CMSC 628 Introduction to Mobile Computing Nilanjan Banerjee

CMSC 628: Introduction to Mobile Computing Nilanjan Banerjee University of Maryland Baltimore County, MD Mobile Systems Programming (Acknowledgment to Deepa Shinde and Cindy Atheron 1

Socket Programming TCP and UDP 2

Socket programming Goal: learn how to build client/server application that communicate using sockets Socket API • introduced in BSD 4. 1 UNIX, 1981 • explicitly created, used, released by apps • client/server paradigm • two types of transport service via socket API: – unreliable datagram – reliable, byte streamoriented socket 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 3

TCP 4

Socket-programming using TCP Socket: a door between application process and endend-transport protocol (UCP or TCP) TCP service: reliable transfer of bytes from one process to another controlled by application developer controlled by operating system process socket TCP with buffers, variables internet socket TCP with buffers, variables controlled by application developer controlled by operating system host or server 2: Application Layer 5

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 application viewpoint TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server 2: Application Layer 6

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. 7

Socket programming with TCP Example client-server app: 1) client reads line from standard input (in. From. User stream) , sends to server via socket (out. To. Server 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 (in. From. Server stream) Client process client TCP socket 8

Client/server socket interaction: TCP Server Client (running on hostid) create socket, port=x, for incoming request: welcome. Socket = Server. Socket() TCP wait for incoming connection request connection. Socket = welcome. Socket. accept() read request from connection. Socket write reply to connection. Socket close connection. Socket setup create socket, connect to hostid, port=x client. Socket = Socket() send request using client. Socket read reply from client. Socket close client. Socket 9

Example: Java client (TCP) import java. io. *; import java. net. *; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modified. Sentence; Create input stream Create client socket, connect to server Create output stream attached to socket Buffered. Reader in. From. User = new Buffered. Reader(new Input. Stream. Reader(System. in)); Socket client. Socket = new Socket("hostname", 6789); Data. Output. Stream out. To. Server = new Data. Output. Stream(client. Socket. get. Output. Stream()); 10

Example: Java client (TCP), cont. Create input stream attached to socket Buffered. Reader in. From. Server = new Buffered. Reader(new Input. Stream. Reader(client. Socket. get. Input. Stream())); sentence = in. From. User. read. Line(); Send line to server out. To. Server. write. Bytes(sentence + 'n'); Read line from server modified. Sentence = in. From. Server. read. Line(); System. out. println("FROM SERVER: " + modified. Sentence); client. Socket. close(); } } 11

Example: Java server (TCP) import java. io. *; import java. net. *; class TCPServer { Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket public static void main(String argv[]) throws Exception { String client. Sentence; String capitalized. Sentence; Server. Socket welcome. Socket = new Server. Socket(6789); while(true) { Socket connection. Socket = welcome. Socket. accept(); Buffered. Reader in. From. Client = new Buffered. Reader(new Input. Stream. Reader(connection. Socket. get. Input. Stream())); 12

Example: Java server (TCP), cont Create output stream, attached to socket Data. Output. Stream out. To. Client = new Data. Output. Stream(connection. Socket. get. Output. Stream()); Read in line from socket client. Sentence = in. From. Client. read. Line(); capitalized. Sentence = client. Sentence. to. Upper. Case() + 'n'; Write out line to socket out. To. Client. write. Bytes(capitalized. Sentence); } } } End of while loop, loop back and wait for another client connection 13

UDP 14

Socket programming with 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 application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server UDP: transmitted data may be received out of order, or lost 15

Client/server socket interaction: UDP Server (running on hostid) create socket, port=x, for incoming request: server. Socket = Datagram. Socket() read request from server. Socket write reply to server. Socket specifying client host address, port number Client create socket, client. Socket = Datagram. Socket() Create, address (hostid, port=x, send datagram request using client. Socket read reply from client. Socket close client. Socket 16

Example: Java client (UDP) Client process Input: receives packet (TCP received “byte stream”) Output: sends packet (TCP sent “byte stream”) client UDP socket 17

Example: Java client (UDP) import java. io. *; import java. net. *; Create input stream Create client socket Translate hostname to IP address using DNS class UDPClient { public static void main(String args[]) throws Exception { Buffered. Reader in. From. User = new Buffered. Reader(new Input. Stream. Reader(System. in)); Datagram. Socket client. Socket = new Datagram. Socket(); Inet. Address IPAddress = Inet. Address. get. By. Name("hostname"); byte[] send. Data = new byte[1024]; byte[] receive. Data = new byte[1024]; String sentence = in. From. User. read. Line(); send. Data = sentence. get. Bytes(); 18

Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port Datagram. Packet send. Packet = new Datagram. Packet(send. Data, send. Data. length, IPAddress, 9876); client. Socket. send(send. Packet); Send datagram to server Datagram. Packet receive. Packet = new Datagram. Packet(receive. Data, receive. Data. length); Read datagram from server client. Socket. receive(receive. Packet); String modified. Sentence = new String(receive. Packet. get. Data(), 0, receive. Packet. get. Length()); System. out. println("FROM SERVER: " + modified. Sentence); client. Socket. close(); } } 19

