Chapter 2: Application layer 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 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

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

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

Challenges Quality of Service (QoS) Need Internet connection 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

Challenges Security Integration into global telephone number system 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

QoS Deal with Jitter Bandwidth 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

Chapter 2: Application layer 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 2.6 P2P file sharing VOIP 2.7 Socket programming with TCP 2.8 Socket programming with UDP 2: Application Layer

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

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

Socket programming with TCP 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 2: Application Layer

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

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 1 2 3 4 2: Application Layer

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

What is a Socket? socket returns an integer (socket descriptor) 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

Socket Structure (Client) AF_INET 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 }; // 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 100 101 102 103 … … 1A 3C 4B 2D Big-Endian (Network Byte Order) 2: Application Layer

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 2: Application Layer

Remote Host Structure struct hostent { 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 hostent *hp; hp = gethostbyname(“mail.cs.ucf.edu”); “132.170.108.1” 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

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

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

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 int sockfd, new_fd; struct sockaddr_in local_addr, remote_addr; // assign local_addr (remember to use hton() to assign server port!) 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)) New socket discriptor Following commun. through this 2: Application Layer

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

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

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 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

Socket programming with UDP 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 application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server 2: Application Layer

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

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

Chapter 2: Summary Most importantly: learned about protocols 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 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