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![Lecture 3: 9-4-01 17
#include <netinet/in.h>
/* Internet address structure */
struct in_addr {
u_long s_addr; /* 32-bit IPv4 address */
}; /* network byte ordered */
/* Socket address, Internet style. */
struct sockaddr_in {
u_char sin_family; /* Address Family */
u_short sin_port; /* UDP or TCP Port# */
/* network byte ordered */
struct in_addr sin_addr; /* Internet Address */
char sin_zero[8]; /* unused */
};
Internet Addressing Data Structure
• sin_family = AF_INET selects Internet address family](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-17-2048.jpg)
![Lecture 3: 9-4-01 18
Byte Ordering
• Big Endian
• Sun Solaris, PowerPC, ...
• Little Endian
• i386, alpha, ...
• Network byte order = Big Endian
128 2 194 95
union {
u_int32_t addr; /* 4 bytes address */
char c[4];
} un;
/* 128.2.194.95 */
un.addr = 0x8002c25f;
/* c[0] = ? */
c[0] c[1] c[2] c[3]
95 194 2 128](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-18-2048.jpg)
![Lecture 3: 9-4-01 28
Socket I/O: read()
• read can be used with a socket
• read blocks waiting for data from the client but
does not guarantee that sizeof(buf) is read
int fd; /* socket descriptor */
char buf[512]; /* used by read() */
int nbytes; /* used by read() */
/* 1) create the socket */
/* 2) bind the socket to a port */
/* 3) listen on the socket */
/* 4) accept the incoming connection */
if((nbytes = read(newfd, buf, sizeof(buf))) < 0) {
perror(“read”); exit(1);
}](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-28-2048.jpg)
![Lecture 3: 9-4-01 33
Socket I/O: write()
• write can be used with a socket
int fd; /* socket descriptor */
struct sockaddr_in srv; /* used by connect() */
char buf[512]; /* used by write() */
int nbytes; /* used by write() */
/* 1) create the socket */
/* 2) connect() to the server */
/* Example: A client could “write” a request to a server
*/
if((nbytes = write(fd, buf, sizeof(buf))) < 0) {
perror(“write”);
exit(1);
}](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-33-2048.jpg)
![Lecture 3: 9-4-01 38
Socket I/O: recvfrom()
• read does not provide the client’s address to the UDP server
int fd; /* socket descriptor */
struct sockaddr_in srv; /* used by bind() */
struct sockaddr_in cli; /* used by recvfrom() */
char buf[512]; /* used by recvfrom() */
int cli_len = sizeof(cli); /* used by recvfrom() */
int nbytes; /* used by recvfrom() */
/* 1) create the socket */
/* 2) bind to the socket */
nbytes = recvfrom(fd, buf, sizeof(buf), 0 /* flags */,
(struct sockaddr*) &cli, &cli_len);
if(nbytes < 0) {
perror(“recvfrom”); exit(1);
}](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-38-2048.jpg)
![Lecture 3: 9-4-01 49
int fd, next=0; /* original socket */
int newfd[10]; /* new socket descriptors */
while(1) {
fd_set readfds;
FD_ZERO(&readfds); FD_SET(fd, &readfds);
/* Now use FD_SET to initialize other newfd’s
that have already been returned by accept() */
select(maxfd+1, &readfds, 0, 0, 0);
if(FD_ISSET(fd, &readfds)) {
newfd[next++] = accept(fd, ...);
}
/* do the following for each descriptor newfd[n] */
if(FD_ISSET(newfd[n], &readfds)) {
read(newfd[n], buf, sizeof(buf));
/* process data */
}
}
Socket I/O: select()
• Now the web server can support multiple connections...](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-49-2048.jpg)
![Lecture 3: 9-4-01 51
A Few Programming Notes:
Building a Packet in a Buffer
struct packet {
u_int32_t type;
u_int16_t length;
u_int16_t checksum;
u_int32_t address;
};
/* ================================================== */
char buf[1024];
struct packet *pkt;
pkt = (struct packet*) buf;
pkt->type = htonl(1);
pkt->length = htons(2);
pkt->checksum = htons(3);
pkt->address = htonl(4);](https://image.slidesharecdn.com/lecture03-250424163159-8e0a9040/75/lecture03-on-socket-programming000000-ppt-51-2048.jpg)
![Number_Guessing_Game_Dsbsbssbzboc[1].pptx](https://cdn.slidesharecdn.com/ss_thumbnails/numberguessinggamedoc1-251206215042-a076fc05-thumbnail.jpg?width=640&height=640&fit=bounds)
lecture03 on socket programming000000.ppt
- 1. 15-441: Computer Networking Lecture3: Application Layer and Socket Programming
- 2. Lecture 3: 9-4-012 Lecture Overview • Application layer • Client-server • Application requirements • Background • TCP vs. UDP • Byte ordering • Socket I/O • TCP/UDP server and client • I/O multiplexing
