Date: Mon, 11 Aug 2014 10:40:10 -0700 From: hiren panchasara <hiren.panchasara@gmail.com> To: Vlad Zolotarov <vladz@cloudius-systems.com> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: TCP Rx window auto sizing relies on TCP timestamp option? Message-ID: <CALCpEUG5cChteYa0JAYcf9gg%2Bfkd0vGkzBJ7DMw97NYXitMfUw@mail.gmail.com> In-Reply-To: <53E8B424.2000904@cloudius-systems.com> References: <53E8B424.2000904@cloudius-systems.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Mon, Aug 11, 2014 at 5:16 AM, Vlad Zolotarov <vladz@cloudius-systems.com> wrote: > Hi, I have the most strange question about the TCP Rx window auto sizing > implementation in a FreeBSD networking stack. > When I looked at the FreeBSD code (hash > 9abce0e567c9a5a0520cdd94d5c633c7baf9a184) I noticed that > the mentioned above feature will not be "enabled" if there isn't a TCP > timestamp option present in the current TCP session: > > See sys/netinet/tcp_input.c: line 1813 in tcp_do_segment() function: > > if (V_tcp_do_autorcvbuf && > *to.to_tsecr* && <-------- this is what I'm > talking about > (so->so_rcv.sb_flags & SB_AUTOSIZE)) > > So, if i read the code correctly, if there isn't a TS option (negotiated and > thus present in every received packet) the receive socket buffer won't grow > thus preventing the growth of the Rx window. > If that's the case this is very strange since TS option is not promised and > even more - in many cases it won't be present. > For example in Linux this feature is disabled by default (controlled by > /proc/sys/net/ipv4/tcp_timestamps). > This is how I actually noticed the problem the first place: I ran iperf test > where Linux was an initiator and a transmitter (iperf -c) FreeBSD box was a > receiver (iperf -s) and I noticed that the Rx window wasn't opening up > because Linux box hasn't negotiated the TS option in the SYN. As a result, > the throughput numbers were significantly lower compared to Linux-to-Linux > setup (Linux uses a Dynamic Right-Sizing (DRS) algorithm > http://public.lanl.gov/radiant/pubs.html#DRS, which doesn't rely on TS). > > Could anybody comment on this, pls.? > Did I miss anything? > Is it true that FreeBSD assumes that TS option is always present and if not > how can I cause an Rx Window to open up when TS option hasn't been > negotiated? In my limited understanding, we (FreeBSD) want to base Rx window size increase on accurate RTT which is not possible if TS option is not negotiated. And that's why this requirement. Do you know how does it work in Linux? cheers, Hiren
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CALCpEUG5cChteYa0JAYcf9gg%2Bfkd0vGkzBJ7DMw97NYXitMfUw>