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The i option flag runs the report at a faster rate to reveal packet loss that can occur only during network congestion.
Network Diagnostics Background
This flag instructs MTR to send one packet every n seconds. The default is 1 second, so setting it to a few tenths of a second 0. Open WinMTR, type the destination host in the box as prompted, and select the start option to begin generating report data. Because MTR reports contain a great deal of information, they can be difficult to interpret at first. The following example is a report from a local connection to google. The report was generated with mtr --report google.
This uses the report option, which sends 10 packets to the host google. Without the --report option, mtr will run continuously in an interactive environment. The interactive mode reflects current round trip times to each host. In most cases, the --report mode provides sufficient data in a useful format. Each numbered line in the report represents a hop. Hops are the Internet nodes that packets pass through to get to their destination. The names for the hosts e. If you want to omit the rDNS lookups you can use the --no-dns option, which produces output similar to the following:.
Beyond simply seeing the path between servers that packets take to reach their host, MTR provides valuable statistics regarding the durability of that connection in the seven columns that follow. The Snt column counts the number of packets sent.
The next four columns Last , Avg , Best , and Wrst are all measurements of latency in milliseconds e. Last is the latency of the last packet sent, Avg is average latency of all packets, while Best and Wrst display the best shortest and worst longest round trip time for a packet to this host. In most cases, the average Avg column should be the focus of your attention. The final column, StDev , provides the standard deviation of the latencies to each host.
The higher the standard deviation, the greater the difference is between measurements of latency. Standard deviation allows you to assess if the mean average provided represents the true center of the data set, or has been skewed by some sort of phenomena or measurement error. If the standard deviation is high, the latency measurements were inconsistent. After averaging the latencies of the 10 packets sent, the average looks normal but may in fact not represent the data very well.
If the standard deviation is high, take a look at the best and worst latency measurements to make sure the average is a good representation of the actual latency and not the result of too much fluctuation. In most circumstances, you may think of the MTR output in three major sections. The hops in between are the routers the packet traverses to reach its destination.
The last 3 hops belong to the data center where your Linode resides. Any hops in the middle are intermediate hops.
When running MTR locally, if you see an abnormality in the first few hops near the source, contact your local service provider or investigate your local networking configuration. Conversely, if you see abnormalities near the destination you may want to contact the operator of the destination server or network support for that machine. Unfortunately, in cases where there are problems on the intermediate hops, both service providers will have limited ability to address the issue.
When analyzing MTR output, you are looking for two things: loss and latency. If you see a percentage of loss at any particular hop, that may be an indication that there is a problem with that particular router. This can give the illusion of packet loss when there is in fact no loss. If that hop shows a loss of 0. In this case, the loss reported between hops 1 and 2 is likely due to rate limiting on the second hop. Although traffic to the remaining eight hops all touch the second hop, there is no packet loss. If the loss continues for more than one hop, than it is possible that there is some packet loss or routing issues.
Remember that rate limiting and loss can happen concurrently. In this case, take the lowest percentage of loss in a sequence as the actual loss:. You can assume that the third and fourth hop is likely losing some amount of traffic because no subsequent host reports zero loss. When different amounts of loss are reported, always trust the reports from later hops.
Some loss can also be explained by problems in the return route. Packets will reach their destination without error, but have a hard time making the return trip. Resist the temptation to investigate or report all incidences of packet loss in your connections.
The Internet protocols are designed to be resilient to some network degradation, and the routes that data takes across the Internet can fluctuate in response to load, brief maintenance events, and other routing issues. In addition to helping you assess packet loss, MTR will also help assess the latency of a connection between your host and the target host. By virtue of physical constraints, latency always increases with the number of hops in a route.
However, the increases should be consistent and linear. When evaluating MTR reports for potentially problematic connections, consider earlier fully functional reports as context in addition to known connection speeds between other hosts in a given area. The connection quality may also affect the amount of latency you experience for a particular route. Predictably, dial-up connections will have much higher latency than cable modem connections to the same destination. The following MTR report shows a high latency:. The amount of latency jumps significantly between hops 3 and 4 and remains high.
This may point to a network latency issue as round trip times remain high after the fourth hop. From this report, it is impossible to determine the cause although a saturated peering session, a poorly configured router, or a congested link are frequent causes. Unfortunately, high latency does not always mean a problem with the current route.
A report like the one above means that despite some sort of issue with the 4th hop, traffic is still reaching the destination host and returning to the source host. Latency could be caused by a problem with the return route as well. The return route will not be seen in your MTR report, and packets can take completely different routes to and from a particular destination.
In the above example, while there is a large jump in latency between hosts 3 and 4 the latency does not increase unusually in any subsequent hops. From this it is logical to assume that there is some issue with the 4th router.
ICMP rate limiting can also create the appearance of latency, similar to the way that it can create the appearance of packet loss:. At first glance, the latency between hops 4 and 5 draws attention. However after the fifth hop, the latency drops drastically. The actual latency measured here is about 40ms.
In cases like this, MTR draws attention to an issue which does not affect the service. Consider the latency to the final hop when evaluating an MTR report.