CTIS 490 DISTRIBUTED SYSTEMS

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CTIS 490DISTRIBUTED SYSTEMS

• WEEK 5
• DISTRIBUTED SYSTEMS
• NAMING

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INTRODUCTION

• Names are used to share resources, to uniquely
  identify entities, to refer to locations, etc.
• Name resolution allows a process to access the
  named entity.
• To resolve names, it is necessary to implement a
  naming system.
• The difference between naming in distributed
  systems and non-distributed systems lies in the
  way naming systems are implemented.
• In distributed systems, the implementation of a
  naming system is itself often distributed across
  multiple machines.
• One of the largest distributed naming services in
  use today is the Internet Domain Name System
  (DNS).

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NAMES AND ADDRESSES

• A name in a distributed system is a string of
  bits or characters that is used to refer to an
  entity.
• An entity in a distributed system can be
  anything
• Host
• Printer
• Disk
• File
• Process
• Mailbox
• Newsgroup
• Web page

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NAMES AND ADDRESSES

• To be able to use an entity, an access point is
  necessary.
• An access point is a special kind of entity in a
  distributed system.
• The name of an access point is called an address.
• An entity can offer more than one access point.
  As a comparison, a telephone is an access point
  of a person, whereas the telephone number is an
  address.
• An entity may change its access points. For
  example, when a mobile computer moves, it is
  assigned a different IP address than the one it
  had before.

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NAMES AND ADDRESSES

• A name for an entity that is independent from its
  address is often much easier and more flexible to
  use (location independence).
• A naming system maintains a name-to-address
  binding, which is in its simplest form is just a
  table of (name, address) pairs.
• However, in distributed systems that span large
  networks, a centralized table is not going to
  work.
• There are three classes of naming systems
• Unstructured (Flat) Naming
• Structured Naming
• Attribute-Based Naming

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UNSTRUCTURED NAMING

• Unstructured names do not contain any information
  on how to locate the access point of its
  associated entity.
• In general, unstructured naming systems are used
  in Local Area Networks.
• There are several approaches
• Broadcasting
• Multicasting
• Home-based
• Hierarchical

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BROADCASTING

• Local-Area Networks and Local-Area Wireless
  Networks offer broadcasting facilities.
• Locating an entity in such environment is easy
• A message containing the identifier of the entity
  is broadcast to each machine, and each machine is
  requested to check whether it has that entity.
• For example, the Internet Address Resolution
  Protocol (ARP) can be used to find the Ethernet
  (MAC) address of a machine when given an IP
  address.
• However, broadcasting becomes inefficient when
  the network grows.
• Network bandwidth is wasted
• Hosts are interrupted

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MULTICASTING

• Multicasting can also be used to locate entities.
  A multicast address can be used as a general
  location service for multiple entities.
• For example, the Internet supports network-level
  multicasting by allowing hosts to join a specific
  multicast group. Such groups are identified by a
  multicast address.
• When a host sends a message to a multicast
  address, the network layer provides a best-effort
  service to deliver that message to all group
  members.
• For example, when employee connects her laptop to
  a corporate network, it is dynamically assigned
  an IP address, and it joins a specific multicast
  group. Then, a process can locate the laptop by
  issuing a multicast message.

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FORWARDING POINTERS

• Another approach to locating entities is to make
  use of forwarding pointers.
• When an entity moves from A to B, it leaves
  behind in A a reference to its new location at B.
• But, a chain for a highly mobile entity can
  become very long, and all intermediate locations
  in a chain will have to maintain their part of
  the chain of forwarding pointers.
• Furthermore, if the forwarding pointer is lost,
  the chain link is broken.

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HOME-BASED APPROACHES

• A popular approach to supporting mobile entities
  in large-scale networks is to introduce a home
  location, which keeps track of the current
  location of an entity.
• In general, home location is chosen to be the
  place where an entity was created.
• Home-based approach is used in Mobile IP.
• Each mobile host uses a fixed IP address. All
  communication to that IP address is initially
  directed to the mobile hosts home agent.
• Whenever the mobile host moves to another
  network, it requests a temporary address that it
  can use for communication. This care-of address
  is registered at the home agent.

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MOBILE IP
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HIERARCHICAL APPROACHES

• In a hierarchical approach, a network is divided
  into a collection of domains.
• There is a single top-level domain that spans the
  entire network.
• Each domain can be subdivided into multiple,
  smaller sub-domains.
• A lowest-level domain, called a leaf domain,
  typically corresponds to a Local-Area Network or
  a cell in a mobile telephone network.
• Each domain D has an associated directory node
  that keeps track of the entities in that domain.
  This leads to a tree of directory nodes.
• There is a location record in the directory node
  to keep track of an entity, and it contains
  entitys current address.

