> Is the offer answer model to be followed only when
> the body is described using SDP??
> 
> i.e. if I have INVITE with no body and the 200 OK
> response also has no body, then is this a valid SIP
> sequence? Since I am conveying all the information
> in SIP headers like Subject etc. I don't need to use a
> body and I am wondering if this will violate the 
> offer-answer model defined in RFC 3261.

It violates the model discussed within rfc3261
section 13.2.1; however you potentially 
could avoid the violation by clearly indicating 
what can and can't be used to setup the session.
I assume that someone will correct me if rfc3261
forbids this.

Including an Accept header without the 
"application/sdp" clearly communicates
that "application/sdp" shall not be
used within INVITE/2xx/ACK to setup
the session.

Including your newly defined option-tag 
within Require or Supported would clearly
indicate the alternative mechanism that
could be used to communicate or retrieve 
the session data.

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