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Today: NamingExample: File NamesResolving File Names across MachinesName Space DistributionName Space Distribution ExampleName Space DistributionImplementing Name ResolutionRecursive Name ResolutionWhich is better?Communication costsThe DNS Name SpaceDNS ImplementationX.500 Directory ServiceThe X.500 Name Space (1)The X.500 Name Space (2)LDAPCS677: Distributed OSComputer ScienceLecture 9, page 1Today: Naming•Names are used to share resources, uniquely identify entities and refer to locations•Need to map from name to the entity it refers to–E.g., Browser access to www.cnn.com–Use name resolution•Differences in naming in distributed and non-distributed systems–Distributed systems: naming systems is itself distributed•How to name mobile entities?CS677: Distributed OSComputer ScienceLecture 9, page 2Example: File Names•Hierarchical directory structure (DAG)–Each file name is a unique path in the DAG –Resolution of /home/steen/mbox a traversal of the DAG•File names are human-friendlyCS677: Distributed OSComputer ScienceLecture 9, page 3Resolving File Names across Machines•Remote files are accessed using a node name, path name•NFS mount protocol: map a remote node onto local DAG–Remote files are accessed using local names! (location independence)–OS maintains a mount table with the mappingsCS677: Distributed OSComputer ScienceLecture 9, page 4Name Space Distribution•Naming in large distributed systems–System may be global in scope (e.g., Internet, WWW)•Name space is organized hierarchically–Single root node (like naming files)•Name space is distributed and has three logical layers–Global layer: highest level nodes (root and a few children)•Represent groups of organizations, rare changes–Administrational layer: nodes managed by a single organization•Typically one node per department, infrequent changes–Managerial layer: actual nodes•Frequent changes–Zone: part of the name space managed by a separate name serverCS677: Distributed OSComputer ScienceLecture 9, page 5Name Space Distribution Example•An example partitioning of the DNS name space, including Internet-accessible files, into three layers.CS677: Distributed OSComputer ScienceLecture 9, page 6Name Space Distribution •A comparison between name servers for implementing nodes from a large-scale name space partitioned into a global layer, as an administrational layer, and a managerial layer.•The more stable a layer, the longer are the lookups valid (and can be cached longer)Item Global Administrational ManagerialGeographical scale of network Worldwide Organization DepartmentTotal number of nodes Few Many Vast numbersResponsiveness to lookups Seconds Milliseconds ImmediateUpdate propagation Lazy Immediate ImmediateNumber of replicas Many None or few NoneIs client-side caching applied? Yes Yes SometimesCS677: Distributed OSComputer ScienceLecture 9, page 7Implementing Name Resolution•Iterative name resolution–Start with the root–Each layer resolves as much as it can and returns address of next name serverCS677: Distributed OSComputer ScienceLecture 9, page 8Recursive Name Resolution •Recursive name resolution–Start at the root–Each layer resolves as much as it can and hands the rest to the next layerCS677: Distributed OSComputer ScienceLecture 9, page 9Which is better?•Recursive name resolution puts heavy burden on gobal layer nodes–Burden is heavy => typically support only iterative resolution•Advantages of recursive name resolution–Caching possible at name servers (gradually learn about others)•Caching improves performance•Use time-to-live values to impose limits on caching duration•Results from higher layers can be cached for longer periods•Iterative: only caching at client possibleCS677: Distributed OSComputer ScienceLecture 9, page 10Communication costs•The comparison between recursive and iterative name resolution with respect to communication costs–Recursive may be cheaperCS677: Distributed OSComputer ScienceLecture 9, page 11The DNS Name Space•The most important types of resource records forming the contents of nodes in the DNS name space.Type of recordAssociated entityDescriptionSOA Zone Holds information on the represented zoneA Host Contains an IP address of the host this node representsMX Domain Refers to a mail server to handle mail addressed to this nodeSRV Domain Refers to a server handling a specific serviceNS Zone Refers to a name server that implements the represented zoneCNAME Node Symbolic link with the primary name of the represented nodePTR Host Contains the canonical name of a hostHINFO Host Holds information on the host this node representsTXT Any kind Contains any entity-specific information considered usefulCS677: Distributed OSComputer ScienceLecture 9, page 12DNS Implementation •An excerpt from the DNS database for the zone cs.vu.nl.CS677: Distributed OSComputer ScienceLecture 9, page 13X.500 Directory Service•OSI Standard•Directory service: special kind of naming service where:–Clients can lookup entities based on attributes instead of full name–Real-world example: Yellow pages: look for a plumberCS677: Distributed OSComputer ScienceLecture 9, page 14The X.500 Name Space (1)•A simple example of a X.500 directory entry using X.500 naming conventions.Attribute Abbr. ValueCountry C NLLocality L AmsterdamOrganization L Vrije UniversiteitOrganizationalUnit OU Math. & Comp. Sc.CommonName CN Main serverMail_Servers -- 130.37.24.6, 192.31.231,192.31.231.66FTP_Server -- 130.37.21.11WWW_Server -- 130.37.21.11CS677: Distributed OSComputer ScienceLecture 9, page 15The X.500 Name Space (2)•Part of the directory information tree.CS677: Distributed OSComputer ScienceLecture 9, page 16LDAP•Lightweight Directory Access Protocol (LDAP)–X.500 too complex for many applications–LDAP: Simplified version of X.500–Widely used for Internet services–Application-level protocol, uses TCP–Lookups and updates can use strings instead of OSI encoding–Use master servers and replicas servers for performance improvements–Example LDAP implementations: •Active Directory (Windows 2000)•Novell Directory services•iPlanet directory services (Netscape)•Typical uses: user profiles, access privileges, network


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