Name Services

1
Name Services

• Name, Identifier and Addresses
• Name Space and Name Resolution
• Domain Name System and Organization
• Domain Queries and Servers
• Directory and Discovery Services
• Directory Service X.500
• Discovery Services
• Locating Mobile Entities

2
Name, Address and Identifier

• The Problem
• A process requests to make a connection (access)
  to a named entity in a distributed system. How???
• Large number of entities and widely distributed
• Performance minimize the searching cost and
  delay
• Entity can be anything in the system, i.e.,
  processes, resources, data, hardware, etc..
• Three problems needed to be considered
• Naming, location and the route to the entity
• Routing networking issue (find the shortest path
  for connection)
• Name gt address gt location (naming convention)
• If we know the address, then we know the location
• Name a string of bits or characters that is used
  to refer to an entity
• Human-readable name /etc/passwd and www.cdk4.net
• System dependent codes IP address, LAN address
  (mapping name to its address)
• Similar to a path name. Once you have identified
  the starting node, then you know the location of
  the referring entity

3
Name, Address and Identifier

• Address
• Access to an entity through an access point
• The name of an access point is called an address
• An entity may have multiple access points
• An address may be a name for referring to an
  entity if mobility or reallocation is infrequent
• Mobility may change the access point
  (location-dependent), i.e., enter into another
  network
• Identifier a unique ID to identify an entity (a
  name maps to an identifier)
• Name gt identifier (entity) gt address gt
  location
• Requirements of a true identifier
• 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

4
Names and Resources

• Large number of entities gt needs a
  convention/method to organize them
• Currently, different name systems are used for
  different types of resources
• resource name identifies
• file pathname file within a given file system
• process process id process on a given computer
• port port number IP port on a given computer
• I.e., Uniform Resource Identifiers (URI) offer a
  general solution for any type of resources in the
  Internet. There two main classes
• URL (Uniform Resource Locator)
• Typed by the protocol field (http, ftp, nfs,
  etc.)
• Part of the name is service-specific
• Resources cannot be moved between domains
  (identifier ? address)
• URN (Uniform Resource Name)
• Requires a universal resource name lookup service
  - a DNS-like system for all resources

5
Composed Naming Domains
Fr. Dollimore
Name server
System address Vs. network address
6
Name Services

• Naming services
• Name (identifier) gt address gt location
• Name resolution
• Receiving a name and then identify the referring
  entity and then its address to access to (simply
  based on the name, we may not be able to identify
  the location of the entity)
• A name point to an entry which contains the
  address of the named entity
• Problems
• How to locate the entry for the named entity? How
  to organize them?
• Unification
• Different systems use the same naming scheme
• Integration
• Integration of different naming systems
• Naming system requirements
• Handle arbitrary (large) number of names, long
  lifetime, fault tolerance, high availability,
  tolerance mistrust (what is mistrust?)
• How to achieve the above requirements?
• Good naming system and structured organization to
  facilitate management and searching (i.e., using
  a graph or a tree)

7
Name Spaces

• Naming convention and examples
• surname and first name
• In Unix, hierarchical name, /etc/passwd
• Relative name ../images/figure1.jpg the
  servers host name and the server directory to
  which this pathname is referred are taken by a
  browser
• Domain names www.cdk4.net (a group of named
  entities)
• Aliases a symbolic name allowing a convenient
  name to be substituted for a more complicated
  name (may not be unique globally)
• Name space the collection of all valid names
  recognized by the system
• Naming domain is a name space for which there
  exists a single overall administrative authority
  for assigning names within it
• Some of the names in the name space may be
  unbounded (valid but currently no entity is
  referring to)

8
Name Spaces

• Structured organization A name space can be
  represented as a labelled, directed graph (naming
  graph) with two types of nodes, leaf nodes and
  directory node
• Leaf node is an entity that can be referred to
  or it contains the address of an entity
• Directory node has a number of outgoing edges
  each label with a name. It stores a directory
  table in which an outgoing edge is represented
  as a pair (edge label, node identifier) for
  searching an entity
• The directory node with no incoming edge is
  called root
• The graph needs to be acyclic, why???
• Path absolute path Vs. relative path
• Name global name Vs local name
• A naming graph may be managed by one or more name
  servers

9
Naming Graph

• A general naming graph with a single root node

Note two paths to n5
Fr. Tanenbaum
10
Name Resolution (Logical)

