Network Programming Using Internet Sockets

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Network Programming Using Internet Sockets
Jong-Hoon Youn CS372 Winter 2001
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A closer look at network structure

• network edge applications and hosts
• network core
• routers
• network of networks

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The network edge

• end systems (hosts)
• run application programs
• e.g., WWW, email
• at edge of network?

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Network edge connection-oriented service

• TCP service RFC 793
• reliable, in-order byte-stream data transfer
• loss acknowledgements and retransmissions
• flow control
• sender won?t overwhelm receiver
• congestion control
• senders slow down sending rate? when network
  congested

• Goal data transfer between end sys.
• handshaking setup (prepare for) data transfer
  ahead of time
• set up state? in two communicating hosts
• TCP - Transmission Control Protocol
• Internet?s connection-oriented service

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Network edge connectionless service

• Goal data transfer between end systems
• UDP - User Datagram Protocol
• Internet's connection-less service
• unreliable data transfer
• no flow control
• no congestion control

• App?s using TCP
• HTTP (WWW), FTP (file transfer), Telnet (remote
  login), SMTP (email)
• App?s using UDP
• streaming media, teleconferencing, Internet
  telephony

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The Network Core

• mesh of interconnected routers
• the fundamental question how is data transferred
  through net?
• Circuit switching dedicated circuit per call
  telephone net
• packet-switching data sent thru net in discrete
  chunks?

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Sockets

• Basic definition - "endpoint of communication"
• Sockets are dominant TCP/IP application API
• Allows connect streams (TCP) or discrete messages
  (UDP) between processes on same machine or cross
  network.

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Creating a Socket

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int socket(int domain, int type, int protocol)
• domain AF_INET, AF_UNIX
• type SOCK_STREAM, SOCK_DGRAM
• protocol 0 the default protocol for the given
  address family and socket type.
• This call returns a socket descriptor that can be
  used as a parameter to reference the socket in
  subsequent system calls, or -1 on error.

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Styles of Communication

• Datagram Sockets (SOCK_DGRAM)
• connectionless and unreliable UDP
• Messages may be lost, duplicated or received out
  of order.
• Stream Sockets (SOCK_STREAM)
• connection-oriented and reliable TCP
• The transport protocol ensure that data is
  delivered in order without loss, error or
  duplication.

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Binding a Socket to a Port

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int bind(int sockfd, struct sockaddr my_addr,
  int addrlen)
• sockfd the socket descriptor returned by
  socket().
• my_addr a pointer to a struct sockaddr that
  contains information about the local address
• addrlen sizeof (struct sockaddr)
• bind associates the given socket with a port on
  your local machine. It returns -1 on error.

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• include ltnetinet/in.hgt
• struct sockaddr_in
• uint8_t sin_len / length of structure (16) /
• sa_family_t sin_family / AF_INET /
• in_port_t sin_port / 16-bit Port number /
• struct in_addr sin_addr / 32-bit Internet
  address /
• char sin_zero8 / not used /
• struct in_addr
• in_addr_t s_addr / 32-bit IPv4 address /
• / it used to be a union /

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• sockaddr_in example

length
AF_INET
16
AF_INET
16-bit port
7777
32-bit IPv4
2160141825
address
(address)
(Unused)
(Unused)
Fixed length (16 bytes)
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• ltExamplegt
• int sockfd
• struct sockaddr_in my_addr
• sockfd socket(AF_INET, SOCK_STREAM, 0)
• / do some error checking! /
• my_addr.sin_family AF_INET / host byte order
  /
• my_addr.sin_port htons(7777)
• / short, network byte order /
• my_addr.sin_addr.s_addr inet_addr("132.241.5.10"
  )
• bzero((my_addr.sin_zero), 8) / zero the rest
  of the struct /
• bind(sockfd, (struct sockaddr )my_addr,
• sizeof(struct sockaddr))
• / don't forget your error checking for
  bind() /

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Listening for connections

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int listen(int sockfd, int queuelen)
• sockfd the socket descriptor returned by
  socket().
• queuelen the maximum number of queue connection
  that will be allowed.
• Listen initializes a queue for waiting connection
  requests. It returns -1 on error.

