Name Services

1
Name Services

• Bina Ramamurthy

2
Introduction

• In a distributed system names are used to refer
  to a wide variety of resources such as computers,
  services, remote objects, and files as well as
  users.
• Names facilitate communication and resource
  sharing.
• Names are used for identification as well as for
  describing attributes.
• Naming Service provides data about named objects
  in a distributed systems.
• Directory and discovery service locates objects
  that satisfy a given description.
• We will discuss services needed to look up
  resources from names and description.

3
Names, Addresses and other attributes

• Any process that requires access to a specific
  resource must posses a name or identifier for it.
  Ex URL http//www.cdk3.net/
• A name has to be looked up before it can be used.
• A name is said to be resolved when it is
  translated into data about the resource or object.

4
Names and Binding

• Names are bound to the attributes of named
  objects (and not to any specific
  implementations.)
• The association between a name and an object is
  called binding.
• Services are written to map between names and the
  attributes of objects they refer to.
• Example domain name? Domain name Service (DNS)
  maps ? attributes of the host computer

5
Composed naming domains used to access a resource
from a URL
6
Composed Domain Name

• A URL is composed of names from three naming
  domains IP (IP address), port number (IPC
  domain, socket), and file name (file system).
• All three have to successfully resolved to get at
  the resource.
• An IP address must be looked up to obtain the
  network address such as an Ethernet address.
  Similarly port number and file name have to be
  resolved by the file system.
• In the last slide an URL is resolved by a DNS
  into IP address, and then ARP into Ethernet
  address.

7
Universal Resource (L,I,N and C)

• URL universal Resource Locator is the principle
  means of identifying web resources.
• Advantages Scalability and efficiency.
• Disadvantages dangling links on removal of a
  resource.
• URLs are particular type of URI (UR Identifier).
  Another type of URI is URN (UR Name).
• URNs are intended to solve the dangling link
  problem and to provide richer modes of finding
  resources on the web.
• URN lookup service will provide URN given a URL.
• Of course the owner registers the resource with
  the lookup service when it moves.
• URC (UR Characteristics) is for looking up a
  resource that matches a description of one or
  more of its attributes Ex authorLeslie
  Lamport

8
Universal Resource Names

• Is an abstraction over URLs.
• A URN is of the form
• urnnameSpacenameSpace-specificName
• urndoi10.555/music-pop-1234
• Refers to publication called music-pop-1234 in
  the naming scheme of publisher known as 10.555 in
  the Digital Object Identifier (doi) scheme
  (www.doi.org).
• Another urn example
• urndcs.gormenghast.ac.ukTR2000-56 can be used
  to get the latest information about technical
  report TR2000-56 registered with the URL lookup
  service gormenghast.ac.uk department of CS (dcs)

9
Server Types

• Connectionless vs. connection-oriented
• Stateless vs. stateful
• Concurrent processing vs. iterative processing

Iterative Connection-oriented
Iterative Connectionless
Concurrent Connection-oriented
Concurrent Connectionless
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Name Service

• A name service stores a collection of one or more
  naming contexts sets of bindings between
  textual names and attributes for objects such as
  users, computers, services and remote objects.
  (Definition)
• Major operation resolve a name to look up
  attributes from a given name. (Operational
  Requirements)
• Other operations required creating new binding,
  deleting bindings, listing bound names and adding
  and deleting contexts.

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General Name Service Requirements

• Originally Name Service was simple since it was
  to bind names to addresses in a single domain.
  Interconnection of networks and increased scale
  of distributed systems have produced a much
  larger name-mapping problem.
• To handle arbitrary number of names and to server
  arbitrary number of administrative organizations.
• A long lifetime.
• High availability
• Fault isolation
• Tolerance of mistrust

12
Design Issues

• Main design issues for Name services are (Taking
  DNS as case study)
• Name spaces
• Name Resolution
• The domain name system

13
Name Spaces

• A name space is a collection of all valid names
  recognized by a particular service.
• Name may be bound to an object or unbound.
• Name spaces require syntactic definition.
• For example two is not an unix process name
  whereas 2 is, and is not acceptable as a DNS
  computer name.
• Name spaces are typically hierarchical.
• DNS names are called domain names examples
  bruno.dcs.qmw.ac.uk (computer) dcs.qmw.ac.uk,
  com, purdue.edu (domains).
• Not case sensitive ac.uk is same as AC.UK

14
Name Space (contd.)

• Aliases DNS allows aliases in which one domain
  name may stand for another.
• A very popular alias is that for a web server
  www.dcs.qmw.ac.uk stands for copper.dcs.qmw.ac.uk
• This has the advantage that the clients can refer
  to the web server by one generic name.

15
Combining Name Spaces

• DNS provides a global and homogeneous name space
  in which a given name refers to the same entity,
  no matter which process on which computer looks
  up the name.
• But some other name services allow distinct name
  space and even heterogeneous name spaces to be
  embedded in them. Issues merging, heterogeneity,
  and customization.
• We will look at heterogeneity in DCE.

16
DCEs Heterogeneous Namespace

• DCE allows heterogeneous namespaces to be
  embedded within it. DCE names are called
  junctions which are similar to mount points in
  NFS and Unix.
• Example consider
• //dcs.qmw.ac.uk/principals/Jean.Dollimore
• //dcs.qmw.ac.uk denotes a context called a cell.
• Next component principals is a junction which
  is a context containing security principals in
  which final component is Jean.Dollimore can be
  looked up.
• Another Example //dcs.qmw.ac.uk/files/pub/report
  s/TR2000-99 where files is the junction point.
• Principals and files are the root for the
  heterogeneous names space.

17
Name Resolution

• Resolution is an iterative process whereby a name
  is repeatedly presented to the naming contexts.
• A naming context either maps name on to a set of
  primitive attributes or it maps it on to a
  further naming context and a derived name to be
  presented to the context.
• Example1 /etc/passwd in which etc is presented
  to context / and paswd is presented to context
  /etc.
• Example 2 www.dcs.qmw.ac.uk in which the alias
  is resolved to another domain name such as
  copper.dcs.qmw.ac,uk which is further resolved to
  produce IP address.

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Name Servers and Navigation

• Any name service stores a very large database.
• Data is partitioned into servers according to its
  domain. In DNS most of the entries are local
  computers.
• Partitioning of the data implies that the local
  name server cannot answer all the enquiries
  without the help of other name servers. For
  example, the name server in the cse.buffalo.edu
  would not be able to IP address of a computer in
  cs.purdue.edu domain unless it is cached.
• Process of locating naming data from among more
  than one name server in order to resolve a name
  is called navigation. Ex Iterative Navigation

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Iterative navigation
20
Non-recursive and recursive server-controlled
navigation
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Caching

• Client name resolution software and servers
  maintain a cache of previous name resolutions.
• Server may use data from its own cache or other
  server cache it is authorized to access.
• Caching is key to performance and fault tolerance.

22
The Domain Name System

• The Domain Name System (DNS) is a name service
  whose principal naming database is used across
  the internet.
• It was designed by Mockapetris(1987) to replace
  the original internet naming scheme which did not
  scale well.
• Objects named by DNS are primarily computers for
  which IP addresses are stored as attributes.
• Millions of names are bound by Internet DNS,
  lookup are made from around the world. This is by
  hierarchical partitioning of name database,
  replication and caching.

23
Domain Names

• Top level organizational domains are partitioned
  geographically and organizationally. edu , uk
• DNS Queries host name resolution and well known
  services.

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Figure 9.4DNS name servers
Note Name server names are in italics, and the
corresponding domains are in parentheses.
Arrows denote name server entries