RFC 1912 conveys basic rules for determining appropriate TTL values.

Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all.

* Default IP packets Time To Live ( TTL ) values

* Gnutella TTL and Hops header values used for preventing loops and monitoring of network topology

* Low-power Schottky TTL ( LS ) – used the higher resistance values of low-power TTL and the Schottky diodes to provide a good combination of speed ( 9. 5ns ) and reduced power consumption ( 2 mW ), and PDP of about 20 pJ.

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.

* The TTL field of IPv4 has been renamed to Hop Limit, reflecting the fact that routers are no longer expected to compute the time a packet has spent in a queue.

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.

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.

For example a 300 second TTL would help key records expire in 5 minutes to help ensure these records are flushed world wide quickly.

Mutable ( and therefore unauthenticated ) IPv4 header fields are DSCP / TOS, ECN, Flags, Fragment Offset, TTL and Header Checksum.

TTL chips are now are also made in surface-mount packages.

Successors to the original bipolar TTL logic often are interchangeable in function with the original circuits, but with improved speed or lower power dissipation.

TTL inputs are the emitters of a multiple-emitter transistor.

TTL inputs are sometimes simply left floating to provide a logical " 1 ", though this usage is not recommended.

TTL outputs are typically restricted to narrower limits of between 0 V and 0. 4 V for a " low " and between 2. 6 V and 5 V for a " high ", providing 0. 4V of noise immunity.

Designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy, but level-shifting devices are required between the two logic families.

The TTL " totem-pole " output structure often has a momentary overlap when both the upper and lower transistors are conducting, resulting in a substantial pulse of current drawn from the power supply.

V < sub > CC </ sub > and Ground are carryovers from TTL logic and that nomenclature has been retained with the introduction of the 54C / 74C line of CMOS.

The increased low output level can cause troubles when TTL logic circuits are driven.

Schottky metal – semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series, where they are employed as clamps in parallel with the collector-base junctions of the bipolar transistors to prevent their saturation, thereby greatly reducing their turn-off delays.

Going on these figures alone TTL logic gates are limited to perhaps 2 to 10, depending on the type of gate, while CMOS gates have DC fan-outs that are generally far higher than is likely to occur in practical circuits ( e. g. using NXP Semiconductor specifications for their HEF4000 series CMOS chips at 25 ° C and 15 V gives a fan-out of 34 thousand ).

The 7400 series of transistor-transistor logic ( TTL ) integrated circuits are historically important as the first widespread family of TTL integrated circuit logic.

An integrated circuit made in CMOS is not a TTL chip since it uses field-effect transistors ( FETs ) and not bipolar junction transistors, but similar part numbers are retained to identify similar logic functions and electrical ( power and I / O voltage ) compatibility in the different subfamilies.

Variations of and successors to the basic TTL family, which has a typical gate propagation delay of 10ns and a power dissipation of 10 mW per gate, for a power – delay product ( PDP ) or switching energy of about 100 pJ, include:

It was built from standard TTL chips and Programmed Array Logic chips, with four boards per CPU module.

Other accessories for the FM3A include the Nikon MD-12 motor drive ( automatic film advance up to 3. 2 frames per second ), as well as the Nikon MF-16 databack ( sequential numbering, time or date stamping on the film ), and the various compatible Nikon TTL flashes ( SB-15, SB-16b, SB-20, etc.

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.

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.

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.

In the DUNDi configuration one can limit the number of consecutive lookups between peers by setting the TTL.

Frequently starting with a value like 128 ( Windows ) or 64 ( Linux ), routers decrement this and discard a packet when the TTL value has reached zero, returning the ICMP error message ICMP Time Exceeded.

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

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

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

The maximum value of this current is about 1. 6 mA for a standard TTL gate.

Waiting for a trigger, starting the ADC, looking up the time, waiting for the ADC to finish, move value to RAM, switch multiplexer, get TTL input, let DAC proceed with voltage ramp.

The use of a TTL value of 0 will still overwrite the users local resolver cache which may cause conflicts with previously authenticated services.

The continuous process of monitoring, analyzing, redesigning and implementing is used to intensify TTL value production as conditions change and new opportunities emerge.

$ TTL 1h ; default expiration time of all resource records without their own TTL value