Example: Java server (UDP) import java. io. *; import java. net. *; Create datagram socket at port 9876 class UDPServer { public static void main(String args[]) throws Exception { Datagram. Socket server. Socket = new Datagram. Socket(9876); byte[] receive. Data = new byte[1024]; byte[] send. Data = new byte[1024]; while(true) { Create space for received datagram Receive datagram Datagram. Packet receive. Packet = new Datagram. Packet(receive. Data, receive. Data. length); server. Socket. receive(receive. Packet); 20

Example: Java server (UDP), cont String sentence = new String(receive. Packet. get. Data()); Get IP addr port #, of sender Inet. Address IPAddress = receive. Packet. get. Address(); int port = receive. Packet. get. Port(); String capitalized. Sentence = sentence. to. Upper. Case(); send. Data = capitalized. Sentence. get. Bytes(); Create datagram to send to client Write out datagram to socket } Datagram. Packet send. Packet = new Datagram. Packet(send. Data, send. Data. length, IPAddress, port); server. Socket. send(send. Packet); } } End of while loop, loop back and wait for another datagram 2: Application Layer 21

Package org. apache. * Description 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 Wi. Fi (802. 11 wireless Ethernet) on the Android platform. Not all devices are equipped with Wi. Fi capability, particularly as Android makes headway in the "flipphone" 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. Required Packages

How does the Bluetooth protocol work? discovery pairing Service Discovery RFComm Scanning for other BT Devices --- 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 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? Bluetooth. Adapter Bluetooth. Device Access to the local Bluetooth device and its properties Access to any Bluetooth device (usually remote) Bluetooth. Server. Socket interface for the server-end Bluetooth. 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 Bluetooth. Adapter 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 on. Activity. Result() callback will be RESULT_OK. Bluetooth adapter = Bluetooth. Adapter. get. Default. Adapter(); if(adapter == null) { //Device does not support Bluetooth. } if(!adapter. is. Enabled()) { Intent enable. BT = new Intent(Bluetooth. Adapter. ACTION_REQUEST_ENABLE); start. Activity. For. Result(enable. BT, REQUEST_ENABLE_BT); }

Discovering devices – 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 Set<Bluetooth. Device> paired. Devices = adapter. get. Bonded. Devices(); if (paired. Devices. size() > 0) { for(Bluetooth. Device device: paired. Devices) { //get access to the devices name through device. get. Name(); //get access to the devices MAC address through device. get. Address(); } } //discovering devices adapter. start. Discover(); private final Broadcast. Receiver m. Receiver = new Broadcast. Receiver() { public void on. Receive(Context context, Intent intent) { String action = intent. get. Action(); if(Bluetooth. Device. ACTION_FOUND. equals(action)) { Bluetooth. Device device = intent. get. Parcelable. Extra(Bluetooth. Device. EXTRA_DEVICE); //get the name of the device through device. get. Name(); //get the MAC address of the device through device. get. Address(); } } Intent. Filter filter = new Intent. Filter(Bluetooth. ACTION_FOUND); register. Receiver(m. Receiver, filte); //register for broadcast receiver when a BT device is found.

Enabling Discovery – 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. Intent discoverable = new Intent(Bluetooth. Adapter. BLUETOOTH_ACTION_DISCOVERABLE); Discoverable. put. Extras(Bluetooth. Adapter. EXTRA_DISCOVERABLE_DURATION, 300); start. Activity(discoverable);

Connecting to a device (server-side) – Just like a TCP socket called Bluetooth. Server. Socket – 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 Name of the service public class Accept. Connection extends Thread{ private final Bluetooth. Server. Socket soc; public Accept. Connection() { try { soc = adapter. listen. Using. Rfcomm. With. Service. Record(NAME, UDID); } catch(IOException e){} } public void run() { Bluetooth. Socket socket = null; while(true) { try { soc. accept(); } catch(IOException e) { break; } if(soc != null) { //spawn another thread to manage the connection } } 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 Client. Thread extends Thread { Bluetooth. Socket temp = null; public Client. Thread(Bluetooth device) { try { temp = device. create. Rfcomm. Socket. To. Service. Record(UDID); }catch(Exception e) { } } public void run() { adapter. cancel. Discover(); try { temp. connect(); } catch(Exception e) { } //manage the connection } }

Data transfer using the server/client socket – Attach an Input. Stream and an Output. Stream to the socket – Use read(byte[]) and write(byte[]) to read and write --- both are blocking calls public class Client. Thread extends Thread { Bluetooth. Socket temp = null; public Client. Thread(Bluetooth device) { try { temp = device. create. Rfcomm. Socket. To. Service. Record(UDID); }catch(Exception e) { } } public void run() { byte[] buffer = new byte[1024]; int numbytes; adapter. cancel. Discover(); try { numbytes = temp. read(buffer); //do whatever you want with the bytes } catch(Exception e) { } //manage the connection } }