# A Multicast Routing Scheme for the Internet: Simulation and Experimentation in Large-Scale Networks
## 2 Related Work and Our Contribution  
### Brief Overview of Multicast Routing in the Internet  
The first multicast routing schemes standardised were dependent of the unicast protocols, either distance vector-based such as the Distance-Vector Multicast Routing Protocol (DVMRP) [13] or link state-based such as Multicast Open Shortest Path First (MOSPF) [14]. These schemes were dependent on the underlying unicast routing and presented high scalability limits and complexity. Thus, they have been replaced by the Core-Based Trees (CBT) [15] and the Protocol Independent Multicast (PIM) [16] which perform independently from the unicast scheme. It worth mentioning that two variants of multicast schemes exist: Dense Mode (DM) and Sparse Mode (SM). Dense mode assumes that a large amount of receivers wants to join the multicast group, and so floods the network with a sort of multicast session setup packets, and prunes back those network subnets that fail to respond or respond with no interest in joining the multicast group. Examples of them are DVMRP and PIM-DM [17]. Sparse mode assumes that very few of the receivers connected to the internet want to join the multicast group, and so waits until a receiver specifically joins the group. In turn, sparse mode defines two different service models, namely Any Source Multicast (ASM) and Single Source Multicast (SSM). ASM explicitly builds unidirectional shared trees rooted at a given router called the rendezvous point (RP) per group which, in turn, establishes the shortest path routes to the specific multicast sources. Receivers subscribe then to the multicast group \(G\) but not to any specific source; thus, the multicast channel is defined as \((*,G)\). PIM-SM [16] is an example of an ASM scheme. On the contrary, SSM builds trees that are rooted in the specific multicast source, and receivers subscribe to a specific source \(S\) and multicast group \(G\), thus defining the multicast channel as \((S,G)\). PIM-SSM [18] is an example of an SSM scheme. In general, SSM provides better end-to-end performance and network resource utilisation as well as implicit security but the routing state they require increases linearly with the number of sources [19]. In the context of research, prior work on multicast schemes mainly focuses on reducing their complexity and storage requirement. Authors in [20] propose the Yet Another Multicast (YAM) scheme which relies on a limited scope broadcast in the neighbourhood of the joining node to find the closest portion of the MDT to join. Such a search is based on an incremental discovery scheme which expands the limit of the neighbourhood up to find the MDT. During such a search, YAM identifies multiple paths and selects the one that can provide the required QoS. In [21], the authors improve the scalability limits of the YAM scheme and propose the QoS sensitive multicast internet protocol (QoSMIC), which, in case the local search fails, involves the use of the so-called Manager Node. Such nodes are the administrators of specific multicast groups and store the MDT information (i.e., very similar to the RP nodes in the ASM model). Such a scheme implies the use of specific nodes which introduce possible scalability, resilience, and memory concerns. A two-step scheme is proposed in [22]: the first step is carried out to obtain an RP and a candidate MDT; the second step is performed to generate a modified MDT rooted at the RP and the optimal MDT is obtained by comparing the original and the modified MDT. Like QoSMIC, this scheme also uses RP nodes, which again can cause scalability, resilience, and memory issues. With the introduction of Software Defined Networking (SDN) [23], several recent works propose this new architecture to deploy multicast in the internet. Two surveys of existing and proposed multicast routing algorithms in SDN are available in [24; 25]. Allthese works claim that SDN enables the use of more complex and efficient algorithms for multicasting as the centralised SDN controller has a global view of the whole topology.