XPost: comp.os.linux.networking   
   From: spam@andyfurniss.entadsl.com   
      
   Mark Hobley wrote:   
   > My LAN is ethernet based. However, for connectivity to the internet, I have   
   > an Edimax 3g-6200n router, which connects to the internet via a 3g mobile   
   > broadband stick.   
   >   
   > Can a 3g mobile broadband stick route the 1500 byte packets arriving from   
   > the LAN without fragmentation, or should I reduce the size of the MTU for   
   > outbound traffic? If so, what should I reduce it to? (My internet service   
   > provider is Three Mobile, if that matters.)   
      
   Don't know, but if it works OK for all sites then I guess so.   
      
   That's a bit oversimplistic because I have no idea about 3G devices, eg.   
   they may be doing some mss clamping that reduces the size of packets in   
   tcp connections.   
      
   Tcpdump/wireshark etc will show this - but don't just rely on looking at   
   one connection as you may be just seeing how they have set things up   
   rather than learning about your ISP.   
      
   I have seen threads where people claim better results by using smaller   
   MTU due to smaller packets having more chance of getting through rf   
   noise type loss, so experimenting if you have problems could improve   
   things - if you don't have problems I wouldn't bother.   
      
      
   >   
   > My next question is ...   
   >   
   > Will Linux allow me to configure two MTU sizes on a single network interface,   
   > depending on whether or not the destination address is local or remote?   
   >   
   > If I need to reduce the MTU for outbound traffic, can I keep using a 1500   
   > byte MTU for LAN traffic?   
      
   Like Rick has said you can do things with ip route. Another way would be   
   to use iptables to mss clamp non local tcp connections.   
      
   >   
   > (The computers have only one ethernet connection onto a single subnet. There   
   > is a single ethernet based router (Netgear) before the Edimax unit, which   
   > forwards internet bound traffic via the Edimax unit).   
      
   A quick test I just did using ip route shows it is possible to just   
   modify the default route, which I assume is all you would need.   
      
   Normally if you set mtu on eth then the tcp advertised mss will also   
   adjust so that incoming tcp packets are also limited in size. Using ip   
   route this doesn't happen, but you can use an extra commant to specify.   
      
   Testing on this PC which already has a default route setup to   
   192.168.0.1, the command -   
      
   ip route change dev eth0 default via 192.168.0.1 mtu 1000 advmss 960   
      
   seems to do the trick and leaves traffic to local net unchanged.   
      
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