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From: kasten@EUROPA.INTERLAN.COM (Frank Kastenholz)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  connect "collisions" in TCP
Message-ID: <9101302235.AA04289@europa.InterLan.Com>
Date: 30 Jan 91 22:35:10 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 42


 > From tcp-ip-RELAY@NIC.DDN.MIL Wed Jan 30 17:15:41 1991
 > From: usc!cs.utexas.edu!calvert@ucsd.edu  (Kenneth L. Calvert)
 > Organization: U. Texas CS Dept., Austin, Texas
 > Subject: connect "collisions" in TCP
 > Sender: tcp-ip-relay@nic.ddn.mil
 > To: tcp-ip@nic.ddn.mil
 > 
 > 
 > In TCP, the result of a "connect collision" (i.e., two peers
 > "simultaneously" attempting to open a connection to the same pair
 > of ports) is a single connection.  I have two questions:
 > 
 > 0.  My impression is that the Berkeley Unix sockets interface (to TCP)
 > precludes deliberate use of this "feature".  Have I missed something?
 > (The implementation does the right thing when SYN is received in state
 > SYN_SENT).
I'm not sure what you mean by this question. This "feature" is a function
of the protocol, not the interface to your local protocol implementation.
The "feature" is deliberately "used" by TCP implementations to prevent
there being two Transmission Control Blocks in a host for the same
connection -- a state which is very bad.

 > 1. Can anyone cite an application or higher-level protocol that
 > makes use of (or could, if it were possible) this fact,
 > i.e. permits users to establish connections symmetrically?
 > I can think of one possibility: the FTP data connection,
 > but I don't think it works that way.

Generally, the client/server model of communications that we use
precludes the possibility of a "connect collision". Assuming that
it was possible to cause such a thing to happen, I am not sure why
one would explicitly want to do so -- the end result is the same
regardless of whether there was such a "conenct collision" or not --
a full duplex connection.

 > 
 > Don't need to do this myself -- just curious.

 > Ken Calvert
 > calvert@cs.utexas.edu
 >