Sockets
(Part I)
LESSON 21

The Socket Interface:

• The Berkeley socket interface provides generalized functions that support network communication using many possible protocols.
• Socket calls refer to all TCP/IP protocols as a single protocol family (protocol suite).
• The calls allow a programmer to specify the type of service required, rather than the name of a specific protocol.
• The socket interface was created since an API (application program interface) for network connections is not standardized, it’s design lies outside the scope of a protocol suite.

Windows Socket Interface (WinSock):

• The WinSock specification was defined to provide a Berkeley Sockets (BSD) 4.3 API for TCP/IP on Microsoft Windows.
• It, like the true BSD socket interface, provides APIs to resolve address and service differences.
• The WinSock specification also provides a set of asynchronous extensions that allow applications to send and receive message-based notices (arrival of new data).
• Currently it is in version 2.0 and provides communication connections (multiple protocol stacks) for many hardware and software technologies.
• Some hardware and software technologies include:
  • IPX/SPX
  • OSI protocols (X.500)
  • DNS
  • ATM
  • ISDN
  • Wireless connections (CDPD)

Basic I/O Functions in UNIX:

• UNIX and other operating systems provide a basic set of system functions used for I/O operations on files and other devices.
• Most operating systems provide similar variations to the five standard I/O operations that BSD UNIX uses.
• I/O Functions:
  • Open - prepare for input / output.
  • Close - terminate the use of a device.
  • Write - transfer data from memory to an output device.
  • Read - transfer data from an input device to memory.
  • Lseek - position the head of a disk drive to a specific place on the disk.

File and Socket Descriptors:

• A socket is a generalized UNIX file access mechanism that provides an endpoint for communication.
• Descriptors (maintained in the descriptor tables) are kept per process by the operating system to point to internal data structures for files and sockets.
• Descriptors are small integer values.
• File Descriptor:
  • Bound to a file when open is called.
• Socket Descriptor:
  • Created using open, but does not bind it to a destination.
  • Unbounded - UDP specifies destination every time.
  • Bounded - TCP specifies destination during an open system call.

• After a socket has been created (using open), additional system calls are required to specify the details of it’s use.
  • Passive Socket - used by a server to wait for calls.
  • Active Socket - used by a client to initiate a connection.

Communication Endpoints:

• TCP/IP protocols define communication endpoints which consist of:
  • An IP address
  • A protocol port number.
• Sockets support all possible endpoint types and address formats used by families of protocols.
• The socket abstraction defines an address family for each type address (allowing many for each protocol family).
  • PF_INET (protocol family) - TCP/IP
  • AF_INET (address family) - TCP/IP
  • AF_PUP - Xerox Corp. PUP
  • AF_APPLETALK - Apple Corp.
  • AF_UNIX - General UNIX file system.

Generic Address Structure:

• The generalized format that all endpoints use is:

  (address family, endpoint address in that family)

• Address families - can be one of the previously mentioned AF types, or others.
• Endpoint addresses - are stored in predefined data structures defined by the address family.

• Sockaddr structure (sa)
  (2-byte sa_family, 14-byte address):
  • Should only be used as an overlay with code only referencing the sa_family field.

TCP/IP Sockaddr_in Structure:

• When representing a TCP/IP communication endpoint, an application uses the sockaddr_in structure.
• Based directly on the sockaddr structure, it contains:
  • An Address Family (8-bits)
  • An IP Address (32-bits)
  • A protocol port number (8-bits).

Socket System Calls:
Socket

• Socket (af, type, protocol):
  • Creates a socket on demand (placing it in an unconnected state), returns an integer identifying the socket (descriptor), and specifies:
    • Address Family (af) - particular address of the family.
  • Type - Type of communication socket:
    • SOCK_STREAM - connection-oriented
    • SOCK_DGRAM - connection-less
    • SOCK_RAW - access to low-level protocols or network interfaces.
  • Protocol - Accommodates multiple protocols within a family.

Socket System Calls:
Socketpair

• Socketpair (af, type, protocol, sarray):
  • Not meaningful within TCP/IP, it is designed to implement an UNIX pipe.
  • The calling process creates both endpoints for the communication simultaneously using the sockets specified in the socket array parameter (sarray).

Socket System Calls:
Close

• Close (socket descriptor):
  • When a client or server finishes with a socket, calls close to deallocate it’s resources.
  • The connection immediately terminates unless several processes share the same socket.
  • It then decrements the reference count (closing it completely when reference count = 0).

