1CS677: Distributed OSComputer ScienceLecture 9, page 1Today: Naming• Names are used to share resources, uniquely identifyentities 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-distributedsystems– 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-friendly2CS677: 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 server3CS677: 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-scalename space partitioned into a global layer, as an administrational layer, and amanagerial layer.• The more stable a layer, the longer are the lookups valid (and can be cachedlonger)SometimesYesYesIs client-side caching applied?NoneNone or fewManyNumber of replicasImmediateImmediateLazyUpdate propagationImmediateMillisecondsSecondsResponsiveness to lookupsVast numbersManyFewTotal number of nodesDepartmentOrganizationWorldwideGeographical scale of networkManagerialAdministrationalGlobalItem4CS677: 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 layer5CS677: Distributed OSComputer ScienceLecture 9, page 9Which is better?• Recursive name resolution puts heavy burden on goballayer 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 nameresolution with respect to communication costs– Recursive may be cheaper6CS677: Distributed OSComputer ScienceLecture 9, page 11The DNS Name Space• The most important types of resource records forming the contentsof nodes in the DNS name space.Contains any entity-specific information considered usefulAny kindTXTHolds information on the host this node representsHostHINFOContains the canonical name of a hostHostPTRSymbolic link with the primary name of the represented nodeNodeCNAMERefers to a name server that implements the represented zoneZoneNSRefers to a server handling a specific serviceDomainSRVRefers to a mail server to handle mail addressed to this nodeDomainMXContains an IP address of the host this node representsHostAHolds information on the represented zoneZoneSOADescriptionAssociatedentityType ofrecordCS677: Distributed OSComputer ScienceLecture 9, page 12DNS Implementation• An excerptfrom theDNSdatabase forthe zonecs.vu.nl.7CS677: 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 fullname– 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 entryusing X.500 naming conventions.130.37.21.11--WWW_Server130.37.21.11--FTP_Server130.37.24.6, 192.31.231,192.31.231.66--Mail_ServersMain serverCNCommonNameMath. & Comp. Sc.OUOrganizationalUnitVrije UniversiteitLOrganizationAmsterdamLLocalityNLCCountryValueAbbr.Attribute8CS677: Distributed OSComputer ScienceLecture 9, page 15The X.500 Name Space (2)• Part of thedirectoryinformation 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