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Applications

Computer Networks

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Where are we?

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Important Note

Although the ISO OSI Reference Model defines a session and presentation layer, they are often integrated into

some other layer in practice. In many cases they are simply not

used.

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Separation of Duties

Network Transfer bits/bytes Operates at the application’s request

Applications What data to transmit When to transmit data Where to transmit data to Meaning of bits/bytes

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Client/Server

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Characteristics

Client Actively initiates contacts with server Uses different source port for each

connectionServer

Passively awaits connections from clients

Uses the same source port for all clients

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Use of Protocol Ports

Each service given unique port number, PServer

Inform Operating System it is using port P Waits for requests to arrive

Client Forms request Sends request to port P on server computer

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Protocol Ports Key Idea

Each application is assigned a unique port identifier. A server must specify the identifier when it begins execution (application startup). A client must

specify the identifier when it requests the network software to contact the server.

Protocol software on the server computer uses the identifier to direct incoming

requests to the correct server application.

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Recall: Application Multiplexing

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Example: IP address and TCP port pairs

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Interacting with Protocol Software

Client or server uses transport protocols

Protocol software resides in OSApplications outside the OSMechanism to bridge the two

Application Programming Interface (API)

The book: TCP/IP Illustrated, Volume 2 - Stevens

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Application Programming Interface

Part of the operating systemPermits applications to use protocolsDefines

Operations allowed Arguments for each operation

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Example API: Sockets

Originally designed for BSD UNIX To use with TCP/IP protocols

Now industry standard available on many operating systems

e.g. winsock

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Sample Socket Procedure

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Example Application: DNS

Internet communication requires IP addresses

Humans prefer to use computer namesAutomated system available to translate

names to addressesKnown as Domain Name System (DNS)Base specs in RFC 1034 and RFC 1035

The book: DNS and Bind - Albitz and Liu

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DNS Functionality

Given Name of a computer

Return Computer’s internet (IP) address

Method Distributed lookup Client contact server(s) as necessary

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Domain Name Syntax

Alphanumeric segments separated by dots

Examples: www.depaul.edu aharp.is-net.depaul.edu www.research.att.com

Most significant part on the right

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Obtaining a Domain Name

Organization Chooses a desired name Must be unique Registers with central authority Placed under one top-level domain

Names subject to international law Trademarks Copyright

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Top-Level Domains

.com commercial organization .edu educational institution .gov government organization .mil military group .net major network provider .org organization other than above .arpa temporary ARPA domain (still used) .int international organization country code A country (e.g. .uk or .tw)

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Example Name Structure

First level is .comSecond level is company nameThird level is division within a

companyFourth level either

company subdivision individual computer

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DNS Illustrated

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Key Concept

The number of segments in a domain name corresponds to the naming hierarchy. There is no universal standard for this

hierarchy; each organization can choose its own naming convention. Furthermore,

names within an organization do not need to follow a uniform pattern; individual

groups within the organization can choose a hierarchical structure that is appropriate

for that group.

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DNS Client/Server Interaction

Client known as resolverMultiple DNS servers usedArranged in a hierarchyEach server corresponds to

contiguous part of naming hierarchy

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Inter-Server Links

All domain name servers are linked together to form a unified system. Each server knows how to reach a

root server and how to reach servers that are authorities for names further down the hierarchy.

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DNS Hierarchy Illustrated

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DNS Record Types

Stored with each nameResolver must specify the type in

lookup request Type A (address): IP address for name Type MX (mail exchanger): IP address of mail server for

the name Type CNAME (Computer Name): alias to another name

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Example Application: Internet Email

Email address text string which specifies destination mailbox e.g. jtk@depaul.edu, jtk@aharp.is-

net.depaul.edu

Mail message format header

identifies sender, receiver, contentsformat is keyword: information

blank line body

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MIME

Original email restricted to ASCII textUsers desire to send

image files audio clips compiled (binary) programs

Solution Multi-purpose Internet Mail Exchange (MIME) Defined in RFC 1521 and RFC 1522

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MIME [continued]

Allows transmission of binary data multimedia files (video/audio clips) multiple types in a single message mixed formats

Backward compatible

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MIME Encoding

Sender inserts additional header lines encodes binary data in (printable) ASCII

Sent like standard messageReceiver

interprets header lines Extracts and decodes parts

Separate standards for content and encoding

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MIME Example

Header lines addedMIME-Version: 1.0Content-Type: Multipart/Mixed; Boundary=xxxsep

Specifies Using MIME version 1.0 Line xxxsep appears before each

message part

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Simple Mail Transfer Protocol (SMTP)

Runs on top of TCPUsed between

Mail transfer program on sender’s computer Mail server on recipient’s computer

Specifies how Client interacts with server Recipients specified Message is transferred

Defined in RFC 821

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Mail Transfer Illustrated

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Computers Without Mail Servers (SMTP)

Typically small, personal computer not continuously connected to the network

To receive email, user must establish mailbox on large computer (server) access mailbox as necessary

Post Office Protocol (POP) often used defined in RFC 1939

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POP Illustrated

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Example Application: Remote Login w/ TELNET

Provides interactive access to computer from a remote site

Text-oriented interface User

invokes client specifies remote computer

Client forms TCP connection to server passes keystrokes over connection displays output on screen

Defined in RFC 854

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Example Application: File Transfer Protocol (FTP)

Complete file copyUses TCPSupports binary or text file transfersLarge set of commandsUntil 1995, it was the major source

of packets on the InternetDefined in RFC 959

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FTP Illustrated

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Example Application: World Wide Web (WWW)

Web pages can contain text, images, imbedded

objects and links standard authoring format is HTML links use URL tags transferred using HTTP

See http://www.w3c.org for all the details

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HyperText Markup Language

Document is free-formatEmbedded tags give display formatTags (often appear in pairs)

Paragraph <P> and </P> Line break <BR> Headings <H1>, <H2> <IMG src=“jtk.jpg” border=“0”> <A

href=“http://www.depaul.edu”>DePaul</A>

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Uniform Resource Locator (URL)

Symbolic representationEmbedded in HTML documentBrowser

hides text of link from user associates link with item on page makes items selectable

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URLs Illustrated

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HyperText Transfer Protocol (HTTP)

Web server makes web pages availableServer uses port 80 by defaultWeb client (browser) requests pagesCreates a TCP connection to serverHTTP sits on top of TCPHTTP v1.1 defined in RFC 2068

major enhancement over v1.0: single TCP connection for multiple HTTP requests

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Browser Operation