F5 Intelligent DNS – Optimizing the Mobile Core

Martin J Brewer, Manager, DNS Product Management

F5 has been very active in the service provider footprint for DNS, especially for high performance authoritative or caching resolver functions. In addition to those use cases, now F5 is able to augment them with what we call Intelligent DNS. In essence we’re taking our ability to use our monitoring functions (which are part of F5 BIG-IP GTM) and apply them so that DNS responses for DNS queries for the Mobile Core reflect the availability and health of the services which sit behind those DNS query names.

So what does this mean in real terms? With these optimizations, subscribers will benefit through increased service reliability and higher performance while Service Providers receive higher customer satisfaction. If subscribers are happy, you get loyalty and you see repeat users by making sure the service works optimally, every time.

Perhaps first we need to talk about how a regular Mobile Core operates during connection setup for a core which uses static DNS. As you can see from the diagram, we have a simplified view of the core which is present in 3G and 4G network designs. On the left you’ll see the mobile device, or smartphone, and on the right you’ll see the Packet Gateways (PGWs) or Gateway GPRS Support Nodes (GGSNs). These are the gateways between the Service Provider core network and the internet and other IP services.  Now for a device to get connectivity to the internet, it needs to initiate a connection. In 4G/LTE for example, this is performed through the Management Mobility Entity (MME). Now, you’ve probably seen in the Settings area on your smartphone that there’s a reference to an “APN” or Access Point Name. What that APN does is tell the network which service your device needs to establish a data connection. And that APN name is specified as a DNS name.

So how does it work?

That query ends up turning into a DNS request which is emitted by the MME so it can set up the connection. So it might look something like “apn.carrier.com” and once resolved by the DNS service inside the Mobile Core, it will resolve to an IP address, like 192.168.1.1. Now, the DNS service itself will pass not just one, but a complete list of all of the possible gateway addresses. Some DNS services might also randomize the order of the list so that the MME might spread the load on the PGWs. Sounds reasonable doesn’t it?

Well, there are a few problems with that approach:

What if the PGW unexpectedly went down? The DNS system has now directed the MME to an unreachable resource. Now it isn’t the end of the world, but it results in a delay in the connection setup and the MME times-out on the PGW that’s no longer in service. This impacts the customer experience and by extension, customer satisfaction.

And remember, that until that PGW is removed from the DNS records, it will continue to serve bad DNS answers. To solve this, manual intervention is required, and that increases OpEx for the carrier.

And the of course there’s the loading of those PGWs.

Let’s take a look….

Just because the DNS service might have randomized the reply order to set some form of load distribution, this doesn’t equate to an even distribution in reality. Data sessions from mobile devices last anywhere from a few seconds to tens of hours, and the loading they place on the PGW is totally determined by how much data the client sends or receives. Placing additional capacity onto an already maxed PGW simply decreased connection speed for the mobile device and again, that leads to poor customer satisfaction.

So isn’t there a better way?

Using BIG-IP GTM with Intelligent DNS, the DNS service is made into a dynamic directory with intelligent answers based on service availability and loading.

The big improvement is that BIG-IP is able to monitor services through a variety of real-time health checks.

The system is highly customizable and allows a Mobile Core administrator the ability to use GTP Echo monitors (these are built right in to the BIG-IP GSLB engine) which check for active availability of the protocol that carries the data.

They can also use External monitors which can factor in other parameters such as live loading information or consider environmental information, such as device temperature and equipment status.

Using these optimizations, the DNS engine considers each IP address it monitors and assigns scores to each so that it may give an optimal reply for any given DNS query. Using this scoring mechanism, BIG-IP GTM can intelligently provide a single IP address as a reply to any given query based on its real-time feed of PGW availability and health.

So what are the benefits?

If a PGW fails unexpectedly, the BIG-IP Intelligent DNS service identifies the unavailability and stops providing DNS responses which contain that PGW’s IP address. Through loading data, the Intelligent DNS service provides answers for the most appropriate PGW in real-time. No longer will a subscriber be placed onto an overloaded PGW. All of this means there’s reduced OpEx through the automation of DNS records to service availability, together with an optimal user experience.

Stop by the F5 Booth at Mobile World Congress in Barcelona from February 24^th through 27^th for a demonstration of F5 Intelligent DNS in action. We’re at hall 5 in booth 5G11.