PPP Compression Control Protocol (CCP) and Compression Algorithms
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CCP Configuration Options and Compression Algorithms
CCP configuration options are used for only one purpose: to negotiate the type of compression to be used by the two devices, and the specifics of how that algorithm is to be employed. The device initiating the negotiation sends a Configure-Request with one option for each of the compression algorithms it supports. The other device compares this list of options to the algorithms it understands. It also checks for any specific details relevant to the option to see if it agrees on how that algorithm should be used. It then sends back the appropriate reply (Ack, Nak or Reject) and a negotiation ensues until the two devices come up with a common algorithm both understand. If so, compression is turned on; otherwise, it is not enabled.
The CCP configuration options begin with a Type value that indicates the compression algorithm. When the Type value is 0, this indicates that the option contains information about a special, proprietary compression algorithm not covered by any RFC standards, which can be used if both devices understand it. Several values from 1 to 254 indicate compression algorithms that have been defined for use with CCP. Table 30 shows the most common values of the Type field, including the compression algorithm each corresponds to and the number of the RFC that defines it:
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