Multiplayer Game Programming Berkeley Socket API Chapter 3.ppt

Multiplayer Game Programming

1. Multiplayer Game Programming
Chapter 3
Berkeley Socket API

2. Chapter 3
Objectives
Socket Creation and Initialization
–How to create and bind a socket
–Platform differences
Datagram Transmission
–How to use sockets foår UDP
Stream Transmission
–How to use sockets for TCP
Blocking and Non blocking IO
–How to use sockets in realtime

3. Sockets
Berkeley Socket API
Originally from BSD UNIX 4.2
Ported in some way to almost every language and
operating system

4. Operating System Differences
POSIX: Linux, MacOSX, PlayStation, BSD
–<sys/socket.h>
–Socket is a file descriptor ( int )
Windows: Xbox
–WinSock2
–<Winsock2.h>
•Define WIN32_LEAN_AND_MEAN before
including <Windows.h>
–Socket is a SOCKET

5. Windows requires explicit startup and shutdown of
WinSock2
Windows Housekeeping
int WSAStartup(
WORD wVersionRequested,
LPWSADATA lpWSAData );
int WSACleanup();

6. Creating A Socket
SOCKET socket(
int af,
int type,
int protocol );
Returns new socket if successful
-1 if unsuccessful
–INVALID_SOCKET

7. Address Family
???INSERT TABLE 3.1 PLEASE

8. Socket Type
How will data be passed
–Tied to protocol
???INSERT TABLE 3.2 PLEASE

9. Protocol
???INSERT TABLE 3.3 PLEASE

10. When return value of -1 indicates error
–retrieve true error code in platform dependent
manner
–Do it quickly- another error in same thread will
change vlaue
Windows
int WSAGetLastError();
POSIX
int errno ;
Error Reporting

11. Binding Socket To Address
Registers address for use by socket
Actual type of sockaddr can change based on
protocol family, etc.
–From a time before classes and inheritance
–Different types must be cast to sockaddr
int bind(
SOCKET sock,
const sockaddr *address,
int address_len );

12. SOCKADDR
struct sockaddr {
uint16_t sa_family;
char sa_data[14];
};
struct sockaddr_in {
short sin_family;
uint16_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};

13. IN_ADDR IPv4 Address
Specify IP Address as a 32-bit number, two 16-bit numbers or four
8-bit numbers ( most common )
INADDR_ANY allows binding to all local interfaces on the host
struct in_addr {
union {
struct {
uint8_t s_b1,s_b2,s_b3,s_b4;
} S_un_b;
struct {
uint16_t s_w1,s_w2;
} S_un_w;
uint32_t S_addr;
} S_un;
};

14. Byte Order
Network Byte Order is Big Endian
Your platform is probably Little Endian
Use conversion functions when manually filling in or
reading address structures
uint16_t htons( uint16_t hostshort );
uint16_t ntohs( uint16_t netshort);
uint32_t htonl( uint32_t hostlong );
uint32_t ntohl( uint32_t netlong);

15. Creating Address From String
hostname: dns name or ip address
servname: port number or service name
hints: can specify address family, etc
res: results. Must be deallocated with freeaddrinfo
int getaddrinfo(
const char *hostname,
const char *servname,
const addrinfo *hints,
addrinfo **res);

16. Datagram Transmission
Returns number of bytes sent or -1 for error
to: destination host address
–Same format as when creating socket
Most common way to send UDP Packets
int sendto(
SOCKET sock,
const char *buf, int len,
int flags,
const sockaddr *to, int tolen );

17. Datagram Reception
Returns number of bytes received or -1 for error
from: output param assigned address of sending host
–Do not fill with an address before invoking
–Does not select incoming datagram based on address
•ONLY USED AS OUTPUT PARAM
Most common way to receive UDP Packets
int recvfrom(
SOCKET sock,
char *buf, int len,
int flags,
sockaddr *from, int *fromlen );

18. Listening for Stream Connections
backlog: number of connections that can be pending
acceptance
Returns 0 for no error
Server calls to prepare for incoming TCP handshake
int listen(
SOCKET sock,
int backlog );

19. Initiating Stream Connection
Sockadder: target host’s address
–Same format as when creating socket
Client calls to initiate TCP handshake
int connect(
SOCKET sock,
const sockaddr *addr,
int addrlen );

20. Stream Connections
Server calls to complete TCP handshake
Returns a new socket representing stream
–Encapsulates reliability buffers, flow and congestion control
values, and other state
–Use only new socket for sending and receiving
addr: output param assigned address of initiating host
SOCKET accept(
SOCKET sock,
sockaddr* addr,
int* addrlen );

21. Stream Transmission
Returns number of bytes sent or -1 for error
No address necessary
–Address stored in socket by connection
Copies data into send buffer for transmission
–Sent when socket decides– Nagle Algorithm!
Most common way to send TCP data
int send(
SOCKET sock,
const char *buf,
int len,
int flags );

22. Stream Reception
Returns number of bytes received or -1 for error
Most common way to receive TCP Packets
No param to store sender address
–data can only come from connected socket on
remote host
int recv (
SOCKET sock,
char *buf,
int len,
int flags );

23. Blocking
Some actions halt thread or block waiting for
completion
–Sending: when no room in send buffers
–Receiving: when no incoming data available
–Connecting: until handshake complete
–Accepting: until handshake complete
Problem for real-time simulation like game
Problem when when communicating with multiple
remote hosts sending data

24. Working Around Blocking
Multithreading
–Use separate thread for each connection
Non-blocking I/O
–Enable non-blocking mode and poll for updates
Select
–Block on all relevant sockets at once

25. Multithreading
Does not scale well for large numbers of clients
???INSERT FIGURE 3.1 PLEASE

26. Non-Blocking I/O
Non-Blocking sockets return control immediately
when asked to do operations that would block
–Return value is -1, but error code is indicative
•Windows: WSAEWOULDBLOCK
•POSIX: EAGAIN
–Can try again next frame to
•Send
•Receive
•Accept
•Connect

27. Windows
POSIX
Enabling Non-Blocking I/O
int ioctlsocket(
SOCKET sock,
long cmd,
u_long *argp );
int fcntl(int sock, int cmd, ...);

28. Blocking with Select
Returns number of ready sockets or negative error.
Sets are modified by function
Initialize sets to contain all relevant sockets
At return, sets contain sockets that can perform
operations without blocking
int select(
int nfds,
fd_set *readfds,
fd_set *writefds,
fd_set *exceptfds,
const timeval *timeout );

29. Select Socket Set
FD_ZERO(fd_set* set )
–Initializes set
FD_SET( SOCKET s, fd_set* set )
–Add socket to set
FD_ISSET( SOCKET s, fd_set* set )
–Tests if a socket is set
FD_CLR( SOCKET s, fd_set* set )
–Removes socket from set