XPost: comp.os.linux.networking   
   From: spam@andyfurniss.entadsl.com   
      
   Mark Hobley wrote:   
   > On Sun, 22 May 2011 06:01:14 +0000, Jorgen Grahn wrote:   
   >   
   >> And then you have GTP tunneling between the routers (SGSN, GGSN) in the   
   >> 3G network. These frequently pull stupid stunts, too.   
   >>   
   >> I don't know ... you also seem to say that IP fragmentation doesn't work   
   >> for you.  That's so abnormal that perhaps you should debug that first.   
   >   
   > I'm trying :)   
   > What is also weird is that my system has been working with previous kernel   
   > versions. It has only just become broken again.   
   >   
   > I'll try dropping another 12 bytes off the ip route command and retest:   
   >   
   > ip route change default via 10.0.0.1 mtu lock 1400   
   >   
   > Am I right in that the below datagram is wrong, because the mss is 1460,   
   > which is above the value of the mtu in the routing table? Shouldn't the   
   > value of the mtu have been factored in here, and a new mss calculated based   
   > on the mtu?   
   >   
   > 03:43:04.320740 IP venus.markhobley.yi.org.52201>   
   >> news.eternal-september.org.nntp: Flags [S], seq 2304145392, win 14600,   
   >> options [mss 1460,sackOK,TS val 47122576 ecr 0,nop,wscale 5], length 0   
   >   
      
   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   
   - don't send >1400.   
      
   If you want to fix up your wan use iptables mss clamping on the gateway   
   if you can.   
      
   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.   
      
   --- SoupGate-Win32 v1.05   
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