1. A Simple Network Analogy

2. Components of our Imaginary Mail Network  Assistant  Warehouses of documents stored in folders  Warehouse address operators  Packets (envelopes) of information  Local Mail Room  Gateway mail routing center  Regional/national mail routing centers

3. Purpose of our Mail Network  Your assistant is to request documents (by mail) and interpret them for you.

4. Limitations and obstacles  We can’t remember addresses, only names.  We can only mail to addresses.  We use small envelopes.  We have an envelope/hr. limit.

5. Limitations and obstacles (cont’d)  Warehouse worker is fast, but not very bright.  We must specify the document location within the warehouse.  Warehouses can be reorganized, shut down, or renamed overnight.  While the document language is supposedly standardized, some document authors use slang terms.

6. The Perfect Process  Give assistant warehouse name, document name and location.  Assistant contacts warehouse address operator for address.  Assistant mails warehouse requesting the document.  Warehouse mails assistant the document in pieces.  Assistant assembles the document.  If pieces are missing, assistant mails warehouse to re-send.  Assistant interprets document and presents to you.

7. Structure

8. Packet - Named

9. Structure

11. Packet - Addressed

12. Structure

13. Analogies  Mailing method: Transfer Protocol (e.g., http)  Warehouse Name: Domain Name (e.g., www.education.umd.edu)  Document location: Pathname  Document name: Document name (often includes type or language)  Document language: Hypertext Markup Language  MailingMethod://WarehouseName/DocLoc/DocName : Uniform Resource Locator (URL)

14. Analogies  Warehouse Address Operator: Domain Name Server (DNS)  Missing pieces re-send: Transfer Control Protocol (TCP)  Mail Room Routing Rules: Internet Protocol (IP)  Numerical Address: IP address  Local Mail Room: Local/Wide Area Network (LAN/WAN)  Packet: Data Packet

15. Analogies  Warehouse: Web Server  Mail Routing Centers: Routers  Assistant: Browser

16. Structure

17. Structure - Internet

18. Packet - Unresolved Request

19. Packet - Request to DNS

20. Structure - Internet

21. Structure - Internet (DNS Request)

22. Packet - Response from DNS

23. Packet - Unresolved Request

24. Packet - Addressed Request

25. Structure - Internet (Request with IP)

26. Structure - Internet (Routing Protocol)

27. Structure - Internet (Request rec’d)

28. Structure - Internet (Doc handled)

29. Structure - Internet (Req. honored)

30. Structure - Internet (Packet sending)

35. Structure - Internet (Packets sent)

36. Structure - Internet

38. The Perfect Process - Internet  Type in your browser a transfer protocol, domain, pathname and document (some missing can be assumed).  Browser contacts DNS for Domain <> IP Address resolution.  Browser uses IP address to mail server your request.  Server mails result to your browser in packets of data.  Browser assembles the data.  If pieces are missing, TCP allows for resending of packets.  Browser interprets document and displays result.

39. Summary  The fact that the resulting received file can contain links puts the hyper in the hypertext.  Document size matters to transmission speed and error reduction.  Work computers (at CoE) are still on a relatively high speed connection; modem links will be considerably slower.  The internet is a relatively simple idea, but a complex set of connections.  Graphics and other multimedia are disproportionately large compared to their text counterparts.