Jim's answer looks reasonable to me.  But a quick additional note.
Many implementations do disconnect.  They destroy any connections with
connections to that host.  That is why some firewalls often create
problems by sending host unreachable's.
 
Jim Wowchuk <[log in to unmask]> wrote:
 
>>Can someone verify my belief that if an active TCP/IP connection should
>>receive an ICMP Host Unreachable, the connection will be dropped?
>>
>>Or is this hidden from the application layer and ignored?
 
>From "TCP/IP Illustrated, Volume 1, The Protocols", by W. Richard Stevens,
>Addison-Wesley, 1994:
 
>"The ICMP 'Host Unreachable' error message is sent by a router when it
>receives an IP datagram that it cannot deliver or forward."
 
>For TCP, "a received host unreachable or network unreachable is effectively
>ignored, since these two errors are considered transient.  It could be that
>an intermediate router has gone down and it can take the routing protocols a
>few minutes to stabilize on an alternative route.  During this period either
>of these two ICMP errors can occur, but they must not abort the connection.
>Instead, TCP keeps trying to send the data that caused the error, although
>it may eventually time out."
 
>Later if the connection does timeout, then the error message will be changed
>to reflect the presence of these ICMP errors.
 
>Good question - logic suggested the solution above, but I've seen little on
>the mechanisms used.  Also don't confuse the program Ping with viewing TCP
>protocols - it is pure ICMP!  The problem is more pronounced though if the
>failure occurs at *your* primary gateway!
 
>Coincidently, I had the very same problem yesterday setting up an NT
>Advanced Server to act as a router between LAN and WAN (PPP). And
>surprisingly (:^), Microsoft included incorrect instructions in their help
>file, omitting a parameter to be manually added with RegEdit32.
 
>Cheers.
>----
>Jim "seMPEr" Wowchuk
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