The ToS field, which stands for Type of Service (ToS), is a data field in the IP header of the IP protocol and the IPv4 protocol in which the services of the datagram are defined. With the ToS information computers can specify network relevant service types. Different parameters like the bandwidth, the transmission speed or the reliability, the delay or the data throughput of the transmission can be defined. In addition, the priority treatment of datagrams, the throughput type and the allocation of resources in the routers can be specified.
Structure of the service field
The ToS data field comprises 8 bits. The first three bits represent the priority control (Precedence). With this precedence control, datagrams with high urgency can be routed faster because they are not queued in the router queue. Various values are defined for the Precedence field, but only some are used in practice. Namely, 000 for IP packets from endpoints and 110 or 111 for controlling IP packets between routers. The following four bits determine the handling of the IP data packet.
The first bit provides for minimizing a delay (Delay, D) during transmission, the next bit is for maximizing throughput (Througput, T), the third is for maximizing reliability (Reliability, R), and the fourth is for minimizing transmission cost (Cost, C). The desired quality of service is enabled by setting the respective bit. The remaining bit is reserved in IPv4 of the IP header for future versions.
The ToS field can also be used to assign specific bit combinations to certain services, since some services are to be transmitted in a throughput-optimized manner, others in a cost-optimized manner, and still others as quickly as possible. Algorithms for the various services have been described for this purpose. For example, a minimum delay is recommended for Telnet, which corresponds to a ToS bit combination (for the bits D, T, R, C) of 1000. The File Transfer Protocol( FTP), on the other hand, is characterized by maximum data throughput, which corresponds to a bit combination of 0100. E-mails can be characterized with the highest reliability by the bit combination 0010 (Any IGP) and news in the lowest cost by 0001( NNTP).