The Application Programming Interface API and the Socketprogramming

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TheApplication Programming Interface (API) and
the Socket-programming
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Application Program Interface (API)
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SMB DCOM

• Server Message Block
• Server Message Block (SMB) is an
  application-level network protocol mainly applied
  to shared access to files, printers, serial
  ports, and miscellaneous communications between
  nodes on a network. It also provides an
  authenticated Inter-process communication
  mechanism. It is mainly used by Microsoft Windows
  equipped computers, where it's known simply as
  "Microsoft Windows Network".
• Distributed Component Object Model
• Distributed Component Object Model (DCOM) is a
  Microsoft proprietary technology for software
  components distributed across several networked
  computers to communicate with each other. DCOM,
  which originally was called "Network OLE",
  extends Microsoft's COM, and provides the
  communication substrate under Microsoft's COM
  application server infrastructure. It has been
  deprecated in favor of Microsoft .NET.

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Socket-programming

• Socket a door between application process and
  end-end-transport protocol (UDP or TCP)
• TCP service reliable transfer of bytes from one
  process to another

controlled by application developer
controlled by application developer
controlled by operating system
controlled by operating system
internet
host or server
host or server
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Sockets

• An abstraction of a network I/O queue
• Embodies one side of a communication channel
• Same interface regardless of location of other
  end
• Could be local machine (called UNIX socket) or
  remote machine (called network socket)
• First introduced in 4.2 BSD UNIX big innovation
  at time
• Now most operating systems provide some notion of
  socket
• Using Sockets for Client-Server (C/C
  interface)
• On server set up server-socket
• Create socket, Bind to protocol (TCP), local
  address, port
• Call listen() tells server socket to accept
  incoming requests
• Perform multiple accept() calls on socket to
  accept incoming connection request
• Each successful accept() returns a new socket for
  a new connection can pass this off to handler
  thread
• On client
• Create socket, Bind to protocol (e.g. TCP),
  remote address, port
• Perform connect() on socket to make connection
• If connect() successful, have socket connected to
  server

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RAW SOCKET

• Definition
• Raw socket is a computer networking term used to
  describe a socket that allows access to packet
  headers on incoming and outgoing packets. Raw
  sockets are usually used at the transport or
  network layers.
• Usually raw sockets always receive packets with
  the header included (as opposed to non-raw
  sockets, which strip the header ). Whether or not
  a header is automatically prepended to outgoing
  packets is usually a configurable socket option.
• Raw sockets are not a programming language-level
  construct, they are part of the underlying
  operating system's networking API.

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RAW SOCKET cont.

• Work
• Raw sockets bypass the transport layer (TCP or
  UDP). With IPv4, raw sockets are used to
  access ICMPv4, IGMPv4, and to read and write
  IPv4 datagrams containing a protocol field that
  the kernel does not process. With IPv6 raw
  sockets will be used for ICMPv6 and to read and
  write IPv6 datagrams containing a Next
  Header field that the kernel does not process.
  All data sent via raw sockets must be in network
  byte order and all data received via raw
  sockets will be in network byte order. This
  differs from the IPv4 raw sockets, which did not
  specify a byte ordering and used the host's
  byte order for certain IP header fields.

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RAW SOCKET cont.

• Problem
• Due to the fact that raw sockets allow users to
  make packet headers themselves, their power can
  be abused to perform feats such as IP address
  spoofing
• When Windows XP was first released in 2001 with
  raw socket support implemented into the Winsock
  interface, the media attacked Microsoft saying
  that raw sockets are only of use to hackers to
  pull off TCP reset attacks.

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RAW SOCKET cont.

• If a hacker had the ability to forge a packet's
  information, he or she could create a SYN packet
  with a fake, or spoofed, return address. In this
  case, the host computer would receive the SYN
  packet from the client computer (hacker), read
  the return address, and send the SYN-ACK packet
  to a fake return address. If there were no
  computer at the spoofed address, the host
  computer would sit and wait for several minutes
  before realizing that no one was connecting to
  it. However, during that time, the host computer
  would have a port open, waiting for a returning
  ACK. Because there are only so many ports
  available for connecting client computers, a
  hacker could quickly use up all of the host
  computer's resources

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Socket Example (Java)

• server //Makes socket, binds addr/port,
  calls listen() ServerSocket sock new
  ServerSocket(6013) while(true) Socket
  client sock.accept() PrintWriter pout
  new PrintWriter(client.getOutputStream(),true
  ) pout.println(Here is data sent to
  client!) client.close()
• client // Makes socket, binds addr/port,
  calls connect() Socket sock new
  Socket(169.229.60.38,6013) BufferedReader
  bin new BufferedReader( new
  InputStreamReader(sock.getInputStream)) String
  line while ((line bin.readLine())!null)
  System.out.println(line) sock.close()

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Another 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())

Create input stream
Create client socket, connect to server
Create output stream attached to socket
Study guide be able to write a
simple Client-server program in Java.
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Another 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(
)))
Create welcoming socket at port 6789
Wait, on welcoming socket for contact by client
Create input stream, attached to socket
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Distributed Applications

• How do you actually program a distributed
  application?
• Need to synchronize multiple threads, running on
  different machines
• No shared memory, so cannot use testset
• One Abstraction send/receive messages
• Already atomic no receiver gets portion of a
  message and two receivers cannot get same message
• Interface
• Mailbox (mbox) temporary holding area for
  messages
• Includes both destination location and queue
• Send(message,mbox)
• Send message to remote mailbox identified by mbox
• Receive(buffer,mbox)
• Wait until mbox has message, copy into buffer,
  and return

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Using Messages Send/Receive behavior

• When should send(message,mbox) return?
• When receiver gets message? (i.e. ack received)
• When message is safely buffered on destination?
• Right away, if message is buffered on source
  node?
• Actually two questions here
• When can the sender be sure that the receiver
  actually received the message?
• When can sender reuse the memory containing
  message?
• Mailbox provides 1-way communication from T1?T2
• T1?buffer?T2
• Very similar to producer/consumer
• Send V, Receive P
• However, cant tell if sender/receiver is local
  or not!

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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 byte1024 byte
receiveData new byte1024 String
sentence inFromUser.readLine() sendData
sentence.getBytes()
Create input stream
Create client socket
Translate hostname to IP address using DNS
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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 byte1024 byte
sendData new byte1024 while(true)
DatagramPacket
receivePacket new
DatagramPacket(receiveData, receiveData.length)
serverSocket.receive(receivePacket)
Create datagram socket at port 9876
Create space for received datagram
Receive datagram