[lug] VoIP quality issues and MTU

[Hugh Brown]

I don't know about MTU, but in a previous life they did voip over a vpn
and 80ms latency was the target the vendor said we needed to hit to
maintain average call quality.

FWIW,

Hugh

On Fri, 23 Sep 2005, Daniel Webb wrote:

> I've been having minor problems with VoIP sound quality and dropouts whenever
> my internet connection is saturated ever since I started using it.  I have my
> own QoS stuff running using iptables classification and tc, and I think I have
> everything set right (outgoing queue is a small pfifo for the VoIP stuff,
> txqueue is small on the ethernet device), and it improved a lot but there was
> still a jittery sound whenever the connection was saturated, especially with
> the outgoing side saturated.  Today I tried setting the MTU very small (200)
> in desperation, and now the problems have gone.  I have read plenty of stuff
> on QoS and VoIP, and never saw anything about changing the MTU.  Did I just
> miss something?  I know what MTU is and how it works, I'm just surprised that
> if it makes that much difference on a standard DSL connection why I've never
> read about changing it for this purpose.
>
> The actual numbers are:
>    - txqueuelen = 2 for eth0 on router, 0 on vlan0 and vlan1
>      (set with "ip link set eth0 qlen 2")
>    - mtu = 200 on the asterisk server and the router
>      (set with "ip link set <whatever> mtu 200")
> For the outgoing qdisc queue for VoIP packets:
>    - "tc qdisc add dev $EXT_IF parent 1:$CLASS_PRIO pfifo limit 10"
>
> My router is a Linksys WRT54G v.2.0, and my internet connection is 1.5M/384k
> DSL.  I'm not very experienced with the hardcore networking aspects of this,
> so I'm a little confused where the bottleneck is and where I want it to be.
>
> If I understand right, I want to use a tc qdisc/class to throttle slightly
> below my DSL modem's speed so that a queue doesn't build up on the DSL modem.
> There should be a queue for each priority class, and then the qdisc gets to
> pick which of the class queues it will send to the network device.
>
> Thinking of the worst case scenario with an MTU of 1500, I imagine the qdisc
> sending out a 1500 byte packet from a file upload, and instantly after that
> asterisk sends out a packet.  I'm assuming the VOIP packet is going to have to
> wait for that file packet to clear the device, so for my connection that will
> take about 50ms if I assume 300kbps:
>    1500 bytes / 30000 kBps = 50 ms
> which means this would cause 50 ms of jitter, which doesn't seem that bad.  So
> why does decreasing MTU help when nothing else seems to?
>
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