One day recently I was messing with the XCAP server, trying to set the Call Forward timeout. In the process I triggered the UE to send a USSD request to the IMS.
Huh, I thought, “I wonder how hard it would be to build a USSD Gateway for our IMS?”, and this my friends, is the story of how I wasted a good chunk of my weekend trying (and failing) to add support for USSD.
You might be asking “Who still uses USSD?” – The use cases for USSD are pretty thin on the ground in this day and age, but I guess balance query, and uh…
But this is the story of what I tried before giving up and going outside…
Routing
First I’d need to get the USSD traffic towards the USSD Gateway, this means modifying iFCs. Skimming over the spec I can see the Recv-Info: header for USSD traffic should be set to “g.3gpp.ussd” so I knocked up an iFC to match that, and route the traffic to my dev USSD Gateway, and added it to the subscriber profile in PyHSS:
Easy peasy, now we have the USSD requests hitting our USSD Gateway.
The Response
I’ll admit that I didn’t jump straight to the TS doc from the start.
The first place I headed was Google to see if I could find any PCAPs of USSD over IMS/SIP.
And I did – Restcomm seems to have had a USSD product a few years back, and trawling around their stuff provided some reference PCAPs of USSD over SIP.
So the flow seemed pretty simple, SIP INVITE to set up the session, SIP INFO for in-dialog responses and a BYE at the end.
With all the USSD guts transferred as XML bodies, in a way that’s pretty easy to understand.
Being a Kamailio fan, that’s the first place I started, but quickly realised that SIP proxies, aren’t great at acting as the UAS.
So I needed to generate in-dialog SIP INFO messages, so I turned to the UAC module to generate the SIP INFO response.
My Kamailio code is super simple, but let’s have a look:
request_route {
xlog("Request $rm from $fU");
if(is_method("INVITE")){
xlog("USSD from $fU to $rU (Emergency number) CSeq is $cs ");
sl_reply("200", "OK Trying USSD Phase 1"); #Generate 200 OK
route("USSD_Response"); #Call USSD_Response route block
exit;
}
}
route["USSD_Response"]{
xlog("USSD_Response Route");
#Generate a new UAC Request
$uac_req(method)="INFO";
$uac_req(ruri)=$fu; #Copy From URI to Request URI
$uac_req(furi)=$tu; #Copy To URI to From URI
$uac_req(turi)=$fu; #Copy From URI to To URI
$uac_req(callid)=$ci; #Copy Call-ID
#Set Content Type to 3GPP USSD
$uac_req(hdrs)=$uac_req(hdrs) + "Content-Type: application/vnd.3gpp.ussd+xml\r\n";
#Set the USSD XML Response body
$uac_req(body)="<?xml version='1.0' encoding='UTF-8'?>
<ussd-data>
<language value=\"en\"/>
<ussd-string value=\"Bienvenido. Seleccione una opcion: 1 o 2.\"/>
</ussd-data>";
$uac_req(evroute)=1; #Set the event route to use on return replies
uac_req_send(); #Send it!
}
So the UAC module generates the 200 OK and sends it back.
“That was quick” I told myself, patting myself on the back before trying it out for the first time.
Huston, we have a problem – Although the Call-ID is the same, it’s not an in-dialog response as the tags aren’t present, this means our UE send back a 405 to the SIP INFO.
Right. Perhaps this is the time to read the Spec…
Okay, so the SIP INFO needs to be in dialog. Can we do that with the UAC module? Perhaps not…
But alas real life came back to rear its ugly head, and this adventure will have to continue another day…
Update: Thanks to a kindly provided PCAP I now know what I was doing wrong, and so we’ll soon have a follow up to this post named “Successes in cobbling together a USSD Gateway” just as soon as I have a weekend free.
So far in our lab we’ve got connectivity between to points, but we’re not carrying any useful data on top of it.
In the same way that TCP is great, but what makes it really useful is carrying application layers like HTTP on top, MTP3 exists to facilitate carrying higher-layer protocols, like ISUP, MAP, SCCP, etc, so let’s get some traffic onto our network.
ISUP is the ISDN User Part, ISUP is used to setup and teardown calls between two exchanges / SSPs – it’s the oldest and the most simple SS7 application to show off, so that’s what we’ll be working with today.
If you’ve not dealt much with ISDN in the past, then that’s OK – we’re not going to deep dive into all the nitty gritty of how ISDN Signaling works, but we’ll just skim the surface to showing how SS7/Sigtran transports the ISUPpackets. So you can see how SS7 is used to transport this protocol.
You can think of it a lot like SIP, which is if not the child of ISUP, then it at least bares a striking resemblance.
So let’s look at an ISUP call flow:
The call is initiated with an Initial Address Message (IAM), akin to a SIP INVITE, sent by the SSP/Exchange of the calling party to the SSP/Exchange of the called party. When the remote party starts to ring, the remote exchange sends an Address Complete (ACM), which is similar to a 100 TRYING in SIP. Once the remote party answers, the remote exchange sends back an Answer Message (ANM), and our call starts, just like a 200 OK.
Rather than SDP for transferring media, timeslots or predefined channels / circuits are defined, each identified by a number, which both sides will use for the media path.
Finally whichever side terminates the call will send a Release (REL) message, which is confirmed with the Release Complete (RLC).
I told you we’d be quick!
So that’s the basics of ISUP, in our next post we’ll do some PCAP analysis on real world ISUP flows!
Recently I’ve been working on open source Diameter Routing Agent implementations (See my posts on FreeDiameter).
With the hurdles to getting a DRA working with open source software covered, the next step was to get all my Diameter traffic routed via the DRAs, however I soon rediscovered a Kamailio limitation regarding support for Diameter Routing Agents.
