Mobile Adhoc Network is fully autonomous in which mobile node plays dual role as node itself and as a router to forward packets. Fully distributed operation in which there is no any centralized authority. Network structure are fully dynamic in which every nodes is free to move anywhere with different speed.As distributive and dynamic approach of MANET, security often needs to be considered as an important one. There are also some more critical issues like packet loss due to mobility and also cause link breakage. In this paper we focus on malicious behavior of AODV under attacks which are mainly WormHole, BlackHole and GrayHole in network layer. In our proposed work, we only analyzed behavior of BlackHole and GrayHole attacks under AODV protocol and show the effects of both on network layer. We have also tested malicious behavior of AODV under above attacks using various performance parameters like throughput, packet delivery ratio, normalized network load and end to end delay using different simulation parameters.

MANET is a self organized, self configurable network having no infrastructure, and in which the mobile nodes move arbitrarily. The mobile nodes can receive and relay packets as a router. Routing is a critical issue and an efficient routing protocol makes the MANET reliable. The provision of quality of service (QoS) guarantees is much more challenging mainly due to node mobility and resource constraints. Security is an essential requirement in mobile ad hoc network (MANETs). Compared to wired networks, MANETs are more vulnerable to security attacks due to the lack of a trusted centralized authority and limited resources. In practice, most TCP deployments have been carefully designed in the context of wired networks. Ignoring the properties of wireless Ad Hoc Networks can lead to TCP implementations with poor performance. Wireless Mobile Ad-hoc networks offer challenges to TCP's congestion control mechanism related to its inability of distinguishing between losses induced by congestion and others types of losses. This article extensively and exclusively studies the issues involved in Adhoc network which can be exploded as further research purpose.

Ad hoc networks are characterized by multihop wireless connectivity, frequently changing network topology and the need for efficient dynamic routing protocols. I compare the performance of two prominent on demand routing protocols for mobile ad hoc networks—Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV). We demonstrate that even though DSR and AODV share similar on-demand behaviour, the differences in the protocol mechanics can lead to significant performance differentials. The AODV out-perform DSR in the normal situation but in the constrained situation DSR out-performs AODV, the degradation is as severe as (30%) in AODV whereas DSR degrades marginally (10%) as observed through simulation [7].

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