## 2 Related Work
### Zone-Based Multicast Protocol  
Zone routing protocol (ZRP) divides the network into multiple zones, where each node exchanges routing information only within its zone, while routes to nodes outside the zone are established through inter-zone routing [6]. The zone size is dynamically adjustable, allowing efficient route setup based on network changes. ZRP employs a method that forwards messages from the source outward and further across covered routing areas, reducing route traffic. The key advantage of this protocol is its ability to minimize control traffic caused by periodic flooding of routing information packets, thereby conserving bandwidth. However, redundancy in routing areas is a notable drawback of ZRP.  
Multicast zone routing protocol (MZRP) operates by having the source node construct a multicast tree within its routing zone and gradually extend this tree to nodes outside the zone [24]. Multicast route request (MRREQ) is categorized into two types: unicast and broadcast requests, based on the source node's information. If the source node has valid routes to nodes in the tree, it sends a unicast MRREQ to the multicast tree and waits for a multicast route reply (MRREP). The unicast MRREQ is forwarded by intermediate nodes, establishing reverse paths in the multicast routing table. If the source node lacks routes for the group, it broadcasts the MRREQ through the source node's broadcast tree to neighboring nodes. Upon receiving an MRREQ for the multicast group, the target node sends an MRREP back to the source and waits for a multicast route activation (MRACT) message from the source to activate the new multicast tree branch. A limitation of this protocol is the significant waiting time for nodes outside the source routing zone to join the multicast group.  
Modified secure zone routing protocol (MSZRP) follows the fundamental operation of ZRP while incorporating several security features to address potential vulnerabilities [25]. MSZRP employs encryption and digital signatures to ensure authentication, integrity, and confidentiality. During inter-zone routing, routers within each zone exchange routing information. These messages are encrypted during transmission, and each router verifies the integrity of the received routing information. Transmitted messages are encrypted, and mechanisms for destination node authentication and packet integrity checks prevent packet tampering or forgery. Encryption keys and signature information are periodically updated without relying on a centralized management system or server.  
Zone-based hierarchical link state routing (ZHLS) adopts a hierarchical and zone-based approach to link-state routing, aimed at efficient routing [26]. ZHLS defines a two-tier network topology structure of node-level topology and zone-level topology. Accordingly, there are two types of LSP (link state packet): node-level and zone-level LSP. Each node handles routing based on link-state information within its zone, maintaining a routing table for its zone. It maintains a routing table for its zone, and the updated information is propagated to other nodes in the same zone whenever link states within a zone change. Inter-zone routing is managed at a higher hierarchy. Area Routers in each zone transmit link-state information in their zone to the upper layer, which computes inter-zone paths and updates the routing tables. In ZHLS, each node synchronizes link-state information between the upper and lower layers whenever the link-state changes. Through periodic updates, each node reflects the link-state changes of both layers, dynamically adjusting its routes. ZHLS aims to minimize costs and optimize bandwidth efficiency when setting routes. However, ZHLS suffers from high overhead due to link-state information dissemination and significant latency in link-state updates.