Translating Addresses Reading: Section 4.1 and 9.1Goals of Today’s LectureDomain Name System (DNS)Outline: Domain Name SystemHost Names vs. IP addressesSeparating Naming and AddressingStrawman Solution #1: Local FileStrawman Solution #2: Central ServerSlide 9DNS Root ServersTLD and Authoritative DNS ServersDistributed Hierarchical DatabaseUsing DNSExampleRecursive vs. Iterative QueriesDNS CachingNegative CachingDNS Resource RecordsDNS ProtocolReliabilityInserting Resource Records into DNSPlaying With Dig on UNIXBoot-Strapping an End HostHow To Bootstrap an End Host?Avoiding Manual ConfigurationKey Ideas in Both ProtocolsNeed Yet Another Kind of IdentityMAC Address vs. IP AddressMAC Addresses on a LANBootstrapping ProblemBroadcastingResponse from the DHCP ServerDynamic Host Configuration ProtocolDeciding What IP Address to OfferSoft State: Refresh or ForgetSo, Now the Host Knows ThingsSending Packets Over a LinkAddress Resolution Protocol TableExample: A Sending a Packet to BBasic StepsHost A Learns the IP Address of BSlide 42Host A Decides to Send Through RHost A Sends Packet Through RSlide 45R Decides how to Forward PacketRouter R Wants to Forward PacketR Sends Packet to BSlide 49Conclusion1Translating AddressesReading: Section 4.1 and 9.1COS 461: Computer NetworksSpring 2007 (MW 1:30-2:50 in Friend 004)Jennifer RexfordTeaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/2Goals of Today’s Lecture•Three different kinds of addresses–Host names (e.g., www.cnn.com)–IP addresses (e.g., 64.236.16.20)–MAC addresses (e.g., 00-15-C5-49-04-A9)•Protocols for translating between addresses–Domain Name System (DNS)–Dynamic Host Configuration Protocol (DHCP)–Address Resolution Protocol (ARP)•Two main topics–Decentralized management of the name space–Boot-strapping an end host that attaches to the ‘net3Domain Name System (DNS)Proposed in 1983 by Paul Mockapetris4Outline: Domain Name System•Computer science concepts underlying DNS–Indirection: names in place of addresses–Hierarchy: in names, addresses, and servers–Caching: of mappings from names to/from addresses•DNS software components–DNS resolvers –DNS servers •DNS queries–Iterative queries–Recursive queries•DNS caching based on time-to-live (TTL)5Host Names vs. IP addresses•Host names–Mnemonic name appreciated by humans–Variable length, alpha-numeric characters–Provide little (if any) information about location–Examples: www.cnn.com and ftp.eurocom.fr•IP addresses–Numerical address appreciated by routers–Fixed length, binary number–Hierarchical, related to host location–Examples: 64.236.16.20 and 193.30.227.1616Separating Naming and Addressing•Names are easier to remember–www.cnn.com vs. 64.236.16.20•Addresses can change underneath–Move www.cnn.com to 173.15.201.39–E.g., renumbering when changing providers•Name could map to multiple IP addresses–www.cnn.com to multiple replicas of the Web site•Map to different addresses in different places–Address of a nearby copy of the Web site–E.g., to reduce latency, or return different content•Multiple names for the same address–E.g., aliases like ee.mit.edu and cs.mit.edu7Strawman Solution #1: Local File•Original name to address mapping–Flat namespace–/etc/hosts –SRI kept main copy–Downloaded regularly•Count of hosts was increasing: moving from a machine per domain to machine per user–Many more downloads–Many more updates8Strawman Solution #2: Central Server•Central server–One place where all mappings are stored–All queries go to the central server•Many practical problems–Single point of failure–High traffic volume–Distant centralized database–Single point of update–Does not scaleNeed a distributed, hierarchical collection of servers9Domain Name System (DNS)•Properties of DNS–Hierarchical name space divided into zones–Distributed over a collection of DNS servers•Hierarchy of DNS servers–Root servers–Top-level domain (TLD) servers–Authoritative DNS servers•Performing the translations–Local DNS servers–Resolver software10DNS Root Servers•13 root servers (see http://www.root-servers.org/)•Labeled A through MB USC-ISI Marina del Rey, CAL ICANN Los Angeles, CAE NASA Mt View, CAF Internet Software C. Palo Alto, CA (and 17 other locations)I Autonomica, Stockholm (plus 3 other locations)K RIPE London (also Amsterdam, Frankfurt)m WIDE TokyoA Verisign, Dulles, VAC Cogent, Herndon, VA (also Los Angeles)D U Maryland College Park, MDG US DoD Vienna, VAH ARL Aberdeen, MDJ Verisign, ( 11 locations)11TLD and Authoritative DNS Servers•Top-level domain (TLD) servers–Generic domains (e.g., com, org, edu)–Country domains (e.g., uk, fr, ca, jp)–Typically managed professionallyNetwork Solutions maintains servers for “com”Educause maintains servers for “edu”•Authoritative DNS servers–Provide public records for hosts at an organization–For the organization’s servers (e.g., Web and mail)–Can be maintained locally or by a service provider12Distributed Hierarchical Databasecom edu org acukzwarpaunnamed rootbarwest eastfoo myaccamusrin-addr123456generic domains country domainsmy.east.bar.eduusr.cam.ac.uk12.34.56.0/2413Using DNS•Local DNS server (“default name server”)–Usually near the end hosts who use it–Local hosts configured with local server (e.g., /etc/resolv.conf) or learn the server via DHCP•Client application–Extract server name (e.g., from the URL)–Do gethostbyname() to trigger resolver code•Server application–Extract client IP address from socket–Optional gethostbyaddr() to translate into name14requesting hostcis.poly.edugaia.cs.umass.eduroot DNS serverlocal DNS serverdns.poly.edu123456authoritative DNS serverdns.cs.umass.edu78TLD DNS serverExampleHost at cis.poly.edu wants IP address for gaia.cs.umass.edu15Recursive vs. Iterative Queries•Recursive query–Ask server to get answer for you–E.g., request 1 and response 8•Iterative query–Ask server who to ask next–E.g., all other request-response pairsrequesting hostcis.poly.eduroot DNS serverlocal DNS serverdns.poly.edu123456authoritative DNS serverdns.cs.umass.edu78TLD DNS server16DNS Caching•Performing all these queries take time–And all this before the actual communication takes place–E.g., 1-second latency before starting Web download•Caching can substantially reduce overhead–The top-level
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