![](bg3.png)
read requests compose read quorums and the servers
on writing operations compose write quorums.
3.1 PAN Operations
PAN provides to clients reading and writing opera-
tions. Servers, acting as agents, are responsible for
receiving and replying requests from clients. In writ-
ing operations, agents deliver updates received from
clients with the collaboration of nodes in the StS. In
reading operations, agents consult a read quorum be-
fore respond to the client. These operations are illus-
trated in Figure 2 and occur as described:
WRITE - as part of the mechanism of writing, all
servers have a buffer that stores the latest update of
each data. Upon receiving a write request, the agent
updates its data and adds it to its buffer. Periodically,
nodes propagate data from the buffer to other nodes.
After some time, all servers have the updated value.
READ - the agent chooses nodes to compose the read
quorum based on predefined read quorum size. Then,
it forwards the request along with its local copy of
the searched data. Thus, members of the read quo-
rum reply to the agent if their data is more updated
than that from the agent. In this case, the agent waits
for replies, and in the lack of them, it responds to the
client with its own data as conclusion that the data it
holds is the most updated. Further, if servers in the
quorum receive a data more updated than theirs, they
revise their local copy and add it to the buffer in order
to be disseminated in the next round of propagation.
Figure 2: Read and write operations in PAN.
To explain PAN’s operation, we assume a net-
work of 20 nodes, in which ten are part of the StS
as illustrated in Figure 2. Node n
4
sends a write re-
quest to node n
8
. Node n
8
, the agent, propagates this
write to other nodes following the gossiping protocol.
Write operations are propagated to a certain number
of servers, being this number called fanout (F). It is
previously configured, and servers follow the same
value.
In Figure 2, F equals two servers and the size of
the read quorum four. Client n
17
issues a read oper-
ation. Then, agent n
18
consults the read quorum in
order to collect the updated value. Nodes n
11
and n
12
participate in both operations, being the intersection
between read and write quorums. Intersection nodes
have the most updated value to client n
17
.
4 ATTACKS ON PAN
The success of writing operations is related to the col-
laboration of all servers involved in the dissemination.
Thus, members of the StS may hinder the progress of
updates in several ways, such as refusing to propa-
gate the updates or making updates to progress slowly.
This can occur if nodes are selfish and want to save
their resources, or if nodes are malicious and want to
degrade the system. Further, nodes can manipulate
values that are being updated, causing inconsistencies
in the system.
Servers consulted by agents in reading operations
may deliberately not respond to requests, forcing the
agent to send its own data to client. They can also
modify the value of data and send this wrong value.
In the second case, generated by active attackers, the
problem is critical because in addition to providing
clients erroneousinformation, servers can trust in data
sent by agents and also update their data.
This work examines particularly three attacks:
lack of cooperation and timeout and data manipula-
tion. Considering the characteristics of PAN, these
types of attacks are more harmful than others for op-
erations in quorum systems. The operation of such
attacks are described below.
4.1 Lack of Cooperation
Lack of cooperation attacks occurs on both opera-
tions. Figure 3(a) illustrates the behavior of a com-
promised agent in a read operation. A client issues a
read request of a data v to server s
0
, which is a selfish
node. As a misbehaving agent, s
0
waits until the op-
eration timeout expires and replies the client with its
own data, that might be outdated.
When the selfish node is an intermediate node, it
simply stops responding to the agent. In Figure 3(b),
the client requests a data v to the server s
0
, that con-
sults the read quorum composed by s
1
, s
2
and s
3
.
Server s
2
is a selfish node and does not respond to
the agent. Under this situation, a reading operation
IMPLICATIONS OF MISBEHAVING ATTACKS ON PROBABILISTIC QUORUM SYSTEM FOR MANETs
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