A 4-bit field in the header of IPv4 network packets.

Allows values from 0 to 15 to be assigned to each packet of network traffic in order to request special handling (for example, minimize delay, maximize throughput). The ToS field is being phased out in favor of Differentiated Services Code Point (DSCP) (see RFC 1349)

TOS field values (from iana)

TOS Value	Description			Reference
---------	--------------------------	---------
  0000		Default				[Obsoleted by tp://www.ietf.org/rfc/rfc2474.txt]RFC2474]
  0001		Minimize Monetary Cost		[Obsoleted by tp://www.ietf.org/rfc/rfc2474.txt]RFC2474]
  0010		Maximize Reliability		[Obsoleted by tp://www.ietf.org/rfc/rfc2474.txt]RFC2474]
  0100		Maximize Throughput		[Obsoleted by tp://www.ietf.org/rfc/rfc2474.txt]RFC2474]
  1000		Minimize Delay			[Obsoleted by tp://www.ietf.org/rfc/rfc2474.txt]RFC2474]
  1111		Maximize Security		[Obsoleted by tp://www.ietf.org/rfc/rfc2474.txt]RFC2474]