## 3 Predict and Forward Scheme Based on Node Profile  
At present, traditional routing algorithms of opportunistic networks select suitable relay nodes from the surrounding neighbors through their network addresses instead of a specific profile that is defined by node attributes. When the destination address is not available by message carriers, the Profile-based network model provides an effective method to identify the destination node by evaluating the attribute characteristics of nodes. Consequently, we propose Prelearn, a new profile-based message model for opportunistic networks, and Predict and Forward, an efficient routing-delivery algorithm for Prelearn messages. The proposed scheme consists of two phases: studying the state of the network to find the destination profile by collecting and comparing the attribute characteristics of nodes, and making delivery decisions for Prelearn messages through the collected statistics.  
As shown in Figure 1, different users in a social scenario contain different node attributes and belong to different node profiles, which are identified by different attribute characteristics of nodes. If a source user needs to send an advertisement to the lightest user in the network, the Prelearn message collects the attribute characteristics of users, and finally, it will be sent to the users that belong to the destination profile with the minimum value of the weight attribute. In the transmission process, the message delivery function is applied to find the destination profile, the Prelearn message model is used to carry and transport information, and the node in the network can handle computing tasks. Certainly, the detailed process for data transmission will be described in the following sections. Moreover, in order to improve the understanding of the Predict and Forward scheme, we give the following important definitions strictly.  
**Definition 1 (node attributes).**_The node attributes represent the social attributes of the users in an OppNet, such as their physical characteristics, places of residence, workplaces or hobbies._  
**Definition 2 (the attribute characteristics of nodes).**_The attribute characteristics of nodes represent the attribute values of nodes in the networks, for example, the interest attribute includes the attribute characteristics of nodes (or the attribute values of nodes): movie, sport, music or reading._  
**Definition 3 (message delivery function).**_Defined by the source node, this is a special function that is used to evaluate and compare the attribute characteristics of nodes, and to find the destination profile that meets the requirements of the source node._  
**Definition 4 (destination profile).**_The destination profile is multiple nodes or a group of nodes. The attribute characteristics of nodes belonging to the destination profile, satisfy the message delivery function. Actually, the destination profile is a set of destinations, such as the blue eye profile; this means a set of users with blue eyes._  
**Definition 5 (the optimal candidate).**_The optimal candidate is a node that plays an important role in the data transmission process, such as the destination node or a node that frequently encounters the destination._