RFC 793, "Transmission Control Protocol", September 1981

Note: This RFC has been obsoleted by RFC 9293

Note: This RFC has been updated by RFC 1122, RFC 3168, RFC 6093, RFC 6528

Errata ID: 1567
Status: Rejected
Type: Technical
Publication Format(s) : TEXT
Reported By: Constantin Hagemeier
Date Reported: 2008-10-11
Rejected by: Wesley Eddy
Date Rejected: 2012-05-29

Section 3.9 says:

SEND Call
LISTEN STATE
      If the foreign socket is specified, then change the connection
      from passive to active, select an ISS.  Send a SYN segment, set
      SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT state.  Data
      associated with SEND may be sent with SYN segment or queued for
      transmission after entering ESTABLISHED state.  The urgent bit if
      requested in the command must be sent with the data segments sent
      as a result of this command.  If there is no room to queue the
      request, respond with "error:  insufficient resources".  If
      Foreign socket was not specified, then return "error:  foreign
      socket unspecified".

It should say:

SEND Call
LISTEN STATE
      If the foreign socket is specified, then change the connection
      from passive to active, select an ISS.  Send a SYN segment, set
      SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT state.  Data
      associated with SEND may be sent with SYN segment or queued for
      transmission after entering ESTABLISHED state. If data is sent with
      the SYN segment, SND.NXT is advanced.    The urgent bit if
      requested in the command must be sent with the data segments sent
      as a result of this command.  If there is no room to queue the
      request, respond with "error:  insufficient resources".  If
      Foreign socket was not specified, then return "error:  foreign
      socket unspecified".

Notes:

If data is sent, SND.NXT has to be advanced.

But there are more problems.
What WND to set in the SYN segment? Set RCV.WND=1?
How much data may be put into the segment? There is no SND.WND yet.
What about PUSH? May the data be queued if a PUSH is given?
--VERIFIER NOTES--
As discussed on the TCPM Working Group mailing list in 2012:

According to the errata, there is some ambiguity in RFC 793 regarding data in SYNs.
This requires changes beyond the suggested errata text, and thus we believe
this issue should be done in a separate RFC.

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