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Re: [Openvpn-users] Re: MULTI: packet dropped due to output saturation (multi_add_mbuf)


  • Subject: Re: [Openvpn-users] Re: MULTI: packet dropped due to output saturation (multi_add_mbuf)
  • From: James Yonan <jim@xxxxxxxxx>
  • Date: Thu, 21 Jul 2005 09:27:35 -0600 (MDT)

On Thu, 21 Jul 2005, Jonas Truemper wrote:

> Hey,
> 
> Jonas Truemper wrote:
> > 
> > MULTI: packet dropped due to output saturation (multi_add_mbuf)
> > 
> 
> does no one have any idea??

This occurs when you have a TCP-connected client on a lower bandwidth 
link, and the OpenVPN server accumlates an excessive backlog of packets 
because it is receiving them faster than it is able to send them to the 
client.

This usually occurs on a TAP-based bridged VPN with TCP clients, and too
much broadcast traffic on the bridged LAN.  Unicast traffic over TCP is
always throttled, so each party to such a connection will throttle down to
the speed that the link can support.

The problem is that broadcast traffic is not throttled, so if you have a 
network with a lot of broadcasts, it can easily overwhelm the link speed 
of a client (even a broadband client).

Two possible solutions (for the TAP-based VPN with clients getting 
saturated by excessive broadcast traffic):

(1) Use a tool such as ebtables to filter out the broadcasts you don't 
need to be forwarded over the VPN.

(2) Use "dev tun" mode instead of "dev tap" mode which will not forward 
broadcast traffic over the VPN.

James


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