[PATCH] Add a new TCP_IGNOREIDLE socket option

[John Baldwin]

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):

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.

-- 
John Baldwin