All TCP messages get filtered in Ip6::PassToHost

[Irving-cl]

With current implementation of ip6 filtering and TCP handling, when OPENTHREAD_CONFIG_TCP_ENABLE is enabled, TCP messages will only be handled by the OT stack and will never be passed to the host.

In Ip6::PassToHost:

#if OPENTHREAD_CONFIG_TCP_ENABLE
        // Do not pass TCP message to avoid dual processing from both
        // OpenThread and POSIX TCP stacks.
        case kProtoTcp:
            error = kErrorNoRoute;
            ExitNow();
#endif

There are some cases where we hope the TCP messages being passed to the host. For example, I'm using iperf3. The iperf client will first send a TCP message when starting the test. With current implementation, the message will be blocked here and not go to the host. Shall we consider to add some mechanism to allow part of the TCP messages go to the host while the others will be handled by OT stack.

@abtink @samkumar

[jwhui]

So far, we have not had a use case for needing to handle TCP sessions inside of OT. That's why we simply disable TCP handling within OT on posix-based systems.

If we have specific use cases for needing to handle TCP sessions both inside and outside of OT, then we'll need to figure out how to implement that.

[Irving-cl]

Make sense. I'm actually working on NCP and considering if we should disable TCP in NCP build.

When I disabled OT_TCP in NCP, I found that OPENTHREAD_CONFIG_DNS_CLIENT_OVER_TCP_ENABLE depends on it. But as I understand, DNS client is mainly for single-chip cases (like cli). When we use NCP, there must be a host. And a seperate dns client should run on the host. So I think we should disable both of them on NCP. Thoughts?

[jwhui]

Yes, I think we can rely on DNS client on host for now.