Socket System Calls:
Bind

• Bind (socket, localaddr, addrlen):
  • Socket is created without any association to local or destination addresses, so a program uses bind to establish a local address for it.
    • Socket - integer descriptor of the socket.
    • Localaddr - structure that specifies the local address to be bound.
    • Addrlen - integer length of the address (in bytes).

Socket System Calls:
Connect

• Connect (socket, destaddr, addrlen):
  • Binds a permanent destination to a socket placing it in a connected state.
  • Sockets using connection-less service do not have to use connect (specify the address in every datagram), but may.
• Socket - socket descriptor.
• Destaddr - socket_addr structure (also includes protocol port number) specifying the destination address.
• Addrlen - length of destination address (in bytes).

Socket System Calls:
Shutdown

• Shutdown (socket, direction):
  • The shutdown function applies to full-duplex sockets (connected using a TCP socket) and is used to partially close the connection.
    • Socket - socket descriptor of a connected socket.
    • Direction - direction in which shutdown is desired
      • 0 = terminate further input.
      • 1 = terminate further output.
      • 2 = terminate input / output (close).

Output Socket Calls:
Write

• Write (socket, buffer, length):
  • Uses a connected socket to transmit data over the connection. Write blocks until data is transferred, then send an error code notifying the application if the write succeeded.
    • Socket - descriptor, can also be a file descriptor.
    • Buffer - address of the data to be sent.
    • Length - number of bytes to be sent.

Output Socket Calls:
Requiring a Connected Socket

• Writev (socket, iovector, vectorlen):
  • Performs a ‘gather-write’ using the iovector structure.
• Send (socket, message, length, flags):
  • Uses flags for out-of-band signaling, debugging, etc.

Output Socket Calls:
Not Requiring a Socket Connection

• Sendto (socket, message, length, flags, destaddr, addrlen):
  • Functions the same as an unconnected Send.
• Sendmsg (socket, messagestruct, flags):
  • Places parameters in the message-structure to be sent.

Input Socket Calls:
Read

• Read (descriptor, buffer, length):
  • Used to receive (read) data from an established connection (connected socket).
    • Descriptor - IP of socket or file from which to read.
    • Buffer - address in memory of where to store read data.
    • Length - maximum number of bytes to read.

Input Socket Calls:
Requiring a Connected Socket

• Readv (descriptor, iovector, vectorlen):
  • Performs a ‘gather-read’ of non-contiguous locations, using the iovector structure.
• Recv (socket, message, length, flags):
  • Flags allow for look-ahead reading from sockets, etc.

Input Socket Calls:
Not Requiring a Socket Connection

• Recvfrom (socket, buffer, length, flags, fromaddr, addrlen):
  • Unconnected read, same as Recv.
• Recvmsg (socket, messagestruct, flags):
  • Message parameters are received through the message-structure.

Server-Specific Socket Calls:
Listen

• Listen (socket, qlength):
  • Server creates a socket, binds it to a well-known port, and waits for requests.
  • To avoid rejecting service requests that cannot be handled, a server queue is created using Listen.
  • It provides a mechanism to create the queue and then listen for incoming connections (passive mode).
  • Listen only works with sockets using a reliable stream service.
    • Socket - Integer descriptor.
    • Qlength - length of the request queue for that socket (max. = 5)

Server-Specific Socket Calls:
Accept

• Accept (socket, addr, addrlen):
  • Bind associates a socket with port, but that socket is not connected to a foreign destination.
  • When a request comes in, Accept establishes the full connection.
  • It blocks until a connection request arrives.
    • Addr - pointer to the sockaddr structure.
    • Addrlen - pointer to integer size of address.

Server-Specific Socket Calls:
Fork

• Fork is not directly related to communications sockets, but it is essential because servers use it to create and handle concurrent connections for the same service.
• Fork ()
  • It does not take any arguments, but if successful, returns the new child process identifier to the parent (server) process.

Order of Socket System Calls:
Client Side

• Client Side (depends on connection type):
  • Socket
  • Connect
  • Write (may be repeated)
  • Read (may be repeated)
  • Close

Order of Socket System Calls:
Server Side

• Server Side (depends on connection type):
  • Socket
  • Bind
  • Listen
  • Accept
  • Read (may be repeated)
  • Write (may be repeated)
  • Close (go back to Accept)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1995 Illusion Industries Inc.