CGS 3763 Operating Systems Concepts Spring 2013

Dan C. Marinescu

Office: HEC 304

Office hours: M-Wd 11:30 - 12:30 AM

Last time: Solutions to HW1 Student questions for week 4 More network concepts

Today: Sockets Client-server systems Intermediaries, trusted intermediaries Examples of client-server systems: event service, email, X-windows, Web

Next time Heterogeneity and impact on client server-systems Remote Procedure Calls (RPCs)

Reading assignments Chapters 3 and 4 of the textbook Chapters 3 and 4 textbook slides

Lecture 15 – Monday, February 11, 2013

2Lecture 15

Multiplexing and de-multiplexing

Lecture 15 3

Application Layer

Transport Layer

Network Layer

HTTP FTP TELNET

TCP UDP

IP

NFS RPC DNS SNTP

Data Link Layer

Ethernet WirelessSatellite

Ports and socket

Lecture 15 4

Host

Process

Socket

Input message queue

Output message queue

Port Internet

5

Host

Process

Network Network

RouterNetwork interface

Port

IP address = (NetworkId, HostId)

Message delivery to processes

Host

Process

Socket

Input message queue

Output message queue

PortInternet

Sockets and portsLecture 15

Communications in Client-Server Systems

Sockets Remote Procedure Calls Remote Method Invocation (Java)

Lecture 15 6

Sockets A socket is defined as an endpoint for communication Concatenation of IP address and port The socket 161.25.19.8:1625 refers to port 1625 on host

161.25.19.8 Communication takes place between a pair of sockets

Lecture 15 7

Socket Communication

Lecture 15 8

Client -server systems

Lecture 15 9

10

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 15

Lecture 15 11

Three-wayhandshake

Client Process

Client socket

Main Thread of

Server Process

Data

Server socket

Newsocket

New Thread of

Server Process

Internet

Examples of client-server systems

Event service – the publish-subscribe paradigm. Events are named The process/thread generating events publishes them – sends

them to the server which queues the events The clients subscribe to different events

X-windows Electronic mail as a trusted intermediary Files systems as trusted intermediaries The Word Wide Web The Domain Name System

Lecture 15 12

Lecture 15 13

1414

Intermediaries

What if the sender and the receiver of a message are not active at the same time?

Intermediaries support buffered communication and allow more flexibility the intermediary may decide how to sort messages

The sender and the receiver may: Push a message Pull a message

Example: the mail service: The sender pushes a message into his/her outbox The outbox pushes it to the inbox of the recipient The recipient pulls it whenever s(he) wants

The publish/subscribe paradigm the sender notifies an event service when it produced a message. Recipients subscribe to the events and when the events occur the messages are delivered

Lecture 15

Lecture 15 15

16

Trusted intermediary

Trusted service acting as an intermediary among multiple clients. Enforces modularity a fault of one client does not affect other

clients. Examples:

File systems Mail systems

Thin-clients a significant part of client functionality is transferred to the intermediary.

Lecture 15

Thin clients In a thin client/server system, the only software installed on the thin

client is the user interface, certain frequently used applications, and a networked operating system. By simplifying the load on the thin client, it can be a very small, low-powered device giving lower costs to purchase and to operate per seat.

The server, or a cluster of servers has the full weight of all the applications, services, and data. By keeping a few servers busy and many thin clients lightly loaded, users can expect easier system management and lower costs, as well as all the advantages of networked computing: central storage/backup and easier security.

Because the thin client is relatively passive and low-maintenance, but numerous, the entire system is simpler and easier to install and to operate.

As the cost of hardware plunges and the cost of employing a technician, buying energy, and disposing of waste rises, the advantages of thin clients grow. From the user's perspective, the interaction with monitor, keyboard, and cursor changes little from using a thick client.

Lecture 15 17

18

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 1984.

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 15

19

Editor is a client of File service which is a client of Block-storage serviceFile service is a trusted intermediary.

Lecture 15

20

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 15

21

D ata

S erver residence tim e.W eb page is created onthe fly

U ser's H TTP request S YN

S YN

A C K

A C K + H TTP request

TC P connectionestab lishm ent

R TT

U ser's H TTP requestfor an im age

S erver residence tim e.Im age is re trived fromdisk

D ata

H TTP request

A C K

A C K

Browser W eb Server

Lecture 15

22

Threadhandlingan HTTPrequest

W eb Server

H T T P request

R esourceR eposito ry

Loca l cache

Threadhandlingan HTTPrequest

H T T Presponse

W ebBrowser

C lient

P ersisten t/N onP ersistent

H TTPconnection

C lien t

TC Pport

H T T P request

H T T P response

A n H TTP requst con ta ins one o f the fo llow ing m ethods:G E T - ge t a resourceH E A D - verify the link and cond itions o f a resourceP O S T - input to a resource , usua lly a C G I scrip tP U T - s to re a resource a t the serverD E LE TE - de le te a resourceTR AC E - inc lude a ll headers in a response

W eb Cache

S am ple H TTP sta tus code in a response100 - C ontinue200 - O K205 - R eset C onnection301 - M oved P erm anently402 - Paym ent R equried404 - N ot Found405 - M ethod N ot A llowed407 - P roxy A uthentica tion R equ ired415 - U nsupported M ed ia Type500 - In terna l S erver E rro r504 - G ateway T im eout505 - H TTP version N ot S upported

Lecture 15

Client server interactions in HTTP

Lecture 15 23

HTTP clientWeb Browser

HTTPserver

TCPport80

request

response

HTTP clientWeb Browser

HTTPserver

Proxy

request to proxy

response to proxy

response to client

HTTP clientWeb Browser

HTTPserver

Tunnel

request to server

request to server

response to client

response to client

request to server