SIP subscribe notify publish forwarding

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FreeSBC can forward SIP Subscribe, Notify and Publish messages between users and registrars (PBX).
Forwarding of these messages follow the same rules as forwarding of SIP registration.

Contents

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 package (independently).


SIP Subscribe/Publish forwarding

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

Subscribe request overview.png
Subscribe response overview.png


SIP Notify forwarding

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

Notify request overview.png
Notify response overview.png


Contact remapping

When forwarding a SIP Subscribe message from a user to the registrar, FreeSBC will remap the contact to make itself the contact. This way, the Registrar will send response messages and Notify events through FreeSBC, 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, FreeSBC 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.

Subscribe topology hiding.png

SIP authorization

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

NAT traversal

  • FreeSBC handles SIP Subscribe NAT traversal to allow interaction between SIP User Agents from public and private networks.
    • FreeSBC 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:

References

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