Protocol Field Compression. Normally, PPP-encapsulated packets are transmitted with a two-byte protocol field. For example, IPv4 packets are transmitted with the protocol field set to 0x0021, and MPLS packets are transmitted with the protocol field set to 0x0281. For all protocols with identifiers in the range from 0x0000 through 0x00ff, PFC enables routers to compress the protocol field to one byte, as defined in RFC 1661, The Point-to-Point Protocol (PPP). PFC allows you to conserve bandwidth by transmitting less data. See also ACFC.