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Accepting a connection

• include ltsys/socket.hgt
• int accept(int sockfd, void addr, int addrlen)
• sockfd the listening socket descriptor
• addr the address of peer socket on new
  connection (returns client ip/port).
• addrlen length of addr in bytes
• Accept takes the first pending connection request
  off the queue and returns a socket descriptor for
  a new, connected socket. If there are no pending
  connection requests, accept blocks. It returns -1
  on error.

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• ltExamplegt
• int sockfd, new_fd, sin_size
• struct sockaddr_in their_addr
• . . . socket()
• . . . bind()
• listen(sockfd, 5)
• sin_size sizeof(struct sockaddr_in)
• new_fd accept(sockfd, their_addr, sin_size)

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Establishing a connection

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int connect(int sockfd, struct sockaddr
  serv_addr, int addrlen)
• sockfd the socket descriptor returned by
  socket().
• serv_addr a pointer to a struct sockaddr that
  contains information about the server address
• addrlen sizeof (struct sockaddr)
• connect will return -1 on error

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Send(), receive()

• int send(int sockfd, char msg, int len, int
  flags)
• sockfd socket descriptor you want to send data
  to
• msg a pointer to the data
• len the length of that data in bytes
• flags usually 0 (cf. MSG_OOB).
• int recv(int sockfd, char buf, int len, int
  flags)
• sockfd the socket descriptor to read from
• buf the buffer to read the information into
• len the maximum length of the buffer
• flags usually 0 (cf. MSG_PEEK).

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sendto(), receivefrom()

• int sendto(int sockfd, char msg, int len, int
  flags, struct sockaddr to, int tolen)
• to a pointer to a struct sockaddr which contains
  the destination IP address and port.
• tolen sizeof(struct sockaddr).
• int recvfrom(int sockfd, char buf, int len, int
  flags, struct sockaddr from, int fromlen)
• from a pointer to a struct sockaddr that will be
  filled with the IP address and port of the
  origin.
• fromlen the length of the address stored in from.

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• Return values
• send(sendto) returns the number of byte accepted
  by the transport layer, or -1 if some error is
  detected.
• recv(recvfrom) returns the number of bytes
  actually received, or -1 if some error is
  detected.
• By default, all the reading and writing calls are
  blocking
• Read calls do not return until at least one byte
  of data is available for reading.
• Write calls block until there is enough buffer
  space to accept some or all of the data being
  sent.

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Closing connection

• int close(sockfd)
• This will prevent any more reads and writes to
  the socket. Anyone attempting to read or write
  the socket on the remote end will receive an
  error.

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Byte order conversion

• There are two types short(two bytes) and long
  (four bytes)
• htons() Host to Network Short
• htonl() Host to network Long
• ntohs() Network to Host short
• ntohl() Network to Host Long
• Some hosts (such as the VAX), have a host order
  for integer values that reverse the order of
  bytes. gt put your bytes in network order before
  you put them on the network (portable).

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little-endian byte order
increasing memory address
address A1
address A
high-order byte
low-order byte
MSB
LSB
16-bit value
high-order byte
low-order byte
address A1
address A
big-endian byte order
increasing memory address
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Other useful functions

• Convert an IP address in number-and-dot notation
  into an unsigned long.
• struct sockaddr_in ina
• ina.sin_addr.s_addr inet_addr(132.241.5.10?)
• Convert string IP addresses to unsigned long.
• printf(s?, inet_ntoa(ina.sin_addr))

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• include ltnetdb.hgt
• struct hostent gethostbyname(char name)
• returns a pointer to the filled struct hostent,
  or NULL on error.
• struct hostent
• char h_name / official name of the host /
• char h_aliases / alterminate names for the
  host /
• int h_addrtype / usually AF_INET /
• int h_length / length of the address in
  bytes(4) /
• char h_addr_list / network addresses for the
  host /
• define h_addr h_addr_list0