With default window size of 64KB, and a delay of 75 msec, you should only get around 7Mbps of throughput with TCP.
You would need a window size of about 1MB in order to fill up the 100 Mbps link. 1/0.75 = 13.333 (how many RTTs in a second) 13.333 * 65535 * 8 = 6,990,225.24 (about 7Mbps) You would need to increase the window to 1,048,560 KB, in order to get around 100Mbps. 13.333 * 1,048,560 * 8 = 111,843,603.84 (about 100 Mbps) *Pablo Lucena* *Cooper General Global Services* *Network Administrator* *Office: 305-418-4440 ext. 130* *[email protected] <[email protected]>* On Thu, Nov 5, 2015 at 6:31 PM, Bob Evans <[email protected]> wrote: > Eric, > > I have seen that happen. > > 1st double check that the gear is truly full duplex....seems like it may > claim it is and you just discovered it is not. That's always been an issue > with manufactures claiming they are full duplex and on short distances > it's not so noticeable. > > Try to perf in both directions at the same time and it become obvious. > > Thank You > Bob Evans > CTO > > > > > > Hello NANOG, > > > > We've been dealing with an interesting throughput issue with one of our > > carrier. Specs and topology: > > > > 100Mbps EPL, fiber from a national carrier. We do MPLS to the CPE > > providing > > a VRF circuit to our customer back to our data center through our MPLS > > network. Circuit has 75 ms of latency since it's around 5000km. > > > > Linux test machine in customer's VRF <-> SRX100 <-> Carrier CPE (Cisco > > 2960G) <-> Carrier's MPLS network <-> NNI - MX80 <-> Our MPLS network <-> > > Terminating edge - MX80 <-> Distribution switch - EX3300 <-> Linux test > > machine in customer's VRF > > > > We can full the link in UDP traffic with iperf but with TCP, we can reach > > 80-90% and then the traffic drops to 50% and slowly increase up to 90%. > > > > Any one have dealt with this kind of problem in the past? We've tested by > > forcing ports to 100-FD at both ends, policing the circuit on our side, > > called the carrier and escalated to L2/L3 support. They tried to also > > police the circuit but as far as I know, they didn't modify anything > else. > > I've told our support to make them look for underrun errors on their > Cisco > > switch and they can see some. They're pretty much in the same boat as us > > and they're not sure where to look at. > > > > Thanks > > Eric > > > > >
