Quality of service (QoS)
Mechanisms that classify network traffic and give chosen classes, such as voice and video, priority for bandwidth, delay, jitter and loss when links are congested.
Quality of Service (QoS) is the set of techniques a network uses to treat some traffic better than other traffic when capacity runs short. Packets are classified, marked and queued so that delay-sensitive flows such as voice over IP get the bandwidth they need and suffer less latency, jitter and loss, while bulk transfers wait. The two classic IETF models are Integrated Services (RFC 1633), which reserves resources per flow, and Differentiated Services (RFC 2474 and RFC 2475), which marks each packet with a class in the IP header and lets each router apply a per-hop behaviour to that class.
QoS is an availability tool. It does not encrypt or authenticate anything, and it cannot create capacity; it only decides who waits. It is closely related to traffic shaping, which smooths the rate at which traffic is sent. The security concern is trust in the markings: if an endpoint can mark its own traffic as high priority, a user or an attacker can push ahead of real-time traffic, so networks commonly re-mark traffic where it enters from untrusted devices.
Exam relevance: a scenario about call quality degrading under load is likely to point to QoS, alongside the performance metrics named in the ISC2 exam outline. Candidates are expected to keep QoS in the availability column, not the confidentiality one.