Networking & Internet
Time to Live
Definition
Time to Live (TTL) is a mechanism that limits the lifespan or lifetime of data in a computer or network. In an IP packet, TTL is an 8-bit field. Each router that forwards the packet decrements the TTL value by one. If the TTL value reaches zero, the router discards the packet.
Why It Matters
TTL is a crucial safety mechanism that prevents data packets from circulating endlessly on the internet in the event of a routing loop. It ensures that undeliverable packets eventually "die" and are removed from the network.
Contextual Example
The `traceroute` utility works by sending packets with progressively increasing TTL values. It sends a packet with TTL=1, which is discarded by the first router. Then TTL=2, discarded by the second, and so on, to map out the path.
Common Misunderstandings
- TTL is a hop count, not a measure of time in seconds.
- In the context of DNS, TTL has a different meaning: it specifies how long a DNS resolver should cache a DNS record before querying for it again.