TCPIP programming

1
TCP/IP programming

• Unix TCP/IP socket programming

2
Tele-Marketing (motivating example)
customer
store

• phone line
• information
• phone number
• extension
• Dial call
• talk
• hang-up

• phone line
• setup
• phone number
• extension
• call-waiting
• Wait for calls
• Answer calls
• talk
• hang-up

3
Internet sockets terminology

• phone line
• phone number
• extension
• call waiting
• dial
• answer
• talk
• hang-up

• socket()
• IP address
• port number
• bind()
• listen()
• connect()
• accept()
• send() / recv()
• close ()

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Sockets architecture
application
user
kernel
socket
TCP/UDP
IP (network)
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The client-server model
request
Port 2356
Port 23
reply
telnet
Client 1
IP132.66.48.37
Port 20
ftp
Port 3325
Client 2
Server
IP132.66.48.12
IP132.66.2.1
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Internet Sockets

• File descriptor
• two (main) types
• TCP (SOCK_STREAM)
• UDP (SOCK_DGRAM)
• Structure
• Byte order
• compatibility (host vs. network)
• conversion routines

sa_family sa_data
sock_addr
sa_family port IP_addr zero
sock_addr_in
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SOCKET ( ) - Get the file descriptor
int socket (int domain, int type, int protocol)

• Domain what type of communication
• AF_INET
• Type semantics of communication
• SOCK_STREAM (TCP)
• SOCK_DGRAM (UDP)
• Protocol which protocol to use
• set to 0
• OUTPUT socket file descriptor.
• return -1 for error

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BIND ( ) - What port am I on
int bind ( int sockfd, struct sockaddr
myaddr, int addrlen)

• Associates a socket with a port on local machine.
• port numbers 1024-65535.
• Unspecified
• port (set port0) chooses an available port.
• IP (set IP address to INADDR_ANY) returns local
  IP address.
• OUTPUT none
• return -1 for error

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CONNECT ( ) - Hey, you!
int connect ( int sockfd, struct sockaddr
serv_addr, int addrlen)

• Associates a socket with a port on local machine.
• Not necessary to bind() before connect().
• OUTPUT none
• return -1 for error

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LISTEN ( ) - Will somebody please call me?
int listen( int sockfd, int backlog)

• backlog - number of connections allowed in
  incoming queue.
• OUTPUT none
• return -1 for error

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ACCEPT ( ) - Thank you for calling
int accept ( int sockfd, void addr, int
addrlen)

• returns a new file descriptor.
• The communication (send recv) uses the new file
  descriptor.
• sockfd can be used to accept more calls.
• OUTPUT file descriptor
• return -1 for error

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send ( ) and recv ( ) - Talk to me!
int recv ( int sockfd, void buf, int
len unsigned int flags)
int send ( int sockfd, const void msg,
int len int flags)

• sockfd is the one to read from.
• buf points to the buffer to read the data into,
  whose maximum length is len.
• returns number of bytes read.

• sockfd is where to send.
• msg points to the data, whose length is len.
• Returns number of bytes sent.

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close ( ) and shutdown ( ) - Done!
int close (int sockfd)
int shutdown (int sockfd, int how)

• prevents read and writes from sockfd.
• Any attempt to read/write results in an error.

• selects how to close sockfd.
• how has the following values
• 0 - disallow receives.
• 1- disallow sends.
• 2 - disallow receive send

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Summary
server
client

• socket()
• bind()
• listen()
• accept()
• send()/recv()
• close()

• socket()
• connect()
• send()/ recv()
• close()

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sendto ( ) and recvfrom ( ) - Talk to me UDP!
int recvfrom (int sockfd, void buf, int
len unsigned int flags const struct sockaddr
from int fromlen)
int sendto ( int sockfd, const void msg,
int len int flags const struct sockaddr
to int tolen)

• Combines the send and receive with IP address.
• No need for listen(), accept() and connect().
• unreliable communication.

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getpeername( ) - Who am I talking with?
int getpeername( int sockfd, struct sockaddr
addr, int addrlen)
Return the IP address of the other party to
sockfd.
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Blocking

• what is blocking
• perform both accept() and recv().
• non-blocking socket.
• CPU wastage.
• The poll command
• Allows to wait on a few events.

int poll(struct pollfd fds unsigned long
nfds int timeout)
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Poll command

• Pollfd structure
• file descriptor (int fd)
• requested events (short events)
• returned events (short events)
• Information
• returns only when one or more request events
  occur.
• Can have multiple events.
• flags are bits set.
• more information in "man poll"
• System view
• The execution waits for the events to occur.
• No wastage of CPU.

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Naming and addressing

• There is more then one level of addressing.
• Domain name (www.tau.ac.il)
• IP address (132.66.16.6)
• MAC address (ethernet card number)
• Why do we need different addresses.
• How can we look up an address.

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Domain Name System (DNS)

• A level of abstraction in naming.
• domain name www.amazon.com
• IP address 208.216.182.15
• struct hostent gethostbyname( const char name)
• name is the domain name.
• struct hostent
• h_name official host name
• h_alises alternate names
• h_addrtype usually AF_INET
• h_length length of address in bytes
• h_addr_list IP address
• h_addr first adress in h_addr_list

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How does DNS work?

• Old system one file that matched names to IP
  addresses.
• Current system
• call a resolver with name.
• resolver send a UDP packet to DNS server.
• DNS server either has the IP address locally
  (/etc/hosts), or
• fetches it from another DNS server.
• Name server - recursive query
• DNS name space
• hierarchical
• case insensitive
• alpha-numeric

mansour.tau.ac.il
edu-server.net
mit.edu
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Address Resolution Protocol (ARP)

• Host is connected to a LAN
• Ethernet address (48 bits)
• Maps an IP address to an Ethernet address
• Data link level communication uses Ethernet
  address
• Why a simple look-up table is not good enough.

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ARP
host name
host name

• FTP calls gethostbyname
• FTP asks TCP .
• TCP asks to IP.
• IP converts the IP address to a next hop address
  (ARP).
• ARP sends ARP request, a broadcast packet
• IP address
• Its IP address
• Its Ethernet number
• The destination checks the ARP request.
• The "right" host sends ARP reply.

resolver
ftp
IP address
TCP
IP
ARP
Ethernet driver
Ethernet driver
Ethernet driver
ARP
IP
ARP