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--></style></head><body lang=EN-GB link=blue vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Hi,</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I have a question regarding basic SIP flow for establishing a call session, and wonder if anyone can help me clarify.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>For normal call set up, INVITE-OK-ACK, from RFC3261, I believe the ACK’s R-URI should be a copy of the Contact header of the OK message. </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>However in RFC3665 sections 3.1 and 3.2 for example, the ACK’s R-URI missed the parameters from the Contact header of the OK, e.g.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>In OK the Contact header is:</p><div><pre><span style='color:black'> Contact: <sip:bob@client.biloxi.example.com;transport=tcp><o:p></o:p></span></pre></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>And ACK the R-URI is:</p><div><pre><span style='color:black'>ACK sip:bob@client.biloxi.example.com SIP/2.0<o:p></o:p></span></pre></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Which has got the parameter ‘transport=tcp’ removed – why is this?</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>In this case if the proxy connected to Bob handles ACK in the usual way as it would for all in-dialog requests, i.e. based on routing headers only and not extra processing, it’ll try to relay the ACK message to Bob using the default transport UDP which is not expected and will fail. </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Can anyone help explain why the parameters are removed in RFC3665 examples please? Have I missed anything? Thank you very much!</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Yufei</p></div></body></html>