Re: Data in SYN and SYN acknowledgement packets

["Guy Harris" <guy () alum mit edu>]

Justin Robinson said:
> Can someone please confirm that these packets are not supposed to have a
>  payload?

No, they can't confirm that, because it's not true.

To quote RFC 793 "Transmission Control Protocol":

  Sequence Number:  32 bits

      The sequence number of the first data octet in this segment (except
      when SYN is present). If SYN is present the sequence number is the
      initial sequence number (ISN) and the first data octet is ISN+1.

and

  LISTEN STATE

        If active and 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.

so it's perfectly legitimate to put data in a SYN segment.



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