[Twisted-Python] TCP KeepAlive

Brian Warner warner at lothar.com
Sun Oct 2 15:18:56 EDT 2005

> 1. When the TCP connection times-out, is pb.Copyable.stoppedObserving()
> called?

I think you mean pb.Cacheable.stoppedObserving, right? pb.Copyable is

I see a call to stoppedObserving() inside Broker.connectionLost, so I'd
hazard a guess and say yes, when the TCP connection is lost (which could be
due to timeout, or the other end closing the connection, or the near end
closing the connection), all the current Cacheables will have their
stoppedObserving methods invoked.

Remember, however, that TCP timeouts are somewhat tricky, and rather
"forgiving": they are intended to ignore "transient" network failures that
only last a few minutes (once upon a time, when the internet was a slower
place than it is now, and the phrase "five nines" referred to a dubious but
still lucky poker hand, connection losses of several minutes at a time were
no cause for alarm, and certainly not a reason to abandon your hard-won
RFC88 Remote Job Entry Protocol session).

The primary timer is a "short" exponential-backoff retransmission timer for
data that has been sent but not yet acknowledged. It depends upon the kernel,
but I tend to see this one give up on the connection after 5 to 15 minutes of

If you set the SO_KEEPALIVE option, a second timer is activated which
basically pings the remote host every once in a while (although it does this
within the context of the TCP session, so it will also detect a remote host
that has shut down your connection but for which the FIN somehow went
missing). This helps detect remote hosts that have powered off abruptly (so
they weren't able to kill off processes and thus send FIN packets to
terminate the TCP connections), and failures in intermediate routers (or,
more commonly, a NAT box which has forgotten about the connection because it
hasn't sent any traffic for 10 or 20 minutes). If the connection is busy,
that is if each end sends some data to the other every couple of minutes,
then the normal retransmission timer will catch connectivity losses after 5
to 15 minutes.

The default interval for the keepalive timer tends to be one the order of 2
hours, plus it tries several times if it can't get through, which adds
another 10 or 15 minutes. So even with SO_KEEPALIVE turned on, it may be
hours before an otherwise-idle connection is detected as being broken.

This drove me crazy on the buildbot, because many of the buildslaves are
behind NAT boxes with short (20 minute) connection timeouts, and because the
buildslaves can be idle for days at a time (waiting for someone to make an
SVN checkin). So I added some application-level keepalives. The actual scheme
I used is kind of weird, I'm not sure I would recommend it for new code, but
the basic idea was to simply add a 'def remote_doNothing(self): pass' method
to a pb.Referenceable at the far end, set up a timer to invoke
target.callRemote("doNothing") once every 10 minutes, and then let the TCP
retransmission timer take care of the rest. On top of that, you could reduce
the extra traffic by keeping track of your normal callRemote invocations and
setting an .activity flag, and then only send the remote_doNothing if there
hadn't been any normal activity in the last 10 minutes. You could also detect
connectivity losses faster by starting a shorter (perhaps 3 minute) timer
when you send the doNothing, and if it doesn't complete (i.e. its Deferred
doesn't fire) before that timer expires, abandon the connection with
target.broker.transport.loseConnection() . If you choose this short timeout,
remember that it could be triggered accidentally if you have a method which
pushes a large amount of data over a slow (dialup) connection: this bit us
several times when the buildslave was dumping _trial_temp/test.log up to the
master, because it filled the Broker's transmit buffer with data that took 5
to 10 minutes to send, and the app-level keepalive message got queued after
that, so the response couldn't possibly get back in time.

Anyways, probably a larger answer than you really wanted :). Hope you (or
someone else, some day) finds it useful.


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