1 / 21

Communication Networks

Communication Networks. Recitation 2. TCP/IP Socket Programming Cont. Outline. Send/Receive data Terminating a connection Dealing with blocking calls Client-server model summary. Sending / Receiving Data. With a connection (SOCK_STREAM): int count = send(sock, &buf, len, flags);

rose-dudley
Download Presentation

Communication Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Communication Networks Recitation 2 Comnet 2006 - Recitation 2 - Sockets

  2. TCP/IP Socket ProgrammingCont. Comnet 2006 - Recitation 2 - Sockets

  3. Outline Send/Receive data Terminating a connection Dealing with blocking calls Client-server model summary Comnet 2006 - Recitation 2 - Sockets

  4. Sending / Receiving Data • With a connection (SOCK_STREAM): • int count = send(sock, &buf, len, flags); • count: # bytes transmitted (-1 if error) • buf: char[], buffer to be transmitted • len: integer, length of buffer (in bytes) to transmit • flags: integer, special options, usually just 0 • int count = recv(sock, &buf, len, flags); • count: # bytes received (-1 if error) • buf: void[], stores received bytes • len: max # bytes received • flags: integer, special options, usually just 0 Comnet 2006 - Recitation 2 - Sockets

  5. Example of send() … connect(sock, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr)); … char *msg = "Comnet is the course!"; int len, bytes_sent; len = strlen(msg); bytes_sent = send(sock, msg, len, 0); Comnet 2006 - Recitation 2 - Sockets

  6. Sending / Receiving Data (cont’d) • Without a connection (SOCK_DGRAM): • int count = sendto(sock, &buf, len, flags, &addr, addrlen); • count, sock, buf, len, flags: same as send • addr: struct sockaddr, address of the destination • addrlen: sizeof(addr) • int count = recvfrom(sock, &buf, len, flags, &addr, &addrlen); • count, sock, buf, len, flags: same as recv • name: struct sockaddr, address of the source • namelen: sizeof(name): value/result parameter Comnet 2006 - Recitation 2 - Sockets

  7. Sending/Receiving notes • send( ) might send only a fraction of the buffer • Need to push the rest • See sendall( ) in Beej’s guide • recv( ) might get several messages together, or a fraction of a message • Need to be able to identify message’s beginning and end Comnet 2006 - Recitation 2 - Sockets

  8. Closing connection • When finished using a socket, the socket should be closed: • status = close(s); • status: 0 if successful, -1 if error • s: the file descriptor (socket being closed) • Closing a socket • closes a connection (for SOCK_STREAM) • frees up the port used by the socket Comnet 2006 - Recitation 2 - Sockets

  9. Skipping the bind() • SOCK_DGRAM: • if only sending, no need to bind. The OS finds a port each time the socket sends a packet • if receiving, need to bind • SOCK_STREAM: • The OS binds a port to the client during connection setup Comnet 2006 - Recitation 2 - Sockets

  10. Dealing with blocking calls • Many of the functions we saw block until a certain event • accept: until a connection comes in • connect: until the connection is established • recv, recvfrom: until a packet (of data) is received • send, sendto: until data is pushed into socket’s buffer • For simple programs, blocking is convenient • What about more complex programs? • multiple connections • simultaneous sends and receives • simultaneously doing non-networking processing Comnet 2006 - Recitation 2 - Sockets

  11. Dealing with blocking (cont’d) • Options: • create multi-process or multi-threaded code • turn off the blocking feature using the fcntl() file-descriptor control function • use the select() function call. Comnet 2006 - Recitation 2 - Sockets

  12. fcntl() function call • int fcntl(int filedes, int cmd, args ...); • status: -1 if error • filedes: argument is an open file descriptor • cmd: specifies the operation to be performed • arg: depends upon the value of cmd sockfd = socket(PF_INET, SOCK_STREAM, 0); fcntl(sockfd, F_SETFL, O_NONBLOCK); Comnet 2006 - Recitation 2 - Sockets

  13. Using select() • What does select do? • can be permanent blocking, time-limited blocking or non-blocking • input: a set of file-descriptors • output: info on the file-descriptors’ status • i.e., can identify sockets that are “ready for use”: calls involving that socket will return immediately Comnet 2006 - Recitation 2 - Sockets

  14. select() function call • int status = select(nfds, &readfds, &writefds, &exceptfds, &timeout); • status: # of ready objects, -1 if error • nfds: 1 + largest file descriptor to check • readfds: set of descriptors to check if read-ready • writefds: set of descriptors to check if write-ready • exceptfds: set of descriptors to check if an exception is registered • timeout: time after which select returns, even if nothing ready - can be 0 or  Comnet 2006 - Recitation 2 - Sockets

  15. To be used with select(): • For sets, select uses a structure, struct fd_set • it is just a bit-vector • if bit i is set in [readfds, writefds, exceptfds], select will check if file descriptor (i.e. socket) i is ready for [reading, writing, exception] • Before calling select: • FD_ZERO(&fdvar): clears the structure • FD_SET(i, &fdvar): to check file desc. i • After calling select: • int FD_ISSET(i, &fdvar): boolean returns TRUE iff i is “ready to read” • Standard input (STDIN) is file descriptor 0 Comnet 2006 - Recitation 2 - Sockets

  16. Timeout parameter struct timeval { int tv_sec; // seconds int tv_usec; // microseconds }; • Set timeout to NULL for no timeout • Set both to 0 for immediate timeout Comnet 2006 - Recitation 2 - Sockets

  17. Ready to read • Listening port is read-ready if accept won’t block • Closed connections are read-ready but recv( ) returns 0 bytes Comnet 2006 - Recitation 2 - Sockets

  18. Other useful functions • bzero(char* c, int n): 0’s n bytes starting at c • gethostname(char *name, int len): gets the name of the current host • gethostbyname(const char *name): converts host’s name to structure containing long integer char hostname[128]; struct hostent *he; gethostname( hostname, sizeof( hostname) ); printf( “My hostname:%s\n”, hostname ); he = gethostbyname( “www.yahoo.com” ); printf( “Address: %s\n”, inet_n_ntoa( he->h_addr ) ); Comnet 2006 - Recitation 2 - Sockets

  19. Useful functions (cont.) • gethostbyaddr(char *addr, int len, int type): converts IP hostname to structure containing long integer • inet_aton(const char *cp, struct in_addr *inp): converts dotted-decimal char-string to binary (network order) • inet_ntoa(const struct in_addr in): converts binary to dotted-decimal string struct hostent *he; struct in_addr addr; inet_aton( “66.94.230.32”, &addr ); printf( “Address: %s\n”, inet_ntoa( addr.sin_addr ) ); he = gethostbyaddr( &addr, sizeof( addr ), AF_INET ); printf( “Name: %s\n”, he->h_name ); Comnet 2006 - Recitation 2 - Sockets

  20. UDP Client-Server UDP Server socket() UDP Client socket() bind() bind() recvfrom() blocks data (request) sendto() process request data (reply) sendto() recvfrom() close() close() Comnet 2006 - Recitation 2 - Sockets

  21. TCP Client-Server TCP Server socket() bind() TCP Client listen() socket() connection establishment 3-way handshake accept() connect() blocks until connection from client write() data (request) read() process request data (reply) read() write() end-of-file notification close() read() Comnet 2006 - Recitation 2 - Sockets close()

More Related