What Is Time to Live (TTL)? TTL Definition

What Is Time To Live (TTL)? | TTL Definition

**What Is Time To Live (TTL)? | TTL Definitionwhat-is-the-ttl**

The term "Time To Live" (TTL) refers to the amount of time or "hops" that a packet is set to exist inside a network before being discarded by a router. TTL is also used in other contexts including CDN caching and DNS caching.

What Is Time To Live (TTL)? | TTL Definition

What Is Time To Live (TTL) in Networking?

How Does TTL Work?

When a packet of information is created and sent over the Internet, there is a risk that it will continue bouncing from router to router indefinitely. To mitigate this possibility, packets are designed with an expiration called a Time To Live or hop limit. Packet TTL can also be useful in determining how long a packet has been in circulation, allowing the sender to receive information about the packet’s path across the Internet.

Each packet has a place where it stores a numerical value that defines how long it should keep moving through the network. Each time a router receives a packet, it subtracts one from the TTL count and then passes it along to the next location in the network. If the TTL count reaches zero after a subtraction, the router will discard the packet and send an ICMP message back to the originating host.

Both of the commonly used network commands ping and traceroute make use of TTL. When using the traceroute command, a stream of packets with incrementally increasing TTLs is sent across the Internet toward the destination. Because each step along the connection is the final stop for one of the packets, each location will return an ICMP message to the sender after discarding the packet. The time it takes for the ICMP message to return to the sender is then used to determine the time it takes to reach each successive hop along the network.

icmp-traceroute-diagram

In What Other Circumstances Is TTL Used?

Apart from tracking the path that packets take across the Internet,

Time To Live is used in the context of caching information for a specific period of time.

Rather than measuring time in hops between routers,

each of which can take a variable amount of time,

some networking use cases operate in a more conventional manner.

CDNs such as a private CDN network typically use a Time To Live (TTL) to determine how long cached content should be served from a CDN edge server before fetching a fresh copy from an origin server.

By properly defining the amount of time between origin server pulls,

a CDN is able to serve up-to-date content without constantly repeating requests to the origin.

This optimization allows a CDN to efficiently serve content closer to the user while minimizing the bandwidth required from the origin.

In the context of a DNS record, TTL is a numerical value that defines how long a DNS caching server can serve a DNS record before reaching out to the authoritative DNS server and obtaining a fresh copy of the record.