After discussion with a super-smart colleague, we think we have figured out what's wrong, but are yet to prove this. What is likely happening is that the keep-alive interval is only taken into account when no keep-alive ACK is received. So after 2 hours, if no ACK was received to this first keep-alive packet, a second packet would be sent after 75 seconds and repeatedly at 75 second intervals until an ACK was received.

It turns out that the reason the interval is taken into account only when it is greater than the keep-alive time is due to the way the Linux keep-alive mechanism works, as described on my other question.

After discussion with a super-smart colleague, we think we have figured out what's wrong, but are yet to prove this. What is likely happening is that the keep-alive interval is only taken into account when no keep-alive ACK is received. So after 2 hours, if no ACK was received to this first keep-alive packet, a second packet would be sent after 75 seconds and repeatedly at 75 second intervals until an ACK was received.

It turns out that the reason the interval is taken into account only when it is greater than the keep-alive time is due to the way the Linux keep-alive mechanism works, as described on my other question.

After discussion with a super-smart colleague, we think we have figured out what's wrong, but are yet to prove this. What is likely happening is that the keep-alive interval is only taken into account when no keep-alive ACK is received. So after 2 hours, if no ACK was received to this first keep-alive packet, a second packet would be sent after 75 seconds and repeatedly at 75 second intervals until an ACK was received.

We will try and prove this with DummyNet or similar.

What this doesn't answer though is why the interval is taken into account when it is greater than the keep-alive time.

After discussion with a super-smart colleague, we think we have figured out what's wrong, but are yet to prove this. What is likely happening is that the keep-alive interval is only taken into account when no keep-alive ACK is received. So after 2 hours, if no ACK was received to this first keep-alive packet, a second packet would be sent after 75 seconds and repeatedly at 75 second intervals until an ACK was received.

It turns out that the reason the interval is taken into account only when it is greater than the keep-alive time is due to the way the Linux keep-alive mechanism works, as described on my other question.