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From: karels%okeeffe@UCBVAX.BERKELEY.EDU.UUCP
Newsgroups: mod.protocols.tcp-ip
Subject: Re: UDP vs. ICMP dest unreachable messages
Message-ID: <8702112015.AA08974@okeeffe.Berkeley.EDU>
Date: Wed, 11-Feb-87 15:15:10 EST
Article-I.D.: okeeffe.8702112015.AA08974
Posted: Wed Feb 11 15:15:10 1987
Date-Received: Thu, 12-Feb-87 19:28:57 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
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Approved: tcp-ip@sri-nic.arpa

ICMP unreachable messages are reported to users of UDP sockets in 4.3
if and only if the socket is "connected"; that is, that the remote
address is bound as well as the local address.  Otherwise, it is unreasonable
to report errors even though the local address matches that in an ICMP
error message.  The error may well refer to a datagram other than the most
recently sent, in which case it is likely to be confusing at best.

This is used in the UNIX resolver code to detect the abscence of a local
server; it depends on receiving the "port unreachable" error.  On the other
hand, the same binding causes late messages from one server to be discarded
after "connecting" to the next of a series of choices.  This isn't a problem
in the standard installation, with only one server choice (the server on the
same host).  The UNIX nameserver does not take advantage of ICMP error returns,
in part because it runs multi-threaded, processing other requests while
awaiting a reply to a recursive query.  However, recent additions to the BIND
server will enable it to measure response time of multiple servers for a domain.
It will then choose the fastest server, which will not include one that
was recently unreachable if there are alternatives.

Recent questions about the ordering of root servers in BIND configuration
files are no longer interesting.  Current servers use the configuration
file to reach the root servers initially, which they then query about
the root domain.  That information is then used as long as it is valid.

		Mike