ToS and DSCP

In order to provide Quality of Service, there should be a way to distinguish priority of each arriving packet according to DiffServ Architecture.
In IP packet header there is special group of 8 bits (from 8th to 15th respectively in header) dedicated for that purpose. They are called Type of Service.

In Precedence could be one of fallowing values, which informs about packet type :

• 000 – routine
• 001 – priority
• 010 – immediate
• 011 – flash
• 100 – flash-override
• 101 – critical
• 110 – internetwork control
• 111 – network

Other bits inform that :

• 11 – packet is delay sensitive (D)
• 12 – packet wants high rate (T)
• 13 – packet wants high reliability (R)
• 14 – mainly unsued
• 15 – mainly unsued

In DiffServ architecture there was defined Differentiated Services Code Point in ToS field.

It allows to define 64 different classes. The mapping between IP Precendence and DSCP is as fallows:

First 3 bits from DSCP agrees with IP Precedence.
Thanks to ToS and DSCP we know packets service requirements.