2: Application Layer1Chapter 2Application LayerComputer Networking: A Top Down Approach, 4thedition. Jim Kurose, Keith RossAddison-Wesley, July 2007. A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved2: Application Layer2Chapter 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 2.9 Building a Web server2: Application Layer3Creating a network appwrite programs that run on (different) end systems communicate over network e.g., web server software communicates with browser softwareNo need to write software for network-core devices Network-core devices do not run user applications applications on end systems allows for rapid app development, propagationapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysical2: Application Layer4Application architectures Client-server Peer-to-peer (P2P) Hybrid of client-server and P2PNetwork architecture is fixed and provides a specific set of services to applications2: Application Layer5Client-server architectureserver: always-on host permanent IP address server farms for scalingclients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each otherclient/serverexamples: web,ftp,telnet,email2: Application Layer6Pure P2P architecturenoalways-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addressesHighly scalable but difficult to managepeer-peerexample: file distribution (bit torrent), IPTV (PPLive)Challenges to Increased Use of P2P Architectures Most residential ISPs have been dimensioned for more downstream traffic than upstream traffic P2P applications shift upstream traffic from servers to residential ISPs, stressing the system P2P applications tend to be highly distributed and open, making them hard to secure Increased incentives to users to volunteer bandwidth, storage, and computational resources2: Application Layer72: Application Layer8Hybrid of client-server and P2PSkype voice-over-IP P2P application centralized server: finding address of remote party: client-client connection: direct (not through server) Instant messaging chatting between 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 buddies2: Application Layer9Processes communicatingProcess: 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 messagesClient process: process that initiates communicationServer process: process that waits to be contacted Note: applications with P2P architectures have client processes & server processes2: Application Layer10Sockets 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 processprocessTCP withbuffers,variablessockethost orserverprocessTCP withbuffers,variablessockethost orserverInternetcontrolledby OScontrolled byapp developer API: (1) choice of transport protocol; (2) ability to fix a few parameters2: Application Layer11Addressing processes to receive messages, process must have identifier host device has unique 32-bit IP addressQ:does IP address of host suffice for identifying the process?2: Application Layer12Addressing processes to receive messages, process must have identifier host device has unique 32-bit IP addressQ:does IP address of host on which process runs suffice for identifying the process?A:No, manyprocesses can be running on same hostidentifierincludes 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: 128.119.245.12 Port number: 80 more shortly…2: Application Layer13App-layer protocol defines Types of messages exchanged, e.g., request, response Message syntax: what fields in messages & how fields are delineated Message semantics meaning of information in fields Rules for when and how processes send & respond to messagesPublic-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP use a well-known portProprietary protocols: e.g., Skype should not use a well-known port2: Application Layer14What 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 transferTiming some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”Throughput Rate at which the sending application can deliver data to the receiving application some apps (e.g., multimedia) require minimum amount of throughput to be “effective” other apps (“elastic apps”) make use of whatever throughput they get Security Encryption, data integrity, …2: Application Layer15Transport service requirements of common appsApplicationfile transfere-mailWeb documentsreal-time audio/videostored audio/videointeractive gamesinstant messagingData lossno lossno lossno
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