By: Dr' Vasos Vassiliou and Pavlos Antoniou

1
CS420 Tutorials on High Speed Multimedia and
Multiservice Networks TCP/IP Socket Programming
in C

• By Dr. Vasos Vassiliou and Pavlos Antoniou
• Spring 2008

2
Overview

• Introduction
• TCP/IP Basic
• UNIX/C Sockets
• UDP Sockets
• TCP Sockets
• Utility Functions
• Other Socket API
• Summary

3
Internet Protocol (IP)

• Datagram (packet) protocol
• Best-effort service
• Loss
• Reordering
• Duplication
• Delay
• Host-to-host delivery (not application-to-applica
  tion)

4
IP Address

• 32-bit identifier
• Identifies a host interface (not a host)

5
Transport Protocols

• Best-effort is not sufficient !!!
• Add services on top of IP
• User Datagram Protocol (UDP)
• Data checksum
• Best-effort
• Transmission Control Protocol (TCP)
• Data checksum
• Reliable byte-stream delivery
• Flow and congestion control

6
Ports

• Identifying the ultimate destination
• IP addresses identify hosts
• Host has many applications
• Ports (16-bit identifier)

Application WWW E-mail Telnet
Port 80 25 23
194.42.16.25
7
TCP/IP Byte Transport

• TCP/IP protocols transports bytes
• Application protocol provides semantics

Application
Application
byte stream
byte stream
Here are some bytes. I dont know what they mean.
Ill pass these to the app. It knows what to do.
TCP/IP
TCP/IP
8
Socket

• How does one speak TCP/IP?
• Sockets provides interface to TCP/IP
• Generic interface for many protocols

9
Socket Types
10
Socket

• Identified by protocol and local/remote
  address/port
• Applications may refer to many sockets
• Socket abstraction
• Berkeley Sockets (traditional)
• Generic
• multiple families
• address representation independence)
• Uses existing I/O programming interface as much
  as possible
• Sockets work with Unix I/O services just like
  files, pipes
• Sockets have special needs
• establishing a connection
• specifying communication endpoint addresses

11
TCP/IP Sockets

• mySock socket(family, type, protocol)
• TCP/IP-specific sockets
• Socket reference
• File (socket) descriptor in UNIX
• Socket handle in WinSock

12

• struct sockaddr
• unsigned short sa_family / Address family
  (e.g., AF_INET) /
• char sa_data14 /
  Protocol-specific address information /
• struct sockaddr_in
• unsigned short sin_family / Internet
  protocol (AF_INET) /
• unsigned short sin_port / Port
  (16-bits) /
• struct in_addr sin_addr / Internet
  address (32-bits) /
• char sin_zero8 / Not used /
• struct in_addr
• unsigned long s_addr / Internet
  address (32-bits) /

Generic
IP Specific
sockaddr
Family
Blob
8 bytes
2 bytes
4 bytes
2 bytes
Family
sockaddr_in
Port
Not used
Internet address
13
Clients and Servers

• Client Initiates the connection
• Server Passively waits to respond

Server Jane
Client Bob
Hi, Bob. Im Jane
Hi. Im Bob. Nice to meet you, Jane.
14
Functions needed

• Specify local and remote communication endpoints
• Initiate a connection (client)
• Wait for incoming connection (server)
• Send and receive data
• Terminate a connection gracefully
• Error handling

15
TCP Client/Server Interaction
Server starts by getting ready to receive
client connections

• Server
• Create a TCP socket
• Assign a port to socket
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

16
Creating a Socket

• int socket(int family, int type, int proto)
• system call returns a socket descriptor(small
  integer) -1 on error
• allocates resources needed for a communication
  endpoint
• does not deal with endpoint addressing
• family specifies the protocol family
• AF_INET for TCP/IP
• type specifies the type of service(communication)
• SOCK_STREAM, SOCK_DGRAM
• protocol specifies the specific protocol
• usually 0 which means the default

17
Declaration for socket function
18
Specifying an Endpoint Address

• Remember that the sockets API is generic
• There must be a generic way to specify endpoint
  addresses
• TCP/IP requires an IP address and a port number
  for each endpoint address.
• Other protocol suites(families) may use other
  schemes.
• Generic socket addresses(The C function that make
  up the sockets API expect structures of type
  sockaddr.)
• struct sockaddr
• unsigned short sa_family
  //specifies the address type
• char sa_data14
  //specifies the address value

19
Assigning an address to a socket

• int bind(int socket, (struct sockaddr ) address,
  int address_length)
• The bind() system call is used to assign an
  address to an existing socket.
• bind returns 0 if successful or -1 on error
• Example
• struct sockaddr_in sin
• int s
• s socket(AF_INET, SOCK_DGRAM, 0)
• sin.sin_family AF_INET
• sin.sin_port htons(9999)
• sin.sin_addr.s_addr INADDR_ANY
• bind(s, (struct sockaddr )sin, sizeof(sin))

