The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Feature Protocols

PPP Link Quality Monitoring/Reporting (LQM/LQR) 1 2 3 4 PPP Encryption Control Protocol (ECP) and Encryption Algorithms

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:

Table 30: PPP Compression Control Protocol (CCP) Compression Algorithms

CCP Option Type Value	Defining RFC	Compression Algorithm (As Given in RFC Title)
0	—	Proprietary
1 and 2	1978	PPP Predictor Compression Protocol
17	1974	PPP Stac LZS Compression Protocol
18	2118	Microsoft Point-To-Point Compression (MPPC) Protocol
19	1993	PPP Gandalf FZA Compression Protocol
21	1977	PPP BSD Compression Protocol
23	1967	PPP LZS-DCP Compression Protocol (LZS-DCP)
26	1979	PPP Deflate Protocol

PPP Link Quality Monitoring/Reporting (LQM/LQR) 1 2 3 4 PPP Encryption Control Protocol (ECP) and Encryption Algorithms

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