Which IPv4 header field carries the IP QoS classification marking?

The ToS (Type of Service) field in the IPv4 header is used for QoS classification. It allows network devices to differentiate between different types of traffic and apply appropriate QoS policies, such as prioritization or bandwidth allocation. This field is now more commonly referred to as the Differentiated Services Code Point (DSCP) field, which uses the first 6 bits of the original ToS byte. ID (Identification) is used for fragment reassembly. TTL (Time to Live) prevents routing loops by decrementing at each hop and discarding packets when it reaches zero. FCS (Frame Check Sequence) is a Layer 2 field used for error detection in Ethernet frames, not an IPv4 header field.