Introduction to Socket Programming

1
Introduction to Socket Programming
2
Overview of TCP

• Features of TCP that are relevant to the network
  programmer
• Addressing
• Segment Structure
• Connection establishment
• Connection release
• State transition

3
Why is the emphasis on TCP ?

• The following use TCP
• BGP (Routing)
• HTTP (Web traffic)
• SMTP (Email)
• FTP (File transfer)
• NNTP (Newsgroups)
• Telnet (Remote login)

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Addressing

• TCP uses the concept of source and destination
  port numbers which act as the TSAPs and uniquely
  identify a particular communicating process on a
  host
• Usually server applications use what are known as
  Well known port numbers which are defined by a
  standards body.
• IANA assigns these port numbers. Most systems
  allow only privileged users to communicate on
  these ports.
• Most operating systems allow only privileged
  applications to communicated on these ports.
• They range from 0 through 1023

5
Continued

• Registered ports are those that are
  administratively assigned by a standards body to
  some specific applications.
• They are assigned by IANA
• They range from 1024 through 49151
• Private ports are those that do not have any
  specific application associated with them.
• Used by applications that are ephemeral in nature
• They range from 49152 through 65535

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TCP Segment Header
7
TCP Connection ManagementThree-way Handshake
8
Establishing a Connection

• TCP uses a three way handshake mechanism for
  connection establishment.
• Each side chooses an initial sequence number that
  needs to be acknowledged by its peer
• Sequence number of the packets that are part of a
  connection are obtained using the formula
• Seq(Pn) Seq(Pn-1) Length(Pn-1)
• Ack(LPn) Seq(RPn-1) Length(RPn-1) 1
• The side that sends the SYN first is said to do
  an active open and its peer is said to do a
  passive open
• It is possible for both the sides to do an active
  open

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Releasing A Connection

• TCP connections are full duplex (they can be
  thought of as two simplex connections).
• Each simplex connection is released
  independently.
• Two-army problem avoided using timers.
• The side sending the FIN first is said to do an
  active close however it is possible for both
  sides to do an active close
• A closing of a TCP connection means no more data
  in the direction of the active close

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TCP State transition diagram
Use netstat command to view them
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• TIMED-WAIT
• QUIET TIME
• FIN_WAIT2
• RESET SEGMENTS
• SIMULTANEOUS OPEN/CLOSE
• MAXIMUM SEGMENT SIZE

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The SOCKET Interface

• What is the socket interface all about ?
• It is a protocol independent interface to
  multiple
• transport layer primitives.
• Where is it used ?
• In order to write applications which need to
  communicate
• with other applications.
• History of socket interface
• The first implementation was released along with
  BSD 4.2
• Never mind

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• What are the advantages of using this interface
  ?
• Syntax of the API functions is independent of the
  protocol being used. Ex- TCP/IP and UNIX domain
  protocols can be used by applications using a
  common set of functions.
• Gives way to better portability of applications
  across protocol suites.
• Hides the finer details of the protocols from
  application programs thereby yielding faster and
  bug free application development
• Sockets are referenced through socket descriptors
  which can be passed directly to UNIX system
  I/O calls.
• File I/O and socket I/O are exactly similar from
  the programmer perspective.

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Organisation of networking code
User process
Socket Interface
Socket implementation
Protocol Layer
(TCP/IP,IPX,UNIX)
Media Driver
Media (Ethernet,FDDI,Wireless)
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Components of the Socket API

• Address structure
• Initialisation/Shutdown
• Connection management
• Data exchange(I/O)
• I/O multiplexing
• Optional parameters
• Signal handling
• Protocol behavior manipulation through ioctl

• Descriptor manipulation through fcntl
• Nonblocking I/O
• Hostname and address conversions
• Data manipulation functions
• Client-server design choices

