Transcript PPT

Network Applications
Example applications
Symbolic names and the Domain
Name System (DNS)
Network applications
• End-user applications
– Email, 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.
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
DNS Design: Hierarchy Definitions
root
org
edu com uk
net
gwu ucb
cmu
cs
cmcl
ece
bu mit
• 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
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 subunits of the organisation
Simple
example name
space
Top level domains
com
edu
gov
mil
net
org
int
arpra
countrycode
commercial organisation
educational institution
government organisation
military group
network support centre
organisation other than the above
international organisation
temporary ARPA domain
a country
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
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
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
Record types
• A – Address type
– 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?
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
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
Iterative and recursive resolution
remote
server
application
(client) recursive
local
server
iterative
other
server
remote
remote
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
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
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