1. Prepared by Dr: Naglaa Fathi Mohammed Soliman
Name service
Prepared by
Dr: Naglaa Fathi Mohammed Soliman

2. Outline
General concepts
Domain Name System (DNS)

3. 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.
Names facilitate communication and resource sharing.
Names are used for identification as well as for describing attributes.
For many purposes, names are preferable to identifiers
because the binding of the named resource to a physical location can be changed
because they are more meaningful to users
Names (e.g. URLs) are bound to objects (e.g. web pages). Names must be resolved before the corresponding objects can be invoked. The use of addresses as names is deprecated because it prevents the relocation of the named objects.

4. Names, Addresses and other attributes
Any process that requires access to a specific resource must possess a name or identifier for it.
Ex: human-readable names are file names such as /etc/passwd,
and Internet domain names such as
The term identifier is sometimes used to refer to names that are interpreted only by programs (Ex. NFS file handles)
Names can be pure or non-pure.
Pure names are simply uninterpreted bit patterns.
Pure names always have to be looked up before they can be of any use.

5. At the other extreme from a pure name is an object’s address.
Addresses are efficient for accessing objects, but objects can sometimes be relocated, so addresses are inadequate
Non-pure names contain information about the object that they name; in particular, they may contain information about the location of the object.
A name is said to be resolved when it is translated into data about the resource or object.

6. Names and Binding
The association between a name and an object is called binding.
Name ↔binding ↔ {Object, Attribute (IP addressing)}
Services are written to map between names and the attributes of objects they refer to.
Example:
Domain name Service (DNS) maps domain names to the attributes of the host computer (IP)
CORBA Naming service maps the name of a remote object onto its remote object reference.

7. Composed naming domains used to access a resource from a URL
Figure 9.1
URL
Resource ID (IP number, port number, pathname)
WebExamples/earth.html
8888
DNS lookup
(Ethernet) Network address
2:60:8c:2:b0:5a
ARP lookup
file
Web server
Socket *

8. Figure 9.1 shows the domain name portion of a URL resolved first via the DNS into an IP address.
and then, at the final hop of Internet routing, via ARP to an Ethernet address for the web server.
The last part of the URL is resolved by the file system on the web server to locate the relevant file.

9. Names and services
Many of the names used in a DS are specific to some particular service .
Client uses service name to perform an operation up one a named object or resource.
Names are also needed to refer to entities in a distributed system that are beyond the scope of any single service.(e.g. users, computers and services)
All these names must be readable.

10. Uniform Resource Identifier
It used to identify resources on the web and other internet resources such as electronic mailbox.
URL: Uniform Resource Locator
This term is reserve for identifiers that are resource locators,
identifies a web page at the given path (‘/’) on the host
URN: Uniform Resource Names
URNs are URIs that are used as pure resource name rather than locator
For example, the URI: mid:0E4FC272-5C02-11D9-B115-

11. Name Service and the DNS
A name service stores a collection of one or more naming contexts – sets of bindings between textual names and attributes for objects.
Provides a general naming scheme for entities (such as users and services) that are beyond the scope of a single service.
Major operation: resolve a name - to look up attributes from a given name
Other operations required: creating new binding, deleting bindings, listing bound names and adding and deleting contexts.

12. Name management is separated from other services largely because of the openness of DS, which brings the following motivations:
Unification : it is convenient for resources managed by different services to use the same naming scheme ( URI).
Integration : it is not possible to predict the scope of sharing DS. So, without a common name service, the administrative domain may use entirely different naming conventions.

13. General Name Service Requirements
Name services were originally quite simple, since they were designed only to meet the need to bind names to addresses in a single management domain, corresponding to a single LAN or WAN.
The interconnection of networks and the increased scale of distributed systems have produced a much larger name-mapping problem.
Handle arbitrary number of names and to serve arbitrary number of administrative organizations.
A long lifetime
High availability
Fault isolation
Tolerance of mistrust

14. Name services: Design Issues
Name spaces
Name Resolution
The domain name system

15. Name Spaces
A name space is a collection of all valid names recognized by a particular service.
Allow simple but meaningful names to be used.
Potentially infinite number of names.
A full domain name is a sequence of labels separated by dots (.). The domain names are always read from the node up to the root.

16. Hierarchic name space
Names may have an internal structure that represents their position in a hierarchic name space.
Hierarchic name space advantages:
Each part of a name is resolved relative to a separate context, and the same name may be used with different meaning in different contexts (directory).
it is potentially infinite
Different context can be managed by different people.

17. Name Space

18. Aliases
An alias is a name defined to denote the same information as another name.
An alias allows a convenient name to be substituted for a more complicated one.
The DNS allows aliases in which one domain is defined to stand for another.
The reason for having aliases is to provide for transparency.
Aliases are generally used to specify the names of machines that runs the web server or an FTP server.
Example:
might be an alias for

19. Aliases (con.) Advantages:
Client can refer to the web server by a generic name that does not refer to a particular machine.
If the web server is moved to another computer, all that needs to be done is to update the alias in the DNS database.

20. Domain
A domain is a subtree of the domain name space. The name of the domain is the domain name of the node at the top of the subtree.
The figure below shows some domains. Note that a domain may itself be divided into domains (or subdomains as they are sometimes called).

