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From: brescia@CCV.BBN.COM.UUCP
Newsgroups: mod.protocols.tcp-ip
Subject: Re: UDP vs. ICMP dest unreachable messages
Message-ID: <8702111436.AA06857@ucbvax.Berkeley.EDU>
Date: Wed, 11-Feb-87 09:22:20 EST
Article-I.D.: ucbvax.8702111436.AA06857
Posted: Wed Feb 11 09:22:20 1987
Date-Received: Thu, 12-Feb-87 06:38:45 EST
References: <8702110804.AA27296@topaz.rutgers.edu>
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> (you can ignore an 'unreachable' because it may indicate a transient
>  routing problem) [paraphrased - mb]

You really need to be advocating to look at the subcodes returned by ICMP
destiination unreachable, because you can usually trust the 'host dead' type
returned from some gateways when trying to talk to hosts on arpanets.  Yes,
you would do well to ignore ICMP net unreachable if you suspect routing
flurries (often the case nowadays).  With UDP domain lookups however, could
you not use that as an indication to try another address, even if you keep
retransmitting to the original one?  You need not worry about "bothering" too
many servers in this case, because the 'unreachable' is a response which tells
you that you did not reach that server.

Also, you should be explicit in your reasoning about the 'port unreachable'
subcode.  Do you mean to try again because the server too busy and did not get
another server listen up again, or give up because there is not now nor will
there ever be a server at that host (because the service host changed).

I think you should use the broadcast approach for connection setup, since you
supposedly don't care which of the equivalent servers you reach.  If, for
example, you try to contact one from the set { A, B, C }, and you get an
unreachable from A, try B next, and only forget A if the reply code was 'host
dead'.

Of course, your implementation on the arpanet (AHIP) interface does recognize
arpanet host-dead messages, doesn't it?

mike