timetolive

Time To Live (TTL) is a reference concept in computing that defines the maximum lifespan of data, packets, or entries within a system. TTL is used in several related but distinct contexts, including IP networking, Domain Name System (DNS), caching, and messaging systems. The common purpose is to bound how long information can remain valid or be propagated before it is discarded or refreshed.

In IP networking, TTL is a field in the IP header (an 8-bit value in IPv4). It specifies how many hops a packet is allowed to traverse. Each router that handles the packet decrements the TTL by one; when TTL reaches zero, the packet is discarded and typically an ICMP Time Exceeded message is sent back to the sender. This mechanism prevents packets from circulating indefinitely in the presence of routing loops and helps to bound delay and resource use. The TTL value can be set by the sender or by routing protocols and devices along the path.

In DNS, TTL applies to cached responses. Each resource record includes a TTL that indicates how long resolvers and caches may reuse the answer before querying authoritative servers again. A higher TTL reduces load and latency but can delay visibility of updates; a lower TTL increases freshness at the cost of more frequent lookups. Negative caching similarly uses TTL to determine how long a non-existent domain result is cached.

TTL concepts also appear in web caching and messaging systems. HTTP caches use TTL-like semantics via Cache-Control directives or Expires headers. In message queues and distributed caches, TTL can define expiration times for messages or entries.

Overall, TTL settings balance data freshness with performance, and the optimal value depends on context, requirements, and policy.