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In computer and computer network technology, time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions that a unit of data (e.g. a record) can experience before it should be discarded.
Time to live of IP datagrams
In IPv4, time to live (TTL) is an 8-bit field in the Internet Protocol (IP) header. It is the 9th octet of 20. The time to live value can be thought of as an upper bound on the time that an IP datagram can exist in an internet system. The TTL field is set by the sender of the datagram, and reduced by every host 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 ( 11 - Time Exceeded) is sent back to the sender. The purpose of the TTL field is to avoid a situation in which an undeliverable datagram keeps circulating on an internet system, and such a system eventually becoming swamped by such immortal datagrams.
In theory, 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 hop. To reflect this practice, the field is named hop limit in IPv6.
See also
External links
- http://serversniff.net/ipinfo.php Serversniff.net shows initial TTLs of a host's IPv4-Datagramms
- http://www.httpsniffer.com/http/1431.htm Max-Forwards as TTL header in HTTP.
- http://rfc-gnutella.sourceforge.net/developer/testing/messageArchitecture.html Gnutella TTL and Hops header values used for preventing loops and monitoring of network topology.
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