Application Layer - Local and Wide Area Networks - Lecture Slides, Slides of Social Work

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Introduction to LAN/WAN

Application Layer

Application Layer Topics ) Domain Name System (DNS)

)^ Electronic Mail (E-mail)

)^ World Wide Web

(7.3) Docsity.com

Domain Name System(DNS) ) Simple file-based solution works well for smallnetwork ) Larger network– Updates become a headache– Name conflicts would give people ulcers– DNS: one distributed hierarchical mapping “database” ) DNS Steps:– Application program calls

resolver^ procedure, passes parameters (e.g.

gethostbyname( )

)

• Resolver sends UDP packet to local DNS server– Local DNS server looks up name and returns IP address– Calling application (e.g. email, web) then uses IP address

Domain Name Space

)^ Managing large and constantly changing human-readable sets of names and IP addresses is a non-trivialproblem )^ 200 top level domains (

.edu, .gov, .uk,..

Etc)

)^ Two categories of domains:– generic and countries )^ Initially 6 generic domains–^ com (

commercial

) ,^ edu^ (educational),

gov^ (government),

mil

(military),^ net

(network providers),

org^ (non-profit), and

int

(int. organizations) ) Countries: one for each country as defined in ISO 3166

Domain Name Space

)^ Getting second-level domain like

EmmanuelAgu.com

is

easy– Simply contact registrar to find out if name is taken– Registrar’s: www.networksolutions.com– Registrar also checks for trademark infringements– If available, Emmanuel simply pays small fee and launches ) Domain is named by path upward from it to namedroot, separated by periods– E.g: Engr. Department at Sun Microsystems (

eng.sun.com

)

• Due to hierarchy, no potential conflicts with

eng.yale.edu

)^ Sub-domains applications to manager:– E.g. New VLSI dept. at Yale (

vlsi.cs.yale.edu

) simply talks to

the manager (system admin) of (

cs.yale.edu )

Docsity.com

Resource Records

)^ Information about

domain-IP address

mappings

are stored as resource records ) So DNS lookup returns complete resourcerecord ) Could contain more than

domain-IP address

mappings ) Resource record is five tuple of (

Domain name,

Time_to_live, Class, Type, Value

Name Servers: Zones

)^ Usually one primary nameserver per zone )^ Possibly many secondary nameservers )^ Primary nameservers read records from file on disk )^ Secondaries get information from primary nameserver

Resolving Process

)^ When resolver gets query, passes it to one of localnameservers )^ If domain under queried nameserver, returns^ authoritative record )^ Authoritative

comes from file, not cached (outdated) )^ If requested domain is remote, send message to top-levelname server for requested domain )^ E.g: first time request for

linda.cs.yale.edu

Overview of resolving process