XPost: comp.os.linux.networking   
   From: rick.jones2@hp.com   
      
   In comp.protocols.tcp-ip Andy Furniss  wrote:   
      
   > 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 think there must be an exception in there somewhere - until it was   
   found to be a vector for an amplification attack, years ago HP-UX did   
   a "hybrid" PathMTU discovery by default.  When it first started   
   talking to a remote IP, it would send IP datagrams with DF cleared -   
   however, at the same time, it would send an ICMP Echo Request to the   
   remote with the DF bit set.  It would lather, rinse and repeat until   
   it could get the Echo reply without generating any datagram too big   
   messages.  I believe that was to "deal" with black holes where the   
   ICMP Datagram Too Big messages were being filtered.   
      
   rick jones   
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