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EPC IMS / VoLTE LTE Mobile Networks RFCs & Standards SDM

Getting to know the PCRF for traffic Policy, Rules & Rating

A look at the PCRF in EPC Networks, and the Charging Rules that lay at the heart of it all.

Misunderstood, under appreciated and more capable than people give it credit for, is our PCRF.

But what does it do?

Most folks describe the PCRF in hand wavy-terms – “it does policy and charging” is the answer you’ll get, but that doesn’t really tell you anything.

So let’s answer it in a way that hopefully makes some practical sense, starting with the acronym “PCRF” itself, it stands for Policy and Charging Rules Function, which is kind of two functions, one for policy and one for rules, so let’s take a look at both.

Policy

In cellular world, as in law, policy is the rules.

For us some examples of policy could be a “fair use policy” to limit customer usage to acceptable levels, but it can also be promotional packages, services like “free Spotify” packages, “Voice call priority” or “unmetered access to Nick’s Blog and maximum priority” packages, can be offered to customers.

All of these are examples of policy, and to make them work we need to target which subscribers and traffic we want to apply the policy to, and then apply the policy.

Charging Rules

Charging Rules are where the policy actually gets applied and the magic happens.

It’s where we take our policy and turn it into actionable stuff for the cellular world.

Let’s take an example of “unmetered access to Nick’s Blog and maximum priority” as something we want to offer in all our cellular plans, to provide access that doesn’t come out of your regular usage, as well as provide QCI 5 (Highest non dedicated QoS) to this traffic.

To achieve this we need to do 3 things:

  • Profile the traffic going to this website (so we capture this traffic and not regular other internet traffic)
  • Charge it differently – So it’s not coming from the subscriber’s regular balance
  • Up the QoS (QCI) on this traffic to ensure it’s high priority compared to the other traffic on the network

So how do we do that?

Profiling Traffic

So the first step we need to take in providing free access to this website is to filter out traffic to this website, from the traffic not going to this website.

Let’s imagine that this website is hosted on a single machine with the IP 1.2.3.4, and it serves traffic on TCP port 443. This is where IPFilterRules (aka TFTs or “Traffic Flow Templates”) and the Flow-Description AVP come into play. We’ve covered this in the past here, but let’s recap:

IPFilterRules are defined in the Diameter Base Protocol (IETF RFC 6733), where we can learn the basics of encoding them,

They take the format:

action dir proto from src to dst

The action is fairly simple, for all our Dedicated Bearer needs, and the Flow-Description AVP, the action is going to be permit. We’re not blocking here.

The direction (dir) in our case is either in or out, from the perspective of the UE.

Next up is the protocol number (proto), as defined by IANA, but chances are you’ll be using 17 (UDP) or 6 (TCP).

The from value is followed by an IP address with an optional subnet mask in CIDR format, for example from 10.45.0.0/16 would match everything in the 10.45.0.0/16 network.

Following from you can also specify the port you want the rule to apply to, or, a range of ports.

Like the from, the to is encoded in the same way, with either a single IP, or a subnet, and optional ports specified.

And that’s it!

So let’s create a rule that matches all traffic to our website hosted on 1.2.3.4 TCP port 443,

permit out 6 from 1.2.3.4 443 to any 1-65535
permit out 6 from any 1-65535 to 1.2.3.4 443

All this info gets put into the Flow-Information AVPs:

With the above, any traffic going to/from 1.23.4 on port 443, will match this rule (unless there’s another rule with a higher precedence value).

Charging Actions

So with our traffic profiled, the next question is what actions are we going to take, well there’s two, we’re going to provide unmetered access to the profiled traffic, and we’re going to use QCI 4 for the traffic (because you’ll need a guaranteed bit rate bearer to access!).

Charging-Group for Profiled Traffic

To allow for Zero Rating for traffic matching this rule, we’ll need to use a different Rating Group.

Let’s imagine our default rating group for data is 10000, then any normal traffic going to the OCS will use rating group 10000, and the OCS will apply the specific rates and policies based on that.

Rating Groups are defined in the OCS, and dictate what rates get applied to what Rating Groups.

For us, our default rating group will be charged at the normal rates, but we can define a rating group value of 4000, and set the OCS to provide unlimited traffic to any Credit-Control-Requests that come in with Rating Group 4000.

This is how operators provide services like “Unlimited Facebook” for example, a Charging Rule matches the traffic to Facebook based on TFTs, and then the Rating Group is set differently to the default rating group, and the OCS just allows all traffic on that rating group, regardless of how much is consumed.

Inside our Charging-Rule-Definition, we populate the Rating-Group AVP to define what Rating Group we’re going to use.

Setting QoS for Profiled Traffic

The QoS Description AVP defines which QoS parameters (QCI / ARP / Guaranteed & Maximum Bandwidth) should be applied to the traffic that matches the rules we just defined.

As mentioned at the start, we’ll use QCI 4 for this traffic, and allocate MBR/GBR values for this traffic.

Putting it Together – The Charging Rule

So with our TFTs defined to match the traffic, our Rating Group to charge the traffic and our QoS to apply to the traffic, we’re ready to put the whole thing together.

So here it is, our “Free NVN” rule:

I’ve attached a PCAP of the flow to this post.

In our next post we’ll talk about how the PGW handles the installation of this rule.

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