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Outline  Introduction  Different Mechanisms Broadcasting Multicasting Forward Pointers Home-based approach Distributed Hash Tables Hierarchical approaches  Ongoing Research  Introduction  Different Mechanisms Broadcasting Multicasting Forward Pointers Home-based approach Distributed Hash Tables Hierarchical approaches  Ongoing Research 2

Introduction ► 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 3

Introduction 4 ► 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

Broadcasting 5  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

Multicasting 6  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

Forward Pointers 7 ► 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:

Forward Pointers 8 ► 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

Home based approach 9 ►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

Home based approach 10 ► Pros: ► Scalability ► Cons: ► Communication latency ► Contacting the entity will become impossible if the home location doesn’t exist ►Poor geographical stability

Distributed Hash Tables 11 ►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

Distributed Hash Tables 12 ► 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 i th entry of node n will contain successor((n+2 i-1 )mod 2 m ) ► At node n, send query for key k to largest successor/finger entry <= k if none exist, send query to successor(n)

Distributed Hash Tables 13

Distributed Hash Tables 14 iFt[i] iFt[i] iFt[i] (i) 80 Finger Table(ii) 16 Finger Table (iii) 32 Finger Table

Distributed Hash Tables 15

Distributed Hash tables 16 ► 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

Hierarchical approaches 17 ►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

Hierarchical approaches 18 ► Look Up: Performance will be degraded when the search continues until the root node is reached

Ongoing Research 19 ► 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.

Questions? 20

Thank you! 21