3.2.2.1 Destination Unreachable: RFC-792

The following additional codes are hereby defined:

6	destination network unknown
7	destination host unknown
8	source host isolated
9	communication with destination network administratively prohibited
10	communication with destination host administratively prohibited
11	network unreachable for type of service
12	host unreachable for type of service

A host SHOULD generate Destination Unreachable messages with code:

2	(Protocol Unreachable), when the designated transport protocol is not supported; or
3	(Port Unreachable), when the designated transport protocol (e.g., UDP) is unable to demultiplex the datagram but has no protocol mechanism to inform the sender.

A Destination Unreachable message that is received MUST be reported to the transport layer. The transport layer SHOULD use the information appropriately; for example, see Sections 4.1.3.3, 4.2.3.9, and 4.2.4 below. A transport protocol that has its own mechanism for notifying the sender that a port is unreachable (e.g., TCP, which sends RST segments) MUST nevertheless accept an ICMP Port Unreachable for the same purpose.

A Destination Unreachable message that is received with code 0 (Net), 1 (Host), or 5 (Bad Source Route) may result from a routing transient and MUST therefore be interpreted as only a hint, not proof, that the specified destination is unreachable [IP:11]. For example, it MUST NOT be used as proof of a dead gateway (see Section 3.3.1).