计算机网络Chapter2-cut.ppt

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 Our goals: conceptual, implementation aspects of network application protocols transport-layer service models client-server paradigm peer-to-peer paradigm learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS programming network applications socket API Some network apps E-mail Web Instant messaging Remote login P2P file sharing Multi-user network games Streaming stored video clips Internet telephone Real-time video conference Massive parallel computing Client-server architecture server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other Processes communicating Process: program running within a host. within same host, two processes communicate using inter-process communication (defined by OS). processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted Sockets process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process Addressing processes identifier includes both IP address and port numbers associated with process on host. Example port numbers: HTTP server: 80 Mail server: 25 to send HTTP message to web server: IP address: 2 Port number: 80 more shortly… to receive messages, process must have identifier host device has unique32-bit IP address Q: does IP address of host on which process runs suffice for identifying the process? Answer: NO, many processes can be running on same host What transport service