3.3.8 Error Reporting

Wherever practical, hosts MUST return ICMP error datagrams on detection of an error, except in those cases where returning an ICMP error message is specifically prohibited.

DISCUSSION:

A common phenomenon in datagram networks is the "black hole disease": datagrams are sent out, but nothing comes back. Without any error datagrams, it is difficult for the user to figure out what the problem is.