Socket Programming

References:redKlyde’s tutorial setWinsock2 for games (gamedev.net)

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Introduction (javadoc)

TCP provides a reliable, point-to-point communication channel that client-server applications on the Internet use to communicate with each other. To communicate over TCP, a client program and a server program establish a connection to one another. Each program binds a socket to its end of the connection. To communicate, the client and the server each reads from and writes to the socket bound to the connection.

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Client/Server

Normally, a server runs on a specific computer and has a socket that is bound to a specific port number. The server just waits, listening to the socket for a client to make a connection request.The client knows the hostname of the machine on which the server is running and the port number to which the server is connected.To make a connection request, the client tries to rendezvous with the server on the server's machine and port.

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Client/Server (cont)

If everything goes well, the server accepts the connection. On the client side, if the connection is accepted, a socket is successfully created and the client can use the socket to communicate with the server. The client and server can now communicate by writing to or reading from their sockets.

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Socket Definition

A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent.

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Winsock 2.2

There are different ways to program with WinSock. the very basic UNIX/Berkeley type functions, the Microsoft’s Windows specialized version of the basic functions, orthe Object Orientated MFC version.

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Learning Objectives

Creating a socket (and VC setup)Making the connectionSending and receiving dataMultiple host connection [Optional multi-thread implementation]

Integrating with game loop

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redKlyde (Tutorial #1)

VC setting: add WS2_32.lib to projectWSAData: hold info about our network capabilityWSAStartup (…): start up winsock

mysock = socket (AF_INET, SOCK_STREAM,0);

creates a TCP socket for Internet; fails if too many sockets are created

closesocket (mysock);

Every winsock function will return SOCKET_ERROR if error occurredWSAGetLastError() to get the errorcode [see msdn for details]

WSACleanup(): last call of winsock

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Tutorial 1Winsock v2.2

AF_INET: Socket for internet

SOCK_STREAM: TCP socket[SOCK_DGRAM: UDP socket]

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redKlyde (Toot #2 – Server)

Host/net byte ordering of numbers (next page)Bind: associate a port to socket to send/receive dataSocket address structure [sockaddr_in]: port no., Internet protocol, …Port no: stay above 2024

Listen (listensock, backlog)Clientsocket = accept (listensock, 0,0)

A blocking call [use debugger to verify]

Backlog: the number of clients you can keep waiting before calling accept()

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[Endianness]

Big endianNetwork protocolhton, ntoh Host-network

conversion

Little endianWindows

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hton & ntoh

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Sizeof (short) = 4 bytesSizeof (long) = 8 bytes

htons (7654) = 58909htonl (7654) = 3860660224

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In fact:If the sending and receiving ends are of the same endianness, there is no need to convert

1D E6 1D E6 7654

However, if not so, only through ntoh/hton can guarantee the correctness of the info…

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Bind the socket to port 7654

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redKlyde (Toot#2 – client)

Which server to connect to?“host entry” structure: gethostbyname (servername)Socket address [sockaddr_in]: port, host_addr

Connect (sock, sockaddr_in, size)

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Tut 2(c)

Or IP “140.129.25.1

94”

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redKlyde (Toot#3)

Send/recv fixed length messageMacros for earlier commandsNbytes = recv (clientsock, buffer,buffersize,0)Nbytes = send (clientsock,buffer,size,0);

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Tut3 (server)

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Tut3 (client)

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redKlyde (Toot#4)

How about variable length message?TCP concatenates all packets into one big chunkUse escape character to decipher…Send the size of message first, then recv accordingly

!? Why the size won’t be mixed with the coming messages?!This is the simple protocol we set for this example.

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Tut4 (server)

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Tut4 (client)

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Tut5: Overview

Thread: a sequence of instructions that may execute in parallel with other threadsMulti-threaded AP Accept thread: accept clients connecting to the

server socket; add client socket to masterset Main thread: polling from selected sets to

process variable-length input

Shared memory: FD_SET (set of client sockets)

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redKlyde (Toot#5 – server)

Blocking calls: accept, recvNonblocking calls: send, connect, …FD_SET (file descriptor set): a list of sockets

Data member: fd_count, fd_array

Macros for FD_SET: FD_SET (sock, &set) FD_ZERO (&set) FD_CLR (sock,&set) FD_ISSET(sock,&set)

Select: create a list of sockets that already have data to readException handling:

Clean disconnect: the client closes the socket and exits [no error]

dropped connection [an error will occur]

Remove from FD_SET, close the corresponding socket

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Setting Up

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Mutex: mutual exclusion algorithms used in concurrent programming to avoid the simultaneous use of a common resource

Code-generation: debug multi-threaded

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Accepting thread

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Main Thread1

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Main Thread2

Error check…

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Error (dropped connection)

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Error (clean disconnect)

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Client called closesocket

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Command Summary

WSAStartup; WSACleanupWSAGetLastError(); socket(); closesocket(); bind(); listen(); accept(); gethostbyname(), connect() ntohl(), ntohs(), htonl(), htons() send(), recv() select() FD_SET, FD_ZERO, FD_CLR, FD_ISSET

Networking for Game Programmers

Reference: Glenn Fieldler’s Blog

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Topics

TCP vs. UDPSending & Receiving Packets (UDP)UDP (Virtual) ConnectionUDP flow control

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