[PATCH] Add a new TCP_IGNOREIDLE socket option

[Sepherosa Ziehau]

On Thu, Jan 24, 2013 at 12:15 AM, John Baldwin <jhb at 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 at 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

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