[Sipping] Question regarding reliable provisional response retransmission

[DarshanBildikar 70559 <darshanb at huawei.com>]

The SIP RFC says that 

"A proxy has the option of canceling a transaction when there is a gap of 3 minutes between responses in a transaction.  To prevent cancellation, the UAS MUST send a non-100 provisional response at every minute, to handle the possibility of lost provisional responses"

Assume a B2BUA has generated a request on behalf of a caller. When the callee responds with a 180 Ringing message (with SDP) the B2BUA negotiates the callee to the caller (who might be connected to a media server, for example) so that a Ring back tone can be played or an IVR session ensues. 

My queries are as follows

1) RFC 3262 states that reliable provisional responses are to be retransmitted every two and a half minutes. Why is this really required since the response is after all "reliable" !

2) Should the retransmitted provisional response always contain the same SDP? i.e. Should I only consider the first SDP and ignore SDP's in subsequent messages?

Any help is appreciated.

******************************************************************************************
 This email and its attachments contain confidential information from HUAWEI, which is intended only for the person or entity whose address is listed above. Any use of the information contained herein in any way (including, but not limited to, total or partial disclosure, reproduction, or dissemination) by persons other than the intended recipient(s) is prohibited. If you receive this e-mail in error, please notify the sender by phone or email immediately and delete it!
 *****************************************************************************************


_______________________________________________
Sipping mailing list  https://www1.ietf.org/mailman/listinfo/sipping
This list is for NEW development of the application of SIP
Use sip-implementors at cs.columbia.edu for questions on current sip
Use sip at ietf.org for new developments of core SIP