Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ICMP_UNREACH_PORT
Message-ID: <40051@think.UUCP>
Date: 3 May 89 23:40:00 GMT
References: <8905021609.AA00846@lear.ultra.com>
Sender: news@think.UUCP
Reply-To: barmar@kulla.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 24

In article <8905021609.AA00846@lear.ultra.com> wayne@ultra.UUCP (Wayne Hathaway) writes:
>And second, what is IP doing talking about ports anyway???

I think that ICMP is providing this as a service to higher-level
protocols that have no way of doing this on their own.  It's not
necessarily a waste of bandwidth; when using an unreliable datagram
protocol, a client is likely to retransmit the operation if it doesn't
get a response.  One ICMP Port Unreachable message is better than a
half dozen retransmitted UDP packets.  Of course, if the protocol is
one-way with no response expected, this is an extraneous packet;
however, that style is generally only used with broadcasts, which also
don't prompt the port unreachable response, so it's probably OK.

>  Also I note that TCP does
>not seem to ever send an ICMP_UNREACH_PORT.)

TCP doesn't need it, because it has its own protocol for indicating an
invalid port (it sends a RST packet).

Barry Margolin
Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar