SIP Registration 101: Components and Examples
Articles - 3 min read

SIP Registration 101: Components and Examples

How do service providers like Telnyx know where their users are located and how to make contact with them? Within the Session Initiation Protocol, there is a process called SIP Registration, which allows service providers to identify the phones of their customers, thus providing them insight into where to send phone calls. In other words, registration is the process by which service providers data mine their customers’ locations.

The Components of SIP Registration

  • User Agent: The SIP entity that interacts with the user e.g. soft phone.
  • SIP URL: The SIP address that identifies a user – it usually consists of a username and domain name, similar to our email addresses. SIP: bob@yourcompany.com i.e., SIP:bob@8.8.8.8
  • SIP Registrar Server: A server belonging to the service provider that accepts Register requests and will have a database that maintains all the user's registrations in a binding table. This server is usually collated with the proxy and redirect servers.
  • Location Server: Registrar servers store their customers' locations to this server.
  • Binding table: This table contains the details on where to reach the user such as numbers, IP address, Ports, etc.
  • Proxy Server: A server that handles incoming invitations.
  • Redirect Server: A server that provides alternative locations where the user can be reachable.
  • IP Authentication: A check by the Registrar to confirm the user’s details.

A Failing Registration Example with User/Auth Authentication

Let's assume that we've created a connection with User Auth authentication called “HOME” and our public IP is 4.4.4.4.
The registration attempt will come from the public IP to Telnyx. Below is a TShark trace of a register attempt from a User Agent configured with the credentials shown above:

4.4.4.4 ->192.76.120.10  SIP 724 Request: REGISTER sip:4.4.4.4;transport=UDP  (1 binding) |
192.76.120.10 -> 4.4.4.4  SIP 547 Status: 401 Unauthorized |
4.4.4.4 ->192.76.120.10  SIP 724 Request: REGISTER sip:4.4.4.4;transport=UDP  (1 binding) |
192.76.120.10 -> 4.4.4.4  SIP 547 Status: 401 Unauthorized |

As you can see, attempts to register are met with a “401 Unauthorized”. This is because the registrar server attempts to know more information about the User Agent. At that point, the User Agent will send another REGISTER message. If we get another “401 Unauthorized” it means that we did not configure our parameters properly i.e., username or password. If this is the case, you will see the following status on the Telnyx portal:

A Successful SIP Registration Example with Username/Password Authentication

Let's assume that we've created a connection with User Auth authentication called “HOME” and our public IP is 4.4.4.4.
The SIP registration attempt will come from the public IP to Telnyx. Below is a TShark trace of a register attempt from a User Agent configured with the credentials shown above:

4.4.4.4 ->192.76.120.10  SIP 724 Request: REGISTER sip:4.4.4.4;transport=UDP  (1 binding) |
192.76.120.10 -> 4.4.4.4  SIP 547 Status: 401 Unauthorized |
4.4.4.4 ->192.76.120.10  SIP 724 Request: REGISTER sip:4.4.4.4;transport=UDP  (1 binding) |
192.76.120.10 -> 4.4.4.4  SIP 547 Status: 200 OK  (1 binding) |

As you can see, attempts to register are met with a “401 Unauthorized”. This is because the registrar server attempts to  know more information about the User Agent. At that point, the User Agent will send another REGISTER message. If we get a 200 OK it means that we have registered successfully and we will see the following status on the Telnyx portal:
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