Socket Programming in C

1
Socket Programming in C

• Slides Adapted on
• Jörn Altmanns Slides

2
Questions that will be Addressed

• What mechanisms are available for a programmer
  who writes network applications?
• How to write a network application that sends
  packets between hosts (client and server) across
  an IP network?
• Answer socket API

IP Network
Client
Server
3
Socket ProgrammingTable of Contents

1. Network Application Programming Interface
   Sockets and Internet Sockets
2. Network Programming Tips
3. Client-Server Architecture
4. Example Client Programming
5. Example Server Programming
6. Network Programmers Mistakes

4
Layers of the IP Protocol Suite
Application Layer
Transport Layer
Network Layer
Link Layer
5
Protocol Suite Location

• Internet Protocol Layer

Application Layer
Transport Layer (TCP, UDP)
Network Layer (IP)
Link Layer
6
Network API

• Operating system provides Application Programming
  Interface (API) for network application
• API is defined by a set of function types, data
  structures, and constants
• Desirable characteristics of the network
  interface
• Simple to use
• Flexible
• independent from any application
• allows program to use all functionality of the
  network
• Standardized
• allows programmer to learn once, write anywhere
• Application Programming Interface for networks is
  called socket

7
Sockets

• Sockets provide mechanisms to communicate between
  computers across a network
• There are different kind of sockets
• DARPA Internet addresses (Internet Sockets)
• Unix interprocess communication (Unix Sockets)
• CCITT X.25 addresses
• and many others
• Berkeley sockets is the most popular Internet
  Socket
• runs on Linux, FreeBSD, OS X, Windows
• fed by the popularity of TCP/IP

8
Internet Sockets

• Support stream and datagram packets (e.g. TCP,
  UDP, IP)
• Is Similar to UNIX file I/O API (provides a file
  descriptor)
• Based on C, single thread model
• does not require multiple threads

9
Types of Internet Sockets

• Different types of sockets implement different
  communication types (stream vs. datagram)
• Type of socket stream socket
• connection-oriented
• two way communication
• reliable (error free), in order delivery
• can use the Transmission Control Protocol (TCP)
• e.g. telnet, ssh, http
• Type of socket datagram socket
• connectionless, does not maintain an open
  connection, each packet is independent
• can use the User Datagram Protocol (UDP)
• e.g. IP telephony
• Other types exist similar to the one above

10
Network Programming Tips

• Byte Ordering
• Naming
• Addressing

11
Byte Ordering of Integers

• Different CPU architectures have different byte
  ordering

D3
F2
Integer representation (2 byte)
12
Byte Ordering Problem

• Question What would happen if two computers with
  different integer byte ordering communicate?

• Answer
• Nothing if they do not exchange integers!
• But If they exchange integers, they would get
  the wrong order of bytes, therefore, the wrong
  value!
• Example

Message is Hello,1
13
Byte Ordering Solution

• There are two solutions if computers with
  different byte ordering system want to
  communicate
• They must know the kind of architecture of the
  sending computer(bad solution, it has not been
  implemented)
• Introduction of a network byte order. The
  functions are
• uint16_t htons(uint16_t host16bitvalue)
• uint32_t htonl(uint32_t host32bitvalue)
• uint16_t ntohs(uint16_t net16bitvalue)
• uint32_t ntohs(uint32_t net32bitvalue)
• Note use for all integers (short and long),
  which are sent across the network
• Including port numbers and IP addresses

14
Network Programming Tips

• Byte Ordering
• Naming
• Addressing

15
Naming and Addressing

• Host name
• identifies a single host (see Domain Name System
  slides)
• variable length string (e.g. www.berkeley.edu)
• is mapped to one or more IP addresses
• IP Address
• written as dotted octets (e.g. 10.0.0.1)
• 32 bits. Not a number! But often needs to be
  converted to a 32-bit to use.
• Port number
• identifies a process on a host
• 16 bit number

16
Client-Server Architecture
response
Client
Server
request

• Client requests service from server
• Server responds with sending service or error
  message to client

17
Simple Client-Server Example
response
Client
Server
request
socket()connect()send()recv()close()
socket()bind()listen()accept()recv()send()
recv()close()
Connectionestablishment
Data request
Data response
End-of-file notification
18
Example Client Programming

• Create stream socket (socket() )
• Connect to server (connect() )
• While still connected
• send message to server (send() )
• receive (recv() ) data from server and process it
• Close TCP connection and Socket (close())

19
socket() Initializing Socket

• Getting the file descriptor
• int chat_sock
• if ((chat_sock socket(AF_INET, SOCK_STREAM,
  0)) lt 0)
• perror("socket")
• printf("Failed to create socket\n")
• abort ()
• 1.parameter specifies protocol/address family
• 2.parameter specifies the socket type
• Other possibilities SOCK_DGRAM
• 3.parameter specifies the protocol.
• 0 means protocol is chosen by the OS.

