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ERROR_PROTOCOL_UNREACHABLE
ERROR_PROTOCOL_UNREACHABLE is more specific than a general host outage. In IPv4 ICMP terminology, “protocol unreachable” means the destination could be reached at the IP layer but the indicated upper-layer protocol was not available for the delivered datagram.
Protocol is not the same as port
The IP protocol field identifies transports such as TCP or UDP. A TCP or UDP port is interpreted only after the matching transport protocol accepts the packet. Therefore a protocol-unreachable result is conceptually earlier than ERROR_PORT_UNREACHABLE.
What to inspect
- The protocol number or socket protocol selected by the application.
- The socket family/type/protocol combination used to create the endpoint.
- Raw-socket or tunneling code that constructs IP headers.
- ICMP errors and the quoted original IP header.
- Middleboxes performing protocol translation or filtering.
For IPv4, RFC 792 assigns destination-unreachable code 2 to protocol unreachable. IPv6 represents unsupported next-header processing differently, including Parameter Problem handling. Capture the address family before comparing packet-level evidence.
Microsoft: system error codes · IETF: ICMP protocol unreachable · IETF: ICMPv6 error processing
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