"Fig-1 Showing working of TCP Checksum(Adapted from Stevens, W. R. (1994). TCP/IP Illustrated, Volume 1. Reading, MA:Addison-Wesley)Architecture to solve problem To solve many NP complete problems, such as the traveling salesman or the satisÆabilityproblem, a common technique is to generate a large number of candidate solutions and then testthe candidates for their adequacy. Because the candidate solutions can be tested in parallel,effective computer architecture for these problems is one that supports simultaneous evaluationof many tests. An example of such a machine is illustrated in Fig. 1a. Here, the computerconsists of a collection of target nodes connected to a network, where each of the target nodescontains arithmetic and logic unit (ALU) that is capable of performing the desired test and anetwork interface (NIF) that allows the node to send and receive messages across the network.7 A single home parasite node initiates the computation, sends messages to the targets directingthem to perform the tests, and tabulates the results.Fig-2 Showing Working of parasitic computing(Adapted from Stevens, W. R. (1994). Working iilustrated, Volume 1. Reading, MA:Addison-Wesley)IMPLEMENTATION USING TCP Sending a message over an internet is a very sophisticated process as the message isprocessed across many layers from HTTP then to TCP then to IP layer, going through data linklayer finally to the physical layer and in the same manner the message is constructed back at thedestination. To implement this concept of parasitic computing we can choose to exploitprocessing theoretically any of these layers but below TCP layer it is not very beneficial. 8Till now there has been only one implementation, which has exploited this concept ofparasitic computing. Idea is to use some feature of the protocol in such a manner that remotemachines respond to the request unknowingly that they are involved in solving a complexproblem and they believe that they are responding to a simple application request over TCPconnection.The main target problems for such distributed environments are NP-completeproblems i.e. non-deterministic polynomial problems. These problems are such that their stepscannot be expressed in terms of polynomial time and therefore to know the right solutions onehas to evaluate many possible alternatives. The property, which can be exploited here, is that allthe alternative solutions can be evaluated in parallel and therefore different machines across theInternet can be used simultaneously for evaluation thousands of possible candidate solutions forany such problem. Like in this case the protocol, which is being used for this purpose, is TCP. Tounderstand the implementation one first needs to have a brief idea of TCP checksum.TCPCHECKSUMOne feature of the TCP protocol that is very important is the implementation of parasiticcomputing is the checksum property. Checksum is that part of TCP layer operation that isresponsible for insuring integrity of packet data being sent over the Internet. Before a packet isreleased to the IP layer of the sending computer, TCP divides the packet information into a seriesof 16-bit words and then creates a one?s complement binary sum of these words. The resultingso-called “checksum” value is a unique representation of the totality of information in thatpacket. The bit-wise binary complement of this checksum is then stored in the TCP headerbefore the packet is sent. When the packet arrives at the receiving computer, the TCP layer thereperforms its own binary sum of all the information in the packet including the checksumcomplement. If the packet was received without corruption, the resultant sum should be a 16-bitvalue with all bits equal to 1 since the original checksum (i.e., the total arrived at by the sendingcomputer) and its exact complement would be added together forming a unitary value. 9 "