[Jack Jackson <jack@xxxxxxxxxxxxxxx>]

At 10:29 AM 5/17/2006, Damodaran  Sampath wrote:

> 2) I remember i observed a scenario with ethereal output like of the form:
>
> ftp > 1111 [FIN, ACK] Seq=x-1 Ack=z Win=some number Len=0
> ftp > 1111 [ACK] Seq=x Ack=y Win=some number Len=0
> ftp > 1111 [ACK] Seq=x Ack=y+1 Win=some number Len=0
> ftp > 1111 [ACK] Seq=x Ack=y+2 Win=some number Len=0
> ftp > 1111 [ACK] Seq=x Ack=y+3 Win=some number Len=0
> ftp > 1111 [FIN, ACK] Seq=x-1 Ack=y+4 Win=some number Len=0  <<<<<
> ftp > 1111 [RST] Seq=x Ack=0 Win=some number Len=0    <<<<<
>
> Here in the above form of output there is 1 more ACK expected before the 
> marked entry [FIN, ACK]. But for some reasons this FIN, ACK is generated 
> with sequence number 1 less and followed by a RST control.
>
> Could you please help on finding out what is wrong here ?

The second FIN is a retransmission of the first FIN.  Apparently no ACK has 
been received for the FIN.  The RST is most likely sent because the TCP 
stack is shutting down the connection due to the FIN not being acknowledged.

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