Algorithms and protocols

Transcription

1 : Chapter 11 by David G. Messerschmitt : Algorithms and protocols by David G. Messerschmitt 1

2 Algorithm Specified sequence of steps that accomplish a designated task in a finite number of steps Representation: simple algorithm: flowchart complicated algorithm: program 3 Example: one turn at monopoly Start turn Throw dice Move token number of squares indicated on dice Move to jail square Yes Land on go to jail? No Finish turn Do not move; follow policies for square (like pay rent ) 4 2

3 Algorithm building blocks Start Start Start Decision Programming languages support these three building blocks Test Finish Finish Finish Sequence Selection Loop 5 Protocol Distributed algorithm realized by two or more modules to coordinate their actions or accomplish some shared task Module interoperability requires a protocol Prescribed order of method invocations Part of interface documentation 6 3

4 Monopoly players protocol Player 1 Player 2 One-turn algorithm This is a protocol interaction diagram Time 7 Application and infrastructure The application defines its own application-level protocols Internally, the network uses protocols to implement the services it provides 8 4

5 Three simple protocols One-way message: send-receive Two-way interaction: request-response Push: publish-subscribe 9 Client Server send receive request response subscribe responses Time 10 5

6 Example: HTTP ❶ User activates URL ❷ HTTP request HTTP client (browser) HTTP server ❹ Browser displays document (if HTML) or invokes helper application ❸ HTTP response (embedded document) HTML documents 11 Example: Stock trading Customer Customer wants to purchase 10 shares of MSFT if price is below $90 StockTrader What is application protocol? 12 6

7 : Locating things by David G. Messerschmitt Three ways of locating things Name Joe Bloe Address 1299 Hearst St, Berkeley, CA Reference Postmaster of Berkeley CA 14 7

8 Name Symbolic (character string) representation Easy for people to remember or guess Identifies but does not locate directly Distinction important for mobile entities Not unique: entities can have more than one name (called aliases) 15 Hierarchical names Hierarchy makes names easier to remember or guess Host domain names: info.sims.berkeley.edu designates administrative hierarchy File names: c:\my Documents\Docs\Resume.doc designates folder hierarchy 16 8

9 Address Route or path to entity is directly specified, or can be inferred Independent of who or what is locating entity Topological specification 17 Example Path from to is (1,2,2,2,1,1,1,1) Not an address, because it depends on starting point 18 9

10 Example (1,1) (1,2) (1,3) (1,4) (1,4) (2,1) (2,2) (2,3) (2,4) (2,5) Address of is (2,5) Route from can be inferred 19 Reference Abstract representation of an entity Interaction is with representation infrastructure arranges redirection to actual entity especially appropriate for things that move 20 10

11 Client Reference Server address 21 Name services 1. name 3. interaction 2. address or reference Name service 22 11