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1、2: Application Layer,1,Computer Networking: A Top Down Approach, 2011.9.5,4th edition. Jim Kurose, Keith Ross,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 Sock
2、et programming with TCP 2.8 Socket programming with UDP,2: Application Layer,3,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 po
3、pular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS programming network applications socket API,2: Application Layer,4,Some network apps,e-mail web instant messaging remote login P2P file sharing multi-user network games streaming stored video clips,voice over IP real-time video confer
4、encing Gridcomputing (网格计算),2: Application Layer,5,Creating a network app,write programs that run on (different) end systems communicate over network e.g., web server software communicates with browser software little software written for devices in network core network core devices do not run user
5、applications applications on end systems allows for rapid app development, propagation,2: Application Layer,6,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 Socket programming with TCP
6、 2.8 Socket programming with UDP 2.9 Building a Web server,2: Application Layer,7,Application architectures,Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P,2: Application Layer,8,Client-server architecture,server: always-on host permanent IP address server farms(服务器群 ) for scaling c
7、lients: communicate with server may be intermittently(间歇地 ) connected may have dynamic IP addresses do not communicate directly with each other,2: Application Layer,9,Pure P2P architecture,no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP
8、 addresses example: GnutellaHighly scalable but difficult to manage,2: Application Layer,10,Hybrid of client-server and P2P,Skype voice-over-IP P2P application centralized server: finding address of remote party: client-client connection: direct (not through server) Instant messaging chatting betwee
9、n two users is P2P centralized service: client presence detection/location user registers its IP address with central server when it comes online user contacts central server to find IP addresses of buddies（伙伴）,2: Application Layer,11,Processes communicating(进程通信),Process: program running within a h
10、ost. within same host, two processes communicate using inter-process communication (defined by OS). processes in different hosts communicate by exchanging messages（报文）,Client process: process that initiates communication Server process: process that waits to be contacted,Note: applications with P2P
11、architectures have client processes & server processes,2: Application Layer,12,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
12、socket at receiving process,Internet,controlled by OS,controlled by app developer,API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later),2: Application Layer,13,Addressing(寻址) processes,to receive messages, process must have identifier host device has un
13、ique 32-bit IP address Q: does IP address of host on which process runs suffice for identifying the process?,2: Application Layer,14,Addressing processes,to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address of host on which process runs suffic
14、e for(满足需求) identifying the process? A: No, many processes can be running on same host,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 gaia.cs.umass.edu web server: IP address: 128.119
15、.245.12 Port number: 80 more shortly,2: Application Layer,15,App-layer protocol defines,Types of messages exchanged, e.g., request, response Message syntax: what fields in messages & how fields are delineated(描述) Message semantics(语义) meaning of information in fields Rules for when and how processes send & respond to messages,Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary(专有) protocols: e.g., Skype,2: Application Layer,16,What transport service does an app need?,
