netdev - Re: SO_RCVBUF doesn't change receiver advertised window

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Message-ID: <f47983b00801161127u5fa4de0bgf53bb3b8fc57a3c7@mail.gmail.com>
Date:	Wed, 16 Jan 2008 14:27:44 -0500
From:	"Ritesh Kumar" <ritesh@...unc.edu>
To:	"Bill Fink" <billfink@...dspring.com>
Cc:	netdev@...r.kernel.org
Subject: Re: SO_RCVBUF doesn't change receiver advertised window

On 1/16/08, Bill Fink <billfink@...dspring.com> wrote:
> On Tue, 15 Jan 2008, Ritesh Kumar wrote:
>
> > Hi,
> >     I am using linux 2.6.20 and am trying to limit the receiver window
> > size for a TCP connection. However, it seems that auto tuning is not
> > turning itself off even after I use the syscall
> >
> > rwin=65536
> > setsockopt(sock, SOL_SOCKET, SO_RCVBUF, &rwin, sizeof(rwin));
> >
> > and verify using
> >
> > getsockopt(sock, SOL_SOCKET, SO_RCVBUF, &rwin, &rwin_size);
> >
> > that RCVBUF indeed is getting set (the value returned from getsockopt
> > is double that, 131072).
>
> Linux doubles what you requested, and then uses (by default) 1/4
> of the socket space for overhead, so you effectively get 1.5 times
> what you requested as an actual advertised receiver window, which
> means since you specified 64 KB, you actually get 96 KB.
>
> > The above calls are made before connect() on the client side and
> > before bind(), accept() on the server side. Bulk data is being sent
> > from the client to the server. The client and the server machines also
> > have tcp_moderate_rcvbuf set to 0 (though I don't think that's really
> > needed; setting a value to SO_RCVBUF should automatically turnoff auto
> > tuning.).
> >
> > However the tcp trace shows the SYN, SYN/ACK and the first few packets as:
> > 14:34:18.831703 IP 192.168.1.153.45038 > 192.168.2.204.9999: S
> > 3947298186:3947298186(0) win 5840 <mss 1460,sackOK,timestamp 2842625
> > 0,nop,wscale 5>
> > 14:34:18.836000 IP 192.168.2.204.9999 > 192.168.1.153.45038: S
> > 3955381015:3955381015(0) ack 3947298187 win 5792 <mss
> > 1460,sackOK,timestamp 2843649 2842625,nop,wscale 2>
> > 14:34:18.837654 IP 192.168.1.153.45038 > 192.168.2.204.9999: . ack 1
> > win 183 <nop,nop,timestamp 2842634 2843649>
> > 14:34:18.837849 IP 192.168.1.153.45038 > 192.168.2.204.9999: .
> > 1:1449(1448) ack 1 win 183 <nop,nop,timestamp 2842634 2843649>
> > 14:34:18.837851 IP 192.168.1.153.45038 > 192.168.2.204.9999: P
> > 1449:1461(12) ack 1 win 183 <nop,nop,timestamp 2842634 2843649>
> > 14:34:18.839001 IP 192.168.2.204.9999 > 192.168.1.153.45038: . ack
> > 1449 win 2172 <nop,nop,timestamp 2843652 2842634>
> > 14:34:18.839011 IP 192.168.2.204.9999 > 192.168.1.153.45038: . ack
> > 1461 win 2172 <nop,nop,timestamp 2843652 2842634>
> > 14:34:18.840875 IP 192.168.1.153.45038 > 192.168.2.204.9999: .
> > 1461:2909(1448) ack 1 win 183 <nop,nop,timestamp 2842637 2843652>
> > 14:34:18.840997 IP 192.168.1.153.45038 > 192.168.2.204.9999: .
> > 2909:4357(1448) ack 1 win 183 <nop,nop,timestamp 2842637 2843652>
> > 14:34:18.841120 IP 192.168.1.153.45038 > 192.168.2.204.9999: .
> > 4357:5805(1448) ack 1 win 183 <nop,nop,timestamp 2842637 2843652>
> > 14:34:18.841244 IP 192.168.1.153.45038 > 192.168.2.204.9999: .
> > 5805:7253(1448) ack 1 win 183 <nop,nop,timestamp 2842637 2843652>
> > 14:34:18.841388 IP 192.168.2.204.9999 > 192.168.1.153.45038: . ack
> > 2909 win 2896 <nop,nop,timestamp 2843655 2842637>
> > 14:34:18.841399 IP 192.168.2.204.9999 > 192.168.1.153.45038: . ack
> > 4357 win 3620 <nop,nop,timestamp 2843655 2842637>
> > 14:34:18.841413 IP 192.168.2.204.9999 > 192.168.1.153.45038: . ack
> > 5805 win 4344 <nop,nop,timestamp 2843655 2842637>
> >
> > As you can see, the syn and syn ack show rcv windows to be 5840 and
> > 5792 and it automatically increases for the receiver to values 2172
> > till 4344 and more in the later part of the trace till 24214.
>
> Since the window scale was 2, the final advertised receiver window
> you indicate of 24214 gives 2^2*24214 or right around 96 KB, which
> is what is expected given the way Linux works.
>
>                                                 -Bill

Thanks for the explanation Bill. That surely clears part of my doubt.
However, why doesn't linux advertise 24214 in the SYN packets? I was
hoping that the moment I setup a RCVBUF, linux would pre-allocate
buffers and drop any autotuning. Doesn't the above behavior count as
autotuning?


Ritesh
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