IP Specification of TOS

The IP specification includes a definition of a Type of Service field in an IP packet header. This is intended to be used by upper-layer protocols to pass information to the Internet layer about how to optimize routing for the packet.

Network topology means that there are often a number of available routes between the source and destination of a packet. Some routes are more reliable than others. Some are expensive, with high call setup or usage charges, while some are low-cost but slow. The most suitable route for a packet depends on the application and user, and might even vary with other factors such as the time of day. For example, if you are a system administrator monitoring a remote system, you need to receive alarm traffic as rapidly as possible regardless of the cost, because the cost of routing the alarm is significantly lower than the cost incurred by a system problem. However, if you start to get a document by ftp from the same system at the end of your working day, intending to use it the following day, a low-cost, slow route is sufficient.

The Internet Layer has no direct knowledge of how to optimize a route for a given application or user. The TOS facility was intended to provide hints about how best to route a packet, influencing both queueing algorithms and routing. It contains a 3-bit precedence field and a 4-bit TOS field. The setting of precedence field indicates one of the following values for the precedence:

• Network

• Internet

• Critical

• Flash-override

• Flash

• Immediate

• Priority

• Routine

The possible settings of the TOS field are:

• Minimize delay

• Maximize throughput

• Maximize reliability

• Minimize monetary cost

• Normal service

The TOS facility has not been widely used in the past, but the Internet Engineering Task Force (IETF) is now working to modify the definition of TOS and to encourage its use.