[NCLUG] Bandwidth throttling when SIP connection is in progress.

[Scott Scriven]

* John Gilmore <jgilmore at glycou.com> wrote:
> HTB doesn't have an equivalent to the "condition" match, so I'd 
> have to create or modify the rules when a SIP connection 
> started, and destroy or modify them when it stopped.

No, that's not necessary.  When your high-priority SIP bucket is 
active, it will automatically tone down the other traffic 
appropriately.

I had a voip phone for a while...  and a roommate who insisted on 
running 30+ torrents all the time.  He kept the connection 
completely saturated and refused to stop, so I fixed it.  He was 
bogging things down so much that ping times to my ISP ranged from 
1000ms to 6000ms, and the phone was completely unusable.

With a bit of QoS on my router, I reduced latency by two orders 
of magnitude and got the phone working nicely.  As a bonus, my 
ssh sessions also became responsive, no matter how overloaded the 
network got.

> My understanding is that since my upstream provider (USCable) 
> doesn't implement QOS, to eliminate latency for the SIP calls 
> would require that I throttle bandwidth to quite a bit lower 
> than the actual limit - say 700KBps on a 1.2Mbps connection.

I've found it works well to limit upstream bandwidth use to about 
15/16ths of the capacity, though really anything from about 90% 
to 95% is probably good.

Basically, that gives you just enough wiggle room to prevent lost 
packets, so your router decides what gets sent, instead of random 
chance.

> Mostly this is a desire on my part to avoid the time-consuming 
> empirical testing ...

I've done a bit of voip testing under load, and made something 
which works.  It's primitive, but you're free to use it if you 
like.  I've put a shortened version online:

  http://narthex.xyzz.org/qos-init.sh

You'll need to adjust the part which matches your voip packets.  
I had a voip appliance on its own IP address, so matching was 
easy.  You may need to do it based on tcp port instead, or 
whatever other mechanism is appropriate.  Also, be sure to set 
CEIL to about 90% of your uplink capacity.


-- Scott