COT 4600 Operating Systems Spring 2011 Dan C. Marinescu Office: HEC 304 Office hours: Tu-Th 5:00 – 6:00 PM

Last time: URLs Soft modularity  Procedure call conventions and the memory map; the stack  Errors  Strongly-typed languages help enforce modularity. Enforced modularity; message passing and the client-server model.  Example of a client –server system: WWW Today:  Today  Client-server organization  Intermediaries; Trusted Intermediaries; Thin clients  Issues  Heterogeneity; little-endian and big-endian representation  Timing, response time.  Examples: Event service, X11; Trusted intermediaries: , File Systems, Web  Remote Procedure Call  Next time Strategies for name resolution Case study DNS Lecture 11 – Thursday, February 17, 2011 Lecture 112

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4 Client/service organization Not only separates functions but also enforces this separation!!  No globally-shared state (e.g., through the stack)  Errors can only propagate from client to service and vice-versa only if the messages are not properly checked.  A client can use time-outs to detect a non-responsive service and take another course of action.  The separation of abstraction from implementation is clearer; the client needs only to know the format of the message, the implementation of the service may change. Lecture 11

Examples of client-server systems Event service X-windows Electronic mail as a trusted intermediary Files systems as trusted intermediaries The Word Wide Web The Domain Name System Lecture 115

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8 Example: the X-windows (X11) X11  software system and network protocol that provides a GUI for networked computer. Developed as part of Project Athena at MIT in Separates  the service program  manipulates the display from  the client program  uses the display. An application running on one machine can access the display on a different computer. Clients operate asynchronously, multiple requests can be sent  the display rate could be much higher than the rate between the client and the server. Lecture 11

9 Editor is a client of  File service which is a client of  Block-storage service File service is a trusted intermediary. Lecture 11

10 A client-service system  the World Wide Web The information in each page is encoded and formatted according to some standard, e.g.  images: GIF, JPEG,  video: MPEG  audio: MP3 The web is based upon a “pull” paradigm. The server has the resources and the client pulls it from the server. The Web server also called an HTTP server listens at a well known port, port 80 for connections from clients. The HTTP protocol uses TCP to establish a connection between the client and the server. Some pages are created on the “fly” other have to be fetched from the disk. Lecture 11

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13 Client server interactions in HTTP Lecture 11

14 RPC semantics At least once  the client stub resends a message up to a given number of times until it receives a message from the server; is no guarantee of a response  the server may end up executing the a request more than once  suitable for side-effect free operations At most once  a message is acted upon at most once.  If the timeout set for receiving the response expires then an error code is delivered to the client.  The server must keep a history of the time-stamps of all messages. Messages may arrive out of order…..  suitable for operations which have side effects Exactly once  implement the at most once and request an acknowledgment from the server. Lecture 11