# What is Quality of Service (QoS)?

> Quality of service (QoS) is the use of network policies and resource-management mechanisms to provide different forwarding treatment to selected traffic.

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## Content

A QoS design may target throughput, delay, delay variation, or packet loss for traffic classes. It becomes most visible when flows compete for constrained links, buffers, or processing capacity.

Network devices can classify traffic, mark packets, meter rates, queue and schedule transmission, shape bursts, police excess traffic, or select paths. In differentiated services, the Differentiated Services Code Point (DSCP) in the IP header is used to select a per-hop behavior; each administrative domain controls classification, marking, conditioning, and mapping.

### Key points

- **Policy basis:** Translate application needs into measurable classes that protect essential control, voice, video, transactional, bulk, and default traffic according to operational requirements.
- **Enforcement:** Validate or remark untrusted classifications at the boundary, configure queues, schedulers, shapers, and policers consistently along the managed path.
- **Verification:** Measure latency, variation, loss, throughput, queue drops, and behavior during congestion and failover instead of assuming a packet marking creates a service guarantee.
- **Security consideration:** Untrusted endpoints may mark traffic for preferred treatment. Boundary conditioning helps prevent theft of service and resource-exhaustion effects on other classes.
- **Important limitation:** QoS cannot create missing capacity or guarantee end-to-end performance across unmanaged domains. Poor prioritization can starve other necessary traffic, and the mechanisms do not authenticate or encrypt application data.

### Related terms

[Network latency](<https://yellowcube.eu/glossary/network-latency/>) · [Packet loss](<https://yellowcube.eu/glossary/packet-loss/>) · [Network traffic](<https://yellowcube.eu/glossary/network-traffic/>) · [Multiprotocol Label Switching (MPLS)](<https://yellowcube.eu/glossary/multiprotocol-label-switching/>) · [Wide area network (WAN)](<https://yellowcube.eu/glossary/wide-area-network/>)

### Sources

[RFC 2475: An Architecture for Differentiated Services](https://www.rfc-editor.org/info/rfc2475) · [ITU-T Y.1540: IP Packet Transfer and Availability Performance Parameters](https://www.itu.int/rec/T-REC-Y.1540/en) · [ITU-T Y.1541: Network Performance Objectives for IP-Based Services](https://www.itu.int/rec/T-REC-Y.1541/en)

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