Distribution & Delivery Global Viewpoint – December 2022

SDN Power

Software Defined Networks (SDNs) are making a big splash in broadcasting, but why are they proving so popular?

SDNs are a relatively new concept in IT as they have separated the control and data planes. This is new because the successful operation requires some “knowledge” of the underlying network resulting in a centralized control which is different from traditional IT networking methods as they use automated routing protocols such as ECMP (Equal Cost Multi-Path), where routers would automatically route IP packets from one domain to the next.

Automated routing has worked well for IP networks, but the methodology is starting to come under some strain as they lack granular control. This is especially important when broadcasters need to route time sensitive packets similar to those found in streaming media.

Traditional IP routers may appear to be a little crude in elements of their operation. This is demonstrated when a link starts to become oversubscribed resulting in congestion. At this point the router has one of two options, it either relays a message back to the sender to tell it to stop transmitting, or it drops the queued packets. Both of which are disastrous for broadcasters.

The first solution requires the transmitting equipment and all the connected infrastructure to be fully compliant with a protocol such as ECN (Explicit Congestion Notification), which is a big ask and cannot be relied upon. Furthermore, by stopping the transmitter from sending, we’re just moving the congestion upstream. And the second solution of dropping packets is just not worth thinking about for broadcasters. Consequently, we have resorted to overengineering infrastructures to take into consideration high peak demand. But we need to avoid this design philosophy if we are to build efficient networks.

Where SDNs excel is in their visibility of the whole network (within its domain). This differs from traditional IT networks using protocols such as ECN as they only have visibility of the domain of their immediate links. Re-routing is possible when protocols such as ECMP are adopted, but they largely rely on packet drop before they are instigated.

Network routers and switches often employ monitoring functionality using protocols including NetFlow. These can provide statistics about the links including datarates which are then made available to a network administrator or the control plane of an SDN. Although the network administrator can monitor the stats and re-route the network to improve efficiency and reduce the risk of congestion and hence packet loss, it is often a manual and reactive process. SDNs can receive the NetFlow information from every router and switch in their domain and determine how the whole network is behaving.

By observing the interaction of the whole network, the SDN can re-route IP flows to under-utilized links and flag potential congestion points quickly to avoid packet loss. In other words, the SDN can dynamically control the network as well as monitor it. Therefore, we can design networks from a position of flow utilization knowledge and experience instead of having to build for peak demand, thus greatly improving efficiency. And as the requirements of the network change, the network itself can efficiently and reliably change.