Time to live (TTL) or hop limit is a mechanism which limits the lifespan or lifetime of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded or revalidated. In computer networking, TTL prevents a data packet from circulating indefinitely. In computing applications, TTL is commonly used to improve the performance and manage the caching of data.
The original DARPA Internet Protocol's RFC document describes [1] TTL as:
The Time to Live is an indication of an upper bound on the lifetime of an internet datagram. It is set by the sender of the datagram and reduced at the points along the route where it is processed. If the time to live reaches zero before the internet datagram reaches its destination, the internet datagram is destroyed. The time to live can be thought of as a self destruct time limit.
IP packetsEdit
Under the Internet Protocol, TTL is an 8-bit field. In the IPv4 header, TTL is the 9th 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.[2][3]
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 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 (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. To reflect this practice, the field is renamed hop limit in IPv6.
DNS recordsEdit
TTL of a DNS answer resolving google.com, seen in Wireshark
TTLs also occur in the Domain Name System (DNS), where they are set by an authoritative name server 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. TTL for NXDOMAIN (non-existent domain) responses is set from the minimum of the MINIMUM field of the SOA record and the TTL of the SOA itself, and indicates how long a resolver may cache the negative answer.[4][jargon]
Shorter TTLs can cause heavier loads on an authoritative name server, 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 reduce possible disruptions.
The units used are seconds. An older common TTL value for DNS was 86400 seconds, which is 24 hours. A TTL value of 86400 would mean that, if a DNS record was changed on the authoritative nameserver, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the last update by client.
Newer DNS methods that are part of a disaster recovery (DR) system may have some records deliberately set extremely low on TTL. For example, a 300-second TTL would help key records expire in 5 minutes to help ensure these records are flushed quickly worldwide. This gives administrators the ability to edit and update records in a timely manner. TTL values are "per record" and setting this value on specific records is sometimes honored automatically by all standard DNS systems worldwide. However, a problem persists in that some caching DNS nameservers set their own TTLs regardless of the authoritative records, thus it cannot be guaranteed that all downstream DNS servers have the new records after the TTL has expired.
HTTPEdit
Time to live may also be expressed as the date and time on which a record expires. The Expires: header in HTTP responses, the Cache-Control: max-age header field in both requests and responses and the expires field in HTTP cookies express time-to-live in this way.
^"DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION". IETF. September 1981.
^. Archived from the original on 2013-02-12. Retrieved 2013-02-19.
^"IP OPTION NUMBERS". 2012-11-30. Retrieved 2013-02-19. The current recommended default time to live (TTL) for the Internet Protocol (IP) is 64 [RFC791], [RFC1122].
^<mark.andrews@cmis.csiro.au>, Mark Andrews (1998). "Negative Caching of DNS Queries (DNS NCACHE)". tools.ietf.org. doi:10.17487/RFC2308. Retrieved 2018-11-12.
External linksEdit
Gnutella TTL and Hops header values used for preventing loops and monitoring of network topology
October 20, 2023
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For the film see Time to Live film Time to live TTL or hop limit is a mechanism which limits the lifespan or lifetime of data in a computer or network TTL may be implemented as a counter or timestamp attached to or embedded in the data Once the prescribed event count or timespan has elapsed data is discarded or revalidated In computer networking TTL prevents a data packet from circulating indefinitely In computing applications TTL is commonly used to improve the performance and manage the caching of data Contents 1 Description 2 IP packets 3 DNS records 4 HTTP 5 See also 6 References 7 External linksDescription EditThe original DARPA Internet Protocol s RFC document describes 1 TTL as The Time to Live is an indication of an upper bound on the lifetime of an internet datagram It is set by the sender of the datagram and reduced at the points along the route where it is processed If the time to live reaches zero before the internet datagram reaches its destination the internet datagram is destroyed The time to live can be thought of as a self destruct time limit IP packets EditUnder the Internet Protocol TTL is an 8 bit field In the IPv4 header TTL is the 9th 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 2 3 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 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 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 To reflect this practice the field is renamed hop limit in IPv6 DNS records Edit nbsp TTL of a DNS answer resolving google com seen in WiresharkTTLs also occur in the Domain Name System DNS where they are set by an authoritative name server 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 TTL for NXDOMAIN non existent domain responses is set from the minimum of the MINIMUM field of the SOA record and the TTL of the SOA itself and indicates how long a resolver may cache the negative answer 4 jargon Shorter TTLs can cause heavier loads on an authoritative name server 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 reduce possible disruptions The units used are seconds An older common TTL value for DNS was 86400 seconds which is 24 hours A TTL value of 86400 would mean that if a DNS record was changed on the authoritative nameserver DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the last update by client Newer DNS methods that are part of a disaster recovery DR system may have some records deliberately set extremely low on TTL For example a 300 second TTL would help key records expire in 5 minutes to help ensure these records are flushed quickly worldwide This gives administrators the ability to edit and update records in a timely manner TTL values are per record and setting this value on specific records is sometimes honored automatically by all standard DNS systems worldwide However a problem persists in that some caching DNS nameservers set their own TTLs regardless of the authoritative records thus it cannot be guaranteed that all downstream DNS servers have the new records after the TTL has expired HTTP EditTime to live may also be expressed as the date and time on which a record expires The Expires header in HTTP responses the Cache Control max age header field in both requests and responses and the expires field in HTTP cookies express time to live in this way See also EditHop telecommunications Ping networking utility tracerouteReferences Edit DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION IETF September 1981 Default TTL Values in TCP IP Archived from the original on 2013 02 12 Retrieved 2013 02 19 IP OPTION NUMBERS 2012 11 30 Retrieved 2013 02 19 The current recommended default time to live TTL for the Internet Protocol IP is 64 RFC791 RFC1122 lt mark andrews cmis csiro au gt Mark Andrews 1998 Negative Caching of DNS Queries DNS NCACHE tools ietf org doi 10 17487 RFC2308 Retrieved 2018 11 12 External links EditDefault IP packets Time To Live TTL values Gnutella TTL and Hops header values used for preventing loops and monitoring of network topology Retrieved from https en wikipedia org w index php title Time to live amp oldid 1175682487, wikipedia, wiki, book, books, library,
