## 4 Security Threats
### Negotiation  
#### 4.2.1 Negotiation Obstruction  
The vast majority of the approaches found in the literature use a common control channel on which to base their negotiation procedure ([33, 34, 35]). Either centralized [36], or distributed [33, 35, 37], such spectrum sharing schemes require communication between network entities to operate successfully. In case this sharing is done in a centralized manner, nodes (competitors) use this channel to send their spectrum requests to the central node assigned with this task. Similarly, when allocations are determined in a distributed way, nodes use the common channel to exchange requests and other operational details. Obviously, this common channel must be accessible by all nodes of a Dynamic Spectrum Access (DSA) network and it is best if it is interference-free and characterized by high availability. If any of these requirements are not satisfied, a DSA network's normal operation could be jeopardized or be under significant threat. To begin with, an attacker may attempt to occupy or even destroy this common control channel, which is the backbone of a spectrum sharing mechanism, with the goal of disrupting a DSA network's normal and optimal operation [38]. Among his alternatives would be to _jam_[39] the channel by simply transmitting a strong signal over that channel. As a consequence, no spectrum requests (or other operational details) could successfully reach their destination and the respective sharing mechanism would remain without input. Although switching to a new common control channel to continue the negotiation process could give a temporal solution to the problem, this approach cannot be characterized as the most efficient. In such a case, all the negotiating parties should get in touch with each other to agree upon a new channel to move to; as this is also a type of negotiation and, in any case, it requires a control channel to be achieved, we get into a loop, or we need to have predetermined a set of \"new\" channels to move to when the current one is rendered unavailable. Unfortunately, a number of issues arise here that make the implementation of such a technique extremely difficult. Namely, efficiently detecting jamming attacks with minimum ambiguity is not trivial, while both resynchronization and common channel reassignment is costly, especially if such attacks are too frequent for this cost to be amortized over time. Exactly the same problem could occur and identical statements would hold if, instead of jamming, a malicious node chose to realize a _flooding attack_ against the common control channel [40]. According to that attack, (dummy) packets are continuously transmitted over a channel with the goal of heavily congesting it and, as a result, significantly increasing packet transmission delays and packet loss rates (to the point of making the control channel useless). However, there are also sharing mechanisms that avoid basing their operation solely on a common control channel. The negotiating parties here have to exchange both requests and other information regarding their operation over the channel currently assigned to them [41]. In such cases, it is more difficult for an attacker to obstruct the negotiation process, but it is still possible. For instance, by carefully monitoring the flowing traffic and predicting when the next desirable \"control\" packet will be transmitted, an adversary could obstruct the negotiation process by simply colliding with the latter. Note that more discriminating and sophisticated attacks may be more difficult to undertake, and may even have lower success rate, or take longer to achieve results, but they are much harder to detect or prove as non-conformant and therefore may go undetected for long periods of time, or more generally be effective longer. Instead, continuous jamming and similar non-refined techniques cannot be overcome with communication techniques, but are more obvious and the jammer can more easily be identified and stopped by external intervention.