Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bbn!bbn.com!mckenzie
From: mckenzie@bbn.com (Alex McKenzie)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: terminating connections
Message-ID: <35454@bbn.COM>
Date: 1 Feb 89 16:42:45 GMT
References: <8901311831.AA27258@tove.umd.edu>
Sender: news@bbn.COM
Reply-To: mckenzie@labs-n.bbn.com (Alex McKenzie)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
Lines: 27

TCP and ISO TP handle the closing of a connection differently because of
different assumptions about the environment in which they will be
embedded.

TCP assumes that it (alone) is responsible for the reliability of the
data transport; a user application which doesn't want to be concerned
with transport reliability may be built directly on top of it.  This
might even be an application which has only a one-way data flow.  The
application can just hand a lot of data to the TCP module and the give
the module a command like "when you are done sending the data, close the
connection".  So TCP better be sure that all the data has been correctly
received by the receiver befor the connection is closed and the TCP
resources are deallocated.

ISO TP assumes that it is in the middle of a protocol stack.  It is
responsible for ensuring reliable transmission (let's not argue about
this phrase - I know it is not literally correct) but that higher layers
of protocol will be doing their own handshaking to be sure their
resources are not prematurely deallocated.  In particular, since the
Transport layer "knows" that the Session layer will perform a handshake
which must be completed before either Session entity asks the Transport
layer to close the connection, no value is added by having the Transport
entities also shake hands; the pipe is truely empty when the "Close
Transport" command comes down from Session.

Hope this helps,
Alex McKenzie