Time to Live (TTL) is a networking concept that defines the lifespan or maximum number of hops that a data packet or record can take as it travels across a network before it is discarded. The main purpose of TTL is to prevent data packets from circulating indefinitely in the event of routing errors or loops, which could otherwise congest and degrade network performance.
How TTL Works
• Each data packet sent over a network includes a TTL value in its header.
• This value is set by the sender and usually starts as a number between 1 and 255.
• Every time the packet passes through a router (a “hop”), the TTL value is decreased by one.
• If the TTL reaches zero before the packet reaches its destination, the packet is discarded by the router, and an ICMP “Time Exceeded” message is typically sent back to the sender.
• This mechanism ensures that undeliverable or misrouted packets do not persist on the network, helping to maintain network health and efficiency.
TTL is also used outside of packet routing:
• DNS Caching: TTL determines how long a DNS record is cached by a resolver before it must be refreshed from the authoritative server. A lower TTL means more frequent updates but increased DNS traffic; a higher TTL means less frequent updates but potentially outdated information.
• CDN Caching: In content delivery networks, TTL specifies how long cached content should be served from an edge server before it is refreshed from the origin server.