infotage.net

So another day devoted to traffic shaping, particularly as it relates to voip. That means playing with traffic shaping rules, often with a distinct lack of documentation, and then testing to see if they make a difference. So today I learned about HFSC after reading a quote in a discussion about HTB CBQ VOIP and latency, “HFSC may be the best solution, but there aren’t many examples/much support for it yet.” Basically the problem is that CBQ and HTB introduce latency which is very problematic for VOIP. Using the Hierarchical Fair Service Curve Scheduler one can guarantee latencies (as far as leaving the device). So I put it into place and so far it’s looking like a great replacement for HTB. I’ve measured losses in packet drops and jitter. The main downside is that lack of decent documentation. Right now I’d say the setup I’m testing is actually less complex than HTB because it requires fewer parameters. With HTB I was tweaking cburst, r2q, quantum, and more to try and achieve the results I wanted. HFSC works pretty well with just the guaranteed rate, the maximum rate, and the latency you wish to ensure. It doesn’t have a prio parameter so thought has to be given to the latency of various classes. It also doesn’t support anything like burst so while a good choice for a system needing to prioritize voip it may not be ideal for a webserver which does not need to prioritize any time sensitive traffic.

Most of the info on HFSC was godawful but here are a few sites that I feel helped push me in the right direction.

• http://linux-ip.net/articles/hfsc.en/
• http://www.voip-info.org/wiki/view/QoS+Linux+with+HFSC
• http://automatthias.wordpress.com/2006/06/30/hfsc-and-voip/
• http://www.cs.cmu.edu/~hzhang/HFSC/
• http://trash.net/~kaber/hfsc/
• http://mailman.ds9a.nl/pipermail/lartc/2006q4/019780.html
• http://wsched.sourceforge.net/examples.html (this may not be helpful)

Also, I note that it is rarely mentioned that the leaf classes by default implement a pfifo qdisc beneath them.  This can be replaced with another qdisc, or even adding a pfifo to that the stats can be reviewed (sfq would be another likely choice for certain kinds of traffic).

3 comments so far

"The black art of traffic shaping: HFSC is your friend" was written on February 13, 2007 and filed in all, gnu / linux