1 chapter 6: name services. 2 introduction name services and dns discovery services summary
TRANSCRIPT
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Chapter 6: Name
Services
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Name Services
Introduction
Name services and DNS
Discovery services
Summary
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Objectives To understand the need for naming systems in DS
To study the design requirements for distributed name services
To understand the operation of the Internet naming service – DNS
To be familiar with the role of discovery services in mobile and ubiquitous computer systems
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The Role Of Names And Name Services
Resources - 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 objectE.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 numberE.g ./etc/passwd, http://www.cdk3.net/
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The Role Of Names And Name Services (Cont.)
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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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
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file
Web serverSocket
Composed Naming Domains Used To Access A Resource From A URL
http://www.cdk3.net:8888/WebExamples/earth.html
URL
Resource ID (IP number, port number, pathname)
138.37.88.61 WebExamples/earth.html8888
DNS lookup
Figure 9.1
(Ethernet) Network address
2:60:8c:2:b0:5a
ARP lookup
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Names And Resources
Currently, different name systems are used for each type of resource:resource name identifiesfile pathname file within a given file systemprocessprocess id process on a given computerport port number IP port on a given computer
Uniform Resource Identifiers (URI) offer a general solution for any type of resource. 2 main classes:
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Names And Resources (Cont.)
There are two main classes (Cont.):URL Uniform Resource Locator
• typed by protocol field (http, ftp, nfs, etc.)
• part of the name is service-specific• resources cannot be moved between
domainsURN Uniform Resource Name
• requires a universal resource name lookup service
• DNS-like system for all resources
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Names And Resources (Cont.)
More on URNsformat:urn:<nameSpace>:<name-within-namespace>examples:a) urn:ISBN:021-61918-0b) urn:dcs.qmul.ac.uk:TR2000-56resolution: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 dcs.qmul.ac.uk - it would return a url for the relevant document
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Name Resolution in an Iterative navigation
A client iteratively contacts name servers NS1–NS3 in order to resolve a nameClient 1
2
3
NS2
NS1
NS3
Nameservers
Figure 9.2
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: Resolves file name iteratively on a component by component basis.
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Non-recursive And Recursive Server-controlled Navigation
A name server NS1 communicates with other name servers on behalf of a client
Recursiveserver-controlled
1
23
5
4
client
NS2
NS1
NS3
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34client
NS2
NS1
NS3
Non-recursiveserver-controlled
Figure 9.3
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 Internet Domain Name System
Def : a distributed hierarchical naming system for computers, services, or any resource
connected to the Internet or a private network
To replace original scheme - host name ad adds were kept in single master file and downloaded by ftp.
Did not scale to a large number of computer.
Local organization wished to administer themselves
A general name service was needed
Name structure reflects administrative structure of 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
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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
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DNS - The Internet Domain Name System (Cont.)
Is designed for use in multiple implementation – have its own space nameCom – commercial organization
Edu - educational institutions
Gov - goverment agencies
DNS naming data are divided into zones that contains following data :Attribute data for names in a domain and less any sub domains
names and adds of at least 2 names servers that provided authoritative data for zone
names of server that hold authoritative data for sub-domains
Zone management parameter – governing the caching and replication
2 types of server that provide authoritative dataPrimary/master – reads zone data directly from local master file
Secondary – download zone data from primary server
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DNS - The Internet Domain Name System (Cont.)
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– IP address (which may not entirely up to date)
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DNS Name Servers
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(ac.uk)
dns0.dcs.qmw.ac.uk(dcs.qmw.ac.uk)
alpha.qmw.ac.uk(qmw.ac.uk)
dns0-doc.ic.ac.uk(ic.ac.uk)
ns.purdue.edu(purdue.edu)
ukpurdue.edu
ic.ac.uk
qmw.ac.uk...
dcs.qmw.ac.uk*.qmw.ac.uk
*.ic.ac.uk*.dcs.qmw.ac.uk
* .purdue.edu
ns1.nic.uk(uk)
ac.uk...
co.uk
yahoo.com ....
Figure 9.4
authoritative path to lookup:
jeans-pc.dcs.qmw.ac.uk
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Values for the most significant segment of a domain name. DNS does not distinguish between
names in upper or lower case.
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Graphical illustrates one way DNS hierarchy might be structured in corporation. Name for
individual computers can be added to the diagram.
DNS Hierarchy
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2 examples of ways the domain name hierarchy can be divided among three servers. Each organization chooses how to divide names among its servers.
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DNS In Typical Operation
a.root-servers.net(root)
ns0.ja.net(ac.uk)
dns0.dcs.qmw.ac.uk(dcs.qmw.ac.uk)
alpha.qmw.ac.uk(qmw.ac.uk)
dns0-doc.ic.ac.uk(ic.ac.uk)
ns.purdue.edu(purdue.edu)
ukpurdue.edu
ic.ac.uk
qmw.ac.uk...
dcs.qmw.ac.uk*.qmw.ac.uk
*.ic.ac.uk*.dcs.qmw.ac.uk
* .purdue.edu
ns1.nic.uk(uk)
ac.uk...
co.uk
yahoo.com ....
client.ic.ac.uk
IP: alpha.qmw.ac.uk
2
3IP:dns0.dcs.qmw.ac.uk
jeans-pc.dcs.qmw.ac.uk ?
