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1
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
46
SummarySummary
Domain Name SystemMaps name to IP addressUses on-line serversUses caching for efficiency
Three e-mail transfer protocolsSMTPPOP3
IMAP