[RndTbl] Curious traceroute (actually, mtr)

[David Milton]

This is normal for many busy routers. It’s a self defence mechanism where ICMP is always low priority (and UDP echo even more so) when compared to normal TCP/UDP transit traffic. ICMP must be handled directly by the router rather than “transit” traffic which these days is handled directly by the hardware. Routing takes priority and for busy routers it’s not uncommon for ICMP to show longer response time or packet loss.

> On Jul 6, 2023, at 23:57, Brian Lowe <brian2 at groupbcl.ca> wrote:
> 
> When I do an mtr from my home to Wikipedia, the host ae20.3518.ear1.Chicago3.level3.net <http://ear1.chicago3.level3.net/> shows a significant packet loss:
> 
>                               My traceroute  [v0.92]
> laptop (192.168.1.1)                                2023-07-06T23:29:23-0500
> Keys:  Help   Display mode   Restart statistics   Order of fields   quit
>                                          Packets               Pings
> Host                                    Loss%   Snt   Last   Avg  Best  Wrst
> 1. gateway                               0.0%    21    0.4   0.4   0.3   0.4
> 2. (ip address redacted)                 0.0%    21    8.8   9.0   8.0  11.6
> 3. rc2nr-be103-1.wp.shawcable.net <http://rc2nr-be103-1.wp.shawcable.net/>        0.0%    20    8.9   9.4   8.5  10.1
> 4. 24.244.61.17                          0.0%    20    9.9   9.9   8.4  12.1
> 5. 24.244.60.173                         0.0%    20    9.4   9.7   8.9  11.3
> 6. rc2nr-be1.wp.shawcable.net <http://rc2nr-be1.wp.shawcable.net/>            0.0%    20    9.9   9.4   8.4  10.2
> 7. rc3sc-be26.wp.shawcable.net <http://rc3sc-be26.wp.shawcable.net/>           0.0%    20    9.8  10.0   9.0  12.3
> 8. rc4ec-be25-1.il.shawcable.net <http://rc4ec-be25-1.il.shawcable.net/>         0.0%    20   26.2  25.4  24.1  28.2
> 9. lag-104.ear5.Chicago2.Level3.net <http://lag-104.ear5.chicago2.level3.net/>      0.0%    20   25.3  25.3  23.9  28.8
> 10. ae20.3518.ear1.Chicago3.level3.net <http://ear1.chicago3.level3.net/>  78.9%    20   26.9  25.6  24.2  26.9
> 11. WIKIMEDIA-F.ear1.Chicago3.Level3.net <http://wikimedia-f.ear1.chicago3.level3.net/> 0.0%    20   24.9  25.5  24.2  27.5
> 12. xe-5-2-1.cr2-codfw.wikimedia.org <http://xe-5-2-1.cr2-codfw.wikimedia.org/>     0.0%    20   74.3  75.2  73.5  77.8
> 13. text-lb.codfw.wikimedia.org <http://text-lb.codfw.wikimedia.org/>          0.0%    20   74.5  73.7  69.4  76.7
> 
> What I suspect is happening is only ICMP packets are being lost, not every packet going through that host, since Wikipedia doesn't seem to have degraded performance. Anyone care to offer other explanations or speculation?
> 
> Brian
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