Basic Information About Time To Live (TTL)

What Is TTL?

Time to live (TTL), also known as hop limit, is a mechanism that limits the lifetime or lifespan of data on a computer or network. It refers to the time or “hops” that a packet will exist within the network before being dropped by a router.

TTL determines how long the computer refreshes its DNS related information. It can be implemented as a timestamp or counter attached to or embedded in data. Once the specified period or event count has passed, the data is discarded or re-validated.

In computer networking, TTL prevents packets from looping indefinitely. In computing applications, TTL is typically used to improve performance and manage data caching.

How Does TTL Work?

The primary function of TTL is to manage packets of information related to DNS requests.

When users create and send a packet of information over the Internet, it may continue to pass the information from the router to the router indefinitely. The packet is designed with expiration to reduce this possibility, which is called the time to live or hop limit. TTL can also determine how long the packet is in circulation and allow senders to receive information about packet paths over the Internet.

Each packet has a location that stores a numerical value telling how long it should continue to move around the network. When the router gets a packet, it subtracts one packet from the TTL count and passes the packet to the next location in the network. If the TTL count is equal to zero at any point after subtracting, the router will drop the packet and send the ICMP message back to the original host.

TTL is used for both common network command pings and traceroutes. When using trace routing commands, packets flow with higher-order TTLs are sent to the destination over the Internet. Each step of the connection is the last stop of one of the packets, so each location returns an ICMP message to the sender after dropping the packet. The time that the ICMP message is returned to the sender is then used to determine the time it takes to reach each continuous hop along with the network.

What Does TTL Use For?

In addition to tracking the route packets transmitted over the Internet, TTL is used in the context of cached information over a while. Some network use cases run in a more traditional way than measuring hop times between routers, which may take time to change between each router.

CDN, like Cloudflare CDN, typically uses TTL to determine how long cached content should be available before a new copy is obtained from the CDN Edge server. By setting the amount of time between the source server pulls correctly, the CDN can provide updated content without requesting continuous propagation back to the source. This optimization allows the CDN to deliver content closer to the user effectively while reducing the bandwidth required by the source.

In the context of DNS records, TTL is a numeric value that tells how long the DNS cache server spent serving DNS records before contacting the authoritative DNS server and obtaining a new copy of the records.

Conclusion

Now, whether or not you’ve ever heard of TTL, after reading this article, you should have a holistic understanding of it.