XPost: comp.os.linux.networking   
   From: spam@andyfurniss.entadsl.com   
      
   Mark Hobley wrote:   
      
   > Right. I have done a tcpdump, at first examining the internet bound stuff:   
      
   Remember - don't just test one address, try the f.root-servers.net that   
   Morten suggested.   
      
   > So, with PMTUD disabled, I can send and receive packets of 1480 to and from   
   yahoo,   
   > presumably fragmentation and reassembly is taking place en-route and I   
   cannot see this.   
      
   If you use the -v switch for tcpdump you will see whether the replies   
   have DF set or not.   
      
   >   
   > With PMTUD enabled, my maximum packet size if 1392.   
   >   
   > Now, if PMTUD is disabled, why do I now have a limit of 1480?   
      
   That 1480 is IP data so add 20 = 1500 it's a bit hit or miss whether   
   you'll get a server on the internet to answer to a > 1500.   
      
   ICMPs with DF set aren't the same as other packets with DF set because   
   AIUI ICMP packets aren't allowed to generate further ICMPs so there will   
   be no PMTUD to help them get through.   
      
   I would have expected packets   
   > larger than this to become fragmented and transmitted successfully.   
      
    > 1500 works for me to f.root-servers.net but not yahoo, even if the   
   target doesn't answer tcpdump should show you outgoing fragged echo request.   
      
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