Socket Programming
Carsten Griwodz
Socket programming
1
What are sockets?
❒
❒
A way in which operating systems make
network programming possible
❍ Offer an API for networking to applications
❍ Hide the details of networking from applications
Standard: Berkeley Socket API
❍ Introduced in early Unix versions
❍ Used in Linux, Unix, Windows, MacOS, OpenVMS,
OS/400, …
❍ Not limited to Internet protocols
Socket programming
2
What are sockets?
❒
An API between the application and the
network
❍ Applications send data through the socket over
the network to a remote application
❍ Applications receive data through the socket from
a remote application
❍ Applications control the time and amount of
sending and receiving
❍ Operating systems implement the details
Socket programming
3
1
What are sockets?
Process
Socket
Control block with
buffers,
variables
Application Controlled by
the appliction
Process
Controlled by
the operating system
Control block with
buffers,
variables
Socket
Internet
Socket programming
4
Socket programming
5
What are sockets?
❒
At the application level
❍ Datatype: int
❍ Very similar for
• all protocols
• file descriptors
• stdin, stdout
❒
Share functions with
❍ Reading and writing files
❍ Writing to the screen
❍ Reading from the keyboard
What are sockets?
❒
Refer to structures in the operating system
❍ The protocol in use
• TCP, UDP, …
❍
Protocol-specific state information
❍
An open, connected TCP socket needs also
• Address of the own machine and application
•
•
•
•
Address of the remote machine and application
Buffers
Variables
Timers
Socket programming
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2
Typical use of sockets
❒
❒
TCP and UDP communication
Transport layer protocols
❍ End-to-end communication
❍ Rely on the routing functions of IP
• Host addressing
• Path finding
• Host identification
❍
IP in turn relies on the link layer
• Packet forwarding from host to host
Add transport layer functions
❍ Add addressing of applications
❍
Socket programming
7
Why introduce sockets now?
After the Link layer lectures, you expect
Networking layer lectures
❒ Instead, you get a sockets lecture
❍ The main goal of sockets is to provide the services
of the transport layer to applications
❒ Why?
❍ It is possible to emulate link layer and networking
layer functions on layer 4
❍ You are expected to emulate link layer functions in
the mandatory assignment 2
❒
Socket programming
8
Layer emulation
PPPoE – PPP over Ethernet
❍ Used by Internet providers who charge by the
minute
❒ IP over ATM
❍ ATM is capable of end-to-end communication
❍ But few people have it
❍ Integrate ATM networks into the greater Internet
❒
Socket programming
9
3
Typical use of sockets
❒
❒
Sockets can provide access to
❍ Network layer
❍ Link layer
But this is useless in assignments
❍ Prohibited to normal users
❍ Major differences between operating systems
Socket programming
10
Connection-oriented service
Goal: data transfer between
❒
end systems
❒
❍ Connection
Start of communication
❍
❍
❍
❍
Handshaking
Initial preparation of data
transfer
Creates a state in the
two machines that
communicate
End systems know their
communication partners
During communication
❍
❍
❒
End system expects
messages from connected
end system
End system know when
messages belong to the
connection
End of communication
❍ Teardown
❍
New handshake required for
re-establishing connection
Socket programming
11
Transport layer: TCP
Connection-oriented
service
❒ TCP
❍
❍
Transmission Control
Protocol
Connection-oriented
service of the Internet
TCP offers
❒ Connections
❍
Handshake, end-system state,
teardown
❒ Reliable, ordered, streamoriented data transfer
❍
Loss: acknowledgements and
retransmissions
❒ Flow control
❍
Send not faster than receiver
can receive
❒ Congestion control
❍
Send slower when the network
is congested.
Socket programming
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4
Connectionless service
Goal: data transfer between
❒
end systems
❒ As before!
