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 18.104.22.168, 22.214.171.124, 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).