The Importance of Data Center Bridging Priority-base Flow Control Testing

August 22, 2012

Data Center Bridging (DCB) Priority-based Flow Control (PFC) is a standard that was recently completed by the IEEE DCB Working Group. It builds upon IEEE 802.3X flow control by breaking the pause up into eight different traffic classes based on the VLAN priority field. This allows congested devices to independently pause different traffic classes without disrupting traffic classes that are not causing congestion.

Prior to the definition of PFC, Ethernet flow control testing was not very interesting. It focused on making sure that the timers all worked correctly, but the technology as a whole simply involved turning on and off transmission for an entire link. With the introduction of PFC, you now have multiple layers of complexity to the testing. The technical staff at the UNH-IOL needs to ensure that devices properly allow different traffic classes to continue to pass traffic while pausing other traffic classes. Ethernet flow control was almost never used in a real network prior to PFC, because it would seriously degrade the network performance on a large real world network.

One of the common failures that we have seen already in early PFC enabled devices is that they cannot simultaneously support all eight traffic classes. This is very important, if the intention is to actually see wide spread use in real data centers, then supporting more than two or three traffic classes will be vital. Another common failure that we see is that devices do not implement the priority_enable_vector field properly. This field allows devices to say which pause timer for a device to ignore or look at. This is important for devices to implement correctly so that efficiencies can be realized in a large network. When a device uses the field properly, it can help the receiver of a PFC frame to skip directly to only the pause timers that need to be followed and ignore the pause timers that do not matter.

This is only a select couple of common failures that we have seen in DCB devices; there are many other test cases that we test that can cause issues with interoperability and deployment. Make sure that your device is compliant by testing with the DCB Consortium at UNH-IOL.