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From: TAYBENGH@NUSDISCS.BITNET
Newsgroups: comp.protocols.tcp-ip
Subject: why UDP send ICMP port unreachable message?
Message-ID: <9103011243.AA10028@ucbvax.Berkeley.EDU>
Date: 2 Mar 91 03:48:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 16

Hi netlanders,
        After reading thru the implementation of TCP/IP from
the book The Design and Implementation of the 4.3BSD OS (published by
Addison-Wesley) and the source code of BSD4.3 TCP/IP, I am puzzled of
the purpose of ICMP Port Unreachable error message sent by UDP that appeared
in the routine udp_input(). According to my understanding, this ICMP message
is never received by UDP, nor is reported to the user if the user send a
UDP message to a non-existent server (destination port). So why it is sent
by the UDP? Did I misunderstand sth?
        Also did TCP use this message to inform the client when it tries to
initiate a connection to a non-existent server?
        Could somebody please explain this? Thanks a lot.

- Beng Hang (email: taybengh@nusdiscs.bitnet)
  Dept of Information Systems and Computer Science
  National University of Singapore