1

5.2 FLAT NAMING

Tejaswini Kalluri

2Outline Introduction

Different Mechanisms

• Broadcasting

• Multicasting

• Forward Pointers

• Home-based approach

• Distributed Hash Tables

• Hierarchical approaches

Ongoing Research

3Introduction

► Names

- To access resources, uniquely identify entities, refer to

locations

► Address

- The name of the access point through which entity can be

accessed

► Identifiers

- Names that uniquely identify entities in a distributed

system

4Introduction

► Flat names:

• Identifiers are just random bits of strings

• Also called as unstructured names

• Doesn’t have any information on how to locate access point of

associated entity

• Different mechanisms are available to locate an entity given

only its identifier

5Broadcasting Broadcasting

Most common in Local Area Networks

Message containing identifier of entity is broadcast to all the machines in the

network

Machine that has access point for the requested entity responds to the message

► Pros

► Simple and easy to implement

► Efficient for small networks

► Cons

► Inefficient when the network grows

► Network bandwidth wasted by requests

Solution: Multicasting

6Multicasting Multicasting

Restricted group of hosts receive the request for entity

The group is called multicast group and is identified by multicast address

Host that has access point for the requested entity responds to the

message

► Pros

► Bandwidth wastage will be less compared to broadcasting

► Efficient for small groups

► Cons

► Scalability

7Forward Pointers

► Popular approach to locate mobile entities

► This is not like looking up an address, instead the client request will be

forwarded along a chain to the actual object

► Mechanism:

► When an entity moves from location A to location B, it leaves behind a

reference to its new location at B in A.

► By following the chain of forward pointers, message will be sent from client

to the actual object

► Example:

8Forward Pointers

► Pros:

► Transparency – The only thing the client sees of an object is a client stub,

to which locations the client stub forwards its invocations are hidden from

the client

► Cons:

► Chains will be long for highly mobile entities

► All intermediate locations in a chain will have to maintain their part of the

chain of forward pointers

► Vulnerability to broken links

Solution: Home-based approach

9Home based approach► Used as a fall back mechanism for location services using forward pointers

► Makes use of home location that keeps track of the current location of the entity

► Mechanism

► Each mobile host uses a fixed IP address

► All communication to that host will be directed

to mobile host’s home agent

► When host moves to a new location the new

care of address will be updated at the home

agent

► When a home agent receives a packet for the

mobile host, it will be either forwarded or tunneled

to the current location depending on the network location

10Home based approach

► Pros:

► Scalability

► Cons:

► Communication latency

► Contacting the entity will become impossible if the home location doesn’t

exist

► Poor geographical stability

11Distributed Hash Tables► Used in peer to peer systems

► A hash table allows to insert, lookup and delete objects with keys

► A distributed hash table allows to do the same in distributed setting (objects=files)

► Mechanism

► Each node has an m-bit random identifier

► Each entity has a m-bit random key

► An entity with key k is located on a node with the smallest identifier that satisfies

id >=k

► This id is referred to as successor of k denoted as succ(k)

► The major task is key lookup

► Two approaches: linear approach and finger table

12Distributed Hash Tables► Linear approach:

► Each node keeps track of its predecessor and successor

► When key k needs to be resolved, request will be forwarded to one of the two

neighbors – whichever one is appropriate

► Drawback: Poor performance, non scalable

► Finger table approach

► Each node has a finger table with m entries

► The ith entry of node n will contain successor((n+2i-1)mod 2m)

► At node n, send query for key k to largest successor/finger entry <= k if none

exist, send query to successor(n)

13Distributed Hash Tables

14Distributed Hash Tablesi Ft[i]

0 96

1 96

2 96

3 96

4 96

5 112

6 16

i Ft[i]

0 32

1 32

2 32

3 32

4 32

5 80

6 80

i Ft[i]

0 45

1 45

2 45

3 45

4 80

5 80

6 96

(i) 80 Finger Table (ii) 16 Finger Table (iii) 32 Finger Table

15Distributed Hash Tables

16Distributed Hash tables

► Pros:

► Efficiency of lookups and inserts

► Load balanced

► Joining and leaving is simple

► Cons:

► Underlying network should be taken into account

Example: Nodes in different places

17Hierarchical approaches

► Network is divided into a collection of domains

► Each domain subdivided into smaller subdomains

► Lowest level domain is called a leaf domain

► Each domain D has an associated directory node dirt(D) that keeps track entities in

that domain, leading to a tree of directory nodes

► Root directory node knows about all the entities in the network

18Hierarchical approaches

► Look Up:

Performance will be degraded when the search continues until the root node is reached

19Ongoing Research► Forward pointers

► Research is going on to develop new chain reduction mechanisms. One such

mechanism is to update client’s reference when the most recent location is

found

► Distributed Hash Tables

► Since the DHT is dependent on underlying network, researchers are working

to propose efficient ways to make a DHT aware of its underlying network.

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Questions?

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Thank you!