115-744: Computer NetworkingL-7 NamingL -7; 11-5-04© Srinivasan Seshan, 2004 2Next Lecture: DNS, P2P & P2P Apps • Readings• [JBSM01] DNS Performance and the Effectiveness of Caching• [BLR04] A Layered Naming Architecture for the Internet• [S+01] Chord: A Scalable Peer-to-peer Lookup Service for Internet Applications• [G+03] The Impact of DHT Routing Geometry on Resilience and ProximityL -7; 11-5-04© Srinivasan Seshan, 2004 3Naming• How do we efficiently locate resources?• DNS: name IP address• Service location: description host• Other issues• How do we scale these to the wide area?• How to choose among similar services?L -7; 11-5-04© Srinivasan Seshan, 2004 4Overview• DNS• Server Selection and CDNs• P2P Lookup Overview• Centralized/Flooded Lookups• Routed Lookups – Chord• Comparison of DHTs• Layered Naming2L -7; 11-5-04© Srinivasan Seshan, 2004 5Obvious Solutions (1)Why not centralize DNS?• Single point of failure• Traffic volume• Distant centralized database• Single point of update• Doesn’t scale!L -7; 11-5-04© Srinivasan Seshan, 2004 6Obvious Solutions (2)Why not use /etc/hosts?• Original Name to Address Mapping• Flat namespace• /etc/hosts • SRI kept main copy• Downloaded regularly• Count of hosts was increasing: machine per domain machine per user• Many more downloads• Many more updatesL -7; 11-5-04© Srinivasan Seshan, 2004 7Domain Name System Goals• Basically building a wide area distributed database• Scalability• Decentralized maintenance• Robustness• Global scope • Names mean the same thing everywhere• Don’t need• Atomicity• Strong consistencyL -7; 11-5-04© Srinivasan Seshan, 2004 8DNS RecordsRR format: (class, name, value, type, ttl)• DB contains tuples called resource records (RRs)• Classes = Internet (IN), Chaosnet (CH), etc.• Each class defines value associated with typeFOR IN class:• Type=A• name is hostname• value is IP address• Type=NS• name is domain (e.g. foo.com)• value is name of authoritative name server for this domain• Type=CNAME• name is an alias name for some “canonical” (the real) name• value is canonical name• Type=MX• value is hostname of mailserver associated with name3L -7; 11-5-04© Srinivasan Seshan, 2004 9DNS Design: Hierarchy Definitionsrootedunetorgukcomgwu ucb cmu bumitcsececmcl• 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.eduL -7; 11-5-04© Srinivasan Seshan, 2004 10DNS Design: Zone Definitionsrootedunetorgukcomcagwu ucb cmu bumitcsececmclSingle nodeSubtreeComplete Tree• Zone = contiguous section of name space• E.g., Complete tree, single node or subtree• A zone has an associated set of name serversL -7; 11-5-04© Srinivasan Seshan, 2004 11DNS Design: Cont.• Zones are created by convincing owner node to create/delegate a subzone• Records within zone stored multiple redundant name servers• Primary/master name server updated manually• Secondary/redundant servers updated by zone transfer of name space• Zone transfer is a bulk transfer of the “configuration” of a DNS server – uses TCP to ensure reliability• Example:• CS.CMU.EDU created by CMU.EDU administratorsL -7; 11-5-04© Srinivasan Seshan, 2004 12Servers/Resolvers• Each host has a resolver• Typically a library that applications can link to• Local name servers hand-configured (e.g. /etc/resolv.conf)• Name servers• Either responsible for some zone or…• Local servers• Do lookup of distant host names for local hosts• Typically answer queries about local zone4L -7; 11-5-04© Srinivasan Seshan, 2004 13DNS: Root Name Servers• Responsible for “root” zone• Approx. dozen root name servers worldwide• Currently {a-m}.root-servers.net• Local name servers contact root servers when they cannot resolve a name• Configured with well-known root serversL -7; 11-5-04© Srinivasan Seshan, 2004 14DNS Message FormatIdentificationNo. of QuestionsNo. of Authority RRsQuestions (variable number of answers)Answers (variable number of resource records)Authority (variable number of resource records)Additional Info (variable number of resource records)FlagsNo. of Answer RRsNo. of Additional RRsName, type fields for a queryRRs in response to queryRecords for authoritative serversAdditional “helpful info that may be used12 bytesL -7; 11-5-04© Srinivasan Seshan, 2004 15DNS Header Fields• Identification• Used to match up request/response• Flags• 1-bit to mark query or response• 1-bit to mark authoritative or not• 1-bit to request recursive resolution• 1-bit to indicate support for recursive resolutionL -7; 11-5-04© Srinivasan Seshan, 2004 16Typical ResolutionClientLocal DNS serverroot & eduDNS serverns1.cmu.edu DNS serverwww.cs.cmu.eduNS ns1.cmu.eduwww.cs.cmu.eduNS ns1.cs.cmu.eduA www=IPaddrns1.cs.cmu.eduDNSserver5L -7; 11-5-04© Srinivasan Seshan, 2004 17Typical Resolution• Steps for resolving www.cmu.edu• Application calls gethostbyname() (RESOLVER)• Resolver contacts local name server (S1)•S1queries root server (S2) for (www.cmu.edu)•S2returns NS record for cmu.edu (S3)• What about A record for S3?• This is what the additional information section is for (PREFETCHING)•S1queries S3for www.cmu.edu•S3returns A record for www.cmu.edu• Can return multiple A records what does this mean?L -7; 11-5-04© Srinivasan Seshan, 2004 18Lookup MethodsRecursive query:• Server goes out and searches for more info (recursive)• Only returns final answer or “not found”Iterative query:• Server responds with as much as it knows (iterative)• “I don’t know this name, but ask this server”Workload impact on choice?• Local server typically does recursive• Root/distant server does iterativerequesting hostsurf.eurecom.frgaia.cs.umass.eduroot name serverlocal name serverdns.eurecom.fr123456authoritative name serverdns.cs.umass.eduintermediate name serverdns.umass.edu78iterated queryL -7; 11-5-04© Srinivasan Seshan, 2004 19Workload and Caching• What workload do you expect for different servers/names?• Why might this be a problem? How can we solve this problem?• DNS responses are cached • Quick response for repeated translations• Other queries may reuse some parts of lookup• NS records for domains •
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