[PDF] - 18-1 Client-Server Computing Protocols Communications protocol : a PDF Document

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CSE 143 Java – Autumn 2001

The Internet From 50,000 Feet

Overview

Topics
Packet switched networks
TCP/IP
Name servers - DNS
Higher-level protocols: file transfer (FTP), mail (SMTP, POP), news

(NNTP), web (HTTP)

For further information…
Internet section of your favorite bookstore
The Internet Book by Douglas Comer (P-H, 3rd ed, 2000)

Browsing the Web

Enter a web address in a browser
Web page appears
What just happened?
How did your machine find the machine on the net with the page?
How did the packets get transferred across the net?
What’s going on here?

Circuit Switching & Packet Switching

Circuit switching
End-to-end circuit set up for communication
Traditional analog telephone system
Packet switching
Digital messages
Bits of message are chopped up into short packets
Packets are shipped from source to destination independently of

each other

Wires are shared by different sessions simultaneously
Method used by the internet (dates back to ARPANet in late 60’s)

Now replacing traditional circuit switching for voice, etc.

Local Area Networks

Computers connected in a small area (office, house)
Network is high-bandwidth, reliable
Typical connection technology
Ethernet
Wireless

Internet

Idea: Local networks are connected to internet a router
Packets are transmitted by injecting them into the network
Packets are self-identifying (source/destination addresses, etc.)
Network is responsible for forwarding packets – routers

Internet

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Client-Server Computing

Much of the communication on the net is some sort of client-

server computing

Server: a program that runs continuously, waiting for

requests (directory lookup, email, web server)

Client: program that sends a request to a server

client server

send me my email here it is

Protocols

Communications protocol: a standard specifying the kinds of

messages exchanged between particular kinds of clients and servers

Examples:
SMTP (outgoing mail), POP (incoming mail)
IMAP (different email protocol)
LDAP (directory lookup)
FTP (file transfers)
HTTP (web server requests)
HTTPS (secure web server)
SSL (Secure Sockets Layer)

Protocol Stack

Actual communication is done via intermediate protocols
Fundamental ones:
IP: Internet protocol – basic packet delivery
TCP: Transmission Control Protocol – message transmission

Hardware IP TCP Application Hardware IP TCP Application

Hardware connection Virtual (software) connection

IP – Internet Protocol

Packet delivery from one internet host to another
Packet size is fairly small – few thousand bytes
IP handles details of routing from source to destination
Each router in the net has routing tables with its current best guess
f how to forward packets to move them towards their destinations
Packet delivery is on a “best efforts” basis
No guarantees – packets will be discarded if something goes wrong

(transmission errors, router unable to keep up with traffic volume, etc.)

Internet Addresses

Every machine on the internet has a unique IP address
Current IP addresses are 32-bits
Typically written as 4 decimal numbers, one per byte. Examples

128.95.4.112 (www.cs.washington.edu) 208.48.26.246 (www.nytimes.com)

IP addresses may be
static – fixed address assignment
dynamic – assigned when computer establishes connection to the

net (DHCP)

IP addresses are assigned in blocks
128.95.4.nnn belong to cs.washington.edu

TCP – Transmission Control Protocol

Provides reliable delivery of arbitrary length messages
Sending TCP agent breaks message up into packets and

transmits via IP

Packets contain time stamps, serial numbers, and other header info

in addition to message data

Receiving TCP agent reassembles packets into messages

and acknowledges receipt

TCP handles retransmission of missed packets
Packets may arrive out of order & there may be duplicates
Most basic internet services are built on TCP

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Naming Computers

Numeric IP addresses are fine for computers; not people
Computers on the internet typically have names
Each machine has (at least one) name

perkins-pc.cs.washington.edu www.amazon.com

Names are organized into domains – typically allocated to an
rganization or part of an organization

washington.edu; cs.washington.edu; amazon.com; sun.com; java.sun.com whitehouse.gov

Each name is associated with an IP address
No particular connection between names and specific IP addresses;

up to organization to assign addresses

Name Lookup - DNS

A domain name must be translated to an IP address before

communication can be established

Every domain on the net runs a Domain Name Server (DNS)
Handles translation requests for that domain (both local & outside)
To translate a non-local name, contact its domain server
Contact DNS for cnn.com to get IP address for www.cnn.com
If all-else fails, contact top-level domain servers to get
rganization domain server IP address
com, edu, net, org, gov, etc.

Higher-Level Protocols

Many basic protocols are built on top of TCP
Protocols describe detailed formats of messages specific to that

protocol

Non-TCP protocols exist for other uses (streaming media,

datagrams, etc)

An IP connection can specify a port-number
Common protocols have “well-known” port numbers

21 (ftp) 25 (smtp) 80 (http) 110 (pop3) 119 (nntp)

When a server process starts up, it specifies the port number

it responds to

URLs

A Uniform Resource Locator is a naming convention that

uniquely specifies a resource on the web

Specifies the protocol
Specifies the domain name
May specify a specific item in that domain

ftp://ftp.cs.washington/edu/directorynames/filename http://www.washington.edu/computing/index.html

Web browsers and other programs that understand URLs

contain client software to handle the appropriate protocols

HTTP

One of the most widely used protocols on the web
HyperText Transfer Protocol
Simple protocol built on top of TCP – uses port 80

telnet www.washington.edu 80 GET /index.html

HTML

HyperText Markup Language
Simple formatting language that

has text and control codes

Browser interprets control codes

when it renders the page

One particular kind of control code:

hyperlink that specifies another page

n the web by giving its URL
Example

<HTML> <HEAD> <TITLE>A Simple Web Page</TITLE> </HEAD> <BODY> <P>Here are some words</P> <P>For more words, see the <A HREF=“www.washington.edu> UW web site</A></P> </BODY> </HTML>

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Java Applets

One of the items that a web page can refer to is a Java

Applet – a compiled Java program that extends Applet or JApplet

Applet tag looks something like this

An applet is a normal Java program except
It runs in the browser’s window (doesn’t have its own main method)
It has security restrictions
It can access information from the applet tag and can communicate

with other applets running on the same page

Applet Interface

An applet is an class that extends
java.applet.Applet (AWT) or
javax.swing.JApplet (Swing)
Applets should override these methods as needed

init( ) – called when applet is first loaded into the browser; create GUI components here start( ) – called when the applet becomes visible and should do its thing stop( ) – called when applet becomes temporarily invisible (window minimized or applet scrolled out of view, for example) destroy( ) – called when browser unloads the applet

Java Network Programming APIs

The java.net library classes make it easy to access the web
Class URL – simple interface to download an object given a URL or
pen a stream to read or write to a URL
Class Socket – supports connections to specific ports on a given

internet host

Uses ordinary Java stream I/O to communicate

Other classes to do low-level network programming, etc.
If you want to program a web server, one option is Java

servlets or Java server pages

Details beyond the scope of CSE143, alas

Summary

Key concepts
Packet switching & routing
Layered protocols

TCP/IP Higher-level protocols (mail, news, web, streaming media, …)

Domain names

DNS – map domain names to IP addresses

URLs
Applets