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I have been running MTR to/from one of my servers and noticed something that looks odd to me. Because I am not really into this I will give you three outputs:

This is from the server to my home location:

My traceroute  [v0.75]
prag341.server4you.de (0.0.0.0)                                                                                                                         Sat Apr 16 12:31:36 2011
Keys:  Help   Display mode   Restart statistics   Order of fields   quit
                                                                                                                                        Packets               Pings
 Host                                                                                                                                 Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. v9-609a.s4y14.fra.routeserver.net                                                                                                  0.0%   143    6.6   2.9   0.7  15.6   2.4
 2. 217.118.16.161                                                                                                                     0.0%   143    0.7   5.7   0.4  67.3  13.2
 3. 217.118.16.25                                                                                                                      0.0%   143    3.3   5.3   3.3  63.5   8.6
 4. 194.25.211.53                                                                                                                      0.0%   143    3.4   5.5   3.2  61.1   9.1
 5. vie-sb2-i.VIE.AT.NET.DTAG.DE                                                                                                       0.7%   143   17.8  21.7  17.6 131.1  14.8
    vie-sb2-i.VIE.AT.NET.DTAG.DE
 6. at-vie05b-ri1-pos-5-0.aorta.net                                                                                                    0.7%   143   18.7  18.4  17.6  23.8   0.9
 7. at-vie05b-ri2-ge-2-1-9.aorta.net                                                                                                   0.0%   143   17.9  18.6  17.5  41.7   2.6
 8. at-vie01a-rd1-xe-1-0-0.aorta.net                                                                                                   0.0%   143   18.2  21.1  17.3 104.1  12.0
 9. at-vie-sk11-pe01-vl-20.upc.at                                                                                                      0.0%   143   18.2  20.6  17.7  55.7   7.0
10. at-vie-sk11-pe02-vl-1.upc.at                                                                                                       0.0%   143   17.8  19.6  17.3  55.2   6.6
11. ???

This is from my home location to the server:

My traceroute  

[v0.80]
joe-desktop (0.0.0.0)                                                                                                                                   Sat Apr 16 14:27:54 2011
Keys:  Help   Display mode   Restart statistics   Order of fields   quit
                                                                                                                                        Packets               Pings
 Host                                                                                                                                 Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. 192.168.1.1                                                                                                                        0.0%    87    0.2   0.2   0.2   0.2   0.0
 2. ???
 3. 84.116.4.33                                                                                                                        0.0%    86    9.7   9.0   6.3  27.3   3.5
 4. at-vie-sk11-cia01-vl-2070.upc.at                                                                                                   0.0%    86   22.7  22.8  20.0  52.2   4.7
 5. at-vie-sk11-pe01-vl-2069.upc.at                                                                                                    0.0%    86   47.6  23.9  20.2  47.6   5.8
 6. at-vie01a-rd1-vl-2042.aorta.net                                                                                                    0.0%    86   21.7  25.0  20.1  61.7   8.5
 7. de-fra03a-rd1-xe-9-2-0.aorta.net                                                                                                   0.0%    86   21.3  22.8  19.6  44.0   5.0
 8. 84.116.132.154                                                                                                                     0.0%    86   20.2  22.8  19.3  41.0   4.1
 9. tge-5-1-0-353a.cr2.fra.routeserver.net                                                                                             0.0%    86   38.6  27.4  20.9 120.2  16.0
10. 217.118.16.130                                                                                                                     0.0%    86   23.7  26.9  20.8  73.0   9.8
11. 217.118.16.26                                                                                                                      0.0%    86   25.5  28.8  22.9  85.1  11.8
12. 217.118.16.165                                                                                                                    81.2%    86   68.2  37.5  25.0  68.2  10.3
13. prag341.server4you.de                                                                                                              0.0%    86   35.7  27.1  24.0  49.3   4.3

And this is from another server (amazon ec2) to the server:

