Mehraghdam Draxler and used the YANG model for describing the

Mehraghdam draxler and used the yang model for

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Mehraghdam (Draxler) and Karl [13 , 14] used the YANG model for describing the service structure. In this work, the YANG data modeling language is designed to pro- vide a hierarchical model of configuration and to provide the run-time data for the Network configuration protocol (NETCONF). It also supports data instances in differ- ent formats. In another work from the same authors [15] , a context-free language is introduced to be used in formal- izing the chaining requests. b ) Service chain Composition (SC-C) In the context of NFV, a network service is defined as an entity that is composed of an ordered list of VNFs. The second step in SFC is how to concatenate the differ- ent VNFs efficiently in order to compose a network service (VNF-FG) in the most appropriate way, with respect to the requester goals. As an example, consider a service chain request with initial data rate of 1Gbps and five VNFs: VNF1 to VNF5. Assume VNF2 is a load balancer that splits the incoming traffic flow into two equal streams. Also, assume that the relative outgoing traffic rate of VNF3 with respect to the incoming traffic is 150%. The ordering is tied to the de- pendencies between VNFs; in this example, VNF2, VNF3 and VNF4 depend on VNF1 and therefore must be exe- cuted after it, and VNF3 must be executed after VNF2. Based on the dependencies, only a limited number of valid service chains are possible. Fig.6 shows two possible ser- vice chain compositions (VNF-FGs) for this example. The different chain composition affects the performance of the network service; therefore, the aim is to achieve an effi- cient service chain composition with regard to the service provider’s objectives. Fig. 6. Service chain composition
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Optimal Network Function Virtualization and Service Function Chaining: A Survey 709 Mehraghdam et al. [15] proposed a greedy algorithm for chin composition with the aim of minimizing the to- tal data rate of the resulting chain. In this regard, at each step, the VNF that its ratio of outgoing to incoming data rate is less, is chained first. Beck and Botero [16 , 17] proposed a heuristic algorithm for joint composition and placement of the service chains. At each step, it composes a single VNF in the service chain, and simultaneously places it on the underlying network. If a VNF cannot be embedded, the algorithm goes back to the last successful step and tries to do an alternative mapping. c ) VNF-FG Placement (VNF-P) For a set of requested network services, the aim of VNF-P is placing the VNFs on suitable nodes in the net- work with regard to the given objectives. Each VNF is defined with a type including computing, storage or net- working; therefore, it has to be allocated into the physi- cal node with the same type. In practice, the orchestrator leaning on the VNF managers and the VIM, evaluates all of the conditions to assign VNFs to the physical resources.
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