## 4 Multicast Routing Approaches in DTN
### Buffer-Efficient Multicast Routing in DTN
#### 4.2.3 Discussion  
Table 2 shows the performance comparison of buffer-efficient multicast routing in DTN. All the schemes with buffer management mechanisms achieve a considerably high delivery ratio. The flooding mechanism always incurs a high delivery cost. Some proposed solutions addressed the buffer issues by adopting a buffer management mechanism in their multicast routing approaches. They used local information, such as the type of message, message TTL, hop counts, and received time-to drop messages in storage. For example, the buffer management scheme in RMDA [47] prioritizes multicast messages and dropped unicast messages more frequently to free the buffer. Otherwise, network information, such as the number of message copies and the number of received ACKs, can be used as metrics to control the number of messages remaining in the network. There is also the combination of local and network-based information as metrics to remove messages in case of a full buffer to accommodate any new incoming messages. For example, SCBMR [44] combines the  
Figure 10: Thin trees with fewer participating nodes [53].  
number of replicas with message TTL information to preserve node storage for messages that are not well-distributed in the network. A delay-tolerant IoT network often consists of resource-constrained IoT devices with limited buffer space. The nodes' buffers will be easily overwhelmed with multicast messages, but they still need to guarantee that intended receivers receive a DTN multicast message. Any routing scheme cannot just drop the oldest message without ensuring the message's coverage in the network.  
Another category of buffer-efficient multicast in DTN exploits dynamic multicast tree generation to reduce the number of message replication or the number of message relays. The single-copy multicasting approach ensures that only a single copy of each multicast message is maintained in the network when the message is forwarded along each tree branch. For example, the Non-Replication Multicasting Scheme [54] and SMTBR [53] exploit contact information to build a multicast tree to achieve a minimum number of message replicas in the network. However, tree-based multicast routing approaches are not flexible and require global knowledge to construct the multicast tree.  
[PERSON] et al. [55] presented a survey on buffer management strategies in DTN and classified them based on protocol dependency and type of drop policy. The authors summarized that major strategies use a dynamic dropping policy due to the mobility characteristics of DTN, which are non-protocol specific, that enable customization and optimization across protocols. However, this survey did not focus on buffer management strategies in DTN multicasting. Provisioning multicast routing approaches with an efficient buffer management scheme improve the overall network performance. Limited storage capacity is always the major constraint of the devices in WSN. [PERSON] et al. [56] proposed a buffer management algorithm for a delay-tolerant WSN in the IoT environment, namely SmartGap. SmartGap drops the low priority messages with minimal impact on the overall performance. Buffer-efficient multicast routing strategies are applicable to delay tolerant WSN to accommodate data dissemination IoT application, which involves a large number of wireless sensor nodes