CPSC 441 Tutorial

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CPSC 441 Tutorial

• Mingwei Gong

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Contact info

• www.cpsc.ucalgary.ca/gongm/CPSC441
• gongm_at_cpsc.ucalgary.ca

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Assignment 1 socket programming

• Whats a socket?
• A way to speak to other programs using standard
  Unix file descriptors
• Sockets represents end points in a line of
  communication.
• Using these line of communication between two
  points are established.
• Jeez, everything in Unix is a file!

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Referencing A socket

• When a new file is opened using open()
• Next available lowest file descriptor is
  returned.
• This is a handle by which all future operations
  can use to reference.
• Same way an handle is generated for a socket when
  a socket is opened.
• Socket pair four-tuple local IP
  addresslocal port, foreign IP address foreign
  port

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Types of Sockets

• Two types of Internet Sockets
• Stream sockets (Analogy phone)
• Reliable two-way connected communication streams.
  
• SOCK_STREAM
• Datagram Sockets (Analogy mail)
• Connectionless communication, best effort
• SOCK_DGRAM

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Necessary Background Information POSIX data
types

• int8_t signed 8bit int
• uint8_t unsigned 8 bit int
• int16_t signed 16 bit int
• uint16_t unsigned 16 bit int
• int32_t signed 32 bit int
• uint32_t unsigned 32 bit int
• u_char, u_short, u_int, u_long

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More POSIX data types

• sa_family_t address family of socket address
  structure
• socklen_t length of struct, normally
  uint32_t
• in_addr_t IPv4 address,
• normally uint32_t
• in_port_t IP port number,
• normally uint16_t

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Socket Address Structures

• Predefined data structures
• struct sockaddr_in
  / INET socket addr info /
• unit8_t sin_len /length of
  structure(16)/ sa_family_t sin_family / set me
  to AF_INET /
• in_port_t sin_port / 16 bit TCP or
  UDP port num, /
• /network byte order/
• struct in_addr sin_addr /
  32 bit IPv4 address /
• /network byte order/
• char sin_zero8 / not used /
• IP address (32-bit numeric representation)
• struct in_addr
• in_addr_t s_addr /
  32 bit IPv4 address,
• /network byte order /

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Generic Socket Address Structure

• generic socket address.
• struct sockaddr
• unit8_t sin_len
• sa_family_t sin_family / address family
  AF_xxxx value /
• char sa_data14 / protocol- spe
  cific address /

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sockaddr_in
sockaddr
sin_len
sa_len
sa_family
sa_data
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Byte Ording Functions

• Integers
• little endian least significant bit first
• big endian most significant bit first
• The terms little-endian and big-endian
  indicate which end of the multibyte value, the
  little end or the big end, is stored at the
  starting address of the value

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Data Types and structures

• There are two type of byte ordering
• Most significant byte first. (big-endian)
• Least significant byte first. (little-endian)
• Former is called Network byte ordering.
• Latter is called host byte ordering

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Network Byte Order Functions

• h host byte order n network byte
  order
• s short (16bit) l long
  (32bit)
• uint16_t htons(uint16_t)
• uint16_t ntohs(uint_16_t)
• uint32_t htonl(uint32_t)
• uint32_t ntohl(uint32_t)

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Conversion of Natives

• htons() -- "Host to Network Short"
• htonl() -- "Host to Network Long"
• ntohs() -- "Network to Host Short"
• ntohl() -- "Network to Host Long"
• sin_addr and sin_port need to be in network byte
  order.
• sin_family need not be in network byte order.