- 3. Lecture 3: 9-4-013 Applications and Application-Layer Protocols • Application: communicating, distributed processes • Running in network hosts in “user space” • Exchange messages to implement app • e.g., email, file transfer, the Web • Application-layer protocols • One “piece” of an app • Define messages exchanged by apps and actions taken • User services provided by lower layer protocols application transport network data link physical application transport network data link physical application transport network data link physical
- 4. Lecture 3: 9-4-014 Client-Server Paradigm Typical network app has two pieces: client and server application transport network data link physical application transport network data link physical Client: • Initiates contact with server (“speaks first”) • Typically requests service from server, • For Web, client is implemented in browser; for e-mail, in mail reader Server: • Provides requested service to client • e.g., Web server sends requested Web page, mail server delivers e-mail request reply
- 5. Lecture 3: 9-4-015 Ftp: The File Transfer Protocol • Transfer file to/from remote host • Client/server model • Client: side that initiates transfer (either to/from remote) • Server: remote host • ftp: RFC 959 • ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user at host
- 6. Lecture 3: 9-4-016 Ftp: Separate Control, Data Connections • Ftp client contacts ftp server at port 21, specifying TCP as transport protocol • Two parallel TCP connections opened: • Control: exchange commands, responses between client, server. “out of band control” • Data: file data to/from server • Ftp server maintains “state”: current directory, earlier authentication FTP client FTP server TCP control connection port 21 TCP data connection port 20
- 7. Lecture 3: 9-4-017 Ftp Commands, Responses Sample Commands: • sent as ASCII text over control channel • USER username • PASS password • LIST return list of files in current directory • RETR filename retrieves (gets) file • STOR filename stores (puts) file onto remote host Sample Return Codes • status code and phrase • 331 Username OK, password required • 125 data connection already open; transfer starting • 425 Can’t open data connection • 452 Error writing file
- 8. Lecture 3: 9-4-018 What Transport Service Does an Application Need? Data loss • Some apps (e.g., audio) can tolerate some loss • Other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing • Some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Bandwidth • Some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” • Other apps (“elastic apps”) make use of whatever bandwidth they get
- 9. Lecture 3: 9-4-019 Transport Service Requirements of Common Apps no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5Kb-1Mb video:10Kb-5Mb same as above few Kbps elastic no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no file transfer e-mail web documents real-time audio/ video stored audio/video interactive games financial apps Application Data loss Bandwidth Time Sensitive
- 10. Lecture 3: 9-4-0110 Lecture Overview • Application layer • Client-server • Application requirements • Background • TCP vs. UDP • Byte ordering • Socket I/O • TCP/UDP server and client • I/O multiplexing
- 11. Lecture 3: 9-4-0111 Server and Client TCP/UDP IP Ethernet Adapter Server TCP/UDP IP Ethernet Adapter Clients Server and Client exchange messages over the network through a common Socket API Socket API hardware kernel space user space ports
- 12. Lecture 3: 9-4-0112 User Datagram Protocol(UDP): An Analogy Postal Mail • Single mailbox to receive messages • Unreliable • Not necessarily in-order delivery • Each letter is independent • Must address each reply Example UDP applications Multimedia, voice over IP UDP • Single socket to receive messages • No guarantee of delivery • Not necessarily in-order delivery • Datagram – independent packets • Must address each packet Postal Mail • Single mailbox to receive letters • Unreliable • Not necessarily in-order delivery • Letters sent independently • Must address each reply
- 13. Lecture 3: 9-4-0113 Transmission Control Protocol (TCP): An Analogy TCP • Reliable – guarantee delivery • Byte stream – in-order delivery • Connection-oriented – single socket per connection • Setup connection followed by data transfer Telephone Call • Guaranteed delivery • In-order delivery • Connection-oriented • Setup connection followed by conversation Example TCP applications Web, Email, Telnet
- 14. Lecture 3: 9-4-0114 Network Addressing Analogy 412-268-8000 ext.123 Central Number Applications/Servers Web Port 80 Mail Port 25 Exchange Area Code 412-268-8000 ext.654 IP Address Network No. Host Number Telephone No 15-441 Students Clients Professors at CMU Network Programming Telephone Call Port No. Extension