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Socket address structure

• This contains the protocol specific addressing
  information that is passed from the user process
  to the kernel and vice versa
• Each of the protocols supported by a socket
  implementation have their own socket address
  structure sockaddr_suffix
• Where suffix represents the protocol family
• Ex sockaddr_in Internet/IPv4 socket address
  structure
• sockaddr_ipx IPX socket address structure

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• The generic socket address structure
• sockaddr
• address family
• protocol specific data
• The internet/IPv4 socked address structure
• sockaddr_in
• in_family Internet address family
• sin_port Transport layer Port Number
• in_addr sin_addr IP address
• sin_zero8 Padding

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Datatypes required by POSIX

• int8_t signed 8-bit integer - ltsys/types.hgt
• uint8_t unsigned 8-bit integer - ltsys/types.hgt
• int16_t signed 16-bit integer - ltsys/types.hgt
• uint16_t unsigned 16-bit integer - ltsys/types.hgt
• int32_t signed 32-bit integer - ltsys/types.hgt
• uint32_t unsigned 32-bit integer - ltsys/types.hgt
• sa_family_t address family of - ltsys/socket.hgt
• socklen_t length of socket address structure
  -ltsys/socket.hgt
• in_addr_t IPv4 address, normally uint32_t
  ltnetinet/in.hgt
• in_port_t TCP/UDP port, normally uint16_t
  ltnetinet/in.hgt

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Data manipulation functions

• Byte ordering
• Network byte order
• Host byte order
• htons(l), ntohs(l)
• Memory content initialization
• memset(buffer,value,buffersize)
• Data copying and comparison
• memcpy(dest,src,num_of_bytes)
• memcmp(buffer1,buffer2,num_of_bytes)

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Continued..

• IP address notation conversion
• Integer notation
• Dotted decimal notation
• status inet_aton(ddstring_pointer,address_pointer
  )
• Returns 1 on success 0 on error
• ddstring_pointer inet_ntoa(address_pointer)
• address_pointer inet_addr(ddstring_pointer)
  deprecated

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Initialisation and Shutdown

• sockfd socket(domain, type, protocol)
• domain is the protocol/address family
  AF_INET,AF_IPX..
• type is the the type of service
  SOCK_DGRAM,SOCK_STREAM
• protocol is the specific protocol that is
  supported by the protocol family specified(as
  param1)
• Returns a fresh socket descriptor on success, 1
  on error
• status close(sockfd)
• Flushes(supposed to) the pending I/O to disk
• Returns 1 on error

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Connection Management

• status bind(sockfd,ptr_to_sockaddr,sockaddr_size)
  
• Associates the sockaddr with sockfd
• The rules for successful binding depend on the
  protocol family of the socket(specified during
  call to socket)
• Necessary for receiving connections on STREAM
  socket
• status listen(sockfd,backlog)
• Notifies the willingness to accept connections
• backlog Maximum number of established connections
  yet to be notified to their respective user
  processes(calls to accepts)
• On unbounded sockets an implicit bind is done
  with IN_ADDRANY and a random port as the address
  and port parameters respectively
• Above calls return 1 on error

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Continued

• connfd accept(sockfd,ptr_to_sockaddr,ptr_to_sockad
  dr_size)
• Blocks till a connection gets established on
  sockfd and returns a new file descriptor on which
  I/O can be performed with the remote entity
• Fills the sockaddr and size parameters with the
  address information (and its size respectively)
  of the connecting entity
• bind and listen are assumed to have been called
  on sockfd prior to calling accept
• status connect(sockfd, ptr_to_sockaddr,
  sockaddr_size)
• Initiates a new connection with the entity
  addressed by sockaddr in case of a STREAM socket
• Sets the default remote address for I/O in case
  of DGRAM socket
• Above calls return 1 on error

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Illustrative Applications

• Application 1
• Iterative tcp server - echoserver.c
• tcp client - echoclient.c
• I/O using send/receive
• Illustrate basic socket calls

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Continued

• Application 2
• simultaneous tcp server - echoserver-select.c -
  multiple ports
• Same client
• Illustrate canonical/noncanonical mode of input