# Building Scalable and Secure Multicast Delivery Infrastructure in a Local Area Network
## 2 The Motivation
### Multicast Delivery Mechanism and Its Weaknesses  
IP multicasting is a method for group communication. The number of receiving hosts may vary between one to several thousands. Thus, this technique scales well in a larger receiver community where the number of hosts is dynamically changing. Irrespective of the number of receiving hosts,the transmitting host is expected to send a packet only once. Network hardware like switches and routers replicate the packets to be sent to multiple receivers [2]. IP multicasting consists of three components: an IP multicast group address, a multicast distribution tree, and the tree creation algorithm. IP multicast address is a logical address used by source and receiving hosts for group communication. In an IPv4 network, any address from the Class D address range is used for this purpose. In an IPv6 network, the prefix FF00::/8 is used for multicast communication. After a multicast address is identified to be used for group communication, a multicast distribution tree must be constructed for this group communication. In this paper, we use Core-based Tree (CBT) to construct the distribution tree that is presented in Section 4. In a traditional multicast operation, an active source host may not know about the receivers of the group. IP multicasting is a useful mechanism in distributing management data in a LAN network. Management data include frequent updating of host OS, security patches, switches and routers' OS updates and patches, new configuration updates, etc. Thus, in a corporate environment, host and network devices can be updated from one or more central locations without visiting each of the places. Even though multicasting is extensively used in a LAN, it poses several challenges. We list them here:  
* Any rogue host may pose as a legitimate server and deploy malicious scripts to end-hosts and network hardware. This script may be used by an attacker (either an insider or an outsider) to launch different attacks at a later time. Thus, the network must have a mechanism to detect the authenticity of a source. * In a traditional multicast scheme, there is no admission control. Any host with malicious intent may join the multicast tree and listen for OS and security patch updates. Then, they can launch an insider attack based on the history of the patches that were applied. * In a corporate network, PCs and network hardware are replaced constantly. Thus, membership revocation is an important issue. An attacker may use the old profile of a decommissioned host or a network hardware to join the network, and launch an attack. Thus, any robust multicast implementation in a corporate network must address the above-mentioned issues. Traditionally, several crypto-techniques are used to address these issues. Shared-key cryptography like Data Encryption Standard (DES) or Advanced Encryption Standard (AES) has been in use for a long time. A single key is used by every host and network devices. Message Authentication Code (MAC) or its crypto-variant HMAC (keyed-hash message authentication code) along with the shared-key is used for data integrity and authentication. Even though the use of a shared-key scheme solves the admission control problem, it does not identify a source node. A legitimate host with a malicious intent may join the network using the shared-key and pretend to be a server to push malicious scripts to every end-hosts and network devices. Key-revocation and key-installation are other major issues. The current practice is to install the keys manually. It may be impossible to automate this process, unless the automation process uses a secure side-channel. Again, the establishment of the secure side-channel has similar challenges. In this case, the use of side-channel consumes an additional bandwidth and opens the door for further attacks. Manual key installation is possible in a smaller network; however, the process cannot be scaled to a larger network. Whenever a host or a device left the network, a new key needs to be generated and installed on to every device.