From: lacos@caesar.elte.hu   
      
   On Sun, 26 Sep 2010, Mark Hobley wrote:   
      
   > On Sat, 25 Sep 2010 18:53:41 +0000, Mark Hobley wrote:   
   >   
   >> The application that is communicating to the foreign host, may use a   
   >> smaller segment size for its communications to the foreign host   
   >   
   > Actually, this raises an interesting question: Supposing that I am using   
   > HTTP protocol, via a remote proxy to a webserver as follows:   
   >   
   > Machine A	Machine B	Machine C   
   > Browser		HTTP Proxy	Web Server   
   >   
   > Machines A and B above have a Maximum Segment Size of 1500. However, the   
   > web server machine (C) has a smaller Maximum Segment Size (lets say   
   > 512).   
   >   
   > Does Machine B tell Machine A that the Maximum Segment Size is 512, so   
   > that queries from the browser are enclosed in segments of that size? Or   
   > does the browser still send its queries in segments of 1500, and the   
   > proxy server fragments them into smaller 512 segments? If the proxy   
   > server is fragmenting them, then presumably this defeats the object of   
   > using PMTUD in the first place, in which case, wouldn't it be better to   
   > not bother with PMTUD?   
      
   The discovery and fragmentation work in the IP layer. The HTTP proxy works   
   in the TCP (and application) layers. Once Machine B is reached, the IP   
   layer's job is done. That job recommences towards Machine C. The proxy is   
   not a router, but an application; it can rewrite requests, for example.   
      
   Machine A and Machine B are constant links in the above chain. Suppose a   
   TCP connection services multiple proxy requests (persistent connections   
   from A to B), and the path between them doesn't change for considerable   
   time. Machine B connects to lots of different HTTP servers (or further   
   proxies) scattered all over the net. Machine C is variable, and thus each   
   type of B-C link is different. Therefore it makes sense for the discovered   
   A-B and B-C PMTU's to differ and to have different lifetimes.   
      
   Or something like that.   
      
   lacos   
      
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