SDN: What is behind Software Defined Networking?

The composition and administration of network structures poses great challenges to many companies. Since conventional networks based on physical hardware only rarely meet the requirements of modern companies, the choice of external Infrastructure-as-a-Service solutions (Iaas) is becoming more and more common. Compared to traditional in-house infrastructures, these cloud services, which give customers access to virtualised computer resources, are characterised by a high degree of flexibility and excellent cost control – unlike a fixed hardware framework, the desired resources can be scaled at any time at the push of a button.

Definition

Software Defined Networking (SDN) is a network concept that enables the central, intelligent management and control of individual hardware components using software. The use of open protocols such as OpenFlow allows access to network devices such as switches, routers or firewalls that would otherwise not be controllable due to proprietary firmware.

In most cases, the provisioning and scaling of virtual resource – both on the part of the customer and on the part of the provider – is accomplished with the help of software, without the need for manual access to the individual physical network components. The underlying network concept is also known as Software Defined Networking (SDN).

What is Software Defined Networking (SDN)?

Software Defined Networking describes a network architecture that enables a purely software-based management of the network. For this purpose, the control plane implemented as standard in the hardware components of the control logic is abstracted from the hardware – in this context, one also speaks of the intelligence of the hardware, which is nothing more than its specific operating software (firmware). Put simply, the SDN concept stands for the separation of infrastructure and its configuration.

The data plane, on the other hand, remains part of the individual network devices (i.e. all routers, switches, and firewalls integrated in the network). With SDN, however, its task is exclusively to forward packets, which is why it requires little computing power. Among other things, this has the advantage that the devices do not require any elaborately developed firmware and are generally much cheaper than other network concepts.

The task field of the abstracted control plane, which is responsible for the proper data traffic in the SDN architecture and therefore has to carry out all relevant analyses, is considerably more complex. However, detached from the hardware and implemented in centralised software, it is highly programmable in software defined networking and therefore much more flexible in terms of network administration than is the case with other architectures.

How does SDN function?

A specific communication interface between control plane and data plane is required for the SDN software operating on the control layer to send instructions for proper packet traffic to the embedded network components. The best known solution for this is OpenFlow. Managed by the Open Networking Foundation (ONF), the communication protocol is the first standardised interface between the control and data levels of a software-defined networking architecture. In many SDN networks, it replaces the individual interfaces of the network devices, which also reduces the dependency on the hardware manufacturers.

Note

OpenFlow is by far the most common, but by no means the only protocol for managing software defined networks: With NETCONF (RFC 6241), BGP (Border Gateway Protocol), XMPP (Extensible Messaging and Presence Protocol), OVSDB (Open vSwitch Database Management Protocol) and MPLS-TP (MPLS Transport Profile) there are alternatives which do not replace the standard protocol one to one, but which can nevertheless play a decisive role in the implementation of software defined networks. Proprietary protocols from Cisco Systems and Nicira are also used in some architectures.

Once communication between hardware and software is established, the administrator can quickly and easily get a good overall view of the network via the control layer and the respective SDN software, and manage the network devices through the central, software based control. This allows data streams to be managed with much greater efficiency than in networks where the various components each have their own control logic – greatly simplifying virtualisation and resource scalability. This is also facilitated by the fact that routing and topology information is no longer distributed in fragments across all routers, but instead converges at a central location.

What SDN models are available?

The ideas and approaches for implementing SDN structures vary depending on the provider or operator and communication standard used. However, it is not always possible to draw a sharp dividing line between the individual models, so it could be that a software defined network has elements from different approaches.

Symmetrical vs. Asymmetrical SDN

Although the software-based principle basically provides the best possible centralisation of network device intelligence, there are SDN approaches where the control plane’s scope is distributed across multiple control units. With such an asymmetrical model, the individual systems typically have the minimal information required for immediate operation, so that they will continue to function even if the central control unit fails. However, compared to the traditional symmetrical model, this also creates unnecessary informational redundancies.

 

Basic principle

Advantages

Disadvantages

Symmetric SDN approach

Maximum centralisation of intelligence

Avoidance of redundancies; relief of the individual components

Availability and stability of the network stand and fall with the central control unit

Asymmetric SDN approach

Distribution of intelligence

The individual function even in the event of control logic failure

Redundancies of information; management of the network is more complex

Host-based vs. Network-based SDN

Another way to characterise Software Defined Networking is to look at the position of the control logic. In highly virtualised environments, for example, it makes sense to have the control plane processes handled by the system on which the hypervisor, i.e. the virtual machine manager, is hosted. If the SDN software is run on this host system, you can be sure that the necessary capacities are available for the resulting data load. The alternative to this host-based approach is to distribute SDN processing to dedicated routers as is common in traditional networks and therefore handle it based in a network.

