Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46CS640Introduction to Computer NetworksDNSDec 1st, 1999Prof. Lawrence H. LandweberProf. Jun MuraiNames and Addresses•Host should be identified before communicating–A host is identified by unique IP address–A host is identified by unique Name (for user convenience)•Names–variable length and mnemonic–easy for humans to remember–contains no information about the host location•Addresses–fixed length–easy for computers to process–tied to routingName Space and Resolution•Name Space–defines set of possible names–flat versus hierarchical•Naming system–maintains collections of a set of name to value bindings•Resolution mechanism–mechanism to get a corresponding value (IP address) from a name (Hostname)•Name server–one implementation of a resolution mechanism–widely used in the InternetA simple exampleTCPweb browserprogramIPuserwww.sfc.keio.ac.jpwww.sfc.keio.ac.jp133.27.4.212133.27.4.212ProvidingName to Valuemapping serviceNameServerName Service in Internet•How to manage Name Space ?–flat vs hierarchical –Who names hosts ?–What kind of mapping data to maintain ?–How to maintain the mapping database ?•How to resolve the names of all over the Internet ?–How to look up ?–Who will answer ?•RFC (Standard 13)–RFC 1034 “Domain names – concepts and facilities”–RFC 1035 “Domain Names - Implementation and Specification”Domain tree jp orguk・or ad coac…………ccTLDHierarchical name spaceExample: www.sfc.keio.ac.jpsfcu-tokyokeioSLDcnncomRoot Domainwww.sfc.keio.ac.jpccwidegTLDTLD: Top Level DomaingTLD: generic TLDccTLD: Country Code TLDSLD: Second Level DomainDelegations•Names are unique in domain•Each domain is maintained by the domain registry–The root domain is maintained by ICANN/IANA–Top level domains are maintained by TLD registries•jp ccTLD domain : JPNIC•.COM gTLD domain : NSI •Registry for a domain delegates its sub domains to lower registries–JPNIC delegates “keio.ac.jp” domain to Keio UniversityDomains and ZonesPartition hierarchy into data administration unit called “zones”Each zone implemented by a set of name servers jp eduuk・or ad coac…………sfcu-tokyokeiocnncomRoot zonewww.sfc.keio.ac.jpccwidejp zonejp.ad zoneZones and name servers•Name-value mapping information are maintained by each zone–“domain” is name administrative boundary–“zone” is mapping data administrative boundary•A set of responsible name servers are running for each zone•A server can be responsible for multiple zonesName Servers•Each name server maintains a name-value mapping information called “resource records” of a zone it is responsible for.•Each name server resolves names and answers to queries based on the resource records it maintains.•Name servers have a mechanism to synchronize and update the resource records for a zone among primary and secondary servers.Resource Records•A resource record contains;<Name, Value, Type, Class, TTL>•Name/Value –not necessarily hostnames to IP addresses–depend on “TYPE”•Class = IN–allows other entities to define a new class•TTL (Time To Live)–How long the RR is valid–optionalTypes or RR•A (Address) –Name=hostname / Value = IP address•CNAME (Canonical NAME)–Name = hostname / Value = canonical name–Used for aliasing•NS (Name Server) –Name=domain name / Value = hostname of Name server for that domain•MX (Mail eXchange)–Name=domain name / Value = hostname of Mail server for that domain, preference=dd•SOA (Start Of Authority) –Name = domain name / Value = several information of name servers and data it maintains•PTR (domain name PoinTeR)–Name = IP address / Value = hostname–Used for “reverse” lookupRR Examples (1)•RR in “Root zone” name servers<jp, ns1.nic.ad.jp, NS, IN>< ns1.nic.ad.jp, 202.12.30.33, A, IN>•RR in “JP zone” name servers<ad.jp, ns0.nic.ad.jp, NS, IN><ns0.nic.ad.jp, 202.12.30.131, A, IN><ac.jp, ns0.nic.ad.jp, NS, IN><ns0.nic.ad.jp, 202.12.30.131, A, IN><keio.ac.jp, ns0.keio.ac.jp, NS, IN><ns0.keio.ac.jp, 133.27.4.121, A, IN><wide.ad.jp, ns.wide.ad.jp, NS, IN><ns.wide.ad.jp, 203.178.136.63, A, IN>RR Examples (2)•RR in “keio.ac.jp zone” name server<sfc.keio.ac.jp, ns1.sfc.keio.ac.jp, NS, IN><ns1.sfc.keio.ac.jp, 133.27.4.2, A, IN><cc.keio.ac.jp, kogwy.cc.keio.ac.jp, NS, IN><kogwy.cc.keio.ac.jp, 131.113.1.1, A, IN>•RR in “sfc.keio.ac.jp” name server<ccz02.sfc.keio.ac.jp, 133.27.4.212, A, IN><www.sfc.keio.ac.jp, ccz02.sfc.keio.ac.jp, CNAME, IN><sfc.keio.ac.jp, mail.sfc.keio.ac.jp, MX, IN>Name resolutionTCPweb browserprogramIPuserwww.sfc.keio.ac.jp133.27.4.212・jpackeio1. root zone(root server)2. jp zone(ns.nic.ad.jp)3. ac.jp zone(ns.nic.ad.jp)4. keio.ac.jp zone(ns0.keio.ad.jp)sfc5. sfc.keio.ac.jp zone(ns1.sfc.keio.ac.jp)LocalNameServer.jpac.jpkeio.ac.jpsfc.keio.ac.jpwww.sfc.keio.ac.jp133.27.4.212Name serverswww.sfc.keio.ac.jp133.27.4.212bootstrap ?•How the “web browser” program find a local server ?•How the local server find the root server which it sends a query first ?Local and Root•Each host somehow should know the local name server(s) which answers all queries.–/etc/resolv.conf (in one unix implementation)•Each local name server somehow should know the Root name servers to send query first.–bootstrap information called “Root cache”–currently manually distributed from InterNICRoot name servers•RR in Root cache<. , A.ROOT-SERVERS.NET, NS, IN, 3600000><A.ROOT-SERVERS.NET, 198.41.0.4, A, IN, 3600000><. , B.ROOT-SERVERS.NET, NS, IN, 3600000><B.ROOT-SERVERS.NET, 128.9.0.107, A, IN, 3600000>•Root name servers–Theoretically all queries come to the root server first–Connectivity from a local server to one of the root server is mandatory for name lookup–caching helps for better performanceList of the root name serversname org city type urla InterNIC Herndon,VA, US comhttp:/ / www.internic.orgb ISIMarina del Rey,CA, USedu http:/ / www.isi.edu/c PSInet Herndon,VA, US comhttp:/ / www.psi.net/d UMDCollege Park,MD, USedu http:/ / www.umd.edu/e NASA Mt View, CA, US usg http:/ / www.nasa.gov/f
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