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Re: ping packet loss when size gt 1500



Chris Davies on 18/10/10 13:15, wrote:
Adam Hardy <adam.ant@cyberspaceroad.com> wrote:
I tried lowering the MTU to 1400 but it made no difference.

So ping -s 1372 failed? I thought you said it worked up to -s 1472
(packets of 1500 bytes)?

When you drop your MTU to 1400, that means that your local data packets
are fragmented as necessary to stay within the maximum packet size. If
you generate data with a "do not fragment" ("DF") label attached to it,
you have to ensure that each packet is no larger than the available
size. (All fairly obvious so far, I trust.)

If something between you and the destination is eating the "Hey you're
MTU's too big for this link" messages then there's no way of handling
overly large packets - as far as the recipient is concerned they have
been discarded en route.

Remember that although your packets might be the right size, there is
(AFAICR) nothing there that limits the size of the packets being returned
to you. And if the "too big" ICMP messages relating to data from the
far end back to you are being discarded then the sender can't know it
needs to reduce the packet size in order to reach you.

So if there is a "black hole router" between me and the server I'm having problems with, I still haven't identified it. When I said I reduced my MTU, that was just on the one machine that runs the software that communicates so poorly with the server at mktgw1.ibllc.com

I did detect a host at hop 5 on the traceroute to the server which was losing packets according to my monitoring programs, but now the traceroute has changed and that server's not on the route. Actually now I'm checking again, it is back. British Telecom are playing around obviously.

I have a question about traceroute that might be relevant. I haven't figured out the traceroute options because on lenny, my traceroute doesn't complete. The last hop to mktgw1.ibllc.com doesn't show - it just counts stars up to 30. But on windows it does complete at 23 - where mktgw1.ibllc.com is the last hop / target. Does this betray anything?

lenny traceroute (from gateway 192.168.0.2)
 1  192.168.1.1 (192.168.1.1)  0.577 ms  1.303 ms  1.282 ms
 2  217.32.146.168 (217.32.146.168)  7.520 ms  7.742 ms  8.567 ms
 3  217.32.146.222 (217.32.146.222)  10.190 ms  11.197 ms  11.333 ms
 4  213.120.177.58 (213.120.177.58)  11.760 ms  12.444 ms  13.441 ms
 5  213.120.176.62 (213.120.176.62)  14.376 ms  14.843 ms  15.810 ms
 6  213.120.176.182 (213.120.176.182)  16.794 ms  6.072 ms  6.350 ms
7 acc2-10GigE-0-2-0-4.l-far.21cn-ipp.bt.net (109.159.249.225) 7.532 ms acc2-10GigE-0-2-0-5.l-far.21cn-ipp.bt.net (109.159.249.227) 7.672 ms acc2-10GigE-0-0-0-4.l-far.21cn-ipp.bt.net (109.159.249.194) 9.562 ms 8 core2-te0-14-4-0.ealing.ukcore.bt.net (109.159.249.157) 10.984 ms core2-te0-11-5-0.ealing.ukcore.bt.net (109.159.249.141) 11.736 ms core2-te0-14-4-0.ealing.ukcore.bt.net (109.159.249.157) 12.092 ms 9 transit2-xe0-1-0.ealing.ukcore.bt.net (194.72.17.82) 12.067 ms 13.004 ms 13.899 ms
10  t2c2-ge11-0-0.uk-eal.eu.bt.net (166.49.168.61)  14.442 ms  15.320 ms  16.005 ms
11 so-6-0-2.edge1.London2.Level3.net (212.113.11.253) 17.451 ms 17.698 ms 18.323 ms
12  ae-32-52.ebr2.London2.Level3.net (4.68.117.62)  27.142 ms  8.414 ms  19.868 ms
13  ae-3-3.ebr1.London1.Level3.net (4.69.141.189)  8.819 ms  9.165 ms  10.011 ms
14 ae-100-100.ebr2.London1.Level3.net (4.69.141.166) 10.478 ms 11.449 ms 12.280 ms 15 ae-41-41.ebr1.NewYork1.Level3.net (4.69.137.66) 81.797 ms 82.604 ms ae-42-42.ebr1.NewYork1.Level3.net (4.69.137.70) 83.437 ms
16  ae-4-4.ebr1.NewYork2.Level3.net (4.69.141.18)  83.826 ms  84.588 ms  85.464 ms
17 ae-1-51.edge2.NewYork2.Level3.net (4.69.138.195) 86.162 ms 86.585 ms 87.297 ms 18 mci-level3-oc48-washington1.level3.net (4.68.127.22) 95.632 ms mci-level3-xe.newyork2.Level3.net (4.68.110.106) 87.545 ms 0.ge-2-0-0.BR3.NYC4.ALTER.NET (204.255.173.53) 87.854 ms 19 0.ae3.XL4.NYC4.ALTER.NET (152.63.16.186) 88.335 ms 88.235 ms 0.ae2.XL4.NYC4.ALTER.NET (152.63.3.117) 88.418 ms 20 0.so-7-1-0.XL4.BOS4.ALTER.NET (152.63.0.221) 91.572 ms 92.351 ms 0.ge-7-0-0.XL3.BOS4.ALTER.NET (152.63.0.165) 92.589 ms 21 POS7-0-0.GW12.BOS4.ALTER.NET (152.63.22.181) 91.460 ms 91.469 ms POS6-0-0.GW12.BOS4.ALTER.NET (152.63.22.177) 92.245 ms 22 interactivebrokers-gw.customer.alter.net (208.192.181.62) 100.881 ms 101.943 ms 102.812 ms
23  * * *
24  * * *
25  * * *
26  * * *
27  * * *
28  * * *
29  * * *
30  * * *

