Date: Thu, 29 Nov 2001 12:09:53 +0800 From: "¼B¾JÂ×" <cfliu@realtek.com.tw> To: <freebsd-net@freebsd.org> Subject: Does 4.4 kernel supports TCP simultaneous open? Message-ID: <002801c1788b$b2dc3dd0$a22314ac@RTCN3848>
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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,=20
case TCPS_SYN_RECEIVED:
.....
if (tp->t_flags & TF_NEEDFIN) {
tp->t_state =3D TCPS_FIN_WAIT_1;
tp->t_flags &=3D ~TF_NEEDFIN;
} else {
tp->t_state =3D TCPS_ESTABLISHED;
callout_reset(tp->tt_keep, tcp_keepidle,=20
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 =3D tcp_drop(tp, ECONNRESET);
rstreason =3D BANDLIM_UNLIMITED;
goto dropwithreset;
}
drops a packet with SYN bit set in SYN_RCVD state. I think this would =
break TCP's simultaneous open.=20
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.=20
Am I correct?
David.
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<DIV><FONT size=3D2>Hi, I am tracing the FreeBSD 4.4 kernel and I found =
that its=20
TCP seems NOT be able to do simultaneous open according to the =
source code.=20
In tcp_input.c, e</FONT><FONT size=3D2>ven though code near line =
#1750,=20
</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>case TCPS_SYN_RECEIVED:</FONT></DIV>
<DIV><FONT size=3D2> .....</FONT></DIV>
<DIV><FONT size=3D2> if (tp->t_flags & TF_NEEDFIN)=20
{<BR> tp->t_state =3D=20
TCPS_FIN_WAIT_1;<BR> tp->t_flags &=3D=20
~TF_NEEDFIN;<BR> } else {<BR> tp->t_state =
=3D=20
TCPS_ESTABLISHED;<BR> callout_reset(tp->tt_keep,=20
tcp_keepidle, <BR> =
tcp_timer_keep, tp);<BR> }</FONT></DIV>
<DIV> </DIV>
<DIV><FONT size=3D2>do change state from SYN_RCVD state to ESTABLISHED, =
however,=20
near line 1700 , the code fragment
<DIV> </DIV>
<DIV><FONT size=3D2>if (thflags & TH_SYN) {<BR> tp =3D =
tcp_drop(tp,=20
ECONNRESET);<BR> rstreason =3D =
BANDLIM_UNLIMITED;<BR> goto=20
dropwithreset;<BR> }<BR></FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2> drops a packet with SYN bit set in SYN_RCVD =
state. I=20
think this would break TCP's simultaneous open. </FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Since upon receiving a SYN segment in SYN_SENT =
state, TCP=20
switches to SYN_RCVD state and sends a (ACK,SYN) segment to the peer, if =
the=20
peer drops segments with SYN bit set during SYN_RCVD state, the =
simultaneous=20
open mechanism of TCP would break. </FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Am I correct?</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV>David.</DIV>
<DIV> </DIV></FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2> </DIV></FONT></BODY></HTML>
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