Time to Live

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The time to live ( TTL , German lifetime ) is the period of validity that is given to data in computer networks .

Internet Protocol

With the Internet Protocol (IP), the TTL of a data packet indicates the remaining maximum number of hops passed (an intermediate station such as a router ) in the network. Originally, each intermediate station was supposed to reduce the TTL value by the length of time the packet stayed in seconds, but at least by one. In practice, the TTL value indicates the number of hops, since the TTL of the Internet Protocol is very rarely implemented according to the Internet standard. In IPv6 , the TTL was replaced by a hop limit of the same size and a time limit was given to higher protocols, but the old name is often still used. The data field for the TTL is one octet (8-bit field), so the maximum possible value is 255. If a data packet has not yet reached its destination after its TTL has expired, it is discarded. The last station should send the ICMP response type 11: Time exceeded with code 0: Time to live exceeded in transit to the sender. This method is used for. B. in the Traceroute program .

Domain Name System

In the Domain Name System (DNS), the TTL of each resource record indicates how long (in seconds) a name resolution that has just taken place is likely to remain valid. During this period, DNS caching can be used. After the TTL has expired, the client should discard the respective name resolution and repeat it if necessary.

While a short TTL can lead to a considerable load on servers, a long TTL delays necessary updates accordingly. A TTL of 86400 seconds, which is not unusual, means, for example, that a change is only completed after 24 hours at the latest. In contrast, dynamic DNS services use TTL values ​​of typically 60 seconds so that the new data is used promptly after (more frequent) address changes.

Individual evidence

  1. ↑ a b RFC 8200 - Internet Protocol, Version 6 . Internet Engineering Task Force. See July 28, 2017. Accessed March 27, 2020.
  2. ↑ RFC 791 - Internet Protocol . Internet Engineering Task Force . S. September 13, 1981. Retrieved June 5, 2020.
  3. ↑ traceroute (8) - Linux man page. Retrieved June 5, 2020 .