Outline ! Socket basics ! TCP sockets ! Socket details ! Socket - PDF document

Outline ! Socket basics ! TCP sockets ! Socket details ! Socket options Operating Systems ! Final notes ! Project 3 Sockets Socket Basics Ports ! An end-point for a IP network connection ! Numbers (vary in BSD, Solaris): – what the application layer “plugs into” – 0-1023 “reserved”, must be root – programmer cares about Application – 1024 - 5000 “ephemeral” Programming Interface (API) – however, many systems allow > 5000 ports ! End-point determined by two things: N (50,000 is correct number) – Host address (IP address) - name of machine ! /etc/services: – Port number - location of process ftp 21/tcp ! Two end-points determine a connection: telnet 23/tcp socket pair finger 79/tcp – ex: 206.62.226.35,p21 + 198.69.10.2,p1500 snmp 161/udp – ex: 206.62.226.35,p21 + 198.69.10.2,p1499 Network Communication Sockets and the OS ! UDP : User Datagram Protocol – no acknowledgements User – no retransmissions Socket – out of order, duplicate possible Operating System – connectionless (Transport Layer) ! TCP : Transmission Control Protocol – reliable (in order, all arrive, no duplicates) ! User sees “descriptor”, integer index – flow control – like: FILE * , or file index – connection – returned by socket() call (more later) – duplex – (Project 3 uses TCP) 1

Socket Details Addresses and Sockets ! Structure to hold address information Unix Network Programming , W. Richard Stevens, 2nd edition,  1998, Prentice Hall ! Functions pass address from app to OS – bind() – connect() ! Socket address structure – sendto() ! TCP client-server ! Functions pass address from OS to app ! Misc stuff – accept() – setsockopt(), getsockopt() – recvfrom() – fcntl() Server TCP Client-Server Socket Address Structure socket() “well-known” bind() port struct in_addr { in_addr_t s_addr; /* 32-bit IPv4 addresses */ listen() Client }; accept() struct sock_ addr_in { socket() (Block until connection ) “Handshake” unit8_t sin_ len; /* length of structure */ connect() sa_family_t sin_family; /* AF_INET */ Data (request) send() in_port_t sin_port; /* TCP/UDP Port num */ recv() struct in_ addr sin_addr; /* IPv4 address */ Data (reply) send() char sin_zero[8]; /* unused */ recv() } End-of-File close() recv() ! Are also “generic” and “IPv6” socket structures close() socket() Server Functions int socket(int family , int type , int protocol ); socket() Create a socket, giving access to transport layer service. “well-known” bind() port ! family is one of listen() – AF_INET (IPv4), AF_INET6 (IPv6), AF_LOCAL (local Unix), – AF_ROUTE (access to routing tables), AF_KEY (new, for encryption) accept() ! type is one of (Block until connection ) “Handshake” – SOCK_STREAM (TCP), SOCK_DGRAM (UDP) – SOCK_RAW (for special IP packets, PING, etc. Must be root) Data (request) N setuid bit ( -rws--x--x root 1997 / sbin/ping*) recv() ! protocol is 0 (used for some raw socket options) Data (reply) send() ! upon success returns socket descriptor End-of-File – similar to a file descriptor or semaphore id recv() – returns -1 if failure close() 2

bind() listen() int bind(int sockfd , const struct sockaddr * myaddr , socklen_t addrlen ); Assign a local protocol address (“name”) to a socket . int listen(int sockfd , int backlog ); ! sockfd is socket descriptor from socket() Change socket state for TCP server. ! myaddr is a pointer to address struct with: ! sockfd is socket descriptor from socket() – port number and IP address ! backlog is maximum number of incomplete – if port is 0, then host will pick ephemeral port connections N not usually for server (exception RPC port-map) – historically 5 – IP address != INADDR_ANY (multiple nics) – rarely above 15 on a even moderate web server! ! addrlen is length of structure ! Sockets default to active (for client) ! returns 0 if ok, -1 on error – change to passive to OS will accept connection – EADDRINUSE (“Address already in use”) close() accept() int close(int sockfd) ; int accept(int sockfd , struct sockaddr cliaddr , socklen_t * addrlen ); Close socket for use. Return next completed connection. ! sockfd is socket descriptor from socket() ! sockfd is socket descriptor from socket() ! closes socket for reading/writing ! cliaddr and addrlen return protocol address – returns (doesn’t block) from client – attempts to send any unsent data ! returns brand new descriptor, created by OS – socket option SO_LINGER ! if used with fork(), can create N block until data sent N or discard any remaining data concurrent server (more later) – -1 if error connect() Client functions int connect(int sockfd , const struct sockaddr * servaddr , socklen_t addrlen ); Connect to server. ! sockfd is socket descriptor from socket() Client ! servaddr is a pointer to a structure with: socket() “Handshake” – port number and IP address connect() – must be specified (unlike bind() ) Data (request) send() ! addrlen is length of structure Data (reply) ! client doesn’t need bind() recv() – OS will pick ephemeral port End-of-File close() ! returns socket descriptor if ok, -1 on error 3

Sending and Receiving Socket Options ! setsockopt (), getsockopt () int recv(int sockfd , void * buff , size_t mbytes , int flags ); ! SO_LINGER – upon close, discard data or block until sent int send(int sockfd , void * buff , size_t mbytes , int flags ); ! SO_RCVBUF, SO_SNDBUF – change buffer sizes ! Same as read() and write() but for flags – for TCP is “pipeline”, for UDP is “discard” – MSG_DONTWAIT (this send non-blocking) ! SO_RCVLOWAT, SO_SNDLOWAT – MSG_OOB (out of band data, 1 byte sent ahead) – how much data before “readable” via select () – MSG_PEEK (look, but don’t remove) ! SO_RCVTIMEO, SO_SNDTIMEO – MSG_WAITALL (don’t give me less than max ) – timeouts – MSG_DONTROUTE (bypass routing table) fcntl() Socket Options (TCP) ! ‘File control’ but used for sockets, too ! TCP_KEEPALIVE ! Signal driven sockets – idle time before close (2 hours, default) ! Set socket owner ! TCP_MAXRT ! Get socket owner – set timeout value ! Set socket non-blocking ! TCP_NODELAY flags = fcntl(sockfd, F_GETFL, 0); – disable Nagle Algorithm flags |= O_NONBLOCK; fcntl(sockfd, F_SETFL, flags); ! Beware not getting flags before setting! ! (Should not need for project 3) Concurrent Servers Project 3: Macro Shell Text segment Parent sock = socket() ! Distributed Shell (1) connect /* setup socket */ int sock; ! Client/Server while (1) { Server Client int newsock; (2) ls newsock = accept(sock) ! Non-interactive fork() (3) fork() – command line args and exec() if child Child (4) data read(newsock) – get-opt.c int sock; until exit Server int newsock; ! Uses TCP sockets } – listen.c and talk.c ! Close sock in child, newsock in parent ! Security – password ! Reference count for socket descriptor 4