Sockets Programming - Stanford University
Sockets ProgrammingCS144 Review Session 1April 4, 2008Ben Nham
Announcements New classroom for lecture– 200‐034, MW 4:15‐5:30 Simple client assignment due Wednesday, 4/9 Use newsgroup for general quesNons– su.class.cs144– Refresh your newsgroup list from the server if youcan’t find it
Simple Client Overview Opens TCP socket to other host– Does DNS lookup if necessary Reads request from command line, appending"\r\n", and sends it through the socket Echoes response to stdout Demo: Sending HTTP request to SCShomepage– ./sc www.scs.stanford.edu 80 "GET /"
Sockets and TCP/IPApplicaNonSockets PortPortIPIP AddressLinkHardware Interface In TCP/IP:– Endpoint has unique (TCPport, IP address) pair– ConnecNon between twoendpoints is idenNfied bythe pair [(IP, port)src, (IP,port)dst] All Unix I/O streams arereferenced by descriptors– Socket maps a descriptorto an endpoint– ConnecNng sockets allowsus to connect endpointsand do I/O
Socket API for Clientsocketbindconnectint socket(int domain, int type, int protocol) Returns a descriptor associated with a new endpointint bind(int sd, struct sockaddr *addr,u int addr len) Set addr/port of endpoint for socket descriptor sdOpNonal for client (lets the kernel choose some availableport with the default IP address)int connect(int sd, struct sockaddr *addr,u int addr len) send/recvint send(int sd, void *buf, int len, int flags)int recv(int sd, void *buf, int len, int flags) shutdownConnect to desNnaNon address port endpointTwo‐way communicaNonint shutdown(int sd, int how) ParNal or complete connecNon teardown
Sockets API for Serverint socket(int, int, int)int bind(int, struct sockaddr *,u int)socketbindint listen(int sd, int backlog) Wait for a client to connect to this portlistenint accept(int sd, struct sockaddr*addr, u int *addr len) Accept connecNon, returning a new descriptorfor this (IP, port)src – (IP, port)dst pairacceptint send(int, void *, int)int recv(int, void *, int)send/recvint shutdown(int, int)shutdown
ClientTypical Request‐ResponseClient‐Server PairServersocketsocketbindbind"Can I connect?""Yes. Can I connect?"listenconnect"Yes."acceptRequest: "GET /"send/recvshutdownsend/recvOne side starts shutdownshutdown
ClientTypical Request‐ResponseClient‐Server ectACKacceptRequest: "GET /"send/recvshutdownsend/recvOne side starts shutdownshutdown
Example DayNme Server See the posted dayNme.c Problems– Doesn’t check return values of system calls You should check the return value and use perror() orfpring(stderr, strerror(errno)) to print out aninformaNve error message– Doesn’t handle mulNple clients simultaneously– Problems with re‐using the same port (usesetsockopt—covered in next review session)
Main Structures Generic socket addressstruct sockaddr {u short sa family;char sa data[14];}; TCP/UDP IPv4 specific address – convenience parallel struct for sockaddrstruct sockaddr in {u short sa family;// usually AF INETu short sin port;struct in addr sin addr; // see belowchar sin zero[8];// zero out}; IP Addressstruct in addr {u long s addr;};// Network byte orderNext two slides by Clay Collier
Useful FuncNonsstruct hostent *gethostbyname(const char *name) Converts domain names and dojed‐quad IP addresses intonumerical IP addresses via DNSstruct servent *getservbyname(const char *name,const char *proto) Query /etc/services to find expected protocol and port for aservice Example: getservbyname("hjp", NULL) to find it resides ontcp port 80getsockname(.), getpeername(.) Gets IP address and port for source/desNnaNon
More Notes ParNal sends and receives– Receive might not return with all the bytes requested Endian issues– All shorts/ints going over the wire must be encodedusing htons/htonl– All shorts/ints being read from the wire must bedecoded using ntohs/ntohl– Why don’t we have to worry about endianness fortext? For arbitrary binary data?
Grading No set rubric yet Mostly funcNonality– Test yourself by sending (for example) hjprequests Some style points– Don’t write everything in main– Handle parNal receives and other edge cases– Check return values of system calls
Resources IPC tutorial Man pages Outside references– Beej’s Sockets Tutorial:hjp://beej.us/guide/bgnet/– Unix Network Programming by Stevens Newsgroup Office hours
Sockets and TCP/IP In TCP/IP: – Endpoint
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