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維基百科，自由的百科全書
存活時間（英語：Time To Live，簡寫TTL）是電腦網路技術的一個術語，指一個封包在經過一個網路時，可傳遞的最長距離（躍點數）。每當封包經過一個路由器時，其存活時間就會被減一。當其存活時間為0時，路由器便會取消封包並傳送一個ICMP TTL封包給原封包的發出者。其設計目的是防止封包因不正確的路由表等原因造成的無限循環而無法送達及耗盡網路資源。
DNS
在網域名稱系統 (DNS)中亦有利用「存活時間」這個概念，用以設定網域名稱紀錄的最長快取時間。
原文出處：存活時間 - 維基百科，自由的百科全書
Time to live
Time to live (TTL) or hop limit is a mechanism that limits the lifespan or lifetime of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data packet from circulating indefinitely. In computing applications, TTL is used to improve performance of caching or to improve
privacy.
IP packets
Under the Internet Protocol, TTL is an 8-bit field. In the IPv4 header, TTL is the 9th octet of 20. In the IPv6 header, it is the 8th octet of 40. The maximum TTL value is 255, the maximum value of a single octet. A recommended initial value is 64.
The time-to-live value can be thought of as an upper bound on the time that an IP In theory, under IPv4, time to live is measured in seconds, although every host that passes the datagram must reduce the TTL by at least one unit. In practice, the TTL field is reduced by one on every TTLs also occur in the Domain Name System (DNS), where they are set by an Shorter TTLs can cause heavier loads on an authoritative nameserver, but can be useful when changing the address of critical services like Web servers or MX records, and therefore are often lowered by the DNS administrator prior to a service being moved, in order to minimize disruptions. The units used are seconds. An older common TTL value for DNS was 86400 seconds, which is 24 hours. A TTL value of 86400 would mean that, if a DNS record was changed on the authoritative nameserver, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the change. Newer DNS methods that are part of a DR ( Time to live may also be expressed as a date and time on which a record expires. The
datagram can exist in an Internet system. The TTL field is set by the sender of the datagram, and reduced by every router on the route to its destination. If the TTL field reaches zero before the datagram arrives at its destination, then the datagram is discarded and an ICMP error datagram (
hop. To reflect this practice, the field is renamed hop limit in IPv6. DNS records
authoritative name server for a particular resource record. When a caching (recursive) nameserver queries the authoritative nameserver for a resource record, it will cache that record for the time (in seconds) specified by the TTL. If a stub resolver queries the caching nameserver for the same record before the TTL has expired, the caching server will simply reply with the already cached resource record rather than retrieve it from the authoritative nameserver again. TTL for NXDOMAIN responses is set from the minimum of the MINIMUM field of the SOA record and the TTL of the SOA itself, and indicates how long a resolver may cache the negative answer. [ jargon] HTTP
Expires: header in HTTP responses and the expires field in HTTP cookies express time-to-live in this way. See also
References
The current recommended default time to live (TTL) for the Internet Protocol (IP) is 64 [RFC791], [RFC1122].
External links