Troubleshoot slow TCP Throughput in L2MPLS Link

asked 2018-10-06 09:29:17 +0000

srijit92 gravatar image

updated 2018-10-06 10:06:52 +0000

Hi, We have a 155 mbps L2MPLS link from a service provider and distance is <100 kms with a steady latency of 5 ms. The service provider is using WAN Killer UDP traffic pump on both ends and 155 mbps is passing properly and declaring it as successful delivery. But when I am using any applications like FTP or Browser to transfer files between two locations (using TCP) the speed is stuck between 30-45 mbps max. Even after several (>15) simultaneous downloads the speed fluctuates between 90-105 mbps. The network diagram is attached - View I did some google search and found that window size could be a factor but not sure for such a low latency link it would be a problem. Is there any possible options from the service provider end (like increase buffer size) to get the resolution of the problem? We used to have a same link from different provider previously and I saw 140 up and 145 down FTP transfers between LAN to LAN PCs. Any help is really appreciated.

PCAP Files (FTP RX & TX)

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Analyzing such kind of problems (TCP performance) assumes deep diving into traces. It's hard to tell anything just looking at symptoms list. So please share PCAPs if you're able to do so.

Packet_vlad gravatar imagePacket_vlad ( 2018-10-06 09:37:34 +0000 )edit

I have added PCAP files for FTP transfer for both upload and download here -

Captured at same time from both client and server

srijit92 gravatar imagesrijit92 ( 2018-10-06 10:06:32 +0000 )edit

Hi,seems your traces are a little bit hard to analyze. As they always only show one side.

Even merging the files is no option, as it seems they are different sessions.

But in short I think you have packet loss, or at least some retransmissions, which could have strong impact at tcp performance. (A more detailed answer can be given, if we see the whole session in a trace)

Christian_R gravatar imageChristian_R ( 2018-10-06 12:01:39 +0000 )edit

Thanks for your quick reply. The download is of a single session and upload is of separate session. FTP Download Session Download - Client --> Captured at client side Download - Server --> Captured at server side

FTP Upload Session Upload - Client --> Captured at client side Upload - Server --> Captured at server side

I will go through the link you shared. Regarding packet loss it's very minimal.

srijit92 gravatar imagesrijit92 ( 2018-10-06 12:18:52 +0000 )edit

Can you please do a better capture where each packet of the session is traced. Maybe you have used a wrong capture filter.

Christian_R gravatar imageChristian_R ( 2018-10-06 16:25:41 +0000 )edit