TCP TIME_WAIT Assassination
<p><strong>I just looked up the <a href="https://www.rfc-editor.org/rfc/rfc1337" rel="nofollow noreferrer">RFC1337</a> about TIME_WAIT Assassination and this is a portion of it.</strong></p>
<blockquote>
<p>Figure 1 shows an example of TIME-WAIT assassination.  Segments 1-5<br />
are copied exactly from Figure 13 of RFC-793, showing a normal close<br />
handshake.  Packets 5.1, 5.2, and 5.3 are an extension to this<br />
sequence, illustrating TWA.   Here 5.1 is <em>any</em> old segment that is<br />
unacceptable to TCP A.  It might be unacceptable because of its<br />
sequence number or because of an old PAWS timestamp.  In either case,
TCP A sends an ACK segment 5.2 for its current SND.NXT and RCV.NXT.<br />
Since it has no state for this connection, TCP B reflects this as RST
segment 5.3, which assassinates the TIME-WAIT state at A!</p>
<p>**</p>
<pre><code>    RFC 1337                 TCP TIME-WAIT Hazards                  May 1992
           TCP A                                                TCP B
       1.  ESTABLISHED                                          ESTABLISHED
           (Close)
       2.  FIN-WAIT-1  --&gt; &lt;SEQ=100&gt;&lt;ACK=300&gt;&lt;CTL=FIN,ACK&gt;  --&gt; CLOSE-WAIT
       3.  FIN-WAIT-2  &lt;-- &lt;SEQ=300&gt;&lt;ACK=101&gt;&lt;CTL=ACK&gt;      &lt;-- CLOSE-WAIT
                                                                (Close)
       4.  TIME-WAIT   &lt;-- &lt;SEQ=300&gt;&lt;ACK=101&gt;&lt;CTL=FIN,ACK&gt;  &lt;-- LAST-ACK
       5.  TIME-WAIT   --&gt; &lt;SEQ=101&gt;&lt;ACK=301&gt;&lt;CTL=ACK&gt;      --&gt; CLOSED
      - - - - - - - - - - - - - - - - - - - - - - - - - - - -
       5.1. TIME-WAIT   &lt;--  &lt;SEQ=255&gt;&lt;ACK=33&gt; ... old duplicate
       5.2  TIME-WAIT   --&gt; &lt;SEQ=101&gt;&lt;ACK=301&gt;&lt;CTL=ACK&gt;    --&gt;  ????
       5.3  CLOSED      &lt;-- &lt;SEQ=301&gt;&lt;CTL=RST&gt;             &lt;--  ????
          (prematurely)

**
</code></pre>
</blockquote>
<p><strong>Now, what confuses me is, in TCP/IP Illustrated volume1, what it says is:</strong></p>
<blockquote>
<p>Any delayed segments that arrive for a connection while it is in the
2MSL wait are discarded.</p>
</blockquote>
<p><strong>So, why did the TCP A in RFC 1337's figure 1 ACK the old duplicate segment?</strong></p>