You see, when Kamailio’s C Diameter Peer module makes a decision as to where to route a request, it looks for the active Diameter peers, and finds a peer with the suitable Vendor and Application IDs in the supported Applications for the Application needed.
Unfortunately, a DRA typically only advertises support for one application – Relay.
This means if you have everything connected via a DRA, Kamailio’s CDP module doesn’t see the Application / Vendor ID for the Diameter application on the DRA, and doesn’t route the traffic to the DRA.
The fix for this was twofold, the first step was to add some logic into Kamailio to determine if the Relay application was advertised in the Capabilities Exchange Request / Answer of the Diameter Peer.
I added the logic to do this and exposed this so you can see if the peer supports Diameter relay when you run “cdp.list_peers”.
With that out of the way, next step was to update the routing logic to not just reject the candidate peer if the Application / Vendor ID for the required application was missing, but to evaluate if the peer supports Diameter Relay, and if it does, keep it in the game.
I added this functionality, and now I’m able to use CDP Peers in Kamailio to allow my P-CSCF, S-CSCF and I-CSCF to route their traffic via a Diameter Routing Agent.
Something that’s kind of great is that the current generation of Ericsson RRUs and Nokia RRUs, use the same power connector – The Amphenol “Amphe-OBTS” series connector.
Construction and wiring of these connectors is the same for both, and with one little trick, we can use the connector for both Ericsson and Nokia RRUs (Airscale and later).
The connectors are not quite universal, in order to use it in both you need to knock off a small pin on the connector, I’d suggest doing this before you assemble it, put the connector on it’s back, facing upwards, and hit this with a screwdriver / chisel and it’ll pop off with very little effort.
Assembling the connectors starts by working out the diameter of the grommet you need to fit your cable, the connector comes with the grommet for 9-14mm, but in the bag you’ll usually get grommets for 6-9mm cable and 14-18mm cable.
Grab the correct one for your cable diameter, and pop into the black fingered cage (‘gland adapter’) shown in the bottom right of the below photo.
Next we line all the parts up along the cable and screw it all together:
The end-cap is actually very useful for stopping the female end of the connector from spinning when you’re assembling the cable, so don’t throw it away!
Next we’ll need to define our rt_pyform config, this is a super simple 3 line config file that specifies the path of what we’re doing:
DirectoryPath = "." # Directory to search
ModuleName = "script" # Name of python file. Note there is no .py extension
FunctionName = "transform" # Python function to call
The DirectoryPath directive specifies where we should search for the Python code, and ModuleName is the name of the Python script, lastly we have FunctionName which is the name of the Python function that does the rewriting.
Now let’s write our Python function for the transformation.
The Python function much have the correct number of parameters, must return a string, and must use the name specified in the config.
The following is an example of a function that prints out all the values it receives:
Note the order of the arguments and that return is of the same type as the AVP value (string).
We can expand upon this and add conditionals, let’s take a look at some more complex examples:
def transform(appId, flags, cmdCode, HBH_ID, E2E_ID, AVP_Code, vendorID, value):
print('[PYTHON]')
print(f'|-> appId: {appId}')
print(f'|-> flags: {hex(flags)}')
print(f'|-> cmdCode: {cmdCode}')
print(f'|-> HBH_ID: {hex(HBH_ID)}')
print(f'|-> E2E_ID: {hex(E2E_ID)}')
print(f'|-> AVP_Code: {AVP_Code}')
print(f'|-> vendorID: {vendorID}')
print(f'|-> value: {value}')
#IMSI Translation - if App ID = 16777251 and the AVP being evaluated is the Username
if (int(appId) == 16777251) and int(AVP_Code) == 1:
print("This is IMSI '" + str(value) + "' - Evaluating transformation")
print("Original value: " + str(value))
value = str(value[::-1]).zfill(15)
The above look at if the App ID is S6a, and the AVP being checked is AVP Code 1 (Username / IMSI ) and if so, reverses the username, so IMSI 1234567 becomes 7654321, the zfill is just to pad with leading 0s if required.
Now let’s do another one for a Realm Rewrite:
def transform(appId, flags, cmdCode, HBH_ID, E2E_ID, AVP_Code, vendorID, value):
#Print Debug Info
print('[PYTHON]')
print(f'|-> appId: {appId}')
print(f'|-> flags: {hex(flags)}')
print(f'|-> cmdCode: {cmdCode}')
print(f'|-> HBH_ID: {hex(HBH_ID)}')
print(f'|-> E2E_ID: {hex(E2E_ID)}')
print(f'|-> AVP_Code: {AVP_Code}')
print(f'|-> vendorID: {vendorID}')
print(f'|-> value: {value}')
#Realm Translation
if int(AVP_Code) == 283:
print("This is Destination Realm '" + str(value) + "' - Evaluating transformation")
if value == "epc.mnc001.mcc001.3gppnetwork.org":
new_realm = "epc.mnc999.mcc999.3gppnetwork.org"
print("translating from " + str(value) + " to " + str(new_realm))
value = new_realm
else:
#If the Realm doesn't match the above conditions, then don't change anything
print("No modification made to Realm as conditions not met")
print("Updated Value: " + str(value))
In the above block if the Realm is set to epc.mnc001.mcc001.3gppnetwork.org it is rewritten to epc.mnc999.mcc999.3gppnetwork.org, hopefully you can get a handle on the sorts of transformations we can do with this – We can translate any string type AVPs, which allows for hostname, realm, IMSI, Sh-User-Data, Location-Info, etc, etc, to be rewritten.
NB-IoT introduces support for NIDD – Non-IP Data Delivery (NIDD) which is one of the cool features of NB-IoT that’s gaining more widespread adoption.
Let’s take a deep dive into NIDD.