❒
During communication
❍ No connection
❍
Start of communication
❍
❍
❍
No connection setup
No preparation for data
transfer
Programs must expect
messages at all times
❍
❍
❒
No state in the machines
Senders don’t know whether
messages are expected
Sender must identify itself in
each message
End of communication
❍ No teardown
❍
Just stop sending
Socket programming
13
Transport layer: UDP
Connectionless
service
❒
UDP
❍
❍
User Datagram Protocol
Connectionless service of
the Internet
UDP offers
❒ No connections
❍
❒
Send immediately
Unreliable, unordered, packetoriented data transfer
❍
❍
Loss: messages are simply lost
Messages arrive exactly as send
❒
No flow control
❒
No congestion control
❍
❍
Send as fast as programs want
to
Ignore network problems
Socket programming
14
Socket programming
15
Using TCP sockets
5
Creating and using a TCP socket
Handshaking
Initial preparation of data transfer
❒ Create a state in the two machines that
communicate
❒
❒
Socket programming
16
socket()
❒
❒
❒
The application claims a socket
Call to the function socket() creates a transport
control block, and returns a reference to it
The kind of socket is determined at this time
❍
TCP, UDP, …
int sock;
sock = socket(PF_INET, SOCK_STREAM,
IPPROTO_TCP);
Socket programming
17
socket()
sock=socket(domain,type,protocol)
❒
❒
PF_INET, SOCK_STREAM and IPPROTO_TCP are
constants that are defined in the included files
Many other possibilities exist
❍
❍
❍
❒
❒
domain: PF_UNIX, PF_INET, PF_INET6, …
type: SOCK_STREAM, SOCK_DGRAM,…
protocol: IPPROTO_TCP, IPPROTO_UDP…
PF_INET and SOCK_STREAM request a TCP socket
PF_INET and SOCK_DGRAM request a UDP socket
Socket programming
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6
socket()
❒
A TCP state machine is created in closed state
CLOSED
LISTEN
SYN RCVD
SYN SENT
ESTABLISHED
FIN WAIT 1
CLOSING
CLOSE WAIT
FIN WAIT 2
TIME WAIT
LAST ACK
Socket programming
19
Creation of a TCP connection
❒
One application must be the active one
❍
❍
❒
The other application must be passive
❍
❍
❍
❒
Take the initiative in creating the connection
This side is called the client
Be prepared for accepting connections
Expect someone else to takes the initiative
This side is called the server
This use of the words client and server is not entirely
consistent with everyday use
❍
For programming this is conventional
Socket programming
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Socket programming
21
Addressing in the Internet
❒
Addressing hosts
Solved by the network layer
IP addresses
❍ IPv4: 32 bit
❍ IPv6: 128 bit
❍
❍
❒
Addressing applications
Solved by the transport layer
Ports
❍ Size: 16 bit
❍
❍
❒
Details in lectures on layers 3 and 4
7
Addressing in applications
❒
Addressing hosts
❒
Addressing applications
❍
❍
Translate the name of the host to an IP address
Defined by the application programmer
Socket programming
22
TCP server steps
❒
❒
❒
Goal: Move the TCP
state machine to a
waiting state
Host address is known
Define an application
address
❍ The port
/* declarations */
struct sockaddr_in serveraddr;
/* Clear the structure */
bzero(&serveraddr,
sizeof(serveraddr));
/* Set the address family */
serveraddr.sin_family = AF_INET;
/* Allow all own addresses to receive */
serveraddr.sin_addr.s_addr = INADDR_ANY;
/* Add the port number */
serveraddr.sin_port = htons(2009);
Socket programming
23
struct sockaddr_in
❒ Define
IP addresses and port numbers in
the way the Berkeley socket API needs it
❒ htons()
❍
❍
Host-to-network short
Translate an integer value to network format
❒ AF_INET
❍
Constant indicating that Internet protocols will be used
❒ INADDR_ANY
❍
❍
Constant meaning any (Internet) address
In this context: any own Internet address
Socket programming
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8
Binding and listing
❒
❒
