Date: Wed, 17 Mar 1999 13:03:43 +0100 From: Stefan Bethke <stefan.bethke@hanse.de> To: boards yan <boardyan@yahoo.com> Cc: freebsd-net@FreeBSD.ORG Subject: Re: exhausted Message-ID: <652340.3130664623@d225.promo.de> In-Reply-To: <19990317050858.7049.rocketmail@web704.mail.yahoo.com>
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boards yan <boardyan@yahoo.com> wrote:
> I tried to set the tcp socket Option(SO_RCV,TCP_MAXSEG), but failed
> all the time.
>
> The program seems quite normal.
> However, the supervision using tcpdump shows that
> the option is unchanged. :-(
>
> Can anybody give me a reason?
[ code deleted ]
TCP_MAXSEG:
Quoting from Steven's TCP/IP Illustrated, Vol. II, pp. 1023:
A process can only decrease the MSS. When a TCP socket is created,
tcp_newtcpcb initializes t_maxseg to its default of 512. When a SYN is
received from the other end with an MSS option, tcp_input calls tcp_mss,
and t_maxseg can be set as high as the outgoing interface MTU (minus 40
bytes for the default IP and TCP headers), which is 1460 for an Ethernet.
Therefore, after a call to socket but before a connection is established, =
a
process can only decrease the MSS from its default of 512. After a
connection is established, the process can decrease the MSS from whatever
value was selected by tcp_mss.
What do you expect from SO_RCVBUF? With the following code it does what it
should:
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
void main(void)
{
int fd;
struct sockaddr_in sin;
int nrbuf;
if ((fd =3D socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror ("socket");
return;
}
nrbuf =3D 65536l;
if (setsockopt(fd, SOL_SOCKET, SO_RCVBUF, &nrbuf, sizeof(nrbuf)) < 0) =
{
perror ("setsockopt(SO_RCVBUF)");
return;
}
sin.sin_len =3D sizeof(sin);
sin.sin_family =3D AF_INET;
sin.sin_addr.s_addr =3D htonl(0x7f000001);
sin.sin_port =3D htons(23);
if (connect(fd, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
perror ("connect");
return;
}
close (fd);
}
tcpdump output:
12:54:05.561009 localhost.3828 > localhost.telnet: S
3917041307:3917041307(0) win 65535 <mss 16344,nop,wscale
1,nop,nop,timestamp 3761197 0,nop,nop,cc[|tcp]> (DF)
Without the setsockopt, tcpdump shows:
12:49:59.008210 localhost.3783 > localhost.telnet: S
3863968019:3863968019(0) win 16384 <mss 16344,nop,wscale
0,nop,nop,timestamp 3760704 0,nop,nop,cc[|tcp]> (DF)
Stefan
--
M=FChlendamm 12 | Voice +49-40-256848, +49-177-3504009
D-22089 Hamburg | e-mail: stefan.bethke@hanse.de
Germany | stb@freebsd.org
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