Network Virtualization and SDN

Definition:

• Network virtualization (NV), similar to compute virtualization, creates a logical network separating network functions from the hardware by simulating network functionality where many virtual instances run on a single hardware platform.
• In addition to multiple software instances running on a physical platform, NV provides an abstraction of the underlying network through encapsulation and tunneling mechanisms.
• NV is not a new topic. VLANs creating segmentation of broadcast domains, VRFs creating multiple virtual routers on a physical router, and Virtual Device Contexts creating multiple switches on a physical switch are widely deployed in today’s networks. From tunneling and encapsulation perspective, we see VPNs creating logical secure connection to corporate network over the Internet, OTV (Overlay Transport Virtualization) by Cisco providing L2 adjacency between data centers over IP packets, VXLAN (Virtual Extensible LAN) tunnels providing VLAN abstraction over IP networks and others. Another addition to these examples may be the efforts to replace or fool the STP (Spanning Tree Protocol), which blocks the redundant links in order to prevent loops. Such examples are TRILL (called FabricPath by Cisco), and vPC (by Cisco).
• In any network, the main functionality is to forward the packets per the needs of the VMs and the hosted applications. Legacy networks fulfill these needs with manual reconfiguration of the network, where the complexity of interaction between networking, security, storage and compute teams makes this task very slow and fit only for small and static environments. Another approach taken by legacy networks is making sure that VLANs are extended within the local network and even to remote data centers (through OTV for example). However, stable and secure implementations of Data Centers require segmentation and a flat L2 network is hard to achieve.

Where does SDN fit in NV?

• SDN is at the very heart of NV. SDN does steering of the packets per the needs of the VMs and the hosted applications in an abstracted or virtualized fashion. We call this SDN approach to NV and see two approaches by SDN vendors to achieve this goal. The first one is to program the abstracted underlay fabric directly through a southbound API such as OpenFlow, or proprietary methods. Opendaylight SDN Controller and HP VAN (Virtual Application Networks) SDN controller using OpenFlow and Cisco ACI (Application Centric Infrastructure) where Cisco Nexus 9K fabric speaks proprietary methods with Cisco APIC (Application Policy Infrastructure Controller) are dominant products in the market. The second approach is to encapsulate the L2 frames in L3 IP packets and then tunnel them to the destination VMs. We call this overlay approach which treats the underlay IP network as an abstract. This is a favored model for environments, where underlay SDN is not possible, because the underlay fabric is not OpenFlow or proprietary methods capable. Currently VXLAN implementation is widespread in the market and overlay approach uses VXLAN. The leader in SDN implementation with overlay approach is VMWare NSX solution.
• While overlays provide agility and abstraction without upgrading the underlay, this approach may have challenges in enforcing QoS and mitigating the effects of elephant flows on mice flows, because encapsulation prevents differentiated services to be provided on flows which are identified by the content of the header that is encapsulated. Another challenge is VMs speaking to physical servers in a hybrid environment where not all the compute is virtualized. Gateways between VMs and physical networks are needed for this case. This VXLAN gateway, if implemented on a VM in software, may be subject to insufficient processing power. Therefore, VXLAN VTEPs (Virtual Tunnel End Point) are needed in the underlay fabric, which in effect means purchasing hardware for this purpose. Another challenge may be that underlay network may not be designed for high availability and reliability and may not be able to provide multiple redundant paths across any two server endpoints. If the reliability of the underlay is not adequate, overlay approach seeing this network as an abstract will suffer for sure. Finally, as overlay SDN solution grows and evolves, physical underlay may not be able to adapt automatically to the changes. Therefore, overlay based solutions require a non-oversubscribed physical underlay network.

Solutions Available Today:

• By supporting open standards such as the OpenFlow Networking Standard, OpenDaylight will deliver a common open source framework and platform for SDN across the industry for customers, partners and developers.  The first release is called Hydrogen released in February 2014 and the planned release is called Beryllium to be released in February 2016.
• VMware started SDN by acquiring Nicira. The product is VXLAN based overlay solution. VMware NSX, can run on any physical IP network. Technically, one can leverage any switch vendor such as HP, Brocade, Arista, Juniper, etc. This is a big plus for VMware NSX as it does not require costly hardware upgrades to get SDN features in existing networks.
• Following the SDN wave, Cisco defined its SDN strategies around acquisitions. Cisco acquired Insieme . Cisco's SDN solution, Cisco ACI, is a combination of software and hardware, rather than purely in software.  Cisco ACI requires the new generation Nexus 9000 switches, and is controlled by Cisco APIC (Application Policy Infrastructure Controller). For existing Nexus 7000 and 5000 switches Cisco ASICs are being road-mapped for installation to enable Cisco ACI.
• HP has been in SDN business since 2007 offering SDN App Store and has made all of its FlexFabric series of switches OpenFlow capable. HP VAN SDN and VMWare NSX SDN is an example of  federation of underlay and overlay. HP VAN and NSX federation made possible by OVSDB (Open Virtual Switch Data Base) unites the strengths of overlay and underlay based SDN NV.

It will be interesting to see how adoption of OpenDaylight will go and how the battle between Cisco ACI (where Cisco dominates the switch deployment) and HP VAN-VMWare NSX federation (where VMWare dominates the compute virtualization and HP being the # 2 in Enterprise Networking and # 1 in x86 server deployment) will end. It will be interesting to see who will win.