TcpSocket::connect does not properly handle an already connected socket

[ryan-summers]

It appears that smoltcp does not properly handle an unexpected ACK when in the SYNC-SENT state. This results in the TCP socket being in an unusable state.

These conditions are described here, but the basics are that a previous connection (which was not properly terminated with a FIN) results in the server having some established TCP state (server is in ESTABLISHED).

When the client sends the SYN packet, the server acknowledges with an ACK#x, which is invalid to receive in the SYNC-SENT state. smoltcp should recognize this state and instead send a RST to the server to properly terminate the previous connection. It should then restart the SYN handshake to establish a connection with the server.

Sample of wireshark capture showing the error

[ryan-summers]

@whitequark I have a fix that basically just aborts() the connection if an ACK is received after SYN (without the SYN bit set). The result is that the end user has to re-connect() the socket, but keeps the details and statefulness to a minimum in the smoltcp TCP implementation. Would we rather implement logic to restart the 3-way handshake after the RST is sent?

[whitequark]

but keeps the details and statefulness to a minimum in the smoltcp TCP implementation.

I like this approach. If it turns out to be insufficient, we can always go for a more complex one later.