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Chapter 2: Application layer

Explore the challenges faced in VoIP communication, such as Quality of Service (QoS), security concerns, and integration with traditional phone systems. Learn about solutions and best practices to overcome these challenges.

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Chapter 2: Application layer

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

  2. Definition • also called IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband • the routing of voice conversations over the Internet or through any other IP-based network Cisco IP Phone 7941G 2: Application Layer

  3. Big Picture • Modes of operation: • PC to PC • PC to phone • Phone to PC • Phone to Phone • Traffic go through Packet Switched Network instead of Public Switched Telephone Network (PSTN) From Wikipedia, the free encyclopedia 2: Application Layer

  4. Challenges • Quality of Service (QoS) • Internet provides best of service • No guarantee for latency, jitter… • Need Internet connection • Home broadband is not reliable • Power issue • VOIP phone, Cable Modem/DSL, Computer • Primary reason for not using VOIP for emergency calls • Second reason is location identification is hard for VOIP 2: Application Layer

  5. Challenges • Security • Most unencrypted • VOIP spam challenges • Integration into global telephone number system • Emergency call availability & functionality • Power, Internet connection • Call routing, location service 2: Application Layer

  6. QoS • Deal with Jitter • Smoothed by playback buffer • Will cause more delay in playback • Too much delayed packets will be discard (dropped) • Bandwidth • 64 kbps or less • Depends on codec and use of silence suppression 2: Application Layer

  7. 2.1 Principles of network applications 2.2 Web and HTTP Internet gaming 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS Chapter 2: Application layer • 2.6 P2P file sharing • VOIP • 2.7 Socket programming with TCP • 2.8 Socket programming with UDP 2: Application Layer

  8. 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

  9. Socket programming goal: learn how to build client/server applications that communicate using sockets socket: door between application process and end-end-transport protocol application application socket controlled by app developer process process transport transport controlled by OS network network link Internet link physical physical Application Layer

  10. 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

  11. TCP Socket Programming Using Python • Python is simpler than C to learn • The 6-th edition textbook uses Python to illustrate the example • Our Unix server support Python and Java programming • Python is a scripting language • No need to compile • Python tutorial • http://www.sthurlow.com/python/ • http://anh.cs.luc.edu/python/hands-on/handsonHtml/handson.html 2: Application Layer

  12. TCP connection setup create socket, connect to hostid, port=x create socket, port=x, for incoming request: clientSocket = socket() serverSocket = socket() wait for incoming connection request connectionSocket = serverSocket.accept() send request using clientSocket read request from connectionSocket write reply to connectionSocket read reply from clientSocket close connectionSocket close clientSocket Client/server socket interaction: TCP client server Application Layer

  13. Example app: TCP client Python TCPClient from socket import * serverName = ’servername’ #put in server name here serverPort = 12000 #port number > 1024 clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’) clientSocket.send(sentence) modifiedSentence = clientSocket.recv(1024) print ‘From Server:’, modifiedSentence clientSocket.close() create TCP socket for server, remote port 12000 No need to attach server name, port Application Layer

  14. Example app: TCP server Python TCPServer from socket import * serverPort = 12000 serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((‘’,serverPort)) # ‘’ represent symbolic name, meaning all available interfaces serverSocket.listen(1) print ‘The server is ready to receive’ while 1: connectionSocket, addr = serverSocket.accept() sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close() create TCP welcoming socket server begins listening for incoming TCP requests loop forever server waits on accept() for incoming requests, new socket created on return read bytes from socket (but not address as in UDP) close connection to this client (but not welcoming socket) Application Layer

  15. 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

  16. 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

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

  18. C Programming Header Files #include <stdio.h> #include <stdlib.h> #include <string.h> #include <netdb.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <unistd.h> • Enough for our socket programming in our Unix server, maybe not enough for other computers 2: Application Layer

  19. 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 • FILE *fid; fid=fopen(“test.txt”, “rt”); • 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

  20. 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

  21. 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(&(local_addr.sin_zero), '\0', 8); // zero the rest of the struct Local host info bind(sockfd, (struct sockaddr *)&local_addr, sizeof(struct sockaddr)); If you do not designate a specific client port, then no need to use the ‘Bind’ API in client code 2: Application Layer

  22. Remote Host Structure feather.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 */ www.cs.ucf.edu “10.173.204.109” struct hostent *hp; hp = gethostbyname(“www.cs.ucf.edu”); struct sockaddr_in remote_addr; remote_addr.sin_family = AF_INET; remote_addr.sin_port = htons(80); // short, network byte order (big-endian) 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

  23. 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]; …. numByteSend=send(sockfd, sendStr, sendByteNum), 0); … recvNumByte = recv(sockfd, recvStr, MaxDataSize, 0); close(sockfd); 2: Application Layer

  24. Complete Client Code Outline int sockfd; struct sockaddr_in local_addr, remote_addr; sockfd = socket(AF_INET, SOCK_STREAM, 0); Assign remote_addr variable (Page 22) connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr); char sendStr[100], recvStr[100]; …. numByteSend=send(sockfd, sendStr, sendByteNum), 0); … recvNumByte = recv(sockfd, recvStr, MaxDataSize, 0); close(sockfd); 2: Application Layer

  25. Partial Send() and recv() Due to multiple packets in transmission #include <sys/types.h> #include <sys/socket.h> int sendall(int sockfd, 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(sockfd, 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

  26. Socket Programming in Server • Still need to initialize local_addr struct (page 21) • No need to initialize remote_addr struct (page 22) • No need to connect() a remote host • Instead, need to listen() on specified port (server port) • Accept() a connection request • Generate a new socket for one connection • Support multiple connections 2: Application Layer

  27. Complete Server Code Outline int sockfd, new_fd; struct sockaddr_in local_addr, remote_addr; assign local_addr (see Page 21 local host part. Change “local_addr.sin_port = 0;” to “local_addr.sin_port = htons(serverPort);” ) socket(…); // create empty socket descriptor bind(…); //fill in local address and assigned port to the socket descriptor listen(sockfd, backLog); // backLog is the max no. of connections in queue new_fd = accept(sockfd, (struct sockaddr *)&remote_addr, &sizeof(struct sockaddr_in)) char sendStr[100], recvStr[100]; …. recvNumByte = recv(new_fd, recvStr, MaxDataSize, 0); ….. numByteSend=send(new_fd, sendStr, sendByteNum), 0); … close(new_fd); close(sockfd); New socket discriptor Following commun. through this 2: Application Layer

  28. 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 (fork() returns 0 in 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 …………. } parent child parent 2: Application Layer

  29. Fork() • Tuotrial on fork(): http://www.erlenstar.demon.co.uk/unix/faq_2.html • System call fork() is used to create child process. It returns a process ID. After a new child process is created, both processes will execute the next instruction following the fork() system call. • On success: • PID of the child process is returned in the parent's thread of execution • 0 is returned in the child's thread of execution 2: Application Layer

  30. 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

  31. 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

  32. 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

  33. 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 • Some applications • Web • Email • DNS • Internet gaming, VOIP • P2P • socket programming 2: Application Layer

  34. 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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