[SIPForum-discussion] a question about re-invite

[Anthony Orlando]

>From RFC3261.  A re-Invite is used to modify an existing session.  If A wants to transfer the call to C then you would user the Refer method.  Refer (no pun intended) to rfc 3515


14 Modifying an Existing Session
A successful INVITE request (see Section 13) establishes both a
dialog between two user agents and a session using the offer-answer
model. Section 12 explains how to modify an existing dialog using a
target refresh request (for example, changing the remote target URI
of the dialog). This section describes how to modify the actual
session. This modification can involve changing addresses or ports,
adding a media stream, deleting a media stream, and so on. This is
accomplished by sending a new INVITE request within the same dialog
that established the session. An INVITE request sent within an
existing dialog is known as a re-INVITE.
Note that a single re-INVITE can modify the dialog and the
parameters of the session at the same time.
Either the caller or callee can modify an existing session.
The behavior of a UA on detection of media failure is a matter of
local policy. However, automated generation of re-INVITE or BYE is
NOT RECOMMENDED to avoid flooding the network with traffic when there
is congestion. In any case, if these messages are sent
automatically, they SHOULD be sent after some randomized interval.
Note that the paragraph above refers to automatically generated
BYEs and re-INVITEs. If the user hangs up upon media failure, the
UA would send a BYE request as usual

--- On Thu, 6/4/09, abhishek.chattopadhyay at wipro.com <abhishek.chattopadhyay at wipro.com> wrote:

From: abhishek.chattopadhyay at wipro.com <abhishek.chattopadhyay at wipro.com>
Subject: Re: [SIPForum-discussion] a question about re-invite
To: fanyanping at huawei.com, discussion at sipforum.org
Date: Thursday, June 4, 2009, 4:21 AM

Hello Nancy,

SIP RE-Invite method is a mechanism of refreshing the session primarily and not changing the session itself.

Moreover if suppose A and B are in session and somehow B wants A to talk to C then the method refer would be used.
There are other implementations where a re-invite is sent to achieve the same objective. This depends upon implementation largely.
The fact all such cases where re-invite is used in place of refer to achieve similar functionalities there are serious resource compromises to mitigate.

Moving further 
Answer to question one: 
If I consider A,B,C are all UAC then no as because C would not be ready to receive a response for the request it never generated and would discard it. 
Such a scenario can be seen where A receives a Request from C and then generates the Invite towards B on behalf of C.

Answer to question two:
Call-id, to tag, from tag are session wise unique in the space and time, they might not be used by other clients as they would be used to identify dialog and session between agents.


Regards 
ABHISHEK


-----Original Message-----
From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of fanyanping
Sent: Wednesday, June 03, 2009 9:38 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] a question about re-invite

Hi all，
my understanding of a SIP reinvite is that it modifies the media session. Is
it capable of modifying the address of the send too? for example， A is
talking to B, can A send a reinvite to B with C's address instead of its own
address?
another example，A is talking to B，recorded as dialog1, C knows the
information of the dialog1,  can C sends a reinvite to B instead of the
dialog1, the dialog bewteen C and B has the same Call-ID, from-tag, to-tag
as dialog1?


Best Regards.
nancy

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