The case against IP for IoT
In the over 40 years since IP was standardized, we’ve shoehorned many things onto IP, but IP was never designed or optimized for low power, low throughput applications.
For the battery life of an IoT device to be measured in years, it has to be very selective about what power hungry operations it does. Transmitting data over the air is one of the most power-intensive operations an IoT device can perform, so we need to do everything we can to limit how much data is sent, and how frequently.
Use Case – NB-IoT Tap
Let’s imagine we’re launching an IoT tap that transmits information about water used, as part of our revolutionary new “Water as a Service” model (WaaS) which removes the capex for residents building their own water treatment plant in their homes, and instead allows dynamic scaling of waterloads as they move to our new opex model.
If I turn on the tap and use 12L of water, when I turn off the tap, our IoT tap encodes the usage onto a single byte and sends the usage information to our rain-cloud service provider.
So we’re not constantly changing the batteries in our taps, we need to send this one byte of data as efficiently as possible, so as to maximize the battery life.
If we were to transport our data on TCP, well we’d need a 3 way handshake and several messages just to transmit the data we want to send.
Let’s see how our one byte of data would look if we transported it on TCP.
That sliver of blue in the diagram is our usage component, the rest is overhead used to get it there. Seems wasteful huh?
Sure, TCP isn’t great for this you say, you should use UDP! But even if we moved away from TCP to UDP, we’ve still got the IPv4 header and the UDP header wasting 28 bytes.
For efficiency’s sake (To keep our batteries lasting as long as possible) we want to send as few messages as possible, and where we do have to send messages, keep them very short, so IP is not a great fit here.
Enter NIDD – Non-IP Data Delivery.
Through NIDD we can just send the single hex byte, only be charged for the single hex byte, and only stay transmitting long enough to send this single byte of hex (Plus the NBIoT overheads / headers).
In summary – the more sending your device has to do, the more battery it consumes. So in a scenario where you’re trying to maximize power efficiency to keep your batter powered device running as long as possible, needing to transmit 28 bytes of wasted data to transport 1 byte of usable data, is a real waste.
Delivering the Payload
NIDD traffic is transported as raw hex data end to end, this means for our 1 byte of water usage data, the device would just send the hex value to be transferred and it’d pop out the other end.
To support this we introduce a new network element called the SCEF –Service Capability Exposure Function.
From a developer’s perspective, the SCEF is the gateway to our IoT devices. Through the RESTful API on the SCEF (T8 API), we can send and receive raw hex data to any of our IoT devices.
When one of our Water-as-a-Service Taps sends usage data as a hex byte, it’s the software talking on the T8 API to the SCEF that receives this data.
Data of course needs to be addressed, so we know where it’s coming from / going to, and T8 handles this, as well as message reliability, etc, etc.
This is a telco blog, so we should probably cover the MME connection, the MME talks via Diameter to the SCEF. In our next post we’ll go into these signaling flows in more detail.
If you’re wondering what the status of Open Source SCEF implementations are, then you may have already guessed I’m working on one!
Hopefully by now you’ve got a bit of an idea of how NIDD works in NB-IoT, and in our next posts we’ll dig deeper into the flows and look at some PCAPs together.
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.
Way back in part 2 we discussed the basic routing logic a DRA handles, but what if we want to do something a bit outside of the box in terms of how we route?
For me, one of the most useful use cases for a DRA is to route traffic based on IMSI / Username. This means I can route all the traffic for MVNO X to MVNO X’s HSS, or for staging / test subs to the test HSS enviroment.
FreeDiameter has a bunch of built in logic that handles routing based on a weight, but we can override this, using the rt_default module.
In our last post we had this module commented out, but let’s uncomment it and start playing with it:
In the above code we’ve uncommented rt_default and rt_redirect.
You’ll notice that rt_default references a config file, so we’ll create a new file in our /etc/freeDiameter directory called rt_default.conf, and this is where the magic will happen.
A few points before we get started:
This overrides the default routing priorities, but in order for a peer to be selected, it has to be in an Open (active) state
The peer still has to have advertised support for the requested application in the CER/CEA dialog
The peers will still need to have all been defined in the freeDiameter.conf file in order to be selected
So with that in mind, and the 5 peers we have defined in our config above (assuming all are connected), let’s look at some rules we can setup using rt_default.
Intro to rt_default Rules
The rt_default.conf file contains a list of rules, each rule has a criteria that if matched, will result in the specified action being taken. The actions all revolve around how to route the traffic.
So what can these criteria match on? Here’s the options:
Item to Match
Code
Any
*
Origin-Host
oh=”STR/REG”
Origin-Realm
or=”STR/REG”
Destination-Host
dh=”STR/REG”
Destination-Realm
dr=”STR/REG”
User-Name
un=”STR/REG”
Session-Id
si=”STR/REG”
rt_default Matching Criteria
We can either match based on a string or a regex, for example, if we want to match anything where the Destination-Realm is “mnc001.mcc001.3gppnetwork.org” we’d use something like:
#Low score to HSS02
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss02" += -70 ;
Now you’ll notice there is some stuff after this, let’s look at that.
We’re matching anything where the destination-host is set to hss02 (that’s the bit before the colon), but what’s the bit after that?
Well if we imagine that all our Diameter peers are up, when a message comes in with Destination-Realm “mnc001.mcc001.3gppnetwork.org”, looking for an HSS, then in our example setup, we have 4 HHS instances to choose from (assuming they’re all online).
In default Diameter routing, all of these peers are in the same realm, and as they’re all HSS instances, they all support the same applications – Our request could go to any of them.
But what we set in the above example is simply the following:
If the Destination-Realm is set to mnc001.mcc001.3gppnetwork.org, then set the priority for routing to hss02 to the lowest possible value.
