Let’s say you’ve got a SIP phone on your desk at the office and at home.
What we could do is create a different username and password for home & work, and then setup some time based forward rules to ring the office from 9-5 and home outside of that.
You could register both with the same username and password, and then unplug the one at home before you leave to work, get to work, plug in your office phone, unplug it before you leave to go home, and when you get home plug back in your home phone, or if multi-device registration is supported, register both and have incoming calls ring on both.
Admittedly, platforms that support this are the exception, not the rule, but the RFC does allow it.
The other little known feature in SIP Registration is that you can query the SIP Registrar to get the list of Addresses on Record.
So there you go, factoids about SIP REGISTER method!
I’ve talked about using the UAC module, but as promised, here’s how we can use the UAC module to send SIP REGISTER requests to another SIP server so we can register to another SIP proxy.
Let’s say we’re using Kamailio to talk to a SIP Trunk that requires us to register with them so they know where to send the calls. We’d need to use Kamailio UAC module to manage SIP Registration with our remote SIP Trunk.
But Kamailio’s a proxy, why are we sending requests from it? A proxy just handles messages, right? Proxies don’t originate messages, it’s true, and Kamailio can be a proxy, but with the UAC module we can use Kamailio as a Client instead of a server. Keep in mind Kamailio is what we tell it to be.
Before we can go spewing registrations out all over the internet we need to start by getting a few things in place;
First of which is configuring UAC module, which is something I covered off in my last post,
Once we’ve got a database connection in place and UAC module loaded, then we can configure an entry in the uacreg table in the database, in my example I’m going to be registering to an Asterisk box on 192.168.1.205, so I’ll insert that into my database:
mysql> INSERT INTO `uacreg` VALUES (NULL,'myusername','myusername','192.168.1.205','myusername','192.168.1.205','asterisk','myusername','mypassword','','sip:192.168.1.205:5060',60,0,0);
Note: If you’re using a later version of Kamailio (5.4+) then the DB schema changes and you may want something like this:
Having a look at the fields in our table makes it a bit clearer as to what we’ve got in place, setting flags to 0 will see Kamailio attempt registration. Make sure the auth_proxy is a SIP URI (Starts with sip:) and leave the auth_ha1 password empty as we haven’t calculated it.
I can see it’s registered, but when I call it it’s not ringing, what’s wrong?
It’s a question I get every so often, and it generally comes down to a misunderstanding in the way the SIP Register mechanism works.
When a UA registers to a SIP server it includes an “Expires:” header, which means it’s registration will expire after that time.
It doesn’t mean it’ll be active that whole time, just that for the time specified it intends to be at that address, but life, and networks, often have other plans.
Let’s jump out of SIP for a minute and imagine you’re going to give me a package, I leave you a note saying:
I’ll be waiting outside the station in a trench-coat under the lamp post between 7:00 and 7:15
You get there at 7:12 but you can’t deliver the package. I’m nowhere to be seen.
The note I left says I’ll be there during that time, but I’ve disappeared, and no you can’t hand the package to me.
Just because you have a note saying I’ll be there, doesn’t mean I still will be. It was my intention to be there, but I’m obviously not.
The SIP register is the same as the note left on the desk. I intended to be there, but I’m now obviously not, and I haven’t had a way to reach you to let you know this has changed, or I myself don’t know.
You see this in SIP from time to time, generally it’s due to the connection the UA is coming from dropping or it’s public IP changing.
For example, a REGISTER is sent with an Expires of 3600 seconds (An hour) to a SIP switch from IP address 18.104.22.168.
Half an hour later your connection drops.
As far as the SIP switch is concerned it’s going to send any incoming messages to 22.214.171.124, as 126.96.36.199 said it’d be there for the next hour.
So even though the connection is dropped to 188.8.131.52 the SIP Switch has no way of knowing this and continues to forward any traffic for that user to 184.108.40.206 until the 3600 seconds is up since it last tried to REGISTER.
Same thing could happen if our UA is behind a NAT and the external IP changes or the connection is changed. The UA doesn’t know anything has changed, so no REGISTER is sent to refresh, and messages from the SIP server are sent to the old address.
A lot of SIP switching platforms allow you to view register status, but just keep in mind it doesn’t mean the device is still answerable at that address, only that it intended to be.