Van Jacobson TCP/IP header compression reduces the size of the TCP/IP headers to as few as three bytes. This can be a significant improvement on slow serial lines, particularly for interactive traffic.
The IP-Compression-Protocol Configuration Option is used to indicate the ability to receive compressed packets. Each end of the link must separately request this option if bi-directional compression is desired.
The PPP Protocol field is set to the following values when transmitting IP packets:
Value (in hex) 0021 Type IP. The IP protocol is not TCP, or the packet is a fragment, or cannot be compressed. 002d Compressed TCP. The TCP/IP headers are replaced by the compressed header. 002f Uncompressed TCP. The IP protocol field is replaced by the slot identifier.