TTLexceeded

TTLexceeded is a term used in computer networking to describe the event that occurs when a data packet's Time to Live (TTL) value expires before the packet reaches its destination, prompting a router to drop the packet and to send an ICMP Time Exceeded message back to the source. The phrase is not an official standard, but it is widely used in network engineering discussions to refer to TTL expiry events and their diagnostic value.

In IPv4, each router decrements TTL by one as the packet is forwarded; when TTL reaches zero, the router discards the packet and returns an ICMP Time Exceeded message (type 11). In IPv6, a similar mechanism exists via ICMPv6 Time Exceeded messages.

TTLexceeded is exploited in traceroute and related diagnostic tools. By sending packets with incrementally higher TTL values, operators elicit responses from successive hops, allowing the path to a destination to be mapped and potential routing loops to be identified.

Limitations include rate limiting or blocking of ICMP messages by routers or firewalls, which can mask TTL-exceeded reports. Some networks may not generate replies for security reasons. TTL-based diagnostics can also be affected by asymmetric paths and load balancing, which can complicate interpretation of the results.

See also TTL, ICMP, traceroute, path MTU discovery.