Editing Time to live (section)

==IP packets==
Under the [[Internet Protocol]], TTL is an 8-bit field. In the [[IPv4 header]], TTL is the 9th [[octet (computing)|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.<ref>{{cite web |url=http://www.map.meteoswiss.ch/map-doc/ftp-probleme.htm |title=Default TTL Values in TCP/IP |access-date=2013-02-19 |archive-date=2013-02-12 |archive-url=https://web.archive.org/web/20130212114759/http://www.map.meteoswiss.ch/map-doc/ftp-probleme.htm |url-status=dead }}</ref><ref>{{cite web |url=https://www.iana.org/assignments/ip-parameters/ip-parameters.xml |quote=The current recommended default time to live (TTL) for the Internet Protocol (IP) is 64 [RFC791], [RFC1122]. |title=IP OPTION NUMBERS |date=2012-11-30 |access-date=2013-02-19}}</ref>

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 [[router (computing)|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 [[Internet Control Message Protocol]] (ICMP) error datagram ([[ICMP Time Exceeded|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 "immortals".

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 [[Hop (networking)|hop]]. To reflect this practice, the field is renamed ''hop limit'' in [[IPv6]].