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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.

Srijit B
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    WireShark can not diagnose speed issues unless they are caused by some other software that is using your bandwidth. Do basic troubleshooting which involves disabling all security software and trying a different computer (obviously hard wired to the ISP's equipment). If both fail to give you good results, ask them to send you a technician but you would probably be liable if their technician connects their laptop and gets a perfect speed without changing anything in the installation. – Julie Pelletier Aug 07 '16 at 05:26
  • @Julie - I have upload the file to a server having more than 200 mbps uplink and enough free bandwidth was there for link testing.. The testing mechanism was first a simple speedtest, then ftp upload, iperf. unstable upload giving average of 16 mbps and not going between 5 to 18 mbps.. Also checked when I am downloading a file while upload is in progress then download is also falling drastically to 8 to 10 mb and total of up+down is between 25 to 30 mbps at all time.. In a simple 150 mb file upload to several server there are average 500+ re transmissions as seen from wireshark. – Srijit B Aug 08 '16 at 02:52
  • That would seem to indicate that the connection is half-duplex. Since your own physical connection is a Gbps Ethernet link, the ISP is probably at fault, but you should do as I said and test it with another separate computer to be 100% sure the problem is not on the computer. – Julie Pelletier Aug 08 '16 at 04:02
  • I have already checked with different computer and using auto negotiation on it takes 1 gbps full by default on computer but result is same poor upload. – Srijit B Aug 08 '16 at 16:44
  • Then, as I already mentioned, if you're sure of your diagnostics, you will not be able to go further than that and will need to get them to send you a tech to check it out. – Julie Pelletier Aug 08 '16 at 16:49
  • So... any updates? – Julie Pelletier Aug 12 '16 at 17:34

2 Answers2

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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.

iMil
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  • RED (Random Early Detection) will randomly drop TCP to keep buffers from filling up and causing congestion. This will purposely slow TCP to prevent congestion on the network. This could even be on the site router. – Ron Maupin Aug 07 '16 at 16:44
  • @iMil - There is no fragmentation taking place as check from your given link.Link for PCAP - http://www.filedropper.com/wiresharkpcapng23a7996d-d285-4da2-816b-b1d2d743bde620160808080235a04252 http://www.filedropper.com/zz_1 – Srijit B Aug 08 '16 at 02:54
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Finally the problem is solved. There was issue at Service Provider Router where policy was applied. They reapplied the policy and gave a burst of 2 mb and now upload is perfect.

Srijit B
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