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From: Lixia@XX.LCS.MIT.EDU (Lixia Zhang)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: A couple of simple questions.
Message-ID: <12352652943.15.LIXIA@XX.LCS.MIT.EDU>
Date: Sun, 22-Nov-87 10:09:13 EST
Article-I.D.: XX.12352652943.15.LIXIA
Posted: Sun Nov 22 10:09:13 1987
Date-Received: Tue, 24-Nov-87 06:48:43 EST
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	I'll leave the version-7 description to BBN people, and only say a
few words about your second question (about the PSN-PSN protocol, or
IMP-IMP protocol as it used to be called).
	It is a link layer protocol providing reliable datagram deliveries
(i.e. every packet will be delivered, but possibly out-of-order).  It has
nothing to do with connection concept at the network layer.  ARAPNET runs a
DATAGRAM protocol internally, but provides a virtual-circuit interface to
the host by a reliable network end-to-end (i.e. between entry-exit IMPs)
protocol.
	Therefore some of your words are still correct -- unreliable IP
runs on top of the reliable ARPANET, and indeed causes high overhead in
many cases.  ARPANET opens an end-to-end connection to deliver data between
two hosts (which can be IP gateways), and blocks the interface to the host
whenever the PSN(IMP) runs out of the resources (buffers, control table
entries, whatever).  And that is why packets always get lost at the
gateways: the IP gateway has no mechanism to stop incoming packets, so it
has to drop them when they can't be forwarded.

Lixia
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