# A Combinational Buffer Management Scheme in Mobile Opportunistic Network
## 2 Related Works
### Single Standard  
The delay of the message is unavoidable and it may take a certain amount of time since the mobile opportunistic network is intermittent; therefore, the node must carry the message until it contacts another node that can act as the relay node. For example, [PERSON] et al. [13] use the delay of the messages as a criterion for evaluating utility values for messages, and propose an optimal buffer management strategy based on a distributed algorithm to statistically learn the approximate global network knowledge. Its purpose is to minimize the average delivery delay or to maximize the average delivery ratio. Another important factor in the assessment criteria of the message is the TTL of the message. The delivery probability increases as the message survival time grows longer. The common method is to remove a packet when its TTL value is zero. For instance, [PERSON] et al. [14] define the TTL value as a timeout value and specify to delete messages whose lifecycle exceeds TTL to reduce congestion in the node buffer.  
Analogously, the number of forwarding hops of the message directly affects the network overhead in the opportunistic network. The goal of the buffer management is that the messages arrive at the destination node with the fewer number of hops, which not only reduces the message delay in the network, but also reduces the network overhead. [PERSON] and [PERSON] [9] propose a priority scheduling policy in which the priority of the message in the node is based on the distance from the source to the destination. [PERSON] et al. [15] propose a new message scheduling strategy for epidemic and two-hop forwarding routing in Delay Tolerant Networks (DTNs), so that each message has the best delivery ratio when it forwards or drops. [PERSON] et al. [16] propose the Prioritized Epidemic (PREP) routing. This strategy uses a density gradient method to delete messages based on the source-to-destination time cost, that is, the density gradient decreases with the increasing distance from the destination.  
Note that the probability of the message arriving at the destination node increases as the number of message copies increases. However, excessive copies of messages in the network also lead to buffer congestion. Hence, how to delete the message copy and ensure the delivery ratio of the message is a central issue. For example, [PERSON] et al. [11] drop the message with the largest number of copies to minimize the influence of message drop, which results in high delivery ratio in a DTN. Similarly, the author treats DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities that determine how packets should be replicated and deleted in the system in [17].  
The size of the message also determines the importance of the message in the node buffer. For example, possibility of congestion will be alleviated if the larger and useless messages are deleted [18]. [PERSON] et al. propose dropping the message in the node if its size is larger than or equal to the incoming message in [19]. In the same year, [PERSON] et al. [10] also introduce a variety of existing buffer management policies and performance evaluation indicators, and propose a Drop Largest (DLA) sorting strategy that drops large-size messages when the node buffer becomes congested. [PERSON] et al. propose a knapsack-based message scheduling and drop strategy for DTN in [20], and calculate utility values of those messages by quantifying the impact of replicating or dropping them. Based on the result of [20], the work in [21] further decides which message should be deleted based on the backpack problem when the buffer overflows, so as to maximize the utility of network resources. Similarly, [PERSON] et al. [22] propose an optimal adaptive buffer management scheme in the situations where the bandwidth is limited and messages vary in size.