From: pk@pk.invalid   
      
   On Wednesday 3 June 2009 17:43, Pascal Hambourg wrote:   
      
   > Hello,   
   >   
   > David Schwartz a écrit :   
   >>   
   >> I can't figure out what you're talking about. What does "enable PMTU   
   >> on our end-points" mean? You mean enable PMTU detection?   
   >   
   > PMTU discovery, I guess.   
   >   
   >> If the   
   >> endpoints do PMTU detection, it doesn't matter what the middle does.   
   >> The endpoints will figure out the largest packet that arrives   
   >> unfragmented.   
   >   
   > Unless some device in the middle behaves badly and does not send an ICMP   
   > "fragmentation needed (and DF set)" (or ICMPv6 "packet too big" if using   
   > IPv6) message when discarding too big a packet. As an example I can   
   > think about some xDSL architectures cascading PPPoE links (MTU 1492) and   
   > L2TP tunnels with MTU 1500 at the link layer level without intermediate   
   > routing.   
      
   Right, or another problem is when there is some device in the return path   
   that blocks such ICMP packets, so the source doesn't receive the PMTU   
   information, thus breaking the PMTU discovery process.   
      
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