Todays Agenda

1
TCP and UDP based Socket Programming in C
2
Contact Info
Avinash Gautam Chamber Number 3254-G Email
avinash_at_bits-pilani.ac.in Chamber Consultation
Hours Saturday 900 950 AM
3
Definition and Components

• Sockets are nothing but a way to speak to other
  programs using standard file descriptors
• Everything in UNIX is a file. What it means is
  that when UNIX programs do any sort of I/O, they
  do it by reading or writing to a file descriptor
• A file descriptor is simply an integer associated
  with an open file
• But this file can be a network connection, a
  FIFO, a pipe, a terminal, a real on-the-disk file
  etc
• So when you want to communicate to another
  program you are going to do it through a file
  descriptor

4
Definition and Components (Contd.)

• How to get a file descriptor?
• socket() system call returns socket descriptor
• Communicate through it using send() and recv()
  socket calls
• Two type of internet sockets
• Stream Sockets
• Datagram Sockets
• Stream sockets are referred to as "SOCK_STREAM"
  and datagram sockets are referred to as
  "SOCK_DGRAM" respectively. Datagram sockets are
  sometime called connection less sockets

5
Internet Sockets

• Stream sockets
• Reliable
• Two way communication streams
• What uses stream sockets?
• Web browser using http to get web pages from the
  server
• How do stream sockets achieve reliability?
• Answer TCP (RFC 793)
• Datagram sockets
• Why are they connectionless?
• Why are they unreliable?
• Answer They use IP for routing, but they dont
  use TCP,
• they use UDP (RFC 768)

6
Internet Sockets (Contd.)

• Why datagram sockets are connectionless?
• Don't maintain an open connection
• Build a packet, attach an IP header on it with
  destination information, and send it out
• They are generally used for packet-by-packet
  transfer of information
• Any Examples?
• tftp, bootp etc
• How do these programs even work if a datagram is
  lost?
• Each program has its own protocol running on the
  top of UDP
• tftp protocol says that for each packet that gets
  sent, the recipient has to send back an
  acknowledgement. If the sender doesn't get a
  reply and timer goes out, it will retransmit the
  packet until it gets an acknowledgment

7
Data types and ordering

• Various data types are used by the socket
  interface
• Most simplest is our friend a regular int which
  is used as a socket descriptor. We will see more
  in rest of the sections
• Byte Ordering
• Most significant byte first also called Network
  Byte Order
• Least significant byte first also called Host
  byte Order
• Some machines store their numbers in NBO some
  dont
• If I say something has to be in NBO then you need
  to change it to HBO by calling htons()
• htons() is host-to-network-short

8
Socket Structure

• struct sockaddr
• unsigned short sa_family // address family,
  AF_xxx
• char sa_data14 // 14 bytes of protocol
  address
• sa_family this remains AF_INET for stream and
  datagram sockets
• sa_data contains destination address and port
  number for the socket

9
Socket Structure

• Parallel structure to sockaddr
• 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 // same size as
  sockaddr
• sin_zero is just used to pad the structure to the
  length of a structure sockaddr and hence is set
  to all zeros with the function memset()
• Important you can cast sockaddr_in to a pointer
  of type struct sockaddr and vice versa
• sin_family corresponds to sa_family and should be
  set to AF_INET.
• sin_port and sin_addr must be in NBO

10
NBO HBO conversion functions

• Two types that can be converted
• short (two bytes)
• long (two bytes)
• Primary conversion functions
• htons() // host to network short
• htonl() // host to network long
• ntohs // network to host short
• ntohl() // network to host long
• Very Important Even if your machine is
  Big-Endian m/c, but you put your bytes in NBO
  before putting them on to the network for
  portability

11
Dealing with IP Addresses

• Suppose you have a structure
• struct sockaddr_in ina, and you want to store ip
  address 10.12.110.57 in it
• How will you do it? I mean this is in dotted
  notation
• inet_addr() at rescue this function converts IP
  address in dotted notation into an unsigned long,
  example-
• ina.sin_addr.s_addr inet_addr(10.12.110.57)
• inet_addr returns the IP in NBO, you need not
  call htonl()
• inet_addr returns -1 on error
• Note In binary -1 corresponds to IP address
  255.255.255.255, which is actually a broadcast
  address and is wrong
• PLZ HANDLE ERRORS PROPERLY

12
Dealing with IP Addresses (Contd.)

• Convert binary representation of IP address back
  to string IP
• Use inet_ntoa() // network to ascii
• Example
• printf(s, inet_ntoa(ina.sin_addr))
• inet_ntoa accepts structure in_addr not a long.
• At this point function return a pointer to a
  char, which points to a statically stored char
  array with in inet_ntoa() so that each time you
  call inet_ntoa() it will overwrite the last IP
  address you asked for.