• Names are managed by name servers which maintains
  the directory table (organized as a tree/graph)
• Given a path name N ltlabel-1, label-2, ,
  label-ngt
• Iterative searching resolution starts at node N
  of the naming graph (normally the root)
• Search the directory table for label-1 to return
  the identifier node that label-1 is referring to
• Then go to the returned identifier and repeat the
  process until label-n is reached
• The search may cover multiple name spaces linked
  (mounted) together
• A directory node has an edge pointing to a node
  in another name space (handled by another server)
• The searching delay depends on how to organize
  the tree/graph and the partition of the graph
• Additional overhead is the reorganization of the
  graph
• Under which situations, we need to reorganization
  graph?

11
Linking and Mounting
Fr. Tanenbaum

• The concept of a symbolic link explained in a
  naming graph
• A node contains a path name

12
Linking and Mounting

• Mounting remote name spaces through a specific
  access protocol

Fr. Tanenbaum
13
Name Space Distribution (i.e., DNS)

• Divide the whole space into layers to facilitate
  the searching
• Some assumptions (i.e., reallocation and renaming
  are infrequent for some of the entities)
• Global layer
• Root and its children
• Relative stable (seldom change in locations and
  number)
• Administrational layer
• Directory nodes managed within an organization
• Managerial layer
• The nodes may change regularly (delete,
  reallocate and insert)
• What can be done after making such
  classification?
• Due to high degree of stability, the performance
  and availability can be improved by replication
  and caching (i.e., the higher layers)
• Difference between replication and caching???
• What will be the problem if all of them are
  changing frequently?

14
Name Space Distribution (i.e., DNS)

• An example partitioning of the DNS name space,
  including Internet-accessible files, into three
  layers.

Fr. Tanenbaum
A zone managed by a name server
15
Name Space Distribution (i.e., DNS)
Fr. Tanenbaum

• A comparison between name servers for
  implementing nodes from a large-scale name space
  partitioned into a global layer, an
  administrational layer, and a managerial layer

16
Name Resolution Implementation

• Name spaces are distributed across multiple name
  servers
• A client passes a name for resolution
• Where to start? Normally indicated in the request
  (starting node)
• How to process the request in the distributed
  name servers?
• Methods
• Iterative
• Recursive
• Non-recursive

17
Iterative Navigation
Fr. Dollimore

• DNS Client presents entire name to servers,
  starting at a local server, NS1.
• If NS1 has the requested name, it is resolved
  (has an entry indicating the location of the
  referring entity), else NS1 suggests contacting
  NS2 (a server for a domain that includes the
  requested name)

18
Iterative Navigation

• The principle of iterative name resolution

Fr. Tanenbaum
19
Recursive Name Resolution

• The principle of recursive name resolution

Fr. Tanenbaum
20
Non-recursive Recursive Server-controlled
Navigation
Fr. Dollimore
A name server NS1 communicates with other name
servers on behalf of a client

• DNS offers recursive navigation as an option, but
  iterative is the standard technique
• Recursive navigation must be used in domains that
  limit client access to their DNS information for
  security reasons

21
Caching in Name Resolution
Fr. Tanenbaum

• Recursive name resolution of ltnl, vu, cs, ftpgt.
  Name servers cache intermediate results for
  subsequent lookups

22
Navigation Cost
Fr. Tanenbaum

• The comparison between recursive and iterative
  name resolution with respect to communication
  costs
• Time delay and cost in communication

23
DNS - Domain Name System

• A distributed naming database for Internet
• Name structure reflects administrative structure
  of the Internet
• Organized as a hierarchical tree
• Rapidly resolves domain names to IP addresses
• Exploits caching heavily
• Typical query time 100 milliseconds
• Scales to millions of computers
• Partitioned and replicated database
• Caching
• Resilient to failure of a server
• Replication
• Basic DNS algorithm for name resolution (domain
  name -gt IP number)
• Look for the name in the local cache
• Try a superior DNS server, which responds with
• Another recommended DNS server
• The IP address (which may not be entirely up to
  date)

24
DNS Server Functions

• The Internet DNS is primarily for
• Host name resolution, i.e., host name -gt IP
  address
• Mail host location
• Reverse resolution, i.e., IP address -gt host name
• Host Information, i.e., operating systems,
  machine architecture
• Well-know services, i.e., telnet, FTP, etc.
• Caches the results of previous searches until
  they pass their time to live (TTL)
• How to determine the value for TTL?
• What need to be done if the IP address
  information changes before the TTL expired?
• The add/delete of a name is done by an
  authoritative administrator manually editing the
  name database (replicated)

