Does 4.4 FreeBSD kernel supports TCP simultaneous open?

[cfliu]

Hi, I am tracing the FreeBSD 4.4 kernel and I found that its TCP seems NOT be able to do simultaneous open according to the source code. In tcp_input.c,  even though code near line #1750,   case TCPS_SYN_RECEIVED:    .....   if (tp->t_flags & TF_NEEDFIN) {    tp->t_state = TCPS_FIN_WAIT_1;    tp->t_flags &= ~TF_NEEDFIN;   } else {    tp->t_state = TCPS_ESTABLISHED;    callout_reset(tp->tt_keep, tcp_keepidle,           tcp_timer_keep, tp);   }   do change state from SYN_RCVD state to ESTABLISHED, however, near line 1700 , the code fragment   if (thflags & TH_SYN) {   tp = tcp_drop(tp, ECONNRESET);   rstreason = BANDLIM_UNLIMITED;   goto dropwithreset;  }    drops a packet with SYN bit set in SYN_RCVD state. I think this would break TCP's simultaneous open.   Since upon receiving a SYN segment in SYN_SENT state, TCP switches to SYN_RCVD state and sends a (ACK,SYN) segment to the peer, if the peer drops segments with SYN bit set during SYN_RCVD state, the simultaneous open mechanism of TCP would break.   Am I correct?   David.