❒
❒
❒
Goal: Move the TCP
state machine to a
waiting state
Host address is known
Define an application
address
❍ The port
Add address information
to socket
Go into waiting state
/* bind the address to the socket */
bind(request_sock,
(struct sockaddr *)&serveraddr,
sizeof serveraddr);
/* active listening on the socket */
listen(request_sock, SOMAXCONN);
/* wait for connections */
clientaddrlen =
sizeof(struct sockaddr_in);
sock = accept(request_sock,
(struct sockaddr *)&clientaddr,
&clientaddrlen);
ACCEPT FUNCTION BLOCKS
Socket programming
25
Server: bind(), listen(), accept()
❒
Moving a server into listen state
listen
CLOSED
bind
LISTEN
accept
SYN RCVD
SYN SENT
ESTABLISHED
FIN WAIT 1
CLOSING
CLOSE WAIT
FIN WAIT 2
TIME WAIT
LAST ACK
Socket programming
26
Server: bind(), listen()
❒
bind()
❍
❍
Tells the socket on the server side which address and port number
to listen to
A machine can have several addresses
❒ listen();
❍
❍
Prepares the server for listening to connect requests, and initializes
a queue for connect requests
The second parameter defines how long the queue should be
❒ SOMAXCONN
❍
❍
The length of the queue of connection requests
Defined in the includes
Socket programming
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9
Server: accept()
❒
❒
❒
❒
❒
Returns a new socket that the server can use to
communicate with the client
Take the first connect request from the connect
request queue
Wait for the next connect request to arrive if the
queue is empty
clientaddr contains information about the client
clientaddrlen must be initialized, so accept
knows the size of clientaddr
Socket programming
28
TCP client steps
Goal: Move the server’s and client’s state
machines to established state
❒ Server’s host name is known
❒ Server’s host address is unknown
❒ Use the same application address
❍ The port
❒
Socket programming
29
Preparing the client for connecting
❒
sockaddr_in
serveraddr
❍
❒
Needed to specify the
server’s address for the
socket
gethostbyname()
❍
❍
❍
Takes a hostname
Returns information
about that hostname
Including its IP address
char* machine = “login.ifi.uio.no”;
struct hostent *hostp;
struct sockaddr_in serveraddr;
int sock;
/* Look in DNS for the IP address of the name */
if ((hostp = gethostbyname(machine)) == 0)
{
fprintf(stderr,“Unknown machine %s\n",machine);
exit(1);
}
bzero((void *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
memcpy(&serveraddr.sin_addr,
hostp->h_addr, hostp->h_length);
serveraddr.sin_port = htons(2009);
Socket programming
30
10
Preparing the client for connecting
❒
connect()
❍
❒
sock
❍
❍
❒
Take all the steps to
initiate a connection to
the server
The own socket
Operating systems fills in
own control block
implicitly
/* Connect */
connect(sock, (struct sockaddr*)&serveraddr,
sizeof(struct sockaddr_in));
serveraddr
❍
The address of the TCP
server application
Socket programming
31
Client: connect()
❒
Advancing the client into established state
CLOSED
Connect()
LISTEN
Receive SYN+ACK
from server
SYN RCVD
ESTABLISHED
Send SYN
to server
SYN SENT
Send ACK to server
FIN WAIT 1
CLOSING
CLOSE WAIT
FIN WAIT 2
TIME WAIT
LAST ACK
Socket programming
32
Server: accept()
❒
Advancing the server into established state
Send SYN+ACK to client
CLOSED
LISTEN
SYN RCVD
Receive ACK
from client
Receive SYN from client
SYN SENT
ESTABLISHED
FIN WAIT 1
CLOSING
CLOSE WAIT
FIN WAIT 2
TIME WAIT
LAST ACK
Socket programming
33
11
Client: connect()
❒
❒
❒
Connects client socket to a server that is
specified in the address structure
Does NOT return a new socket
The given socket is later used for
communicating
contains information about the
server
❒ The length of serveraddr is not modified
❒ serveraddr
Socket programming
34
Server: accept()
❒
❒
❒
Server has left waiting
state
accept() function
returns