25
DNS (Domain Name System)

• Naming domain is a name space for which there is
  a single administrative authority for assigning
  names within it
• Name of a domain is the common suffix of domain
  names within it
• Name server of a domain is a server which does
  the mapping between domain names and IP addresses
• Domain name space is partitioned both
  organizationally and geographically
• Domains never overlap
• The top-level domains are
• com, edu, gov, mil, net, org, us, uk, hk, cn,
  ..
• The DNS name space is divided into domains
  (zones)
• Each domain has a name server which manages the
  name database of the local domain
• Each name server contains pointers to higher
  level domain servers and caches the addresses of
  other servers
• Each name server contains the names and addresses
  of at least two name servers that provide
  authoritative data for the zone

26
DNS Name Servers

• DNS name resolution model

Root NS
NS
NS
Domain boundary
Local domain NS
Client
destination NS
27
Primary Secondary Servers

• For fault tolerance, there are two servers
  providing authoritative data in a domain
• Primary server the server reads the domain data
  directly from a local master file (edited by
  system administrator)
• Secondary server it down loads the domain data
  from the primary server and communicates with the
  primary periodically to keep its data up-to-date

28
Data Files for Name Servers

• A name server (primary or secondary) uses 3 data
  files for name resolutions
• Name resolution file names to IP addresses
• i.e. db.cs.cityu.edu.hk
• Reverse translation file IP addresses to names
• i.e. db.120.214.144
• Cached database file data learnt from previous
  queries
• i.e. db.cache
• Note db.cs.cityu.edu.hk, db.120.214.144, and
  db.cache are file names

29
Resource Record

• Zone data are stored by name servers in resource
  records
• The data for a zone starts with an SOA (start of
  authority) record containing the zone parameters
• An A (address) represents a particular host in
  the Internet
• MX (mail exchange) is a symbolic link to a node
  representing a mail server
• NS (name server) contains the name of a name
  server that implements the zone represented by
  the node
• CNAME contains the canonical name of a host
• DNS maintains an inverse mapping of IP addresses
  to host names by means of PTR (pointer)
• HINFO contains additional information on a host
• TXT are used for any kind of data that a user
  finds useful to store about the entity
  represented by the node

30
DNS Resource Records
Record type
Meaning
Main contents
A
A computer address
IP number
NS
An authoritative name server
Domain name for server
CNAME
The canonical name for an alias
Domain name for alias
SOA
Marks the start of data for a zone


Parameters governing the zone
WKS
A well-known service description
List of service names and protocols
PTR
Domain name pointer (reverse
Domain name
lookups)
HINFO
Host information
Machine architecture and operating
system
preference, host
MX
Mail exchange
List of lt
gt pairs
TXT
Text string
Arbitrary text
Fr. Dollimore
31
DNS Resource Records
Fr. Tanenbaum
32
DNS Resource Records
Fr. Tanenbaum
An excerpt from the DNS database for the zone
cs.vu.nl
33
DNS Issues

• Name tables change infrequently, but when they
  do, caching can result in the delivery of stale
  data
• Clients are responsible for detecting this and
  recovering
• Its design makes changes to the structure of the
  name space difficult. For example
• Merging previously separate domain trees under a
  new root
• Moving subtrees to a different part of the
  structure (e.g. if Scotland became a separate
  country, its domains should all be moved to a new
  country-level domain

34
Directory and Discovery Services

• DNS given a name gt an IP address (a node
  containing a set of attributes)
• Directory services
• Yellow pages' for the resources in a network
• A node contains an address, a name and a set of
  attributes
• Retrieve the set of entities whose attributes
  satisfy a set of conditions (List out the names
  of all lecturers in CS dept)
• e.g. X.500, LDAP, MS Active Directory Services
• Discovery service a directory service that also
• is automatically updated as the network
  configuration changes
• Meets the needs of clients in spontaneous
  networks
• Discovers services required by a client (who may
  be mobile) within the current scope, for example,
  to find the most suitable printing service for
  image files after arriving at a hotel.
• Examples of discovery services Jini discovery
  service and the 'service location protocol'

