Hi Terry, You are correct - REFER does not have an options-tag defined because its support can be deduced from an Allow header in an INVITE, 200 OK, or OPTIONS response. Also, if the method is used but not understood by the UAS, a 405/501 will be returned.
On the other hand, a PRACK is sent in response to a provisional response sent containing a RSeq header. Since a UAS may silently ignore headers that it does not understand, the 100rel option tag is necessary to establish prior to any provisionoal responses being sent reliably, or a UAS which does not support the extension will never send a PRACK and the UAC which does support it will continue to retransmit the provisional. Finally, if one supports REFER, they should support the basic flows in the sipping transfer draft, since this represents some very common uses of the method. Of course, the call flows documents are not normative. Hope this helps. Thanks, Alan Johnston WorldCom sip:[EMAIL PROTECTED] > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED]] On Behalf Of > Terry L Anderson > Sent: Tuesday, January 22, 2002 12:27 PM > To: SIP Implementors > Subject: [Sip-implementors] Refer Method > > > Does the Refer method extension need an extension options-tag > for use in Require/Supported headers? I do not see one yet > defined. Or is it considered that Allow and OPTIONS is > sufficient to determine support? I am not clear how this > case differs from PRACK method extension where option-tag > '100rel' is defined. > > If an UA supports Refer, should one assume that all transfer > methods suggested in draft-ietf-sip-cc-transfer are usable or > does one need an additional method to determine which > scenarios can be used? > > -- > > ------------------------------------------------------------ > Terry L Anderson mailto:[EMAIL PROTECTED] > Tel:908.582.7013 Fax:908.582.6729 > Lucent Technologies/INS/Voice Over IP Access Networks > Rm 2B-121, 600 Mountain Av, Murray Hill, NJ 07974 http://its.lucent.com/~tla (Lucent internal) http://www.gti.net/tla _______________________________________________ Sip-implementors mailing list [EMAIL PROTECTED] http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
