![Socket Address Structure
struct in_addr {
in_addr_t s_addr; /* 32-bit IPv4 addresses */
};
struct sockaddr_in {
unit8_t sin_len; /* length of structure */
sa_family_t sin_family; /* AF_INET */
in_port_t sin_port; /* TCP/UDP Port num */
struct in_addr sin_addr; /* IPv4 address (above) */
char sin_zero[8]; /* unused */
}
• Are also “generic” and “IPv6” socket
structures](https://image.slidesharecdn.com/sockets-240122205814-1f98d249/85/Introduction-to-sockets-tcp-ip-protocol-ppt-10-320.jpg)
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Introduction to sockets tcp ip protocol.ppt
- 2. Outline • Socket basics • Socket details (TCP and UDP) • Socket options • Final notes
- 3. Socket Basics (1 of 2) • An end-point for an Internet network connection – what the application layer “plugs into” User Application Socket Operating System Transport Layer Internet Protocol Layer • User sees “descriptor” - integer index or object handle – like: FILE *, or file index from open() – returned by socket() call (more later) – programmer cares about Application Programming Interface (API)
- 4. Socket Basics (2 of 2) • End point determined by two things: – Host address: IP address is Network Layer – Port number: is Transport Layer • Two end-points determine a connection: socket pair – ex: 206.62.226.35,p21 + 198.69.10.2,p1500 – ex: 206.62.226.35,p21 + 198.69.10.2,p1499
- 5. Ports • Numbers (typical, since vary by OS): – 0-1023 “reserved”, must be root – 1024 - 5000 “ephemeral” – Above 5000 for general use + (50,000 is specified max) • Well-known, reserved services (see /etc/services in Unix): – ftp 21/tcp – telnet 23/tcp – finger 79/tcp – snmp 161/udp
- 6. Transport Layer • UDP: User Datagram Protocol – no acknowledgements – no retransmissions – out of order, duplicates possible – connectionless • TCP: Transmission Control Protocol – reliable (in order, all arrive, no duplicates) – flow control – Connection-based • While TCP ~95% of all flows and packets, much UDP traffic is games!
- 7. Outline • Socket basics •Socket details (TCP and UDP) • Socket options • Final notes
- 8. Socket Details Outline Unix Network Programming, W. Richard Stevens, 2nd edition, 1998, Prentice Hall • Project 3 Links has samples – C++, Windows and C++, Linux – Java • Code is very similar for Windows • Addresses and Sockets • TCP client-server (talk-tcp, listen-tcp) • UDP client-server (talk-udp, listen-udp) • Misc stuff – setsockopt(), getsockopt() – fcntl()
- 9. Addresses and Sockets • Structure to hold address information • Functions pass address from user to OS bind() connect() sendto() • Functions pass address from OS to user accept() recvfrom()
- 10. Socket Address Structure struct in_addr { in_addr_t s_addr; /* 32-bit IPv4 addresses */ }; struct sockaddr_in { unit8_t sin_len; /* length of structure */ sa_family_t sin_family; /* AF_INET */ in_port_t sin_port; /* TCP/UDP Port num */ struct in_addr sin_addr; /* IPv4 address (above) */ char sin_zero[8]; /* unused */ } • Are also “generic” and “IPv6” socket structures
- 11. TCP Client-Server socket() bind() listen() accept() Server socket() connect() send() recv() Client (Block until connection) “Handshake” recv() send() Data (request) Data (reply) close() End-of-File recv() close() “well-known” port
- 12. socket() int socket(int family, int type, int protocol); Create a socket, giving access to transport layer service. • family is one of – AF_INET (IPv4), AF_INET6 (IPv6), AF_LOCAL (local Unix), – AF_ROUTE (access to routing tables), AF_KEY (new, for encryption) • type is one of – SOCK_STREAM (TCP), SOCK_DGRAM (UDP) – SOCK_RAW (for special IP packets, PING, etc. Must be root) + setuid bit (-rws--x--x root 1997 /sbin/ping*) • protocol is 0 (used for some raw socket options) • upon success returns socket descriptor – Integer, like file descriptor
- 13. bind() • sockfd is socket descriptor from socket() • myaddr is a pointer to address struct with: – port number and IP address – if port is 0, then host will pick ephemeral port + not usually for server (exception RPC port-map) – IP address != INADDR_ANY (unless multiple nics) • addrlen is length of structure • returns 0 if ok, -1 on error – EADDRINUSE (“Address already in use”) int bind(int sockfd, const struct sockaddr *myaddr, socklen_t addrlen); Assign a local protocol address (“name”) to a socket.
- 14. listen() • sockfd is socket descriptor from socket() • backlog is maximum number of incomplete connections – historically 5 – rarely above 15 on a even moderate Web server! • Sockets default to active (for a client) – change to passive so OS will accept connection int listen(int sockfd, int backlog); Change socket state for TCP server.
