XPost: comp.os.linux.networking   
   From: spam@andyfurniss.entadsl.com   
      
   Mark Hobley wrote:   
   > On Sun, 22 May 2011 11:40:16 +0100, Andy Furniss wrote:   
   >   
   >> I see the same, but don't know what the behavior should be. If you set   
   >> mtu on say eth0 then advertised tcp mss will be affected (which in a way   
   >> is setting mtu plus mru for tcp) - maybe the route setting really just   
   >> does mtu alone, which in a way is what you asked for   
   >   
   > That would not be very good at all, because it breaks PMTUD. I would   
   > obviously want the MTU to match the route.   
      
   It doesn't break PMTUD as such - if someone sends you a larger packet   
   with DF set than your ISP can send, then your ISP should send a frag   
   needed. The trouble is it may never reach the sender so it's best to   
   avoid the situation arising (for tcp) by advertising a smaller mss.   
      
   >   
   >> If you want to fix up your wan use iptables mss clamping on the gateway   
   >> if you can.   
   >   
   > I had a quick look through the router configuration settings and I could not   
   > see such an option, so I guess that this facility is not provided.   
      
   Ahh, I see now you don't have shell access. It may be clamping anyway as   
   noted elsewhere. Some routers will clamp to the mtu on the wan/ppp   
   connection by default, or maybe your ISP is doing some clamping.   
      
   I would see if the test upload works after you set mtu on the machines   
   nic rather than the route.   
      
   >   
   >> Using ping to try and work out isp behavior can be misleading - you may   
   >> just be seeing the settings of the server/network you are pinging and   
   >> the pings may get fragged/re-assembled without you knowing.   
   >   
   > With PMTUD enabled, the Don't Fragment flag is set though, isn't it? So this   
   > would mean that we have a single datagram end to end.   
      
   A quick test on my box shows that df is only set for ping when < mtu.   
      
   If the ping is larger it gets fragged and reply is also fragged.   
      
   Assuming iputils ping you can force no frag with ping -M do .....   
      
   If you have set mtu low with route/on nic  this will just refuse to send   
    > than that, so to test you need to set higher mtu, and of course be   
   aware that what you see may just depend on the settings/net of the   
   target rather than your ISP.   
      
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