So that leaves the 3 other Diameter peers with a higher score than HSS02, so HSS02 won’t be used.
Let’s steer this a little more,
Let’s specify that we want to use HSS01 to handle all the requests (if it’s available), we can do that by adding a rule like this:
#Low score to HSS02
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss02" += -70 ;
#High score to HSS01
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss01" += 100 ;
But what if we want to route to hss-lab if the IMSI matches a specific value, well we can do that too.
#Low score to HSS02
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss02" += -70 ;
#High score to HSS01
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss01" += 100 ;
#Route traffic for IMSI to Lab HSS
un="001019999999999999" : dh="hss-lab" += 200 ;
Now that we’ve set an entry with a higher score than hss01 that will be matched if the username (IMSI) equals 001019999999999999, the traffic will get routed to hss-lab.
But that’s a whole IMSI, what if we want to match only part of a field?
Well, we can use regex in the Criteria as well, so let’s look at using some Regex, let’s say for example all our MVNO SIMs start with 001012xxxxxxx, let’s setup a rule to match that, and route to the MVNO HSS with a higher priority than our normal HSS:
#Low score to HSS02
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss02" += -70 ;
#High score to HSS01
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss01" += 100 ;#Route traffic for IMSI to Lab HSS
un="001019999999999999" : dh="hss-lab" += 200 ;
#Route traffic where IMSI starts with 001012 to MVNO HSS
un=["^001012.*"] : dh="hss-mvno-x" += 200 ;
Let’s imagine that down the line we introduce HSS03 and HSS04, and we only want to use HSS01 if HSS03 and HSS04 are unavailable, and only to use HSS02 no other HSSes are available, and we want to split the traffic 50/50 across HSS03 and HSS04.
Firstly we’d need to add HSS03 and HSS04 to our FreeDiameter.conf file:
Then in our rt_default.conf we’d need to tweak our scores again:
#Low score to HSS02
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss02" += 10 ;
#Medium score to HSS01
dr="mnc001.mcc001.3gppnetwork.org" : dh="hss01" += 20 ;
#Route traffic for IMSI to Lab HSS
un="001019999999999999" : dh="hss-lab" += 200 ;
#Route traffic where IMSI starts with 001012 to MVNO HSS
un=["^001012.*"] : dh="hss-mvno-x" += 200 ;
#High Score for HSS03 and HSS04dr="mnc001.mcc001.3gppnetwork.org" : dh="hss02" += 100 ;dr="mnc001.mcc001.3gppnetwork.org" : dh="hss04" += 100 ;
One quick tip to keep your logic a bit simpler, is that we can set a variety of different values based on keywords (listed below) rather than on a weight/score:
Behaviour
Name
Score
Do not deliver to peer (set lowest priority)
NO_DELIVERY
-70
The peer is a default route for all messages
DEFAULT
5
The peer is a default route for this realm
DEFAULT_REALM
10
REALM
15
Route to the specified Host with highest priority
FINALDEST
100
Rather than manually specifying the store you can use keywords like above to set the value
In our next post we’ll look at using FreeDiameter based DRA in roaming scenarios where we route messages across Diameter Realms.
FreeDiameter has been around for a while, and we’ve covered configuring the FreeDiameter components in Open5GS when it comes to the S6a interface, so you may have already come across FreeDiameter in the past, but been left a bit baffled as to how to get it to actually do something.
FreeDiameter is a FOSS implimentation of the Diameter protocol stack, and is predominantly used as a building point for developers to build Diameter applications on top of.
But for our scenario, we’ll just be using plain FreeDiameter.
So let’s get into it,
You’ll need FreeDiameter installed, and you’ll need a certificate for your FreeDiameter instance, more on that in this post.
Once that’s setup we’ll need to define some basics,
Inside freeDiameter.conf we’ll need to include the identity of our DRA, load the extensions and reference the certificate files:
What I typically refer to as Diameter interfaces / reference points, such as S6a, Sh, Sx, Sy, Gx, Gy, Zh, etc, etc, are also known as Applications.
Diameter Application Support
If you look inside the Capabilities Exchange Request / Answer dialog, what you’ll see is each side advertising the Applications (interfaces) that they support, each one being identified by an Application ID.
If two peers share a common Application-Id, then they can communicate using that Application / Interface.
For example, the above screenshot shows a peer with support for the Zh Interface (Spoiler alert, XCAP Gateway / BSF coming soon!). If two Diameter peers both have support for the Zh interface, then they can use that to send requests / responses to each other.
This is the basis of Diameter Routing.
Diameter Routing Tables
Like any router, our DRA needs to have logic to select which peer to route each message to.
For each Diameter connection to our DRA, it will build up a Diameter Routing table, with information on each peer, including the realm and applications it advertises support for.
Then, based on the logic defined in the DRA to select which Diameter peer to route each request to.
In its simplest form, Diameter routing is based on a few things:
Look at the DestinationRealm, and see if we have any peers at that realm
If we do then look at the DestinationHost, if that’s set, and the host is connected, and if it supports the specified Application-Id, then route it to that host
If no DestinationHost is specified, look at the peers we have available and find the one that supports the specified Application-Id, then route it to that host
With this in mind, we can go back to looking at how our DRA may route a request from a connected MME towards an HSS.
Let’s look at some examples of this at play.
The request from MME02 is for DestinationRealm mnc001.mcc001.3gppnetwork.org, which our DRA knows it has 4 connected peers in (3 if we exclude the source of the request, as we don’t want to route it back to itself of course).
So we have 3 contenders still for who could get the request, but wait! We have a DestinationHost specified, so the DRA confirms the host is available, and that it supports the requested ApplicationId and routes it to HSS02.
