How “traceroute” use to identify network troubles

TRACEROUTE Command

Usually, information sent from one computer to another across the network, it does not reach the destination computer in one single jump. In fact, it usually requires several hops or routers to transmit the information. A “traceroute” is a utility program that identifies each router on that way, and the amount of time it takes to get from one computer to another. If there is an interruption in the transfer of data, the “traceroute” will show where in the chain the problem occurred.

On UNIX and Linux computers, the "traceroute" application is available in the shell, while on Windows computers, the "tracert" program can be accessed from DOS(usually command prompt in today).

How Trace route Works

The traceroute command is used to discover the routes that packets actually take when traveling to their destination. Each router along the path is required to decrements the TTL on a packet by at least 1 before forwarding it, so the TTL is effectively a hop count.

If a router finds a TTL value of 1 or 0, it drops the datagram (default send three UDP by router or PC) and sends back an Internet Control Message Protocol (ICMP) Time-Exceeded message to the sender. Traceroute determines the IP address of the first hop by examining the source address field of the ICMP Time-Exceeded message.

To identify the next hop, traceroute sends another three UDP message with a TTL value of 2. The first router decrement the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the Time-Exceeded message to the source. This process continues until the TTL is incremented to a value large enough for the datagram to reach the destination host or until the maximum TTL is reached or the destination host replies with an ICMP Echo Reply.

Running a traceroute

UNIX systems, Mac OS X, and Windows 95 and later On a Unix system, including Mac OS X, run a

traceroute at the command line like this:

traceroute server.name

Windows, the tracert command type command prompt like this:

tracert server.name

How “traceroute” use to identify network troubles

The Traceroute command can be used to help you understand your network infrastructure and troubleshoot network problems. The most common use of the Trace command is to identify where on the network a packet has been dropped on its way to its intended destination. This is very helpful when dealing with a larger network or networks in which there are a several intermediate systems between the source and destination device.

For example, if we can usually reach an Internet site but it is slow today, then a traceroute to that site should show you one or more hops with either long times or marked with "*" indicating the time was really long.

See the below Microsoft command prompt screenshot..

The format of each line is as follows:

Hop RTT1 RTT2 RTT3 Domain Name [IP Address]

• Hops: are the amount of connections in-between the source and destination. There are 12 hops to my PC (walakulu) to destination.
• RTT1, RTT2, RTT3: This is the round-trip time that it takes for a packet to get to a hop and back to your computer (in milliseconds). This is often referred to as latency .Traceroute sends three packets to each hop and displays each time.

We can use to RRT for find the network lettency. Smaller numbers in RTT generally mean better connections.

If traceroute output indicate, a large “jump” in latency from one hop to the next, that could indicate a problem. It could be a saturated (overused) network link; a slow network link; an overloaded router; or some other problem at that hop.

• Domain Name [IP Address]: The domain name, if available, can often help to see the location of a router. If this isn’t available, only the IP address of the router is displayed.

In hear “destination net unreachable” massage display last line because of the(Find yourself......you can search on web)

• If an asterisk (*) appears for RTT, then a packet was not returned within the expected time frame.

1. 1.     One or two asterisks for a hop do not necessarily indicate packet loss at the final destination. Many Internet routers intentionally discard traceroute packets, this used to prevent routers from being impacted by denial-of-service attacks.

1. 2.      Three asterisks followed by the “Request timed out” message may appear for several reasons. 

Asterisks position	Why happened this ?
At the Beginning of a traceroute	Device that doesn’t respond to ICMP or traceroute requests (this is very common situation). See the 1st hope in above screenshot.
At the End of a traceroute	1.      The destination’s firewall or other security device is blocking the request(or source device security, when return route) 2.      There may be a connection problem at that particular system or the next system. See the 10-11 hops in above screenshot.