Doubly Optimal Secure and Protected Multicasting in Hierarchical Sensor Networks

Issues In Multicasting Over Wireless Networks

In case of providing multicasting to wireless nodes, the maintenance of the host group or multicast delivery tree becomes a major issue. In wireless network, we need to provide dynamic group membership but also monitor the movement of the host. This makes determining and maintaining the optimal multicast delivery tree very difficult. The following major issues are involved in multicasting in a mobile environment:

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  Group Membership Management: Wireless nodes may not receive IGMP query, due to point-to-point links, interference, noise, signal attenuation or mobility. Also the use of IGMP significantly increases the overhead due to joint operation and polling of mobile users by routers. An intruder / malicious node may get added into the system and feed false data or block the passage of true data. Typically such a false node is robust in computational capability. The solutions like group key agreements, quorums and per hop authentication are very computationally heavy for sensor nodes in the network.

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  Routing: The topology is dynamic in ad hoc wireless networks. The MH causes a change in routing structure. Also the movement of nodes may change the user density patterns causing a requirement of frequent change in the mode of packet forwarding from sparse to dense mode and vice versa. An intermediary node malfunction maydrop or garble packets in transit. It is necessary to detect and relocate such nodes from the network. Similarly, in case of an intermediarynode outagealternate route needs to be provided.

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  Multicasting for Infrastructure-Based Wireless Networks: Such networks involve base station and routers in fixed topology and also mobile users. Existing multicast routing protocols can be modified for multicasting over such wireless networks.

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  Multicasting for Ad Hoc Wireless Networks: In such networks both users and routers have high mobility that may frequently cause the path between any two users to change during a session. Therefore, the multicasting designed for wired networks cannot be used in ad hoc wireless networks. The multicasting using treerequires minimum BW resources for initialization. A source-based tree or a shared tree among sources and receivers in a group may be used. However, a link / node failure or movement of a node may disrupt the multicast tree by breaking it into two or more sub-trees making group communication intermittent. The various issues and possible solutions (see Figure 2).

Figure 2.

Multicasting issues in ad hoc wireless networks

Secure Multicasting Problem

For many applications, reliable and secure multicasting is an essential requirement. In wireless multicast, security issues arise due to wireless links prone to eavesdropping, inherent broadcast nature of any wireless links & lack of control on receivers and use of flooding for multicast tree / mesh construction. The risks involved are stealing / modification of information, access to unauthorized users by modifying multicast tree / mesh and complete loss of service.

The security issues may be taken care of in many ways such as encryption of packets using symmetric (private-key) or asymmetric (public key) schemes. The existence of an effective key management framework is also paramount for secure routing. But the distribution of key and re-key processes causes significant processing & network overhead due to node mobility and membership changes.

To ensure secure dissemination and sharing of information over a wireless network, we need to restrict the access of information multicast by the source node to the group of authorized nodes only. The scheme needs to take into account mobility of nodes, unreliable links and limited communication & computation power of nodes.