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1. Chapter 5 - Naming

2. 2
Introduction
names play an important role to:
share resources
uniquely identify entities
refer to locations
etc.
an important issue is that a name can be resolved to the entity it
refers to
to resolve names, it is necessary to implement a naming system
in a distributed system, the implementation of a naming system is
itself often distributed, unlike in nondistributed systems
efficiency and scalability of the naming system are the main
issues

3. 3
we discuss how
human friendly names are organized and implemented; e.g.,
those for file systems and the WWW
classes on naming systems
flat naming
structured naming, and
attribute-based naming
Objectives of the Chapter

4. 4
5.1 Names, Identifiers, and Addresses
a name in a distributed system is a string of bits or characters
that is used to refer to an entity
an entity is anything; e.g., resources such as hosts, printers,
disks, files, objects, processes, users, Web pages, ...
entities can be operated on; e.g., a resource such as a printer
offers an interface containing operations for printing a
document, requesting the status of a job, ...
to operate on an entity, it is necessary to access it through its
access point, itself an entity (special)

5. 5
access point
the name of an access point is called an address (such as
IP address and port number as used by the transport layer)
the address of the access point of an entity is also referred
to as the address of the entity
an entity can have more than one access point (similar to
accessing an individual through different telephone
numbers)
an entity may change its access point in the course of time
(e.g., a mobile computer getting a new IP address as it
moves)

6. 6
an address is a special kind of name
it refers to at most one entity
each entity is referred by at most one address; even when
replicated such as in Web pages
an entity may change an access point, or an access point
may be reassigned to a different entity (like telephone
numbers in offices)
separating the name of an entity and its address makes it
easier and more flexible; such a name is called location
independent
there are also other types of names that uniquely identify an
entity; in any case an identifier is a name with the following
properties
it refers to at most one entity
each entity is referred by at most one identifier
it always refers to the same entity (never reused)
identifiers allow us to unambiguously refer to an entity

7. 7
examples
name of an FTP server (entity)
URL of the FTP server
address of the FTP server
IP number:port number
the address of the FTP server may change
there are three classes on naming systems: flat naming,
structured naming, and attribute-based naming

8. 8
5.2 Flat Naming
a name is a sequence of characters without structure; like
human names? may be if it is not Ethiopian name!
difficult to be used in a large system since it must be centrally
controlled to avoid duplication
how are flat names resolved
name resolution: mapping a name to an address or an
address to a name is called name-address resolution
possible solutions: simple, home-based approaches, and
hierarchical approaches

9. 9
1.Simple Solutions

two solutions for LANs: Broadcasting and Multicasting,
and Forwarding Pointers
a. Broadcasting and Multicasting
a computer that wants to access another computer for
which it knows its IP address broadcasts this address
the owner responds by sending its Ethernet address
used by ARP (Address Resolution Protocol) in the
Internet to find the data link address (MAC address) of a
machine
broadcasting is inefficient when the network grows
(wastage of bandwidth and too much interruption to other
machines)
multicasting is better when the network grows - send only
to a restricted group of hosts
multicasting can also be used to locate the nearest
replica - choose the one whose reply comes in first

10. 10
b. Forwarding Pointers
how to look mobile entities
when an entity moves from A to B, it leaves behind a
reference to its new location
advantage
simple: as soon as the first name is located using
traditional naming service, the chain of forwarding
pointers can be used to find the current address
drawbacks
the chain can be too long - locating becomes expensive
all the intermediary locations in a chain have to maintain
their pointers
vulnerability if links are broken
hence, making sure that chains are short and that
forwarding pointers are robust is an important issue

11. 11
2.Home-Based Approaches
broadcasting and multicasting have scalability problems;
performance problems and broken links are problems in
forwarding pointers
a home location keeps track of the current location of an
entity; often it is the place where an entity was created
it is a two-tiered approach
an example where it is used in Mobile IP
each mobile host uses a fixed IP address
all communication to that IP address is initially directly
sent to the host’s home agent located on the LAN
corresponding to the network address contained in the
mobile host’s IP address
whenever the mobile host moves to another network, it
requests a temporary address in the new network
(called care-of-address) and informs the new address to
the home agent

12. 12
when the home agent receives a message for the
mobile host it forwards it to its new address and also
informs the sender the host’s current location for
sending other packets
home-based approach: the principle of Mobile IP

13. 13
problems:
creates communication latency
the host is unreachable if the home does no more exist
(permanently changed); the solution is to register the home
at a traditional name service