35
Directory Services X.500

• A directory entry (record) in X.500 is made up of
  a collection of (attribute, value)
• The collection of all directory entries in an
  X.500 directory service is called a directory
  information base (DIB)
• Each entry is unique by a set of naming
  attributes called relative distinguished name
  (RDN)
• The use of unique names by listing RDNs in
  sequence leads to a hierarchy of the collection
  of directory entries called directory information
  tree (DIT)
• A node can be a directory and a record
• Node N is a directory entry
• Also, it has two children
• X.500 provides two main types of access request
  read and search
• read given a name and a list of attributes (path
  name), it searches the DIT and returns a set of
  attribute values of the named entry
• I.e., /CNL/OVrije Universiteit/OUMath. Comp.
  Sc/CN Main Server
• search given a base name and a filter
  expression, it returns a list of names whose
  entries are below the base node and satisfy the
  filter conditions

36
Directory Services X.500
Fr. Tanenbaum
An example of a X.500 directory entry using X.500
naming conventions
37
Directory Information Tree (DIT)
Fr. Tanenbaum
Node N corresponding to the directory entry shown
in the previous example
38
Directory Information Tree (DIT)
Fr. Tanenbaum
39
X.500 Implementation

• Similar to DNS, the DIT is usually partitioned
  and distributed across several servers called
  DSAs (directory server agents)
• DUA (directory user agent) on each client
  computer
• Light-weight directory access protocol (LDAP) an
  application level protocol implemented on top of
  TCP
• The DSA interface includes operations of add,
  delete and modify of entries.
• Access control is required for both queries and
  updating operations.
• X.500 does not address implementation issues. It
  only defines the interface and specifies
  functions.
• The implementation and application of X.500 is
  still at a pilot stage

40
X.500 Service Architecture
Fr. Dollimore
41
Discovery Services

• When a client enters a new service area,
• The system needs to update its location (and
  system status)
• The object needs to determine how to join it
  (registration) and what are the services provided
  in the space
• I.e., A client enters a hotel carrying a notebook
  and wants to print a document
• A discovery service is a directory service in
  which services in a service are registered and
  looked up by their attributes
• Types of services device directory and service
  directory
• Device directory records the names and addresses
  of co-present devices
• Select one of the devices for connection and to
  require for services
• Service directory records what are the services
  provided in the smart space
• Submit a request (query) for a particular type of
  service
• The system will select one of the devices to
  provide the service to the requesting client

42
Discovery Services

• Directory query operations
• Registration and de-registration of services
• Look-up for services
• Directory discovery problems
• Directory data required by a client are dynamic
  and generated at run-time (as a function of the
  context of the client)
• May be no infrastructure (distributed Vs.
  centralized) to hold the directory server
• The services registered in a directory are
  dynamic too
• The new object may be a service provider
• The protocols used for accessing the directory
  need to be energy sensitive
• Note that a service may disappear spontaneously.
  Why?
• Using leasing a lease is temporary allocation of
  a service by a server to a client. It can be
  renewed by a further request (refresh) from the
  client before the deadline expires

43
The interface to a discovery service
Fr. Dollimore
44
Directory Server Vs. Serverless

• For systems with a fixed infrastructure and the
  directory server is a powerful robust machine
• Directory server maintains a set of description
  of services
• The client issues a multicast request to locate
  the directory server
• The directory server responds with its unicast
  address
• They communicate point-to-point with each other
• If no pre-defined powerful directory server, the
  directory server may be elected from the group of
  devices within the space
• Problems disappear of the elected server
  (re-election). The cost for re-election depends
  on the degree of volatility of the system

45
Directory Server Vs. Serverless

• No fixed infrastructure and no powerful machine
  to be acted as a server
• The participating devices collaborate with each
  other to implement the service directory. How?
• Event synchronization problem all changes have
  to be propagate to all the member before any
  change can be updated (S1-gtS2-gtS3)
• Push model services multicast their description
  regularly. Clients listen for the multicasts and
  run their queries against them
• Pull model clients multicast their queries.
  Devices providing services run the queries
  against their descriptions, and only response
  with any description that match. Clients repeat
  their queries periodically if there is no
  response

46
Push Vs. Pull

• Consideration energy and bandwidth consumption
• Every time a device issues a multicast message,
  bandwidth is consumed and all listening clients
  expend energy receiving the message
• In a pure push model, devices need to multicast
  their services even there is no client
• In a pure pull model, a client can discover
  services as soon as it appears. But, the client
  may receive multiple responses from different
  devices

