Applications

1. Applications

2. DNS – The Domain Name System • Naming system of hosts on the Internet. • DNS servers are hierarchicaly organized. They store databases • There 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.

3. Name Servers Part of the DNS name space showing the division into zones.

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

5. DNS Functions • Queries may be recursive: server that doesn’t 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 name • Providing aliasing: conversion of various names to the real host name. • Load balancing over multiple replicated web servers.

6. Resource Records The principal DNS resource records types.

7. Resource Records Internet TTL Value Domain name A portion of a possible DNS database for cs.vu.nl.

8. Electronic Mail • The user agent (mail reader) • Message transfer (SMTP) • Final delivery (mail downloading)

9. Electronic Mail • Alice invokes her user agent, provides Bob’s 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 Bob’s 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 Bob’s mailbox. • Bob invokes his user agent and reads the message at his convenience.

10. 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, elm • User agent forms the e-mail with the header, and hands it to the transfer agent that forms the envelope required for the transmission.

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

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

13. MIME – Multipurpose Internet Mail Extensions Problems 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).

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

15. MIME The MIME types and subtypes defined in RFC 2045.

16. MIME A multipart message containing enriched and audio alternatives.

17. Message Transfer Simple Mail Transfer Protocol (SMTP) Transferring a message from elinore@abc.com to carolyn@xyz.com.

18. 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.

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

20. The World Wide Web • Static web documents • HTML, XML • Dynamic web documents • Javascript (client side), ASP (server), PHP (server) • URLs – Uniform Resource Locators • Comprises protocol, DNS host name and file name: http://www.etf.bg.ac.yu/~aleksandra • HTTP – The HyperText Transfer Protocol

21. Architectural Overview The parts of the Web model.

22. URL Protocols Some common URLs.

23. HTTP • Client side • Browser 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 side • Accepts TCP connection and receives the file name. • Fetch the file, and sends it to the client. • Finishes TCP connection.

24. HTTP Methods The built-in HTTP request methods.

25. HTTP Methods The status code response groups.

26. HTTP Message Headers Some HTTP message headers.

27. Caching Hierarchical caching with three proxies.

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

29. Content Delivery Network Steps in looking up a URL when a CDN is used.

30. Multimedia • Streaming audio • Streaming video • Internet radio • Voice over IP • Video on demand

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

32. RTSP Methods RTSP commands from the player to the server.

33. Improving QoS at Receiver • Packet loss recovery • Sending redundant packets which are functions of actual packets • Sending redundant packets with the lower quality • Interleaving • Jitter improvement • Incoming packets are buffered • They are played out according to their timestamps and sequence numbers.

34. 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.

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

36. 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.

37. SIP – The Session Initiation Protocol The SIP methods defined in the core specification.

38. SIP Use a proxy and redirection servers with SIP.

39. Comparison of H.323 and SIP