2: Application Layer 1 Chapter 2 Application Layer Computer Networking: A Top Down Approach, 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

2: Application Layer 2 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 applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP

2: Application Layer 3 recv and send  recv may be used instead of read (and is recommended): n = recv(fd, buffer, count, flags);  The first three arguments are the same as for read. The flags argument provides additional flexibility. Interesting flags are MSG_DONTWAIT and MSG_PEEK. Refer to recv(2) for details.

2: Application Layer 4 recv and send  Similarly send may be used instead of write (and is also recommended): n = send(fd, buffer, count, flags);  Interesting flags are MSG_DONTWAIT, MSG_MORE and MSG_DONTROUTE. Refer to send(2) for details.

2: Application Layer 5 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 applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP

2: Application Layer 6 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 7 Client/server socket interaction: UDP Server (running on hostid ) close close() read datagram using recvfrom create socket, clifd = socket() Client Create datagram with server IP and port=x; send datagram via sendto create socket, port= x. srvfd = socket() bind() read datagram using recvfrom write reply using sendto specifying client address, port number

2: Application Layer 8 UDP Programming  The client functions for passing UDP datagrams are: socket(), sendto(), and recvfrom(). It is not necessary for the client to connect() with the server. (Although connect() can be used.)  The server uses socket(), bind(), recvfrom(), and sendto(). The server does not accept() a connection from the client, instead recvfrom() returns the data and client address.

2: Application Layer 9 UDP Programming  The sendto() routine takes a destination address structure as an argument: typedef struct sockaddr SA; struct sockaddr_in srvaddr_in; memset(&srvaddr_in, 0, sizeof(srvaddr_in)); srvaddr.sin_family = AF_INET; srvaddr.sin_port = htons(9000); inet_pton(AF_INET, “ ”, &srvaddr_in.sin_addr.s_addr); sfd = socket(AF_INET, SOCK_DGRAM, 0); sendto(sfd, msg, msglen, 0, (SA *)&srvaddr_in, sizeof(srvaddr_in));

2: Application Layer 10 UDP Programming  recvfrom() returns an address as an argument. The address is used by the server to respond to the proper client: n = recvfrom(sockfd, mesg, mesg_len, 0, (SA *)&cliaddr_in, &len); sendto(sockfd, mesg, n, 0, (SA *)&cliaddr_in, len);  A NULL address may be used in recvfrom() if we do not care about the address.  Refer to udp_server.cpp for a complete example of UDP socket programming (a modified echo server).  In-class Exercise: Write a udp_client.cpp program to communicate with udp_server.cpp. It should prompt the user for input, send the input to the server, and then read and display the response.

2: Application Layer 11 Chapter 2: Summary  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!  specific protocols:  HTTP  FTP  SMTP, POP, IMAP  DNS  P2P: BitTorrent, Skype  socket programming

2: Application Layer 12 Chapter 2: Summary  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 Important themes:  control vs. data msgs  in-band, out-of-band  centralized vs. decentralized  stateless vs. stateful  reliable vs. unreliable msg transfer  “complexity at network edge”