1. Chapter 9 - Applications We will look at three main applications DNS (name services) SMTP (e-mail) HTTP (World Wide Web) Our main focus will be on DNS For most students this chapter should cover familiar material – so we will go through it faster than the other chapters

2. DNS – Domain Name System The problem: IP addresses are fine for routers, but are not exactly user friendly DNS maps user-friendly names into router- friendly addresses

3. Terminology Terminology: binding and resolution (page 624) Name server – a specific implementation of a resolution mechanism that can be queried by sending it a message DNS – the Internet’s naming system Simple example in the next diagram

4. Name server Mail program User TCP IP 2 cs.princeton.edu 192.12.69.5 3 user @ cs.princeton.edu 1 192.12.69.5 4 5

5. Domain Hierarchy Names are processed right-to-left mars.cs.princeton.edu edu is the highest level princeton is the next level cs is within princeton, the next level mars is a server in the CS department, the lowest level

6. Quick Overview A client from say Virginia Tech wants to access a server called “cicada” at Princeton’s CS department The client inputs the URL: cicada.cs.princeton.edu However, neither the client nor his default router know the IP address Now what happens??

7. Overview (continued) Basic idea: the client communicates with a local server A that knows a remote server B which knows more information about.edu names/addresses Server B probably doesn’t have the entire address but knows server C that knows more information about princeton.edu names/address Server C knows the address of cs.princeton.edu

8. Root name server Princeton name server CS name server Local name server Client 1 cicada.cs.princeton.edu 192.12.69.60 8 cicada.cs.princeton.edu princeton.edu, 128.196.128.233 cicada.cs.princeton.edu cicada.cs.princeton.edu, 192.12.69.60 cicada.cs.princeton.edu cs.princeton.edu, 192.12.69.5 2 3 4 5 6 7

9. Name Servers The naming hierarchy is split up into zones Briefly one name server per zone Clients send queries to the name servers Sometimes they get back the answer they seek Sometimes they only get a partial answer and are pointed to another name server where the partial answer might be extended

12. A Hierarchy of Name Servers In effect there is a tree or hierarchy of name servers that need to be queried Study the next diagram Then review the first diagram (presented again) and review what happens in the process of resolving an entire name or URL

13. Root name server Princeton name server Cisco name server CS name server EE name server … …

14. Root name server Princeton name server CS name server Local name server Client 1 cicada.cs.princeton.edu 192.12.69.60 8 cicada.cs.princeton.edu princeton.edu, 128.196.128.233 cicada.cs.princeton.edu cicada.cs.princeton.edu, 192.12.69.60 cicada.cs.princeton.edu cs.princeton.edu, 192.12.69.5 2 3 4 5 6 7

15. Details and other Points There are many more points about this process of course, some of the main ones: Aliases are used for name servers so that the function can be moved to another machine with affecting remote users Not all clients know about the root servers, instead the clients know about a local name server which in turn knows about the root servers

16. More Details The process doesn’t necessary have to repeated each time a URL is requested, instead a cache can be maintained to resolve future queries without having to go through the entire process again

17. Entire Name Translation Process Domain name IP Address Physical Address DNS Server ARP

18. The E-Mail Application SMTP and MIME are the main protocol and format (see page 634) The message format is rather involved and we won’t be going into detail, but study over the example on page 636-7 Message transfer uses SMTP (T for “Transfer”) Study pages 637-8 for details of the process involving mail daemons and gateways

19. Mail reader Mail daemon SMTP/TCP Mail gateway Mail daemon SMTP/TCP Mail reader Mail daemon

20. HTTP Not much is covered in the text about this well known application Two key points that the text does cover are: Persistent connections page 645 Caching page 645-6