5.2 FLAT NAMING

Introduction Different Mechanisms: Broadcasting Multicasting Forward Pointers Home-based approach Distributed Hash Tables Hierarchical approaches

Introduction Flat names: Identifiers are just random bits of strings. Doesn’t have any information on how to locate access point of associated entity. The simplest name space is a flat name space where names are character strings exhibiting no structure. Names defined in a flat name space are called primitive or flat names. Different mechanisms are available to locate an entity given only its identifier. Therefore, flat names are suitable for use either for small name spaces having names for only a few objects or for system-oriented names that need not be meaningful to the users.

Broadcasting 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 Advantages: Simple and easy to implement Efficient for small networks Dis: Inefficient when the network grows Network bandwidth wasted by requests Solution: Multicasting

Multicasting 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 Advantages: Bandwidth wastage will be less compared to broadcasting Efficient for small groups Dis: Scalability

Forward 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. Ex:

Forward Pointers Advantages: 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. Dis: 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 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 Advantages: Scalability Dis: Communication latency Contacting the entity will become impossible if the home location doesn’t exist Poor geographical stability

Distributed Hash Tables 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 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) .

Distributed Hash Tables

Distributed Hash Tables

Distributed Hash tables Advantages: Efficiency of lookups and inserts Load balanced Joining and leaving is simple Dis: Underlying network should be taken into account Ex: Nodes in different places

Hierarchical 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 .

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

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.

Thank you….