21. Naming domains
A naming domain is a name space for which there exist a single overall administrative authority for assigning names with it.
Domains in DNS are collections of domain names.
A domain’s name is the common suffix of the domain names within it. For example, net is a domain name that contains cdk4.net

22. Combining and customizing name spaces
Merging
In which a part of one name space is conveniently embedded in another.
Adding super root
Embed the mounted file systems
Heterogeneity
The DCE allows heterogeneous name spaces to be embedded with it using junction.
Customization
Users sometimes prefer to construct their name spaces independently rather than sharing a single name space

23. Name Resolution
Resolution is an iterative process whereby a name is repeatedly presented to the naming contexts.
The name is first presented to some initial naming context; resolution iterates as long as further context and derived names are output.
Example1: /etc/passwd in which ‘etc’ is presented to context / and ‘passwd’ is presented to context /etc.
Example 2: in which the alias is resolved to another domain name such as copper.dcs.qmw.ac.uk which is further resolved to produce IP address.
Name resolution in the presence of cooperating name servers is called navigation and may be performed by the client or by the server.

24. Name Servers and Navigation
Any name service stores a very large database.
Data is partitioned into servers according to its domain.
Partitioning of the data implies that the local name server cannot answer all the enquiries without the help of other name servers.
Process of locating naming data from among more than one name server in order to resolve a name is called navigation. Ex: Iterative Navigation model(DNS)

25. Navigation types Iterative navigation Multicast navigation
Non-recursive server-controlled navigation
Recursive server-controlled navigation

26. Iterative navigation NS2 2 Name 1 NS1 servers Client 3 NS3
A client iteratively contacts name servers NS1–NS3 in order to resolve a name
NS2
NS1
NS3
Name
servers

27. Non-recursive and recursive server-controlled navigation
Figure 9.3 1 2 3 4
client
NS2
NS1
NS3
Non-recursive
server-controlled
Recursive
server-controlled 1 2 3 5 4
client
NS2
NS1
NS3
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. *

28. Caching
Client name resolution software and servers maintain a cache of previous name resolutions.
How long a resolver caches a DNS response (i.e. how long a DNS response remains valid) is determined by a value called the time to live.
Server may use data from its own cache or other server cache it is authorized to access.
Caching is key to performance and fault tolerance.

29. The Domain Name System A distributed naming database
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
The Domain Name System resolves names on a world-wide scale and represents a considerable engineering achievement.

30. Parts of a domain name
Usually consists of two or more parts (technically labels), separated by dots. .
The rightmost label conveys the top-level domain.
Each label to the left specifies a subdivision, or subdomain of the domain above it.

31. DNS queries Host name resolution Mail host location
When a web browser is given a URL containing the domain name , it makes a DNS enquiry and obtains the corresponding IP address.
Mail host location
Electronic mail software uses the DNS to resolve domain names into the IP address of mail hosts

32. Reverse resolution: return the domain name given an IP address..
Some other types of query that are implemented in some installations but are less frequently used than those just given are:
Reverse resolution: return the domain name given an IP address..
Host information: return the machine type and operating system
Well-known services : return the list of services run by a computer.

33. DNS name servers
The Domain Name System consists of a hierarchical set of DNS servers
Each server holds part of the naming database
Each domain or sub-domain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it
The hierarchy of authoritative DNS servers matches the hierarchy of domains.
At the top of the hierarchy stand the root name-servers: the servers to query when looking up (resolving) a top-level domain name

34. Zones The DNS naming data are divided into zones. Each zone contains:
Attribute data for names in a domain, less any sub- domains administered by lower-level authorities.
The name and addresses of at least two name servers that provide authoritative data for the zone.
The names of name servers that hold authoritative data for delegated sub-domains
Zone management parameters ( governing the caching & replication of zone data.

35. DNS servers and zones
A server may hold authoritative data for zero or more zones.
The DNS architecture specifies that each zone must be replicated authoritatively in at least two servers.

36. Master files
System administrators enter the data for a zone into a master file, which is the source of authoritative data for the zone.
There are two types of servers:
Primary or master server which reads zone data directly from a local master file.
Secondary servers which download zone data from a primary server and communicate periodically with the primary server to check whether their stored version matches that held by the primary server.
The primary sends the latest version if the secondary’s copy is out of date
The value of secondary’s check is one or two a day which is set by administrators as a zone parameters.

37. Basic DNS algorithm for name resolution
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)

38. DNS name servers Figure 9.4 jeans-pc.dcs.qmw.ac.uk *
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) uk
purdue.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
....
Note: Name server names are in italics, and the corresponding domains are in parentheses. Arrows denote name server entries
authoritative path to lookup:
jeans-pc.dcs.qmw.ac.uk *

39. 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) uk
purdue.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
....
Without caching
IP: alpha.qmw.ac.uk 2
Animation: reveals each step in looking up jeans-pc.dcs.qmw.ac.uk from a client in the ic.ac.uk domain
Discuss how caching reduces the number of steps.
client.ic.ac.uk
IP:jeans-pc.dcs.qmw.ac.uk 4
jeans-pc.dcs.qmw.ac.uk ?
IP:ns0.ja.net 1 3
IP:dns0.dcs.qmw.ac.uk *

40. Resource Records

41. Figure 9.6 DNS zone data records
domain name
time to live
class
type
value 1D IN NS
dns0
dns1
cancer.ucs.ed.ac.uk MX
1 mail1.qmul.ac.uk
2 mail2.qmul.ac.uk
domain name ti m
e to l i ve
class
type
value
www 1D IN
CNAME
apr
cot A
dcs 1D IN NS
dns0.dcs
dns0.dc s A
dns1.dcs
dns1.dc ca n c e r. u
.ed .a
c.uk
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

42. 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.

43. References Distributed Systems: Concepts and Design Internet sources
wiki