Copy of The Application Layer

Copy of The Application Layer - Computer Networks 2 The...

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February 13, 2012 Veton Këpuska 1 Computer Networks 2 The Application Layer
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February 13, 2012 Veton Këpuska 2 The Application Layer The layers bellow application layer provide  services to application layer. However, application layer needs support  protocols to allow the applications to function. Example of such protocol is DNS (The  Domain Name System)
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February 13, 2012 Veton Këpuska 3 DNS-The Domain Name System Programs can refer to  Hosts, Mailboxes, Other Network resources, By their network addresses (e.g., IP). 1. These addresses are hard for people to remember. 2. Furthermore, when those addresses (e.g., IPs) change in (e.g.,  moving from one mail server to the other; moving from IPv4 to IPv6,  etc.). Consequently ASCII names were introduced to decouple machine  names from machine addresses. Because network itself understands only numerical addresses, some  mechanism is required to convert the ASCII strings to network  addresses.
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February 13, 2012 Veton Këpuska 4 DNS-The Domain Name System ARPANET: Used a file: hosts.txt that listed all the hosts and their IP addresses.  Every night, all hosts would fetch this file from the site at which it was maintained. This approach worked well because the networks at that time had only few  hundred large timesharing machines. With (hundred of) thousands of minicomputers and PCs connected to the  net, ARPANET approach could no longer continue to work: 1. The size of the file (hosts.txt) would become prohibitively large.  2. Host name conflicts would occur constantly unless names were centrally  managed. Host names can not be centrally managed because of load latency of huge  international network. As a result a solution was invented as a DNS (Domain Name System).
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February 13, 2012 Veton Këpuska 5 DNS-The Domain Name System The core of DNS invention is: Hierarchical domain-based naming scheme,  and Distributed database system for implementing  this naming scheme. DSN is primarily used for: Mapping host names to IP addresses, and E-mail destinations to IP addresses.
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February 13, 2012 Veton Këpuska 6 DNS-The Domain Name System DSN achieves the goal of mapping a name onto IP address  by using a library procedure called  resolver . 1. Application passes a name to  resolver  as a parameter and  gets the IP back. An example of  resolver  is  gethostbyname  function when UDP  was discussed. 1.
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Copy of The Application Layer - Computer Networks 2 The...

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