Fix Slow SFTP/SCP File Upload Rate and Stalling caused by TCP SACK

In brief, sometimes a conflict occurs between some firewalls and the TCP SACK. In my case, the upload was severely affected and reached upload speeds of a few kilobytes over a LAN!

The following command disables TCP SACK and fixes the slow upload on all Ethernet cards:

sudo sysctl -w net.ipv4.tcp_sack=0

To make it permanent, edit /etc/sysctl.conf and add this line

net.ipv4.tcp_sack = 0

Then run:

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About SoCRaT

Research Support and Infrastructure Engineer, OSS & Linux Geek
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