How to send a packet using a binary communication protocol to the tcp port
I am working on a device that will work to measure some readings using sensors. The device is controlled by an Android app. I have to take readings from the TCP layer. This is the code for sending data over TCP
TcpClient.java
import android.util.Log;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;
/**
* Created by shahbaz on 25/4/17.
*/
public class TcpClient {
public static final String SERVER_IP = "192.168.1.76"; //server IP address
public static final int SERVER_PORT = 1800;
// message to send to the server
private String mServerMessage;
// sends message received notifications
private OnMessageReceived mMessageListener = null;
// while this is true, the server will continue running
private boolean mRun = false;
// used to send messages
private PrintWriter mBufferOut;
// used to read messages from the server
private BufferedReader mBufferIn;
/**
* Constructor of the class. OnMessagedReceived listens for the messages received from server
*/
public TcpClient(OnMessageReceived listener) {
mMessageListener = listener;
}
/**
* Sends the message entered by client to the server
*
* @param message text entered by client
*/
public void sendMessage(String message) {
if (mBufferOut != null && !mBufferOut.checkError()) {
mBufferOut.println(message);
mBufferOut.flush();
}
}
/**
* Close the connection and release the members
*/
public void stopClient() {
mRun = false;
if (mBufferOut != null) {
mBufferOut.flush();
mBufferOut.close();
}
mMessageListener = null;
mBufferIn = null;
mBufferOut = null;
mServerMessage = null;
}
public void run() {
mRun = true;
try {
//here you must put your computer IP address.
InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
Log.e("TCP Client", "C: Connecting...");
//create a socket to make the connection with the server
Socket socket = new Socket(serverAddr, SERVER_PORT);
try {
//sends the message to the server
mBufferOut = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);
//receives the message which the server sends back
mBufferIn = new BufferedReader(new InputStreamReader(socket.getInputStream()));
//in this while the client listens for the messages sent by the server
while (mRun) {
mServerMessage = mBufferIn.readLine();
if (mServerMessage != null && mMessageListener != null) {
//call the method messageReceived from MyActivity class
mMessageListener.messageReceived(mServerMessage);
}
}
Log.e("RESPONSE FROM SERVER", "S: Received Message: '" + mServerMessage + "'");
} catch (Exception e) {
Log.e("TCP", "S: Error", e);
} finally {
//the socket must be closed. It is not possible to reconnect to this socket
// after it is closed, which means a new socket instance has to be created.
socket.close();
}
} catch (Exception e) {
Log.e("TCP", "C: Error", e);
}
}
//Declare the interface. The method messageReceived(String message) will must be implemented in the MyActivity
//class at on asynckTask doInBackground
public interface OnMessageReceived {
public void messageReceived(String message);
}
}
Package structure
The format of the packet contains, When communicating with a device on TCP, the boundaries between packets are not identified, in this case, if the packets do not match the sequence or if any of the packets are missing, you can define a new packet using the "header start". So, the first 2 bytes in the packet represent the beginning of the packet.
Beginning of header: A two-byte field indicating the beginning of each packet. 0x55AA is the 2 byte number used to start the header.
Protocol version: a one-byte field to indicate the version of the protocol being used. The version specified in the payload will determine the structure of the payload. At any time, the device will support a version of one protocol. The present version of the protocol is "1.
DSN: Sequence Number is a 1-byte field that uniquely identifies a packet. The package applicant must complete this field in the request field information; the responder must fill in the same unique identifier in the response payload.
Request ID: A single byte field specifies the command ID. The parsing of the payload will be done based on the command ID. If a request is loaded, this field will be non-zero, and if a response is received, it will be equal to zero.
Payload Length: A two-byte field specifies the length of the payload in bytes. It defines the number of bytes following the payload length field. In the payload length, the header length and CRC are not included. Currently, the maximum payload length supported by the gateway is 512 (bytes). CRC: 1 byte field that will be calculated by XORing all bytes and adding an XOR count of 0.
And it works. But according to the docs, I have to send the packet using a binary communication protocol. Including header , payload data , etc. How can I send these parameters to the packet structure? How do I create a package?
Any help is appreciated.
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The main mistake was that I didn't think about the size of primitive data types.
byte = 1 byte
short = 2 bytes
int = 4 bytes
long = 8 bytes
float = 4 bytes
double = 8 bytes
char = 2 bytes
After referencing the size of primitive data types, I realized that we have to keep track of the size and index of the packet because we are dealing with an array of bytes.
TcpPacket.java
public class TcpPacket {
private static int header_start = 0x55AA;
private static int protocol_version = 1;
private PacketUtils packetUtils = new PacketUtils();
public byte[] getHandshakePacket()
{
int request_id = 1;
byte[] header_data = packetUtils.ItoBA2(header_start);
byte[] payload_data = packetUtils.ItoBA4(packetUtils.getDateTime());
byte[] payload_length = packetUtils.ItoBA2(4);
byte[] a_data = new byte[]{header_data[0], header_data[1], (byte) protocol_version, packetUtils.getDSN(), (byte) request_id, payload_length[0], payload_length[1],
payload_data[0], payload_data[1], payload_data[2], payload_data[3]};
byte[] b_data = new byte[]{ packetUtils.getCRC(a_data)};
byte[] packet_data = packetUtils.concatBytes(a_data,b_data);
return packet_data;
}
}
PacketUtils.java
public class PacketUtils {
public byte[] ItoBA4(int value) { // integer to bytes function (return byte array of 4 bytes)
return new byte[] {
(byte)(value >>> 24),
(byte)(value >>> 16),
(byte)(value >>> 8),
(byte)value};
}
public byte[] ItoBA2(int value) { // integer to bytes function (return byte array of 2 bytes)
return new byte[] {
(byte)(value >>> 8),
(byte)value};
}
public byte getDSN() // return one byte random number
{
char[] chars = "1234567890".toCharArray();
StringBuilder sb = new StringBuilder();
Random random = new Random();
for (int i = 0; i < 1; i++) {
char c = chars[random.nextInt(chars.length)];
sb.append(c);
}
byte output = Byte.valueOf(sb.toString());
return output;
}
public byte getCRC(byte[] packet) // required CRC function (return byte)
{
try
{
if (packet == null)
{
//Logger.Error("empty packet received");
return (byte)0;
}
byte XORCheckSum = 0;
byte zeroCount = 0;
byte FFCount = 0;
for (int i = 0; i < packet.length; i++)
{
XORCheckSum ^= packet[i];
if (packet[i] == (byte) 0)
{
zeroCount++;
continue;
}
if (packet[i] == (byte)255)
{
FFCount++;
continue;
}
}
XORCheckSum ^= zeroCount;
XORCheckSum ^= FFCount;
return XORCheckSum;
}
catch (Exception ex)
{
//Logger.Error(ex);
return (byte)0;
}
}
byte[] concatBytes(byte[]...arrays) // concatenate byte arrays
{
// Determine the length of the result array
int totalLength = 0;
for (int i = 0; i < arrays.length; i++)
{
totalLength += arrays[i].length;
}
// create the result array
byte[] result = new byte[totalLength];
// copy the source arrays into the result array
int currentIndex = 0;
for (int i = 0; i < arrays.length; i++)
{
System.arraycopy(arrays[i], 0, result, currentIndex, arrays[i].length);
currentIndex += arrays[i].length;
}
return result;
}
public int getDateTime()
{
int dateInSec = (int) (System.currentTimeMillis() / 1000);
return dateInSec;
}
}
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