TAN et al.: TRUST MANAGEMENT SYSTEM FOR SECURING DATA PLANE OF AD-HOC NETWORKS 7581
its packets are forwarded correctly. Similarly, the trustwor-
thiness measurement approaches in [20], [25], and [26] also
leverage promiscuous mode to collect information. It is worth
noting that the success of such approach d epends on the ability
to access the header and content of a packet. The adoption
of trust management does not obviate the need for crypto-
graphic security mechanisms, such as packet encryption and
encapsulation. That is, the overheard packet cannot be parsed.
Consequently, trust factors can no longer be obtained. To over-
come the shortcoming, an ACK-based collection approach is
presented in this paper.
In trust management, recommendation and reputation are of-
ten considered to facilitate trust evaluation. In [27], nodes have
to provide forwarding services for others to acquire the rep-
utation value, which is used to pay for services. Consequently,
selfish nodes will be isolated as they do not have enough reputa-
tion value to pay for the required forwarding services. Based on
a hidden active probe method [5], the trustworthiness of a node
is evaluated according to multiple reputation values. Similarly,
recommendation is also integrated in the trust mechanism in
[25]. Despite the advantages, the frequent use of recommen-
dation and reputation also leads to vulnerabilities, which are
exploited by two widely recognized attacks: slandering and
harboring. To defend against them, a filtering algorithm b ased
on the idea of our earlier work [28] is proposed.
A trust model based on fuzzy theory is presented in [16], but
it is not based on fuzzy reasoning systems. Schmidt et al. [17]
have proposed a customizable trust evaluation model based
on fuzzy logic for multiagent systems, particularly for the
automated e-commerce markets. Another linguistic fuzzy trust
model based on fuzzy reasoning is also proposed in [29],
although these two models cannot be directly used in ad-hoc
networks because the security requirements of ad-hoc networks
and the application environments of [17] and [29] vary a lot.
Moreover, some issues such as trust decay in ad-hoc networks
should probably be addressed using different approaches.
III. T
RUST FAC TOR COLLECTION
Trust evaluation always starts from collecting trust factors.
It is crucial to decide what kind of information should be
collected. The decision should concern two aspects: 1) The col-
lected information should expose the misbehaviors and anom-
alies, and 2) the collection method should be valid and feasible.
Regarding the first aspect, objectives of the proposed trust
management should be reminisced. Since it aims at enhancing
the data-plane security of ad-hoc networks, we should concen-
trate more on data-plane information. Furthermore, the infor-
mation should reflect the changes of networks when attacks
occur. The data plane is responsible for successfully delivering
packets to destinations at a given timescale. Then, malicious
agents can launch three types of attacks regarding data packets:
tampering, suppressing (delaying), and dropping. To defend
against tampering, data packets should be authenticated by end
nodes to ensure integrity. This requires proper cryptographic
security primitives, which demonstrates that the a doption of
trust mechanisms does not obviate the need for traditional secu-
rity mechanisms. A tampered data packet is usually discarded
Fig. 1. Data-packet ACK.
by receivers due to authentication failure, which implies that
tampering attacks can be somehow viewed as dropping attacks
from the perspective of sender nodes. Thus, the data packet
delivery ratio (PDR) is able to indicate tampering attacks to
some degree. The suppressing attack becomes increasingly dan-
gerous for real-time services. For instance, the suppression of
alert messages may cause car accidents in VANETs. Dropping
attacks such as BlackHole is the most serious threat to the
data plane because it completely disrupts data delivery. More
seriously, the suppressing and dropping attacks cannot be effec-
tively detected and resisted by traditional security mechanisms.
According to the above analysis, we get that all three types of
attacks will affect the average delay ( AD) and delivery ratio
of transm itted data packets. Thus, the AD and data PDR are
selected as the trust factors in this paper.
The second aspect implies that collection methods should be
valid and feasible. As described in Section II, if data packets
are protected by cryptography mechanisms, such as encryption,
the monitoring node is no longer able to obtain the required
information. To avoid this incompatibility, we present a new
approach to collect the AD and PDR of the data packets
transmitted by a path. Within a path, the source node generates
and sends data packets, and is responsible for computing the
AD and PDR. Note that, in this paper, AD and PDR r efer to
the average transmission time and delivery ratio of data packets
forwarded along a path, rather than by a node.
The source node numbers and records its recently sent
packets by a record {seq, time}, where seq is a monotonically
increasing sequence number of d ata packets, and time is when
the packet is sent. After receiving N packets with a larger
sequence number than the previously acknowledged one, the
destination node will reply an acknowledgement (ACK) mes-
sage. The process is shown in Fig. 1.
In Fig. 1, seq
prev
refers to the sequence number of the previ-
ously acknowledged packet, and seq
curr
refers to the one that is
acknowledged currently. ts
i
and td
i
denote when the success-
fully delivered ith data packet is sent and received b y the source
node and the destination node, respectively, for 1≤i≤N .
t
ack
is the time when the current ACK message is received.
An ACK message is comprised of {seq
curr
,N,(seq
h
1
,td
h
1
),
...,(seq
h
j
,td
h
j
),...,(seq
h
m
,td
h
m
)},whereseq
h
1
is the se-
quence number of the h
1
th received data packet after the
previous ACK, for 1 ≤ j ≤ m,1≤ m ≤ N ,and1≤ h
j
≤ N.