Time to live

Time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data (e.g. a packet) can experience before it should be discarded.

Time to live of IP packets

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.

The Unix traceroute command ("tracert" on Windows) depends on the functionality of the TTL field.

Time to live of DNS records

TTLs also occur in the Domain Name System (DNS), where they are set by an authoritative nameserver 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. Nameservers may also have a TTL set for NXDOMAIN (acknowledgment that a domain does not exist); but they are generally short in duration (3 hours at most). 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. A common TTL value for DNS is 86400 seconds, which is 24 hours. A TTL value of 86400 would mean that if a DNS record was changed, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the change.

ee also

* ping
* traceroute

External links

* [http://rfc-gnutella.sourceforge.net/developer/testing/messageArchitecture.html Gnutella TTL and Hops header values used for preventing loops and monitoring of network topology]

List of initial TTL values

*http://members.cox.net/~ndav1/self_published/TTL_values.html
*http://secfr.nerim.net/docs/fingerprint/en/ttl_default.html


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Time Team — Infobox television show name = Time Team format = Archaeology runtime = 60 minutes (including adverts) creator = executive producer = Tim Taylor presenter = Tony Robinson country = United Kingdom network = Channel 4 More4 first aired = 16 January …   Wikipedia

  • Live television — Live TV redirects here. For the British TV station formerly known by this name, see L!VE TV. Live television refers to television broadcast in real time or on a short tape delay basis. It is used in the local news.In general live television was… …   Wikipedia

  • Live Bullet — Live album by Bob Seger the Silver Bullet Band Released April 12, 1976 Recorded Cobo Hal …   Wikipedia

  • Live — (l[i^]v), v. i. [imp. & p. p. {Lived} (l[i^]vd); p. pr. & vb. n. {Living}.] [OE. liven, livien, AS. libban, lifian; akin to OS. libbian, D. leven, G. leben, OHG. leb[=e]n, Dan. leve, Sw. lefva, Icel. lifa to live, to be left, to remain, Goth.… …   The Collaborative International Dictionary of English

  • Time Squad — From left to right, Buck Tuddrussel, Otto Osworth, and Larry 3000 Genre Science fiction Comedy Format Animated se …   Wikipedia

  • Live in Concert (Martina McBride album) — Live in Concert Live album / Video by Martina McBride Released April 29, 2008 …   Wikipedia

  • Time Warner Cable Arena — Former names Charlotte Bobcats Arena (2005–2007) Location …   Wikipedia

  • Time of Judgment — is a series of roleplaying game scenario books for the World of Darkness settings of White Wolf Game Studio. These scenarios are presented as the semi canonical endings of the original World of Darkness, as preparation for the new version of the… …   Wikipedia

  • Live from Death Row — published in May 1995, is a collection of memoirs by American death row inmate Mumia Abu Jamal. Addison Wesley gave Abu Jamal a $30,000 advance for the novel, an action resulting in Maureen Faulkner, Daniel Faulkner s widow, hiring a plane to fl …   Wikipedia

  • Live Aid — infobox music festival music festival name = Live Aid location = London, Philadelphia participation = Madonna, Sting, Phil Collins, Eric Clapton, Bob Geldof, Duran Duran, Neil Young founders = Bob Geldof and Midge Ure years active = 1985 dates =… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.