DSCP

DSCP stands for Differentiated Services Code Point. It is a field in the Internet Protocol header used to classify packets for quality of service (QoS). In IPv4, the DSCP occupies the six most significant bits of the 8-bit differentiated services field, while the remaining two bits are used for ECN (explicit congestion notification). In IPv6, the DSCP value is carried in the Traffic Class field. The DSCP value indicates a Per-Hop Behavior (PHB) that routers along a path should apply to the packet, guiding queuing, policing, shaping, and scheduling decisions.

Implementation usually involves edge devices classifying traffic to assign a DSCP value and intermediate routers enforcing the corresponding PHB. DSCP enables policy-based QoS across multi-hop networks with a compact header field. Common PHBs defined by standards include Expedited Forwarding (EF) for low-latency, high-priority traffic and Assured Forwarding (AF) with multiple classes and drop priorities to signal varying levels of service under congestion.

DSCP values range from 0 to 63 and are organized into classes by policy. The default value is 0 (best effort) and is often preserved to maintain backward compatibility. In practice, DSCP must be preserved across networks that honor it, though some networks may rewrite or reset DSCP at domain boundaries. DSCP is standardized by the IETF; it is widely supported in routers and switches and is used in enterprise networks, service provider QoS, and multimedia delivery.