- 15. Lecture 3: 9-4-0115 Concept of Port Numbers • Port numbers are used to identify “entities” on a host • Port numbers can be • Well-known (port 0-1023) • Dynamic or private (port 1024-65535) • Servers/daemons usually use well- known ports • Any client can identify the server/service • HTTP = 80, FTP = 21, Telnet = 23, ... • /etc/service defines well-known ports • Clients usually use dynamic ports • Assigned by the kernel at run time TCP/UDP IP Ethernet Adapter NTP daemon Web server port 123 port 80
- 16. Lecture 3: 9-4-0116 Names and Addresses • Each attachment point on Internet is given unique address • Based on location within network – like phone numbers • Humans prefer to deal with names not addresses • DNS provides mapping of name to address • Name based on administrative ownership of host
- 17. Lecture 3: 9-4-0117 #include <netinet/in.h> /* Internet address structure */ struct in_addr { u_long s_addr; /* 32-bit IPv4 address */ }; /* network byte ordered */ /* Socket address, Internet style. */ struct sockaddr_in { u_char sin_family; /* Address Family */ u_short sin_port; /* UDP or TCP Port# */ /* network byte ordered */ struct in_addr sin_addr; /* Internet Address */ char sin_zero[8]; /* unused */ }; Internet Addressing Data Structure • sin_family = AF_INET selects Internet address family
- 18. Lecture 3: 9-4-0118 Byte Ordering • Big Endian • Sun Solaris, PowerPC, ... • Little Endian • i386, alpha, ... • Network byte order = Big Endian 128 2 194 95 union { u_int32_t addr; /* 4 bytes address */ char c[4]; } un; /* 128.2.194.95 */ un.addr = 0x8002c25f; /* c[0] = ? */ c[0] c[1] c[2] c[3] 95 194 2 128
- 19. Lecture 3: 9-4-0119 Byte Ordering Functions • Converts between host byte order and network byte order • ‘h’ = host byte order • ‘n’ = network byte order • ‘l’ = long (4 bytes), converts IP addresses • ‘s’ = short (2 bytes), converts port numbers #include <netinet/in.h> unsigned long int htonl(unsigned long int hostlong); unsigned short int htons(unsigned short int hostshort); unsigned long int ntohl(unsigned long int netlong); unsigned short int ntohs(unsigned short int netshort);
- 20. Lecture 3: 9-4-0120 Lecture Overview • Application layer • Client-server • Application requirements • Background • TCP vs. UDP • Byte ordering • Socket I/O • TCP/UDP server and client • I/O multiplexing
- 21. Lecture 3: 9-4-0121 • A socket is a file descriptor that lets an application read/write data from/to the network • socket returns an integer (socket descriptor) • fd < 0 indicates that an error occurred • socket descriptors are similar to file descriptors • AF_INET: associates a socket with the Internet protocol family • SOCK_STREAM: selects the TCP protocol • SOCK_DGRAM: selects the UDP protocol What is a Socket? int fd; /* socket descriptor */ if ((fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) } perror(“socket”); exit(1); }
- 22. Lecture 3: 9-4-0122 TCP IP Ethernet Adapter Web Server Port 80 • For example: web server • What does a web server need to do so that a web client can connect to it? TCP Server
- 23. Lecture 3: 9-4-0123 • Since web traffic uses TCP, the web server must create a socket of type SOCK_STREAM int fd; /* socket descriptor */ if((fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) { perror(“socket”); exit(1); } • socket returns an integer (socket descriptor) • fd < 0 indicates that an error occurred • AF_INET associates a socket with the Internet protocol family • SOCK_STREAM selects the TCP protocol Socket I/O: socket()
- 24. Lecture 3: 9-4-0124 • A socket can be bound to a port int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ /* create the socket */ srv.sin_family = AF_INET; /* use the Internet addr family */ srv.sin_port = htons(80); /* bind socket ‘fd’ to port 80*/ /* bind: a client may connect to any of my addresses */ srv.sin_addr.s_addr = htonl(INADDR_ANY); if(bind(fd, (struct sockaddr*) &srv, sizeof(srv)) < 0) { perror("bind"); exit(1); } • Still not quite ready to communicate with a client... Socket I/O: bind()
- 25. Lecture 3: 9-4-0125 Socket I/O: listen() • listen indicates that the server will accept a connection int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ /* 1) create the socket */ /* 2) bind the socket to a port */ if(listen(fd, 5) < 0) { perror(“listen”); exit(1); } • Still not quite ready to communicate with a client...