So just because we are going through a DRA does not mean we can’t specific which destination host we need, just like we would if we had a direct link between each Diameter peer.
Conversely, if we sent another S6a request from MME01 but with no DestinationHost set, let’s see how that would look.
Again, the request is from MME02 is for DestinationRealm mnc001.mcc001.3gppnetwork.org, which our DRA knows it has 3 other peers it could route this to. But only two of those peers support the S6a Application, so the request would be split between the two peers evenly.
Clever Routing with DRAs
So with our DRA in place we can simplify the network, we don’t need to build peer links between every Diameter device to every other, but let’s look at some other ways DRAs can help us.
Load Control
We may want to always send requests to HSS01 and only use HSS02 if HSS01 is not available, we can do this with a DRA.
Or we may want to split load 75% on one HSS and 25% on the other.
Both are great use cases for a DRA.
Routing based on Username
We may want to route requests in the DRA based on other factors, such as the IMSI.
Our IMSIs may start with 001010001xxx, but if we introduced an MVNO with IMSIs starting with 001010002xxx, we’d need to know to route all traffic where the IMSI belongs to the home network to the home network HSS, and all the MVNO IMSI traffic to the MVNO’s HSS, and DRAs handle this.
Inter-Realm Routing
One of the main use cases you’ll see for DRAs is in Roaming scenarios.
For example, if we have a roaming agreement with a subscriber who’s IMSIs start with 90170, we can route all the traffic for their subs towards their HSS.
But wait, their Realm will be mnc901.mcc070.3gppnetwork.org, so in that scenario we’ll need to add a rule to route the request to a different realm.
DRAs handle this also.
In our next post we’ll start actually setting up a DRA with a default route table, and then look at some more advanced options for Diameter routing like we’ve just discussed.
One slight caveat, is that mutual support does not always mean what you may expect. For example an MME and an HSS both support S6a, which is identified by Auth-Application-Id 16777251 (Vendor ID 10415), but one is a client and one is a server. Keep this in mind!
Answer Question 1: Because they make things simpler and more flexible for your Diameter traffic. Answer Question 2: With free software of course!
All about DRAs
But let’s dive a little deeper. Let’s look at the connection between an MME and an HSS (the S6a interface).
We configure the Diameter peers on MME1 and HSS01 so they know about each other and how to communicate, the link comes up and presto, away we go.
But we’re building networks here! N+1 redundancy and all that, so now we have two HSSes and two MMEs.
Okay, bit messy, but that’s okay…
But then our network grows to 10 MMEs, and 3 HSSes and you can probably see where this is going, but let’s drive the point home.
Now imagine once you’ve set all this up you need to do some maintenance work on HSS03, so need to shut down the Diameter peer on 10 different MMEs in order to isolate it and deisolate it.
The problem here is pretty evident, all those links are messy, cumbersome and they just don’t scale.
If you’re someone with a bit of networking experience (and let’s face it, you’re here after all), then you’re probably thinking “What if we just had a central system to route all the Diameter messages?”
An Agent that could Route Diameter, a Diameter Routing Agent perhaps…
By introducing a DRA we build Diameter peer links between each of our Diameter devices (MME / HSS, etc) and the DRA, rather than directly between each peer.
Then from the DRA we can route Diameter requests and responses between them.
Let’s go back to our 10x MME and 3x HSS network and see how it looks with a DRA instead.
So much cleaner!
Not only does this look better, but it makes our life operating the network a whole lot easier.
Each MME sends their S6a traffic to the DRA, which finds a healthy HSS from the 3 and sends the requests to it, and relays the responses as well.
We can do clever load balancing now as well.
Plus if a peer goes down, the DRA detects the failure and just routes to one of the others.
If we were to introduce a new HSS, we wouldn’t need to configure anything on the MMEs, just add HSS04 to the DRA and it’ll start getting traffic.
Plus from an operations standpoint, now if we want to to take an HSS offline for maintenance, we just shut down the link on the HSS and all HSS traffic will get routed to the other two HSS instances.
In our next post we’ll talk about the Routing part of the DRA, how the decisions are made and all the nuances, and then in the following post we’ll actually build a DRA and start routing some traffic around!
If you work with IMS or Packet Core, there’s a good chance you need DNS to work, and it doesn’t always.
When I run traces, I’ve always found I get swamped with DNS traffic, UE traffic, OS monitoring, updates, etc, all combine into a big firehose – while my Wireshark filters for finding EPC and IMS traffic is pretty good, my achilles heel has always been filtering the DNS traffic to just get the queries and responses I want out of it.
Well, today I made that a bit better.
By adding this to your Wireshark filter:
dns contains 33:67:70:70:6e:65:74:77:6f:72:6b:03:6f:72:67:00
You’ll only see DNS Queries and Responses for domains at the 3gppnetwork.org domain.
This makes my traces much easier to read, and hopefully will do the same for you!
Bonus, here’s my current Wireshark filter for working EPC/IMS:
(diameter and diameter.cmd.code != 280) or (sip and !(sip.Method == "OPTIONS") and !(sip.CSeq.method == "OPTIONS")) or (smpp and (smpp.command_id != 0x00000015 and smpp.command_id != 0x80000015)) or (mgcp and !(mgcp.req.verb == "AUEP") and !(mgcp.rsp.rspcode == 500)) or isup or sccp or rtpevent or s1ap or gtpv2 or pfcp or (dns contains 33:67:70:70:6e:65:74:77:6f:72:6b:03:6f:72:67:00)
Even if you’re not using TLS in your FreeDiameter instance, you’ll still need a certificate in order to start the stack.
Luckily, creating a self-signed certificate is pretty simple,
Firstly we generate your a private key and public certificate for our required domain – in the below example I’m using dra01.epc.mnc001.mcc001.3gppnetwork.org, but you’ll need to replace that with the domain name of your freeDiameter instance.
