Figure 15.1 A distributed multimedia system
Name Services
Updated by Rajkumar Buyya
Redmond Barry Distinguished Professor
• Introduction
• Name services and the DNS
• Directory/Discovery services
• Summary
Most concepts are
drawn from Chapter 13
2
Which one is easy for humans and machines? and why?
74.125.237.83 or google.com
128.250.1.22 or distributed systems website
128.250.1.25 or Prof. Buyya
Disk 4, Sector 2, block 5 OR /usr/raj/Hello.java
.1.25 or .au
Names or Codes, or Numbers?
– Names (when meaningful) are easier to remember than
codes or numbers…
– Number (or sequence codes) are more useful for
structuring data and locating resources by a program..
– Example: IPv4
128.250.29.30
marg. cis. unimelb.edu.au
CIS network
Level 5 subnet
Machine Identifier
Australia
Education Institutions
The University of
Melbourne
Computing and
Information Systems
Machine name
Names or Codes? or Numbers?
– As discussed in file system (hierarchical naming of files) and mounting
at right location.
Which one is better?
Disk 4, Sector 26, Block15
/usr/bin/tar
5
Aim
To introduce the name service as a distinct service
that is used by client processes to obtain attributes
such as the address of resources (computing
systems, files, printers etc.) or objects when given
their name.
6
Learning objectives
To understand the need for naming systems in
distributed systems
To be familiar with the design requirements such as
structure and management of name spaces, and
operations supported by them.
To understand the operation of the Internet naming
service – DNS (Domain Name Service)
To understand structure and operation of directory
service – X.500 Directory Service & LDAP
(Lightweight Directory Access Protocol)
7
1. 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.
Naming is fundamental issue in DS design as it
facilitates communication and resource sharing.
– A name in the form of URL is needed to access a specific web page.
– Processes cannot share particular resources managed by a computer
system unless they can name them consistently
– Users cannot communicate within one another via a DS unless they
can name one another, with email address.
Names are not the only useful means of
identification: descriptive attributes are another.
What are Naming Services?
How do Naming Services facilitate communication
and resource sharing?
– An URL facilitates the localization of a resource exposed on the Web.
e.g., abc.net.au means it is likely to be an Australian entity?
– A consistent and uniform naming helps processes in a distributed
system to interoperate and manage resources.
e.g., commercials use .com; non-profit organizations use .org
.edu, ac.uk or edu.au educational institutes
– Users refers to each other by means of their names (i.e. email) rather
than their system ids
– Naming Services are not only useful to locate resources but also to
gather additional information about them such as attributes
What are Naming Services?
Definition
Key benefits
– Resource localization
– Uniform naming
– Device independent address (e.g., you can move domain name/web
site from one server to another server seamlessly).
In a Distributed System, a Naming Service is a specific service
whose aim is to provide a consistent and uniform naming of
resources, thus allowing other programs or services to localize
them and obtain the required metadata for interacting with
them.
10
The role of names and name services
Resources are accessed using identifier or reference
– An identifier can be stored in variables and retrieved from tables quickly
– Identifier includes or can be transformed to an address for an object
E.g. NFS file handle, CORBA remote object reference
– A name is human-readable value (usually a string) that can be resolved to an
identifier or address
Internet domain name, file pathname, process number
E.g ./etc/passwd, http://www.cdk5.net/
For many purposes, names are preferable to identifiers
– because the binding of the named resource to a physical location is deferred
and can be changed
– because they are more meaningful to users
Resource names are resolved by name services
– to give identifiers and other useful attributes
Role of Names and Naming Services
– Name Resolution
66.102.11.104
100.109.23.104Naming Service
name IP attributes
www.google.com
www.hotmail.com
……..
