By Shaun Behrens • Mar 16, 2018

What You Need to Know About Traceroute

Bonnie and Clyde. Salt and pepper. Peanut butter and jelly.

Some things fit so well together that it is almost impossible to separate them when talking or thinking about them. And so it is with ping and traceroute. Whether you are a network n00b or a veteran administrator, ping and traceroute are probably your first two ports of call when troubleshooting network connectivity or latency issues. Since we covered ping in a previous post, it makes sense that we now take a look at its slightly more capable cousin. 

While ping can tell you if there is a problem, traceroute can help point to where the problem is. And, like ping, the beauty of traceroute lies in its simplicity: it's a tool that any user with access to a command prompt can run. Yet, despite this ease of use, there is a fairly good chance you will misinterpret its results if you do not know how it works.

In this post we'll look at how traceroute does what it does, and then in a future post I will give some tips for interpreting the results. 

Traceroute: It Traces the Route

Most platforms offer traceroute as a tool, such as TRACERT on Windows, or TRACEROUTE on Linux and Mac. These tools all essentially do the same thing: map the  route that data takes from a point in a network to a specific IP server. To get between these two points, data must travel - or "hop" - through a series of devices, such as routers or switches. For each hop on the way to the destination device, traceroute provides the data's Round-Trip Time (RTT) and, when possible, the name and IP address of the device.

How does it do this? Here it gets a bit trickier to explain succinctly, so stay with me!

TTL

Traceroute makes use of a network mechanism called TTL, or "Time to Live". The purpose of TTL is to limit how long data will "live" in an IP network. Each packet of data that is sent out is assigned a TTL value - for example, "30". When a data packet reaches a hop (such as a router) on the way to the destination device, the TTL value is decreased by 1. So, for example, once our data packet with a TTL of "30" has passed through five devices (or hops), it will have a TTL of "25". In other words, it can only make 25 more hops before it runs out of time to live.

If a data packet's TTL reaches "0", the data is not routed further, but is dropped. This way, if there is a problem with routing in the network, the data packet will not be passed around indefinitely. At the point a device drops a packet, it sends an ICMP message back to the source device to let it know the packet has been killed.

Still with me? Hang on just a little longer!

Probing the Hops

Traceroute makes sure that each hop on the way to a destination device drops a packet, and sends back an ICMP error message. Why? Because then it can measure the duration of time between when the data is sent out, and when the ICMP message is received back for each hop. Now you have the RTT, or "Round-Trip Time", for each hop.

It does this by sending multiple waves of data packets out, increasing the TTL for the packets each time.

This diagram depicts how this works in a Windows environment (click to enlarge):

Traceroute_diagram.png

 

An Example

I'll use an example to demonstrate:

You run a traceroute to a destination device and specify a maximum of 30 hops. Traceroute then does the following:

  1. It sends data packets with a TTL of "1" to the destination server. The first network device the data passes through reduces the TTL to "0", and sends back a message that the packets were dropped. Taking into account when we sent the data and when we receive the message back, we now have the RTT for hop #1.
  2. It sends data packets to the destination server, this time with a TTL of "2". As the packets pass through the first hop, their TTL is reduced to "1" and they are passed on to the next device. When they get to the second hop, the router reduces the TTL to "0", and sends back a message again. We now have the RTT for hop #2.

The above steps are repeated, each time increasing the TTL so the data packets get to the next hop, until the data packets either reach the destination device, or the specified maximum of 30 hops is reached. In the end, you have the number of hops to the destination server, how long the round-trip for each hop took, and, in some cases, the name and IP addresses of the devices at each hop.

For the record, the protocol used for the probes differs depending on the environment: Windows uses ICMP Echo Requests, and UNIX-like environments use UDP packets. The response is sent back using ICMP, regardless of environment.

If you're still here: thank you. There were points during that explanation when even I thought of shutting down and going home. But there was a reason for going through all of that. Knowing what happens in the background means we can start making sense of the numbers.

Reading the Results

Although the results of a traceroute look slightly different depending on the tool you are using, the core information is the same. Here, as an example, are the results of TRACERT, the Windows traceroute tool:

Tracert_reuters.png

You can see the number of hops (far left column) from the source device (my pc) to the destination device. For each hop, there are three RTT values (the default of TRACERT is to send 3 data packets to test each hop). Finally, on the right you can see additional information that is available for each device.

You can quickly tell that there does not seem to be a problem with the RTT values in the above example - once the data leaves my local network, they remain constant at between 20 and 25 milliseconds for each hop.

By the way, the hops that return asterisks and a "Request timed out" message (hops 12, 13, and 14) might seem like cause for alarm, but this is not necessarily the case: as long as the traceroute completes correctly (as it did in this case), then the destination device was reached. The asterisks simply indicate that some devices along the way were not configured to provide a reply.

Making Sense of the Numbers

If the traceroute times out on a specific hop and does not recover, then you know exactly where the network problem is. The bad news? This is rarely the case. 

In most situations you will be faced with a set of RTT values that you need to interpret to find the problem. To figure out what could be going on behind the scenes, you not only need to know how traceroute works (which I've tried to cover here), but also which factors influence the results.

We'll look at how to accurately interpret a traceroute in a future post. In the meantime, you can always start your troubleshooting with a good old ping: Get your quick fix with our tool that lets you ping a server from multiple geographic locations.