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TTL can refer either to :
- the lifetime of a packet in a network can do before being released ;
- the expiry time of cached data.
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.
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.
- TTL on Wikipedia