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Figure 1. : Topology of distributed unstructured P2P
In Gnutella, nodes use flooding to forward messages.
If the average of the degrees of the nodes is n and the
c
hops of the messages of the Ping or Query is h, then
when active node sends a message of Ping or Query there
h
would be ( nc
−1)
−1
n ⋅ messages of Ping or Query at
c
nc
−2
the worst[6]. The number is an exponential growth
function on the h. It would consume a lot of bandwidth
while the h is big because there would be a lot of
messages. But when the h is little, the search scopes
would be small and lead to can’t find the needed
resources.
To reduce the number of messages in the
distributed unstructured network, there are three
methods about this problem listed as follows.
(1) Based on improved the mechanism of
forwarding method. This method is improving the
algorithm of flooding by doing not forward message to
all neighbors. Literature[7] put forward Directed BFS. In
this way all nodes preserve statistical data such as the
delay to neighbors. Peer would select some optimal nodes
to sends messages. The number of messages can be
reduced largely by this way. Intelligent-BFS put forward
by Literature[4] is similar to Directed BFS. It sends
messages to the neighbors which have most return results.
k-Walker[5] is that every active nodes sends k different
Walker and every Walker can select next node by random.
If the hops are h there would be k×h messages at worst
and reduce the number of messages largely. Literature[8]
gave a method called hybrid periodical flooding, HPF.
(2) Based on cache. This way contains index cache
and content cache. Literature[1] found that the network
communications on the edge of ISP are reduplicated
mostly and the ratio of hitting the target in the cache
reaches 67 percent while web cache is only 30 to 60
percent. Some systems put the index to normal nodes. All
nodes preserve the indexes about files on the nodes when
there are certain hops between them. When the Peer
receives the query, it can provide response for the
nearby Peer. With locality of query, literature [2,9] put
forward every peer cache the results of search through
itself. Literature[10] studied the KaZaa network by
simulation and showed that content cache could reduce
the demand of bandwidth.
(3) Based on topology optimization. Literature [11]
discussed the mismatch between overlay network and
physical network, has designed a method called
Location-Aware Topology Matching, LTM and assessed
the efficiency of LTM in dynamic and statistically
network. The result of the experiment showed that LTM
can reduce 75 percent of traffic and 65 percent the query
response time. Literature[12] proposed a distributed
solution of non-structural overlay network does not
match the problem ACE (Adaptive Connection
Establishment). The simulations show this method can
reduce the cost of 65 percent for each query and 35
percent response time.
The methods above are cutting high-cost connections,
establishing low-cost connections, but there are short of
proof of the correctness of matching operations, and no
analysis of their matching method would bring forth the
negative impact of net. In addition, all these methods
eliminate mismatching after the mismatching appearance.
This will produce a lot of messages for eliminate
mismatching.
C Distributed Structured P2P
There are a lot of researches about how to construct a
highly structured system because of the poor scalability
of unstructured systems. The point of current researches
is how to find information effectively and most recent
results are based on DHT (Distributed Hash Table) which
is a distributed discovery and routing algorithm. These
algorithms don’t use flooding like Gnutella but through a
distributed hash function. While enter the keywords it
maps to a certain node, and establish a connection with
the node through different rules of routing. The typical
representatives are Chord, CAN [13,14].
Chord use the compatible with the hash function to
assign m bits NodelID to every node and m bits DataID
to every resource. All nodes make up a logistic hoop by
NodleID mod 2 m . The data of k is stored in the first one
whose NodelID>=DataID and the node is called the
successor nodes of the data k and labeled successor(k).
When there are N nodes in system each node preserves
the other O(log (N)) nodes information and store it to
routing table called Finger Table. In this table, the item
i contains the information of node j and meet
j=successor(n+2 i-1 ) where 1