[SIPForum-discussion] a question about re-invite

Thu Jun 25 11:28:41 UTC 2009

HI All,

I have had a scenario were Reinvite changes the Target URI...

Raghul

--- On Mon, 8/6/09, fanyanping <fanyanping at huawei.com> wrote:

From: fanyanping <fanyanping at huawei.com>
Subject: Re: [SIPForum-discussion] a question about re-invite
To: abhishek.chattopadhyay at wipro.com, avorlando at yahoo.com, discussion at sipforum.org
Date: Monday, 8 June, 2009, 1:19 PM

I review RFC 3261, in section  12.2  have a description  as below: 
" For dialogs that have been established with an INVITE, the only target refresh request defined is re-INVITE (see Section 14).  Other extensions may define different target refreshrequests for dialogs established in other ways."

so , i think re-invite can change  the remote target URI

From: abhishek.chattopadhyay at wipro.com [mailto:abhishek.chattopadhyay at wipro.com] 
Sent: Friday, June 05, 2009 7:47 PM
To: fanyanping at huawei.com; avorlando at yahoo.com; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] a question about re-invite
Importance: High

Well, 

That’s a bit tricky for me; coz there is a very thin line separating a dialog from the session. Most of the times if dialog is initiated by invite, and it matures into a session it would be really difficult to accommodate both definitions in such a point and differentiate them cleanly.  And moreover a dialog formed by any other sip request doesn’t needs refreshing etc. so basically I hope it is always what the dialog is meant for and that is mostly the media. ( I mean after all sip signaling is just the way to let two parties talk)
So by far whenever I have conceived about changing the session using re-invite it has always been the media (that’s the SDP).
And yes session should include session and media both but what can change, will be SDP (that’s what I think).

Regards
ABHISHEK 

From: fanyanping [mailto:fanyanping at huawei.com]
Sent: Friday, June 05, 2009 6:03 AM
To: 'Anthony Orlando'; discussion at sipforum.org; Abhishek Chattopadhyay (WT01 - Telecom Equipment)
Subject: RE: [SIPForum-discussion] a question about re-invite

Hi all,

thank you for the answer, ^_^

i have a question that what is  the mean of "modify the session"? just only inlude the SDP Parameters or include the dialog Parameters and SDP Parameters??
in my mind, session should include dialog and media,is it right??

ps :
> 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.

response wil be back to A  according the Via header

From: Anthony Orlando [mailto:avorlando at yahoo.com]
Sent: Friday, June 05, 2009 12:35 AM
To: fanyanping at huawei.com; discussion at sipforum.org; abhishek.chattopadhyay at wipro.com
Subject: RE: [SIPForum-discussion] a question about re-invite
Not arguing the point of using re-invite to "transfer" a call to c.  My points are two
1.) RFC3261 chapter 14 defines the use of a re-invite to modify the session. 
2.) re-invite should not be used to transfer the call.  Refer method should be used.

--- 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: avorlando at yahoo.com, fanyanping at huawei.com, discussion at sipforum.org
Date: Thursday, June 4, 2009, 12:23 PM
Hello Anthony 

I agree that a re-invite can change the remote target URI of a dialog. But I think it wouldn’t be safe enough to assume that changing the remote target of the dialog to such a UA which has no information of the already existing dialog, and is expected to somehow respond to the message.
So in case A is aware then B surely is negotiating with C on behalf of A. An SBC can be a good example of such scenarios where a UAC makes a request on behalf of another UAC. 

I hope it is not difficult assume a scenario where call is intended between A and C however A calls B and B does all the negotiations and re-negotiations with B before passing over the session to A finally.

Further, till now I haven’t experienced any such scenario where such a thing happens in case you came across one, can you please forward the call flow.

Thanks you
Regards
ABHISHEK 

From: Anthony Orlando [mailto:avorlando at yahoo.com]
Sent: Thursday, June 04, 2009 9:40 PM
To: fanyanping at huawei.com; discussion at sipforum.org; Abhishek Chattopadhyay (WT01 - Telecom Equipment)
Subject: Re: [SIPForum-discussion] a question about re-invite

>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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