Date: Thu, 24 Jan 2013 16:10:51 -0500 From: Alfred Perlstein <bright@mu.org> To: John Baldwin <jhb@freebsd.org> Cc: Sepherosa Ziehau <sepherosa@gmail.com>, freebsd-net@freebsd.org, Bjoern Zeeb <bz@freebsd.org> Subject: Re: [PATCH] Add a new TCP_IGNOREIDLE socket option Message-ID: <5101A35B.2060104@mu.org> In-Reply-To: <201301241114.40734.jhb@freebsd.org> References: <201301221511.02496.jhb@freebsd.org> <CAMOc5cwhEEpZn0AM2hiXjpQYujLu%2BnZAb%2Bp%2B=USaE5JsQs6LLQ@mail.gmail.com> <5100EAD3.2090006@networx.ch> <201301241114.40734.jhb@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On 1/24/13 11:14 AM, John Baldwin wrote: > On Thursday, January 24, 2013 3:03:31 am Andre Oppermann wrote: >> On 24.01.2013 03:31, Sepherosa Ziehau wrote: >>> 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? >> No. This is far too dangerous once it makes it into some tuning guide. >> The threat of congestion breakdown is real. The Internet, or any packet >> network, can only survive in the long term if almost all follow the rules >> and self-constrain to remain fair to the others. What would happen if >> nobody would respect the traffic lights anymore? > The problem with this argument is Linux has already had this as a tunable > option for years and the Internet hasn't melted as a result. > >> Besides that bursting into unknown network conditions is very likely to >> result in burst losses as well. TCP isn't good at recovering from it. >> In the end you most likely come out ahead if you decay the restartCWND. >> >> We have two cases primarily: a) long distance, medium to high RTT, and >> wildly varying bandwidth (a.k.a. the Internet); b) short distance, low >> RTT and mostly plenty of bandwidth (a.k.a. Datacenter). The former >> absolutely definately requires a decayed restartCWND. The latter less >> so but even there bursting at 10Gig TSO assisted wirespeed isn't going >> to end too happy more often than not. > You forgot my case: c) dedicated long distance links with high bandwidth. > >> Since this seems to be a burning issue I'll come up with a patch in the >> next days to add a decaying restartCWND that'll be fair and allow a very >> quick ramp up if no loss occurs. > I think this could be useful. OTOH, I still think the TCP_IGNOREIDLE option > is useful both with and without a decaying restartCWND? > Linux seems to be doing just fine with it for what seems to be a long while. Can we get this committed? -Alfred
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?5101A35B.2060104>