IP:ns0.ja.net
1
IP:jeans-pc.dcs.qmw.ac.uk
4
Without caching
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DNS Resource Records
Record type Meaning Main contentsA A computer address IP numberNS An authoritative name server Domain name for serverCNAME The canonical name for an alias Domain name for aliasSOA Marks the start of data for a zone Parameters governing the zoneWKS A well-known service description List of service names and protocolsPTR Domain name pointer (reverse
lookups)Domain name
HINFO Host information Machine architecture and operatingsystem
MX Mail exchange List of <preference, host> pairsTXT Text string Arbitrary text
- A Resource Record (RR) is the basic data element in the domain name system.- Zone data are stored by name servers in files based on resource records.- Each records refer to domain name.
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DNS zone data records
domain name time to live class type value
1D IN NS dns01D IN NS dns1
1D IN NS cancer.ucs.ed.ac.uk
1D IN MX 1 mail1.qmul.ac.uk1D IN MX 2 mail2.qmul.ac.uk
domain name time to l ive class type value
www 1D IN CNAME apricot
apricot 1D IN A 138.37.88.248
dcs 1D IN NS dns0.dcsdns0.dc s 1D IN A 138.37.88.249dcs 1D IN NS dns1.dcs
dns1.dc s 1D IN A 138.37.94.248dcs 1D IN NS cancer.ucs.ed.ac.uk
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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 And Discovery Services
A service that stores collection of bindings between names and attributes
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
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Directory And Discovery Services (Cont.)
Discovery service 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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Printing service
serviceLookup
serviceLookup
Printing service
admin
admin
admin, finance
finance
Client
Mobile client
Corporate infoservice
Network
Service Discovery In Jini
2. Here I am: .....4. Use printing
service
1. ‘finance’ lookup service?
Jini services register their interfaces and descriptions with the Jini lookup services in their scope
Clients find the Jini lookup services in their scope by IP multicast
Jini lookup service searches by attribute or by interface type– The designers of Jini argue convincingly that this the only reliable way to do discovery
3. Requestprinting &receive proxy
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Case Study
GNS case study (Section 9.4)– an early research project (1985) that developed solutions for the problems of:
large name spaces restructuring the name space
X.500 and LDAP (Section 9.5)– a hierarchically-structured standard directory service designed for world-wide use– 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
Trading services (see Section 17.3)– Directories of services with retrieval by attribute searching– Brokers negotiate the contract for the use of a service, including negotiation of
attribute such as quality and quantity of service
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Fig. 9.7: GNS directory tree &value tree for user Peter.Smith
UK FR
AC
QMWDI: 322
Peter.Smith
passwordmailboxes
DI: 599 (EC)
DI: 574DI: 543
DI: 437
Alpha GammaBeta
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Fig. 9.8: Merging trees under a new root
EC
UK FR
DI: 599
DI: 574DI: 543
NORTH AMERICA
US
DI: 642
DI: 457DI: 732
#599 = #633/EC#642 = #633/NORTH AMERICA
Well-known directories:
CANADA
DI: 633 (WORLD)
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Fig. 9.9: Restructuring the directory
EC
UK FR
DI: 599
DI: 574DI: 543
NORTH AMERICA
US
DI: 642
DI: 457DI: 732CANADA
DI: 633 (WORLD)
#633/EC/US
US
#599 = #633/EC#642 = #633/NORTH AMERICA
Well-known directories:
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Fig. 9.10: X.500
service architectu
re
DSA
DSA
DSA
DSA
DSADSADUA
DUA
DUA
- Organized in tree structure - Namely Directory Information Tree (DIT)- Servers are Directory Service Agents (DSAs) - Clients are Directory User Agents (DUAs)
Case Study: X.500 service
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Case Study: X.500 service (Cont.)
Advantages- X.500 is designed to be scalable. If this is achieved in
practice, then it should meet the future needs of large-scale networking better than DNS.
- The main advantage of X.500 is that it is an attribute-based directory service.
- Easy to use: users could address messages to people by quoting their real names and their organizational affiliations, instead of the Domain Name based addresses currently used.
- The mail system would make a search request of X.500 to find the corresponding DNS or other network address of the user’s mailbox.
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Case Study: X.500 service (Cont.)
Disadvantages
- Searching with a wide scope is quite slow - Costly in computing resources- The scope could be limited by the use of the
organizational affiliation. Several alternate mailboxes could be held in the directory server, providing fault tolerant mail delivery.
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Fig. 9.11: Part of X.500 Directory Information Tree
... France (country) Great Britain (country) Greece (country)...
BT Plc (organization) University of Gormenghast (organization)... ...
Department of Computer Science (organizationalUnit)
Computing Service (organizationalUnit)
Engineering Department (organizationalUnit)
...
...
X.500 Service (root)
Departmental Staff (organizationalUnit)
Research Students (organizationalUnit)
ely (applicationProcess)...
...
Alice Flintstone (person) Pat King (person) James Healey (person) ...... Janet Papworth (person)...
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Fig. 9.12: An X.500 DIB Entry
infoAlice Flintstone, Departmental Staff, Department of Computer Science,
University of Gormenghast, GB
commonName Alice.L.Flintstone Alice.Flintstone Alice Flintstone A. Flintstone
surname
Flintstone
telephoneNumber +44 986 33 4604
uid
alf
roomNumber
Z42
userClass
Research Fellow
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Summary
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