What is QoS (Quality of Service)?
Mechanisms that, when a network is congested, set priorities and bandwidth for each type of traffic to limit delay and loss for voice and important business traffic. Packets are labeled with their type using DSCP and similar fields (marking), and equipment along the path varies queue assignment, sending order and dropping order accordingly. QoS does not add bandwidth.
Japanese name: QoS
QoS (Quality of Service) appears in 1 practice question across 1 topic, including Network Design and Operations.
Field: Network Design and Operations
Classification and marking
QoS first sorts packets into types (classes) by address, port number, application and so on. The result is written as a value into the DSCP field of the IP header (6 bits) or the priority in the VLAN tag (PCP, 3 bits) (marking). Equipment along the path can then decide treatment just by looking at this value, without inspecting the packet again in detail. Voice is commonly given EF (value 46), which is forwarded with priority, and other business traffic is given AF classes and the like.
Queuing and scheduling
When an outgoing link is congested, packets that cannot be sent wait in queues. Strict priority queuing, which always sends from the highest-priority queue first, gives the lowest delay, but when there is a lot of priority traffic the other queues cannot send at all. Weighted round robin and weighted fair queuing (WFQ) share bandwidth in proportion to weights, so every class is assured a certain amount of bandwidth. A common combination gives strict priority only to voice, with a cap on its bandwidth, and divides the rest by weights.
Shaping, policing and congestion avoidance
There are two ways to hold the sending rate to a contracted bandwidth or similar limit: shaping, which queues the excess and sends it out gradually, and policing, which drops or re-marks the excess. In addition, dropping everything at once when a queue overflows makes many TCP flows slow down at the same time, so methods such as RED (random early detection) drop some packets at random once a queue starts to grow, signaling congestion to TCP early. QoS works only when the same policy is applied consistently across the equipment along the path.
FAQ
Does configuring QoS make the line faster?
No. QoS decides which traffic gets to use limited bandwidth first when the network is congested. If bandwidth is always short, the line needs to be upgraded. When there is no congestion, QoS settings make almost no difference to speed.
Does QoS work for traffic that crosses the internet?
It works within your own organization's equipment, but ISP equipment on the internet does not necessarily honor the DSCP values you set. To assure quality between sites, it is common to use a private network service whose quality can be guaranteed by contract.