山东大学计算机网络英语课件07Application Layer
88页1、Application LayerChapter 7CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011DNS Domain Name SystemElectronic MailThe WebStreaming Audio and VideoContent DeliveryRevised: August 2011The Application LayerCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Uses transport services to build distributed applicationsPhysicalLinkNetworkTransportApplicationDNS Domain Name SystemThe DNS resolves high-level human readable names for compute
2、rs to low-level IP addressesDNS name space Domain Resource records Name servers CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011The DNS Name Space (1)DNS namespace is hierarchical from the root downDifferent parts delegated to different organizationsThe computer robot.cs.washington.eduCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011The DNS Name Space (2)Generic top-level domains are controlled by ICANN who appoints registr
3、ars to run themThis one was controversialCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Domain Resource Records (1)The key resource records in the namespace are IP addresses (A/AAAA) and name servers (NS), but there are others too (e.g., MX)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Domain Resource Records (2)A portion of a possible DNS database for cs.vu.nl.IP addresses of computersName serverMail gatewaysCN5E by Ta
4、nenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Name Servers (1)Name servers contain data for portions of the name space called zones (circled).One zoneCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Name Servers (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Finding the IP address for a given hostname is called resolution and is done with the DNS protocol.Resolution:Computer requests local n
5、ame server to resolveLocal name server asks the root name serverRoot returns the name server for a lower zoneContinue down zones until name server can answerDNS protocol:Runs on UDP port 53, retransmits lost messagesCaches name server answers for better performanceName Servers (3)Example of a computer looking up the IP for a nameCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Electronic MailCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D.
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