Client
12
Requirements for name spaces
Allow simple but meaningful names to be used
Potentially infinite number of names
Structured
– to allow similar subnames without clashes
– to group related names
Allow re-structuring of name trees
– for some types of change, old programs should continue to work
Management of trust
13
file
Web server
Socket
Composed naming domains used to access a resource from a URL
http://www.cdk5.net:8888/WebExamples/earth.html
URL
Resource ID (IP number, port number, pathname)
138.37.88.61 WebExamples/earth.html8888
DNS lookup
(Ethernet) Network address
2:60:8c:2:b0:5a
ARP lookup
Address Resolution Protocol (ARP)
http://penguin.dcs.bbk.ac.uk/academic/networks/network-layer/arp/index.php
14
Names and resources
• Currently, different name systems are used for each type of
resource:
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
• Uniform Resource Identifiers (URI) offer a general solution for any
type of resource. There two main classes:
URL Uniform Resource Locator (URL)
• typed by the protocol field (http, ftp, nfs, etc.)
• part of the name is service-specific
• resources cannot be moved between domains
URN Uniform Resource Name (URN)
• requires a universal resource name lookup service – a DNS-like system
for all resources
More on URNs (Uniform Resource Names)
format: urn:
examples:
a) urn:ISBN:021-61918-0
b) urn:cloudbus.unimelb.edu.au:TR2005-10
resolution:
a) send a request to nearest ISBN-lookup service – it would return
whatever attributes of a book are required by the requester
b) send a request to the urn lookup service at
cloudbus.unimelb.edu.au – it would return a url for the
relevant document
15
2. Name Services and the Domain Name System
A name service stores a collection of one or more
naming contexts, sets of bindings between textual
names and attributes for objects such as computers,
services, and users.
The major operation that a name service supports is
to resolve names.
Navigation
Namespaces allows for structure in names.
URLs provide a default structure that decompose
the location of a resource in
– protocol used for retrieval
– internet end point of the service exposing the resource
– service specific path
This decomposition facilitates the resolution of the
name into the corresponding resource
Moreover, structured namespaces allows for
iterative navigation…
Navigation is the act of chaining multiple Naming
Services in order to resolve a single name to the
corresponding resource.
17
Iterative navigation
Client
1
2
3
A client iteratively contacts name servers NS1–NS3 in order to resolve a name
NS2
NS1
NS3
Name
servers
Used in:
DNS: Client presents entire name to servers, starting at a local server, NS1.
If NS1 has the requested name, it is resolved, else NS1 suggests
contacting NS2 (a server for a domain that includes the requested name).
NFS: Client segments pathnames (into ‘simple names’) and presents them
one at a time to a server together with the filehandle of the directory that
contains the simple name.
Reason for NFS iterative name resolution
This is because the file service may encounter a symbolic link (i.e. an
alias) when resolving a name. A symbolic link must be interpreted in
the client’s file system name space because it may point to a file in a
directory stored at another server. The client computer must determine
which server this is, because only the client knows its mount points.
Server controlled navigation
In an alternative model, name server coordinates
naming resolution and returns the results to the
client. It can be:
– Recursive:
it is performed by the naming server
the server becomes like a client for the next server
this is necessary in case of client connectivity constraints
– Non recursive:
it is performed by the client or the first server
the server bounces back the next hop to its client
19
Non-recursive and recursive server-controlled navigation
A name server NS1 communicates with other name servers on behalf of a client
Recursive
server-controlled
1
2
3
5
4
client
NS2
NS1
NS3
1
2
34
client
NS2
NS1
NS3
Non-recursive
server-controlled
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.
20
DNS – The Domain Name System
A distributed naming database (specified in RFC 1034/1305)
Name structure reflects administrative structure of the Internet
Rapidly resolves domain names to IP addresses
– exploits caching heavily
– typical query time ~100 milliseconds
Scales to millions of computers
– partitioned database
– caching
Resilient to failure of a server
– replication
Basic DNS algorithm for name resolution (domain name -> 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)
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DNS name servers: Hierarchical organisation
Note: Name server names are in
italics, and the corresponding
domains are in parentheses.
