Saturday, April 5, 2008

Linux Selective Acknowledgment (SACK) CPU Overhead

Last year I tossed an e-mail from the Linux kernel networking list in my "projtodo" folder.

The mail talked about how the Linux TCP stack in particular, and likely all TCP stacks in general, likely had an excessive-CPU attack exposure when confronted with malicious SACK options. I found the mail intriguing but unsatisfying. It was well informed speculation but didn't have any hard data, nor was there any simple way to gather some. Readers of the other posts on this blog will know I really dig measurements. The issue at hand was pretty obviously is a problem - but how much of one?

A few weeks ago I had the chance to develop some testing code and find out for myself - and IBM DeveloperWorks has published the summary of my little project. The executive summary is "its kinda bad, but not a disaster, and hope is on the way". There is had data and some pretty pictures in the article itself.

The coolest part of the whole endeavor, other than scratching the "I wonder" itch, was getting to conjure up a userspace TCP stack from raw sockets. It was, of course, woefully incomplete as it was just meant to trigger a certain behavior in its peer instead of being generally useful or reliable - but nonetheless entertaining.