Popular in Course
Popular in Computer Engineering
This 5 page Class Notes was uploaded by Paxton Okuneva on Saturday September 19, 2015. The Class Notes belongs to CEN 4500 at University of Florida taught by Staff in Fall. Since its upload, it has received 22 views. For similar materials see /class/207041/cen-4500-university-of-florida in Computer Engineering at University of Florida.
Reviews for CEN 4500
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 09/19/15
CEN 4500 Network Fundamentals Chapter 7a The Application Layer The Domain Name System Humans are comfortable with words and phrases network machinery and applications are not For example although the email address string some onesomewhereorg may make sense to an email app user it makes no sense to the email application in use nor the underlying network In order for both the application and network to make use of humanfriendly ASCII strings a mechanism was needed to translate said strings into binary addresses and thus un derstandable to the nonhuman elements of the communi cation task Initially when the Internet was in its infancy and the num ber of addresses in use was comparatively small hosts would have a local file called hoststxt which listed all hosts and their corresponding binary address These files were updated regularly and maintained locally However as the Internet grew two things became obvi ous The host file s maintenance and size would become problematic Hostname conflicts would become commonplace In order to address these concerns and introduce a sem blance of orderto the mushrooming Internet the Domain Name System was invented Following the post office s example of identifying an indi vidual first by area zip code then street then residence and then name the DNS mimics this system by first breaking down the Internet into domains o A domain somewhat like a zip code covers numerous hosts with each host potentially a subdomain The toplevel domains are broken into two types generic and countries I Generic J I Countries gt int colm elt1u gov mII org net us n slim yak 7m ieee alc Clo nce vlu eng cs eng jack kTo nec ai hnda cs csl ns Huh pc24 The top level of the Intemet s Domain Name Space One reason for this dichotomy is that in the early days of the Internet it s maintainers considered it local to the US and didn t foresee it s huge overseas growth Further each domain is broken down into subdomains ad infinitum until the lowestlevel domain contains no further subdomains but only hosts Each level of domain is responsible for allocating subdo mains and the rules that govern them Until recently a single company in the US was responsi ble for allocating and maintaining toplevel domain names however this monopoly was broken in the late 1990s Finally lnternet addresses like postal addresses are re solved backwards ie fgambe cise ufl edu is broken down into edu an entity within the toplevel education domain ufl the University of Florida subdomain cise an internal subdomain within ufl fgamble the receiving entity s identifier The way a sending application resolves an ASCIIstring address into a binary address is by first submitting the ASCIIstring to a local function called a resolver The resolver in turn sends a UDP packet containing the string to a Domain Name Server also known as a DNS The DNS then searches its tables forthe corresponding binary address of the requested domain Should the first DNS be unable to supply the requested bi nary address it then passes the string address to the highestlevel DNS forthe requested domain The highestlevel DNS then forwards the request down to the mostlocal subdomain of which it is aware lfthe subdomain DNS is not aware of the address it too then passes the request down to the most local subdomain DNS of which it is aware This process of passing the request up and down the Do mainNameSystem tree continues until either a binary ad dress or an address not found message is returned Once the binary address is located the DNS server re sponsible for maintaining the address record aka the authoritative record returns it to the requesting party which in turn passes it back to its requesting party and so on until the address record is ultimately received by the originating resolver function Once the resolver receives the record it makes a local or cached copy of it The cached record will have a short lifespan relative to the authoritative record as the assumption is it will soon be out of date Next the resolver then passes the record on to the send ing application and the sending app then establishes a di rect connection with the intended receiver In theory a single Domain Name Server could be used for the entire Internet but the pitfalls of such a setup are obvi ous Accordingly the Domain Name System is broken into zones with each domain s systemadmin responsible for defining zones and each zone having a corresponding domain name server For the DNS system to work every domainlevel machine has a set of resource records associated with it To use Tanenbaum s example a resource record has 5 parts The Domain Name The record s timetolive The record s class The record s type The record s associated value The Domain Name states the domain to which this record applies ie ciseufledu The Record s TimetoLive How long a localcopy of a DNS record should remain active The Record s Class or scope virtually all records are of classtype IN thernet which means they can be viewed publicly The Records Type identifies what kind of record See fig726 on page 625 of Ta nenbaum for a description ofthe most important types The Record s Associated Value depending on the record s type can be a number a domain name or an ASCII string Again refer to fig726 for examples of this field s potential values Required Reading Tanenbaum chapter 72 723
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'