Tag Archives: Ethernet

How’s prioritisation implemented in 802.1Q capabled devices?

It is incredibly difficult to find out whether prioritisation is handled if a NIC is 802.1Q capabled. And if yes, how? All I manage to find out from various literature is that the 802.1Q header supports setting Priority Code Point (PCP), which is made up of 3 bits. But not much is mentioned on whether prioritisation is actually done, end-to-end which I’m interested in. I would imagine that if the MAC supports prioritisation, then there must be some form of mechanisms to prioritise, whether it be multiple queues or what have you. WLAN 802.11e NICs have four queues implemented in hardware. However nothing is mentioned on NICs that support 802.1Q. I know VLAN works fine with just single queue. But I would like to find out how prioritisation is meant to work in 802.1Q!If there is not multiple queues in hardware, guess the stack needs to do something on prioritisation according to the code points. I know priority mappings can be done from IP layer (DiffServ Code Point) to WLAN 802.11e MAC. I guess the same needs to be done too if we want to realise traffic prioritisation. Perhaps it’s time to look into the source code of the Linux stack and see if prioritisation is done there. Product’s information from vendors is always a pain to locate, and always very vague.