[Sip] info-events and "in-order delivery"

[Jeroen van Bemmel <jbemmel at zonnet.nl>]

   The current info-events text says (section 5.6):

"In addition, the INFO method does not define mechanisms for ensuring
  in-order delivery.  While the UAC will increment the CSeq header upon
  the transmission of new INFO messages, the UAS cannot use the CSeq to
  determine the sequence of INFO information.  This is due to the fact
  that there could be gaps in the INFO message CSeq count caused by a
  user agent sending re-INVITES or other SIP messages."


This is not entirely accurate: SIP guarantees that INFO messages are never delivered out-of-sequence. 
Only in case of *overlapping INFO requests* (i.e. sending a next one before having received a final 
response for the previous one) it cannot be guaranteed that all are received.

The partial event mechanism (RFC5263) includes a 'version' attribute to detect such gaps. In case of INFO events, 
I think it is better to simply say: the UAC MUST NOT send an INFO request with a given event type when a previous 
INFO request containing that event type has not yet received a final response.

Regards,
Jeroen


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