47
Locating Mobile Entities

• DNS assumption the changes in addressing
  information is infrequent
• What will be the impact of changing addressing
  information (i.e., reallocation)?
• I.e., moving ftp.cs.vn.nl to a new machine named
  ftp.cs.unisa.edu.au
• Replicated servers and caches
• Need reorganization of the hierarchical tree
• Solutions
• (1) record the address of the new machine in the
  DNS database for cs.vu.nl
• (2) record the name of the new machine in
  ftp.cs.vu.nl (a symbolic link)
• Problems
• (1) Frequent reorganization if reallocation is
  frequent
• (2) The search time is longer
• Entity gt address
• Entity gt entity ID gt address
• Separate naming from location entities by
  introducing identifiers which is a globally
  system recognizable ID that will not be changed
• Locating an entity is handled by a location
  service which accepts an identifier and then
  returns an address of the referring entity

48
Home-Based Approach

• Home location maintains the current location of
  an entity
• Each mobile host has a fixed IP address. All
  communications to that IP address is initially
  directed to the mobile hosts home agent located
  using the fixed IP address
• Whenever the mobile host moves to another
  network, it requests a temporary address from the
  new network, the care-of-address
• The care-of-address is registered at the home
  agent
• If the home agent receives a packet for the
  mobile host, it looks up the hosts current
  location. If it is on the current network, the
  packet is forwarded as usual.
• Otherwise, it is tunneled to the hosts current
  location to the care-of-address. At the same
  time, the sender of the packet us informed of the
  hosts current location
• Problems
• Always need to contact the home agent
• A fixed home agent

49
Home-Based Approach
Fr. Tanenbaum

• The principle of Mobile IP

50
Hierarchical Approach

• A network is divided into a collection of domains
• There is a single top-level (root)
• A lowest-level domain, called a leaf domain,
  contain mobile entities
• Each domain D has an associated directory node
  dir(D) that keeps tracks of the entities in that
  domain
• Each entity currently located in a domain D is
  represented by a location record in the directory
  node dir(D)
• A location record for entity E in the directory
  node N for a leaf domain D contains the entitys
  current address in that domain
• The directory node N for the next higher-level
  domain D that contains D have a location record
  for E containing only a pointer to N
• The directory node of the root has the location
  records for all the entities either as pointers
  or addresses

51
Hierarchical Approach (Lookup)

• A client wishing to locate an entity E issues a
  lookup request to the directory node of the leaf
  domain D in which it resides
• If the directory node of the leaf domain D does
  not contain a location record for the entity E,
  the node forwards the requests to its parent
• Once the request reaches a directory node
  containing a location record for E, the request
  will be forwarded down following the its location
  pointer until the leaf node containing its
  address is reached
• The lookup operation explores the concept of
  locality. What???

52
Hierarchical Approach (Insert delete)

• Insert
• An entity E has created a replica in leaf domain
  D
• The insertion request is initiated by the leaf
  node D to its parent until it reaches a directory
  node M that already contains a location record
  for E
• Node M then stores a pointer in the location
  record for E referring to the child node from
  where the insert request was forwarded
• The process is repeated until leaf node D is
  reached. Then, leaf node D insert the address of
  E into its location record
• Delete
• When address for an entity E in leaf domain D
  needs to be removed, directory node dir(D) is
  requested to remove that address from its
  location record for E
• If the record becomes empty, the record can be
  removed
• If the location record for E at the parent now
  also becomes empty, the record should be removed
  as well and the next higher-level directory node
  should be informed

53
Hierarchical Approach

• Hierarchical organization of a location service
  into domains, each having an associated directory
  node

Fr. Tanenbaum
54
Hierarchical Approach
Fr. Tanenbaum
55
Hierarchical Approach

• Looking up a location in a hierarchically
  organized location service.

Fr. Tanenbaum
56
Hierarchical Approach (insert)

• An insert request is forwarded to the first node
  that knows about entity E.

Fr. Tanenbaum
57
Hierarchical Approach

• A chain of forwarding pointers to the leaf node
  is created

Instead of changing the pointers of the directory
node, a pointer may be created at the leaf node
pointer to the new leaf node of an entity
Fr. Tanenbaum
58
References

• Dollimore Ch. 9
• Tanenbaum Ch. 5