On 1/16/08, Bill Fink <[EMAIL PROTECTED]> 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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
