Date: Thu, 24 Jan 2013 10:31:30 +0800 From: Sepherosa Ziehau <sepherosa@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>, Bjoern Zeeb <bz@freebsd.org> Subject: Re: [PATCH] Add a new TCP_IGNOREIDLE socket option Message-ID: <CAMOc5cwhEEpZn0AM2hiXjpQYujLu%2BnZAb%2Bp%2B=USaE5JsQs6LLQ@mail.gmail.com> In-Reply-To: <201301231115.06393.jhb@freebsd.org> References: <201301221511.02496.jhb@freebsd.org> <CAMOc5czyB=c0fQ%2BHnYdZf0Ym7wPQsXzR-b81yWg%2BLwziZeCQOA@mail.gmail.com> <201301231115.06393.jhb@freebsd.org>
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On Thu, Jan 24, 2013 at 12:15 AM, John Baldwin <jhb@freebsd.org> wrote: > On Wednesday, January 23, 2013 1:33:27 am Sepherosa Ziehau wrote: >> On Wed, Jan 23, 2013 at 4:11 AM, John Baldwin <jhb@freebsd.org> wrote: >> > As I mentioned in an earlier thread, I recently had to debug an issue we were >> > seeing across a link with a high bandwidth-delay product (both high bandwidth >> > and high RTT). Our specific use case was to use a TCP connection to reliably >> > forward a latency-sensitive datagram stream across a WAN connection. We would >> > often see spikes in the latency of individual datagrams. I eventually tracked >> > this down to the connection entering slow start when it would transmit data >> > after being idle. The data stream was quite bursty and would often attempt to >> > transmit a burst of data after being idle for far longer than a retransmit >> > timeout. >> > >> > In 7.x we had worked around this in the past by disabling RFC 3390 and jacking >> > the slow start window size up via a sysctl. On 8.x this no longer worked. >> > The solution I came up with was to add a new socket option to disable idle >> > handling completely. That is, when an idle connection restarts with this new >> > option enabled, it keeps its current congestion window and doesn't enter slow >> > start. >> > >> > There are only a few cases where such an option is useful, but if anyone else >> > thinks this might be useful I'd be happy to add the option to FreeBSD. >> >> I think what you need is the RFC2861, however, you probably should >> ignore the "application-limited period" part of RFC2861. > > Hummm. It appears btw, that Linux uses RFC 2861, but has a global knob to > disable it due to applictions having problems. When it is disabled, > it doesn't decay the congestion window at all during idle handling. That is, > it appears to act the same as if TCP_IGNOREIDLE were enabled. > > From http://www.kernel.org/doc/man-pages/online/pages/man7/tcp.7.html: > > tcp_slow_start_after_idle (Boolean; default: enabled; since Linux 2.6.18) > If enabled, provide RFC 2861 behavior and time out the congestion > window after an idle period. An idle period is defined as the current > RTO (retransmission timeout). If disabled, the congestion window will > not be timed out after an idle period. > > Also, in this thread on tcp-m it appears no one on that list realizes that > there are any implementations which follow the "SHOULD" in RFC 2581 for idle > handling (which is what we do currently): Nah, I don't think the idle detection in FreeBSD follows the RFC2581/RFC5681 4.1 (the paragraph before the "SHOULD"). IMHO, that's probably why the author in the following email requestioned about the implementation of "SHOULD" in RFC2581/RFC5681. > > http://www.ietf.org/mail-archive/web/tcpm/current/msg02864.html > > So if we were to implement RFC 2861, the new socket option would be equivalent > to setting Linux's 'tcp_slow_start_after_idle' to false, but on a per-socket > basis rather than globally. Agree, per-socket option could be useful than global sysctls under certain situation. However, in addition to the per-socket option, could global sysctl nodes to disable idle_restart/idle_cwv help too? Best Regards, sephe -- Tomorrow Will Never Die
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