[SIPForum-discussion] PRACK doubt...

[vinoth]

Hi Gautam,
 
            Once the Session gets established, the communication becomes end to end. Both the parties will know the end point contact address, so that they can reach the end point directly. So there is no necessary for the proxies to send provisional responses within the dialog.

 Regards,
Vinoth Kumar.
Mobax Networks,Coimbatore.



---------- Original Message ----------------------------------
From: Deepanshu <deepanshu at huawei.com>
Date:  Thu, 01 Feb 2007 17:32:45 +0800

>Dear Nithya
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>My understanding for your Question is as follows in blue
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>i)             why a proxy cannot generate reliable provisional responses to requests sent within the context of a dialog. ...and 
>Well, this question is bit dizzy for me too, but the probable reason which I can think of is. If we allow a proxy (working as a UAS) to send a provisional response with in a established dialog (session) then since a provisional response may be SDP parameters so it is prone to change the pre-negotiated media attributes between the UAS and UAC.
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>FYI: And moreover as the RFC3262 only talks about reliable provisional response for SIP INVITE then I can only think of only one SIP request which can be sent after the dialog is established and which can have To-tag, that is SIP re-INVITE
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>ii)             why UAS MUST NOT attempt to send a 100 (Trying) response reliably ..
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>The 100 trying response is hop-by-hop, that is each proxy on the signaling path can generate the 100 trying response. Whereas the non-100 trying response (181, 183) are end to end that is the proxy only forward the response form UAS to UAC. The reliability mechanism described in RFC3261 end-to-end, so the 100-trying response, which is hop-by-hop, is kept out of it.
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>HTH
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>Deepanshu Gautam
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>R&D Engineer
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>Huawei Technologies Co. Ltd.
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