## 1 Introduction  
A Delay Tolerant Network (DTN) presents a new communication model, which due to the nature of the network uses a store-carry-forward approach to deliver messages. One of its main characteristics is that the links between mobile nodes are volatile and may break down for a long period of time at any time. With the development of the mobile Internet, research on DTNs has become a hot research topic. There are many ubiquitous scenarios similar to the communication ways of DTNs, including social networks. In this paper, we mainly focus on the DTNs with social characteristics, called social DTNs. In social DTNs, people with some social relationship tend to be co-located quite regularly and people with same characteristics, including interests, nationality, age, _etc._, usually appear in the same place. As a result, communities will be established by the people who collaborate with each other frequently. In each community, people can be considered as \"friends\" and messages among them can be efficiently delivered. What's more, there are several active persons in the social DTNs who move frequently in different communities and come into contact with many other peoples. These active persons can take on the responsibility of transmitting messages between communities.  
Considering the characteristics of social DTNs, there are several issues that need to be solved. They include: (1) the reduction of messages dropped due to the unreliability of the wireless communication links; (2) the need for a mobility model which can accurately reflect real scenarios; (3) the need for a mechanism which can make full use of the social features of the mobile nodes and the relationships among them. The existing research solutions have not solved these problems efficiently, as will be introduced in the related works. In this paper, we propose a new routing scheme to solve these issues, named CCS-DTN. With CCS-DTN, firstly a mobility model is proposed to describe the social DTN scenarios. Secondly, according to nodes' contact probability, mobile nodes are divided into different clusters. Then different routing mechanisms are designed for intra-cluster and inter-cluster routing. The Spray and Wait scheme is used for the intra-cluster routing while a new forwarding mechanism is proposed for the inter-cluster routing problem. In the proposed mechanism, messages are forwarded to active nodes and then these active nodes adopt the \"carry-forward\" method to transmit messages. Finally, to solve the problem of messages dropped due to unreliability of the wireless communication channels, the messages will be coded by the random linear network coding scheme for multiple transmissions.  
Our major contributions presented in this paper include: (1) a mobility model which can accurately describe real scenarios; (2) a dynamic clustering mechanism which highlights the community characteristics of the social DTNs; (3) a new routing mechanism which takes full advantages of the characteristics of the active nodes; (4) the network coding mechanism employed to reduce the negative effects caused by the unreliability of the wireless communication channels.  
The remainder of this paper is organized as follows: in Section 2, the related works is introduced. The mobility model and the proposed routing mechanism are presented in Section 3. In Section 4, the performance of the solution is evaluated by a mathematical model and simulation experiments. Finally, the conclusion of the paper is summarized in Section 5.