4343 X2 – The Application Layer Tanenbaum Chapter 7

4343 X2 – Outline The Domain Name System The Web Multimedia

4343 X2 – 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 – 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 – 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 – 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 – 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 – 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 – 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 – Name Zones

4343 X2 – 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 – 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 – How it works. 1.Browser determines the URL 2.Browser gets IP address for server 3.Browser makes a connection to server 4.Browser sends a request for the page 5.Server sends the page back 6.The TCP connection is released 7.Browser displays the page

4343 X2 – Web server machine

4343 X2 – 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…