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Chapter 2: Application layer

2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS. Chapter 2: Application layer. 2.6 P2P file sharing 2.7 VOIP 2.8 Socket programming with TCP 2.9 Socket programming with UDP 2.10 Building a Web server. Socket programming.

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Chapter 2: Application layer

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  1. 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS Chapter 2: Application layer • 2.6 P2P file sharing • 2.7 VOIP • 2.8 Socket programming with TCP • 2.9 Socket programming with UDP • 2.10 Building a Web server 2: Application Layer

  2. Socket programming Goal: learn how to build client/server application that communicate using sockets Socket API • introduced in BSD4.1 UNIX, 1981 • explicitly created, used, released by apps • client/server paradigm • two types of transport service via socket API: • unreliable datagram (UDP) • reliable, byte stream-oriented (TCP) 2: Application Layer

  3. process process TCP with buffers, variables TCP with buffers, variables socket socket Socket-programming using TCP Socket: an interface between application process and end-end-transport protocol (UCP or TCP) Why socket?: A Layer seen by application, OS transparent controlled by application developer controlled by operating system internet host or server host or server 2: Application Layer

  4. Client must contact server server process must first be running server must have created socket (door) that accepts client’s contact Client contacts server by: creating client-local TCP socket specifying IP address, port number of server process When client creates socket: client TCP establishes connection to server TCP When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with multiple clients source port numbers used to distinguish clients (more in Chap 3) TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint Socket programming with TCP 2: Application Layer

  5. Many Versions of Socket APIs • Unix socket (berkeley socket) • Winsock • MacTCP • …. • We introduce Unix socket API here • Can program under SUN OS, Linux, etc • A good tutorial on socket programming: • http://beej.us/guide/bgnet/ 2: Application Layer

  6. Unix Descriptor Table Descriptor Table Data structure for file 0 0 1 Data structure for file 1 2 3 Data structure for file 2 4 2: Application Layer

  7. Socket Descriptor Data Structure Descriptor Table Family: AF_INET Service: SOCK_STREAM Local IP: 111.22.3.4 Remote IP: 123.45.6.78 Local Port: 2249 Remote Port: 3726 0 1 2 3 4 2: Application Layer

  8. TCP Client/Server Socket Overview TCP Server socket() TCP Client bind() socket() listen() bind() accept() connection establishment connect() data request recv() send() data reply send() recv() end-of-file notification recv() close() close() 2: Application Layer

  9. What is a Socket? int sockfd; /* socket descriptor */ if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) } perror(“socket”); exit(1); } • socket returns an integer (socket descriptor) • sockfd < 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 2: Application Layer

  10. Socket Structure (Client) AF_INET struct sockaddr_in { short int sin_family; // Address family unsigned short int sin_port; // Port number struct in_addrsin_addr; // Internet address unsigned char sin_zero[8]; // all zero }; // Internet address (Network Byte Order) // (a structure for historical reasons) struct in_addr { unsigned long s_addr; // that's a 32-bit long, or 4 bytes }; IP: 1A.2D.3C.4B 101 103 100 102 … … 4B Big-Endian (Network Byte Order) 1A 3C 2D 2: Application Layer

  11. Bind (Client) int sockfd; struct sockaddr_in local_addr; local_addr.sin_family = AF_INET; local_addr.sin_port = 0; // random assign a port local_addr.sin_addr.s_addr = INADDR_ANY; // use my IP address memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct sockfd = socket(AF_INET, SOCK_STREAM, 0); // create an empty socket bind(sockfd, (struct sockaddr *)&local_addr, sizeof(struct sockaddr); Local host info 2: Application Layer

  12. Remote Host Structure Longwood.cs.ucf.edu struct hostent { char *h_name; /* official name */ char **h_aliases; /* alias list */ int h_addrtype; /* address type */ int h_length; /* address length */ char **h_addr_list; /* address list */ }; #define h_addr h_addr_list[0] /* backward compatibility */ mail.cs.ucf.edu “132.170.108.1” hostent *hp; hp = gethostbyname(“mail.cs.ucf.edu”); struct sockaddr_in remote_addr; remote_addr.sin_family = AF_INET; remote_addr.sin_port = htons(80); // short, network byte order remote_addr.sin_addr = *((struct in_addr *)hp->h_addr); memset(&(remote_addr.sin_zero), '\0', 8); // zero the rest Remote host info 2: Application Layer

