Though hardware NFs are high capacity they are power hungry and incur large

Though hardware nfs are high capacity they are power

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Though hardware NFs are high-capacity, they are power hungry and incur large capital investment. As a result, there are only a limited number of hardware NFs deployed in fixed points in the network. Different hardware NFs have their own placement considerations and thus different placement strategies, as we discuss in this section. TABLE. I summarizes the comparison. A. Independent passive NFs Some passive NFs can work independently to perform a complete task, without the interaction with other NFs. The placement strategies for this kind of NFs either tries to max- imize the utility under the budget constraints or to minimize the total cost while a certain level of service is guaranteed. The passive monitor placement falls in this category. The passive monitor is inside a router or a stand alone device that tap into a communication link. There is an intrinsic trade-off between the monitoring coverage and cost (which consists of the initial capital investment and operating expense). To this end, the proposed solution in [30] carefully places passive monitors and controls their sample rate, without chaning traffic routing pathes. Two integer linear programs (ILPs) are formulated respectively for two conflicting objectives: One is to maximize the fraction of IP traffic being sampled and the other is to minimize the total monitoring cost. The results of both optimization problems determine the number of monitors, their optimal place positions and their sampling rate. It can be further proven that both problems are NP-hard (maximizing monitoring coverage and minimizing cost inher- ent the hardness of SET COVER and MAX k-COVERAGE, respectively). Therefore, greedy heuristics are proposed to solve the optimization problems. One problem of this static placement strategy is that varying traffic matrix renders the previously optimal solution sub- optimal. MeasuRouting [24] addresses this problem by strate- gically routing traffic to fix monitoring points while routing pathes are least disrupted. It is optional after the NF placement. B. Chained NFs Some flows may be required, either by applications or users, to traverse through a given sequence of NFs. This is called service chaining (or policy chaining). For example, the network administrator may specify that all http traffic should follow the service chain: firewall IDS proxy , for security purposes. There are two requirements for service chaining:
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NF type Location Traffic steering Placement objective Independent NFs in-line optional max cov./min cost Chained NFs off-line compulsory min latency TABLE I C OMPARISON BETWEEN INDEPENDENT PASSIVE NF S & CHAINED NF S . Correctness : The sequential order must hold. Efficientcy : Traffic should not traverse unnecessary NFs. Service chaining is more complicated than using inde- pendent passive NFs. Trivially placing NFs in order on the routing path may require a prohibitively large number of NFs. Existing solutions leverage traffic steering to accomplish service chaining.
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