14. 14
3.Hierarchical Approaches
a generalization of the two-tiered approach into multiple
layers
a network is divided into a collection of domains, similar
to DNS
a single top-level domain spans the entire network
each domain can be subdivided into multiple, smaller
domains
the lowest-level domain is called a leaf domain; typically a
LAN
each domain D has an associated directory node dir(D)
that keeps track of the entities in that domain leading to a
tree of directory nodes
the root (directory) node knows about all entities

15. 15
hierarchical organization of a location service into domains, each having an associated directory node

16. 16
each entity is represented by a location record in the
directory node dir(D) to keep track of its whereabouts
a location record for an entity in a leaf domain contains the
entity’s current address; all other high-level domains will
have only pointers to this address; this means the root node
will store only pointers to all entities
an entity may have multiple addresses, for instance, if it is
replicated; a higher level domain containing the two
subdomains where the entity has addresses will have two
pointers

17. 17
an example of storing information of an entity having two addresses in different leaf domains

18. 18
looking up a location in a hierarchically organized location service
example of a look up operation
a client (in Domain D) would like to locate an entity E

19. 19
update operations (i.e., inserting and deleting addresses)
read pages 194 - 195)
another solution is Distributed Hash Tables (DHT)
read pages 188 - 191

20. 20
5.3 Structured Naming
flat names are not convenient for humans
Name Spaces
names are organized into a name space
each name is made of several parts; the first may define the
nature of the organization, the second the name, the third
departments, ...
the authority to assign and control the name spaces can be
decentralized where a central authority assigns only the
first two parts
a name space is generally organized as a labeled, directed
graph with two types of nodes
leaf node: represents the named entity and stores
information such as its address or the state of that entity
directory node: a special entity that has a number of
outgoing edges, each labeled with a name
each node in a naming graph is considered as another entity
with an identifier

21. 21
a general naming graph with a single root node, no
a directory node stores a table in which an outgoing edge is
represented as a pair (edge label, node identifier), called a
directory table
each path in a naming graph can be referred to by the
sequence of labels corresponding to the edges of the path
and the first node in the path, such as
N:<label-1, label-2, ..., label-n>, where N refers to the first
node in the path

22. 22
such a sequence is called a path name
if the first node is the root of the naming graph, it is called an
absolute path name; otherwise it is a relative path name
instead of the path name n0:<home, steen, mbox>, we often
use its string representation /home/steen/mbox
there may also be several paths leading to the same node,
e.g., node n5 can be represented as /keys or
/home/steen/keys
although the above naming graph is directed acyclic graph
(a node can have more than one incoming edge but is not
permitted to have a cycle), the common way is to use a tree
(hierarchical) with a single root (as is used in file systems)
in a tree structure, each node except the root has exactly
one incoming edge; the root has no incoming edges
each node also has exactly one associated (absolute)
path name

23. 23
Name Resolution
given a path name, the process of looking up a name stored
in the node is referred to as name resolution; it consists of
finding the address when the name is given (by following
the path)
Linking and Mounting
Linking: giving another name for the same entity (an alias)
e.g., environment variables in UNIX such as HOME that
refer to the home directory of a user
two types of links (or two ways to implement an alias):
hard link: to allow multiple absolute path names to refer
to the same node in a naming graph
e.g., in the previous graph, there are two different path
names for node n5: /keys and /home/steen/keys

24. 24
the concept of a symbolic link explained in a naming graph
symbolic link: representing an entity by a leaf node and
instead of storing the address or state of the entity, the
node stores an absolute path name
when first resolving an absolute path name stored in a
node (e.g., /home/steen/keys in node n6), name resolution
will return the path name stored in the node (/keys), at
which point it can continue with resolving that new path
name

25. 25
so far name resolution was discussed as taking place
within a single name space
name resolution can also be used to merge different name
spaces in a transparent way
the solution is to use mounting
Mounting
as an example, consider a mounted file system, which
can be generalized to other name spaces as well
let a directory node store the directory node from a
different (foreign) name space
the directory node storing the node identifier is called a
mount point
the directory node in the foreign name space is called a
mounting point, normally the root of a name space
during name resolution, the mounting point is looked up
and resolution proceeds by accessing its directory table

26. 26
consider a collection of name spaces distributed across
different machines (each name space implemented by a
different server)
to mount a foreign name space in a DS, the following are at
least required
the name of an access protocol (for communication)
the name of the server
the name of the mounting point in the foreign name space
each of these names needs to be resolved
to the implementation of the protocol
to an address where the server can be reached
to a node identifier in the foreign name space
the three names can be listed as a URL

