netdev - SO_RCVBUF doesn't change receiver advertised window

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Message-ID: <f47983b00801151236l3b06f6dci35898fee71f6a942@mail.gmail.com>
Date:	Tue, 15 Jan 2008 15:36:15 -0500
From:	"Ritesh Kumar" <ritesh@...unc.edu>
To:	netdev@...r.kernel.org
Subject: SO_RCVBUF doesn't change receiver advertised window

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).

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.

The values for the tcp sysctl variables are given below:
/proc/sys/net/ipv4/tcp_moderate_rcvbuf  0
/proc/sys/net/ipv4/tcp_mem             32768   43690   65536
/proc/sys/net/ipv4/tcp_rmem            4096    87380   1398080
/proc/sys/net/ipv4/tcp_wmem           4096    16384   1398080
/proc/sys/net/core/rmem_max          131071
/proc/sys/net/core/wmem_max         131071
/proc/sys/net/core/wmem_default      109568
/proc/sys/net/core/rmem_default       109568

I will really appreciate your help,

Ritesh
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