When I have connection tracking enabled in the kernel and use the Space Communications Protocol Standard (SCPS), which is a protocol to speed up TCP over satellite, I can no longer start a video call using Polycom ViewStationFX. Using Wireshark, I see the "Connect" message is being dropped. At the same time, I see "nf_ct_q931: packet dropped" being printed to /var/log/messages. This happens because nf_ct_expect_related returns -EBUSY since expect_clash does not return zero. Without SCPS, the call works fine with connection tracking enabled.

Why does expect_clash return nonzero? What is the "clash" it is looking for and is there any reason that SCPS would cause such a clash? Is there any known incompatibility between SCPS and connection tracking?

I am running Linux 2.6.26 on PowerPC.