The UPDATE method allows a UAC to update parameters of a session, such as the SDP and session timers. The UPDATE has no impact on the state of a dialog. In that sense, it is like a re-INVITE, except that it can be sent before the initial INVITE has been completed. This makes it very useful for updating session parameters within early dialogs. The UPDATE method allows a greater control over a SIP session including, but not limited to, the following parameters:
- SDP (for example, to set the media on hold during early media)
- Session timers (for example, to adjust call duration in a prepaid application)
The IMG supports the following for SIP UPDATE:
- an UPDATE request that is received before or after the initial INVITE transaction is completed and when a dialog exists (early or confirmed), in accordance with RFC 3261.
- the validation of an SDP contained in an UPDATE follows the existing restrictions of the offer/answer model (RFC 3264).
- The IMG will reject an UPDATE request that is meant as a FAX re-INVITE. Although it is possible, the RFC does not recommend the use of the UPDATE method in this fashion.
The SIP UPDATE method is accepted without user intervention and cannot be configured or disabled.
The following calls flows are a few possible transactions. For more call flows refer to RFC 3311 and RFC 3261.
IMG handles a UAS UPDATE transaction while in a UAS INVITE transaction
IMG initiates a UAC UPDATE transaction while in a UAS INVITE transaction
IMG handles a UAS UPDATE transaction while in a UAC INVITE transaction
IMG initiates a UAC UPDATE transaction while in a UAC INVITE transaction
IMG handles a UAS UPDATE transaction while in answered state
IMG initiates a UAC UPDATE transaction while in answered state