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in routing table. When search resources CAN uses greedy
algorithm and send the messages to the neighbor node
which is nearest to target according the routing table.
Each node has O(d) neighbor nodes, and the hops of
searching is O(dN 1/d )[14]. The topology of CAN is shown
in figure 3.
The biggest problem of DHT structure is that the
mechanism of maintenance is too complex. In particular
while there are nodes always join or quit the system, it
becomes difficult to maintain. The other shortage of
DHT is that it supports exact keywords search only. It
can’t support the content of complex queries.
D Hybrid Structure P2P
Hybrid structure P2P integrates the advantages of the
centralized topology and distributed topology. In this
system, there are some supernodes which have high
performance such as the ability of compute, bandwidth,
memory and so on. These supernodes have stored the
information of other nodes. Discovery algorithm is
carried out between supernodes only and then the
certain supernode forward the query to the specific
bottom nodes. Hybrid structure P2P is a
semi-distributed and hierarchical structure. Super nodes
make up a high-speed forwarding layer and this super
nodes and normal nodes constitute several levels. The
most typical case is KaZaa. This topology is shown in
figure 4.
Figure 4: hybrid structured P2P
The advantages of hybrid structured are that the
performance and scalability of it can reach a preferable
level. Of course it also relies on the supernodes and it is
easy to be attacked. But as a compromising method
although all aspects of its performance has failed to
reach the optimal level of the current structures, it has
more practical value and attracted many researchers.
The company CA’s statistics indicate that there are
more than 250 million times download on KaZaa. Using
KaZaa to transport files consumes 40 percent of the
Internet bandwidth. It was so successful because it
combines the advantages of Napster and Gnutella. It
uses the distributed structure of Gnutella and so it has
high scalability. It selects nodes which have high
peformances and let these nodes become super nodes.
These super nodes store the information about the
bottom nodes nearest to them and they connect together
to make up an overlay network. As the supernodes have
indexing about certain bottom nodes, the performance
of searching has been improved greatly.
Now, many studies are based on hybrid structure. The
focus is using relevant estimate system and specific trust
computing to select right nodes to be supernodes and
how to dispose the problem while supernodes are
attacked or disabled. Literatures[15] is a typical
document about this problem.
Ⅲ AN IMPROVEMENT BASED ON GUETELLA
It is found that searching always focus on a few
popular resources. So there is a model of probabilistic
transmittance based on Guntella been put forward.
In this model, every node in this system builds a
routing table about popular resources on the basis of the
searching which relevant to itself. Nodes send messages
according to the table.
While they echo ping or query the messages of Pong
or Queryhit should take some additional information
such as processing power, bandwidth, current load,
online time, delay of connection and so on. Source node
renews its routing table after received a response from
target node. The additional information make up a
vector Y, and all components can be divided in two
kinds, positive component (bandwidth), negative
component (delay of connection) based on practical
meaning. In order to be easy to dispose we let all
negative components to be inverse to themself and there
a vector X in which all components is positive
components. Routing table has build n item of routing
of most popular resources based on the ability of the
node such as memory. The popular resources are
towards to a certain node. Every keywords has m targets
and it can be controlled to send c request (c ≤ m, while
c=m sends request to all targets directly). With respect
to a certain target i we pull-in a parameter of
transmission k i . it can be compute by iteration shown in
equation 1.
k i n =f(X,ki n −1
) 1
The function f() needs to be able to correctly reflect the
impact to the probability of every component as
accurately as possible. There are two basic conditions
listed as follows.
∂f
Condition 1: ≥ 0
∂xi
2
Condition 2:
∂ f
≤0
2
∂x
i
( x i
is any component of X)
The probability of the target i is :
i
j
⎧⎪
( k × c)/ ∑ k ( Pi
≤1)
⎫⎪
2
Pi
= ⎨ ⎬
⎩⎪
1( else)
⎭⎪
(j∈ N, N is the set of target nodes toward the certain
keywords)
In theory, nodes in this model transmitting messages
are based on the routing tables not using flooding and so
it has these advantages such as reducing the demand of
bandwidth, improving the efficiency of searching and
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