1. Example applications Symbolic names and the Domain Name System (DNS)
Network Applications
Example applications
Symbolic names and the Domain Name System (DNS)

2. Network applications End-user applications Supporting services
, file transfer, World Wide Web, remote login, audio and video conferencing, directory services, networked games, MUDs
Supporting services
Domain name system, authentication, billing, encryption, time etc.

3. Symbolic names for computers
users can give symbolic text names for computers instead of numeric IP addresses
symbolic name is sequence of alphanumeric stings separated by full-stops
structured as a hierarchical, list of domains with the most significant part on the right
marian.cs.nott.ac.uk
foo.bar.com
number of domains is not fixed

4. DNS Design: Hierarchy Definitions
Each node in hierarchy stores a list of names that end with same suffix
Suffix = path up tree
E.g., given this tree, where would following be stored:
Fred.com
Fred.edu
Fred.cmu.edu
Fred.cmcl.cs.cmu.edu
Fred.cs.mit.edu
root
org
net
edu
com uk
gwu
ucb
cmu bu
mit cs
ece
cmcl

5. Management of domain names
Each organisation registers under a top level domain with the Internet authority
It can then manage its own name space, possibly further devolving authority to sub-units of the organisation

6. Simple example name space

7. Top level domains com commercial organisation
edu educational institution
gov government organisation
mil military group
net network support centre
org organisation other than the above
int international organisation
arpra temporary ARPA domain
country- a country
code

8. New Registrars
Network Solutions (NSI) used to handle all registrations, root servers, etc…
Clearly not the democratic (Internet) way
Large number of registrars that can create new domains  However NSI still handles root servers

9. New Top Level Domains (TLDs)
.info  general info
.biz  businesses
.aero  air-transport industry
.coop  business cooperatives
.name  individuals
.pro  accountants, lawyers, and physicians
.museum  museums
Only new ones active so far = .info, .biz, .name

10. The Domain Name System (DNS)
Applications use the DNS to map symbolic names to numeric addresses
An application becomes a client of a DNS server
Each entry in DNS database contains three items:
A domain name
A record type
Value

11. Record types A – Address type CNAME – canonical name
What is the IP address of a a given domain name?
CNAME – canonical name
What alternative names does a given domain name have?
MX – Mail EXchanger
What is the Mail Exchanger of a given domain name?
NS – Name Server
What is the name server of a given domain name?

12. Distributed DNS servers
DNS database is distributed among servers
Local server contacts other servers if necessary
Root servers are authorities for top-level domains
Other servers are authorities for a part of the hierarchy and also hold references to servers lower down and root servers
Each organisation can decide how to partition authority among servers

14. Client-server interactions
An application acting as a client requests recursive resolution
A server acting as client requests iterative resolution
Replication of servers (especially root servers) optimises performance
Caching exploits temporal locality of reference

15. Iterative and recursive resolution
remote server
application (client)
local server
iterative
remote remote
recursive
other server

16. Workload and Caching DNS responses are cached
Quick response for repeated translations
Other queries may reuse some parts of lookup
NS records for domains
DNS negative queries are cached
Don’t have to repeat past mistakes
E.g. misspellings, search strings in resolv.conf
Cached data periodically times out
Lifetime (TTL) of data controlled by owner of data
TTL passed with every record

17. Course summary and related courses
G52CNN has provided an introduction to fundamental network concepts and techniques
Related ideas are covered in:
G5BIAW - Internet and The World Wide Web
G53ACC - Advanced Computer Communications

18. Exam 2 hours Answer 3 out of 5 questions
Each worth 25 points (maximum 75 points)
There will be a question on each course section
Data Transmission, Packet Transmission, Internetworking and Applications
All lectures examinable