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

2. 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

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

4. 2: Application Layer 4 Challenges r Quality of Service (QoS)  Internet provides best of service  No guarantee for latency, jitter… r Need Internet connection  Home broadband is not reliable r 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

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

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

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

8. 2: Application Layer 8 Socket programming Socket API r introduced in BSD4.1 UNIX, 1981 r explicitly created, used, released by apps r client/server paradigm r two types of transport service via socket API:  unreliable datagram (UDP)  reliable, byte stream-oriented (TCP) Goal: learn how to build client/server application that communicate using sockets

9. 2: Application Layer 9 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 process TCP with buffers, variables socket controlled by application developer controlled by operating system host or server process TCP with buffers, variables socket host or server internet

10. 2: Application Layer 10 Socket programming with TCP Client must contact server r server process must first be running r server must have created socket (door) that accepts client’s contact Client contacts server by: r creating client-local TCP socket r specifying IP address, port number of server process r When client creates socket: client TCP establishes connection to server TCP r 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

11. 2: Application Layer 11 Many Versions of Socket APIs r Unix socket (berkeley socket) r Winsock r MacTCP r …. r We introduce Unix socket API here  Can program under SUN OS, Linux, etc  A good tutorial on socket programming: http://beej.us/guide/bgnet/

12. 2: Application Layer 12 Socket Descriptor Data Structure Descriptor Table 0 1 2 3 4 Family: AF_INET Service: SOCK_STREAM Local IP: 111.22.3.4 Remote IP: 123.45.6.78 Local Port: 2249 Remote Port: 3726 Family: AF_INET Service: SOCK_STREAM Local IP: 111.22.3.4 Remote IP: 123.45.6.78 Local Port: 2249 Remote Port: 3726

13. 2: Application Layer 13 TCP Client/Server Socket Overview socket() bind() listen() accept() send() recv() close() socket() TCP Client connect() send() recv() close() connection establishment data request data reply end-of-file notification TCP Server bind()

14. 2: Application Layer 14 What is a Socket? r 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 ” ); r AF_INET: associates a socket with the Internet protocol family r SOCK_STREAM: selects the TCP protocol r SOCK_DGRAM: selects the UDP protocol int sockfd; /* socket descriptor */ if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) } perror(“socket”); exit(1); }

15. 2: Application Layer 15 Socket Structure (Client) struct sockaddr_in { short int sin_family; // Address family unsigned short int sin_port; // Port number struct in_addr sin_addr; // Internet address unsigned char sin_zero[8]; // all zero }; AF_INET // 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 }; … … Big-Endian (Network Byte Order) 101103 102 1A 2D 3C 4B IP: 1A.2D.3C.4B 100

16. 2: Application Layer 16 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 sockfd = socket(AF_INET, SOCK_STREAM, 0); // create an empty socket bind(sockfd, (struct sockaddr *)&local_addr, sizeof(struct sockaddr)); Local host info

17. 2: Application Layer 17 Remote Host Structure hostent *hp; hp = gethostbyname(“mail.cs.ucf.edu”); “132.170.108.1” 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 */ Longwood.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 mail.cs.ucf.edu Remote host info

18. 2: Application Layer 18 Connect(), send(), recv() by Client connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr); Struct sockaddr  sockaddr_in After connecting to the remote sever…. char sendStr[100], recvStr[100]; …. send(sockfd, sendStr, strlen(sendStr), 0); … recvNumByte = recv(sockfd, recvStr, MaxDataSize, 0); close(sockfd); Blocking call Remote host info Local host socket

19. 2: Application Layer 19 Partial Send() and recv() Due to multiple packets in transmission #include 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 }

20. 2: Application Layer 20 Socket Programming in Server r No need to connect() a remote host r Need to listen() on specified port r Accept() a connection request  Generate a new socket for one connection Support multiple connections int sockfd, new_fd; struct sockaddr_in local_addr, remote_addr; // assign local_addr socket(…); bind(…); listen(sockfd, backLog); // backLog is the max no. of connections in queue new_fd = accept(sockfd, (struct sockaddr *)&remote_addr, &sizeof(struct sockaddr_in)) New socket discriptor Following commun. through this

21. 2: Application Layer 21 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 …………. }

22. 2: Application Layer 22 Fork() r Tuotrial on fork(): http://www.erlenstar.demon.co.uk/unix/faq_2.html http://www.erlenstar.demon.co.uk/unix/faq_2.html r 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. r 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

23. 2: Application Layer 23 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 VOIP r 2.8 Socket programming with TCP r 2.9 Socket programming with UDP r 2.10 Building a Web server

24. 2: Application Layer 24 Socket programming with UDP UDP: no “connection” between client and server r no handshaking r sender explicitly attaches IP address and port of destination to each packet r server must extract IP address, port of sender from received packet UDP: transmitted data may be received out of order, or lost application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

25. 2: Application Layer 25 UDP Socket Programming r sockfd = socket(AF_INET, SOCK_DGRAM, 0) r No connect(), accept() r Send()  sendto(), recv()  recvfrom()  Sendto() includes target address/port SOCK_STREAM (tcp)

26. 2: Application Layer 26 Chapter 2: Summary r Application architectures  client-server  P2P  hybrid r application service requirements:  reliability, bandwidth, delay r Internet transport service model  connection-oriented, reliable: TCP  unreliable, datagrams: UDP Our study of network apps now complete! r specific protocols:  HTTP  FTP  SMTP, POP, IMAP  DNS r Some applications  Web  Email  DNS  Internet gaming, VOIP  P2P r socket programming

27. 2: Application Layer 27 Chapter 2: Summary r typical request/reply message exchange:  client requests info or service  server responds with data, status code r message formats:  headers: fields giving info about data  data: info being communicated Most importantly: learned about protocols r control vs. data msgs  in-band, out-of-band (ftp) r centralized vs. decentralized r stateless vs. stateful r reliable vs. unreliable msg transfer r “complexity at network edge”