1 cmsc628: introduction to mobile computing nilanjan banerjee mobile systems programming...
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1
CMSC628: Introduction to Mobile Computing
Nilanjan Banerjee
Mobile Systems Programming (Acknowledgment to Deepa Shinde and Cindy Atheron
University of MarylandBaltimore County, MD
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Socket Programming
TCP and UDP
3
Socket programming
Socket API• introduced in BSD4.1
UNIX, 1981• explicitly created, used,
released by apps • client/server paradigm • two types of transport
service via socket API: – unreliable datagram – reliable, byte stream-
oriented
a host-local, application-created,
OS-controlled interface (a “door”) into which
application process can both send and
receive messages to/from another
application process
socket
Goal: learn how to build client/server application that communicate using sockets
4
TCP
2: Application Layer5
Socket-programming using TCP
Socket: a door between application process and end-end-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one process to another
process
TCP withbuffers,
variables
socket
controlled byapplicationdeveloper
controlled byoperating
system
host orserver
process
TCP withbuffers,
variables
socket
controlled byapplicationdeveloper
controlled byoperatingsystem
host orserver
internet
2: Application Layer6
Socket programming with TCP
Client must contact server• server process must first
be running• server must have
created socket (door) that welcomes client’s contact
Client contacts server by:• creating client-local TCP
socket• specifying IP address,
port number of server process
• When client creates socket: client TCP establishes connection to server TCP
• When contacted by client, server TCP creates new socket for server process to communicate with client– allows server to talk
with multiple clients– source port numbers
used to distinguish clients
TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server
application viewpoint
7
Stream jargon
• A stream is a sequence of characters that flow into or out of a process.
• An input stream is attached to some input source for the process, eg, keyboard or socket.
• An output stream is attached to an output source, eg, monitor or socket.
8
Socket programming with TCP
Example client-server app:
1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream)
2) server reads line from socket
3) server converts line to uppercase, sends back to client
4) client reads, prints modified line from socket (inFromServer stream)
outT
oSer
ver
to network from network
inFr
omS
erve
r
inFr
omU
ser
keyboard monitor
Process
clientSocket
inputstream
inputstream
outputstream
TCPsocket
Clientprocess
client TCP socket
9
Client/server socket interaction: TCP
wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()
create socket,port=x, forincoming request:welcomeSocket =
ServerSocket()
create socket,connect to hostid, port=xclientSocket =
Socket()
closeconnectionSocket
read reply fromclientSocket
closeclientSocket
Server (running on hostid) Client
send request usingclientSocketread request from
connectionSocket
write reply toconnectionSocket
TCP connection setup
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Example: Java client (TCP)
import java.io.*; import java.net.*; class TCPClient {
public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;
BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
Createinput stream
Create client socket,
connect to server
Createoutput stream
attached to socket
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Example: Java client (TCP), cont.
BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close(); } }
Createinput stream
attached to socket
Send lineto server
Read linefrom server
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Example: Java server (TCP)
import java.io.*; import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
Createwelcoming socket
at port 6789
Wait, on welcomingsocket for contact
by client
Create inputstream, attached
to socket
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Example: Java server (TCP), cont
DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); } } }
Read in linefrom socket
Create outputstream,
attached to socket
Write out lineto socket
End of while loop,loop back and wait foranother client connection
14
UDP
15
Socket programming with UDP
UDP: no “connection” between client and server• no handshaking• sender explicitly attaches IP address and port of
destination to each packet• server must extract IP address, port of sender from
received packet
UDP: transmitted data may be received out of order, or lost
application viewpoint
UDP provides unreliable transfer of groups of bytes (“datagrams”)
between client and server
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Client/server socket interaction: UDP
closeclientSocket
Server (running on hostid)
read reply fromclientSocket
create socket,clientSocket = DatagramSocket()
Client
Create, address (hostid, port=x,send datagram request using clientSocket
create socket,port=x, forincoming request:serverSocket = DatagramSocket()
read request fromserverSocket
write reply toserverSocketspecifying clienthost address,port number
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Example: Java client (UDP)
sendP
ack
et
to network from network
rece
iveP
ack
et
inF
rom
Use
r
keyboard monitor
Process
clientSocket
UDPpacket
inputstream
UDPpacket
UDPsocket
Output: sends packet (TCP sent “byte stream”)
Input: receives packet (TCP received “byte stream”)
Clientprocess
client UDP socket
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Example: Java client (UDP)
import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Createinput stream
Create client socket
Translate hostname to IP
address using DNS
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Example: Java client (UDP), cont.
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData(),0,receivePacket.getLength()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }
}
Create datagram with data-to-send,
length, IP addr, port
Send datagramto server
Read datagramfrom server
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Example: Java server (UDP)
import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Createdatagram socket
at port 9876
Create space forreceived datagram
Receivedatagra
m
2: Application Layer21
Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }
}
Get IP addrport #, of
sender
Write out datagramto socket
End of while loop,loop back and wait foranother datagram
Create datagramto send to client
Required Packages
Package Description
org.apache.*Represents a number of packages that provide fine control and functions for HTTP communications. You might recognize Apache as the popular open source Web server.
android.net
Contains additional network access sockets beyond the core java.net.* classes. This package includes the URI class, which is used frequently in Android application development beyond traditional networking.
android.net.http Contains classes for manipulating SSL certificates.
android.net.wifi
Contains classes for managing all aspects of WiFi (802.11 wireless Ethernet) on the Android platform. Not all devices are equipped with WiFi capability, particularly as Android makes headway in the "flip-phone" strata of cell phones from manufacturers like Motorola and LG.
android.telephony
Contains classes required for managing and sending SMS (text) messages. Over time, an additional package will likely be introduced to provide similar functions on non-GSM networks, such as CDMA, or something like android.t lephony.cdma.