Processing continues
/* bind the address to the socket */
bind(request_sock,
(struct sockaddr *)&serveraddr,
sizeof serveraddr);
/* active listening on the socket */
listen(request_sock, SOMAXCONN);
/* wait for connections */
clientaddrlen =
sizeof(struct sockaddr_in);
sock = accept(request_sock,
(struct sockaddr *)&clientaddr,
&clientaddrlen);
SERVER CAN CONTINUE
Socket programming
35
read() and write()
❒
The call read(sock, buffer, n);
❍ Reads n characters
❍ From socket sock
❍ Stores them in the character array buffer
❒
The call write(sock, buffer, n);
❍ Writes n characters
❍ From character array buffer
❍ To the socket sock
Socket programming
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12
read() and write()
❒
State stays the same
CLOSED
LISTEN
SYN RCVD
SYN SENT
ESTABLISHED
read
write
FIN WAIT 1
CLOSING
CLOSE WAIT
FIN WAIT 2
TIME WAIT
LAST ACK
Socket programming
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read() and write()
TCP sender
char buffer[2000];
int retval;
buffer[0] = ‘a’;
buffer[1] = ‘b’;
buffer[2] = ‘c’;
…
❒
Typical
❍
retval == -1
❍
retval == 0
❍
retval n < 2000
• Some kind of error, look at errno
• The connection has been closed
• You tried to send too fast
• Only n bytes have been sent
• Try sending the reset
/* write abcdefghij */
retval = write( sock, buffer, 2000 );
❍
retval == 2000
• All bytes have been sent
❒
Very untypical
❍
❍
❍
retval < -1
retval > 2000
Both cases:
• Have you used char retval instead
of int retval?
Socket programming
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read() and write()
TCP receiver
char buffer[2000];
int retval;
…
❒
Typical
❍
retval == -1
❍
retval == 0
❍
retval n < 2000
• Some kind of error, look at errno
/* write abcdefghij */
retval = read( sock, buffer, 2000 );
• The connection has been closed
• Only n bytes have been received
• No new data has arrived recently
• Try sending the rest
❍
retval == 2000
• All bytes have been received
❒
Very untypical
❍
❍
❍
retval < -1
retval > 2000
Both cases:
• Have you used char retval instead
of int retval?
Socket programming
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13
Closing of sockets
Server
Client
/* Close the socket */
close(sock);
❒
Note that the semantics of close depends
❍
❍
❒
/* Close both sockets */
close(sock);
close(request_sock);
on the kind of protocol
some possible extra settings
All data that has not been read yet may be thrown
away
Socket programming
40
close(), initiator
❒
Return to close state
CLOSED
LISTEN
SYN RCVD
One side calls close
Send FIN to
other side
close()
ESTABLISHED
FIN WAIT 1
Recv ACK
CLOSING
Recv FIN
Send ACK
FIN WAIT 2
TIME WAIT
SYN SENT
Timeout
CLOSE WAIT
LAST ACK
Socket programming
close(), other
❒
41
side
Return to close state
CLOSED
SYN RCVD
Recv ACK
FIN WAIT 1
LISTEN
ESTABLISHED
CLOSING
SYN SENT
Recv FIN
Send ACK
Notify application
CLOSE WAIT
close()
Send FIN
FIN WAIT 2
TIME WAIT
LAST ACK
Socket programming
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14
Concurrency
❒
Must solve several tasks at once
❍
Wait for data from communication partners
❍
❍
❍
Read data typed by the user
Listen for client connections (servers only)
Socket programming
43
Concurrency
❒
Use fork to create several processes
❒
Use select
❒
Use threads
❍
Each process waits for one event
❍
One process waits for several events
❍
Similar to fork
Socket programming
44
fork()
❒ int fork();
❒
A standard approach for servers
❍
Perform a blocking wait such as
❍
When it returns
❍
The parent process
• new_sock = accept(listen_sock);
• Call fork()
• Closes new_sock
• Wait for more connections on listen_sock
❍
The child process
• Closes listen_sock
• Communicates with the client using new_sock
Socket programming
45
15
select()
❒
❒
❒
int select( int max_fd,
fd_set* read_set,
fd_set* write_set,