If you’re using freeDiameter as part of another software stack (Such as Open5Gs) the below filenames will contain the config for that particular freeDiameter components of the stack:
The Wiki on the Sangoma documentation page is really out of date and can’t be easily edited by the public, so here’s the skinny on how to setup a Sangoma transcoding card on a modern Debian system:
apt-get install libxml2* wget make gcc
wget https://ftp.sangoma.com/linux/transcoding/sng-tc-linux-1.3.11.x86_64.tgz
tar xzf sng-tc-linux-1.3.11.x86_64.tgz
cd sng-tc-linux-1.3.11.x86_64/
make
make install
cp lib/* /usr/local/lib/
ldconfig
At this point you should be able to check for the presence of the card with:
sngtc_tool -dev ens33 -list_modules
Where ens33 is the name of the NIC that the server that shares a broadcast domain with the transcoder.
Successfully discovering the Sangoma D150 transcoder
If instead you see something like this:
root@fs-131:/etc/sngtc# sngtc_tool -dev ens33 -list_modules
Failed to detect and initialize modules with size 1
That means the server can’t find the transcoding device. If you’re using a D150 (The Ethernet enabled versions) then you’ve got to make sure that the NIC you specified is on the same VLAN / broadcast domain as the server, for testing you can try directly connecting it to the NIC.
I also found I had to restart the device a few times to get it to a “happy” state.
It’s worth pointing out that there are no LEDs lit when the system is powered on, only when you connect a NIC.
Next we’ll need to setup the sngtc_server so these resources can be accessed via FreeSWITCH or Asterisk.
Config is pretty simple if you’re using an all-in-one deployment, all you’ll need to change is the NIC in a file you create in /etc/sngtc/sngtc_server.conf.xml:
<configuration name="sngtc_server.conf" description="Sangoma Transcoding Manager Configuration">
<settings>
<!--
By default the SOAP server uses a private local IP and port that will work for out of the box installations
where the SOAP client (Asterisk/FreeSWITCH) and server (sngtc_server) run in the same box.
However, if you want to distribute multiple clients across the network, you need to edit this values to
listen on an IP and port that is reachable for those clients.
<param name="bindaddr" value="0.0.0.0" />
<param name="bindport" value="9000" />
-->
</settings>
<vocallos>
<!-- The name of the vocallo is the ethernet device name as displayed by ifconfig -->
<vocallo name="ens33">
<!-- Starting UDP port for the vocallo -->
<param name="base_udp" value="5000"/>
<!-- Starting IP address octet to use for the vocallo modules -->
<param name="base_ip_octet" value="182"/>
</vocallo>
</vocallos>
</configuration>
With that set we can actually try starting the server,
Again, all going well you should see something like this in the log:
Well, there’s another concept I haven’t introduced yet, and that’s ChargerS, this is a concept / component we’ll dig into deeper for derived charging, but for now just know we need to add a ChargerS rule in order to get CDRs rated:
Well, if you’ve got CDR storage in StoreDB enabled (And you probably do if you’ve been following up until this point), then the answer is a MySQL table, and we can retrive the data with:
sudo mysql cgrates -e "select * from cdrs \G"
For those of you with a bit of MySQL experience under your belt, you’d be able to envisage using the SUM function to total a monthly bill for a customer from this.
Of course we can add CDRs via the API, and you probably already guessed this, but we can retrive CDRs via the API as well, filtering on the key criteria:
This would be useful for generating an invoice or populating recent calls for a customer portal.
Maybe creating rated CDRs and sticking them into a database is exactly what you’re looking to achieve in CGrateS – And if so, great, this is where you can stop – but for many use cases, there’s a want for an automated solution – For your platform to automatically integrate with CGrateS.
If you’ve got an Asterisk/FreeSWITCH/Kamailio or OpenSIPs based platform, then you can integrate CGrateS directly into your platform to add the CDRs automatically, as well as access features like prepaid credit control, concurrent call limits, etc, etc. The process is a little different on each of these platforms, but ultimately under the hood, all of these platforms have some middleware that generates the same API calls we just ran to create the CDR.
So far this tutorial has been heavy on teaching the API, because that’s what CGrateS ultimately is – An API service.
Our platforms like Asterisk and Kamailio with the CGrateS plugins are just CGrateS API clients, and so once we understand how to use and interact with the API it’s a breeze to plug in the module for your platform to generate the API calls to CGrateS required to integrate.
I build phone networks, and unfortunately, I’m not able to be everywhere at once.
This means sometimes I have to test things in networks I may not be within the coverage of.
To get around this, I’ve setup something pretty simple, but also pretty powerful – Remote test phones.
Using a Raspberry Pi, Intel NUC, or any old computer, I’m able to remotely control Android handsets out in the field, in the coverage footprint of whatever network I need.
This means I can make test calls, run speed testing, signal strength measurements, on real phones out in the network, without leaving my office.
Base OS
Because of some particularities with Wayland and X11, for this I’d steer clear of Ubuntu distributions, and suggest using Debian if you’re using x86 hardware, and Raspbian if you’re using a Pi.
Setup Android Debug Bridge (adb)
The base of this whole system is ADB, the Android Debug Bridge, which exposes the ability to remotely control an Android phone over USB.
You can also do this over WiFi, but I find for device testing, wired allows me to airplane mode a device or disable data, which I can’t do if the device is connected to ADB via WiFi.
There’s lot of info online about setting Android Debug Bridge up on your device, unlocking the Developer Mode settings, etc, if you’ve not done this before I’ll just refer you to the official docs.
