Type of Service (TOS) is an 8 bit field in the IP header which can be used for differentiating the treatment for that packet. TOS is an older method not used as much any more because it has been superseded by the Class of Service (COS). This redefining of the TOS field is called the Differentiated Services (DiffServ) Framework.
COS uses 6 bits in the DiffServ field, called the DiffServ Code Point (DSCP). 6 bits allows 64 classes, which can be the predefined classes or manually chosen. The queueing and forwarding treatment of the IP packet is called Per Hop Behaviour (PHB).
The last 2 bits in the DiffServ field is the ECN - Explicit Congestion Notification. This can be used to signal congestion.
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