Traceroute is a network diagnostic tool used to track the route taken by a packet on an IP network from source to destination. It also calculates and displays the amount of time each hop took. Traceroute uses Internet Control Message Protocol (ICMP) echo packets with variable time to live (TTL) values in case of windows machine, it uses UDP packet in case of Linux.
Before executing the command traceroute, lets discuss few very important and basic facts.
Each IP packet that we send on the internet has got a field called as TTL. TTL stands for Time To Live. Although its called as Time To Live, its not actually the time in seconds, but its something else.TTL is not measured by the no of seconds but the no of hops. Its the maximum number of hops that a packet can travel through across the internet, before its discarded.
Hops are nothing but the computers, routers, or any devices that comes in between the source and the destination
What if there was no TTL at all?. If there was no TTL in an IP packet, the packet will flow endlessly from one router to another and on and on forever searching for the destination. TTL value is set by the sender inside the IP packet ( the person using the system, or sending the packet, is unaware of these things, but is automatically handled by the operating system ).If the destination is not found after traveling through too many routers in between ( hops ) and TTL value becomes 0 (which means no further travel) the receiving router will drop the packet and informs the original sender.
Original sender is informed that the TTl value exceeded and it cannot forward the packet further.
Let's say I need to reach 8.8.8.8 IP address, and my default TTL value is 30 hops (in case of windows) or 64 hops (in case of linux). Which means I can travel a maximum of 30 hops to reach my destination, before which the packet is dropped.But how will the routers in between determine the TTL value limit has reached. Each router that comes in between the source and destination will go on reducing the TTL value before sending to the next router. Which means if i have a default TTL value of 30, then my first router will reduce it to 29 and then send that to the next router across the path.The receiving router will make it 28 and send to the next and so on. If a router receives a packet with TTl of 1 (which means no more further traveling, and no forwarding ), the packet is discarded. But the router which discards the packet will inform the original sender that the TTL value has exceeded.
The information send by the router receiving a packet with TTL of 1 back to the original sender is called as "ICMP TTL exceeded messages". Of course in internet when we send something to a receiver, the receiver will come to know the address of the sender.Hence when an ICMP TTL exceeded message is sent by a router, the original sender will come to know the address of the router.
Traceroute makes use of this TTL exceeded messages to find out routers that come across your path to destination(Because these exceeded messages send by the router will contain its address).
But how does Traceroute uses TTL exceeded message to find out routers/hops in between?
You might be thinking, TTL exceeded messages are only send by the router that receives a packet with TTL of 1. That's correct, every router in between you and your receiver will not send TTL exceeded message. Then how will you find the address of all the routers/hops in between you and your destination. Because the main purpose of Traceroute is to identify the hops between you and your destination.
Lets take an example diagram of the whole process in the below diagram, where a sender does a traceroute towards one of the servers a remote location.
So let's say I want to do a traceroute to google's publicly available DNS server(8.8.8.8). My traceroute command and its result will look something like the below.
trace route 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
After looking at the output we might get so many questions like why the max hop is 64, why we are getting three different time for each hop, we will discuss all this point soon.
When I have executed that command of traceroute 8.8.8.8, what my computer does is to make a UDP packet (Yeah its UDP, because I am running this on linux machine ). This UDP packet will contain the following things.
- My Source Address (Which is my IP address)
- Destination address (Which is 8.8.8.8)
- And A destination UDP port number which is invalid. Means the traceroute utility will send packet to a UDP port in the range of 33434 to 33534, Which is normally unused.
So Let's see how this thing works.
Step 1: My Source address will make a packet with destination ip address of 8.8.8.8 and a destination port number between 33434 to 33534. And the important thing it does it to make the TTL Value 1
Step 2: Of course my packet will reach my gateway server. On seeing receiving the packet my gateway server will reduce the TTL by 1 (All routers/hops in between does this job of reducing the TTL value by 1). Once the TTL is reduced by the value of 1 (1-1= 0), the TTL value becomes zero. Hence my gateway server will send me back a TTL Time exceeded message. Please remember that when my gateway server sends a TTL exceeded message back to me, it will send the first 28 byte header of the initial packet i send.
