Internet Core Protocols: The Definitive Guide

Time-to-Live

Another validation service provided by IP is checking to see if a datagram has outlived its usefulness. This is achieved through a Time-to-Live field provided in the IP datagram’s header. When a system generates an IP packet, it stores a value in the Time-to-Live header field. Every system that forwards the packet decreases the value of the Time-to-Live field by one, before sending the datagram on. If the Time-to-Live value reaches zero before the datagram gets to its final destination, then the packet is destroyed.

The purpose of the Time-to-Live field is to keep datagrams that are caught in an undeliverable loop from tying up network resources. Let’s assume that a pair of routers both have bad information in their routing table, with each system pointing to the other for final delivery. In this environment, a packet would be sent from one router to the other, which would then return the packet, with this process repeating forever. Meanwhile, more packets may be introduced to this network from external devices, and after a while, the network could become saturated.

But by using a Time-to-Live field, each of these routers would decrement the value by one every time it forwarded a packet. Eventually the Time-to-Live value would reach zero, allowing the datagram to be destroyed. This safeguard prevents routing loops from causing network meltdowns.

The strict definition of the Time-to-Live field states that the value is a measure of time in seconds, or any forwarding act ...