  13. Connect(), send(), recv() by Client connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr); Remote host info Local host socket Structsockaddr  sockaddr_in After connecting to the remote sever…. Blocking call char sendStr[100], recvStr[100]; …. send(sockfd, sendStr, strlen(sendStr), 0); … recvNumByte = recv(sockfd, recvStr, MaxDataSize, 0); close(sockfd); 2: Application Layer

  14. Partial Send() and recv() Due to multiple packets in transmission #include <sys/types.h> #include <sys/socket.h> int sendall(int s, char *buf, int *len) { int total = 0; // how many bytes we've sent int bytesleft = *len; // how many we have left to send int n; while(total < *len) { n = send(s, buf+total, bytesleft, 0); if (n == -1) { break; } total += n; bytesleft -= n; } *len = total; // return number actually sent here return n==-1?-1:0; // return -1 on failure, 0 on success } 2: Application Layer

  15. Non-Blocking Call and Select() fcntl(sockfd, F_SETFL, O_NONBLOCK); Con: poll data, waste CPU time fcntl(sockfd, F_SETFL, O_ASYNC); // set to asynchronous I/O Set the socket to do asynchronous I/O. When data is ready to be recv()'d on the socket, the signal SIGIO will be raised. With multiple sockets, use select() to determine which socket gives the signal SIGIO. 2: Application Layer

  16. Socket Programming in Server • No need to connect() a remote host • Need to listen() on specified port • Accept() a connection request • Generate a new socket for one connection • Support multiple connections listen(sockfd, backLog); // backLog is the number of connections in queue new_fd = accept(sockfd, (struct sockaddr *)&remote_addr, &sizeof(struct sockaddr_in)) New socket discriptor Following commun. through this 2: Application Layer

  17. Socket Programming in Server: fork() for multi-connection service • while(1) { // main accept() loop • sin_size = sizeof(struct sockaddr_in); • new_fd = accept(sockfd, (struct sockaddr *)&remote_addr, &sin_size); • printf("server: got connection from %s\n", inet_ntoa(remote_addr.sin_addr)); • if (!fork()) { // this is the child process • close(sockfd); // child doesn't need the listener • send(new_fd, "Hello, world!\n", 14, 0); • close(new_fd); exit(0); • } • close(new_fd); // parent doesn't need this • } See the following link for tuotrial on fork(): http://www.erlenstar.demon.co.uk/unix/faq_2.html 2: Application Layer

  18. 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS Chapter 2: Application layer • 2.6 P2P file sharing • 2.7 VOIP • 2.8 Socket programming with TCP • 2.9 Socket programming with UDP • 2.10 Building a Web server 2: Application Layer

  19. UDP: no “connection” between client and server no handshaking sender explicitly attaches IP address and port of destination to each packet server must extract IP address, port of sender from received packet UDP: transmitted data may be received out of order, or lost UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server application viewpoint Socket programming with UDP 2: Application Layer

  20. UDP Socket Programming • sockfd = socket(AF_INET, SOCK_DGRAM, 0) • No connect(), accept() • Send()  sendto(), recv()  recvfrom() • Sendto() includes target address/port SOCK_STREAM (tcp) 2: Application Layer

  21. Application architectures client-server P2P hybrid application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP Our study of network apps now complete! Chapter 2: Summary • specific protocols: • HTTP • FTP • SMTP, POP, IMAP • DNS • socket programming 2: Application Layer

  22. typical request/reply message exchange: client requests info or service server responds with data, status code message formats: headers: fields giving info about data data: info being communicated Most importantly: learned about protocols Chapter 2: Summary • control vs. data msgs • in-band, out-of-band (ftp) • centralized vs. decentralized • stateless vs. stateful • reliable vs. unreliable msg transfer • “complexity at network edge” 2: Application Layer

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