TTL meaning and definition

Time is a critical factor to be considered almost for everything. Every project, business, process, fixing, etc., you try is marked by a specific time. Computing and networking are not exceptions. Lots of processes must happen in determined periods of time, actually in milliseconds, to be considered efficient and successful. 

TTL meaning and definition

The letters TTL are the initials for time-to-live. It is the value that points to the exact period of time or number of hops that data packet is configured to be alive on a network or in the cache memory. Once that time expires, or it hops the number of times, routers will discard it. There are different kinds of data-chunks, and they all work with their specific TTL. Meaning, the time such data will be held in a device to complete determined tasks or functions.  

How does TTL work?

The huge amount of packets that are sent daily all across the Internet needed a way to be controlled. Otherwise, you could have them traveling around routers permanently. A limit of time or expiration on every data packet was the measure to avoid this and to get information about them. For instance, to know how long they have been around and track their route on the Internet. 

Packets have a purpose, a final destination, but they have to travel through different network points to reach it. Within the data packet’s design, there is a spot where the TTL value is stored.

When routers receive a packet, they can obtain the TTL value. If there is remaining time/hops, the packet will be passed to the network’s next point. But if the lecture of this value shows the packet’s TTL is expired (zero seconds remaining), routers won’t pass it anymore. 

Instead of transfer it, routers will dispatch an ICMP message (Internet Control Message Protocol). The kind of message used for reporting IP errors or diagnosis. ICMP messages are addressed directly to the IP address source where that packet was emitted.

Time is really revealing! Every ICMP message sent from the different routers to inform the packet’s expiration will take a specific time to arrive in the packet’s source. Through that time is possible to track the hops packets did while alive on the network (route).

What else is TTL used for?

TTL is an important value to control the time that information will be valid. Limiting its time life reduces the amount of data going around not to stress systems. TTL also helps to keep everything up to date. 

  • Networking. TTL stops packets not to travel indefinitely. It helps to know the time packets will be around and to track their route on the Internet.
  • Content Delivery Network (CDN) caching. Networks use TTL to know the time their cache servers should save all the content they distribute (docs, photos, video, etc.). Once the TTL is over, servers discard the information they have. Then they make a new request to primary servers for getting updated data and caching them again. The same process will be repeated when the new TTL expires. CDNs are big networks with a presence (servers, data centers…) in different points of the planet. TTL helps them to keep copies of the content time enough not to saturate with constant requests from their primary servers. They optimize the process, reduce bandwidth and serve faster.
  • Domain Name System (DNS) caching. TTL value shows the time DNS resolving servers have to hold DNS records in their cache. There are different records involved, and everyone has their own TTL. When the TTL of those records is over, resolving servers must require a new records’ copy to the authoritative DNS server. They will cache the records again until the new TTLs expire.

Recommended article: DNS propagation – meaning


No doubt, time is helpful for accomplishing tasks and functions efficiently. And it is absolutely revealing too – a good partner for you to have everything under control.