Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!att!hriso!devildog!jrallen
From: jrallen@devildog.UUCP (Jon Allen)
Newsgroups: comp.protocols.tcp-ip
Subject: TCP Protocol Question: reusing connections
Keywords: SYN, RST, Connection Establishment
Message-ID: <1125@devildog.UUCP>
Date: 27 Feb 90 20:46:10 GMT
Sender: jrallen@devildog.UUCP
Reply-To: jrallen@devildog.UUCP (Jon Allen)
Distribution: usa
Organization: AT&T - Piscataway, NJ
Lines: 81


I am having trouble understanding how the TCP protocol works when a 
connection is closed and the same connection is reused soon after 
closing (same end port numbers).  [I am certainly no TCP protocol
guru!]

The sequence of events is a connection is established from machine A
to machine B.  This connection is closed and the same connection is
opened a short time later, however a RST occurs.  I have been told
that this happens because the next SYN sequence number is within
the 'window'.  The following is an actual trace:

	.
	.
	.

Packet #: 20
ether: IP    machineA          -> machineB              64    1007.728    14.233
   ip: TCP   8201.0064 -> 8201.003c
  tcp: 1023    -> CMDSRV   FIN ACK 
  tcp: seq:  29252636  ack:   9361313  win: 4096  len:    0  

Packet #: 21
ether: IP    machineB            -> machineA            64    1009.939     2.211
   ip: TCP   8201.003c -> 8201.0064
  tcp: CMDSRV  -> 1023     ACK 
  tcp: seq:   9361313  ack:  29252637  win: 8192  len:    0  
#
# Connection is closed at this point.  The next SYN is an attempt
# to open the same connection (same address and port numbers).
#
	.
	.
	.

Packet #: 24
ether: IP    machineA          -> machineB              64    2615.763  1588.359
   ip: TCP   8201.0064 -> 8201.003c
  tcp: 1023    -> CMDSRV   SYN 
  tcp: seq:  29254401  ack:         0  win: 4096  len:    0  

Packet #: 25
ether: IP    machineB            -> machineA            64    2618.096     2.334
   ip: TCP   8201.003c -> 8201.0064
  tcp: CMDSRV  -> 1023     RST ACK 
  tcp: seq:   9361313  ack:  29252637  win: 8192  len:    0  

Packet #: 26
ether: IP    machineB            -> machineA            64    2619.358     1.262
   ip: TCP   8201.003c -> 8201.0064
  tcp: CMDSRV  -> 1023     RST ACK 
  tcp: seq:         0  ack:  29254402  win:    0  len:    0  

	.
	.
	.

The above trace was edited for brevity and only the last part of the
connection shutdown sequence is shown.  However, it seems to conform
to the TCP spec.  After reading the TCP RFC several times, I cannot
seem to understand how the sequence numbers should come into play
on a reused connection.  This trace brings up several questions:

	1) Why is the RST generated by machineB?  I have been told
	   it is because the SYN sequence number is within the 
	   ack# plus the window size.

	2) But if this is true then wouldn't some flakiness occur?
	   For example, if the window size was small enough that the 
	   SYN flagged packet would be outside the window and work,
	   wouldn't raising the window size then cause it to fail (can't
	   window size be adjustable on some machines?)?
	   This doesn't seem right to me.

If someone could provide me with some enlightenment, I would 
appreciate it.

-Jon

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