Tag Archives: AVP

Diameter Routing Agents – Part 5 – AVP Transformations

EPC, IMS / VoLTE, LTE, Mobile Networks, RFCs & Standards, SDM, Software, , , , , , ,

Having a central pair of Diameter routing agents allows us to drastically simplify our network, but what if we want to perform some translations on AVPs?

For starters, what is an AVP transformation? Well it’s simply rewriting the value of an AVP as the Diameter Request/Response passes through the DRA. A request may come into the DRA with IMSI xxxxxx and leave with IMSI yyyyyy if a translation is applied.

So why would we want to do this?

Well, what if we purchased another operator who used Realm X, and we use Realm Y, and we want to link the two networks, then we’d need to rewrite Realm Y to Realm X, and Realm X to Realm Y when they communicate, AVP transformations allow for this.

If we’re an MVNO with hosted IMSIs from an MNO, but want to keep just the one IMSI in our HSS/OCS, we can translate from the MNO hosted IMSI to our internal IMSI, using AVP transformations.

If our OCS supports only one rating group, and we want to rewrite all rating groups to that one value, AVP transformations cover this too.

There are lots of uses for this, and if you’ve worked with a bit of signaling before you’ll know that quite often these sorts of use-cases come up.

So how do we do this with freeDiameter?

To handle this I developed a module for passing each AVP to a Python function, which can then apply any transformation to a text based value, using every tool available to you in Python.

In the next post I’ll introduce rt_pyform and how we can use it with Python to translate Diameter AVPs.

Diameter Droplets – The Flow-Description AVP and IPFilterRules

EPC, LTE, Mobile Networks, RFCs & Standards, SDM, , , ,

When it comes to setting up dedicated bearers, the Flow-Description AVP is perhaps the most important,

The specially encoded string (IPFilterRule) in the FlowDescription AVP is what our P-GW (Ok, our PCEF) uses to create Traffic Flow Templates to steer certain types of traffic down Dedicated Bearers.

So let’s take a look at how we can lovingly craft an artisanal Flow-Description.

The contents of the AVP are technically not a string, but a IPFilterRule.

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

Which are in turn based loosely off the ipfw utility in BSD.

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) in most scenarios.

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,
For example to match a single port you could use 10.45.0.0/16 1234 to match anything on port 1234, but we can also specify ranges of ports like 10.45.0.0/16 0 – 4069 or even mix and match lists and single ports, like 10.45.0.0/16 5060, 1000-2000

Protip: using any is the same as 0.0.0.0/0

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!

Keep in mind that Flow-Descriptions are typically sent in pairs as a minimum, as you want to match the traffic into and out of the network (not just one way), but often there can be quite a few sent, in order to match all the possible traffic that needs to be matched that may be across multiple different subnets, etc.

There is an optional Options parameter that allows you to set things like to only apply the rule to open TCP sessions, fragmentation, etc, although I’ve not seen this implemented in the wild.

Example IP filter Rules

permit in 6 from 10.98.254.0/24 5061 to 10.98.0.0/24 5060
permit out 6 from 10.98.254.0/24 5060 to 10.98.0.0/24 5061

permit in 6 from any 80 to 172.16.1.1 80
permit out 6 from 172.16.1.1 80 to any 80

permit in 17 from 10.98.254.0/24 50000-60100 to 10.98.0.0/24 50000-60100
permit out 17 from 10.98.254.0/24 50000-60100 to 10.98.0.0/24 50000-60100

permit in 17 from 10.98.254.0/24 5061, 5064 to 10.98.0.0/24  5061, 5064
permit out 17 from 10.98.254.0/24 5061, 5064 to 10.98.0.0/24  5061, 5064

permit in 17 from 172.16.0.0/16 50000-60100, 5061, 5064 to 172.16.0.0/16  50000-60100, 5061, 5064
permit out 17 from 172.16.0.0/16 50000-60100, 5061, 5064 to 172.16.0.0/16  50000-60100, 5061, 5064

For more info see:

RFC 6773 – Diameter Base Protocol – IP Filter Rule

3GPP TS 29.214 section 5.3.8 Flow-Description AVP

MSISDN Encoding - Brought to you by the letter F

MSISDN Encoding in Diameter AVPs – Brought to you by the letter F

EPC, LTE, RFCs & Standards, , ,

So this one knocked me for six the other day,

MSISDN AVP 700 / vendor ID 10415, used to advertise the subscriber’s MSISDN in signaling.

I formatted the data as an Octet String, with the MSISDN from the database and moved on my merry way.

Not so fast…

The MSISDN AVP is of type OctetString.

This AVP contains an MSISDN, in international number format as described in ITU-T Rec E.164 [8], encoded as a TBCD-string, i.e. digits from 0 through 9 are encoded 0000 to 1001;

1111 is used as a filler when there is an odd number of digits; bits 8 to 5 of octet n encode digit 2n; bits 4 to 1 of octet n encode digit 2(n-1)+1.

ETSI TS 129 329 / 6.3.2 MSISDN AVP

Come again?

In practice this means if you have an odd lengthed MSISDN value, we need to add some padding to round it out to an even-lengthed value.

This padding happens between the last and second last digit of the MSISDN (because if we added it at the start we’d break the Country Code, etc) and as MSISDNs are variable length subscriber numbers.

1111 in octet string is best known as the letter F,

Not that complicated, just kind of confusing.

Diameter Dispatches – Origin-State-Id AVP

EUTRAN, LTE, Mobile Networks, RFCs & Standards, Software,

The Origin-State-Id AVP solves a kind of tricky problem – how do you know if a Diameter peer has restarted?

It seems like a simple problem until you think about it.
One possible solution would be to add an AVP for “Recently Rebooted”, to be added on the first command queried of it from an endpoint, but what if there are multiple devices connecting to a Diameter endpoint?

The Origin-State AVP is a strikingly simple way to solve this problem. It’s a constantly incrementing counter that resets if the Diameter peer restarts.

If a client receives a Answer/Response where the Origin-State AVP is set to 10, and then the next request it’s set to 11, then the one after that is set to 12, 13, 14, etc, and then a request has the Origin-State AVP set to 5, the client can tell when it’s restarted by the fact 5 is lower than 14, the one before it.

It’s a constantly incrementing counter, that allows Diameter peers to detect if the endpoint has restarted.

Simple but effective.

You can find more about this in RFC3588 – the Diameter Base Protocol.