TCP Window Scaling issue

Zach Hill zach.reborn at gmail.com
Thu Jul 24 16:14:58 UTC 2014


Hello,


I know this isn't precisely on topic but I'm having an issue that I could
use some assistance with.


I'm currently seeing a very interesting issue for a single server. File
transfers from Server A to Server B are relatively slow and not using up
much of the circuit. Upon further inspection the TCP window size remains at
default 65535 and window scaling doesn't negotiate. What's interesting is
this is only affecting a single server and only when traffic is going over
the WAN circuit. Testing from Server A to any server on it's network shows
it is negotiating window scaling just fine.

Below I'll try and draw out a better idea of what is happening. Let the
letters represent the server in question and let the .# represent which
subnet they are on to show whether transversal of the WAN circuit is
occurring.

Server A.1 -> Server B.2 = No window scaling

Server A.1 -> Server C.1 = Window scaling

Server B.2 -> Server A.1 = Window scaling

Server C.1 -> Server B.2 = Window scaling

The net result here is when window scaling is properly being used I'm
seeing about 30-40 Mbps of bandwidth usage, without scaling I'm only seeing
2.8Mbps. Any thoughts?


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