- 26. Lecture 3: 9-4-0126 Socket I/O: accept() • accept blocks waiting for a connection int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ struct sockaddr_in cli; /* used by accept() */ int newfd; /* returned by accept() */ int cli_len = sizeof(cli); /* used by accept() */ /* 1) create the socket */ /* 2) bind the socket to a port */ /* 3) listen on the socket */ newfd = accept(fd, (struct sockaddr*) &cli, &cli_len); if(newfd < 0) { perror("accept"); exit(1); } • accept returns a new socket (newfd) with the same properties as the original socket (fd) • newfd < 0 indicates that an error occurred
- 27. Lecture 3: 9-4-0127 Socket I/O: accept() continued... struct sockaddr_in cli; /* used by accept() */ int newfd; /* returned by accept() */ int cli_len = sizeof(cli); /* used by accept() */ newfd = accept(fd, (struct sockaddr*) &cli, &cli_len); if(newfd < 0) { perror("accept"); exit(1); } • How does the server know which client it is? • cli.sin_addr.s_addr contains the client’s IP address • cli.sin_port contains the client’s port number • Now the server can exchange data with the client by using read and write on the descriptor newfd. • Why does accept need to return a new descriptor?
- 28. Lecture 3: 9-4-0128 Socket I/O: read() • read can be used with a socket • read blocks waiting for data from the client but does not guarantee that sizeof(buf) is read int fd; /* socket descriptor */ char buf[512]; /* used by read() */ int nbytes; /* used by read() */ /* 1) create the socket */ /* 2) bind the socket to a port */ /* 3) listen on the socket */ /* 4) accept the incoming connection */ if((nbytes = read(newfd, buf, sizeof(buf))) < 0) { perror(“read”); exit(1); }
- 29. Lecture 3: 9-4-0129 TCP IP Ethernet Adapter 2 Web Clients TCP Client • For example: web client • How does a web client connect to a web server?
- 30. Lecture 3: 9-4-0130 struct sockaddr_in srv; srv.sin_addr.s_addr = inet_addr(“128.2.35.50”); if(srv.sin_addr.s_addr == (in_addr_t) -1) { fprintf(stderr, "inet_addr failed!n"); exit(1); } Converting a numerical address to a string: Dealing with IP Addresses • IP Addresses are commonly written as strings (“128.2.35.50”), but programs deal with IP addresses as integers. struct sockaddr_in srv; char *t = inet_ntoa(srv.sin_addr); if(t == 0) { fprintf(stderr, “inet_ntoa failed!n”); exit(1); } Converting strings to numerical address:
- 31. Lecture 3: 9-4-0131 Translating Names to Addresses • Gethostbyname provides interface to DNS • Additional useful calls • Gethostbyaddr – returns hostent given sockaddr_in • Getservbyname • Used to get service description (typically port number) • Returns servent based on name #include <netdb.h> struct hostent *hp; /*ptr to host info for remote*/ struct sockaddr_in peeraddr; char *name = “www.cs.cmu.edu”; peeraddr.sin_family = AF_INET; hp = gethostbyname(name) peeraddr.sin_addr.s_addr = ((struct in_addr*)(hp->h_addr))->s_addr;
- 32. Lecture 3: 9-4-0132 Socket I/O: connect() • connect allows a client to connect to a server... int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by connect() */ /* create the socket */ /* connect: use the Internet address family */ srv.sin_family = AF_INET; /* connect: socket ‘fd’ to port 80 */ srv.sin_port = htons(80); /* connect: connect to IP Address “128.2.35.50” */ srv.sin_addr.s_addr = inet_addr(“128.2.35.50”); if(connect(fd, (struct sockaddr*) &srv, sizeof(srv)) < 0) { perror(”connect"); exit(1); }
- 33. Lecture 3: 9-4-0133 Socket I/O: write() • write can be used with a socket int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by connect() */ char buf[512]; /* used by write() */ int nbytes; /* used by write() */ /* 1) create the socket */ /* 2) connect() to the server */ /* Example: A client could “write” a request to a server */ if((nbytes = write(fd, buf, sizeof(buf))) < 0) { perror(“write”); exit(1); }
- 34. Lecture 3: 9-4-0134 Review: TCP Client-Server Interaction socket() bind() listen() accept() write() read() read() TCP Server close() socket() TCP Client connect() write() read() close() connection establishment data request data reply end-of-file notification from UNIX Network Programming Volume 1, figure 4.1
- 35. Lecture 3: 9-4-0135 UDP IP Ethernet Adapter NTP daemon UDP Server Example Port 123 • For example: NTP daemon • What does a UDP server need to do so that a UDP client can connect to it?