Before we plug in the phones we’ll need to setup the software on our remote testing machine, which is simple enough:
Now we can plug in each of the remote phones we want to use for testing and run the command “adb devices” which should list the phones with connected to the machine with ADB enabled:
[email protected]:~$ adb devices
List of devices attached
ABCDEFGHIJK unauthenticated
LMNOPQRSTUV unauthenticated
You’ll get a popup on each device asking if you want to allow USB debugging – If this is going to be a set-and-forget deployment, make sure you tick “Always allow from this Computer” so you don’t have to drive out and repeat this step, and away you go.
Lastly we can run adb devices again to confirm everything is in the connected state
Scrcpy
scrcpy an open-source remote screen mirror / controller that allows us to control Android devices from a computer.
In our case we’re going to install with Snap (if you hate snaps as many folks do, you can also compile from source):
After SSHing into the box, we can just run scrcpy and boom, there’s the window we can interact with.
If you’ve got multiple devices connected to the same device, you’ll need to specify the ADB device ID, and of course, you can have multiple sessions open at the same time.
scrcpy -s 61771fe5
That’s it, as simple as that.
Tweaking
A few settings you may need to set:
I like to enable the “Show taps” option so I can see where my mouse is on the touchscreen and see what I’ve done, it makes it a lot easier when recording from the screen as well for the person watching to follow along.
You’ll probably also want to disable the lock screen and keep the screen awake
Some OEMs have an additonal tick box if you want to be able to interact with the device (rather than just view the screen), which often requires signing into an account, if you see this toggle, you’ll need to turn it on:
Ansible Playbook
I’ve had to build a few of these, so I’ve put an Ansible Playbook on Github so you can create your own.
In our last post we introduced the CGrateS API and we used it to add Rates, Destinations and define DestinationRates.
In this post, we’ll create the RatingPlan that references the DestinationRate we just defined, and the RatingProfile that references the RatingPlan, and then, as the cherry on top – We’ll rate some calls.
For anyone looking at the above diagram for the first time, you might be inclined to ask why what is the purpose of having all these layers?
This layered architecture allows all sorts of flexibility, that we wouldn’t otherwise have, for example, we can have multiple RatingPlans defined for the same Destinations, to allow us to have different Products defined, with different destinations and costs.
Likewise we can have multiple RatingProfiles assigned for the same destinations to allow us to generate multiple CDRs for each call, for example a CDR to bill the customer with and a CDR with our wholesale cost.
All this flexibility is enabled by the layered architecture.
Define RatingPlan
Picking up where we left off having just defined the DestinationRate, we’ll need to create a RatingPlan and link it to the DestinationRate, so let’s check on our DestinationRates:
From the output we can see we’ve got the DestinationRate defined, there’s a lot of info returned (I’ve left out most of it), but you can see the Destination, and the Rate associated with it is returned:
So after confirming that our DestinationRates are there, we’ll create a RatingPlan to reference it, for this we’ll use the APIerSv1.SetTPRatingPlan API call.
In our basic example, this really just glues the DestinationRate_AU object to RatingPlan_VoiceCalls.
It’s worth noting that you can use a RatingPlan to link to multiple DestinationRates, for example, we might want to have a different RatingPlan for each region / country, we can do that pretty easily too, in the below example I’ve referenced other Destination Rates (You’d go about defining the DestinationRates for these other destinations / rates the same way as we did in the last example).
One last step before we can test this all end-to-end, and that’s to link the RatingPlan we just defined with a RatingProfile.
StorDB & DataDB
Psych! Before we do that, I’m going to subject you to learning about backends for a while.
So far we’ve skirted around CGrateS architecture, but this is something we need to know for now.
To keep everything fast, a lot of data is cached in what is called a DataDB (if you’ve followed since part 1, then your DataDB is Redis, but there are other options).
To keep everything together, databases are used for storage, called StorDB (in our case we are using MySQL, but again, we can have other options) but calls to this database are minimal to keep the system fast.
If you’re an astute reader, you may have noticed many of our API calls have TP in method name, if the API call has TP in the name, it is storing it in the StoreDB, if it doesn’t, it means it’s storing it only in DataDB.
Why does this matter? Well, let’s look a little more closely and it will become clear:
ApierV1.SetRatingProfile will set the data only in DataDB (Redis), because it’s in the DataDB the change will take effect immediately.
ApierV1.SetTPRatingProfile will set the data only in StoreDB (MySQL), it will not take effect until it is copied from the database (StoreDB) to the cache (DataDB).
After we define the RatingPlan, we need to run this command prior to creating the RatingProfile, so it has something to reference, so we’ll do that by adding:
The last piece of the puzzle to define is the RatingProfile.
We define a few key things in the rating profile:
The Tenant – CGrateS is multitenant out of the box (in our case we’ve used tenant named “cgrates.org“, but you could have different tenants for different customers).
The Category – As we covered in the first post, CGrateS can bill voice calls, SMS, MMS & Data consumption, in this scenario we’re billing calls so we have the value set to *call, but we’ve got many other options. We can use Category to link what RatingPlan is used, for example we might want to offer a premium voice service with guaranteed CLI rates, using a different RatingPlan that charges more per call, or maybe we’re doing mobile and we want a different RatingPlan for use when Roaming, we can use Category to switch that.
The Subject – This is loosely the Source / Calling Party; in our case we’re using a wildcard value *any which will match any Subject
The RatingPlanActivations list the RatingPlanIds of the RatingPlans this RatingProfile uses
So let’s take a look at what we’d run to add this:
Okay, so at this point, all going well, we should have some data loaded, we’ve gone through all those steps to load this data, so now let’s simulate a call to a Mobile Number (22c per minute) for 123 seconds.
