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am_gli_287451's avatar
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Oct 01, 2018

Slow Download Speed through F5



we are facing an issue regarding download speed. A virtual F5, licensed for 200M has a VS configured. Behind the VS, there's a server (10G-NIC) with a 1GB test file.


If the client is downloading the file directly (100Mbit NIC), its speed is constantly around 100M. If the client tries the download through the F5, the speed varies and ends up around 76M with the default tcp-profile, and around 84 with tcp-lan-optimized.


Here you can see a chart of the NIC - the first and third attempt are directly, second and fourth via F5:


On F5, it looks like this:



I assume now - given that the server can handle 10G, but the F5 TMM instance is only capable of 100M - the server tries to increase the speed over these 100M - which you can see in the light blue peaks. The TMM instance can't handle the packets, drops them, and the server needs to retransmit. This seems to happen more often and I think that this could be the reason for this low output rate.


Does this sound plausible?


Is there any way to tweak this? E.g. with TCP window-size or similar towards the server?


Or do you think the problem is somewhere else? (Pagle is off, working only with these both default profiles).


There are no irules / ASM / APM bound to this VS. Does only SSL-Offloading, nothing else.


Thanks in advance!


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