Elementary TCP Sockets

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Elementary TCP Sockets

• Unix Network Programming
• Ch 4

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Elementary Socket functions
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Socket Function

• To perform network I/O, first thing a process
  must do is call the socket function

include ltsys/socket.hgt int socket(int family,
int type, int protocol) - returns non-negative
descriptor if ok, -1 on error
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Connect funciton

• The connect function is used by a TCP client to
  establish a connection with a TCP server
• include ltsys/socket.hgt
• int connect(int sockfd, const struct sockaddr
  servaddr, socklen_t addrlen)
• Returns 0 if ok, -1 on error
• Sockfd is a socket descriptor returned by the
  socket function
• 2nd 3rd args are the socket address structures,
  must contain the address of the server to
  communicate with
• The client does not have to call bind
• The kernel chooses both an ephemeral port and the
  source IP address if necessary.

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Bind function

• The bind funtion assigns a local protocol address
  to a socket.
• With IP, combination of 32-bit (IPv4 or 128-bit
  for IPv6) address, along with a 16-bit TCP or UDP
  port number.
• include ltsys/socket.hgt
• int bind(int sockfd, const struct sockaddr
  myaddr, socklen_t addrlen)
• Servers bind to their well-known port when they
  start
• A process can bind a specific IP address to its
  socket
• Normally, however, a client does not bind an IP
  address, so that client can then respond on any
  interface available on the host

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Listen function

• The listen function is called only by a TCP
  server and it performs 2 actions
• Converts an unconnected (active) socket into a
  passive socket (indicates kernel should accept
  incoming connect requests directed to this socket
• 2nd argument specifies the maximum number of
  connections kernel should queue for this socket
• include ltsys/socket.hgt
• int listen(int sockfd, int backlog)

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Listen function

• Normally called after both the socket and bind
  function, only by the server of course
• Backlog - for a given listening socket, the
  kernel maintains 2 queues
• An incomplete connection queue, which contains an
  entry for each SYN that has arrived from a client
  for which server is awaiting completion of the
  TCP 3-way handshake
• A completed connection queue, entry for each
  client with whom 3-way handshake has completed.
• Figure 4.7, pg. 105

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Accept function

• Accept is called by a TCP server to return the
  next completed connection from the front of the
  completed connection queue.
• If completed queue is empty, the process is put
  to sleep.
• include ltsys/socket.hgt
• int accept(int sockfd, struct sockaddr cliaddr,
  socklen_t addrlen)
• Returns non-negative descriptor if OK, -1 on
  error
• The cliaddr and addrlen args are used to return
  the protocol address of the connect peer process
  (the client).

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Fork and exec functions

• We will look at building a concurrent server
• Need to create a new child process to handle each
  incomming client request/transaction
• fork function is the only way in Unix to create a
  new process
• include ltunistd.hgt
• pid_t fork(void)
• Returns 0 in child, process ID of child in
  parent, -1 on error
• Called once but returns TWICE
• Once in the parent process (returns child process
  id),
• and once in the child process (return of 0)

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More Forking

• All descriptors open in the parent before the
  call to fork() are shared with the child after
  fork returns.
• Including the connected socket file description
  returned by accept

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Exec function

• Only way in which an executable program file on
  disk can be executed in Unix is for an existing
  process to call one of the 6 exec functions

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Concurrent Servers

• When a client request can take some time to
  service, don't want to take away time for
  handling connections to service a single client
• Handle the communication with multiple clients at
  the same time
• Simplest way to write a concurrent server under
  Unix is to fork a child process to handle each
  client.

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Concurrent Servers
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Close function

• Close() function used to close a socket and
  terminate a TCP connection
• include ltunistd.hgt
• int close(int sockfd)
• Returns 0 if ok, -1 on error
• Default action of close with a TCP socket
  description is to mark the socket as closed and
  return tot he process immediately.
• Socket descriptor is no longer usable to the app
  process at this point
• But TCP will try to send any data that is already
  queued, and once flushed begin the normal TCP
  termination sequence.