We cheated a fair bit, to show something that worked, but it’s not something you’d probably want to use in real life, loading static CSV files gets us off the ground, but in reality we don’t want to manage a system through CSV files.
Instead, we’d want to use an API.
Fair warning – There is some familiarity expected with JSON and RESTful APIs required, we’ll use Python3 for our examples, but you can use any programing language you’re comfortable with, or even CURL commands.
So we’re going to start by clearing out all the data we setup in CGrateS using the cgr-loader tool from those imported CSVs:
redis-cli flushall
sudo mysql -Nse 'show tables' cgrates | while read table; do sudo mysql -e "truncate table $table" cgrates; done
cgr-migrator -exec=*set_versions -stordb_passwd=CGRateS.org
sudo systemctl restart cgrates
So what have we just done? Well, we’ve just cleared all the data in CGrateS. We’re starting with a blank slate.
In this post, we’re going to define some Destinations, some Rates to charge and then some DestinationRates to link each Destination to a Rate.
But this time we’ll be doing this through the CGrateS API.
Introduction to the CGrateS API
CGrateS is all API driven – so let’s get acquainted with this API.
I’ve written a simple Python wrapper you can find here that will make talking to CGRateS a little easier, so let’s take it for a spin and get the Destinations that are loaded into our system:
import cgrateshttpapi
CGRateS_Obj = cgrateshttpapi.CGRateS('172.16.41.133', 2080) #Replace this IP with the IP Address of your CGrateS instance...
destinations = CGRateS_Obj.SendData({'method':'ApierV1.GetTPDestinationIDs','params':[{"TPid":"cgrates.org"}]})['result']
#Pretty print the result:
print("Destinations: ")
pprint.pprint(destinations)
All going well you’ll see something like this back:
Initializing with host 172.16.41.133 on port 2080
Sending Request with Body:
{'method': 'ApierV2.Ping', 'params': [{'Tenant': 'cgrates.org'}]}
Sending Request with Body:
{'method': 'ApierV2.GetTPDestinationIDs', 'params': [{"TPid":"cgrates.org"}]}
Destinations from CGRates: []
So what did we just do? Well, we sent a JSON formatted string to the CGRateS API at 172.16.41.133 on port 2080 – You’ll obviously need to change this to the IP of your CGrateS instance.
In the JSON body we sent we asked for all the Destinations using the ApierV1.GetTPDestinationIDs method, for the TPid ‘cgrates.org’,
And it looks like no destinations were sent back, so let’s change that!
Note: There’s API Version 1 and API Version 2, not all functions exist in both (at least not in the docs) so you have to use a mix.
Adding Destinations via the API
So now we’ve got our API setup, let’s see if we can add a destination!
To add a destination, we’ll need to go to the API guide and find the API call to add a destination – in our case the API call is ApierV2.SetTPDestination and will look like this:
So we’re creating a Destination named Dest_AU_Mobile and Prefix 614 will match this destination.
Note: I like to prefix all my Destinations with Dest_, all my rates with Rate_, etc, so it makes it easy when reading what’s going on what object is what, you may wish to do the same!
So we’ll use the Python code we had before to list the destinations, but this time, we’ll use the ApierV2.SetTPDestination API call to add a destination before listing them, let’s take a look:
If we post this to the CGR engine, we’ll create a rate, named Rate_AU_Mobile_Rate_1 that bills 22 cents per minute, charged every 60 seconds.
Let’s add a few rates:
CGRateS_Obj.SendData({"method":"ApierV1.SetTPRate","params":[{"ID":"Rate_AU_Mobile_Rate_1","TPid":"cgrates.org","RateSlots":[{"ConnectFee":0,"Rate":22,"RateUnit":"60s","RateIncrement":"60s","GroupIntervalStart":"0s"}]}],"id":1})
CGRateS_Obj.SendData({"method":"ApierV1.SetTPRate","params":[{"ID":"Rate_AU_Fixed_Rate_1","TPid":"cgrates.org","RateSlots":[{"ConnectFee":0,"Rate":14,"RateUnit":"60s","RateIncrement":"60s","GroupIntervalStart":"0s"}]}],"id":1})
CGRateS_Obj.SendData({"method":"ApierV1.SetTPRate","params":[{"ID":"Rate_AU_Toll_Free_Rate_1","TPid":"cgrates.org","RateSlots":[{"ConnectFee":25,"Rate":0,"RateUnit":"60s","RateIncrement":"60s","GroupIntervalStart":"0s"}]}],"id":1})
TPRateIds = CGRateS_Obj.SendData({"method":"ApierV1.GetTPRateIds","params":[{"TPid":"cgrates.org"}]})['result']
print(TPRateIds)
for TPRateId in TPRateIds:
print("\tRate: " + str(TPRateId))
All going well, when you add the above, we’ll have added 3 new rates:
Rate Name
Cost
Rate_AU_Fixed_Rate_1
14c per minute charged every 60s
Rate_AU_Mobile_Rate_1
22c per minute charged every 60s
Rate_AU_Toll_Free_Rate_1
25c connection, untimed
Rates we just created
Linking Rates to Destinations
So now with Destinations defined, and Rates defined, it’s time to link these two together!
Destination Rates link our Destinations and Route rates, this decoupling means that we can have one Rate shared by multiple Destinations if we wanted, and makes things very flexible.
For this example, we’re going to map the Destinations to rates like this:
All going well, you’ll see the new DestinationRate we added.
Here’s a good chance to show how we can add multiple bits of data in one API call, we can tweak the ApierV1.SetTPDestinationRate method and include all the DestinationRates we need in one API call:
In our next post, we’ll keep working our way up this diagram, by creating RatingPlans and RatingProfiles to reference the DestinationRate we just created.
Want more telecom goodness?
I have a good old fashioned RSS feed you can subscribe to.