[SIPForum-discussion] PRACK doubt...

Deepanshu deepanshu at huawei.com
Thu Feb 1 09:32:45 UTC 2007

Dear Nithya

My understanding for your Question is as follows in blue

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.


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

ii)             why UAS MUST NOT attempt to send a 100 (Trying) response reliably ..

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.


Deepanshu Gautam

R&D Engineer

Huawei Technologies Co. Ltd.

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