A source-quench message informs the source that a atagram has been discarded due to congestion in a router or the destination host. The source must slow down the sending of datagrams until the congestion is relieved. One source-quench message is sent for each datagram that is discarded due to congestion.
Whenever a router decrements a datagram with a time-to-live value to zero, it discards the datagram and sends a time-exceeded message to the original source.
When the final destination does not receive all of the fragments in a set time, it discards the received fragments and sends a time-exceeded message to the original source.