Did you get Virtual network based on Overlay+SDN?

Fancy Wang 0620 2021

SDN can provide a global view for the encapsulation of VxLAN tunnels, including how to establish tunnels between VTEPs, the access location of virtual machines, the mapping relationship between VLAN IDs and VNIs, and so on.

There are two sources of the global view: One is that a cloud management platform such as OpenStack directly informs the SDN controller of the virtual machine's access location. In this way, the SDN controller presets the forwarding table on the data plane through the Proactive way. The other is that the SDN controller does not learn the location of the virtual machine by interacting with the CMS, but triggers the hypervisor to notify the controller when the virtual machine goes online. In this way, the SDN controller can only dynamically generate a forwarding table in a reactive way.

The two methods have advantages and disadvantages: In the Proactive method, the matching and forwarding can be done directly on the data plane when the traffic arrives. The real-time pressure on the control channel is small, but this method will push many unused forwarding tables down. When using TOR for tunnel processing, hardware resources will be wasted. In the Reactive mode, the first packet will be delayed or discarded, and the real-time pressure of the control channel is relatively high, but there will only be forwarding tables that need to be used on the data plane. In actual situations, it may be necessary to balance the two methods.

SDN also helps optimize traffic at the edge of the network. The optimization of Layer 2 traffic is reflected in the processing of BUM traffic. With the help of the global view in the controller, ARP Request can be proxied locally. In some cases, DHCP can also be directly processed locally by the SDN controller. The flexibility of SDN (especially OpenFlow) has had a great impact on the boundaries of the network. It has been discovered that Layer 3 traffic does not necessarily have to be processed by physical routers.

The popularity of anycast ideas and the development of distributed routing technologies have made Layer 3 traffic can be routed locally in the vSwitch/TOR, which not only simplifies the network path, but also avoids a single point of failure in routing. However, distributed routing technology may also cause some other problems, such as too complex network edge functions and difficulty in fault location.

When optimizing traffic through SDN, using OpenFlow can gain considerable flexibility. Since communication between VMs still has to rely on a large number of ARPs, the suppression of ARP flooding is a problem that most controllers must solve. Using standard OpenFlow requires the controller to reply to ARP Reply as a proxy.

When the network is large, the pressure on the controller to process ARP will increase dramatically. Therefore, a better way is to use OpenFlow Nicira Extension to preset the content of ARP Reply. In the OVS, the OVS performs a proxy reply to the ARP locally on the data plane. DHCP also relies on broadcast, but its number is far less frequent than ARP. Therefore, in most SDDCN schemes, DHCP is still broadcast to the DHCP server for processing.

However, the OpenFlow controller can also proxy DHCP replies, but because the structure of DCHP is based on UDP, OVS cannot handle DHCP locally in the data plane, and the controller must reply. Distributed routing can be implemented entirely by OpenFlow, or it can be implemented using OpenFlow combined with vRouter. The design of different SDDCN solutions is different. At present, the industry is more accepting the implementation of OpenFlow combined with vRouter, but directly using OpenFlow to implement routing, the overall performance will generally be slightly better.

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