I get the TCP-based one, as the service would still complete the connection handshake, send ACKs, etc - but the UDP one seems indistinguishable from simply dropping the packets.
Maybe back then the designers still expected that hosts would always reply to unwanted packets with an ICMP error, so silently dropped packets were expected to be rare and always indicators of a connection fault?
Though I guess we can proudly say today that UDP:9 is the most widely deployed service on the internet...
Yes, indeed it was expected to reply with ICMP errors when receiving packets to unused ports and the necessity of firewalls was not predicted, because the "barbarians" were not using the Internet yet.
Nowadays the well configured servers send ICMP errors only for the traceroute port range and the badly configured servers, which are more common, do not send any ICMP errors for unused ports.
Maybe back then the designers still expected that hosts would always reply to unwanted packets with an ICMP error, so silently dropped packets were expected to be rare and always indicators of a connection fault?
Though I guess we can proudly say today that UDP:9 is the most widely deployed service on the internet...