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Flowchat
TCP Server
socket()
bind()
TCP Client
listen()
socket()
connection establishment 3-way handshake
accept()
connect()
blocks until connection from client
write()
data (request)
read()
process request
data (reply)
read()
write()
close()
end-of-file notification
read()
close()
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Socket Conceptual View
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Creating a socket1

• In order to create a socket pair following
  function has to be used.
• Format int socket(family, type, protocol)

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Protocol family constants
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Type of socket
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Creating a socket2

• family, type and protocol parameters
• family PF_INET/AF_INET, PF_UNIX/AF_UNIX
• type
• SOCK_DGRAM datagram service (i.e., UDP)
• SOCK_STREAM byte stream service (i.e.,TCP)
• protocol usually 0 for default type

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Assigning an address to a socket

• The bind() system call is used to assign an
  address to an existing socket.
• int bind( int sockfd,
• const struct sockaddr myaddr, int
  addrlen)
• bind returns 0 if successful or -1 on error.

const!
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bind()

• calling bind() assigns the address specified by
  the sockaddr structure to the socket descriptor.
• You can give bind() a sockaddr_in structure
• bind( mysock,
• (struct sockaddr) myaddr,
• sizeof(myaddr) )

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bind() Example

• int mysock,err
• struct sockaddr_in myaddr
• mysock socket(AF_INET,SOCK_STREAM,0)
• myaddr.sin_family AF_INET
• myaddr.sin_port htons( portnum )
• myaddr.sin_addr htonl( ipaddress)
• errbind(mysock, (sockaddr ) myaddr,
  sizeof(myaddr))

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Uses for bind()

• There are a number of uses for bind()
• Server would like to bind to a well known address
  (port number).
• Client can bind to a specific port.
• Client can ask the O.S. to assign any available
  port number.

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Port number - who cares ?

• Clients typically dont care what port they are
  assigned.
• When you call bind you can tell it to assign you
  any available port
• myaddr.port htons(0)

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What is my IP address ?

• How can you find out what your IP address is so
  you can tell bind() ?
• There is no realistic way for you to know the
  right IP address to give bind() - what if the
  computer has multiple network interfaces?
• specify the IP address as INADDR_ANY, this tells
  the OS to take care of things.

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IPv4 Address Conversion

• int inet_aton( char , struct in_addr )
• Convert ASCII dotted-decimal IP address to
  network byte order 32 bit value. Returns 1 on
  success, 0 on failure.
• char inet_ntoa(struct in_addr)
• Convert network byte ordered value to ASCII
  dotted-decimal (a string).

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• A Few Words about Port Numbers
• 1-255 standard services (21 ftp, 25 SMTP, 80
  HTTP)
• 1-1023 available only to system
• 1024-4099 usable by system users
• 5000 - usable only by users

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Listen()

• This function is called after both the socket()
  and bind() functions
• int listen (int sockfd, int backlog)
• notify OS/network ready to accept requests
• backlog max. listen queue size while waiting for
  accept()
• backlog does not block/wait for requests

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Accept()

• int accept(int sockfd, struct sockaddr
  fromaddr, int addrlen)
• use socket (sockfd) for accepting incoming
  connection requests
• create new socket (return value) for data
  exchange w/ client
• block until client connects, cannot selectively
  accept

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Flowchat
TCP Server
socket()
bind()
TCP Client
listen()
socket()
connection establishment 3-way handshake
accept()
connect()
blocks until connection from client
write()
data (request)
read()
process request
data (reply)
read()
write()
close()
end-of-file notification
read()
close()
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connect()

• int connect (int sockfd, const struct sockaddr
  toaddr, int addrlen)
• Returns 0 if OK, -1 on error
• Client issues connect() to establish connection
  with server

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Fork functions

• Pid_t fork(void)Return 0 in child, process ID of
  child in parent
• For ( )
• Connfd accept (listenfd, )
• If ((pidFork())0)
• doit(connfd) //process the request
• close(connfd) //done with this client
• exit(0) //child terminates
• close(connfd) //parent closes connected
  socket

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Sending and Receiving Data

• Connection-oriented
• send() and recv() or read() and write()
• Format
• int send(int sockfd, char buff, int nbytes, int
  flags)
• int recv(int sockfd, char buff, int nbytes, int
  flags)

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Closing a Socket

• Format int close(int sockfd)
• Call this function to close a created socket.
• System takes care of buffered data.