The UPDATE method is used to modify the state of a SESSION WITHOUT changing the state of the dialog. A session is established in SIP using an INVITE REQUEST in an offer/answer manner. Typically, a session offer is made in the INVITE and an answer made in a RESPONSE to the INVITE. In an established session, a re-INVITE is used to update session parameters. However in pending session (INVITE sent but no final response received) UPDATE is used to update session parameters. Basically UPDATE is used to perform QoS and negotiate end-to-end attributes prior to session establishment.

The UPDATE method is used to modify the state of a session without changing the state of the dialog. A session is established in SIP using an INVITE request in an offer/answer manner. Typically, a session offer is made in the INVITE and an answer made in a response to the INVITE. In an established session, a re-INVITE is used to update session parameters. However in pending session (INVITE sent but no final response received) UPDATE is used to update session parameters. Basically UPDATE is used to perform QoS and negotiate end-to-end attributes prior to session establishment.