1. Chapter 9 Applications Giving user-friendly names (instead of router-friendly addresses) is often the 1 st application (middleware) implemented on a network. Name space (defines the set of possible/allowable names) Consists of a set of name-to-value bindings (anything we want the naming system to return) Names versus addresses Location transparent versus location-dependent Global versus local Name Resolution mechanism returns a value when invoked with a name (done by a Name Server.) Internet has a hierarchical naming system, DNS (Domain Name System) Previously a central authority, the (Network Info Center) maintained a flat table (hosts.txt); mailed it out to sites every few days. System Administrators installed it on host. Name resolution was local. Properties 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 Names translated into addresses Hosts: cheltenham.cs.princeton.edu 192.12.69.17 192.12.69.17 80:23:A8:33:5B:9F Files /usr/llp/tmp/foo (server, fileid) Users Larry Peterson llp@cs.princeton.edullp@cs.princeton.edu Examples: Name: Value: 1. User presents hostname to an applic, MP, eg, embedded in an email address or URL) NS MP 2-3. MP engages NS translates to host adr 4-5. MP opens a connection To the host thru TCP (& IP)

2. Domain Naming System DNS implements a Hierarchical Name Space Read names right-to-left Separated by.’s Visualized as a tree: - inodes are domains - leaves are hosts Hierarchy is partitioned into subtrees called zones (admin authority) Each zone implemented by 2 or more name servers –A name server is a program that can be accessed over the Internet for resource records to translate a name to a value educom princeton … mit csee ux01ux04 physics cisco … yahoonasa … nsfarpa … navyacm … ieee govmilorgnetukfr Root name server Princeton name server Cisco name server CS name server EE name server … … Example of domain hierarchy Domain hierarchy partitioned into zones Hierarchy of name servers Big six Non-US countries Managed by InterNIC doesn’t want responsibility of managing a hierarchy, so they remain in princeton zone

3. Resource Records and Resolution Each name server maintains a collection of resource records (Name, Value, Type, Class, TTL) Name  Value: host  IP_addresses (if Type=A) Type specifies how the value is to be interpreted: A:IP-addr; NS:name server that can resolve the name; CNAME:alias; MX: for a mail server Class: allow other entities (not InterNIC=IN) to define types; TTL: how long record is valid Root Server (with TTL ignored) (princeton.edu, cit.princeton.edu, NS, IN) (cit.princeton.edu, 128.196.128.233, A, IN) (cisco.com, thumper.cisco.com, NS, IN) (thumper.ciscoe.com, 128.96.32.20, A, IN)… Princeton Server (cs.princeton.edu, optima.cs.princeton.edu, NS, IN) (optima.cs.princeton.edu, 192.12.69.5, A, IN) (ee.princeton.edu, helios.ee.princeton.edu, NS, IN) (helios.ee.princeton.edu, 128.196.28.166, A, IN) (jupiter.physics.princeton.edu, 128.196.4.1, A, IN) (saturn.physics.princeton.edu, 128.196.4.2, A, IN) (mars.physics.princeton.edu, 128.196.4.3, A, IN) Root name server Princeton name server Cisco name server CS name server EE name server … … CS Server (cs.princeton.edu, optima.cs.princeton.edu, MX, IN) (cicada.cs.princeton.edu, 192.12.69.60, A, IN) (cic.cs.princeton.edu, cicada.cs.princeton.edu, CNAME, IN) (optima.cs.princeton.edu, 192.12.69.5, A, IN) (opt.cs.princeton.edu, optima.cs.princeton.edu, CNAME, IN) (baskerville.cs.princeton.edu, 192.12.69.35, A, IN) (bas.cs.princeton.edu, baskerville.cs.princeton.edu, CNAME, IN) 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