20
IP Address Data Structure

• struct sockaddr_in
• short int sin_family // Address family
• unsigned short int sin_port // Port number
• struct in_addr sin_addr // Internet address
• unsigned char sin_zero8
• struct in_addr
• unsigned long s_addr // 4 bytes
• Padding of sin_zeros struct sockaddr_in has same
  size as struct sockaddr

21
connect() Making TCP Connection to Server

• struct sockaddr_in sin
• struct hostent host gethostbyname (argv1)
• unsigned int server_address (unsigned long )
  host-gth_addr_list0
• unsigned short server_port atoi (argv2)
• memset (sin, 0, sizeof (sin))
• sin.sin_family AF_INET
• sin.sin_addr.s_addr server_address
• sin.sin_port htons (server_port)
• if (connect(chat_sock, (struct sockaddr ) sin,
  sizeof (sin)) lt 0)
• perror("connect")
• printf("Cannot connect to server\n")
• abort()

22
send() Sending Packets

• int send_packets(char buffer, int buffer_len)
• sent_bytes send(chat_sock, buffer,
  buffer_len, 0)
• if (send_bytes lt 0)
• perror (send")
• return 0
• Needs socket descriptor,
• Buffer containing the message, and
• Length of the message
• Can also use write()

23
Receiving PacketsSeparating Data in a Stream
Fixed length record
Fixed length record
A
B
C
D
0
1
3
2
9
4
6
8
7
5
slide through
receive buffer

• Use records (data structures) to partition the
  data stream

24
Receiving Packets

• int receive_packets(char buffer, int buffer_len,
  int bytes_read)
• int left buffer_len - bytes_read
• received recv(chat_sock, buffer
  bytes_read, left, 0)
• if (received lt 0)
• perror (recv")
• if (received lt 0)
• return close_connection()
• bytes_read received
• while (bytes_read gt RECORD_LEN)
• process_packet(buffer, RECORD_LEN)
• bytes_read - RECORD_LEN
• memmove(buffer, buffer RECORD_LEN,
  bytes_read)
• return 0

buffer_len
buffer
bytes_read

• Can also use read()

25
Server Programming Simple

• Create stream socket (socket() )
• Bind port to socket (bind() )
• Listen for new client (listen() )
• While
• accept user connection and create a new socket
  (accept() )
• data arrives from client (recv() )
• data has to be send to client (send() )

26
bind() Assign IP and Port

• struct sockaddr_in sin
• struct hostent host gethostbyname (argv1)
• unsigned int server_address (unsigned long )
  host-gth_addr_list0
• unsigned short server_port atoi (argv2)
• memset (sin, 0, sizeof (sin))
• sin.sin_family AF_INET
• sin.sin_addr.s_addr server_address
• sin.sin_port htons (server_port)
• if (bind(chat_sock, (struct sockaddr ) sin,
  sizeof (sin)) lt 0)
• perror("bind")
• printf("Cannot bind server application to
  network\n")
• abort()

27
bind()

• bind() tells the OS to assign a local IP address
  and local port number to the socket.
• Many applications let the OS choose an IP
  address.
• Use wildcard INADDR_ANY as local address in this
  case.
• At server, user process must call bind() to
  assign a port
• At client, bind() is not required since OS may
  assign available port and IP address
• The server will get the port number of the client
  through the UDP/TCP packet header
• Note Each application is represented by a server
  port number

28
listen() Wait for Connections

• int listen(int sockfd, int backlog)
• Puts socket in a listening state, willing to
  handle incoming TCP connection request.
• Backlog number of TCP connections that can be
  queued at the socket.

