Tag Archives: Registration

SIP Register – Lesser Known Features

In the past we’ve covered what a SIP Registrar does, how to build one, and covered some misconceptions about what being Registered means, but there’s a few little-utilized features of SIP Registration that are quite useful.

A lot of people think there’s a one-to-one relationship between a registration Address on Record, and a username.

That doesn’t have to be the case, there are some platforms that only allow a single registration for a single username, but the RFC itself allows multiple registrations for a single username.

REGISTER requests add, remove, and query bindings.

A REGISTER request can add a new binding between an address-of-record and one or more contact addresses.

Registration on behalf of a particular address-of-record can be performed by a suitably authorized third party.

A client can also remove previous bindings or query to determine which bindings are currently in place for an address-of-record.

RFC 3261 – 10.2

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!

SIP REGISTER status & why it’s not what you think it is.

I can see it’s registered, but when I call it it’s not ringing, what’s wrong?

Support team

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

Half an hour later your connection drops.

As far as the SIP switch is concerned it’s going to send any incoming messages to, as said it’d be there for the next hour.

So even though the connection is dropped to the SIP Switch has no way of knowing this and continues to forward any traffic for that user to 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.

Further reading: