Time-to-live
A counter that limits how long something stays valid.
Origin
IP TTL is in the original RFC 791 (1981). DNS TTLs in RFC 1034 (1987). The cache eviction sense is folklore — every cache implementation calls expiration a "TTL."
Where it shows up in production
- DNS TTL determines how long a record stays cached. A change to a 300s-TTL record can take 5 minutes to propagate.
- Redis EXPIRE Per-key TTL on any data type. Lazy + active expiration combined.
- traceroute Sends packets with TTL=1, 2, 3... — each router along the path generates ICMP TTL-exceeded.
On Semicolony
Sources & further reading
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