Flood-based (proactive) vs. floodless (reactive) SDN

A third software-defined network model focuses on the way information is passed between the control plane and data plane. On the one hand, there is the option for the control instance to forward new information and changes to all participating network nodes through broadcast or multicast. This so-called flood-based or proactive SDN concept is advantageous if the centralisation of intelligence does not play a role, e.g. because a symmetrical approach is used.

However, the more nodes a network has, the higher the network load will be with such a message transmission concept, which results in limited scalability. In larger networks, the floodless or reactive SDN model is therefore a popular alternative: In this case, the control plane ensures the correct functioning of all components by means of a controlled, reactive information transfer in which only the affected devices are clamped. The relevant information is usually obtained from special lookup tables, where distributed hashtag and caching methods are used.

 

Message transmission

Advantages

Disadvantages

flood-based SDN

Broadcast, Multicast

Easy to move; parcels are shipped the shortest way possible

The network load automatically increases with each new node

floodless SDN

Lookup tables

All devices receive information relevant only to them

Problems with delivery of the information automatically lead to delays

What distinguishes Software Defined Networking from the classic network concept?

In the previous sections, the basic differences between a network based on the SDN approach and a classic network have become clear. The crucial point here is undoubtedly the separation of hardware and software, which was unthinkable years ago. Only since 2013 have there been devices that can implement this elementary aspect of software defined networking. It is therefore hardly surprising that future oriented technology has not yet been an issue in many companies.

The following sections therefore summarise the main difference between SDN and traditional networking before concluding with the goals and benefits as well as the concrete applications scenarios of SDN.

The differences between SDN and traditional networking in a tabular overview

Software Defined Networking (SDN) Traditional Networking
Centralized supervisory authority Device-specific control instances
Celar separation between hardware and control level Control of the hardware is integrated in the hardware
Freely programmable control plane Device-specific control plane
Standardized protocols (e.g. via OpenFlow) Manufacturer-specific protocols
Software access to data layer possible Access to the data layer must be made directly on the hardware
Flexible, easily scalable architecture Static, difficult to adapt architecture

What are SDN’s goals and benefits?

Parallel to the demands placed on the computer’s computing power, the demands placed on the performance of networks are also continuously increasing: While digital networks are becoming larger and more complex, the degree of virtualisation and the desire for maximum flexibility and scalability are also increasing at the same time. Since conventional devices, which are equipped with their own intelligence and process a large part of the processes independently, have not been able to meet these demands for some time, the software-defined networking concept was developed. With specific hardware without its own control instance, these goals can be achieved and demands met. The advantages over traditional networks can be summarised as follows:

  • No configuration of individual devices or operating systems required
  • Low maintenance and administration costs for the entire network
  • Lower hardware and operating costs
  • Enables dynamic allocation and monitoring of resources in real time
  • Low dependence on hardware manufacturers

Possible application scenarios for Software Defined Networking

Thanks to its numerous advantages over the classic network concept, SDN is interesting for a large number of applications. Among other things, the software-defined network model is suitable for the following purposes:

  • Quality of Service (QoS): The central overview of all network nodes makes it easier for the administrator to track how often a single connection is used. The administrator can react in real time to the knowledge gained and regulate data traffic accordingly in order to be able to deliver the promised bandwidth to all participants at all times.
  • Manufacturer-independent device management: The focus on a uniform protocol such as OpenFlow makes SDN an excellent solution when devices from different manufacturers are to be combined and managed in a network.
  • Manufacturer-independent functional expansion of the network: The freedom of SDN technology is also a good solution for scenarios in which networks should be easily expandable with new functions at any time – and the independence of device manufacturers also plays into the cards of users.
  • Application-driven packet routing: SDN creates the basis for third-party applications to intervene in packet routing, i.e. change and adjust routes in the network. The prerequisite for this is that the control unit has a suitable interface.
  • Central definition and distribution of security policies: Security policies can be passed on to the individual network switches simply and efficiently through the central control unit.
Note

Together with other software-defined services, virtualised network structures are required, among other things, for setting up a Software Defined Data Centre (SDDC).

Conclusion: Flexible network architectures thanks to software defined networking

It is no coincidence that the SDN approach has been adopted by various network providers in recent years: Software defined networking optimises the basic approach of hardware virtualization by removing manufacturer-specific restrictions and considerably simplifying the administration of a network. By decoupling the logic from the underlying hardware and the associated ability to control the network via software, network operators are well prepared for future developments and challenges in the IT industry.


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