[j-nsp] QoS when there is no congestion

[Andrey Khomyakov]

My understanding was that if you don't have congestion, QoS is simply
useless for a number of reasons:

1. You spend engineering hours writing/debugging policy that will never get
used (we assuming no congestion)
2. QoS on egress will not go into effect (other than incrementing counters)
because there is no buffering at this point. Since your buffers are empty.
Your packets go straight into the tx ring and you can't yank priority
packets out of them to place on the wire before the less important packets.
3. And lastly, it's nearly always cheaper just to get more bandwidth then
to spend time on QoS designs.

In fact, I found in some cases QoS hurts more than it helps, because you
are limiting the amount of bandwidth available to high priority packets
(either priority, policy or shape) and you will have a chance for tail
drops or WRED even on a non congested link.

-Andrey



--Andrey

On Tue, Nov 8, 2016 at 6:01 AM, Tim St. Pierre <tim at communicatefreely.net>
wrote:

> If you have some lower bandwidth links, you may still see a latency
> improvement if you implement QoS.
>
> The important thing here is in your output queues, as strict priority will
> see voice traffic serviced first.  Even if nothing is ever dropped, going
> out the queue first keeps latency low.
>
> -Tim
>
>
> On 2016-11-08 04:48 AM, tim tiriche wrote:
>
>> Hello,
>>
>> Do we need QoS if there is no congestion in the network for Voice/Video
>> traffic?
>>
>> Is there a case where Voice/Video traffic could experience any delay if
>> there were data packets to process before the voip traffic?
>>
>> Would this be a concern on older hardware?
>>
>> for example:
>>
>> [voip][data][data][data][data] --> router
>>
>> -Tim
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>
> --
>
> --
> Tim St. Pierre
> System Operator
> Communicate Freely
> 289-225-1220 x5101
>
>
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