From: rick.jones2@hp.com   
      
   Mark  wrote:   
   > I've encountered the following statement somewhere in the net:   
   > "Although theoretically only 8 bytes of L4 info may be guaranteed in   
   > a fragment, assume complete L4 info is available...". I don't   
   > understand how possible that only 8 bytes of a transport are   
   > guaranteed in a fragment, as the IP fragment can't be less than 46   
   > bytes (minimum payload size for Ethernet frame), and this includes   
   > 20 bytes of IP header and 20bytes of TCP header (not considering   
   > variable-lngth options), UDP will be less.   
      
   > Thus for the first IP fragment we always can expect IP header at tcp   
   > header, while other fragments will carry only IP header + payload.   
      
   > I believe I'm missing something, but I still can't understand why   
   > only 8 bytes can be guaranteed in a fragment? I would appreaciate if   
   > someone helps to clarify this issue. Thanks !   
      
   I seem to recall something about the minimum IPv4 MTU being something   
   like 68 bytes.  IP MTU is IP header + IP payload and does not include   
   layer2 stuff.  And while the IP header is 20 bytes, I think there can   
   be some number of IP options - perhaps 40 bytes worth.  Thus, after a   
   68 byte minimum IPv4 MTU, 20 bytes of IP header and 40 bytes of IP   
   options, there are 8 bytes left for the L4 headers and data.   
      
   From RFC 791 we have:   
      
       Every internet module must be able to forward a datagram of 68   
       octets without further fragmentation.  This is because an internet   
       header may be up to 60 octets, and the minimum fragment is 8 octets.   
      
   rick jones   
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