## 1 Introduction  
In a traditional computing scheme, data processing is performed in the processors, storage takes place in memory, and the network is used for communication. Over the years, there have been several different architectures and technologies, such as centralised and distributed computing, as well as primary, secondary and cache storage devices, but the vast majority of them appear to respect the general idea that the processing, storage, and communication should be performed in the processor, memory, and network, respectively. There have not been many attempts at achieving something different, even if the attempted variant could result in significant advantages. Consider, for example, the case of Processing in Memory (PIM) [1], which modifies slightly the concept of a traditional memory and processor. The specific way of implementing PIM is by coupling the processor very closely with the memory, usually on the same chip. This approach will reduce the need for moving data, which is reflected in lower power consumption, reduced memory latency and increased bus bandwidth. Another interesting combination to attempt is to mix the processing with the network. This concept could be called \"Process-in-Network\" (PIN), which is defined as the possibility that the network processes information as it is being transmitted. The processing is performed directly in the network nodes that are found between the origin and the destination, taking advantage of waiting times in router queues, idle processing capacity in intermediate nodes, and the information that passes through the network. If a link is busy or does not meet the QoS (Quality-of-Service) requirements, and provided that there are sufficient resources in the intermediate node, it is reasonable to consider the possibility of performing processing during the time that the link is unavailable and/or fails to meet the QoS requirements. Today's routers are very fast and they are very efficient at performing routing tasks, but this may not be enough to ensure an adequate data flow. There is still the possibility of a bottleneck. A network could become congested when the buffers are saturated due to limitations in the link capacity. Every network is likely to become congested. If a network is never congested, it may be oversized and that would imply an unjustified investment. PIN could be understood as a kind of countermeasure when the network becomes congested. There is an opportunity for PIN while the link is busy, especially in congested networks. The more congested the network is, the greater the advantages of using PIN. These constraints (either in terms of capacity, delay, cost, power consumption, or any other factor or combination of them) are a primary motivation for seeking alternatives that optimise the transmission of information through the network. Traditional alternatives refer either to mechanisms that attempt to find the best route between the origin and destination or to find solutions that simplify the information in the input terminal nodes to reduce the burden that must be sent to the next hop. Another important idea to consider as part of a PIN is the ability to leverage information as it flows through the network, using information fusion techniques [2], whether based on information at a node or as the result of the aggregation of several nodes. The information upon arrival to the destination is better and richer in comparison with the information that was originally sent. The benefits of PIN concern mainly the following three factors, with the understanding that each of them can occur (or not) concurrently with the others, depending on the scenario and the specific application:  
* A significant reduction in the use of the links because the information is simplified and this therefore reduces the need for network transmission.