TTL (Time To Live) explained

We live in an environment where time is probably one of the most critical factors in our everyday life. Computing and networking are not any different. Many of the processes frequently must happen in a specific period of time. Here comes TTL in hand. In some cases, the task should be finished in milliseconds. Can you imagine that? Let’s make things a little bit more precise and explain what TTL actually is?

What is TTL?

TTL is the short acronym for time-to-live. It refers to the value that points to the exact period of time or number of hops that the data packet is configured to be alive on a network. In some cases, also in the cache memory. When that time expires, or it hops the number of times, routers will discard it. There exist many different varieties of data chunks. Every and each of them operates with their particular TTL. That means the time such data will be held in a device to function or finish a certain task.

How does it work?

If the massive amount of packets is not controlled, they will travel around routers permanently. The way to avoid this is with a limit of time or expiration on every data packet. This allows understanding how long they have been around and track their route on the Internet.

Packets travel through network points with the purpose of reaching their final destination. There is a spot inside the data packets’ design where the TTL value is placed.

Routers receive the TTL value inside the packet. It will pass to the next network point if there is spare time or hops. But if the value of TTL shows that there is no more remaining hops/time, routers won’t pass it anymore.

Instead, routers will send an ICMP (Internet Control Message Protocol) message. This type of message is used to report IP errors or diagnoses and directs to the IP address source, which issued the packet.

It will take a specific time for every ICMP message to arrive at the source. During that time, it is likely to track the hops it made while alive on the network.

TTL and DNS 

TTL in DNS finds its place for the time that the DNS resolvers have to keep the DNS records in their cache. Every DNS record has its assigned TTL value. When it is of the record is longer, there is less chance that the value will change. Therefore other records with a lot and often changes will be with a shorter value.

And because DNS requests are also packets of data, they have their TTL value inside. It would be a very interesting case if they didn’t have such limitations. DNS queries would constantly go from server to server and never finding a destination. TTL value acts as a stop mechanism of a DNS request and prevents endless search for an answer and pointless stress on the Domain Name System. The value begins with a larger number and gets decreased until it comes to zero by the routers.