On Fri, Nov 16, 2012 at 2:16 PM, Tom Metro <[email protected]> wrote: > > Eventually the problem was traced back to an incorrectly configured QoS > setting where the bandwidth limit was set in excess of the WAN speed. > This was apparently filling up the router's buffers, and spilling over > to its ability to pass packets between the WLAN and the LAN.
Sounds like a head of line blocking problem: http://en.wikipedia.org/wiki/Head-of-line_blocking I think Jim mentioned that in passing when I asked about Ethernet flow control as a solution to bufferbloat. (Although he admitted that he was concentrating on "last mile" rather then data center/core network equipment.) The wikipedia article on the subject (http://en.wikipedia.org/wiki/Ethernet_flow_control) has the following to say: "For example, a flow can come into a switch on a higher speed link than the one it goes out, or several flows can come in over two or more links that total more than an output link's bandwidth. These will eventually exhaust any amount of buffering in the switch. However, blocking the sending link will cause all flows over that link to be delayed, even those that are not causing any congestion. This situation is a case of head-of-line blocking, and can happen more often in core network switches due to the large numbers of flows generally being aggregated. Many switches use a technique called Virtual Output Queues to eliminate the HOL blocking internally, so will never send pause frames.[3]" As I understand it, the issue is that Ethernet PAUSE frames are too coarse a mechanism. It works on a physical link rather then flow level. If you have a typical situation where you aggregate multiple links to a single uplink, it's easy to fill up the uplink. When that happens, the link that has been paused has trouble communicating with other (non-uplink) ports because it keeps getting PAUSEd due to the uplink port being permanently full. So an upload to the Internet might cause you to have trouble accessing a local file server or network printer even when theoretically you have plenty of local bandwidth available to you. Bill Bogstad _______________________________________________ bblisa mailing list [email protected] http://www.bblisa.org/mailman/listinfo/bblisa
