Time to live
Encyclopedia
Time to live is a mechanism that limits the lifespan 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. In computer networking, TTL prevents a data packet from circulating indefinitely. In computing
applications, TTL is used to improve performance of caching
or improve privacy
.
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.
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 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), 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 (three 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 record
s, 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. 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, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the change.
Newer DNS methods that are part of a DR (Disaster Recovery) 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 world wide quickly. 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 normally honored automatically by all standard DNS systems world-wide.
Counter
In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...
or timestamp
Timestamp
A timestamp is a sequence of characters, denoting the date or time at which a certain event occurred. A timestamp is the time at which an event is recorded by a computer, not the time of the event itself...
attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data packet from circulating indefinitely. In computing
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
applications, TTL is used to improve performance of caching
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
or improve privacy
Internet privacy
Internet privacy involves the right or mandate of personal privacy concerning the storing, repurposing, providing to third-parties, and displaying of information pertaining to oneself via the Internet. Privacy can entail both Personally Identifying Information or non-PII information such as a...
.
Under the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, TTL is an 8-bit field. In the IPv4 header, TTL is the 9th octet
Octet (computing)
An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:...
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.
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 ICMP
Internet Control Message Protocol
The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
error datagram (11 - Time Exceeded
ICMP Time Exceeded
The Time Exceeded Message is an ICMP message which is generated by a gateway to inform the source of a discarded datagram due to the time to live field reaching zero...
) 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
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
, 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
Hop (networking)
Data packets often have to go through routers , if not several, before they reach their final destination. Each time packets are passed to the next router a hop occurs. .- Hop Count :The distance between two hosts...
. To reflect this practice, the field is renamed hop limit in IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
.
DNS records
TTLs also occur in the Domain Name SystemDomain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS), where they are set by an authoritative nameserver
Name server
In computing, a name server is a program or computer server that implements a name-service protocol. It maps a human-recognizable identifier to a system-internal, often numeric, identification or addressing component....
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 (three 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 record
MX record
A mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...
s, 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. 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, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the change.
Newer DNS methods that are part of a DR (Disaster Recovery) 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 world wide quickly. 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 normally honored automatically by all standard DNS systems world-wide.
HTTP
Time to live may also be expressed as a date and time on which a record expires. TheExpires: header in HTTP responses and the expires field in HTTP cookies express time-to-live in this way.