The MPLS-OPS Archive

[Pradeepa Shastry <Pshastry@in.huawei.com>]

Title: RE: TCP state machine

Hi,

Answer to your question is two fold

1) If you are waiting for SYN_SENT state and waiting for SYN + ACK from peer...

In this case if application says close, FIN will not be send to peer since state is not established (see function TCP_Disconnect ()) and it closes all the resources.

2) If you are in a SYN_SENT state and received SYN + ACK from peer...
Then you moved to ESTABLISHED state and sent final ACK, if that got lost other side still in SYN_RECVD state.
At this point if application running at your end tries to close the session, then you will send a FIN to other side. But the point here is it is not just FIN it is FIN+ACK.

Peer which is in SYN_RCVD state receives a FIN+ACK ,you process the ACK first and moved to ESTABLISHED state and then process the FIN.

I hope this answer to your question.

Thanks and Regards
-Pradeep Shastry

-----Original Message-----
From: Santosh Kumar [mailto:santosh1914@yahoo.com]
Sent: Saturday, February 16, 2002 9:55 PM
To: 'mpls-ops@mplsrc.com'
Cc: 'ahussain@eng.utoledo.edu'
Subject: RE: TCP state machine

To add to Ken's response:
TCP state machine specifies that the only valid
transitions out of "SYN_SENT" state are when you
receive a SYN, an ACK, or when you timeout. Any other
event or message should have no effect on this state.
So, the fact that you received a FIN when you are in
SYN_SENT state has no relevance. It has no meaning to
this state. Hope this helps.

Have a nice day,
Santosh Kumar
Ohio State University

--- "Hull, Kenneth A." <KHULL@alleghenyenergy.com>
wrote:
> I would suggest you build a timer to take either end
> of the session back to
> the previous state if the an appropriate response is
> not received in the
> timer window, and be sure your software ignores any
> extra messages
> corresponding to move form the previous state it may
> receive for the same
> session (same source TCP port) once it moves to the
> next state.
>
> -----Original Message-----
> From: Mohammed [mailto:ipswitching@yahoo.com]
> Sent: Wednesday, February 13, 2002 11:08 AM
> To: mpls-ops@mplsrc.com
> Cc: ahussain@eng.utoledo.edu
> Subject: TCP state machine
>
>
> Hi all,
>
>  I was writing the code for TCP state machine so
> that
> i could use it for running LDP over TCP. This would
> just be a component of the simulator that i am
> working
> on.
>
>  I was looking at the state transition diagram given
> in TCP/IP illustrated Vol 1 by Richard Stevens.
>
>  Please consider the following scenario --
>
>  A TCP software on a peer working on passive open
> mode
> is in "SYN-RCVD" state. It is waiting for an 'ACK'
> from its peer on the other end of the connection.
> There are two things that might happen here ...
>
>  (1) SYN + ACK sent before transition to this state
> was lost, so the other side never received it and
> hence never ACK.
>
>  (2) The other side have received it and had ACk but
> the ACk was lost.
>
>  Anyways 'SYN-RCVD' i waiting for an ACK. But during
> its wait period or even before its time oust and
> sends
> RST ... if it receives an appl:close interrupt then
> it
> is supposed to send a FIN to its peer.
>
>  Now if the peer on the other side is in
>
>  -- "ESTB" state then this would be okay.
> 
>  But if it is "SYN SENT" state .. then how am i
> supposed to handle this FIN.
>
>  Any pointers from TCP developers or any other
> interested professionals are welcome.
>
>  Thanks for your time and help!!!
>  Abrar.
> 
>
> =====
> Abrar Hussain.
> Graduate Student/Network Designer.
> The University of Toledo.
> Toledo,OH.
>
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