a Dynamic versus static Many authors assume that the environment is static

A dynamic versus static many authors assume that the

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a ) Dynamic versus static Many authors assume that the environment is static, while in the today modern networks, the user demands and network conditions are highly dynamic due to the growing usage of mobile devices. As a result, dynamic re- source allocation of the VNFs is a problem of significant importance. In dynamic environments, the resources must be allocated at run time that is a more complex problem. This brings new challenges, because the VNF resources may scale due to dynamic traffic, and the QoS demands may change due to the changes in the service requests. In addition, we should monitor the VNFs for reliability prob- lem and when VNF failures happen, we need to reassign VNFs for corresponding service requests. To overcome the above challenges, new solutions must be considered for the SFC problem. Wang et al. [25] in- vestigated the problem of dynamic function composi- tion, and developed a combinatorial model by defin- ing the dynamic function composition problem as an
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710 Chinese Journal of Electronics 2018 MMKP (Multi-dimension multi-choice 0-1 knapsack prob- lem). They used the Markov approximation method to solve the problem. Callegati et al. [26] propose using Open- Flow for dynamic steering of the traffic flows. They de- scribe the required actions must be programmed in Open- Flow switches in order to achieve the dynamic service chaining. In another work [27] , the SFC problem is mod- eled as Markov decision process (MDP) and a preemp- tive resource allocation strategy is proposed. In addi- tion, authors suggest using Bayesian learning algorithm for predicting the future resource reliability and propose an MDP-based algorithm by considering the concept of asynchronous partitioning. b ) Online versus offline In offline scenarios, a set of services is given and we try to embed them in the network. In online scenarios, service requests arrive one by one and each service is em- bedded in the network as soon as it arrives. In this case, by arriving a new request, the migration and remapping of already embedded VNFs may be necessary. Mijumbi et al. [22] investigate the problem of online placement and scheduling of VNFs, where each service is embedded as it arrives. Mohammadkhan et al. [28] formu- lated the problem of VNF-FG placement to achieve min- imum delay by minimizing the consumption of resources. Then they proposed a heuristic algorithm that solves the problem incrementally for arriving flows without affect- ing existing flows. Lukoszki et al. [29] considered an Online version of the service chain embedding problem (OSCEP), where requests arrive over time. They proposed a deter- ministic online algorithm that is asymptotically optimal in the class of both deterministic and randomized situa- tions. c ) Single versus multiple providers In providing network services, sometimes it is neces- sary to implement a service chain that spans the infras- tructure of more than one service provider. However, in multiple provider scenarios, the restrictions in information exchange and issues in interoperability between different
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