29
Server Example

• define MYPORT 3490 // the port users will be
  connecting to
• define BACKLOG 10 // how many pending
  connections queue will hold
• int main(void)
• int sockfd, new_fd // listen on sockfd, new
  connection on new_fd
• struct sockaddr_in my_addr // my address
  information
• struct sockaddr_in their_addr // connector's
  address information
• int sin_size
• if ((sockfd socket(AF_INET, SOCK_STREAM, 0))
  -1) perror("socket")
• exit(1)
• my_addr.sin_family AF_INET // host byte
  order
• my_addr.sin_port htons(MYPORT) // short,
  network byte order
• my_addr.sin_addr.s_addr INADDR_ANY // auto.
  filled with local IP
• memset((my_addr.sin_zero), '\0', 8)
  // zero the rest of the struct

30

• if (bind(sockfd, (struct sockaddr )my_addr,
  sizeof(struct sockaddr)) -1)
• perror("bind")
• exit(1)
• if (listen(sockfd, BACKLOG) -1)
• perror("listen")
• exit(1)
• while(1) // main accept() loop
• sin_size sizeof(struct sockaddr_in)
• if ((new_fd accept(sockfd, (struct sockaddr
  )their_addr, sin_size)) -1)
• perror("accept")
• continue
• printf("server got connection from s\n",
  inet_ntoa(their_addr.sin_addr))

31
Client Example

• include ltnetinet/in.hgt
• include ltsys/socket.hgt
• define PORT 3490 // the port client will be
  connecting to
• define MAXDATASIZE 100 // max number of
  bytes we can get
• // at once
• int main(int argc, char argv)
• int sockfd, numbytes
• char bufMAXDATASIZE
• struct hostent he
• struct sockaddr_in their_addr // server's
  address information
• if (argc ! 2)
• fprintf(stderr,"usage client hostname\n")
• exit(1)
• if ((hegethostbyname(argv1)) NULL) //
  get the host info
• perror("gethostbyname")

32

• their_addr.sin_family AF_INET //
  host byte order
• their_addr.sin_port htons(PORT) //
  short, network byte order
• their_addr.sin_addr ((struct in_addr
  )he-gth_addr) // already network byte order
• memset((their_addr.sin_zero), '\0', 8) // zero
  the rest of the struct
• if (connect(sockfd, (struct sockaddr
  )their_addr, sizeof(struct sockaddr)) -1)
• perror("connect")
• exit(1)
• if ((numbytesrecv(sockfd, buf, MAXDATASIZE-1,
  0)) -1)
• perror("recv")
• exit(1)
• bufnumbytes '\0'
• printf("Received s",buf)
• close(sockfd)
• return 0

33
I/O Blocking

• socket()
• bind()
• listen()
• while
• accept()
• recv()
• send()
• Simple server has blocking problem
• Suppose 5 connections accepted.
• Suppose next accept() blocks.
• Other connections cannot send and receive.
• Cannot get keyboard input either.

34
select() I/O Multiplexing

• waits on multiple file descriptors and timeout
• returns when any file descriptor
• is ready to be read or
• written or
• indicate an error, or
• timeout exceeded
• advantages
• simple
• application does not consume CPU cycles while
  waiting
• disadvantages
• does not scale to large number of file descriptors

35
Example Server Programming

• create stream socket (socket() )
• Bind port to socket (bind() )
• Listen for new client (listen() )
• While
• Wait for (select() )
• (depending on which file descriptors are ready)
• accept user connection and create a new socket
  (accept() )
• data arrives from client (recv() )
• data has to be send to client (send() )

36
Server Alternative Ways of Handling Many Clients

• Forking a new process for each client fork()
• But, creating new process is expensive.
• Multithreaded implementation have one thread
  handling each client.
• Thread is like a process but light-weighted.

37
Network Programmers Mistakes

• byte ordering
• separating records in streams
• use of select()
• misinterpreting the project specification
• not knowing all available system calls

38
There are more System Calls

• Depends on communication type
• Datagram sockets use recvfrom() and sendto() for
  receiving and sending data
• Closing connection close(), shutdown()
• Convenient functions (on UNIX)
• inet_aton, inet_ntoa
• inet_pton, inet_ntop

39
Literature

• short tutorial Beej's Guide to Network
  Programming
• http//www.ecst.csuchico.edu/beej/guide/net/
• Unix Network Programming, volumes 1 and 2 by W.
  Richard Stevens. Published by Prentice Hall
  ISBNs for volumes 1 and 2 013490012X,
  0130810819.
• Advanced Programming in the Unix Environment by
  W. Richard Stevens. Published by Addison Wesley.
  ISBN 0201563177.
• man pages on a Unix computer