20
listen() connect() -- TCP Sockets

• int listen(int socket, int qlength)
• allows servers to prepare a socket for incoming
  connections
• puts the socket in a passive mode ready to accept
  connections
• informs the OS that the protocol software should
  enqueue multiple simultaneous requests that
  arrive at the socket
• applies only to sockets that have selected
  reliable stream delivery service
• int connect(int, struct sockaddr dest_addr,
  int)
• binds a permanent destination to a socket
• changes the socket state from unconnected state
  to connected state
• The semantics of connect depend on the
  underlaying protocol
• TCP connection (AF_INET, reliabe stream delivery
  service)
• just store the destination address locally
  (connectionless)

21
accept() -- TCP Sockets

• int accept(int, struct sockaddr addr, int
  addrlen)
• needs to wait for a connection
• blocks until a connection request arrives
• addrlen is a pointer to an integer
• when a request arrives, the system fills in
  argument addr with the address of the client that
  has placed the request and sets addrlen to the
  length of the address.
• system creates a new socket, returns the new
  socket descriptor

22
send() and recv() -- TCP Sockets

• int send(int s, const char msg, int len, int
  flags)
• connected socket
• argument flags controls the transmission.
• allows the sender to specify that the message
  should be sent out-of- band messages correspond
  to TCPs urgent data
• allows the caller to request that the message be
  sent without using local routine tables (take
  control of routine)
• int recv(int s, char buf, int len, int flags)
• connected socket
• argument flags allow the caller to control the
  reception
• look ahead by extracting a copy of the next
  incoming message without removing the message
  from the socket

23
close() and shutdown()

• close(int socket)
• For UDP sockets, this will release the ownership
  on the local port that is bound to this socket
• For TCP, this will initiate a two-way shutdown
  between both hosts before giving up port
  ownership.
• shutdown(int socket, int how)
• f the how field is 0, this will disallow further
  reading (recv) from the socket.
• If the how field is 1, subsequent writes (send)
  will be disallowed. The socket will still need to
  be passed to close.

24
Relationship Between Sockets and File Descriptors

• Socket handles are integer values. In UNIX,
  socket handles can be passed to most of the
  low-level POSIX I/O functions.
• read(s, buffer, buff_length) //s could be a
  file descriptor too
• write(s, buffer, buff_length)
• Calling read on an open socket is equivalent to
  recv
• if the socket is UDP, then information about the
  sender of the datagram will not be returned
• Similarly the write function call is equivalent
  to send
• UDP sockets may call connect to use send and
  write
• use the socket library functions instead of the
  file I/O equivalents.

25
Socket interface for Connectionless Iterative
Server
26
TCP Client/Server Interaction
/ Create socket for incoming connections
/ if ((servSock socket(PF_INET,
SOCK_STREAM, IPPROTO_TCP)) lt 0)
DieWithError("socket() failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

27
TCP Client/Server Interaction
echoServAddr.sin_family AF_INET
/ Internet address family /
echoServAddr.sin_addr.s_addr htonl(INADDR_ANY)/
Any incoming interface /
echoServAddr.sin_port htons(echoServPort)
/ Local port / if (bind(servSock,
(struct sockaddr ) echoServAddr,
sizeof(echoServAddr)) lt 0)
DieWithError("bind() failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

28
TCP Client/Server Interaction
/ Mark the socket so it will listen for
incoming connections / if (listen(servSock,
MAXPENDING) lt 0) DieWithError("listen()
failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

29
TCP Client/Server Interaction
for () / Run forever / clntLen
sizeof(echoClntAddr) if ((clntSockaccept(se
rvSock,(struct sockaddr )echoClntAddr,clntLen))
lt 0) DieWithError("accept() failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

30
TCP Client/Server Interaction
Server is now blocked waiting for connection
from a client Later, a client decides to talk to
the server

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

31
TCP Client/Server Interaction
/ Create a reliable, stream socket using
TCP / if ((sock socket(PF_INET,
SOCK_STREAM, IPPROTO_TCP)) lt 0)
DieWithError("socket() failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

32
TCP Client/Server Interaction
echoServAddr.sin_family AF_INET
/ Internet address family /
echoServAddr.sin_addr.s_addr inet_addr(servIP)
/ Server IP address / echoServAddr.sin_port
htons(echoServPort) / Server port /
if (connect(sock, (struct sockaddr )
echoServAddr, sizeof(echoServAddr)) lt 0)
DieWithError("connect() failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

33
TCP Client/Server Interaction
if ((clntSockaccept(servSock,(struct sockaddr
)echoClntAddr,clntLen)) lt 0)
DieWithError("accept() failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

