The time for which a resolver caches a DNS response is determined by a value called the time to live ( TTL ) associated with every record.

The value of the MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer.

If the resulting TTL is 0, the packet is discarded and an ICMP Time To Live exceeded in transit message is sent to the datagram's source address.

Time to live ( TTL ) is a mechanism that limits the lifespan or lifetime of data in a computer or network.

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.

The maximum TTL value is 255, the maximum value of a single octet.

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.

An older common TTL value for DNS was 86400 seconds, which is 24 hours.

TTL values are " per record " and setting this value on specific records is normally honored automatically by all standard DNS systems world-wide.

A dedicated voltage comparator chip such as LM339 is designed to interface with a digital logic interface ( to a TTL or a CMOS ).

Transistor – transistor logic ( TTL ) is a class of digital circuits built from bipolar junction transistors ( BJT ) and resistors.

TTL is notable for being a widespread integrated circuit ( IC ) family used in many applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, synthesizers, etc.

The designation TTL is sometimes used to mean TTL-compatible logic levels, even when not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments.

The tracert ( traceroute ), Pathping commands are implemented by transmitting UDP datagrams with specially set IP TTL header fields, and looking for ICMP Time to live exceeded in transit ( above ) and " Destination unreachable " messages generated in response.

Traceroute works by increasing the TTL value of each successive set of packets sent.

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 ).

Newer DNS methods that are part of a DR ( Disaster Recovery ) system may have some records deliberately set extremely low on TTL.

While it contains no microprocessor, it used the 4004 programming instruction set and its custom TTL was the basis for the Intel 8008, and for practical purposes the system behaves approximately as if it contains an 8008.

In manual mode, a black needle points out the shutter speed recommended by the built-in, open aperture, through-the-lens ( TTL ), silicon photodiode ( SPD ) light meter, with 60 / 40 percent centerweighting, while a translucent green needle shows the actual camera set shutter speed.

For example, every device ( such as an intermediate router ) forwarding an IP datagram first decrements the time to live ( TTL ) field in the IP header by one.

For small-scale logic, designers now use prefabricated logic gates from families of devices such as the TTL 7400 series by Texas Instruments and the CMOS 4000 series by RCA, and their more recent descendants.

In 1973, the KA10 was replaced by the KI10, which used TTL SSI.

They are activated by AC control signals or DC control signals from Programmable logic controller ( PLCs ), PCs, Transistor-transistor logic ( TTL ) sources, or other microprocessor and microcontroller controls.

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.

After introduction in integrated circuit form in 1963 by Sylvania, TTL integrated circuits were manufactured by several semiconductor companies, with the 7400 series by Texas Instruments becoming particularly popular.

TTL was invented in 1961 by James L. Buie of TRW, " particularly suited to the newly developing integrated circuit design technology ", and it was originally named transistor-coupled transistor logic ( TCTL ).

The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family ( SUHL ).

TTL also became important because its low cost made digital techniques economically practical for tasks previously done by analog methods.

The main disadvantage of TTL with a simple output stage is the relatively high output resistance at output logical " 1 " that is completely determined by the output collector resistor.

At low input voltage, the TTL input sources current which must be absorbed by the previous stage.

A TTL input signal is defined as " low " when between 0 V and 0. 8 V with respect to the ground terminal, and " high " when between 2. 2 V and 5 V ( precise logic levels vary slightly between sub-types and by temperature ).

As a noteworthy consequence of this distributed and caching architecture, changes to DNS records do not propagate throughout the network immediately, but require all caches to expire and refresh after the TTL.

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 change.

To limit the impact of DNS poisoning typically a TTL of 0 is used.

* TTL: standard DNS time to live field.