Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pasteur!ucbvax!TOVE.UMD.EDU!sandy
From: sandy@TOVE.UMD.EDU (Sandy Murphy)
Newsgroups: comp.protocols.tcp-ip
Subject: terminating connections
Message-ID: <8901311831.AA27258@tove.umd.edu>
Date: 31 Jan 89 18:31:16 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 25

A few months ago I posted a message to this list asking for comments of the
difference between the ISO TP and TCP connection establishment services --
specifically, the reason for / necessity for TCP's symmetric system.
Encouraged by the number of kind people who had very interesting answers,
I would like to pose a similar question (you knew there was a reason to
ignore such requests!):

TCP and ISO TP differ in how they handle connection termination.  TCP
performs a graceful close, ensuring that data in transit is received 
before the connection is closed.  ISO moves this up to the session layer
with the two session entities doing their own exchange of I'm-done-transmitting
messages.  Since TCP accomplishes the graceful close by FIN messages
that are given sequence numbers one greater than the last data sent,
it looks to me like TCP is doing something similar.  That is, it is 
as if TCP were inserting its own I'm-done-transmitting message in the
data stream.  Because the session layer can count on the transport layer
to deliver data in sequence, it need only check the data for the
I'm-done message.  But TCP must devote considerable processing to
ensure that the FIN is acted upon at the appropriate time, etc.

Does anyone have any comments on whether this belongs in the transport
or session layers, further effects of doing it TCP's (ISO's) way,
agreements/disagreements with my interpretation, etc.

--Sandy Murphy