TCP connection timeout is 20 or 21 seconds on *some* PCs when set to 500ms

Question

I was given 10 new PCs, all (supposedly) with Windows 7 Pro freshly installed and nothing else done to them.

I have a program, coded in Delphi XE2, using Indy 10 components for the networking. I set the "connect timeout" and "read timeout" properties of my TIdTcpCleint to 500ms, set "resuse socket" to 'o/s dependant'" (I also tried a build with it set to No) and leave "use Nagle" (whatever that is set to True (I also tried with false).

Here's the problem: when I run the same .EXE on these PCs and test the case where I pull the network cable, my debug trace shows the connect attempt / connect timeout happening in the same second or the next second (with a granularity of 1 second) - but on others it is 20 or 21 seconds before I see the conenction timeout.

It would seem some of that the PCs are not totally "fresh install" as claimed, although I see no aps installed. Maybe some one installed somethign then removed it, maybe they tried to tweak performance.

Before I reinstall Windows on 10 PCs, can anyone suggest where to look? Does 20 (or 21) seconds ring a bell with regard to TCP Client connect timeout?

[update] I am attempting to connect directly to a specific IP Address, so I am not sure if @Nikolai suggestion to check DNS is relevant. Sorry for not mentioning this originally.

[upperdate] the program does not attempt to keep the socket open. It connects, sends some data & disconnects - repeatedly, for each new piece of data.

Answer 1

Sadly, this is working as intended. The connect did already timeout. Indy made the determination that the connect would fail in the 500 milliseconds that you asked it to. However, that does not guarantee the function will return.

After the connect times out, Indy spins down the connection to release all of its resources. It does this synchronously. This means that you wind up waiting for the underlying TCP operation to fail. This typically takes 20 seconds.

The solution is to call connect in a thread. Believe it or not, this is what Indy already does to implement the timeout. However, when it times out waiting for the thread, it tries to shut down the connection in the main thread. You need to defer that to a worker thread.

As for why it happens immediately on some systems and in 20 seconds on others, it depends on the precise networking configuration. For example, if IPv6 is enabled, the stack may attempt to use an IPv6-to-IPv4 connection, and that may not report down even if the physical interface is down. Immediate detection of connection impossibility is never guaranteed and you shouldn't rely on it.

Answer 2

I've had same problems with INDY in the past (while using D6, year 1998-2000). I changed the component to IP*Works. At that time it was an external component, but as far as I know it is included in XE2. Ip*Works is a bit hard to understand at the beginning but the way they approach to the communication structure is a lot different.

I think that it would be worth to give it a try.