PPP Compression Control Protocol (CCP) and Compression Algorithms
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Compression Algorithm Operation: Compressing and Decompressing Data
Once an algorithm has been successfully negotiated, the compression algorithm is used to compress data before transmission, and to decompress it once received. To compress, the transmitting device takes the data that would normally be put in the Information field of an uncompressed PPP frame and runs it through the compression algorithm. To indicate that a frame has been compressed, the special value 0x00FD (hexadecimal) is placed in the PPP Protocol field. When compression is used with multiple links and the links are compressed independently, a different value is used: 0x00FB. Recall that in a regular uncompressed frame, the Protocol field indicates what layer three protocol the data comes from; since we still need to know this, the original Protocol value is actually prepended to the data before compression. When the data is decompressed, this value is used to restore the original Protocol field, so the receiving device knows what higher layer the data belongs to.
For example, if we use IPCP to encapsulate IP data in PPP, the uncompressed frame would have a value of 0x8021 (hex) in the Protocol field. This value (0x8021) would be placed at the start of the data to be compressed. The compressed data would be put in a PPP frame with a Protocol value of 0x00FD. The receiving device would see the value 0x00FD in the Protocol field, recognize the frame as compressed, decompress it and restore the original frame with 0x8021 as the Protocol value. The PPP general frame format topic covers this in more detail.
In theory, a compression algorithm can put more than one PPP data frame into a compressed PPP data frame. Despite this, many if not most of the algorithms maintain a one-to-one correspondence, putting each PPP data frame into one compressed frame. Note that LCP frames are not compressed, nor are the control frames used for other protocols. For example, a data frame carrying IP traffic would be compressed, but a control frame for IPCP (the Network Control Protocol for IP) would not be.
Compression can be combined with encryption; in this case compression is done before encryption.
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