Subject: [Sipping] Response ambiguity in RFC 3398

From: Tom Taylor <[email-masked]>
Date: Thu, 02 Jun 2005 13:35:26 -0400

RFC 3398 has the following advice for generation of a response when an 
ACM with 'Subscriber Free' is received:

7.2.6 ACM received

    Most commonly, on receipt of an ACM a provisional response (in the
    18x class) SHOULD be sent to the SIP network.  If the INVITE that
    initiated this session contained legitimate and comprehensible
    encapsulated ISUP, then the ACM received by the gateway SHOULD be
    encapsulated in the provisional response.

    If the ACM contains a Backward Call Indicators parameter with a value
    of 'subscriber free', the gateway SHOULD send a '180 Ringing'
    response.  When a 180 is sent, it is assumed, in the absence of any
    early media extension, that any necessary ringback tones will be
    generated locally by the SIP user agent to which the gateway is
    responding (which may in turn be a gateway).

The very next paragraph talks about cases where the ACM indicates 
in-band media.  It concludes with the following:

    After early media transmission has been initiated, the gateway SHOULD
    send a 183 Session Progress response code.

Did the authors intend that a 180 be sent first, followed by the 183, or 
did they intend that 183 is sent instead of 180?  Note that particularly 
if the latter is the case, Study Group 11 has an action to amend 
Q.1912.5 to align it with RFC 3398 on this point.

Tom Taylor

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