What is DSCP?

Understanding DSCP

DSCP (Differentiated Services Code Point) is a 6-bit field in the IP packet header used to classify and prioritize traffic for Quality of Service (QoS). Defined in RFC 2474, it occupies the high-order six bits of the former Type of Service (IPv4) or Traffic Class (IPv6) byte, allowing network devices to give latency-sensitive traffic preferential handling over best-effort data.

The 6 bits yield 64 possible values, mapped to per-hop behaviors (PHBs) that routers and switches apply as packets transit the network. Standard markings include Default/Best Effort (DSCP 0), Expedited Forwarding (EF, DSCP 46) for voice, and Assured Forwarding (AF) classes for prioritized data. Devices read the DSCP value to place packets into appropriate queues, schedule them, and decide drop precedence under congestion, implementing the DiffServ model that scales better than per-flow reservation.

DSCP is relevant to security in two ways. First, QoS for control-plane and security traffic (management sessions, VoIP, encrypted tunnels) ensures these flows survive congestion or denial-of-service pressure, preserving availability, the A in the CIA triad. Second, DSCP markings are untrusted when they cross administrative boundaries: an attacker or misbehaving host can set high-priority markings to starve other traffic, so secure designs re-mark or clear DSCP at network edges and trust markings only from controlled endpoints.

For example, an enterprise running IP telephony marks voice packets with EF (DSCP 46) and signaling with CS3. Edge switches are configured to trust these markings only from registered IP phones; traffic from user PCs is re-marked to best effort. During a heavy backup window that saturates the WAN, routers honor the DSCP markings and keep voice calls clear while bulk data is delayed, and the trust boundary prevents a compromised PC from forging EF marks to hijack bandwidth.