27. 27
example: Sun’s Network File System (NFS) is a distributed file
system with a protocol that describes how a client can access
a file stored on a (remote) NFS file server
an NFS URL may look like nfs://flits.cs.vu.nl/home/steen
-nfs is an implementation of a protocol
-flits.cs.vu.nl is a server name to be resolved using DNS
-/home/steen is resolved by the server
e.g., the subdirectory /remote includes mount points for
foreign name spaces on the client machine
a directory node named /remote/vu is used to store
nfs://flits.cs.vu.nl/home/steen
consider /remote/vu/mbox
this name is resolved by starting at the root directory on
the client’s machine until node /remote/vu, which returns
the URL nfs://flits.cs.vu.nl/home/steen
this leads the client machine to contact flits.cs.vu.nl
using the NFS protocol
then the file mbox is read in the directory /home/steen

28. 28
mounting remote name spaces through a specific process protocol

29. 29
distributed systems that allow mounting a remote file
system also allow to execute some commands
example commands to access the file system
cd /remote/vu
ls -l
by doing so the user is not supposed to worry about the
details of the actual access; the name space on the local
machine and that on the remote machine look to form a
single name space

30. 30
The Implementation of a Name Space
a name space forms the heart of a naming service
a naming service allows users and processes to add,
remove, and lookup names
a naming service is implemented by name servers
for a distributed system on a single LAN, a single server
might suffice; for a large-scale distributed system the
implementation of a name space is distributed over multiple
name servers
Name Space Distribution
in large scale distributed systems, it is necessary to
distribute the name service over multiple name servers,
usually organized hierarchically
a name service can be partitioned into logical layers
the following three layers can be distinguished (according to
Cheriton and Mann)

31. 31
global layer
formed by highest level nodes (root node and nodes close
to it or its children)
nodes on this layer are characterized by their stability, i.e.,
directory tables are rarely changed
they may represent organizations, groups of
organizations, ..., where names are stored in the name
space
administrational layer
groups of entities that belong to the same organization or
administrational unit, e.g., departments
relatively stable
managerial layer
nodes that may change regularly, e.g., nodes representing
hosts of a LAN, shared files such as libraries or binaries,
…
nodes are managed not only by system administrators, but
also by end users

32. 32
an example partitioning of the DNS name space, including Internet-
accessible files, into three layers

33. 33
the name space is divided into nonoverlapping parts, called
zones in DNS
a zone is a part of the name space that is implemented by a
separate name server
some requirements of servers at different layers
performance (responsiveness to lookups), availability (failure
rate), etc.
high availability is critical for the global layer, since name
resolution cannot proceed beyond the failing server; it is also
important at the administrational layer for clients in the same
organization
performance is very important in the lowest layer, since
results of lookups can be cached and used due to the relative
stability of the higher layers
they may be enhanced by client side caching (global and
administrational layers since names do not change often)
and replication; they create implementation problems since
they may introduce inconsistency problems (see Chapter 7)

34. 34
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
Item Global AdministrationalManagerial
Geographical scale of networkWorldwide Organization Department
Total number of nodes Few Many Vast numbers
Responsiveness to lookups Seconds Milliseconds Immediate
Update propagation Lazy Immediate Immediate
Availability requirement Very High High low
Number of replicas Many None or few None
Is client-side caching applied?Yes Yes Sometimes

35. 35
Implementation of Name Resolution
recall that name resolution consists of finding the address
when the name is given
assume that name servers are not replicated and that no
client-side caches are allowed
each client has access to a local name resolver, responsible
for ensuring that the name resolution process is carried out
e.g., assume the path name
root:<nl, vu, cs, ftp, pub, globe, index.txt>
is to be resolved
or using a URL notation, this path name would correspond
to ftp://ftp.cs.vu.nl/pub/globe/index.txt

36. 36
Resolution
mapping a name to an address or an address to a name is
called name-address resolution
Resolver
a host that needs to map an address to a name or a name
to an address calls a DNS client named a resolver
the resolver accesses the closest DNS server with a
mapping request
if the server has the information it satisfies the resolver;
otherwise, it either refers the resolver to other servers
(called Iterative Resolution) or asks other servers to
provide the information (called Recursive Resolution)

