TTL

TTL can refer either to : 

• the lifetime of a packet in a network can do before being released
• the expiry time of cached data

Networking

In networking, the TTL, embedded in the packet, is a usually defined as a number of hops or as an expiration timestamp after which the packet is dropped. It provides a way to avoids network congestion, but releasing packets after they roamed the network too long.

Caching

In the context of caching, TTL (as an unsigned 32-bit integer) being a part of the HTTP response header or the DNS query, indicates the amount of time in seconds during which the ressource can be cached by the requester. 

Learn more

General knowledge

• TTL on Wikipedia

Technical reference

• RFC 2181 on IETF

• RFC1035 on IETF