Problem reading from server socket

[glauber]

Hey guys,

I've implemented a simple TCP server using the ServerSocket class (code below) to communicate with a client. They should just exchange strings. The client comes off-the-shelf with a sensor I need to use and, although I don't have access to its source code, I know all the commands I can R/W from it. The server starts listening and as soon as the client connects to it, the client is supposed to send a string with 3 initialization commands of the type "command 1\r\n command 2\r\n command 3\r\n". \r\n denotes the end of a command.

The problem is, my server can't seem to read this initial string and then the client has a timeout because the server doesn't reply appropriately. I know the initial string packet is being sent because I can see it with a network sniffer (Wireshark). Does anyone have an idea why my server hasn't been able to read it?

Thanks,
Glauber

function main

    import java.net.ServerSocket
    import java.io.*
    
    port = 16544;
    server = [];
    s = [];
    command = [];
    
    try
        % Creates the server socket if port is available
        fprintf(1, ['Waiting for the client to connect...\n']);
        server = ServerSocket(port);

        % Listens on the specified port
        server.setSoTimeout(0); % Wait forever
        s = server.accept; % Start listening
        fprintf(1, 'Server connected\n');

        % Socket options
        s.setKeepAlive(true) % Heartbeat
        s.setTcpNoDelay(true) % Nable's algorithm off

        % R/W streams
        in = BufferedReader(InputStreamReader(s.getInputStream));
        out = DataOutputStream(s.getOutputStream);

        while true

            % read data
            command = in.readLine;
            if ~isempty(command)
                command
            end

            command = char(command);
            
            % write data
            CRLF = [char(13),char(10)];
            switch command
                case 'command 1'
                    out.writeBytes(['reply 1',CRLF]);
                    out.flush;
                case 'command 2'
                    out.writeBytes(['reply 2',CRLF]);
                    out.flush;
            end
        end

        % clean up
        server.close
        s.close
        return

    catch exception
        
        if ~isempty(server)
            server.close
        end

        if ~isempty(s)
            s.close
        end
        
        throw(exception)
    end
end

[toadaly]

You don't have a default case in your switch. My suspicion is that command is neither 'command 1' nor 'command 2', but is instead 'command 1\r' and 'command 2\r'.

[glauber]

My bad - I should have mentioned that the switch is there just as an example of how I intend to implement the writing part. It also allows me to see if the server can write correctly when I debug it through telnet. I have a list of 40+ commands the switch will deal with but I didn't want to spend too much time on it before making sure my server can read them.

The problem is at the read portion. My

command = in.readLine;

variable returns empty, although I can see the packets being sent by the client with Wireshark. Can it be a timing issue? Like, if the client sends the packet right after connecting, could it be that the server is not yet ready to read? If so, what route can I take to solve it?

[glauber]

If it helps, here's a screenshot of the network capture during the conversation. 170.170.6.5 is the client and .50 is the server.

[firer]

I don't see a timing problem here; readLine should block until there's enough data in the buffer. If you're using Wireshark, I think it could be useful if you could confirm that the incoming stream is exactly as you're thinking. I find it strange that the client is sending a FIN+ACK packet right after the connection is established. This is probably closing the connection before anything is sent, but I don't remember all the details of the TCP protocol to confirm you this.

EDIT: You can filter the specific TCP conversation using ContextMenu -> Apply as Filter -> Selected, and can see the TCP data using Context Menu -> Follow TCP Conversation.

[glauber]

firer,

Thx for the reply. The incoming strings are arriving as expected. The problem was elsewhere. While the ServerSocket needs to be opened only once, the socket resulting from the serversocket.accept listener should be renewed continuously to deal with incoming client requests. The client tried to send data after the FIN+ACK package but by that time connection through the first listener socket was already closed. Putting serversocket.accept in a loop solved it. So the correct code should look something like this:

function main

    import java.net.ServerSocket
    import java.io.*
    
    port = 16544;
    server = [];
    s = [];
    command = [];
    
    try
        % Creates the server socket if port is available
        fprintf(1, ['Waiting for the client to connect...\n']);
        server = ServerSocket(port);

        % Listens on the specified port
        server.setSoTimeout(0); % Wait forever
        s = server.accept; % Start listening
        fprintf(1, 'Client connected\n');

        % R/W streams
        in = BufferedReader(InputStreamReader(s.getInputStream));
        out = DataOutputStream(s.getOutputStream);
        
        while true
            
            % read data
            command = in.readLine;
            if ~isempty(command) & command~=-1
                command
            end

            command = char(command);
            
            % write data
            CRLF = [char(13),char(10)];
            switch command
                case 'command 1'
                    out.writeBytes(['reply 1',CRLF]);
                    out.flush;
                case 'command 2'
                    out.writeBytes(['reply 2',CRLF]);
                    out.flush;
            end
            
            % Repeatedly answer client requests (reopens listen socket)
            if isempty(command)
                
                s.close
                s = server.accept; % Start listening

                % R/W streams
                in = BufferedReader(InputStreamReader(s.getInputStream));
                out = DataOutputStream(s.getOutputStream);
            end
        end

        % clean up
        s.close
        server.close
        return

    catch exception
        
        if ~isempty(server)
            server.close
        end

        if ~isempty(s)
            s.close
        end
        
        throw(exception)
    end
end