37. 37
Iterative
a name resolver hands over the complete name to the root
name server
the root name server will resolve the name as far as it can and
return the result to the client; at the minimum it can resolve
the first level and sends the name of the first level name
server to the client
the client calls the first level name server, then the second, ...,
until it finds the address of the entity
the principle of iterative name resolution

38. 38
Recursive
a name resolver hands over the whole name to the root name
server
the root name server will try to resolve the name and if it
can’t, it requests the first level name server to resolve it and
to return the address
the first level will do the same thing recursively
the principle of recursive name resolution

39. 39
Advantages and drawbacks
recursive name resolution puts a higher performance
demand on each name server; hence name servers in the
global layer support only iterative name resolution
caching is more effective with recursive name resolution;
each name server gradually learns the address of each name
server responsible for implementing lower-level nodes;
eventually lookup operations can be handled efficiently
recursive name resolution of <nl, vu, cs, ftp>; name servers cache
intermediate results for subsequent lookups
Server for
node
Should
resolve
Looks
up
Passes to
child
Receives
and caches
Returns to
requester
cs <ftp> #<ftp> -- -- #<ftp>
vu <cs,ftp> #<cs> <ftp> #<ftp> #<cs>
#<cs, ftp>
nl <vu,cs,ftp>#<vu> <cs,ftp> #<cs>
#<cs,ftp>
#<vu>
#<vu,cs>
#<vu,cs,ftp>
root <nl,vu,cs,ftp>#<nl> <vu,cs,ftp>#<vu>
#<vu,cs>
#<vu,cs,ftp>
#<nl>
#<nl,vu>
#<nl,vu,cs>
#<nl,vu,cs,ftp>

40. 40
the comparison between recursive and iterative name resolution with respect to communication
costs; assume the client is in Ethiopia and the name servers in the Netherlands
communication costs may be reduced in recursive name
resolution
Summary
Method Advantage(s)
RecursiveLess Communication cost; Caching is more effective
IterativeLess performance demand on name servers

41. 41
Example - The Domain Name System (DNS)
one of the largest distributed naming services is the Internet DNS
it is used for looking up host addresses and mail servers
hierarchical, defined in an inverted tree structure with the root at
the top
the tree can have only 128 levels

42. 42
Label
each node has a label, a string with a maximum of 63
characters (case insensitive)
the root label is null
children of a node must have different names (to guarantee
uniqueness)
Domain Name
each node has a domain
name
a full domain name is a
sequence of labels
separated by dots (the last
character is a dot; null
string is nothing)
domain names are read
from the node up to the
root
full path names must not
exceed 255 characters

43. 43
Fully Qualified Domain Name (FQDN) or Absolute
terminated by a null string
contains the full name of a host, e.g., cs.aau.edu.et.
usually the last dot is omitted for readability
Partially Qualified Domain Name (PQDN) or Relative
not terminated with a null string
it starts from a node but does not reach the root
used when the name to be resolved belongs to the same
site as the client (the resolver supplies the missing part,
called the suffix to create an FQDN)

44. 44
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 Internet is divided into over 200 top-level domains;
each partitioned into subdomains, ... ; the leaves represent
domains that have no subdomains; a leaf domain may
contain a single host or represent a company and contain
thousands of hosts

45. 45
Hierarchy of Name Servers
storing the information contained in the domain name space
in a single computer is inefficient and unreliable
distribute the information among many computers called
DNS servers
there is a hierarchy of name servers as we have a hierarchy
of names

46. 46
Zone
what a server is responsible for, or has authority over, is
called a zone; zones are nonoverlapping
the server makes a database called a zone file and keeps
all the information for every node under that domain
it can divide its domain into subdomains and delegate part
of its authority to other servers

47. 47
Root Server
a server whose zone consists of the whole tree
it usually does not store the whole information about
domains but delegates its authority to other servers and
keeps references to those servers
there are currently more than 13 root servers, each
covering the whole domain name space and distributed all
around the world
Primary and Secondary Servers
a primary server is one that stores a file about the zone for
which it is an authority; it is responsible for creating,
maintaining, and updating the zone file
a secondary server is one that transfers the complete
information about a zone from another server (primary or
secondary); it does not create or update the file
such arrangement is to create redundancy so that if one
server fails, the other can still serve clients

48. 48
Types of Top-Level Domains
two types: generic domains and country domains; there is a
third one called Inverse Domain (used to map an address to
a name; we will not discuss it further)
Generic Domains
define registered hosts according to their generic
behaviour
LabelDescription
com Commercial organizations
edu Educational institutions
gov Government institutions
int International organizations
mil Military groups
net Network support centers
org Nonprofit organizations

