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 computers 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 registrars 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 Tanenbaum & 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 name 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. Wetherall, 2011Architecture and services The user agent Message formats Message transfer Final delivery Architecture and Services (1)The key components and steps (numbered) to send emailCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Architecture of the email systemArchitecture and Services (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Paper mailElectronic mailEnvelopeMessage (= header and body)The User AgentWhat users see interface elements of a typical user agentCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Message Formats (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Header fields related to message transport; headers are readable ASCII textMessage Formats (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Other header fields useful for user agentsMessage Formats (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011MIME header fields used to describe what content is in the body of the messageMessage Formats (4)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Common MIME content types and subtypesMessage Formats (5)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Putting it all together: a multipart message containing HTML and audio alternatives.One part (HTML)Another(audio)Message Transfer (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Messages are transferred with SMTP (Simple Mail Transfer Protocol)Readable text commandsSubmission from user agent to MTA on port 587One MTA to the next MTA on port 25Other protocols for final delivery (IMAP, POP3)Message Transfer (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Sending a message:From Alice to BobSMTP commands are marked pink . . . (rest of message) . . . Message Transfer (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Common SMTP extensions (not in simple example)Final Delivery (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011User agent uses protocol like IMAP for final deliveryHas commands to manipulate folders / messages rightAlternatively, a Web interface (with proprietary protocol) might be usedThe World Wide WebCN5E by Tanenbaum & Wetherall, Pe