Network Programming with Sockets. Process Management in UNIX

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Network Programming with Sockets

This section is a brief introduction to the basics of net- working programming using the BSD Socket interface on the Unix Operating System.

Processes in Unix

Sockets Stream Sockets Datagram Sockets

System Calls

Process Management in UNIX

• pid = fork();

• Creates a process.

• The calling process is called “parent”, created process is called “child”.

• The child process is identical to the parent pro- cess (One exception: the process number of parent and child are different).

•

fork()

returns value

“0”

to child and

“child’s pro- cess number”

to parent.)

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Process Management in UNIX

• exit(status);

• Terminates a process and returns status to waiting parent.

• Processes are created by the fork system call.

• The child is an identical copy of the parent (exception: the process id is different).

A

child A of A

fork()

Process Management in UNIX

• Result: Process structure is hierarchical.

main() {

pid = fork();

if (pid != 0 ) {

/* original process */

printf(“original process prints this !”);

} else {

/* newly created process */

printf(“new process prints this !”);

}

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BSD Sockets

•

Sockets are endpoints of two-way communication paths between two Unix processes.

•

Sockets are one of many mechanisms in a Unix operating system to establish communication between processes.

•

In our programming assignments, we will make extensive use of sockets to enable processes at different machines to talk to another.

• In fact: Most network applications of the Internet (including: ftp, telnet, mosaic) are written using the

socket interface.

• The abstraction provided by the socket interface is that of a file. A process opens a socket, reads from a socket,

writes to a socket, etc.

Sockets

Process A

Kernel

Network

Process B

Kernel socket

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Socket Types and Families

•

Stream Sockets:

Bidirectional, reliable, sequenced, and unduplicat- ed data flow.

•

Datagram Sockets:

Bidirectional, but unreliable and without guar- antee of sequence or duplication.

•

Raw Sockets:

Allows direct access to underlying communica- tion protocols. (Requires special privileges.)

Internet Family Sockets

“Families” (Domains) of Sockets:

• UNIX (AF_UNIX)

• INTERNET (AF_INET)

• Other: XNS, IMP etc.

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Internet Domain Sockets

• Sockets and the Internet protocol suite:

Socket Layer

Network Interface Internet Protocol (IP)

TCP UDP

Stream Sockets Datagram Sockets User Level

Stream Sockets

• Stream Sockets establish a communication path that provides a “connection-oriented”

service.

• The mode of communication between two processes is asymmetric:

•

One process (the “Client”) requests a connection (connect).

•

The other process (the “Server”) is waiting for a connection (listen) and accepts connection requests (accept).

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Stream Sockets

Client Algorithm:

1. Find the address (=IP address and port number) of the server with which communication is desired.

2. Allocate a socket.

3. Specify that the connection needs an unused protocol port on the local machine.

4. Connect the socket to the server.

5. Communicate with the server (send and receive).

6. Close the connection.

Stream Sockets

Server Algorithm:

1. Create a socket and bind to a well-known address (=IP address and port number) for the service.

2. Place the socket in passive mode, waiting for connections from clients.

3. Accept the next connection request from the socket, and obtain a second socket for the connection.

4. Read and write with client over the second socket.

5. When finished, close the second socket, and continue with Step 3.

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Stream Sockets

•

Sequence of system calls

:

socket connect write read close Client

socket bind listen accept read Server

write close

Datagram Sockets

• “Connectionless” paradigm.

•

Symmetric interface to data exchange.

•

No requirements for connection establishment.

•

Destination address explicitly specified in every

message.

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Datagram Sockets

Client Algorithm:

1. Find the address (=IP address and port number) of the server with which communication is desired.

2. Allocate a socket.

3. Specify that the connection needs an unused protocol port on the local machine.

4. Specify the server to which messages are sent.

5. Communicate with the server (send and receive).

6. Close the socket.

Datagram Sockets

Server Algorithm:

1. Create a socket and bind to a well-known address (=IP address and port number) for the service.

2. Repeatedly read the next request from a client, and send back the response.

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Datagram Sockets

•

Sequence of system calls

: socket

sendto recvfrom close CLIENT

socket bind recvfrom SERVER

sendto close bind

Socket

s = socket(family, type, protocol);

family: AF_INET for Internet Family.

type: SOCK_STREAM, SOCK_DGRAM, SOCK_RAW

protocol: 0 (for default), other protocol number.

• Creates a socket and returns a socket descriptor s.

• s is an index in the open file table which points to a socket structure.

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Bind

• To make the socket addressable from another process, a socket must have a name bound to it.

• The bind call is always used by servers, which need to specify a well-known port number. Connectionless clients also need to assure that the system assigns it a unique address.

• Syntax:

bind(s, name, namelen)

name: local address which contains an Internet ad- dress and a port number; name is of type

“sockaddr_in”.

• Port numbers:

• Internet ports below 1024 are reserved for root.

• If bind is called with port number “0”, operating system will as- sign an unused port number.

Connect

• Syntax:

connect(s, servaddr, addrlen)

servaddr: server address which contains an Inter- net address and a port number; servaddr is of type

“sockaddr_in”.

• For stream-oriented connections, results in connection establishment between the local system and the foreign system.

• Assigns association elements: local_addr, local_process, foreign_addr, and foreign_process.

• After connecting, the process can write to or read from the socket without specifying the foreign address.

• A process that uses connect does not need to call bind.

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Listen

• Syntax:

listen(s, backlog)

backlog: specifies how many outstanding connec- tion requests can be queued while waiting for the server to execute accept.

• Specifies that a connection-oriented server is willing to receive connections.

• Maximum backlog is 5.

Accept

• Syntax:

accept(s, peer, addrlen)

peer: address of the connected peer process (the client).

s:

specifies socket from which a connection should be accepted.

• Used after calling socket to create a socket, bind to specify a local endpoint address, and listen to place it in passive mode.

• Creates a new socket for each new connection request, and returns the descriptor of the new socket to its caller.

• Original Socket: used to accept new connection.

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Write / Read

• Syntax:

write(s, buff, nbytes)

read(s, buff, nbytes)

s: Socket to use for reading/writing. (akin to file de- scriptor)

• Similar to reading and writing to files.

• Used with connection-oriented sockets (which use the connect and accept system calls).

• Returns the number of bytes read or written.

• Also the send call:

send(s, buff, nbytes, flags)

• The flags specify options. (e.g. MSG_OOB, MSG_PEEK, etc.)

Sendto / Recvfrom

• Syntax:

sendto(s,buff,nbytes,flags,to,adrlen)

recvfrom(s,buff,nbytes,flags,from,adrlen)

to (from): Address to (from) which packet is to be sent.

adrlen

:Size of

to

(or

from

) argument.

• For connectionless communication: address is specified for each packet sent by sendto.

• For a connectionless server, recvfrom fills in protocol-specific address of who sent the data into from.

• Returns the number of bytes read or written.

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Types of Servers

iterative

connectionless iterative

connection-oriented

concurrent connection-oriented concurrent

connectionless