TCP port prioritizing on switch/router (QoS)

Question

I have some HP ProCurve series Switch. There is a QoS option for prioritize traffic by TCP/UDP port. I set two ports: one with highest priority and another with lowest. Then I performed tests to ensure that prioritizing really works. I run two instances of Network benchmark application on two computers being in the LAN. One computer acts like server and other like client. In benchmark applications I set two ports configured earlier on the switch. Then I run tests, so that server sends some amount of data to client showing current transmission speed. I assumed that, since one port has higher priority then other, the transmission speed will not be equal, and the traffic going throughout the tcp port with higher priority will be significant faster. Unfortunately there is no difference. The speeds are nearly equal (half of bandwidth) and non prioritizing seems to be applied.

Can anyone explain me why this is not working. I suppose that I misunderstand some QoS basics and this is a reason, but I will be grateful if someone clarify this issue for me.

Answer 1

Your assumption that cince one port has higher priority then other, the transmission speed will not be equal, and the traffic going throughout the tcp port with higher priority will be significant faster is incorrect.

QoS basically says that the different traffic classes will end up in a different "bucket" in a switch or router. How exactly that bucket behaves is defined by the QoS policies. For example, if lowest QoS class is defined as "minimum 10kbs bandwidth", and highest one is defined as "minimum 10mbps bandwidth", than if your total available bandwidth is much higher than these limits, than obviously your QoS classification will not have any effect (e.g. both streams will share the available bandwidth equally).

This is a gross oversimplification of the QoS issue, and I think configuration QoS on ProCurve-class switches is beyond the scope of superuser.com.