The Application Layer

1. The Application Layer Tanenbaum Chapter 7 4343 X2 – 2007

2. Outline • The Domain Name System • Email • The Web • Multimedia 4343 X2 – 2007

3. The Domain Name System • Using IP addresses as absolute machine addresses on the Internet is not very practical. • Computers can frequently change IPs, rendering using an IP address to access the machine useless. • A system of using a name to access the machine was devised to overcome this problem. 4343 X2 – 2007

4. DNS • The name of the machine is related to the IP of the machine, but the IP can be changed without altering the name. • A Domain Name System machine (sometimes known as the Domain Name Server) is responsible for keeping track of the relationships between IP addresses and system names. 4343 X2 – 2007

5. The DNS Name Space • The original name space was split into over 200 top-level domains, including com,edu, int (international), net (network providers), org, gov (US only), mil (US only) and at least one domain for each country. • Recently (2000), biz, info, name and pro were introduced at the top level. 4343 X2 – 2007

6. DNS Issues • It used to be somewhat difficult to get a domain name. • Now anyone with a credit card can buy a .com without ever having to prove that they own a business. • There is no one watching over how domain names have been distributed. 4343 X2 – 2007

7. Domain Records • Each domain record stores the following information: • Domain Name (the name of the domain) • Time to Live (how stable the record is) • Class • Type (A, MX, NS, CNAME, PTR, etc) • Value 4343 X2 – 2007

8. Domain Types • SOA = State of Authority: Information about the domain • A = IP address of the host • MX = Mail exchange: a server willing to accept mail for this domain • NS = Name of a server for this domain • CNAME = Canonical name • PTR = Pointer: an alias for an IP address 4343 X2 – 2007

9. Name Servers • We need more than just one name server for the whole Internet. • The net is split into non-overlapping zones, each of which has a primary name server. The primary name server gets its information off of disk, and shares that information with secondary name servers. • Some name servers for a zone can be located outside of the zone. 4343 X2 – 2007

10. Name Zones 4343 X2 – 2007

11. Resolving Domain Names • The request is passed from Name Server to Name Server until it arrives at the zone where the machine should reside. • The local name servers can give an answer about the machine in question. • It should be noted that an authoritative record is one that comes from the name authority and is always correct, while cached information from a secondary name server may be incorrect. 4343 X2 – 2007

12. The Web • We use URLs (Uniform Resource Locators) to specify information about the data we want to access: • the protocol (http, nntp, ftp, etc) • the name of the machine (dragon.acadiau.ca) • the file containing the page (~dbenoit/index.html) • How does it all work? 4343 X2 – 2007

13. How it works. • Browser determines the URL • Browser gets IP address for server • Browser makes a connection to server • Browser sends a request for the page • Server sends the page back • The TCP connection is released • Browser displays the page 4343 X2 – 2007

14. Web server machine 4343 X2 – 2007

15. Markup Languages • The WWW began with straight text, but then moved to HTML. • HTML is a subset of SGML. • HTML moved through several versions until HTML 4. • Now, the standard is XHTML, essentially HTML 4 written in XML. • We are not sure where things will go from here… 4343 X2 – 2007