Situations can occur in network communication where a datagram travels in a circle, never reaching its destination. This might occur if two routers continually route a datagram back and forth between them, thinking the other should be the next hop to the destination. This is an example of faulty routing information.

The limitations of the routing protocol can result in destinations being unreachable. For example, RIP has a limit on the distance a certain routing information is allowed to travel. The hop limit of RIP is 15, which means that the packet will only be allowed to pass through 15 routers.

In either of these cases, an excessively long route exists. Whether the actual path includes a circular routing path or too many hops, the packet will eventually exceed the maximum hop count. This is also known as reaching its time-to-live (TTL), because the TTL value typically matches the maximum hop count defined by the routing protocol. A TTL value is defined in each datagram. As each router processes the datagram, it decreases the TTL value by one. When the TTL of the datagram value reaches zero, the packet is discarded. ICMP uses a time exceeded message to notify the source device that the TTL of the datagram has been exceeded.