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SIP: In what cases or scenarios, an user agent can send INVITE within an already established Dialog between UAC and UAS?

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As we know, when a session is established between two user agents i.e. UAC (User Agent Client) and UAS (User Agent Server), known as Dialog in SIP terminology. I read that a SIP user agent client can trigger INVITE message towards user agent server in an already established session. This scenario is known as "re-invite" but I am unable to think the case in which it happens.

posted Aug 14, 2016 by Vikram Singh

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1 Answer

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When a dialog is already established between two user agents and then to make any change in the session characteristics is achieved by sending the INVITE message. Since session was already in place and this is the second INVITE, that's why it is known as re-INVITE. This re-INVITE is nothing but an INVITE message with the same Call ID and From header field.

When an ongoing call put on hold, originator of hold sends the INVITE message to the other end and says media will not be transmitted.
Again when originator who earlier has put the call on hold decides to resume the call, again sends INVITE message and informing to other end that media transmission is resumed.

Due to mobility, IP address of the user agent may get change, for an ongoing call , UA would have to send INVITE to other end with the update contact URI and SDP information.

This is all about what I know on INVITE message.

answer Aug 16, 2016 by Vimal Kumar Mishra
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In one of the articles it was mentioned "BYE" SIP message routes end to end and by pass the SIP proxy server. What is it mean ? and when SIP proxy servers release the resources that was reserved during the session establishment ?

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While going through rfc 3665 , I stopped where proxy server was sending 407 to originator of INVITE request message.
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