[TriLUG] More Asterisk Qs.. QoS

Brian Henning brian at strutmasters.com
Tue Jun 20 11:30:25 EDT 2006

Hiya Gang,
   Hardware issues aside, I'm having less and less good times with 
shoving an Asterisk call over a VPN.  Here's what I'm encountering:

When the VPN is established and essentially idle, I get ICMP round-trip 
times of about 70 - 80 ms.  Not bad.
   When a call is in progress, the ping time skyrockets as high as 1250 
ms, and never gets much lower than 300 ms.  Obviously, the longer the 
ping time (assume, for the time being [because I haven't tackled it yet] 
there is no QoS throttling taking place), the worse the problems with 
echo and garble become.
   This testing setup where I'm seeing this behavior is out through our 
cable ISP and back in through our DSL ISP.  Traceroute conks out at hop 
13 in the DSL->Cable direction; I imagine there's a router out there 
that is ignoring those packets.  Hop 13 is on the right class-C, so I 
figure it's only one or two more hops at most to get the rest of the way.
   Enabling or disabling lzo compression on the VPN has no effect.

So essentially, my questions become:
1 - Does this sound like just something I can fix with QoS tweaking?  I 
have to admit embarrassingly that I really have no idea how to start 
adjusting QoS in our PIX-501.
2 - Since the VoIP traffic is shuttling across a VPN, the hardware 
handling the QoS isn't actually going to know it's VoIP traffic anyway; 
all it will see is VPN packets.  I haven't scoured the docs too 
carefully yet, but does OpenVPN have internal QoS ability?  Or is that 
something I should do just outside the VPN, such as with tc for example? 
  I can up the priority of VPN packets themselves, but it won't always 
be VoIP traffic within the VPN.

Continued thanks for all the friendly help. :)


Brian A. Henning

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