计算机网络课件chapter 2a.ppt

Chapter 2: Application Layer Key point: The principles of application-layer protocols HTTP (Hypertext Transfer Protocol) FTP (File Transfer Protocol) SMTP (Simple Mail Transfer Protocol) DNS (Domain Name System) TCP/IP protocol Some jargon A process is a program that is running within a host. Processes running in different hosts communicate with an application-layer protocol A user agent is an interface between the user and the network application. Web: browser E-mail: mail reader streaming audio/video: media player application-layer protocols (cont.) Application-layer protocols one “piece” of an app define messages exchanged by apps and actions taken the type of messages exchanged the syntax of the various message types the semantics of the fields rules for determining when and how a process sends messages and responds to messages user services provided by lower layer protocols Clients and servers Typical network app has two pieces: client and server Processes communicating socket: process’s door two processes communicate by sending data into socket, reading data out of socket API: application programming interface: defines interface between application and transport layer See figure 2.3 Addressing processes Q: how does a process “identify” the other process with which it wants to communicate? IP address of host running other process “port number” - allows receiving host to determine to which local process the message should be delivered What transport service does an app need? Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Transport service requirements of common apps Services provided by Internet transport protocols TCP service: connection-oriented: setup required between client, server reliable transport between sending and receiving process flow control: sender won’t