QoS understanding question

[skulakov1977]

So I read a lot of documentation about QoS, but about one things I have a question - does QoS policy used by router only when interface experience saturation? or router each time look to policy and send packets according QoS policy ?

[shiva_ial]

there are two queues

hardware queue

software queue

harware queue is a assocaited with interface itself and once its get filled , software queue will be used (qos will be used) so qos is kind of software queue to handle congestion

with various type of queuing like FIFO.priority queing, CBWFQ etc.

hardware queue always uses FIFO

[Kevin Dorrell]

Various parts of QoS policies can have an effect before the line is saturated, but it is true that most effect is seen when the link is congested.

For example, consider a link that is being 50% used by FTP, and the FTP is generating large bursts of large packets. Suppose we are also running voice on the same circuit. We do not want the voice packets to get stuck on a queue behind the big FTP packets, especially on a slow line. We can prevent that by defining a priority queue. So, the QoS comes into play even on a line that is not congested.

Some QoS techniques are used to prevent congestion before it happens. For example, WRED can be configured to prevent congestion long before the congestion actually happens. For example, you could set it so that when the line is more than 60% full, it drops just the occasional TCP packet. This causes some TCP sessions to slow down for re-transmission, hence the line falls below the 60 % again.

In short, the QoS policy is active all the time. The only exception is that in some architectures, if the output queue is empty, then the packet goes straight for the output ring without going through all the complex queueing mechanisms.

Kevin Dorrell

Luxembourg

[Joseph W. Doherty]

QoS kicks in when you have a queue of packets. As Shiva notes, there's a hardware queue (FIFO) that needs to overflow before software defined QoS activates.

Congestion can be transient, QoS can be managing queued packets even though the interface doesn't appear saturated. On Cisco routers, QoS counters can indicate transient congesting that's unseen against interface usage statistics.