Showing posts with label TCP. Show all posts
Showing posts with label TCP. Show all posts


Wifi connectivity issues and TCP SACK

So, my Ubuntu 12.10 laptop recently developed an issue where it would not connect to the wifi access point at a local café. My iPhone had no trouble, nor the laptop when booting into Windows 7. Other people using Mac laptops seemed to have no trouble, either.

At a guess, since similar connectivity issues came up at work a few months ago, I decided to turn off TCP SACK (Selective ACKnowledgement), and that caused the wifi connection to start working. What's odd is that SACK is supposed to be negotiated at the start of a connection. Whatever. It works, now.

UPDATE: Well, maybe that wasn't it. Right after doing this, I turned SACK back on to see its effects, and was able to connect to the access point with no trouble.

UPDATE 2: Here's the commandline:
sysctl -w net.ipv4.tcp_sack=0
There was some discussion on ServerFault about TCP SACK, and also an in-depth article by Patrick McManus from 2008.