-Nak and Configure-Reject messages just
as described in the LCP topic, except
the configuration options are particular to CCP.
- Link Maintenance: Code-Reject messages
can be sent to indicate invalid code values in CCP frames. The two new
message types are Reset-Request and Reset-Ack, which are
used to reset the compression (the CCP link) in the event of a detected
failure in decompression.
- Link Termination: A CCP link can be terminated
using Terminate-Request and Terminate-Ack. Again, remember
that like the NCP links, the CCP link is set up within an LCP link,
and closing it doesn't terminate the LCP link which controls PPP overall.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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