and the windows traceroute:
  1     1 ms    <1 ms    <1 ms  isengard.localdomain [192.168.0.2]
  2     4 ms     1 ms     1 ms  192.168.1.1
  3     7 ms     7 ms     6 ms  217.32.146.168
  4     7 ms     7 ms     7 ms  217.32.146.222
  5     9 ms    11 ms     9 ms  213.120.177.58
  6     7 ms     *        8 ms  213.120.176.54
  7     8 ms     7 ms     8 ms  213.120.176.182
8 7 ms 7 ms 7 ms acc2-10GigE-0-1-0-4.l-far.21cn-ipp.bt.net [109.159.249.198] 9 9 ms 8 ms 8 ms core2-te0-14-4-0.ealing.ukcore.bt.net [109.159.249.157] 10 8 ms 8 ms 8 ms transit2-xe11-1-0.ealing.ukcore.bt.net [194.72.17.194]
 11     8 ms     8 ms     8 ms  t2c2-ge13-0-0.uk-eal.eu.bt.net [166.49.168.53]
 12     9 ms     9 ms     8 ms  195.50.91.169
 13     9 ms    13 ms    18 ms  ae-32-52.ebr2.London2.Level3.net [4.68.117.62]
 14     9 ms     8 ms     9 ms  ae-3-3.ebr1.London1.Level3.net [4.69.141.189]
 15     9 ms     7 ms     9 ms  ae-100-100.ebr2.London1.Level3.net [4.69.141.166]
 16    78 ms    78 ms    77 ms  ae-41-41.ebr1.NewYork1.Level3.net [4.69.137.66]
 17    77 ms    77 ms    77 ms  ae-4-4.ebr1.NewYork2.Level3.net [4.69.141.18]
 18    98 ms    78 ms    77 ms  ae-1-51.edge2.NewYork2.Level3.net [4.69.138.195]
 19   105 ms    80 ms    81 ms  4.68.111.138
 20    80 ms    81 ms    81 ms  0.ae3.XL3.NYC4.ALTER.NET [152.63.16.182]
 21    88 ms    87 ms    87 ms  0.so-7-1-0.XL3.BOS4.ALTER.NET [152.63.0.209]
 22    88 ms    92 ms    87 ms  POS6-0-0.GW12.BOS4.ALTER.NET [152.63.22.177]
23 96 ms 95 ms 95 ms interactivebrokers-gw.customer.alter.net [208.192.181.62]
 24    95 ms    96 ms    96 ms  mktgw1.ibllc.com [208.245.107.9]

(The route is slightly different, I copied the traceroute from the windows machine yesterday but took the lenny traceroute today)


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