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HIERARCHICAL APPROACHES
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HIERARCHICAL APPROACHES
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STRUCTURED NAMING

• Unstructured names are good for machines, but
  they are not convenient for humans to use.
• As an alternative, naming systems support
  structured names that are composed from simple,
  human-readable names.
• File naming and Internet host naming use this
  approach.
• These names are resolved to addresses.
• In structured naming, names are commonly
  organized into what is called a name space.

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NAME SPACES

• Name spaces for structured names can be
  represented as a labeled, directed graph with two
  types of nodes
• A leaf node represents a named entity, and it has
  the property that it has no outgoing edges.
• A directory node has a number of outgoing edges,
  each labeled with a name. A directory node stores
  a table in which an outgoing edge is represented
  as a pair (edge label, node identifier). Such a
  table is called a directory table.
• Each path in a naming graph can be referred to by
  a sequence of labels which is called a path name.
• If the first node in a path name is the root, it
  is called an absolute path name, otherwise it is
  called a ralative path name.

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NAME SPACES

• Naming graph is implemented in many file
• systems where labels are separated by /

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DNS NAME SPACE

• A naming service is implemented by name servers.
• If a distributed system is restricted to a local
  area network, a naming service can be implemented
  by using a single name server.
• However, in large-scale distributed systems, it
  is necessary to distribute the implementation of
  a name space over multiple name servers.
• The name space is divided into parts called
  zones, which are implemented by a separate name
  server.
• One of the largest distributed naming services in
  use today is the Internet Domain Name System
  (DNS).
• DNS is primarly used for looking up IP addresses
  of hosts and mail servers.
• The DNS name space is hierarchically organized as
  a rooted tree.

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DNS NAME SPACE
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DNS NAME SPACE
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DNS NAME SPACE

• The global layer is formed by highest-level
  nodes. In this layer nodes usually represent
  organizations.
• The administration layer is formed by directory
  nodes that together are managed within a single
  organization. For example, there may be a
  directory node for each department in an
  organization. The nodes in the administrational
  layer are relatively stable, although changes
  generally occur more frequently than to nodes in
  the global layer.
• The managerial layer consists of nodes that may
  change regularly. For example, nodes representing
  hosts in the Local Area Network belong to this
  layer. This layer also includes user defined
  directories and files.

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NAME RESOLUTION

• Given a path name, the process of looking up a
  name is called name resolution.
• In order start the name resolution process, it is
  necessary to already have access to the directory
  table of the root node of the naming graph.
• Name resolution can be used to merge different
  name spaces in a transparent way.
• A mounted file system corresponds to letting a
  directory node store the identifier of a
  directory node from a different name space,
  referred as foreign name space.
• The directory node storing the node identifier is
  called mount point.
• Accordingly, the directory node in the foreign
  name space is called a mounting point. The
  mounting point becomes the root of a name space,
  and during name resolution starts by accessing
  its directory table.

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MOUNTING REMOTE NAME SPACE

• Sun Microsystems Network File System (NFS) is a
  distributed
• file system that comes with a protocol that
  describes how a
• client can access a file stored on a remote NFS
  server.

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ATTRIBUTE-BASED NAMING

• Unstructured and structured names provide a
  unique and location-independent way of referring
  to entities, and the name refers to only a single
  entity.
• However, location independence and human
  friendliness are not the only criterion for
  naming entities.
• As more information is being made available it
  becomes important to effectively search for
  entities.
• A method used in distributed systems is to
  describe an entity in terms of (attribute, value)
  pairs, which is referred as attribute-based
  naming.
• In attribute-based naming, an entity has a
  collection of attributes.

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DIRECTORY SERVICES

• Attribute-based naming systems are also known as
  directory services.
• With directory services, entities have a set of
  associated attributes that can be used for
  searching.
• For example, in an e-mail system, messages can be
  tagged with attributes for the sender, recipient,
  subject, etc.
• Resource Description Framework (RDF) provides a
  structure to describe the resources.
• If resource descriptions are stored, it becomes
  possible to query that storage in a way that is
  common for many attribute-based naming systems.

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LIGHTWEIGHT DIRECTORY ACCESS PROTOCOL

• Lightweight Directory Access Protocol (LDAP) is a
  networking protocol for querying and modifying
  directory services running over TCP/IP.
• Microsofts Active Directory is an application of
  LDAP for use in Windows environments. Active
  Directory stores information (ex email accounts)
  and settings (ex security) relating to an
  organization.
• A LDAP directory service consists of a number of
  records, referred as directory entries.
• Each record is made up of a collection of
  (attribute, value) pairs.

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LIGHTWEIGHT DIRECTORY ACCESS PROTOCOL
An example of an LDAP directory entry.