I have a 25 mb up/25 mb down Internet Leased Line from Service Provider delivered directly from their MUX Port at their BTS. The problem is that I never get 25 mbps tcp upload speed. TCP download speed is perfectly 25 mbps. Also checked with UDP upload and download that is also 25 mbps. But when ever TCP Upload takes place the speed varies between 10 and 18 giving an average of 16 mbps max.
I have already talked with the provider and they are very casual and just showed UDP upload using WAN Killer software.. I need something strong to prove them.
Please guide me a proper troubleshooting process.. My link is delivered via 1 Gbps full duplex ethernet port both set at auto negotiation at MUX end and Laptop end.
Best Answer
The only parameter you could witness with Wireshark that could explain a speed drop would be packets MTU being too high and leading to fragmentation, here's a well written post on how to detect such a behavior with Wireshark https://ask.wireshark.org/questions/41152/how-to-check-if-fragmentation-is-happening This could be fixed by setting an appropriate MTU on your side.
What you describe though looks like your ISP applies some form of QoS, if by any chance they have an upload service of some kind, or if you own a server on their network, you could try uploading a big enough file and record (screenshot) the obtained speed. Using SpeedTest or any speed calculation service that they don't host themselves won't be a good enough proof as they can blame a third party operator.
I has this kind of issue with my previous operator which did something similar with encrypted (IPsec) traffic and never had them accept this fact.