Chapter 6 Fundamentals of Socket Programming Recommended reading: Comer, Vol 3, Chapter 5

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Chapter 6Fundamentals of Socket Programming
(Recommended reading Comer, Vol 3, Chapter 5)

• Review of C Programming
• Some Data Types
• The following data types are often used in socket
  programming
• u_char unsigned 8-bit character
• u_short unsigned 16-bit integer
• u_long unsigned 32-bit integer
• A structure is a group of different types of
  data. Example
• struct info
• char name20
• int ID
• struct info student60
• student0.name "chan" 11
• student0.ID 1234
• student1.name "cheung"
• student1.ID 5678

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• Casting of Data Types
• Casting is to change the data type of a variable
  or expression to the designated one. Example
• If i is an int, then the data type of the
  following expression is double
• (double) (i1)
• Pointers
• Every memory location has an address
• We can use this address to access the content in
  this memory location.
• A pointer is a variable that stores the memory
  address (not the value) of a data item.
• Declaration of pointer variable
• data-type pointer-variable
• Dereference operator
• pv gives the content stored in the memory
  location with address pv.
• Address operator
• The address storing the first memory byte of the
  variable v is v.

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• Example
• x and y are declared to be of type "float", and p
  is declared to be of type "pointer to float"
• float x, y, p
• The following statements
•   p x
• y p
• are equivalent to
•   y x
• Pointer and Array
• The name of an array is actually a pointer to the
  first element in the array.
• If x is a one-dimensional array,
• the address of the 1st array element is x0 or
  x
• the address of the 2nd array element is x1 or
  (x1)
• Pointer Types
• The 80x86 memory system is divided into segments,
  and each segment has 64 K.
• An address consists of two parts segment and
  offset.

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• Pointer types
• A near pointer only stores the offset which is
  relative to the data segment.
• A far pointer stores both the segment and offset.
• Byte Order
• Different computers may have different internal
  representation of 16 / 32-bit integer (called
  host byte order).
• Examples
• Big-Endian byte order (e.g., used by Motorola
  68000)
• Little-Endian byte order (e.g., used by Intel
  80x86)

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• TCP/IP specifies a network byte order which is
  the big-endian byte order.
• For some WinSock functions, their arguments
  (i.e., the parameters to be passed to these
  functions) must be stored in network byte order.
• WinSock provides functions to convert between
  host byte order and network byte order
• Prototypes for these conversion functions

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• Endpoint Address
• Generic Endpoint Address
• The socket abstraction accommodates many protocol
  families.
• It supports many address families.
• It defines the following generic endpoint
  address
• ( address family, endpoint address in that family
  )
• Data type for generic endpoint address
• TCP/IP Endpoint Address
• For TCP/IP, an endpoint address is composed of
  the following items
• Address family is AF_INET (Address Family for
  InterNET).
• Endpoint address in that family is composed of an
  IP address and a port number.

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• The IP address identifies a particular computer,
  while the port number identifies a particular
  application running on that computer.
• The TCP/IP endpoint address is a special instance
  of the generic one
• Port Number
• A port number identifies an application running
  on a computer.
• When a client program is executed, WinSock
  randomly chooses an unused port number for it.
• Each server program must have a pre-specified
  port number, so that the client can contact the
  server.

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• The port number is composed of 16 bits, and its
  possible values are used in the following manner
• 0 - 1023 For well-known server
  applications.
• 1024 - 49151 For user-defined server
  applications (typical range to be used is 1024 -
  5000).
• 49152 - 65535 For client programs.
• Port numbers for some well-known server
  applications
• WWW server using TCP 80
• Telnet server using TCP 23
• SMTP (email) server using TCP 25
• SNMP server using UDP 161.

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• Data types for TCP/IP endpoint address
• struct in_addr
• u_long s_addr / IP address /
• The IP address is stored in a structure instead
  of an unsigned long (with 32 bits) because of a
  historical reason.
• sockaddr and sockaddr_in are compatible

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• Example 1
• The IP address of a server is 158.182.7.63. Its
  decimal value is 2662729535. We can specify the
  endpoint address for this server as follows
• struct sockaddr_in ServerAddr
• ServerAddr.sin_family AF_INET
• ServerAddr.sin_port 2000
• ServerAddr.sin_addr.s_addr htonl (2662729535)
• Example 2
• We specify the endpoint address for a server as
  follows
• struct sockaddr_in ServerAddr
• ServerAddr.sin_family AF_INET
• ServerAddr.sin_port 2000
• ServerAddr.sin_addr.s_addr htonl(INADDR_ANY)
• where the symbolic constant INADDR_ANY
  represents a wildcard address that matches any of
  the computer's IP address(es).

