TTL (Time to Live) is a term used to describe a number of computer networking principles. The portion of the TCP/IP protocol that establishes an expiry timestamp for each data transmission sent over a network can be referred to as this phrase. It can also refer to the amount of time a computer keeps content in cache before updating it.

TCP/IP

A data file that is sent over the Internet from one machine to another moves from one network to another until it finds its target. A "hop" is the term used to describe every movement of the trip. Each data packet that travels through a network adds a tiny amount of latency, so packets that are unable to reach their target would cause the network to function noticeably worse if they continued to bounce around. Every data transmission is sent with a TTL timer to avoid this from happening and keep it from bouncing around indefinitely.

The TTL count of a data transmission is an 8-bit value (between 1 and 255, usually fixed in the center between 32 and 128) that indicates how many steps it can make a total of before it expires and is destroyed. The TTL value is reduced by 1 for each step a data transmission makes between routers. The gateway discards it if the TTL count drops to 0 before it reaches its target.

Cache

TTL can also be used to describe how long a website retains a copy of data available for access before updating it. For instance, a DNS server will have a TTL value for each record that determines how long it can service a record before updating it. This value is expressed in seconds. TTL entries for these records can be made as low as 30 seconds, but records with lesser precedence can be set for as long as 86,400 seconds (24 hours).

Every object that is stored on a CDN server will have a TTL number that specifies how long it should retain a cached duplicate before checking the parent server for updates. As a result, frequently changed files can be checked frequently by the CDN while other assets can be stored for days or weeks between changes.