l3 Transport and reliability

l3 Transport and reliability - Lecture 3 Transport and...

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Unformatted text preview: Lecture 3: Transport and Reliability Overview • End-to-end model • User datagram protocol (UDP) • Packet checksums • Reliability: sliding window • TCP connection setup • TCP windows, retransmissions, and acknowledgments An Application View of the World client server Internet • Why doesn’t the network help more?- Compress data- Reformat/improve/translate requests- Respond with cached data- Secure communications- Migrate connections across networks End-to-End Model The function in question can completely and correctly be implemented only with the knowledge and help of the application standing at the end points of the communication system. Therefore, providing that questioned function as a feature of the communication system itself is not possible. (Sometimes an incomplete version of the function provided by the communication system may be useful as a performance enhancement.) We call this line of reasoning...“the end-to-end argument.” – Saltzer, Reed, and Clark, 1984 Strong End-to-End The network’s job is to transmit datagrams as efficiently and flexibly as possible. Everything else should be done at the fringes... – [RFC 1958] • Why? Example: File Transfer • What can go wrong?- File might not arrive at receiver- File could be corrupted in transit • How can we be sure a file arrives successfully?- Send acknowledgment to ensure file arrived at destination- Use checksum to ensure data has not been corrupted • Why do something in the middle?- Network often drops packets because of congestion, sometimes corrupts packets because of transmission errors- Cheaper to resend one packet than whole file- ...or to re-send over one link than through whole network Link-by-link reliability • Link-by-link reliability is not sufficient- MIT network did link-by-link ack and checksum- Thought transmission was most likely source or errors- But problem arose from bad RAM in router- Ended up corrupting a lot of MULTICS source code • Principle applies even above the network layer- E.g., Google learned disk error-correction insufficient... GFS keeps its own application-layer checkums End-to-end & net neutrality • “Dumb” network clearly beneficial...- But what if ISPs want to do complex things? • Require ISPs to treat everyone’s traffic equally? • Pro: prevent extortion from last-mile duopoly- Create a hugely profitable business (next Google)?- Seeing $$$s, ISPs could charge you to reach their customers • Con: reduces ISP flexibility to cope with change- E.g., new applications that clog the network • Current FCC strongly in favor of Net neutrality FCC net neutrality history • 2002: Classifies cable modems as information services- Much less regulated than telephone or TV • 2005: FCC issues Internet Policy Statement :- Users should be able to 1) access any lawful content, 2) run any applications, 3) use any devices, and 4) enjoy competition between network, application, and content providers....
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This note was uploaded on 04/02/2012 for the course CS 144 at Stanford.

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l3 Transport and reliability - Lecture 3 Transport and...

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