# Glossary: SIP Registration
### SIP Registrar
A SIP [registrar](https://en.wikipedia.org/wiki/Session_Initiation_Protocol#Registrar)’s role is to accept REGISTER requests with an Address Of Record (URI) and write the associated contact bindings to a location service. The location service is a logical entity and is simply a database that contains a list of AOR to contact address bindings. Very often a registrar functions as a location service. It is also very common for a registrar/location service to be co-located with the proxy server for the same domain. The example below shows a typical registration transaction.
Tmedia supports two modes for SIP Registration:
* The Tmedia forwards per-user registration requests from users
* [Explanation of Tmedia's registration forwarding](/configuration-details/configuration-by-use-case/sip-registration-forwarding.md)
* [Configuration of Tmedia's registration forwarding](/configuration-details/configuration-by-use-case/sip-registration-forwarding/creating-a-sip-domain.md)
* The Tmedia registers itself to the SIP Proxy (per SIP NAP, as explained below in current page)
### Registration per SIP NAP
The Tmedia supports to register a SIP [NAP](/appendices/appendix-a-glossary/glossary-nap.md) to a SIP proxy. Registration, can be used, for example, to authenticate the Tmedia Gateway IP address to a SIP provider proxy or SBC and allow the SIP traffic from the Tmedia to that SIP provider.
* Registration is when Tmedia sends a SIP REGISTER to a Registrar, and gets a response which could be accepted (200 OK) or refused (4xx reason code). Upon refusal, the registration is not stopped and is retried after 5 seconds.
* Even if there is no call on the system, Tmedia will send SIP REGISTER
* Sending SIP REGISTER will happen when the option ["Register to Proxy?"](/configuration-details/configuration-parameters-all.md#parameter-register-to-proxy) is selected for a specific SIP [NAP](/configuration-details/configuration-by-web-portal-category/network-access-points-nap.md)
* If you apply a new configuration and the option ["Register to Proxy?"](/configuration-details/configuration-parameters-all.md#parameter-register-to-proxy) is present it will start sending SIP REGISTER
#### Register with no answer from registrar
If it is not registered already, the Tmedia will send a new SIP REGISTER every 32 + 5 seconds interval until it receive a response from the Registrar
* Over UDP (shown it the message flow bellow), the REGISTER request is retransmitted for 32 seconds before the request transaction timeout, then a new SIP REGISTER is sent after 5 seconds.
* Over TCP, if the SIP registrar does not listen on the TCP port, a new TCP connection is retried every 5 seconds. If the TCP connection is established but no SIP responses returned by the Registrar, the transaction timeout after 32s and then a new SIP REGISTER is sent after 5 seconds.
#### Register expire negotiation
The REGISTER request negotiate the contact binding expiration by adding "expires=3600" parameter to the contact header. The value is hardcoded to 3600 seconds. If the REGISTER response from the Registrar does not change the "expires" parameter:
* The SIP REGISTER will be "valid" for 3600 seconds
* Tmedia will send a new REGISTER (to refresh the registration) after half of the minimum registration negotiated, general use will be at 1800 seconds (3600/2)
The Registrar can change the "expires" contact parameter or the Expires header as shown in the two example below to negotiate a value different from 3600 seconds.
**Negotiation uses case 1: contact expires header**
In the example below, the Registrar changes the "expires" contact parameter.
**Negotiation uses case 2: no contact expires parameter; Expires header**
In the example below, the Registrar removes the "expires" contact parameter and set the Expires header.
[](https://docs.telcobridges.com/wiki/File:SIP_Register_negotiated_expire_time_example_2.png)
#### Register Authentication
* If Authentication is required, the Registrar returns a 401 Unauthorized response with a WWW-Authenticate header
* The WWW-Authenticate header contains nonce to encrypt user’s communications password, user then sends a second REGISTER containing an Authorization header with the user's encrypted password
### References
* [Wikipedia registrar article](http://en.wikipedia.org/wiki/Session_Initiation_Protocol#Registrar)
* [Understanding SIP Registration](https://andrewjprokop.wordpress.com/2014/05/29/understanding-sip-registration/)
* [VoIP Protocols:SIP Call Flow - Toncar.cz](http://toncar.cz/Tutorials/VoIP/VoIP_Protocols_SIP_Call_Flow.html)
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