Applications

DNS The Domain Name SystemNaming system of hosts on the Internet.DNS servers are hierarchicaly organized. They store databasesThere are local, authoritative DNS servers and one root DNS server.Name is also hierarchical (64bits at most each part, 255 bits the whole name). DNS messages sent are sent using UDP.

Name ServersPart of the DNS name space showing the division into zones.

Name Servers How a resolver looks up a remote name linda.cs.yale.edu in eight steps.

DNS FunctionsQueries may be recursive: server that doesnt know the answer ask another server for it, and returns it to the user. Otherwise it just sends info to user about the server who might know.Functions of DNS servers: Providing IP address for the given host nameProviding aliasing: conversion of various names to the real host name.Load balancing over multiple replicated web servers.

Resource RecordsThe principal DNS resource records types.

Resource Records A portion of a possible DNS database for cs.vu.nl.Domain nameTTLInternetValue

Electronic Mail

The user agent (mail reader)Message transfer (SMTP)Final delivery (mail downloading)

Electronic Mail

Alice invokes her user agent, provides Bobs address, composes and sends a message.User agent sends a message to her mail server, where it is placed in the queue.SMTP client on her server opens TCP connection at port 25 to an SMTP server on Bobs mail server. After handshaking, the SMTP client sends the message.The SMTP server receives the message, and the server places the message and places it in Bobs mailbox.Bob invokes his user agent and reads the message at his convenience.

User Agent

Serves for reading e-mails, composing and sending e-mails.Outlook, Netscape messanger, Eudora are GUI agents, while text-based agents were pine, mail, elmUser agent forms the e-mail with the header, and hands it to the transfer agent that forms the envelope required for the transmission.

Message Formats RFC 822RFC 822 header fields related to message transport.

Message Formats RFC 822 Some fields used in the RFC 822 message header.

MIME Multipurpose Internet Mail ExtensionsProblems with international languages:Languages with accents (French, German).Languages in non-Latin alphabets (Hebrew, Russian).Languages without alphabets (Chinese, Japanese).Messages not containing text at all (audio or images).

MIME RFC 822 headers added by MIME specified by RFC 2045-2049.

MIME The MIME types and subtypes defined in RFC 2045.

MIME A multipart message containing enriched and audio alternatives.

Message Transfer Simple Mail Transfer Protocol (SMTP)

Transferring a message from elinore@abc.com to carolyn@xyz.com.

Final Delivery(a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading e-mail when the receiver has a dial-up connection to an ISP.

POP3 and IMAPA comparison of POP3 and IMAP two different protocols for Retrieving messages from mail server.

The World Wide Web

Static web documentsHTML, XMLDynamic web documentsJavascript (client side), ASP (server), PHP (server)URLs Uniform Resource LocatorsComprises protocol, DNS host name and file name: http://www.etf.bg.ac.yu/~aleksandraHTTP The HyperText Transfer Protocol

Architectural Overview The parts of the Web model.

URL ProtocolsSome common URLs.

HTTP

Client sideBrowser determines URL and request IP address from DNS server.Browser opens TCP connection at port 80 to the obtained IP adr. Browser requests for file, gets it and presents it including images.Server sideAccepts TCP connection and receives the file name. Fetch the file, and sends it to the client.Finishes TCP connection.

HTTP MethodsThe built-in HTTP request methods.

HTTP Methods The status code response groups.

HTTP Message HeadersSome HTTP message headers.

CachingHierarchical caching with three proxies.

Content Delivery Networks(a) Original Web page. (b) Same page after transformation.

Content Delivery NetworkSteps in looking up a URL when a CDN is used.

Multimedia

Streaming audioStreaming videoInternet radioVoice over IPVideo on demand

Getting AudioA straightforward way to implement clickable music on a Web page.Alternatively Real Time Streaming Protocol (RTSP) can be used over UDP

RTSP Methods RTSP commands from the player to the server.

Improving QoS at Receiver

Packet loss recoverySending redundant packets which are functions of actual packetsSending redundant packets with the lower qualityInterleavingJitter improvementIncoming packets are bufferedThey are played out according to their timestamps and sequence numbers.

Interleaving to Correct Packet Loss When packets carry alternate samples, the loss of a packet reduces the temporal resolution rather than creating a gap in time.

Buffering of Streaming Media The media player buffers input from the media server and plays from the buffer rather than directly from the network.

Voice over IP

Phone calls are performed over the Internet among possibly mobile users. It can be based on two protocols H.323 and SIP.H323 comprises different protocols for user registration, connection establishment, connection quality negotiation, ringing, while SIP only performs functions of user registration and connection establishment.

SIP The Session Initiation ProtocolThe SIP methods defined in the core specification.

SIP Use a proxy and redirection servers with SIP.

Comparison of H.323 and SIP