Time to live (TTL) in computing and computer networks - the limiting period of time or the number of iterations or transitions for which a data set ( packet ) can exist before it disappears.
TTL (Lifetime) IP Packages
In IPv4, the TTL is an eight-bit IP header field.
Determines the maximum number of hops ( hop , i.e., jump, distance between routers) that a packet can pass. The presence of this parameter does not allow the package to go on the network indefinitely. Each router during routing should decrease the TTL value by one, but some gateways can be configured to ignore this. Packets that have not reached the addressee, but whose lifetime has become zero, are destroyed, and the message ICMP Time Exceeded is sent to the sender. If it is required that the packet was not routed (that is, it was received only in its segment), then TTL = 1 is set. Tracing their paths ( traceroute ) is based on sending packets with different lifetimes. The maximum value of TTL = 255. The usual initial value is TTL = 64 ( Linux , Mac , Android , iOS ), TTL = 128 ( Windows ).
Initially, according to the RFC 791 standard, the lifetime (TTL) in IPv4 should have been measured in seconds (hence the name). Each second of waiting in the queue of a node (for example, a router), as well as each transition to a new node through which the datagram passes, should have reduced the TTL value by one unit. In practice, this did not take root, and the TTL field is simply decremented by one on each transit node ( hop ) through which the datagram passes. In order to reflect this, in the IPv6 protocol , the TTL field has been renamed to “maximum transitions” (Hop Limit).
Switches traditionally operate on the data link layer and do not decrease TTL.
TTL control is often used by providers to detect unauthorized connected device traffic. For example, if you enable the Mobile Access Point ( tethering ) mode on your smartphone, the TTL from the connected devices will be 1 less than expected. This blocks access to these devices.
DNS record lifetime
For DNS records, the “Time to live” parameter determines the time the data was updated when queries are cached . Set in seconds, a typical value is 86,400 seconds, that is, 24 hours. This means that when a DNS record changes, up to 24 hours after the change, DNS servers around the world can issue old data from the cache until it is updated.