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From: guenther@irafs2.ira.uka.de (Guenther Schreiner)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: why UDP send ICMP port unreachable message?
Keywords: BSD, UDP connections, ICMP port unreachable
Message-ID: <91.060.16:34:30@ira.uka.de>
Date: 1 Mar 91 16:34:30 GMT
References: <9103011243.AA10028@ucbvax.Berkeley.EDU>
Sender: news@ira.uka.de (USENET News System)
Reply-To: guenther@irafs2.ira.uka.de (Guenther Schreiner)
Followup-To: <9103011243.AA10028@ucbvax.Berkeley.EDU>
Organization: University of Karlsruhe, FRG
Lines: 29

In article <9103011243.AA10028@ucbvax.Berkeley.EDU>,
TAYBENGH@NUSDISCS.BITNET writes:
> 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?

 Even if it sounds strange, it is possible to establish a UDP "connection".
 If you do so (with the system call connect() and I/O with send()/recvfrom()),
  the I/O system calls will return EADDRNOTAVAIL, ENETDOWN and ENETUNREACH
  after an appropriate incoming ICMP message.
 In opposite to this the I/O calls will NOT return an error if you use a loose
  UDP connection (with sendto() and recvfrom() calls without a connect()).

 Hope this helps,
  Guenther

--
 Guenther Schreiner			University of Karlsruhe, West Germany
  c/o Fakultaet fuer Informatik		Phone: (0721) 608-2749
  Am Fasanengarten 5			UUCP:  ...!seismo!unido!uka!guenther
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					EARN:  GUENTHER at DKAUNI0I
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