Arrows denote name server entries
a.root-servers.net
(root)
ns0.ja.net
(edu.au)
mulga.cis.unimelb.edu.au
(cis.unimelb.edu.au)
abc.unimelb.edu.au
(unimelb.edu.au)
dns0-doc.usyd.edu.au
(usyd.edu.au)
ns.purdue.edu
(purdue.edu)
au
purdue.edu
usyd.edu.au
unimelb.edu.au
…
cis.unimelb.edu.au
*.unimelb.edu.au
*.usyd.edu.au*.cis.unimelb.edu.au
* .purdue.edu
ns1.nic.au
(au)
edu.au
…
com.au
yahoo.com
….
authoritative path to lookup:
raj-pc.cis.unimelb.edu.au
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a.root-servers.net
(root)
ns0.ja.net
(edu.au)
mulga.cis.unimelb.edu.au
(cis.unimelb.edu.au)
alpha.unimelb.edu.au
(unimelb.edu.au)
dns0-doc.usyd.edu.au
(usyd.edu.au)
ns.purdue.edu
(purdue.edu)
au
purdue.edu
usyd.edu.au
unimelb.edu.au
…
cis.unimelb.edu.au
*.unimelb.edu.au
*.usyd.edu.au*.cis.unimelb.edu.au
* .purdue.edu
ns1.nic.au
(au)
edu.au
…
com.au
yahoo.com
….
DNS in typical operation
client.usyd.edu.au
IP: alpha.unimelb.edu.au
2
3IP:mulga.csse.unimleb.edu.au
raj-pc.cis.unimelb.edu.au ?
IP:ns0.ja.net
1
IP:raj-pc.cis.unimelb.edu.au
4
Without caching
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DNS server functions and configuration
Main function is to resolve domain names for
computers, i.e. to get their IP addresses
– caches the results of previous searches until they pass their ‘time to live’
Other functions:
– get mail host for a domain
– reverse resolution – get domain name from IP address
– Host information – type of hardware and OS
– Well-known services – a list of well-known services offered by a host
– Other attributes can be included (optional)
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DNS resource records
Record type Meaning Main contents
A A computer address (IPv4) IPv4 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
PTR Domain name pointer (reverse
lookups)
Domain name
HINFO Host information Machine architecture and operating
system
MX Mail exchange List of
TXT Text string Arbitrary text
AAA A computer address (IPv6) IPv6 number
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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.)
26
Directory services (registration and discovery)
Sometime users wish to find a particular person or resource, but they don’t know
its name, only some of its attributes.
– What is the name of the user with a telephone number 03-83441344?
– What is the name of professor teaching Cloud computing at UniMelb (e.g., ask Google!)
Sometime users require a service, but they are not concerned with what system
entity provides it.
– Where can I print high resolution colour image?
Directory services can help with above situation: they store collections of bindings
and attributes and also looks up entries that match attribute-based specs.
Directory service:- ‘yellow pages’ for the resources in a network
– Retrieves the set of names that satisfy a given description
– e.g. X.500, LDAP, MS Active Directory Services
(DNS holds some descriptive data, but:
• the data is very incomplete
• DNS isn’t organised to search it)
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, the ‘service location protocol’, the ‘simple service discovery protocol’ (part of UPnP),
the ‘secure discovery service’.
27
X.500 Directory Service
X.500 and LDAP (Lightweight Directory Access Protocol)
– a hierarchically-structured standard directory service designed for world-wide use
– X.500 is standardised by ITU (international telecommunication union) and ISO
– accommodates resource descriptions in a standard form and their retrieval for
any resource (online or offline)
– never fully deployed, but the standard forms the basis for LDAP, the Lightweight
Directory Access Protocol, which is widely used – IETF RFC 2251.
– A secure access to directory through authentication is also supported.
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Part of the X.500 Directory Information Tree (DIT)
X.500 Service (root)
Australia (country) India USA
NSW (state) Vic (state)
Govt Private Educational
MonashUniMelb
CSSE Medicine
Staff Students
Object class for NSW govt.
29
Summary
• Names services facilitate communication and resource sharing in distributed
systems.
• They are playing very important role in Distributed systems such as the
Internet, Web, CDNs (Content Delivery Networks), Web Services, Location-
aware services– publication and discovery
• Name services:
– defer the binding of resource names to addresses (and other attributes)
– Names are resolved to give addresses and other attributes
– Goals :
Scalability (size of database, access traffic (hits/second), update traffic)
Reliability
Trust management (authority of servers)
– Issues
exploitation of replication and caching to achieve scalability without compromising the distribution
of updates
navigation methods
• Directory and discovery services:
– ‘yellow pages’ retrieval by attributes
– dynamic resource registration and discovery