# SACT: A New Model of Covert Communication Based on SDN
## 1 Introduction  
Anonymous communication system is a privacy protection technology that hides the communication content or both parties of communication by data forwarding, content encryption, traffic confusion and other measures. The purpose of anonymous communication technology is hiding information, thus attackers, tensors and firewalls may be the enemies of anonymous communication. Most of the anonymous communication usually use a multi hop reverse proxy or resource-shared storage to mask the real address and ensure that the anonymous services are not traceable. Researchers have designed a variety of anonymous communication systems such as [PERSON] et al. [1], [PERSON] et al. [2] and Cranor [3]. Most of these systems use reroute technology for covert communication. These systems insert one or more intermediate nodes in the communication path between the sender and the receiver and rewrite the IP address and other related information to hide the source, destination and entry-exit relationship of the packet while forwarding the data packet. In this way, the IP address obtained by the attacker is not real so that the real communication relationship cannot be inferred. Because of the difficulty in discovering communication nodes and determining communication relationships, hidden channels are gradually adopted by malware to establish anti forensics mechanism or abuse communication services. By marking and comparing the communication packets of the users, the tracking technology of communication layer analyzes the sending and receiving packets of the communication parties andconfirms the correlation between them. These tracking technologies have great advantages in general resource requirements and application scope. A hidden communication path in the communication layer can be created by adding private data to the different characteristics of network traffic. Unused fields in the packet header, packet error rate, and jitter all can be carriers. As an active traffic analysis method [4] in the communication layer, network watermarking [5] can be used to trace the malicious users who communicate illegally through anonymous channels and network attack source through the gangplank chain. However, in the traditional network structure, if the configuration of network equipment needs to be modified after deployment, it will be a complex, costly, and time-consuming work. Due to the complexity of the algorithm, the above method cannot be directly deployed in the traditional network, additional devices such as watermark embedder need to be added in the network environment. It is quite difficult to modify the network equipment which has already been deployed. More importantly, traditional watermarking technology needs to embed deployment points in the network, which is uncomplicated to arouse the vigilance of criminals. The emergence of Software Defined Networks (SDN) [6] promotes the development of anonymous communication tracking technology. SDN separates the control plane from the data plane and improves the programability of the network. The data layer and control layer use open unified interface (such as openflow [7] protocol) for interaction. The tracking scheme in traditional network can be directly deployed in the data layer of SDN, which can effectively reduce the equipment load. SDN not only solves the problem of watermark deployment but also eliminates the side effects of embedding watermark through coding. For attackers, it is hard to be suspicious and vigilant to the watermarked data stream, which is of great significance to the anonymous tracking system. In this paper, we concentrate on the combination of network watermarking technology and SDN to improve the concealment and security of SDN data transmission. There are three challenges:  
* Security: Before the data stream enters the network, the watermark is embedded into the network data flow through the SDN controller. In order to avoid being detected by attackers and reduce data corruption, a secure and rigorous watermark coding scheme is needed.