Patrick Mahan schrieb am 18.02.2010 16:20 (localtime):
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Please clarify my TCP understanding.If I have the window set to 65535 in the header and a MSS of 1460, how often should the receiver send ACK segments? window/MSS, right?How soon you see the ACK is based on two values in the kernel: net.inet.tcp.delacktime net.inet.tcp.delayed_ack The first one controls how soon the peer replies with an ACK if there is no data to send back, ie. it is just a plain ack. Van Jacobson first recommended it in the early days of TCP/IP. Historically, it has been implemented as a 200 ms timer, but in FreeBSD it is a 100 ms timer.
Thank you for your hint. I heard of that but never thought about it, because 100ms is a magnitude higher than my µs LAN delay and since I'm not suffering from extremeley slow speeds.
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a) why disabling net.inet.tcp.rfc1323 gives slightly better rsync throughput than enabledrfc1323 deals with window scaling and timestamp options. Perhaps these are getting in the way?b) why I can't transfer more than 50MB/s over my direct linked GbE boxes.But right now I even don't understand the dump I see. As far as I understand I should only see every 45 data segments one ACK segment. That would clearly explain to me why I can't saturate my GbE link. But I can't imagine this is a uncovered faulty behaviour, so I guess I haven't understood TCP.No we are also seeing similar behavior over the em(4) interface under FreeBSD 8.0-Stable.
Some experimental results:When rsyncing with windows, and FreeBSD is receiver, I see the same ACK ever two segemnts, but speed is at 72MB/s. When FreeBSD is sender and Windows is receiver, it looks more I expected. There are about 20 data segments before a ACK is returned. And there are TCP Window Update Segments, reflecting smaller receiver buffers on the windows side. But this happens at a throughput of 82MB/s!!! So the windows machine is behaving like I understand the TCP flow control.
Any explanation why the FreeBSD machine seems to ignore window size? Thanks, -Harry
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