From: pk@pk.invalid   
      
   On Wednesday 3 June 2009 18:21, Alan S wrote:   
      
   >>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.   
   >   
   > Which is correct here?   
      
   PMTU discovery is done by the endpoints only, and it works fine if there are   
   no black holes in the middle. If there are broken hosts in the middle (that   
   don't send or block ICMP messages on which PMTU discovery relies upon),   
   PMTU discovery won't work, as the endpoints won't be able to gather the   
   necessary information. An alternative algorithm to do PMTU discovery which   
   does not rely on ICMP is described in RFC 4821.   
      
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