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• Sketch of a TCP Client and a TCP Server
• Using TCP, both the client and the server use
  three major steps for communication
• Initialize sockets.
• Communicate between sockets.
• Close the sockets.
• In this section, we study the above three steps
  in detail.
• Remark
• Network communication may be considered as
  network I/O. The above steps are similar to that
  for file I/O
• Open a file.
• Read and write data.
• Close the file.
• Initialize Sockets
• There are three initialization steps
• Initialize WinSock DLL.
• Create a socket.
• Assign an endpoint address to a socket.

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• Initialize WinSock DLL
• Before using WinSock, an application calls
  WSAStartup().
• Then Windows binds to the WinSock DLL and
  allocates the necessary resources to this
  application.
• WSAStartup() requires two arguments
• The 1st argument specifies the version of the
  requested WinSock.
• The 2nd argument returns information about the
  version of the WinSock that is actually used.

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• Example

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• Create a Socket
• A program calls socket() to create a new socket,
  which can then act as the endpoint of the TCP
  connection.
• The function socket() requires three arguments
• The 1st argument is the the protocol family. For
  TCP/IP, specify PF_INET (Protocol Family for the
  InterNET).
• The 2nd argument is the type of socket to be
  created. For stream (TCP) sockets, specify
  SOCK_STREAM.
• The 3rd argument is simply 0.
• The call to socket() returns a socket descriptor
  for the newly created socket.

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• Assign an Endpoint Address to a Socket
• After creating a socket, a server must assign its
  endpoint address to this socket.
• Then the client can identify the server's socket
  and send requests to this server.
• Steps
• The server specifies its endpoint address. In
  other words, it assigns the following three
  attribute values to a structure of type
  sockaddr_in
• protocol family (AF_INET for TCP/IP),
• IP address,
• port number.
• The server calls bind() to bind its endpoint
  address to the newly created socket.

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• Example

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• Communicate Between Sockets
• Main Steps
• Setup a TCP connection
• A client initiates to setup a TCP connection to a
  server.
• The server waits for and accepts connection
  requests from the clients.
• Send and receive data.
• Close the TCP connection.
• In socket programming, a TCP connection and its
  associated socket are closed simultaneously.
  Therefore, we will consider the 3rd step in the
  next subsection.
• Client Initiates a TCP Connection
• After creating a socket, a client calls connect()
  to establish a TCP connection to a server.
• The function connect() has three arguments
• the descriptor of the client's socket
• endpoint address of the server
• length of the 2nd argument.

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• Example

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• Server Listens to Connection Requests
• After creating a socket, the server calls
  listen() to place this socket in passive mode.
• Then the socket listens and accepts incoming
  connection requests from the clients.
• Multiple clients may send requests to a server.
• The function listen() tells the OS to queue the
  connection requests for the server's socket.
• The function listen() has two arguments
• the server's socket
• the maximum size of the queue.
• The function listen() applies only to sockets
  used with TCP.

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• Server Accepts a Connection Request
• The server calls accept() to
• extract the next incoming connection request from
  the queue.
• Then the server creates a new socket for this
  connection request and returns the descriptor of
  this new socket.
• Remarks
• The server uses the new socket for the new
  connection only. When this connection ends, the
  server closes this socket.
• The server still uses the original socket to
  accept additional connection requests.
• The function accept() applies only to stream
  (TCP) sockets.

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• Example

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• Send Data
• Both client and server calls send() to send data
  across a connection.
• The function send() copies the outgoing data into
  buffers in the OS kernel, and allows the
  application to continue execution while the data
  is being sent across the network.
• If the buffers become full, the call to send()
  may block temporarily until free buffer space is
  available for the new outgoing data.

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• Receive Data
• Both the client and server call recv() to receive
  data through a TCP connection.
• Before calling recv(), the application must
  allocate a buffer for storing the data to be
  received.
• The function recv() extracts the data that has
  arrived at the specified socket, and copies them
  to the application's buffer. Two possible cases
• Case 1 No data has arrived
• The call to recv() blocks until data arrives.
• Case 2 Data has arrived
• If the incoming data can fit into the
  application's buffer, recv() extracts all the
  data and returns the number of bytes received.
• Otherwise, recv() only extracts enough to fill
  the buffer. Then it is necessary to call recv() a
  number of times in order to receive the entire
  message. (See the next chapter.)