34
TCP Client/Server Interaction
echoStringLen strlen(echoString) /
Determine input length / / Send the string to
the server / if (send(sock, echoString,
echoStringLen, 0) ! echoStringLen)
DieWithError("send() sent a different number of
bytes than expected")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

35
TCP Client/Server Interaction
/ Receive message from client / if
((recvMsgSize recv(clntSocket, echoBuffer,
RCVBUFSIZE, 0)) lt 0) DieWithError("recv()
failed")

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

36
TCP Client/Server Interaction
close(sock)
close(clntSocket)

• Server
• Create a TCP socket
• Bind socket to a port
• Set socket to listen
• Repeatedly
• Accept new connection
• Communicate
• Close the connection

• Client
• Create a TCP socket
• Establish connection
• Communicate
• Close the connection

37
TCP Tidbits

• Client must know the servers address and port
• Server only needs to know its own port
• No correlation between send() and recv()

• Client
• send(Hello Bob)
• recv() -gt Hi Jane

• Server
• recv() -gt Hello
• recv() -gt Bob
• send(Hi )
• send(Jane)

38
Closing a Connection

• close() used to delimit communication
• Analogous to EOF

• Echo Client
• send(string)
• while (not received entire string)
• recv(buffer)
• print(buffer)
• close(socket)

• Echo Server
• recv(buffer)
• while(client has not closed connection)
• send(buffer)
• recv(buffer)
• close(client socket)

39
Byte-order Transformations

• Byte ordering is a function of machine
  architecture
• Intel little-endian
• Sparc, PowerPC big-endian
• Network order big-endian
• The byte order for the TCP/IP protocol suite is
  big endian.

40
Big-endian byte order
41
Little-endian byte order
42
Network Byte Order Functions
43
Network Byte Order Functions

• Functions
• u_long m ntohl(u_long m)
• network-to-host byte order, 32 bit
• u_long m htonl(u_long m)
• host-to-network byte order, 32 bit
• ntohs(),htons()
• short (16 bit)
• Example
• struct sockaddr_in sin
• sin.sin_family AF_INET
• sin.sin_port htons(9999)
• sin.sin_addr.s_addr inet_addr

44
Address transformation
struct in_addr
char
45
Address transformation

• char inet_ntoa(struct in_addr ip)
• Converts the 32-bit value which is assumed to be
  in network byte order and contained in ip to a
  string
• The pointer returned by inet_ntoa contains this
  string. However, subsequent calls to inet_ntoa
  will always return the same pointer, so copying
  the string to another buffer is recommended
  before calling again.
• unsigned int inet_addr(char str)
• str represents an IP address(dotted-quad
  notation) inet_addr will return it's equivalent
  32-bit value in network byte order.
• This value can be passed into the sin_addr.s_addr
  field of a socketaddr_in structure
• Ex. sin_addr inet_addr(152.14.51.129)
• -1 is returned if the string can not be
  interpreted

46
Obtaining Information About Hosts, etc.

• struct hostent hptr /includes host address in
  binary/
• hptrgethostbyname(char name)
• Ex.gethostbyname(www.csc.ncsu.edu)
• Struct hostent hptr
• hptrgethostbyaddr(char addr,int addrlen, int
  addrtype)
• Ex gethostbyaddr(addr, 4, AF_INET)

47
Obtaining Information About Hosts, etc.

• int inet_addr(char dotdecimal)
• Ex. sin_addr inet_addr(152.14.51.129)
• struct servent sptr / includes port and
  protocol /
• sptrgetservbyname(char name, char proto)
• Ex. getservbyname(smtp, tcp)
• struct protoent pptr / includes protocol
  number /
• pptrgetprotobyname(char name)
• Ex. getprotobyname(tcp)

48
Others

• Include files
• include ltsys/types.hgt
• include ltsys/socket.hgt
• include ltnetinet/in.hgt
• include ltarpa/inet.hgt
• include ltnetdb.hgt
• include ltunistd.hgt
• include ltsignal.hgt
• include ltstdio.hgt
• include ltfcntl.hgt
• include lterrno.h
• include ltsys/time.hgt
• include ltstdlib.hgt
• include ltmemory.hgt
• Compiling and Linking
• Under most versions of UNIX (Linux, BSD, SunOS,
  IRIX) compiling is done as usual
• gcc my_socket_program.c -o my_socket_program
• Solaris
• cc my_socket_program.c -o my_socket_program
  -lsocket -lnsl

49
Summary

• TCP/IP basic
• UNIX/C Sockets
• socket() bind() connect() listen()
  accept() sendto() recvfrom() send() recv()
  read() write()
• some utility functions
• For more Information
• W. Richard Stevens, Unix Network Programming
  Networking APIs Sockets and XTI, Volume 1,
  Second Edition, Prentice Hall, 1998.
• THE network programming bible.