- 15. accept() • sockfd is socket descriptor from socket() • cliaddr and addrlen return protocol address from client • returns brand new descriptor, created by OS • note, if create new process or thread, can create concurrent server int accept(int sockfd, struct sockaddr cliaddr, socklen_t *addrlen); Return next completed connection.
- 16. close() • sockfd is socket descriptor from socket() • closes socket for reading/writing – returns (doesn’t block) – attempts to send any unsent data – socket option SO_LINGER + block until data sent + or discard any remaining data – returns -1 if error int close(int sockfd); Close socket for use.
- 17. TCP Client-Server socket() bind() listen() accept() Server socket() connect() send() recv() Client (Block until connection) “Handshake” recv() send() Data (request) Data (reply) close() End-of-File recv() close() “well-known” port
- 18. connect() • sockfd is socket descriptor from socket() • servaddr is a pointer to a structure with: – port number and IP address – must be specified (unlike bind()) • addrlen is length of structure • client doesn’t need bind() – OS will pick ephemeral port • returns socket descriptor if ok, -1 on error int connect(int sockfd, const struct sockaddr *servaddr, socklen_t addrlen); Connect to server.
- 19. Sending and Receiving int recv(int sockfd, void *buff, size_t mbytes, int flags); int send(int sockfd, void *buff, size_t mbytes, int flags); • Same as read() and write() but for flags – MSG_DONTWAIT (this send non-blocking) – MSG_OOB (out of band data, 1 byte sent ahead) – MSG_PEEK (look, but don’t remove) – MSG_WAITALL (don’t give me less than max) – MSG_DONTROUTE (bypass routing table)
- 20. UDP Client-Server socket() bind() recvfrom() Server socket() sendto() recvfrom() Client (Block until receive datagram) sendto() Data (request) Data (reply) close() “well-known” port - No “handshake” - No simultaneous close - No fork()/spawn() for concurrent servers!
- 21. Sending and Receiving int recvfrom(int sockfd, void *buff, size_t mbytes, int flags, struct sockaddr *from, socklen_t *addrlen); int sendto(int sockfd, void *buff, size_t mbytes, int flags, const struct sockaddr *to, socklen_t addrlen); • Same as recv() and send() but for addr – recvfrom fills in address of where packet came from – sendto requires address of where sending packet to
- 22. connect() with UDP • Record address and port of peer – datagrams to/from others are not allowed – does not do three way handshake, or connection – “connect” a misnomer, here. Should be setpeername() • Use send() instead of sendto() • Use recv() instead of recvfrom() • Can change connect or unconnect by repeating connect() call • (Can do similar with bind() on receiver)
- 23. Why use connected UDP? • Send two datagrams unconnected: – connect the socket – output first dgram – unconnect the socket – connect the socket – ouput second dgram – unconnect the socket • Send two datagrams connected: – connect the socket – output first dgram – ouput second dgram
- 24. Socket Options •setsockopt(), getsockopt() • SO_LINGER – upon close, discard data or block until sent • SO_RCVBUF, SO_SNDBUF – change buffer sizes – for TCP is “pipeline”, for UDP is “discard” • SO_RCVLOWAT, SO_SNDLOWAT – how much data before “readable” via select() • SO_RCVTIMEO, SO_SNDTIMEO – timeouts
- 25. Socket Options (TCP) • TCP_KEEPALIVE – idle time before close (2 hours, default) • TCP_MAXRT – set timeout value • TCP_NODELAY – disable Nagle Algorithm – won’t buffer data for larger chunk, but sends immediately
- 26. fcntl() • ‘File control’ but used for sockets, too • Signal driven sockets • Set socket owner • Get socket owner • Set socket non-blocking flags = fcntl(sockfd, F_GETFL, 0); flags |= O_NONBLOCK; fcntl(sockfd, F_SETFL, flags); • Beware not getting flags before setting!
- 27. Concurrent Servers • Close sock in child, newsock in parent • Reference count for socket descriptor Text segment sock = socket() /* setup socket */ while (1) { newsock = accept(sock) fork() if child read(newsock) until exit } Parent int sock; int newsock; Child int sock; int newsock;
- 28. Project 3: Online Chess • 1) Start server • 2) Client A connects – Client sends handle – Server sends color • 3) Client B connects – Client sends handle – Server sends color • 4) Game starts! • 5) Server sends turn • 6) If clients turn – Client sends move – Server sends OK/Illegal • 7) Else – Server sends opponent move • 8) When checkmate – Server sends winner • 9) Close • 10) Server returns to wait for next game Server Client
Editor's Notes
- #11: Length field makes it easier for OS to handle