49. 49
newly introduced first-level domains
LabelDescription
aero Airlines and aerospace companies
biz Businesses or firms (similar to com)
coop Cooperative business organizations
info Information service providers
museum Museums and other nonprofit organizations
name Personal names (individuals)
pro Professional individual organizations
Country Domains
include one entry for every
country (as defined by ISO) -
two character abbreviations

50. 50
the contents of a node is formed by a collection of resource
records; the important ones are the following
Type of
record
Associated
entity
Description
SOA (start of
authority)
Zone
Holds information on the represented zone, such as an
e-mail address of the system administrator
A (address)Host Contains an IP address of the host this node represents
MX (mail
exchange)
Domain
Refers to a mail server to handle mail addressed to this
node; it is a symbolic link; e.g. name of a mail server
SRV Domain Refers to a server handling a specific service
NS (name
server)
Zone
Refers to a name server that implements the
represented zone
CNAME Node Contains the canonical name of a host
PTR (pointer)Host
Symbolic link with the primary name of the represented
node
HINFO (host
info)
Host
Holds information on the host this node represents;
such as machine type and OS
TXT Any kind
Contains any entity-specific information considered
useful

51. 51
an excerpt from the DNS database for the zone cs.vu.nl
cs.vu.nl
represents the
domain as well
as the zone; it
has 3 name
servers (star, top,
solo) and 3 mail
servers
name server for
this zone with 2
network
addresses
mail server
Web server
FTP server
a single machine
implementing
Web server and
FTP server
laser printer
inverse mapping

52. 52
5.4 Attribute-Based Naming
flat naming: provides a unique and location-independent way
of referring entities
structured naming: also provides a unique and location-
independent way of referring entities as well as human-friendly
names
but do not allow searching entities by giving a description of
an entity
each entity is assumed to have a collection of attributes that
say something about the entity
then a user can search an entity by specifying (attribute, value)
pairs known attribute-based naming
Directory Services
attribute-based naming systems are also called directory
services

53. 53
how are resources described? one possibility is to use RDF
(Resource Description Framework) that uses triplets
consisting of a subject, a predicate, and an object
e.g., (person, name, Alice) to describe a resource Person
whose Name is Alice
Hierarchical Implementations: LDAP
distributed directory services are implemented by combining
structured naming with attribute-based naming
e.g., Microsoft’s Active directory service
such systems rely on the lightweight directory access
protocol or LADP which is derived from OSI’s X.500
directory service
a LADP directory service consists of a number of records
called directory entries (attribute, value) pairs, similar to a
resource record in DNS; could be single- or multiple-valued
(e.g., Mail_Servers)

54. 54
a simple example of an LDAP directory entry using LDAP
naming conventions to identify the network addresses of
some servers
Attribute Abbr.Value
Country C NL
Locality L Amsterdam
Organization O Vrije Universiteit
OrganizationalUnitOU Comp. Sc.
CommonName CN Main server
Mail_Servers -- 137.37.20.3, 130.37.24.6,137.37.20.10
FTP_Server -- 130.37.20.20
WWW_Server -- 130.37.20.20

55. 55
the collection of all directory entries is called a Directory
Information Base (DIB)
each record is uniquely named so that it can be looked up
each naming attribute is called a Relative Distinguished Name
(RDN); the first 5 entries above
a globally unique name is formed using abbreviations of
naming attributes, e.g.,
/C=NL/O=Vrije Universiteit/OU=Comp. Sc.
this is similar to the DNS name nl.vu.cs
listing RDNs in sequence leads to a hierarchy of the collection
of directory entries, called a Directory Information Tree (DIT)
a DIT forms the naming graph of an LDAP directory service
where each node represents a directory entry

56. 56
part of the directory information tree
node N corresponds to the directory entry shown earlier; it
also acts as a parent of other directory entries that have an
additional attribute, Host_Name; such entries may be used
to represent hosts

57. 57
two directory entries having Host_Name as RDN
Attribute Value Attribute Value
Country NL Country NL
Locality Amsterdam Locality Amsterdam
Organization Vrije UniversiteitOrganization Vrije Universiteit
OrganizationalUnitComp. Sc. OrganizationalUnitComp. Sc.
CommonName Main server CommonName Main server
Host_Name star Host_Name zephyr
Host_Address 192.31.231.42 Host_Address 137.37.20.10
read pages 222 - 226 about Decentralized Implementations