Type of Service

Each quality helps gateways determine the best way to route datagrams. A router maintains a ToS value for each route in its routing table. The lowest priority TOS is 0, the highest is 7 - when bits 3, 4, and 5 are all set to 1. The router tries to match the TOS of the datagram to the TOS on one of the possible routes to the destination. If there is no match, the datagram is sent over a zero TOS route.

Table 5: The role of each bit in the IP header TOS 8-bit field
bits 0, 1, 2	Precedence	Some networks treat high precedence traffic as more important traffic. Precedence should only be used within a network, and can be used differently in each network. Typically you do not care about these bits.
bit 3	Delay	When set to 1, this bit indicates low delay is a priority. This is useful for such services as VoIP where delays degrade the quality of the sound.
bit 4	Throughput	When set to 1, this bit indicates high throughput is a priority. This is useful for services that require lots of bandwidth such as video conferencing.
bit 5	Reliability	When set to 1, this bit indicates high reliability is a priority. This is useful when a service must always be available such as with DNS servers.
bit 6	Cost	When set to 1, this bit indicates low cost is a priority. Generally there is a higher delivery cost associated with enabling bits 3,4, or 5, and bit 6 indicates to use the lowest cost route.
bit 7	Reserved for future use	Not used at this time.

For example, if you want to assign low delay, and high reliability, say for a VoIP application where delays are unacceptable, you would use a bit pattern of xxx1x1xx where an ‘x’ indicates that bit can be any value. Since all bits are not set, this is a good use for the bit mask; if the mask is set to 0x14, it will match any TOS packets that are set to low delay and high reliability.