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[Openvpn-users] Multicast fragmenting not handled by OpenVPN ?

  • Subject: [Openvpn-users] Multicast fragmenting not handled by OpenVPN ?
  • From: James MacLean <macleajb@xxxxxxxxxxx>
  • Date: Wed, 21 May 2003 20:59:38 -0300 (ADT)

Hi Folks,

I am seeing a problem which occurs both OpenVPN, GRE and mrouted's 
builtin tunnels.

If a large Multicast packet gets sent to the tunnel, it gets fragmented, 
or atleast broken up :). But when it comes out the other side, it appears 
to never get reconstructed into it original format.

For some reason, this makes tunneled connections get a limited max 
bandwidth of around 270Kbs. Almost like the kernel is getting slowed down 
by them?

If I set the MTU at the originating machine down to something smaller 
like 1100, Linux fragements the packets at the source, they pass through 
the tunnel un-split, and appear to work fine end to end.

This is not only OpenVPN but also on GRE tunnels and mrouted builtin 
tunnels. GRE and mrouted both use ipip.o in the Linux kernel if that 

VIC and Rat appear to not be affected by this because their packets are 
mostly smaller around 512 bytes I think.

To see it in action, Get mp4live running (part of mpeg4ip.sourceforge.net) 
in multicast through a tunnel. I tried both mrouted and pimd. The results 
will be that the fastest throughput you'll get is around 270Kbs :).

Maybe this is just particular to Linux? Or it is expected?

OpenVPN 1.4.1 and RedHat 9.0.

James B. MacLean        macleajb@xxxxxxxxxxx
Department of Education 
Nova Scotia, Canada
B3M 4B2

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