Costeffective Broadcast for Fully Decentralized Peertopeer Networks

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Cost-effective Broadcast for Fully Decentralized
Peer-to-peer Networks

• Marius Portmann
• Aruna Seneviratne

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Peer to Peer Systems

• Two types
• Structured
• Guarantee location of content (if exists)
• Access within bounded number of hops
• Control of data placements and topology
• Unstructured
• Decentalized
• Looser guarantees
• Placement of data and topology is ad-hoc

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Wireless Environments

• Characterized by
• Highly transient node populations
• Wide range of users form non cooperating
  organizations
• Searches on partial information
• Not typically looking for rare information
  replicated at a number of places
• Not a good match for structured systems
• Back to unstructured systems

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Unstructured Systems

• Most widely used application of p2p systems is
  file sharing
• As the placement of data is ad-hoc
• Only random searchers are possible
• Hard to find desired files without wide
  distribution of queries
• Unscalable unless can improve the efficiency of
  searches

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Example - Gnutella

• Gnutella can be considered as pure peer-to-peer
  system
• Fully decentralized and distributed searching
• Operation of Gnutella
• Two types of services
• Searching for files
• Peer discovery
• Implemented with application level broadcasts
• Broadcast is implemented with TTL flooding

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File Location

• A query message is forwarded to all its
  neighbors, except for the one, where it was
  received from
• Each message has a Time To Live (TTL)
• Decremented by one at each visited node
• Message is dropped when TTL0
• Each message has an unique ID
• Node keeps a record of IDs of messages that it
  has seen in the recent past
• Message with the same ID and type as ones that
  that have been received are dropped

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Cost Metric 1

• Define a cost metric for comparison of methods
• number of messages that are generated and
  forwarded
• based solely on the network size and the average
  node degree,
• Estimate the average bandwidth consumption per
  node

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Cost Metric 2
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Flooding - Unscalable

• Resource consumption per node of flooding based
  broadcast can be prohibitively high, even for
  networks of moderate size

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Rumor Mongering or Gossip Protocols

• A class of probabilistic protocols for message
  routing
• Messages are spread in a network much like a
  disease in a susceptible population.
  (epidemiological protocol)
• The neighbors to which messages are forwarded to
  by each node are chosen randomly.
• Trades off reliability and speed for a reduction
  in cost

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Blind Counter Rumor Mongering

• A node n initiates a broadcast
• Send the message m to B neighbors, chosen at
  random
• When a node (p) receives a message m from anther
  node (q)
• If (p has received m no more than F times)
• p sends m to B uniformly randomly chosen
  neighbors that p knows have not yet seen m
• p knows if its neighbor q has already seen the
  message m only if p has sent it to q previously,
  or if p received the message from q

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Cost of BCRM

• Difficult to obtain analytical expressions to
  describe the behavior of a Gossip protocol, even
  for relatively simple topologies
• Can give an upper limit
• bounded by BF- an upper limit for the cost c

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Simulation Results

• Barabási Topology
• Model for generating topology is based on how
  typical p2p networks evolve
• Power-law characteristics
• 1000 nodes with an average node degree of 6
• F and B for the BCRM was set to be 2

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Some More Results

• Trade-off of cost, reliability and time by
  choosing F and B appropriately
• Level of cost reduction depends on the average
  node degree
• The higher the node degree is, the bigger the
  potential for cost reduction

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P2P Network Topologies

• Typical characteristic of peer-to-peer networks
  is a power-law distribution of the node degrees
• most nodes have few links while a small number
  of nodes have a large number of links

From Matei Ripeanu Ian Foster
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Deterministic Rumor Mongering

• Make intelligent decisions as to which of its
  neighbors to forward messages to
• Based it on the node degree of the corresponding
  nodes
• The nodes with the lowest degree are chosen first
  

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Deterministic Rumor Mongering cont.

• When a node p receives a message m from node q
• If (p has received m no more than F times)
• send m to all of its neighbors of degree one, and
• B of the rest of its neighbors with the lowest
  node degree, that p knows have not yet seen m

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Rationale for (1)

• Pendant neighbors, have no other chance to
  receive the message
• These pendant neighbors cannot contribute to the
  further propagation of the message
• not considered for the limit of B messages to be
  forwarded

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Rationale for (2)

• Nodes of high degree receive a large number of
  copies of the same message
• This overhead grows approximately linearly with
  the node degree
• Also with higher parameters B and F.

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Viability

• The only requirement is that each node knows the
  node degree of its immediate neighbors
• Not in conflict with the decentralized nature of
  the networks
• Can easily be integrated
• Gnutella
• a one byte field in the Gnutella message header
• Increasing the minimal message size by less than
  5.

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Some Results

• Performance of Deterministic Rumor Mongering
  compared to Blind Counter Rumor Mongering
• For a given B and F, DRM achieves a significant
  higher reach than the BCRM, within a shorter time
• For a given reach, DRM has a significantly lower
  cost

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Some More Results

• BCRM
• DRM
• (B,F)

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Conclusions

• Unstructured peer-to-peer systems are more
  suitable for wireless environments
• For unstructured systems to be viable, scalable
  methods of searching need to developed
• The obvious way of is to look at alternatives to
  broadcast
• One such scheme that have been used in the past
  in other application is Rumor Mongering
  (Gossiping)
• We show that, Rumor Mongering, can be used as a
  basis for providing an alternative flooding for
  distributing queries in unstructured peer to peer
  systems

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More Information

• Available form mobqos.ee.unsw.edu.au
• M. Portmann, Pipat Sookavatna, Sebstien Ardon
  and Aruna Seneviratne,The Cost of Peer Discovery
  and Searching in the Gnutella Peer-to-peer File
  Sharing Protocol, IEEE ICON 2001, Bangkok,
  September 2001
• M. Portmann, and Aruna Seneviratne, The Cost of
  Application-level Broadcast in a fully
  Decentralized Peer-to-peer Networks, ISCC,
  Italy, July 2002
• M. Portmann, and Aruna Seneviratne,
  Cost-effective Broadcast for Fully Decentralized
  Peer-to-peer Networks, accepted for publication,
  Computer Communication, Special Issue on
  Ubiquitous Computing
• Also related work
• Qin Lv, Sylvia Ratnasamy and Scott Shenker,Can
  Heterogeneity Make Gnutella Scalable?, 1st
  International Workshop on Peer-to-Peer Systems
  (IPTPS '02), Cambridge, MA, USA, March 2002
• Berverly Yang, and Hector Garcia-Molina,Efficien
  t Search in Peer-to-Peer Networks, 1st
  International Workshop on Peer-to-Peer Systems
  (IPTPS '02), Cambridge, MA, USA, March 2002

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