Java TCP Socket Protocol Implementation

Protocol Implementation

The following implementation details the core logic for managing TSocket connections, including matching, establishing, and closing connections.

Matching TSocket Instances

The getMatchingTSocket method identifies a socket based on local and remote port parameters:

protected TSocket getMatchingTSocket(int localPort, int remotePort) {
    lk.lock();
    try {
        for (TSocket ts : activeTSocks) {
            if (ts.localPort == localPort && ts.remotePort == remotePort) {
                return ts;
            }
        }
        for (TSocket ts : listenTSocks) {
            if (ts.localPort == localPort) {
                return ts;
            }
        }
        return null;
    } finally {
        lk.unlock();
    }
}

Passive Connection Handling (PASIU)

The accept method is used in passive mode to wait for and retrieve incoming connections from the queue:

public TSocket accept() {
    lk.lock();
    try {
        log.debug("%1$s->accept()", this);
        if (state != LISTEN) {
            return null;
        }
        while (acceptQueue.empty()) {
            appCV.awaitUninterruptibly();
        }
        log.debug("%1$s->accepted", this);
        return acceptQueue.get();
    } finally {
        lk.unlock();
    }
}

Active Connection Establishment (ACTIU)

In active mode, the connect method initiates the TCP handshake by sending a SYN segment:

protected void connect(int remPort) {
    lk.lock();
    try {
        log.debug("%s->connect(%d)", this, remPort);
        remotePort = remPort;
        proto.addActiveTSocket(this);
        TCPSegment syn = new TCPSegment();
        syn.setSyn(true);
        syn.setPorts(localPort, remotePort);
        this.sendSegment(syn);
        state = SYN_SENT;
        while (state != ESTABLISHED) {
            appCV.awaitUninterruptibly();
        }
    } finally {
        lk.unlock();
    }
}

Connection Termination and Closing

The close method handles the transition of the socket state to CLOSED using FIN segments:

public void close() {
    lk.lock();
    try {
        log.debug("%s->close()", this);
        switch (state) {
            case ESTABLISHED: {
                state = FIN_WAIT;
                TCPSegment fin = new TCPSegment();
                fin.setFin(true);
                fin.setPorts(this.localPort, this.remotePort);
                this.sendSegment(fin);
                while (state != CLOSED) {
                    appCV.awaitUninterruptibly();
                }
                break;
            }
            case CLOSE_WAIT: {
                state = CLOSED;
                TCPSegment fin = new TCPSegment();
                fin.setFin(true);
                fin.setPorts(this.localPort, this.remotePort);
                this.sendSegment(fin);
                appCV.signal();
                proto.removeActiveTSocket(this);
                break;
            }
            default:
                log.error("%s->close: connection does not exist", this);
        }
    } finally {
        lk.unlock();
    }
}