Type of Service

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Type of Service (TOS) is a way of marking TCP/IP packages which makes it possible to prioritize on routers in order to make some types of traffic be transferred before other types of traffic. This is done by setting a byte in the IPv4 header which indicates what type of traffic the packet carries.

Types of service

The predefined Linux TOS values are defined in linux/ip.h and are:

  • Minimize-Delay (minimize delay, decimal value 16, hex value 0x10)
  • Maximize-Throughput (maximize throughput, decimal value 8, hex value 0x08)
  • Maximize-Reliability (maximize reliability, decimal value 4, hex value 0x04)
  • Minimize-Cost (minimize monetary cost, decimal value 2, hex 0x02)
  • Normal-Service (normal service, decimal value 0, hex value 0x00). The default value on most packets is Normal-Service, or 0.

The TOS types listed above are not the one commonly used by ISPs today, see Type of Service (ToS) and DSCP Values.