Most concepts are drawn from Chapter 13
Name Services
Updated by Rajkumar Buyya
Redmond Barry Distinguished Professor
• Introduction
• Name services and the DNS
• Directory/Discovery services • Summary
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 raj@128.250.1.25 or raj@unimelb.edu.au
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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
Level 5 subnet
128.250.29.30 marg.cis. unimelb.edu.au
Machine Identifier
Education Institutions
CIS network
Australia
Machine name
The University of Melbourne
Computing and Information Systems
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
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.
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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)
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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.
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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
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.
Key benefits
– Resourcelocalization
– Uniform naming
– Device independent address (e.g., you can move domain name/web site from one server to another server seamlessly).
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
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Role of Names and Naming Services – Name Resolution
66.102.11.104
Client
name
IP
attributes
www.google.com
www.hotmail.com
……..
Naming Service
100.109.23.104
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
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Composed naming domains used to access a resource from a URL URL
http://www.cdk5.net:8888/WebExamples/earth.html
DNS lookup
Resource ID (IP number, port number, pathname)
138.37.88.61
8888
WebExamples/earth.html
ARP lookup
(Ethernet) Network address
Address Resolution Protocol (ARP)
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2:60:8c:2:b0:5a
Socket
Web server
file
Names and resources
• Currently, different name systems are used for each type of resource:
resource name identifies
More on URNs (Uniform Resource Names)
file pathname file within a given file system
format: urn:
process process id process on a given computer
type of resource. There two main classes:
examples:
port number IP port on a given computer
a)
•
port
Uniform Resource Identifiers (URI) offer a general solution for any
b) resolution:
a) b)
URL Uniform Resource Locator (URL)
urn:ISBN:021-61918-0 urn:cloudbus.unimelb.edu.au:TR2005-10
send a request to nearest ISBN-lookup service – it would return
• typedbytheprotocolfield(http,ftp,nfs,etc.)
whatever attributes of a book are required by the requester
• part of the name is service-specific
send a request to the urn lookup service at
• resourcescannotbemovedbetweendomains
cloudbus.unimelb.edu.au – it would return a url for the
URN Uniform Resource Name (URN)
relevant document
• requires a universal resource name lookup service – a DNS-like system
for all resources
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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.
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Navigation
Namespaces allows for structure in names.
URLs provide a default structure that decompose
the location of a resource in – protocol used for retrieval
Navigation is the act of chaining multiple Naming
– internet end point of the service exposing the resource
Services in order to resolve a single name to the
– service specific path
corresponding resource.
This decomposition facilitates the resolution of the name into the corresponding resource
Moreover, structured namespaces allows for iterative navigation…
Iterative navigation
NS2
Name servers
NS1
NS3
2 1
3
Client
Used in:
A client iteratively contacts name servers NS1–NS3 in order to resolve a name
Reason for NFS iterative name resolution
This is because the file service may encounter a symbolic link (i.e. an
DNS: Client presents entire name to servers, starting at a local server, NS1.
alias) when resolving a name. A symbolic link must be interpreted in If NS1 has the requested name, it is resolved, else NS1 suggests
the client’s file system name space because it may point to a file in a
contacting NS2 (a server for a domain that includes the requested name).
directory stored at another server. The client computer must determine
NFS: Client segments pathnames (into ‘simple names’) and presents them
which server this is, because only the client knows its mount points.
one at a time to a server together with the filehandle of the directory that contains the simple name.
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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
Non-recursive and recursive server-controlled navigation
1
4
1
5
client
client
NS2 2
NS1
3
NS3
NS2
2
4
NS1
NS3
3
Non-recursive server-controlled
Recursive server-controlled
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.
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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
Basic DNS algorithm for name resolution (domain name -> IP number)
• L–oopkafortrititohneendadmaetaibnatshe local cache
• Try a superior DNS server, which responds with:
– caching
– another recommended DNS server
Resilient to failure of a server
– the IP address (which may not be entirely up to date)
– replication
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DNS name servers: Hierarchical organisation
a.root-servers.net
(root)
Note: Name server names are in italics, and the corresponding domains are in parentheses. Arrows denote name server entries
authoritative path to lookup:
ns1.nic.au
(au)
ns.purdue.edu
(purdue.edu)
com.au
edu.au …
au purdue.edu
yahoo.com ….
raj-pc.cis.unimelb.edu.au
ns0.ja.net
(edu.au)
* .purdue.edu
usyd.edu.au
unimelb.edu.au …
abc.unimelb.edu.au
(unimelb.edu.au)
mulga.cis.unimelb.edu.au
(cis.unimelb.edu.au) *.cis.unimelb.edu.au
dns0-doc.usyd.edu.au
(usyd.edu.au)
cis.unimelb.edu.au *.unimelb.edu.au
*.usyd.edu.au
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DNS in typical operation
Without caching
ns1.nic.au
(au)
ns.purdue.edu
(purdue.edu)
a.root-servers.net
(root)
au purdue.edu
yahoo.com ….
com.au
edu.au …
ns0.ja.net
(edu.au)
alpha.unimelb.edu.au
(unimelb.edu.au)
mulga.cis.unimelb.edu.au
(cis.unimelb.edu.au) *.cis.unimelb.edu.au
2 client.usyd.edu.au 4
raj-pc.cis.unimelb.edu.au ?
IP:mulga.csse.unimle22b.edu.au
3
* .purdue.edu
usyd.edu.au
unimelb.edu.au …
dns0-doc.usyd.edu.au
(usyd.edu.au)
IP: alpha.unimelb.edu.au
cis.unimelb.edu.au *.unimelb.edu.au
*.usyd.edu.au
IP:raj-
pc.cis.unimelb.edu.au
IP:ns0.ja.net
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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
A
AAA
NS CNAME SOA
PTR HINFO
MX
TXT Text string
Main contents
IPv4 number
IPv6 number
Domain name for server Domain name for alias Parameters governing the zone
Domain name
Machine architecture and operating system
List of
Meaning
A computer address (IPv4)
A computer address (IPv6)
An authoritative name server The canonical name for an alias Marks the start of data for a zone
Domain name pointer (reverse lookups)
Host information
Mail exchange
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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.)
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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’.
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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)
Govt
Vic (state)
X.500 Service (root)
Australia (country)
India
USA
Object class for NSW govt.
NSW (state)
Private
Educational
UniMelb
Monash
CSSE
Medicine
Staff
Students
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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
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