My traceroute  [v0.75]
flimmit.com (0.0.0.0)                                                                                                                                   Sat Apr 16 12:32:50 2011
Keys:  Help   Display mode   Restart statistics   Order of fields   quit
                                                                                                                                        Packets               Pings
 Host                                                                                                                                 Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. ip-10-48-192-3.eu-west-1.compute.internal                                                                                          0.0%   178    0.4   0.9   0.3  16.4   1.7
    ip-10-48-192-2.eu-west-1.compute.internal
 2. ec2-79-125-0-244.eu-west-1.compute.amazonaws.com                                                                                   0.0%   178    0.5   0.9   0.3  30.8   2.6
    ec2-79-125-0-242.eu-west-1.compute.amazonaws.com
 3. ???
 4. ???
 5. ???
 6. xe-4-1-0.dub10.ip4.tinet.net                                                                                                      36.5%   178    1.9   3.9   1.6  56.8   8.5
 7. xe-4-1-0.dub10.ip4.tinet.net                                                                                                       0.0%   178   12.1   9.7   1.6  92.5  10.5
    xe-0-1-0.lon14.ip4.tinet.net
    xe-2-1-0.lon14.ip4.tinet.net
 8. xe-0-1-0.lon14.ip4.tinet.net                                                                                                       0.0%   177   17.4  17.7  11.1 184.3  24.6
    xe-2-1-0.lon14.ip4.tinet.net
    213.200.77.234
 9. 213.200.77.234                                                                                                                     0.0%   177   25.2  23.7  12.0 162.5  16.0
    tge-4-2-0-0a.cr2.fra.routeserver.net
10. tge-4-2-0-0a.cr2.fra.routeserver.net                                                                                               0.6%   177  178.6  57.1  24.7 178.6  39.0
    217.118.16.26
11. 217.118.16.26                                                                                                                     47.2%   177   32.7  61.1  29.1 164.4  35.4
    217.118.16.165
12. 217.118.16.165                                                                                                                    28.2%   177   28.9  29.8  27.8  48.9   4.2
    prag341.server4you.de
13. prag341.server4you.de                                                                                                              1.1%   177   28.2  28.7  27.7  63.4   2.9

What looks weird to me is this very high loss >80% on the last hop from My home location to the server. The server is responding fine and services run smoothly.

It may be due to my lacking knowledge of networking but it would sound logical to me that loss rates should add up? But I often see MTR outputs where there are high loss rates on the way, but the final target loss is much lower.

So my questions are:

  • In my particular case, is this an indicator of a possible problem I should pay attention to?

  • In general, how do I interpret an output of mtr correctly? Can you recommend a good article / literature on that?

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2 Answers

up vote 6 down vote accepted

The packet loss is not necessary an indication of a problem. Remember those are attempts to communicate with that particular network node directly. Usually those in-between router nodes are only responsible for passing traffik through to another location. They are not required to chat with you directly at all, and one that drops most of your chat should not be a cause for concern. The only important number for you would be how many packets are getting through to your destination.

The most useful information to come out of those reports is the relative data of how far apart nodes are (in terms of packet time), and, even more importantly, how many hops there are so you can get an idea how long different legs of the journey will take for people trying to communicate with your servers. Usually the fewer hops there are, the more efficient the route -- indicating the quality of your ISP.

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ahh, okay! so in other words spot latency bottlenecks? –  The Shurrican Apr 16 '11 at 12:48
    
+1. Very good answer. –  joeqwerty Apr 16 '11 at 12:54
    
The only issue I have is with your second paragraph. The latency (response time) from each successive node is not an indication or a measurement of the total latency in the path. The response times are not cumulative. As you'll see in the trace output there are several nodes with much higher latency than the target node. Still a good answer, though. –  joeqwerty Apr 16 '11 at 13:04
    
You are correct, they are not cumulative, which is why I noted all that data as being relative. It's not an easy guide, but sometimes the relative differences can give you some idea of what might be happening at those locations and if you are a network administrator of any of those nodes gives you the change to judge the relative effectiveness of different segments of your network. Otherwise it does't tell you much except roughly where your traffic is being routed through. For this user it should mean next to nothing. –  Caleb Apr 16 '11 at 13:11
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MTR is good at measuring latency and hops from site to site. It is commmon to have 100% loss starting at the firewall in front of a reachable site. I usually set the interval to 15 seconds or more to lighten the network load. It takes a little longer to get results, but I find the results are more reliable.

Some routers give lower priority to generating the error packet that MTR uses to detect intermediate routers. If the router is busy, they may drop the packet, and wait for the next one. This will cause high drop rates for that router. If there are routers further away with 0% loss things are fine.

Packet losses may also occur when the routes are dynamically changing. Your last trace shows the routes changing over time. Theses should be relatively short term and easily recovered.

Loss rates can be indicative of an overloaded router. If there are connection or packet loss problems, I start investigating at the closest router with a loss rate above 0%.

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