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

Network applications End-user applications Supporting services 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 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

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

Simple example name space

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

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 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?

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) local server iterative remote remote recursive other server

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