Taking a Longer Look at 5G Infrastructure and Services

It seems possible, based on the results of the MWC show, to speculate a bit on what infrastructure and service considerations are likely to arise out of the 5G specs.  “Speculate” is the key word here; I’ve already noted that the show didn’t address the key realities of 5G, IoT, or much anything else.  I also want to point out that we don’t have firm specifications here, and in my view, don’t even have convincing indicators that all the key issues are going to be addressed in the specs that do develop.  Thus, we can’t say if these “considerations” will be considered, outside this blog and those who respond on LinkedIn or to me directly.

Three things that 5G is supposed to do according to both the operators and what I read as “show consensus” are to support a unified service framework for wireline and wireless, support “network slicing” to separate services and operators who share infrastructure, and allow mobile services to incorporate elements of other connectivity resources, including wireline and satellite.  These three factors seem to frame one vision of the future that’s still not accepted widely—the notion of an explicit overlay/underlay structure for 5G.

Traditional networking is based on two notions; that services are built on layers that abstract a given layer from the details in implementing the layers below, and that within a layer the protocols of the layer define the features of the service.  When you have an IP network, for example, you rely on some Level 2 and Level 1 service, but you don’t “see” those layers directly.  You do “see” the features of the IP network in the features of your service.

Overlay/underlay networking is similar to the layered structure of the venerable OSI model, but it extends it a bit.  We have overlay/underlay networking today in “tunnel networks” that build connectivity based on the use of virtual paths or tunnels supported by a protocol like Ethernet or IP, and we now have formalized overlays built using SDN or SD-WAN technology.  Most overlay/underlay networks, in contrast to typical OSI-layer models, don’t rely on any feature of the layer below other than connectivity.  There are no special protocols or features needed.  Also, overlay/underlay networking has from the first been designed to allow multiple parallel overlays on a single underlay; most OSI-modeled networks have a 1:1 relationship between L2 and L3 protocols.

In a 5G model, the presumption of overlay/underlay services would be that there would be some (probably consistent) specification for an overlay, both in terms of its protocols and features.  This specification would be used to define all of the “service networks” that wireline and wireless services currently offer, and so the overlay/underlay framework would (with one proviso I’ll get to) support any “service network” over any infrastructure.  That satisfies the first of our three points.

The second point is also easily satisfied, because multiple parallel overlay networks are exactly what network slicing would demand.  If we expanded the “services” of the underlay network to include some class-of-service selectivity, the overlays could be customized to the QoS needs of the services they represent in turn.

In both SD-WAN and SDN overlays, the connectivity of the overlay is managed independent of the underlay; the OSI model tends to slice across layer boundaries or partition the devices to create overlay/underlay connectivity.  In most SD-WAN applications the presumption is that the edge devices (where the user is attached) terminate a mesh of tunnels that create connectivity.  In SDN, there may be a provision for intermediary steering, meaning that an endpoint might terminate some tunnels and continue others.  For proper 5G support, we need to review these options in the light of another element, which is explicit network-to-network interconnect.

Most protocols have some mechanism for NNI, but these are usually based on creating a connection between those singular top-of-the-stack OSI protocols.  In overlay/underlay networks, an NNI element lives at the overlay level, and simply connects across what might be a uni-protocol (same protocol for the underlay) or a multi-protocol (a different underlay on each side) border.  Alternatively, you could have an underlay gateway that binds the two networks together and harmonizes connectivity and QoS, and this could allow the overlay layer to treat the two as the same network.

The border concept could also describe how an underlay interconnect would be shared by multiple overlays, and that concept could be used to describe how a fiber trunk, satellite link, or other “virtual wire” would be represented in an overlay/underlay structure and how it could be used by multiple services.  On- and off-ramps to links like this are a form of gateway, after all.

The question that’s yet to be addressed here is the role that virtual function hosting might play.  There’s nothing explicitly in 5G discussions to mandate NFV beyond hopefulness.  On the other hand, the existence of an overlay technology could well create the beginning of an NFV justification, or at least a justification for cloud-hosting of these overlay components rather than dedicating devices to that role.  An overlay network should be more agile than the underlay(s) that support it.  That agility could take the form of having nodes appear and disappear at will, based on changes in traffic or connectivity, and also in response to changes in the state of the underlay network.  Virtual nodes fit well into the overlay model, even NFV-hosted virtual nodes.

Beyond that it’s harder to say, not because hosting more features isn’t beneficial but because hosting alone doesn’t justify NFV.  NFV was, from the first, fairly specialized in terms of its mission.  A “virtual network function” is a physical network function disembodied.  There really aren’t that many truly valuable physical network functions beyond nodal behavior.  Yes, you can hypothesize things like virtual firewalls and NATs, but you can get features like that for a few bucks at the local Staples or Office Depot, at least for the broad market.  Moving outside nodal (connectivity-routing) features to find value quickly takes you outside the realm of network functions and into application components.  Is a web server a network function, or a mail server?  Not in my view.

From the perspective of 5G and IoT, though, the requirements for hosting virtual functions or hosting cloud processes are very similar; there is a significant connectivity dimension.  We have done very little work in the NFV space to frame what network model is required to support the kind of function-hosting-and-management role needed.  That work that’s been done in the cloud space has focused on a pure IP-subnet model that’s too simple to address all the issues of multi-tenant functions that have to be securely managed as well.  In fact, the issue of addressing and address management is probably the largest issue to be covered, even in the overlay/underlay model.  If operators and vendors are serious about 5G then they need to get serious about this issue too.