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In our scheme, the SDN switches rerouting packets based on multiple MPLS labels which take the forwarding ports of switches. The latest OpenFlow protocol supports MPLS technology [26], the Push MPLS header actions can push new MPLS headers onto the packet. When a new MPLS tag is pushed on an IP packet, it is the outermost MPLS tag, inserted as a shim header immediately before any MPLS tags or immediately before the IP header. SDN switches encapsulate multiple MPLS labels for each packet that indicates the forwarding port numbers of switches on its route. Energy saving requires fine-grained flow scheduling to shutdown switches and links. SDN switch achieves fine-grained flow scheduling by encapsulating the MPLS labels to take the forwarding information. Thus, it can save energy by rerouting the flows to turn off the idle links and switches.

For example, Fig. 2 shows an example of using MPLS labels to achieve energy saving in the network. We assume that there are only two flows in the topology, which are f1 from h1 to h3 and f2 from h2 to h3. The path of the flow f1 is {h1, s1, s2, s3, s4, s7, h3}, while the path of the flow f2 is {h2, s5, s6, s7, h3}. The controller selects switch s2 to install flow entries to reroute the flow f1. When the packets of flow f1 enter the SDN switch s2, they are encapsulated with 3 MPLS labels, {2,2,2}, each number indicates a forwarding port of the switch on its route. The packets pop up one MPLS label as a forwarding port of switch when they pass through the switch. The new path of the flow f1 is {h1, s1, s2, s5, s6, s7, h3}. Thus, we can achieve energy saving by turning off the switches s3, s4 and the links s2 → s3, s3 → s4, and s4 → s7. Moreover, unlike traditional network MPLS, there is no need to maintain path state in forwarding path exc

Fig. 2. An example of using MPLS label to achieve energy saving in the network. The controller configures the rerouting path for the flow h1→h3 by installing the flow entries at switches s2.

ept on the ingress node, because packets are now routed based on the list of labels they carry [29].

3.3 Flow table aggregation by segment routing

The papers [7,30] propose the segment routing architecture and they detail the operation mechanism of segment routing in wide area networks and data centers. Segment routing also enables finer control of the routing paths and can be used to route traffic through middle boxes. We break up the routing path into segments in order to reduce the flow table entries and improve the flexibility of rerouting. While implementing energy savings, we can aggregate the flow table entries based on the segmented routing between SDN switches. Therefore, we introduce the flow sets to path aggregation and path ID assignment methods based on the coincidence degree of flows on the forwarding paths.

We reused the flow entries by encapsulating the MPLS labels for the flows of those forwarding paths are overlap. Moreover, we also reused the flow entries to pop up the MPLS labels to indicate the switch port number of the forwarding path for the flows. For example, in Fig. 2 we can install a flow table entry at switch s5 to encapsulate MPLS labels for both flow f1 and flow f2 to indicate the forward information.

4 Problem Formulation

The goal of our issue is to achieve energy saving by incremen- tally deploying SDN switches. Firstly, we maximize the NCA with a given SDN switches upgrading budget constraint. Secondly, we maximize energy saving by shutting down idle links and switches with deployed SDN switches. We assume that the SDN forwarding elements are a subset of the nodes in the network. The rest of the nodes in the network run some standard hop-by-hop routing pro- tocol like OSPF [1,31]. Using the information collected by each SDN switch, the SDN controller is able to peer the entire network and be aware of the workload on each link as well as the traffic vol- ume between different source-destination pairs [9]. Then, we can achieve energy saving by flexibly scheduling the flows to turn off links and switches.