Peer to peer network schemes and finding algorithms

Searching in P2P networks
Mohamed Elsharnouby - Istanbul Sehir University

P2P networks
Structured:
- CAN
- Chord
- Tapestry
- Pastry
- Viceroy
Unstructured:
- Freenet
- Gnutella
- BitTorrent

Structured
Pros:
- Can search any resource even if rare
- Search is more efficient as it exploits the
structure
Cons:
- Not very robust and resilient as
unstructured
- Overhead of maintaining the structure with
joining and leaving peers
Pros:
- More resilient to failures
- Better handling of joining/leaving peers
- Allow better optimization of routing by
changing the overlay structure
Cons:
- Rare resources are harder to find if found
at all
- Searching can flood and overload the
whole network
Unstructured

Content
Addressable
Network (CAN)

CAN
Multidimensional Cartesian
coordinate space on a multi-torus
Each peer has a neighbour list
Routing performance is O( × N1/
)

CAN
Joining: by splitting an existing peer’s
zone into half
Neighbour list: transferred from the
old peer - updated for all neighbouring
peers
Leaving: a neighboring peer takes over
its space and the neighbour lists are
updated

CAN improvement
Multiple coordinate spaces (realities)
with different place for each peer,
same place for data
Increasing dimensions: gives better
routing. But both are needed
Overloading zones: more data
availability - fault tolerance - shorter
routing
Topological awareness of IP network
Using multiple hash functions:
increases data availability

Chord
Peers are organized around a circle
according to their ID which is an m-bit
ID assigned by a uniform hashing
function
Each data item is assigned an ID on
the same circle and assigned to its
successor peer
Routing takes O(log N) if peer
information is up to date

Chord
Each peer carries a finger table for
info of peers which are successors of
IDs that increase by a power [ hence
the O(log N) routing ]
Resilience is increased by maintaining
another list of length r of the peer’s
direct successors
Joining and leaving: needs successor
keys to be updated which is done by a
stabilization protocol that runs
periodically in the background

Chord
It needs O(log N) for routing, much
better than CAN
Needs O(log2
N) which is worse than
CAN which requires O(2 x d)
Could make some use of CAN
improvements ideas as multiple
realities
Cannot take into account IP topology

Tapestry
The nth peer that the message
reaches shares a suffix of at least
length n with destination ID
Routing takes O(logb
N) where b is the
base of IDs
Uses multiple roots for each data
object to avoid single points of failure
Robustness is increased by making
the neighbour map maintain two
backup peers in addition to the
primary ones

Pastry
Same as Tapestry
Doesn’t have optimization for locality
of peers
Less efficient replication algorithm

Viceroy
- General Ring: every node is
connected to its successor and
predecessor
- Level Ring: every node is
connected to others on ring
- Butterfly: every level L:
- Down right edge that is
added to a long range
- Down left edge to close
range
- Up edge to close range
Routing performance is O(log N)

Search in Unstructured Networks

Freenet
It uses Steepest Ascent Hill Climbing
with backtracking algorithm
It caches the found file in the path
peers => improvement of routing
Anonymity is one of the main
properties of the network
Least Recently Used (LRU) is the basic
cache replacement algorithm
An enhanced algorithm for cache
replacement could be used for cache
replacement

Freenet
Enhanced-clustering with Random
Shortcut
It uses the concept of small world by
choosing the farthest node in the
cache
If the new added node is closer it
replaces in the cache
If it’s farther with a certain probability
it replaces
The choice of optimum is still an
open question

Gnutella
Routing through the network is mainly
done by flooding (BFS) with certain
TTL and limit of hops
This causes high overload of the
network when too many nodes join
To join a client connects to one of the
peers and broadcasts its content by
flooding as well
A concept of ultra peers with higher
bandwidth is introduced to carry the
network routing and search operations
for its leaves

BitTorrent
A centralized P2P system
It cuts files into pieces of fixed size
(256 Kbytes each) and hashes them
with SHA1 to confirm integrity of data
A client needs to connect to Tracker
that gives the client a set of random
peers having the file needed
A downloaded piece could be seeded
DHT introduced trackerless BitTorrent

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