Review of the Survey: P2P Overlay in MANET


Chapter on P2P over Manet

Peer-to-Peer paradigm in the networks, such as MANET, is quite importante as wireless multi-hop networks don't have centralized servers available or located in the Internet.

There is enough similarity between P2P and MANET, as they both self-organized and decentralized, due to the common nature of their distributed components. They both also have high degree of dynamicity in the system. On the other hands some differences could make some troubles. Main difference is the bandwidth consumption, where P2P are mostly concerned on the IP routing infrastructure, and MANET is usually quite limited in bandwidth, and high traffic will lead to the scalability problems (as could be noticed in the structured overlay networks).

That's why it is very important to study well the challenges, characteristics and design goals between P2P overlays and MANET. This study could help (with proper conditions) to amplify the effect of interaction P2P and MANET.

P2P Overlay Networks

  • Unstructured.
    This type of overlay doesn't impose a rigid relation between the overlay topology and where resources or their indices are stored. Advantages: easy to implement and simplicity, supporting dynamic environment and keyword search. But scalability could be an issue.
    • Hybrid decentralized overlays
      • Napster
      • Publius
      • Bittorent
    • Purely decentralized overlays
      • Free Haven
      • Early Gnutella
    • Partially centralized overlays
      • Kazaa
      • Morpheus
      • OverNet/eDonkey2000
  • Structured.
    Implements DHT, that's why upper bound for the look up is introduced - O(log(n)).
    • CAN
    • Chord
    • Pastry
    • Bamboo

Wireless Multi-hop and mobility

In multi-hop cases the roles of collision and interference become more complex and depend on many factors: radio environment, modulation schemes, transmission power or sensing ranges. As a result, adjacent links and even link further separated, affect each other during transmission and they might have to share the wireless channel.

Another problem area is mobility of nodes, so the disconnecting nodes for a long period and high mobility could be noticed. One of the approach to reduce drawbacks could be digital postal service through store-carry-forward message delivery.

Routing protocols in MANET

Flat routing protocols distribute information as needed to any network node that can be reached or receive information, when hierarchical routing protocols, instead, group nodes together by function into a hierarchy. Flat routing protocol was proved to be more efficient that's why more details study of them could be found below.

  • Topology-based Schemes
    Main focus is on the exploiting information related to the network topology.
    • Reactive protocols
      They builds and maintains routing table at every node to find the way, both, to the destination and back to the source.
      • Ad-hoc On Demand Distance Vector (AODV)
      • Dynamic Source Routing (DSR)
        It obtains and encodes the source route in each packet header to the destination.
    • Proactive protocols
      They attempt to continuously maintain consistent, up-to date routing information from each node to any other node in the network. That's why in highly dynamic networks the overhead of proactive approaches is much higher that in reactive schemes, however routes to any node in the system are always known.
      • Optimized Link State Routing Protocol (OLSR)
      • Better Approach to Mobile Ad-hoc Networking (BATMAN)
        Each node perceives and maintain only the information about the best next hop towards all other nodes. It could be particularly suitable fot the network with low level of connectivity.
  • Geographic-based Routing
    It relies on the geographical position information of the nodes.
    • Greedy Perimeter Stateless Routing (GPSR)
      It tries to bring the message closer to the destination at each step, using only local information.
    • Compas routing
  • Probabilistic Routing
    In this type of routing the probability of physically meeting with other users or visiting particular places is discovered.
    • Probabilistic Routing Protocol (PROPHET)
      It si kind of epidemic approach with delivery predictability.
    • MaxProp
      This method, in addition, exploits information about frequently visited places.

Challenges of Deploying P2P Services in MANET

  • Bandwidth Constrains
    Mobil ad-hoc networks are quite limited in bandwidth. That's why high traffic will lead to scalability problems in multi-hop environment. The main challenge is how to enable efficient overlay services and applications on the resource constrains wireless environment. One of the approach to solve this could be implementation of cross-layering techniques between P2P and the MANET routing layer.
  • P2P Overlay Maintenance
    One of the solution could be netwrok probing, but this introduces high level of congestion and collisions. That's why cross-layer and integrated approaches were applied to handle this challenge (CrossROAD, SSR).
  • Network Resiliency
    DHT experiences a reduces routing performance until the recovery has finished (of course depends on the DHT topology). It's importante to know at which moment start the recovery process, in order not to introduce unnecessary overhead from appearance this node again.
  • Routing Stretch
    Routing stretch metric is the ratio between the cost fo selected route using the overlay=neighbor to the optimal shortest path routing through the MANET. So minimizing routing stretch is a critical issue for a multi-hop environment, as otherwise both delay and packet loss will be increased.
  • Exploiting Heterogeneity
    Some nodes may not wish to act as overlay routers for other nodes (due to some reasons, low battery etc.)
  • Query Propagation
  • Cooperative Content Caching
    Proper caching technique is quite important to reach appropriate level of data availability and reduce pressure on the nodes to store all the data in the system. Distributed caching should be applied to store popular information.
  • Information Distribution and Survival
    Information shouldn't be allowed to be lost in any case. So the evaluation of the minimum number of copies of a specific information is quite important.
  • Security
    Security in P2P over MANET is quite challenging and could require use of public keys, etc, and hence depends on a PKI. PKI sotres digital certificates that attach a public key to the name of the owner by the digital signature of a trusted peers - CAs. But maintaining this introduces some complexity due to the centralized nature of the PKI.
routing_over_manet/p2p_overlay_in_manet.txt · Last modified: 2012/06/05 16:10 by julia
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