- 36. Lecture 3: 9-4-0136 Socket I/O: socket() • The UDP server must create a datagram socket… int fd; /* socket descriptor */ if((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { perror(“socket”); exit(1); } • socket returns an integer (socket descriptor) • fd < 0 indicates that an error occurred • AF_INET: associates a socket with the Internet protocol family • SOCK_DGRAM: selects the UDP protocol
- 37. Lecture 3: 9-4-0137 Socket I/O: bind() • A socket can be bound to a port int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ /* create the socket */ /* bind: use the Internet address family */ srv.sin_family = AF_INET; /* bind: socket ‘fd’ to port 80*/ srv.sin_port = htons(80); /* bind: a client may connect to any of my addresses */ srv.sin_addr.s_addr = htonl(INADDR_ANY); if(bind(fd, (struct sockaddr*) &srv, sizeof(srv)) < 0) { perror("bind"); exit(1); } • Now the UDP server is ready to accept packets…
- 38. Lecture 3: 9-4-0138 Socket I/O: recvfrom() • read does not provide the client’s address to the UDP server int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ struct sockaddr_in cli; /* used by recvfrom() */ char buf[512]; /* used by recvfrom() */ int cli_len = sizeof(cli); /* used by recvfrom() */ int nbytes; /* used by recvfrom() */ /* 1) create the socket */ /* 2) bind to the socket */ nbytes = recvfrom(fd, buf, sizeof(buf), 0 /* flags */, (struct sockaddr*) &cli, &cli_len); if(nbytes < 0) { perror(“recvfrom”); exit(1); }
- 39. Lecture 3: 9-4-0139 Socket I/O: recvfrom() continued... nbytes = recvfrom(fd, buf, sizeof(buf), 0 /* flags */, (struct sockaddr*) cli, &cli_len); • The actions performed by recvfrom • returns the number of bytes read (nbytes) • copies nbytes of data into buf • returns the address of the client (cli) • returns the length of cli (cli_len) • don’t worry about flags
- 40. Lecture 3: 9-4-0140 TCP IP Ethernet Adapter 2 UDP Clients UDP Client Example ports • How does a UDP client communicate with a UDP server?
- 41. Lecture 3: 9-4-0141 Socket I/O: sendto() • write is not allowed • Notice that the UDP client does not bind a port number • a port number is dynamically assigned when the first sendto is called int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by sendto() */ /* 1) create the socket */ /* sendto: send data to IP Address “128.2.35.50” port 80 */ srv.sin_family = AF_INET; srv.sin_port = htons(80); srv.sin_addr.s_addr = inet_addr(“128.2.35.50”); nbytes = sendto(fd, buf, sizeof(buf), 0 /* flags */, (struct sockaddr*) &srv, sizeof(srv)); if(nbytes < 0) { perror(“sendto”); exit(1); }
- 42. Lecture 3: 9-4-0142 Review: UDP Client-Server Interaction socket() bind() recvfrom() sendto() UDP Server socket() UDP Client sendto() recvfrom() close() blocks until datagram received from a client data request data reply from UNIX Network Programming Volume 1, figure 8.1
- 43. Lecture 3: 9-4-0143 UDP IP Ethernet Adapter UDP Server The UDP Server Port 2000 Port 3000 • How can the UDP server service multiple ports simultaneously?
