Also known more formally as
SERVICE TYPE. The TOS is an 8-bit field in a
datagram, or
IP packet header. It was originally divided into five subfields (precedence, D, T, R, and unused). The
IETF redefined the field in the 1990's to accomodate a set of
differentiated services (
DS), containing the 6-bit
codepoint (
DSCP) and 2 bits that are unused. Multiple codepoints map to each service. Backward compatibility is maintained by distinguishing between the first and last 3 bits of the codepoint.
Basically, the TOS field helps the
routing algorithm choose among various paths to a destination based on policies.