> For the complete documentation index, see [llms.txt](https://prosbcdocs.telcobridges.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://prosbcdocs.telcobridges.com/use-cases/applications/sip-subscribe-notify-publish-forwarding.md).

# SIP subscribe notify publish forwarding

### SIP Subscribe overview

The SIP SUBSCRIBE method allows users to subscribe to various services, and get notified of events related to these services. Typical services (packages) that phones will register to are:

* Voice mail
* Presence
* Conference

Each user's device can register to various packages, then receive notifications for each of these packages (independently).<br>

<br>

### SIP Subscribe/Publish forwarding

ProSBC will forward SUBSCRIBE or PUBLISH requests from users to Registrar (PBX)

<figure><img src="https://docs.telcobridges.com/w/images/thumb/5/5f/Subscribe_request_overview.png/700px-Subscribe_request_overview.png" alt=""><figcaption></figcaption></figure>

<figure><img src="https://docs.telcobridges.com/w/images/thumb/5/54/Subscribe_response_overview.png/700px-Subscribe_response_overview.png" alt=""><figcaption></figcaption></figure>

### &#x20;SIP Notify forwarding

ProSBC will forward NOTIFY requests from Registrar (PBX) back to users.

<figure><img src="https://docs.telcobridges.com/w/images/thumb/e/e8/Notify_request_overview.png/700px-Notify_request_overview.png" alt=""><figcaption></figcaption></figure>

<figure><img src="https://docs.telcobridges.com/w/images/thumb/a/a8/Notify_response_overview.png/700px-Notify_response_overview.png" alt=""><figcaption></figcaption></figure>

<br>

### Contact remapping

When forwarding a SIP Subscribe message from a user to the registrar, ProSBC will remap the contact to make itself the contact. This way, the Registrar will send response messages and Notify events through ProSBC, and thus make sure to "hide" private network topology from users (avoid messages being sent directly from registrar to users).

Because the contact's domain is being remapped, ProSBC will also remap the contact's user name to make sure there is not user name conflict between two domains that would have the same user name.

<figure><img src="https://docs.telcobridges.com/w/images/thumb/0/04/Subscribe_topology_hiding.png/700px-Subscribe_topology_hiding.png" alt=""><figcaption></figcaption></figure>

### SIP authorization

SIP authorization requests (401/407 Unauthorized) are forwarded from Registrar to users, allowing users to send the request again with appropriate credentials.\
ProSBC does not perform authorization by itself, only forwards between users and registrars.

### NAT traversal

* ProSBC handles SIP Subscribe NAT traversal to allow interaction between SIP User Agents from public and private networks.
  * ProSBC detects the actual IP from which users are sending messages, so responses and Notify messages can be forwarded properly, even in case users are unaware that they are behind a NAT

### Configuring

SIP Subscribe/Publish/Notify forwarding is automatically available when configuring SIP Domain for [SIP Registration forwarding](/configuration-details/configuration-by-use-case/sip-registration-forwarding.md):

* [Creating a SIP domain](/configuration-details/configuration-by-use-case/sip-registration-forwarding/creating-a-sip-domain.md)

### References

* [Understanding SIP Subscribe](https://andrewjprokop.wordpress.com/2015/03/11/a-deep-dive-into-sip-subscriptions/)
* [IANA SIP Event types](https://www.iana.org/assignments/sip-events/sip-events.xhtml)


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