1.1 application - Application Layer Dr Shengquan Wang Some...

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Application Layer Dr. Shengquan Wang Some slides are modified from Computer Networking: A Top Down Approach, 6th edition. Jim Kurose, Keith Ross. Addison-Wesley, March 2012.
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Review: Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: end-to-end reliable/unreliable transport TCP, UDP network: best-effort global packet delivery IP, routing protocols link: best-effort local packet delivery PPP, Ethernet, Wi-Fi physical: local physical transfer of bits application transport network link physical 2
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Some network apps e-mail web instant messaging remote login P2P file sharing multi-user network games social networks voice over IP streaming stored video clips (YouTube …) real-time video conferencing live broadcasting 3
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Outline Principles of network applications 4
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Creating a network app Write programs that run on (different) end systems communicate over network e.g., web server software communicates with browser software No 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, propagation applicatio n transport network data link physical applicatio n transport network data link physical applicatio n transport network data link physical 5
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Application architectures Client-server Including data centers / cloud computing Peer-to-peer (P2P) 6
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Client-server architecture server: always-on host fixed address data centers for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other client/server 7
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Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses examples: file distribution (BitTorrent) VoIP (Skype) 8
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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 Note: applications with P2P architectures have client processes & server processes 9
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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 suffice for 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.
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