How does the Bluetooth protocol work?
discovery
pairing
RFComm
Scanning for other BTDevices --- inquiry scan Followed by page scan. Take about 15-20 seconds
Authentication process where two devices exchange a pin. Once paired the info is maintained in service discovery db
Service Discovery Every server device publishes a set of service that client connect to
After pairing the devices communicate amongst each other over a RF communication channel
Android implementation overview?
BluetoothAdapter
BluetoothDevice
BluetoothServerSocket
BluetoothSocket
Access to the local Bluetooth device and its properties
Access to any Bluetooth device (usually remote)
Socket interface for the server-end
Socket interface for the client-end
Bluetooth Permissions
• Permission BLUETOOTH is used ONLY for communication– Requesting a connection, accepting a connection, and transferring data
• Permission BLUETOOTH_ADMIN is used for controlling the device– Device discovery, changing the settings of the Bluetooth device etc.
<manifest> <uses permission android:name=“android.permission.BLUETOOTH”> <uses permission android:name=“android.permission.BLUETOOTH_ADMIN”>
</manifest>
Setting up the Bluetooth Adapter
– Use BluetoothAdapter to get a reference to the Bluetooth device• If Bluetooth device is not supported the adapter returns a NULL
– Enable Bluetooth device using an Intent and starting a new Activity with the Bluetooth device
• It does ask the user whether he wants to enable the device• How do you know that the Bluetooth device is enabled? --- the resultcode in
onActivityResult() callback will be RESULT_OK.
Bluetooth adapter = BluetoothAdapter.getDefaultAdapter();if(adapter == null){ //Device does not support Bluetooth.}
if(!adapter.isEnabled()) {Intent enableBT = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);startActivityForResult(enableBT, REQUEST_ENABLE_BT);
}
Discovering devices
Set<BluetoothDevice> pairedDevices = adapter.getBondedDevices();
if (pairedDevices.size() > 0) { for(BluetoothDevice device: pairedDevices) { //get access to the devices name through device.getName();
//get access to the devices MAC address through device.getAddress(); }}
//discovering devicesadapter.startDiscover();private final BroadcastReceiver mReceiver = new BroadcastReceiver() { public void onReceive(Context context, Intent intent) {
String action = intent.getAction();if(BluetoothDevice.ACTION_FOUND.equals(action)) {
BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
//get the name of the device through device.getName();//get the MAC address of the device through device.getAddress();
} } IntentFilter filter = new IntentFilter(Bluetooth.ACTION_FOUND); registerReceiver(mReceiver, filte); //register for broadcast receiver when a BT device is found.
– First step is to find devices that you have already paired with: these are devices you do not need to pair to get connected
– Use a broadcast receiver discover new Bluetooth devices
Enabling Discovery
Intent discoverable = new Intent(BluetoothAdapter.BLUETOOTH_ACTION_DISCOVERABLE);Discoverable.putExtras(BluetoothAdapter.EXTRA_DISCOVERABLE_DURATION, 300);startActivity(discoverable);
– Why do you need to set a device’s Bluetooth to Discoverable• If you are a server and you want client devices to connect to you• If you want other devices to see you in order to pair with you
– You set it up using an Intent• A parameter that you can set up is the time that you want the device to be
discoverable• Default = 120 seconds, 0 forever, max = 3600, < 0 or > 3600 – default is
taken.
Connecting to a device (server-side)
– Just like a TCP socket called BluetoothServerSocket– You wait on an accept() (blocking call) till you receive an
incoming connection request– accept() is blocking so it should happen in a separate thread
from the UI thread
public class AcceptConnection extends Thread{private final BluetoothServerSocket soc;
public AcceptConnection() { try {
soc = adapter.listenUsingRfcommWithServiceRecord(NAME, UDID); } catch(IOException e){} }public void run() { BluetoothSocket socket = null; while(true) {
try {soc.accept();} catch(IOException e) { break; }
if(soc != null) {//spawn another thread to manage the connection
}
}
}}
Name of the service
Unique ID for the service
Connecting to a device (client-end)
– Connect() is a blocking call so needs to happen in a thread separate from the UI thread
– From the remote device, create a Rfcomm channel for data transfer.
public class ClientThread extends Thread { BluetoothSocket temp = null; public ClientThread(Bluetooth device) { try {
temp = device.createRfcommSocketToServiceRecord(UDID); }catch(Exception e) { } }
public void run() { adapter.cancelDiscover(); try { temp.connect(); } catch(Exception e) { }
//manage the connection }}
Data transfer using the server/client socket
– Attach an InputStream and an OutputStream to the the socket– Use read(byte[]) and write(byte[]) to read and write --- both are
blocking calls
public class ClientThread extends Thread { BluetoothSocket temp = null; public ClientThread(Bluetooth device) { try {
temp = device.createRfcommSocketToServiceRecord(UDID); }catch(Exception e) { } }
public void run() { byte[] buffer = new byte[1024]; int numbytes; adapter.cancelDiscover(); try { numbytes = temp.read(buffer); //do whatever you want with the bytes } catch(Exception e) { }
//manage the connection }}
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