fd_set* except_set,
struct timeval* timeout );
❒
❒
Complicated at first
But very useful
Can wait for activity on
many sockets
❍ New connections on request
sockets
❍ New data on connected
sockets
❍ Closing of a connected
socket
❍ Ready-to-send on a socket
Can wait for user input
Can wait for timeouts
Socket programming
46
select()
❒
For servers
❍
int select( int max_fd,
fd_set* read_set,
fd_set* write_set,
fd_set* except_set,
struct timeval* timeout );
❍
❒
Serve many clients at
once
Handle clients that close
connections, clients that
crash, …
For simpler cases
❍
❍
Wait for data from chat
partner
Wait for typing of the
user
Socket programming
47
select()
❒
read_set
❍
int select( int max_fd,
fd_set* read_set,
fd_set* write_set,
fd_set* except_set,
struct timeval* timeout );
❍
❍
❒
write_set
❍
❍
❒
Arriving connect requests
Arriving data
Closing sockets
Not often used
Non-blocking send is
finished
except_set
❍
Hardly ever used
❍
sendto(.,.,.,MSG_OOB)
Socket programming
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16
select()
void wait_for_all(int clientsock[], int clients)
{
fd_set read_set;
int i,act,top=0;
❒
❒
Using only the read_set is
typical
Clear the read_set
❍
FD_ZERO(&read_set);
for( i=0; i<clients; i++ )
{
FD_SET(clientsock[i],&read_set);
top = MAX(top,clientsock[i]);
}
act = select( top+1,
&read_set, NULL, NULL,
NULL);
…
❒
❒
❒
NULL timeout
❍
❒
Must be done every time
Put all sockets into the read
set
Find the highest socket
number, add 1
Means forever
Call select
❍
}
waits for arriving data
Socket programming
49
select()
void wait_some_time(int sec, int usec)
{
struct timeval timeout;
timeout.tv_sec = sec;
timeout.tv_usec = usec;
act = select( 0,
NULL, NULL, NULL,
&timeout );
…
}
can also wait only
for a timeout
❒ select
❍
❒
Without sockets
Timeout parameter
❍
❍
❍
NULL means wait forever
Timeval {5,0} means wait 5
seconds
Timeval {0,0} means don’t
wait at all
Socket programming
50
Compilation of socket programs
❒ IFI’s Solaris machines
❍
❍
gcc client.c –o client –lsocket -lnsl
cc client.c –o client -lsocket –lnsl
❒ IFI’s Linux machines
❍
gcc client.c –o client
❒ Cygwin on Windows
❍
gcc client.c –o client
Socket programming
51
17
The socket API’s error variable errno
❒
extern int errno
❍ (Can be used like) a variable that exists once
❍ Contains the last socket error
❒ void perror(const char* str)
❍ Prints your string str first, and an explanation for errno
afterwards
❒ In /usr/include/errno.h
❍
❍
❍
❍
Defines
EINTR
• Something strange happened, e.g. an interrupt. Nothing
happened. Try again!
ENOTSOCK
• Your sock variable is not a socket at all
EBADFD
• The connection has been closed and you haven’t noticed it
Socket programming
52
Socket options
❒
Some ways to set socket options
❍
setsockopt( int sock, int level,
int optname, void* optvalue,
int optlen )
• Options for reading and writing all types of sockets
• Often used in
setsockopt( request_sock, SOL_SOCKET,
SO_REUSEADDR, &i, sizeof(i));
❍
fcntl( int sock, int optname, long optvalue );
• Options for reading and writing all types of sockets, files,
pipes, user input, graphical output
• Often used in fcntl( sock, F_SETFL, O_NONBLOCK);
to make a socket non-blocking
Socket programming
53
Socket options
❒
Some ways to set socket options
❍
ioctl( int sock, int optname, void* arg);
• Options for all types of file descriptors
• Useful in
int num_bytes;
ioctl( sock, FIONREAD, &num_bytes);
number of bytes that have arrived
to check the
Socket programming
54
18
Summary
❒
Introduced sockets
❒
Provided details for the use of TCP
❍
According to the Berkeley Socket API
Setting up a connection
Reading and writing over a connection
❍ Tearing down a connection
❍ Some ways to approach the concurrency problem
❍ Frequently helpful socket options
❍
❍
Socket programming
55
19