13
Dealing with IP Addresses (Contd.)

• Example
• char ip1, ip2
• ip1 inet_ntoa(ina1.sin_addr) // this is
  192.168.4.14
• ip2 inet_ntoa(ina2.sin_addr) // this is
  10.12.110.57
• printf(Address 1 s\n, a1)
• printf(Address 2 s\n, a2)
• Output
• Address 1 10.12.110.57
• Address 2 10.12.110.57
• If you want to save the address use strcpy() it
  to your own array
• We will learn to convert hostname to IP address
  in sections to come

14
Socket Library Function

• System calls
• startup / close
• data transfer
• options control
• Network configuration lookup
• host address
• ports for services
• Utility functions
• data conversion
• address manipulation
• error handling

15
Network Database Administration functions

• gethostbyname - given a hostname, returns a
  structure which specifies its DNS name(s) and IP
  address(es)
• getservbyname - given service name and protocol,
  returns a structure which specifies its name(s)
  and its port address
• gethostname - returns hostname of local host
• getservbyname, getservbyport, getservent
• getprotobyname, getprotobynumber, getprotobyent
• getnetbyname, getnetbyaddr, getnetent

16
Socket Utility Functions

• ntohs/ntohl - convert short/long from network
  byte order (big endian) to host byte order
• htons/htonl - convert short/long from host byte
  order to network byte order
• inet_ntoa/inet_addr - convert 32-bit IP address
  (network byte order to/from a dotted decimal
  string)
• perror() - print error message (based on errno)
  to stderr
• herror() - print error message for
  gethostbyname() to stderr (used with DNS)

17
Primary Header Files

• Include file sequence may affect processing
  (order is important!)
• ltsys/types.hgt - prerequisite typedefs
• lterrno.hgt - names for errno values (error
  numbers)
• ltsys/socket.hgt - struct sockaddr system
  prototypes and constants
• ltnetdb.h.hgt - network info lookup prototypes and
  structures
• ltnetinet/in.hgt - struct sockaddr_in byte
  ordering macros
• ltarpa/inet.hgt - utility function prototypes

18
Primary Socket System Calls

• socket() - create a new socket and return its
  descriptor
• bind() - associate a socket with a port and
  address
• listen() - establish queue for connection
  requests
• accept() - accept a connection request
• connect() - initiate a connection to a remote
  host
• recv() - receive data from a socket descriptor
• send() - send data to a socket descriptor
• close() - one-way close of a socket descriptor

19
Socket System Calls For Connection-Oriented
Protocols
20
Socket System Calls For Connectionless Protocols
21
Socket Addresses

• Socket address structure from ltnetinet/in.hgt
• struct sockaddr_in (IPv4)

struct sockaddr_in uint8_t sin_len sa_fami
ly_t sin_family in_port_t sin_port struct
in_addr sin_addr char sin_zero8
struct in_addr in_addr_t
s_addr
22

• Socket System Call

include ltsys/types.hgt include
ltsys/socket.hgt int socket (int family, int type,
int protocol)
Socket descriptor -1 if fails
Usually 0 is used. Defined in ltnetinet/in.hgt IPPRO
TO_xxx
AF_INET Internet protocols(IPv4) AF_INET6
IPv6 protocol
AF_UNIX AF_INET
SOCK_STREAM Stream Socket SOCK_DGRAM Datagram
Socket
SOCK_STREAM SOCK_DGRAM

• Creates a network plug point that enables the
  client/server to communicate

23

• Bind System Call
• Servers register their addresses with the system.
• Both CO and CL servers do this before accepting
  client requests.
• Allows a server to specify the IP address and
  port number associated with a socket
• connect System Call
• Enables a client to connect to a server i.e.
  initiate a connection to a remote host
• Client process connects a socket descriptor to
  establish a connection with the server
• Specific communication parameters like buffer
  size, size of data to be exchanged without ACK,
  etc, is agreed

Pointer to a protocol-specific address
include ltsys/types.hgt include
ltsys/socket.hgt int bind(int sockfd,const struct
sockaddr myaddr,int addrlen)
include ltsys/types.hgt include
ltsys/socket.hgt int connect(int sockfd,struct
sockaddr servaddr,int addrlen)
24

• listen System Call
• Used to indicate that it is willing to receive
  connections
• backlog specifies the number of requests that can
  be queued by the system, while it waits for the
  server to execute the accept system call
• accept System Call
• After listen, accept waits for an actual
  connection request from a client
• Takes the first connection request from queue,
  and creates another socket with the same
  properties as sockfd
• accept called to return next completed connection
  from the front of completed connection queue

int listen (int sockfd, int backlog)
include ltsys/types.hgt include
ltsys/socket.hgt int accept (int sockfd, struct
sockaddr peer, int addrlen)
25
Either zero, or ORed with MSG_OOB
Out-of-band MSG_PEEK Peek at data
MSG_DONTROUTE Bypass routing
include ltsys/types.hgt include
ltsys/socket.hgt int write (int connfd, char
buff, int nbytes) int sendto (int sockfd, char
buff, int nbytes, int flags, struct sockaddr
to, int addrlen) int read (int connfd, char
buff, int nbytes) int recvfrom (int sockfd,
char buff, int nbytes, int flags, struct
sockaddr from, int addrlen)

