Re: [LARTC] QoS for VoIP

[Kilian Krause <kk@verfaction.de>]

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Hi Craig,

> I would like to prioritise all VoIP traffic on a linux router. I am new
> to QoS, tc and TOS, to please be gentle.
> 
> My logic works like this:
> 
> 1) identify the VoIP packets
> 2) mark packets using iptables (with TOS?)
> 3) use tc to prioritise the marked packets.
> 
> Is this logic correct? If not, where is it flawed?

it can work, VoIP for me is H323, for SIP i haven't checked yet what
their ports are..

> I understand that VoIP used the udp protocol and has packet sized less
> than 250 bytes. Is simply reducing the MTU on the interfaces good enough
> to give better thoughput, without the lag of larger packets trying to
> pass though?

Well, reducing MTU is most probably not a good idea, as you force more
packets benig sent even when you can keep the overhead to just 1 packet
with large payload. so i wouldn't do that..
VoIP (speaking of H323) is UDP traffic on random ports but can be
limited by using either GnomeMeeting or nmproxy
(http://www.cryogenic.net/nmproxy.html), thouch i haven't tried the
latter so far..

As of GnomeMeeting you can simply use a match for UDP ports 5000-5003
(on a direct connection)...
For OhPhone and OpenMCU etc. the ports may vary.. If you don't use H245
tunneling there's also some TCP ports involved (for GM 30000-30010).

Using a GateKeeper will also have another port-range.. but checking
either source, mailinglist or support info of these products will shed
some light where you'll be most lucky matching their packets..

> Are there any good HOWTOS?

None that i know of, sorry..

Good luck! ;)

-- 
Best regards,
 Kilian

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