Introduction to Socket Programming
1
Overview of TCP
• Features of TCP that are relevant to the network
programmer
– Addressing
– Segment Structure
– Connection establishment
– Connection release
– State transition
2
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
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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
5
TCP Segment Header
6
TCP Connection Management
Three-way Handshake
7
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 it’s 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 it’s 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
10
– TIMED-WAIT
– QUIET TIME
– FIN_WAIT2
– RESET SEGMENTS
– SIMULTANEOUS OPEN/CLOSE
– MAXIMUM SEGMENT SIZE
11
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
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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_zero[8]
Padding ;
};
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Datatypes required by POSIX
• int8_t signed 8-bit integer - <sys/types.h>
• uint8_t unsigned 8-bit integer - <sys/types.h>
• int16_t signed 16-bit integer - <sys/types.h>
• uint16_t unsigned 16-bit integer - <sys/types.h>
• int32_t signed 32-bit integer - <sys/types.h>
• uint32_t unsigned 32-bit integer - <sys/types.h>
• sa_family_t address family of - <sys/socket.h>
• socklen_t length of socket address structure <sys/socket.h>
• in_addr_t IPv4 address, normally uint32_t <netinet/in.h>
• in_port_t TCP/UDP port, normally uint16_t <netinet/in.h>
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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)
19
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
21
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_sockaddr_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 it’s 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
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