• All return the length of data that was read or
  written as the value of the function
• Close System Call

int close (int fd)
26

• Byte ordering routines
• Address conversion routines

include ltsys/types.hgt include
ltnetinet/in.hgt u_long htonl(u_long hostlong)
/ host-to-network, long integer / u_short
htons(u_short hostshort) / host-to-network,
short integer / u_long ntohl(u_long netlong)
/ network-to-host, long integer / u_short
ntohs(u_short netshort) / network-to-host,
short integer /
include ltsys/socket.hgt include
ltnetinet/in.hgt include ltarpa/inet.hgt unsigned
long inet_addr(char ptr) converts a char
string IP address to its 32-bit network
byte-order integer equivalent. char
inet_ntoa(struct in_addr inaddr)
27
Simple TCP Server

• include ltsys/types.hgt
• include ltsys/socket.hgt
• include ltnetinet/in.hgt
• define SERVER_PORT 5888
• int main()
• int sockfd, connfd,clilen,n
• char buf256
• struct sockaddr_in servaddr, cliaddr
• sockfd socket( AF_INET, SOCK_STREAM, 0)
• if (sockid lt 0)
• printf( Server socket error")
  exit(1)
• servaddr.sin_family AF_INET
• servaddr.sin_port htons(SERVER_PORT)
• servaddr.sin_addr.s_addr htonl(INADDR_ANY)
• if(bind(sockfd,(struct sockaddr)servaddr,sizeof(
  server)) lt0 )
• printf(Server Bind Error) exit(1)

28

• listen(sockfd, 5)
• for( )
• clilen sizeof(cliaddr)
• connfdaccept(sockfd, (struct sockaddr )
  cliaddr, clilen)
• if(connfdlt0)
• printf(Server Accept error \n) exit(1)
• printf("Client IP s\n", inet_ntoa(cliaddr.sin_a
  ddr))
• printf("Client Port hu\n", ntohs(cliaddr.sin_po
  rt))
• n read(connfd, buf,256)
• printf("Server read \"s\" d chars\n",
  buf, n)
• write(connfd, Server Got Message, 18)
• close(connfd)

29
Simple TCP Client

• include ltsys/types.hgt
• include ltsys/socket.hgt
• include ltnetinet/in.hgt
• define SERVER_PORT 5888
• define SERVER_IPADDR 172.24.2.4
• int main()
• int sockfd, clifd,len
• char buf256
• struct sockaddr_in servaddr
• sockfd socket( AF_INET, SOCK_STREAM, 0)
• if (sockid lt 0) printf( Server socket
  error") exit(1)
• servaddr.sin_family AF_INET
• servaddr.sin_port htons(SERVER_PORT)
• servaddr.sin_addr.s_addr
  inet_addr(SERVER_IPADDR)
• connect(sockfd,(struct sockaddr)servaddr,
  sizeof(servaddr))
• print(Enter Message \n)

30
Simple UDP Server

• include ltsys/types.hgt
• include ltsys/socket.hgt
• include ltnetinet/in.hgt
• define SERVER_PORT 9988
• int main()
• int sockfd, clilen
• char buf256
• struct sockaddr_in servaddr, cliaddr
• sockfd socket( AF_INET, SOCK_DGRAM, 0)
• servaddr.sin_family AF_INET
• servaddr.sin_port htons(SERVER_PORT)
• servaddr.sin_addr.s_addr htonl(INADDR_ANY)
• if (bind(sockfd,(struct sockaddr)servaddr,sizeof
  (servaddr)) lt0 )
• printf(Server Bind Error) exit(1)
• for( )
• clilen sizeof(cliaddr)
• recvfrom(sockfd,buf,256,0,(struct
  sockaddr)cliaddr,clilen)
• printf(Server Receiveds\n,buf)

31
Simple UDP Client

• include ltsys/types.hgt
• include ltsys/socket.hgt
• include ltnetinet/in.hgt
• define SERVER_PORT 9988
• define SERVER_IPADDR 172.24.2.4
• int main()
• int sockfd,len
• char buf256
• struct sockaddr_in ,cliaddr,servaddr
• servaddr.sin_family AF_INET
• servaddr.sin_port htons(SERVER_PORT)
• servaddr.sin_addr.s_addr
  inet_addr(SERVER_IPADDR)
• sockfd socket( AF_INET, SOCK_DGRAM, 0)
• cliaddr.sin_family AF_INET
• cliaddr.sin_port htons(0)
• cliaddr.sin_addr.s_addr htonl(INADDR_ANY)

Not mandatory
32
THANK YOU