Step 3: On receiving this TTL Time exceeded message, my traceroute program will come to know the source address and other details about the first hop (Which is my gateway server.).
Step 4: Now the traceroute program will again send the same UDP packet with the destination of 8.8.8.8, and a random UDP destination port between 33434 to 33534. But this time i will make the initial TTL 2. This is because my gateway router will reduce it by 1 and then forwards that same packet which send to the next hop/router (the packet send by my gateway to its next hop will have a TTL value of 1).
Step 5: On receiving UDP packet, the next hop to my gateway server will once again reduce it to 1 which means now the TTL has once again become 0. Hence it will send me back a ICMP Time exceeded message with its source address, and also the first 28 byte header of the packet which i send.
Step 6: On receiving that message of TTL Time Exceeded, my traceroute program will come to know about that hop/routers IP address and it will show that on my screen.
Step 7: Now again my traceroute program will make a similar UDP packet with again a random udp port with the destination address of 8.8.8.8. But this time the ttl value is made to 3, so that the ttl will automatically become 0, when it reaches the third hop/router(Please remember that my gateway and the next hop to it, will reduce it by 1 ). So that it will reply me with a TTL Time exceeded message, and my traceroute program will come to know about that hop/routers IP address.
Step 8: On receiving that reply, the traceroute program will once again make a UDP packet with TTL value of 4 this time. If i gets a TTL Time exceeded for that also, then my traceroute program will send a UDP packet with TTL of 5 and so on.
But how will my traceroute program come to know that the final destination of 8.8.8.8 has reached. The traceroute program will come to know about that because, when the original receiver of the packet 8.8.8.8 (remember that all UDP packet had a destination address of 8.8.8.8) gets the request it will send me a message that will be completely different from all the messages of "TTL Time exceeded".
When the original receiver (8.8.8.8) gets my UDP packet, it will send me a "ICMP Destination/PORT Unreachable" message. This is bound to happen because we are always sending a random UDP port between 33434 to 33534. Hence my Traceroute program will come to know that we have reached the final destination and will stop sending any further packets.
Now lets take a Wireshark packet capture to understand this in practical.
From the above output we can see the UDP and the ICMP packet (since I am running the traceroute on my linux machine , I will describe this in more detail later in the post).
Notice the TTL value its first sets to 1 line. It starts from TTL of 1 and then 2, and then 3 so on. One important point to note here is that the trace route sends 3 UDP packet with the same ttl value in different sequence lets say if the first sequence is 1/256, then the second will be 2/256 and third will be 3/512 with the same ttl vale 1 , we can confirm the same from the below output.
But you might be wondering why my server is sending 3 UDP messages with TTL value of 1 and then 2 and then 3.?The reason behind this is to calculate an average Round Trip Time. Traceroute program sends three UDP packets to each hop to measure the exact average round trip time. Round trip time is nothing but the time it took to send and then receive the reply in milliseconds.
So the bottom line is my traceroute program sends three UDP packets to each hop to simply calculate the round trip average. because the traceroute output shows you those three values in its output. Please see the traceroute output more closely. It shows three millisecond values for each hop. To get a clear idea about the round trip time.
traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
1 10.200.200.200 (10.200.200.200) 256.162 ms 251.602 ms 253.542 ms
One more interesting thing to note is that each time my traceroute program is sending a different random UDP port number. This is to identify the reply belonged to which packet. As told before the reply messages send by the hops and destination contains the header of original packet we send, hence traceroute program can accurately calculate the round trip time (For each three UDP packets send to each hop), as it can easily identify the reply and correlate. The random port numbers are sort of identifiers to identify the reply.
The reply messages looks like the below.
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