Type of service code constants for IP. Type of service describes how a packet should be handled.
TOS is an 8-bit record in an IP header which contains a 3-bit precendence field, 4 TOS bit fields and a 0 bit.
The following constants are bit masks which can be logically and'ed with the 8-bit IP TOS field to determine what type of service is set.
Taken from TCP/IP Illustrated V1 by Richard Stevens, p34.
