If your life is a busy as mine is, you often wish that you could be in two or more places at the same time. This is the same for my work and personal lives. While human cloning may be years away (ignoring the legal and ethical aspects), SIP and the concept of multiple, simultaneous registrations offers some assistance.
It works like this. A SIP device advertises itself with a Register message. This registration associates a username with an IP address. For example, let’s pretend that my SIP user name is SIP:aprokop@mycompany.com and my IP address is 192.120.10.10. I run a SIP phone on my iPhone and when it starts it sends a SIP Register message containing that information to my company’s SIP proxy. The proxy will verify my identity before storing my contact information. When another user wishes to call me, the Invite need only specify my user name and the proxy will do the work of forwarding that Invite off to my IP address. This separation of user and IP address is a significant strength of SIP.
Let’s take this example a little further. I told you that I have an iPhone with a SIP client. I also have a PC that runs a SIP soft phone and back at my office I have an Avaya 9641 running in SIP mode. One of the powers of SIP is that every one of my devices can send its own Register message. They will all contain the same user ID, SIP:aprokop@mycompany.com, but the IP address in the three Register messages will be unique. Now, when someone calls me, each of my devices will ring allowing me to take the call in the way that makes sense at that point in time. This is known as multiple registration and call forking and it’s what differentiates SIP from other VoIP protocols such as H.323.
Honesty is the Best Policy
A little truth in advertising is necessary here. What I described above is part of the SIP standard, but that doesn’t mean that every SIP solution implements that. Until recently, Avaya only supported a single SIP registration for a user. My registered iPhone SIP client would be immediately unregistered when my 9641 sent its Register message. Thankfully, all of that changes with the latest and greatest version of Avaya’s Session Manager. Session Manager 6.3 supports up to 10 registered devices for a single SIP address. Other SIP solutions have supported this for a long time so it’s nice that Avaya finally caught up.
While none of this will help when I am supposed to be in two conference calls at the same time or I receive two dinner invitations for the same night (if only I were that popular), it does make my communications life a lot more flexible. So, until human cloning becomes practical, legal, and ethical, this is the best I will get and I will gladly take it.
Tao, Zen, and Tomorrow 
[…] A related discussion can be found here. […]
Hi;
Thanks for explanation.
I have a ques.
How can I register my multiple public ids(say 4 numbers) in one go to my registrar?
Thanks
You need a server that supports multiple REGISTER messages for the same user ID. I am in the enterprise world and an Avaya Aura system will allow up to 10 registration per client.
Seems less of a ‘how’ more of ‘is possible’; you note that a single registration can have multiple UACs with different IPs (what happens if there’s NATing involved?), but not how.
I would love to see you do a more in-depth explanation like you do in your other articles.
How about this? https://andrewjprokop.wordpress.com/2015/04/07/multiple-registration-and-call-forking-with-sip/
What does it mean by IMPI and IMPU. Could you please give me a Example for both identity. user have a one public and one Private Identity. like mobile number – IMPU and IMSI is IMPI.
Please correct me and add more information about identity.
Thanks in advance.