RFC 793, "Transmission Control Protocol", September 1981

Note: This RFC has been obsoleted by RFC 9293

Errata ID: 3305
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Botong Huang
Date Reported: 2012-07-31
Held for Document Update by: Martin Stiemerling
Date Held: 2015-03-24

Section 3.9 says:

p70
SEGMENT ARRIVES
   Otherwise,
   first check sequence number

      SYN-RECEIVED STATE
      ESTABLISHED STATE
      FIN-WAIT-1 STATE
      FIN-WAIT-2 STATE
      CLOSE-WAIT STATE
      CLOSING STATE
      LAST-ACK STATE
      TIME-WAIT STATE

        Segments are processed in sequence.  Initial tests on arrival
        are used to discard old duplicates, but further processing is
        done in SEG.SEQ order.  If a segment's contents straddle the
        boundary between old and new, only the new parts should be
        processed.

        There are four cases for the acceptability test for an incoming
        segment:

        Segment Receive  Test
        Length  Window
        ------- -------  -------------------------------------------

           0       0     SEG.SEQ = RCV.NXT

           0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

          >0       0     not acceptable

          >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
                      or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

It should say:

Added my Martin Stiemerling (TSV AD):
A document update will address this problem and the TCPM working is
expected to find a solution. 

Notes:

Not sure how to correct it systemmatically, so I just present the problem.

If in SYN-RECEIVED state, and received a SYN-ACK packet with no data as:
SEG.SEQ = IRS
SEG.ACK = ISS + 1

However in SYN-RECEIVED state, RCV.NXT = IRS + 1, which means the SYN-ACK packet will fail on the second test above. The SYN-ACK packet will be dropped and a ACK packet is sent in reply. As a result, we lost the SYN part, but it is fine because we've already received SYN packet once. However, we also lost the ACK part which is supposed to be the ACK of our SYN. Thus we will never reach the ESTABLISH state.

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