naming chapter 4. basic naming terminology name: a string of bits or characters that is used to...
TRANSCRIPT
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Naming
Chapter 4
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Basic Naming Terminology
Name: A string of bits or characters that is used to refer to an entity (i.e. hosts, printers, disks, files).
Access point: An entity through which other entities in a distributed system are accessed.
Address: a special kind of name that refers to an access point of an entity.
An entity can – offer more than one access point.– change its access point
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Separation an Entity from its Address– Changing an access point or reassigning an access point
• Reorganizing a distributed system
• Using an address to refer to an entity leads to invalid reference
• Refer to the service by a name independent of the address of its access point
– How to reference an entity that offers more than one access point
• Web services that are distributed across several servers
• Refer to the web service by a name independent of the addresses of its access points
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Naming versus Locating Entities
a) Direct, single level mapping between names and addresses.b) T-level mapping using identities.
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Characteristics of an identifier
– Identifiers: Names that are used to uniquely identify an entity
– Properties of an identifier that helps to defines names that are location independent:
• An identifier refers to at most one entity
• Each entity is referred to by at most one identifier
• An identifier always refers to the same entity
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Flat Naming– Broadcasting and Multicasting
– Applicable in LANs
– Address Resolution Protocol (ARP)
– Forwarding Pointers• When an entity moves from A to B it leaves a forwarding
reference– SSP chains: when an object moves from address space A to B it leaves
behind a client stub in A and installs a server stub that refers to it in B.
– Home-based approaches– Hierarchical approaches
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Forwarding Pointers (1)
The principle of forwarding pointers using (proxy, skeleton) pairs.
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Forwarding Pointers (2)
Redirecting a forwarding pointer, by storing a shortcut in a proxy.
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Home-Based Approaches• Home location keeps track of the current location of an entity
• Mobile IP– Each mobile host uses a fixed IP
– All communication to that entity is directed to the host’s home agent
– This home agent is located on the LAN corresponding to the network address in the mobile host’s IP address
• Drawback: – increased latency in large networks
– Use of a fixed home location
• Solution– register the home at a traditional naming service
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Home-Based Approaches
The principle of Mobile IP.
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Hierarchical Approaches (1)
Hierarchical organization of a location service into domains, each having an associated directory node.
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Hierarchical Approaches (2)
An example of storing information of an entity having two addresses in different leaf domains.
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Hierarchical Approaches (3)
Looking up a location in a hierarchically organized location service.
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Hierarchical Approaches (4)
a) An insert request is forwarded to the first node that knows about entity E.
b) A chain of forwarding pointers to the leaf node is created.
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Structured Naming
– Name Spaces– Name space distribution– Implementing of name resolution– Domain Name System
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Name Spaces (1)
A general naming graph with a single root node.
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Name Spaces (2)
The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks.
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Linking and Mounting (1)
The concept of a symbolic link explained in a naming graph.
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Linking and Mounting (2)
Mounting remote name spaces through a specific process protocol.
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Linking and Mounting (3)
Organization of the DEC Global Name Service
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Name Space Distribution (1)
An example partitioning of the DNS name space, including Internet-accessible files, into three layers.
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Name Space Distribution (2)
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.
Item Global Administrational Managerial
Geographical scale of network Worldwide Organization Department
Total number of nodes Few Many Vast numbers
Responsiveness to lookups Seconds Milliseconds Immediate
Update propagation Lazy Immediate Immediate
Number of replicas Many None or few None
Is client-side caching applied? Yes Yes Sometimes
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Implementation of Name Resolution (1)
The principle of iterative name resolution.
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Implementation of Name Resolution (2)
The principle of recursive name resolution.
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Implementation of Name Resolution (3)
Recursive name resolution of <nl, vu, cs, ftp>. Name servers cache intermediate results for subsequent lookups.
Server for node
Should resolve
Looks upPasses to
childReceives
and cachesReturns to requester
cs <ftp> #<ftp> -- -- #<ftp>
vu <cs,ftp> #<cs> <ftp> #<ftp> #<cs>#<cs, ftp>
ni <vu,cs,ftp> #<vu> <cs,ftp> #<cs>#<cs,ftp>
#<vu>#<vu,cs>#<vu,cs,ftp>
root <ni,vu,cs,ftp> #<nl> <vu,cs,ftp> #<vu>#<vu,cs>#<vu,cs,ftp>
#<nl>#<nl,vu>#<nl,vu,cs>#<nl,vu,cs,ftp>
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Implementation of Name Resolution (4)
The comparison between recursive and iterative name resolution with respect to communication costs.
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The DNS Name Space
The most important types of resource records forming the contents of nodes in the DNS name space.
Type of record
Associated entity
Description
SOA Zone Holds information on the represented zone
A Host Contains an IP address of the host this node represents
MX Domain Refers to a mail server to handle mail addressed to this node
SRV Domain Refers to a server handling a specific service
NS Zone Refers to a name server that implements the represented zone
CNAME Node Symbolic link with the primary name of the represented node
PTR Host Contains the canonical name of a host
HINFO Host Holds information on the host this node represents
TXT Any kind Contains any entity-specific information considered useful
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DNS Implementation (1)
An excerpt from the
DNS database for the zone
cs.vu.nl.
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DNS Implementation (2)
Part of the description for the vu.nl domain which contains the cs.vu.nl domain.
Name Record type Record value
cs.vu.nl NIS solo.cs.vu.nl
solo.cs.vu.nl A 130.37.21.1
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Attribute-based Naming
A simple example of a X.500 directory entry using X.500 naming conventions.
Attribute Abbr. Value
Country C NL
Locality L Amsterdam
Organization L Vrije Universiteit
OrganizationalUnit OU Math. & Comp. Sc.
CommonName CN Main server
Mail_Servers -- 130.37.24.6, 192.31.231,192.31.231.66
FTP_Server -- 130.37.21.11
WWW_Server -- 130.37.21.11
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The X.500 Name Space (1)
Part of the directory information tree.
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The X.500 Name Space (2)
Two directory entries having Host_Name as RDN.
Attribute Value Attribute Value
Country NL Country NL
Locality Amsterdam Locality Amsterdam
Organization Vrije Universiteit Organization Vrije Universiteit
OrganizationalUnit Math. & Comp. Sc. OrganizationalUnitMath. & Comp. Sc.
CommonName Main server CommonName Main server
Host_Name star Host_Name zephyr
Host_Address 192.31.231.42 Host_Address 192.31.231.66
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References
Lecture slides of Distributed Systems principles and paradigms by Andrew Tannenbaum and Van Steen, Prentice Hall India, 2002.