From owner-freebsd-hackers  Tue Jan 27 11:10:52 1998
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From: njs3@doc.ic.ac.uk (Niall Smart)
Date: Tue, 27 Jan 1998 19:10:38 +0000
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To: hackers@FreeBSD.ORG
Subject: TCP_MAXSEG and path MTU discovery
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Sender: owner-freebsd-hackers@FreeBSD.ORG
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Hi folks,

I've been playing with TCP/IP recently, and it looks like the TCP_MAXSEG
option is broken or interacting with path MTU discovery in a non-intuitive
way.  Or maybe I'm way off track here.

Anyway, I'm doing this:

        opt = atoi(argv[3]);
        optlen = sizeof(opt);

        if (setsockopt(fd, IPPROTO_TCP, TCP_MAXSEG, &opt, optlen) < 0)
                perror("setsockopt"), exit(1);
        
        if (connect(fd, (struct sockaddr*) &sin, sizeof(sin)) < 0)
                perror("connect"), exit(1);

        if (getsockopt(fd, IPPROTO_TCP, TCP_MAXSEG, &opt, &optlen) < 0)
                perror("getsockopt"), exit(1);
        else
                printf("%d\n", opt);

With argv[3] == "300", I'm getting this, 146.169.50.56 being my host.

19:04:07.726242 146.169.50.56.1171 > 146.169.46.7.21: S 1185413053:1185413053(0) win 16384 <mss 1460,nop,wscale 0,nop,nop,timestamp[|tcp]> (DF)
19:04:07.728505 146.169.46.7.21 > 146.169.50.56.1171: S 4095294624:4095294624(0) ack 1185413054 win 8760 <mss 1460> (DF)
19:04:07.728603 146.169.50.56.1171 > 146.169.46.7.21: . ack 4095294625 win 17520 (DF)

What happened to my maximum segment size??  (Yes, the program prints 1460)

Regards,

Niall