- 44. Lecture 3: 9-4-0144 int s1; /* socket descriptor 1 */ int s2; /* socket descriptor 2 */ /* 1) create socket s1 */ /* 2) create socket s2 */ /* 3) bind s1 to port 2000 */ /* 4) bind s2 to port 3000 */ while(1) { recvfrom(s1, buf, sizeof(buf), ...); /* process buf */ recvfrom(s2, buf, sizeof(buf), ...); /* process buf */ } UDP Server: Servicing Two Ports • What problems does this code have?
- 45. Lecture 3: 9-4-0145 Socket I/O: select() int select(int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); FD_CLR(int fd, fd_set *fds); /* clear the bit for fd in fds */ FD_ISSET(int fd, fd_set *fds); /* is the bit for fd in fds? */ FD_SET(int fd, fd_set *fds); /* turn on the bit for fd in fds */ FD_ZERO(fd_set *fds); /* clear all bits in fds */ • maxfds: number of descriptors to be tested • descriptors (0, 1, ... maxfds-1) will be tested • readfds: a set of fds we want to check if data is available • returns a set of fds ready to read • if input argument is NULL, not interested in that condition • writefds: returns a set of fds ready to write • exceptfds: returns a set of fds with exception conditions
- 46. Lecture 3: 9-4-0146 Socket I/O: select() int select(int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); struct timeval { long tv_sec; /* seconds / long tv_usec; /* microseconds */ } • timeout • if NULL, wait forever and return only when one of the descriptors is ready for I/O • otherwise, wait up to a fixed amount of time specified by timeout • if we don’t want to wait at all, create a timeout structure with timer value equal to 0 • Refer to the man page for more information
- 47. Lecture 3: 9-4-0147 int s1, s2; /* socket descriptors */ fd_set readfds; /* used by select() */ /* create and bind s1 and s2 */ while(1) { FD_ZERO(&readfds); /* initialize the fd set */ FD_SET(s1, &readfds); /* add s1 to the fd set */ FD_SET(s2, &readfds); /* add s2 to the fd set */ if(select(s2+1, &readfds, 0, 0, 0) < 0) { perror(“select”); exit(1); } if(FD_ISSET(s1, &readfds)) { recvfrom(s1, buf, sizeof(buf), ...); /* process buf */ } /* do the same for s2 */ } Socket I/O: select() • select allows synchronous I/O multiplexing
- 48. Lecture 3: 9-4-0148 TCP IP Ethernet Adapter Web Server Port 80 How can a a web server manage multiple connections simultaneously? Port 8001 More Details About a Web Server
- 49. Lecture 3: 9-4-0149 int fd, next=0; /* original socket */ int newfd[10]; /* new socket descriptors */ while(1) { fd_set readfds; FD_ZERO(&readfds); FD_SET(fd, &readfds); /* Now use FD_SET to initialize other newfd’s that have already been returned by accept() */ select(maxfd+1, &readfds, 0, 0, 0); if(FD_ISSET(fd, &readfds)) { newfd[next++] = accept(fd, ...); } /* do the following for each descriptor newfd[n] */ if(FD_ISSET(newfd[n], &readfds)) { read(newfd[n], buf, sizeof(buf)); /* process data */ } } Socket I/O: select() • Now the web server can support multiple connections...
- 50. Lecture 3: 9-4-0150 A Few Programming Notes: Representing Packets 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: 4-byte integer Length: 2-byte integer Checksum: 2-byte integer Address: 4-byte IP address
- 51. Lecture 3: 9-4-0151 A Few Programming Notes: Building a Packet in a Buffer struct packet { u_int32_t type; u_int16_t length; u_int16_t checksum; u_int32_t address; }; /* ================================================== */ char buf[1024]; struct packet *pkt; pkt = (struct packet*) buf; pkt->type = htonl(1); pkt->length = htons(2); pkt->checksum = htons(3); pkt->address = htonl(4);
- 52. Lecture 3: 9-4-0152 Socket Programming References • Man page • usage: man <function name> • Textbook • Sections 2.6, 2.7 • demo programs written in Java • Unix Network Programming : Networking APIs: Sockets and XTI (Volume 1) • Section 2, 3, 4, 6, 8 • ultimate socket programming bible!