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by wtallis 3767 days ago
Just a reminder for everyone wanting to simulate a bad network connection: setting a static packet-loss percentage, latency, and bandwidth limit does not accomplish that task. Real networks suck due to wireless interference and bufferbloat, which aren't simulated by tools this simple. Optimizing your software to perform well under this simulation may help it work well on a satellite link, but could be counterproductive for common real-world problems.
1 comments
shabble 3767 days ago
are there tools or applicable models out there that do represent the other factors you mention? I've been looking recently at ways to smoke-test my network app (probably separated into parts using docker containers) under adverse network conditions.

There was a nice looking python library that wrapped things that I can't seem to find at the moment. There's https://pypi.python.org/pypi/atcd/0.1.0, but I don't think that's what I was thinking of.

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wtallis 3767 days ago
Direct use of queuing disciplines lets you create bloated buffers and use features of TBF and netem that aren't exposed through this simple wrapper. If you're testing at line rate, bfifo suffices to create bufferbloat, and TBF lets you do rate limiting with the kinds of burst allowances used by many cable ISPs. The IFB module is used to do ingress shaping, which is totally not addressed by this tcconfig tool.

For packet loss, you really just need to create competing traffic in order to cause congestion, preferably realistically bursty competition. Dropping packets based on a fixed probability isn't a realistic simulation of any common real-world cause of packet loss, and generating competing traffic is probably easier than figuring out the right way to approximate its effect using the other loss models implemented by netem (and again not exposed through this simple tool). Simulating the behavior of WiFi under congestion and interference is hard, and I don't think the stock Linux kernel (or any other mainstream OS) has adequate tools to emulate layer 2 retransmits, packet aggregation, and the periodic interference of microwave ovens.

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