From: rick.jones2@hp.com   
      
   rahul.dev.agg@gmail.com wrote:   
   > Thanks for your reply. I am just doing it out of curiosity. I am   
   > trying to understand how path mtu discovery works.   
      
   > Can we have two hosts with different mtu connected to the same link   
   > ? Ethernet supports both 1500 and jumbo frames (9000). So, can we   
   > have one host with mtu = 1500 and other with mtu = 9000 on the same   
   > link.   
      
   > If yes, what would be the pmtu for the path between these nodes ?   
   > Can node A find the mtu of node B on the same link ?   
      
   The starting point is this, in pseudo IEEEspeak (which I've probably   
   botched) with a smattering of IETF style:   
      
       All stations in the same broadcast domain MUST have the same MTU.   
      
   Ethernet has no way to communicate frame size between peers.  If a   
   frame larger than the station is prepared to receive arrives, that   
   frame will be dropped.   
      
   Path MTU is up at the IP layer and uses ICMP messages to communicate   
   MTU between hosts.  The PathMTU logic is run when a router looks to   
   forward the IP datagram - when it goes to send it.  That means it must   
   have received it in the first place.  For IP to "receive" the datagram   
   it must first be received at the layer below it - in this case   
   Ethernet.   
      
   However, if the interface on which it was going to receive the   
   datagram has an MTU/framesize smaller than the size of the datagram   
   sent, the datagram won't be "received" by the IP layer so it cannot be   
   resent, so the Path MTU logic cannot trigger.   
      
   Thus the reason why all stations (hosts, systems, what you will) in   
   the broadcast domain (everything joined at layer 2 eg ethernet) MUST   
   have the same MTU.   
      
   Now, if you have a router (a device making forwarding decisions at   
   layer three - eg IP) it will have a foot in two different broadcast   
   domains.  So long as its feet are the correct size for each broadcast   
   domain, you can have different MTUs on each side of the router.  Then,   
   PathMTU discovery will be able to do its thing.  But when all there   
   are between the two hosts are switches (a device making forwarding   
   decisions at layer 2 - eg Ethernet) there is no way for PathMTU   
   discovery to become invovled in the first place.   
      
   rick jones   
   --   
   oxymoron n, commuter in a gas-guzzling luxury SUV with an American flag   
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