Date: Tue, 27 Jan 1998 21:29:07 +0100 From: "Stefan Bethke" <stefan@promo.de> To: "Niall Smart" <njs3@doc.ic.ac.uk> Cc: hackers@FreeBSD.ORG Subject: Re: TCP_MAXSEG and path MTU discovery Message-ID: <2507803.3094925347@stefan.promo.de>
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--On Die, 27. Jan 1998 19:10 Uhr +0000 "Niall Smart" <njs3@doc.ic.ac.uk>
wrote:
> 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);
> 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)
Quoting from Stevens, TCP/IP Illustrated, Vol. II, pp. 1022:
TCP_MAXSEG option
[...] When a SYN is received from the other end with a MSS option, tcp_input
calls tcp_mss, and t_maxseg can be set as high as the outgoing interface
MTU. [1460 for 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 was selected by tcp_mss.
The logic here seems to be that a SYN with MSS is the equivalent of a
setsockopt(TCP_MAXSEG).
I've haven't read enought to say *why* this is useful, and Net/3 even
doesn't include path MTU discovery, so Stevens doesn't explain it's
implementation.
Stefan
--
Stefan Bethke
Promo Datentechnik | Tel. +49-40-851744-18
+ Systemberatung GmbH | Fax. +49-40-851744-44
Eduardstrasse 46-48 | e-mail: stefan@Promo.DE
D-20257 Hamburg | http://www.Promo.DE/
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