One network connection affects another?

Question

We have a network connectivity issue that appears very surprising (at least to me). I'd expect two network interfaces (e.g., ethernet and wireless) on one PC to behave rather independently from each other, but in our case it seems that this is not the case.

We have a simple setup of a PC (let's call it "A") connected by cross-over ethernet to another PC ("B"). Together, they compose a private network using fixed IP addresses. There are two TCP socket connections that are open all the time - the piece of software running on A acts as the server; that on B as the client.

In addition, A maintains a WLAN connection to another network. This is obviously another interface, which one would expect to be largely independent from the aforementioned ethernet connection (in the WLAN connection, the IP address is obtained by DHCP and from a completely different address space).

Now the issue is: when the WLAN connection either drops or is re-established (manually or due to WLAN instability), the sockets on the ethernet interface start getting errors at the same time. Then, they need to be recreated, which causes an undesired pause in the communications between the two PCs (it's a 24/7 system that we're developing, and we'd expect the direct ethernet connection to never have any breaks -- not even for a second).

In other words: activity on one network interface clearly affects that on the other! Has anyone experienced anything like this? How could this happen, and what can we try to do in order to get rid of it? (Except for disabling the WLAN interface completely, which is not exactly an option, unfortunately.)

A is a Dell Precision M6500 running Windows 7, and B is a Dell Vostro 1510 running Windows XP. The network interfaces are integrated on the motherboard. We have tried to reproduce this on different hardware: sometimes it seems to be possible, sometimes apparently not (for example, Lenovo T400 as A and a self-built desktop PC as B works perfectly) -- perhaps depending on the specific models of the network interfaces and their drivers?!

Answer 1

Whoa! The issue appears to correlate heavily with the F-Secure firewall on A: when the firewall is enabled and the ethernet connection is not trusted (even if the application is allowed of course), then the behavior described above can be observed. However, when the firewall is disabled or the ethernet connection is configured as trusted, then it seems that the problems are gone!