Information about 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.
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.
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.
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.
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.
See also
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 (old pages unfortunately):
- [1]
- [2]
computer is a machine which manipulates data according to a list of instructions.
Computers take numerous physical forms. The first devices that resemble modern computers date to the mid-20th century (around 1940 - 1941), although the computer concept and various machines
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Computers take numerous physical forms. The first devices that resemble modern computers date to the mid-20th century (around 1940 - 1941), although the computer concept and various machines
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as a college campus, industrial complex, or a military base. A CAN, may be considered a type of MAN (metropolitan area network), but is generally limited to an area that is smaller than a typical MAN.
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Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. IPv4 is the dominant network layer protocol on the Internet and apart from IPv6 it is the only standard internetwork-layer protocol
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Internet protocol may refer to:
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- The Internet Protocol, a data-oriented protocol used for communicating data across a packet-switched internetwork
- The Internet protocol suite, a set of communications protocols that implement the protocol stack on which the Internet runs
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In computing, an octet is a grouping of eight bits.
In France, French Canada and Romania, the word octet usually means byte; a megabyte (MB) is called a megaoctet in France, Romania and also French Canada. Bit and Byte are homophones in the French language.
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In France, French Canada and Romania, the word octet usually means byte; a megabyte (MB) is called a megaoctet in France, Romania and also French Canada. Bit and Byte are homophones in the French language.
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The Time Exceeded Message is an ICMP message which is generated by a gateway to inform the source of a datagram that the datagram has been discarded due to the time to live field reaching zero.
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Internet Protocol version 6 (IPv6) is a network layer protocol for packet-switched internetworks. It is designated as the successor of IPv4, the current version of the Internet Protocol, for general use on the Internet.
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traceroute is a computer network tool used to determine the route taken by packets across an IP network. An IPv6 variant, traceroute6, is also widely available.
The traceroute tool is available on practically all Unix-like operating systems.
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The traceroute tool is available on practically all Unix-like operating systems.
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On the Internet, the Domain Name System (DNS) associates various sorts of information with so-called domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. en.wikipedia.
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A name server is a computer server that implements a name service protocol. It will normally map a computer-usable identifier of a host to a human-usable identifier for that host. For example, a Domain Name System (DNS) server might translate the domain name en.wikipedia.
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An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. MX records point to the servers that should receive an e-mail, and their priority relative to each other.
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ping is a computer network tool used to test whether a particular host is reachable across an IP network. It works by sending ICMP “echo request” packets to the target host and listening for ICMP “echo response” replies.
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traceroute is a computer network tool used to determine the route taken by packets across an IP network. An IPv6 variant, traceroute6, is also widely available.
The traceroute tool is available on practically all Unix-like operating systems.
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The traceroute tool is available on practically all Unix-like operating systems.
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