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• Example receive a small message with at most 5
  characters

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• Close the Sockets
• Close a Socket
• Once a client or server finishes using a socket,
  it calls closesocket() to
• terminate the TCP connection associated with this
  socket, and
• deallocate this socket.
• Example

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• Clean Up
• When an application finishes using sockets, it
  must call WSACleanup() to deallocate all data
  structures and socket bindings.
• Example

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• Sequence of Execution of WinSock Functions
• We assume that TCP is used and the server serves
  the clients requests one after the other.
• Sequence of execution of WinSock functions

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• Remarks on WinSock Functions
• WinSock has defined some data structures that can
  be used in WinSock programs.
• Examples
• sockaddr
• sockaddr_in
• WinSock has defined some symbolic constants that
  can be used as arguments to WinSock functions.
• Examples
• AF_INET
• PF_INET
• INADDR_ANY
• SOCK_STREAM (stream socket)
• SOCK_DGRAM (datagram socket)
• To use these symbolic constants and declarations,
  include the header file winsock.h or winsock2.h
• include ltwinsock.hgt
• or
• include ltwinsock2.hgt

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• A Simple and Complete Example
• In this section, we implement the following
  client-server application using TCP
• The client sends the message "Hello" to the
  server.
• The server sends this message back to the client.
• Then both the client and server are terminated.
• Client Program
• include ltstdio.hgt
• include ltwinsock2.hgt
•  
• define WSVERS MAKEWORD(2,0)
•  
• main()
• WSADATA wsadata
• SOCKET s
• struct sockaddr_in ServerAddr
• char message"Hello",
  buf5, bptr
• int i, buflen, count
•  

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• / call connect() to connect to the server /
• ServerAddr.sin_family AF_INET
• ServerAddr.sin_port htons(2000)
• ServerAddr.sin_addr.s_addr
  htonl(2662729535)
• connect(s, (struct sockaddr ) ServerAddr,
  sizeof(ServerAddr))
•   
• / call send() to send a message to the server
  /
• send(s, message, strlen(message), 0)
•  
• / call recv() to receive a message from the
  server /
• bptr buf buflen 5
• recv(s, bptr, buflen, 0)
•  
• / Echo the received message from the server
  /
• for (i0 ilt5 i)
• printf("c", bufi)
• printf("\ns\n", "Bye bye!")
•  

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• Server Program
• include ltstdio.hgt
• include ltwinsock2.hgt
• define WSVERS MAKEWORD(2,0)
•  
• main()
• WSADATA wsadata
• SOCKET s, nsock
• struct sockaddr_in ServerAddr,
  ClientAddr
• char buf5, bptr
• int i, buflen, count
•  
• / call WSAStartup() /
• WSAStartup ( WSVERS , wsadata )
•  
• / call socket() /
• s socket ( PF_INET , SOCK_STREAM , 0 )
•  

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• / call accept() /
• i sizeof ( ClientAddr )
• nsock accept (s, (struct sockaddr )
  ClientAddr , i )
•   
• / call recv() to receive a message from the
  client /
• bptr buf buflen 5
• recv ( nsock , bptr , buflen , 0 )
•  
• / call send() to send the message back to
  the client /
• send ( nsock, buf, strlen(buf), 0)
•  
• / call closesocket() /
• closesocket ( nsock )
• closesocket ( s )
•  
• / call WSACleanup() /
• WSACleanup()

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• Remarks
• For clarity, the above two programs do not check
  any possible error in network communication
  (e.g., whether the remote server can be reached).
• In practice, a sophisticated program should check
  all the possible errors.
• If a program has to receive a large message, it
  should call recv() repeatedly until the entire
  message has been received. The details are
  explained in the next chapter.

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• Tutorial Problems
• Using the network byte order, how is the IP
  address 158.182.7.1 stored in the memory?
• State the items in a generic endpoint address.
• State the items in a TCP/IP endpoint address.
• What is the relationship between the above two
  addresses?
• A WinSock application has to call both
  WSAStartup() and socket() for initialization.
  What is the difference between these two
  functions?
• When should an application call closesocket()?
• When should an application call WSACleanup()?
• What is the main purpose of bind() ?
• Should a TCP client call bind()?
• Should a TCP server call bind()?
• Consider a TCP client-server application.
• Should the client call connect() ?
• Should the server call connect() ?

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• What are the main purposes of listen() ?
• What are the main purposes of accept() ?
• In different executions of a server, is its port
  number always the same?
• In different executions of a client, is its port
  number always the same?
• When you write a server program, should you
  assign a port number to it?
• When you write a client program, should you
  assign a port number to it?
• A program calls send() to send a message. When
  the call returns, does it mean that the entire
  message has been sent out?
• A program calls recv() to receive a message.
  When the call returns, does it mean that the
  entire message has been received?
• If you want to develop a server that can store
  100 waiting requests, how would you do?