XPost: comp.os.linux.networking   
   From: markhobley@yahoo.donottypethisbit.co   
      
   On Wed, 29 Sep 2010 18:57:11 +0000, Jorgen Grahn wrote:   
      
   > ["Followup-To:" header set to comp.protocols.tcp-ip.]   
   >   
   > On Wed, 2010-09-29, Mark Hobley wrote:   
   >> On Wed, 29 Sep 2010 17:16:12 +0000, Mark Hobley wrote:   
   >>   
   >> I have some more weirdness now. If I ping from one host to another,   
   >> connected via ethernet, I would expect a maximum datagram size of 1500.   
   >> However, I actually get a result of 65535, as follows:   
   >>   
   >> # PMTUD is disabled on both hosts:   
   >> echo 0 > /proc/sys/net/ipv4/ip_no_pmtu_disc   
   >>   
   >> ping -s 65507 miranda   
   >> PING miranda.markhobley.yi.org (10.0.0.30) 65507(65535) bytes of data.   
   >> 65515 bytes from miranda.markhobley.yi.org (10.0.0.30): icmp_seq=1   
   >> ttl=64 time=13.8 ms   
   >>   
   >> Does this mean that jumbo frames have become enabled on my network?   
   >   
   > No, it means you should install and use[1] tcpdump to see what this   
   > actually translates to on the Ethernet level.  Hint: IP fragmentation.   
   >   
      
   Right. I have done a tcpdump, at first examining the internet bound stuff:   
      
   With PMTUD enabled:   
      
   21:13:47.557102 IP venus.markhobley.yi.org > ir1.fp.vip.ac4.yahoo.com: ICMP   
   echo request, id 52485, seq 4, length 1392   
   21:13:47.811766 IP ir1.fp.vip.ac4.yahoo.com > venus.markhobley.yi.org: ICMP   
   echo reply, id 52485, seq 4, length 1392   
      
   With PMTUD disabled:   
   21:19:24.453588 IP venus.markhobley.yi.org > ir1.fp.vip.re1.yahoo.com: ICMP   
   echo request, id 58885, seq 6, length 1480   
   21:19:24.976825 IP ir1.fp.vip.re1.yahoo.com > venus.markhobley.yi.org: ICMP   
   echo reply, id 58885, seq 6, length 1480   
      
   So, with PMTUD disabled, I can send and receive packets of 1480 to and from   
   yahoo,   
   presumably fragmentation and reassembly is taking place en-route and I cannot   
   see this.   
      
   With PMTUD enabled, my maximum packet size if 1392.   
      
   Now, if PMTUD is disabled, why do I now have a limit of 1480? I would have   
   expected packets   
   larger than this to become fragmented and transmitted successfully.   
      
   This actually does happen on the LAN. Here are those 65535 long being   
   fragmented at   
   1480 for transportation over ethernet:   
      
   21:34:00.635725 IP venus.markhobley.yi.org > miranda.markhobley.yi.org: ICMP   
   echo request, id 64005, seq 2, length 1480   
   21:34:00.635760 IP venus.markhobley.yi.org > miranda.markhobley.yi.org: icmp   
      
   blah blah blah ...   
      
   21:34:00.640468 IP venus.markhobley.yi.org > miranda.markhobley.yi.org: icmp   
   21:34:00.643685 IP miranda.markhobley.yi.org > venus.markhobley.yi.org: ICMP   
   echo reply, id 64005, seq 2, length 1480   
   21:34:00.643804 IP miranda.markhobley.yi.org > venus.markhobley.yi.org: icmp   
      
   blah blah blah ...   
      
   21:34:00.648989 IP miranda.markhobley.yi.org > venus.markhobley.yi.org: icmp   
   21:34:04.638350 ARP, Request who-has venus.markhobley.yi.org tell   
   miranda.markhobley.yi.org, length 46   
   21:34:04.638376 ARP, Reply venus.markhobley.yi.org is-at 00:11:2f:bc:df:19   
   (oui Unknown), length 28   
      
   I guess upstream have set a limit of 1480, even though they have to fragment   
   to carry this.   
      
   There is another strange thing I noticed. Why does miranda make an arp request   
   for venus at   
   the end of the exchange? I would have thought that miranda already knows the   
   mac address   
   of venus, because she has just been communicating with it.   
      
   Mark.   
      
   --   
   Mark Hobley   
   Linux User: #370818  http://markhobley.yi.org/   
      
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