1 chapter 31-32 internet applications (dns, e-mail)

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Chapter 31-32Chapter 31-32

Internet Applications(DNS, E-mail)

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Internet ApplicationsInternet Applications

Domain Name SystemElectronic mailRemote loginFile transferWorld Wide WebAll use client-server model

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NamesNames

Internet communication requires IP addresses

Humans prefer to use computer namesAutomated system available to translate

names to addressesKnown as Domain Name System (DNS)

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DNS FunctionalityDNS Functionality

GivenName of a computer

ReturnsComputer’s internet address

MethodDistributed lookupClient contacts server(s) as necessary

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Domain Name SyntaxDomain Name Syntax

Alphanumeric segments separated by dotsExamples

www.netbook.cs.purdue.edu

www.eg.bucknell.eduMost significant part on right

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Obtaining a Domain NameObtaining a Domain Name

OrganizationChooses a desired nameMust be uniqueRegisters with central authorityPlaced under one top-level domain

Names subject to international law forTrademarksCopyright

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Some Top-Level DomainsSome Top-Level Domains

Meaning assigned to each

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Within OrganizationWithin Organization

Subdivision possibleArbitrary levels possibleNot standardizedControlled locally by organization

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Example Name StructureExample Name Structure

First level is .comSecond level is company nameThird level is division within companyFourth level either

Company subdivisionIndividual computer

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An ExampleAn Example

AssumeCompany is FoobarHas two divisions

Soap divisionCandy division

Candy division has subdivisionsSoap Division has no subdivisions

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An Example (continued)An Example (continued)

Names in soap division have form

Names in candy division have form

computer.soap.foobar.com

computer.subdivision.candy.foobar.com

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Illustration of Foobar Naming Hierarchy

Illustration of Foobar Naming Hierarchy

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The Point About NamesThe Point About Names

The number of segments in a domain name corresponds to the naming hierarchy. There is no universal standard; each organization can choose how to structure names in its hierarchy. 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 the group.

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DNS Client-Server Interaction

DNS Client-Server Interaction

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

of naming hierarchy

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Two Possible DNS Hierarchies

Two Possible DNS Hierarchies

Choice made by organization

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Inter-Server LinksInter-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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In PracticeIn Practice

DNS uses backup server(s)ISPs and others

Offer DNS service to subscribersSmall organizations and individuals

Only need domain names for computers running servers

Contract with an ISP for domain service

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DNS LookupDNS LookupApplication

Becomes DNS clientSends request to local DNS server

Local serverIf answer known, returns responseIf answer unknown

Starts at top-level serverFollows linksReturns response

Called name resolution

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Interative DNS LookupInterative DNS Lookup

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Caching in DNSCaching in DNS

Server always caches answersHost can cache answersCaching

Improves efficiencyEliminates unnecessary searchWorks well because high locality of reference

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DNS TypesDNS Types

Each entry in server consists ofDomain nameDNS type for nameValue to which name corresponds

During lookup, client must supplyNameType

ServerMatches both name and type

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The Point About TypesThe Point About Types

The domain name system stores a type with each entry. When a resolver looks up a name, the resolver must specify the type that is desired; a DNS server returns only entries that match the specified type.

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Example DNS TypesExample DNS Types

Type A (Address)Value is IP address for named computer

Type MX (Mail eXchanger)Value is IP address of computer with mail server

for nameType CNAME (Computer NAME)

Value is another domain nameUsed to establish alias (www)

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Domain Name Abbreviation

Domain Name Abbreviation

DNS lookup uses full namesUsers desire abbreviationsTechnique

Configure resolver with list of suffixesTry suffixes one at a time

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Example of DNS Abbreviations

Example of DNS Abbreviations

Suffixes arefoobar.comcandy.foobar.com

User enters name walnutResolver tries

walnutwalnut.foobar.comwalnut.candy.foobar.com

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Other Internet Applications

Other Internet Applications

Invoked directly by userE-mailRemote loginFile transferWeb browsing

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Electronic MailElectronic Mail

OriginallyMemo sent from one user to another

NowMemo sent to one or more mailboxes

MailboxDestination point for messages Can be storage or programGiven unique address

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E-mail AddressE-mail Address

Text stringSpecifies mail destinationGeneral form mailbox@computercomputer

Domain name of computerActually type MX

mailboxDestination on the computer

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Use of E-mail AddressUse of E-mail Address

Each electronic mailbox has a unique address, which is divided into two parts: the first identifies a user’s mailbox, and the second identifies a computer on which the mailbox resides. E-mail software on the sender’s computer uses the second part to select a destination; e-mail software on the recipient’s computer uses the first part to select a particular mailbox.

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Mail Message FormatMail Message Format

HeaderIdentifies sender, recipient(s), memo contentsLines of form

keyword:informationBlank lineBody

Contains text of message

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Example E-mail Header Fields

Example E-mail Header Fields

Most header lines optional

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Extending E-mailExtending E-mail

Original e-mailSMTP - message restricted to ASCII text

Users desire to sendImage filesAudio clipsCompiled (binary) programs

SolutionMulti-purpose Internet Mail Extensions (MIME)

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MIMEMIME

Allows transmission ofBinary dataMultimedia files (video/audio clips)Multiple types in single messageMixed formats

Backward compatible

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MIME EncodingMIME Encoding

SenderInserts additional header linesEncodes binary data in (printable) ASCII

Sent like standard messageReceiver

Interprets header linesExtracts and decodes parts

Separate standards for content and encoding

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Example of MIMEExample of MIME

Header lines added

MIME-Version: 1.0

Content-Type: Multipart/Mixed; Boundary=Mime_sep

SpecifiesUsing MIME version 1.0Line Mime_sep appears before each message

part

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MIMEMIME

Although Internet e-mail only transfers text, MIME can be used to transport binary data by encoding it in printed characters. A MIME mail message includes additional information that a receiving application uses to decode the message.

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Mail TransferMail Transfer

Protocol is Simple Mail Transfer Protocol (SMTP)Runs over TCPUsed between

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

Specifies howClient interacts with serverRecipients specifiedMessage is transferred

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Illustration of Mail TransferIllustration of Mail Transfer

ServerRequired to receive mailPlaces message in user’s mailbox

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TerminologyTerminologyMail exploder

ProgramAccepts incoming messageDelivers to multiple recipients

Mailing listDatabaseUsed by exploder

Mail gatewayConnects two mail systems

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Illustration of a Mailing ListIllustration of a Mailing List

Separate permissions forMailing to listAdding/deleting members

Public – anyone can joinPrivate – access restricted by owner

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Illustration of a Mail Gateway

Illustration of a Mail Gateway

Can connect twoHeterogeneous systemsInternet to non-Internet

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Automated Mailing ListsAutomated Mailing Lists

Automated program to handle routine chores of maintaining mailing list: list manager

Used in conjunction with exploderExample expected command:

add mailbox to list

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Computers Without Mail Servers

Computers Without Mail Servers

TypicallySmall, personal computerNot continuously connected to Internet

To receive e-mail, user mustEstablish mailbox on large computerAccess mailbox as necessary

Post Office Protocol (POP) used

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POPPOP

Current version named POP3

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IMAPIMAP

Internet Message Access Protocol (IMAP4 rev1 1996)

A more modern type of protocol for accessing email messages

Leaves mail on server Built in security features

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SummarySummary

Domain Name SystemMaps name to IP addressUses on-line serversUses caching for efficiency

Three e-mail transfer protocolsSMTPPOP3

IMAP