I'm trying to make sense of the following two RFC sections with respect to SIP dialogs and incrementing CSeq sequence numbers:
https://www.rfc-editor.org/rfc/rfc3261#section-12.2.1.1
https://www.rfc-editor.org/rfc/rfc3261#section-12.2.2
The first RFC section says that, with respect to a UAC,
Requests within a dialog MUST contain strictly monotonically increasing and contiguous CSeq sequence numbers (increasing-by-one) in each direction.
However the latter sections states that with respect to a UAS,
It is possible for the CSeq sequence number to be higher than the remote sequence number by more than one. This is not an error condition, and a UAS SHOULD be prepared to receive and process requests with CSeq values more than one higher than the previous received request.
In my scenario, I have a device sending a SIP subscribe to a server. The server can then send zero or more notify request's within the same dialog to that subscriber. Can the server sending the notify's increment the CSeq sequence number by more than one without the subscriber rejecting the request?
Yes. In the real World you're not going to find a SIP user agent rejecting a request due to a CSeq that's skipped a few numbers in the sequence.
Request will get rejected if the CSeq is less than or equal to the CSeq in a previous request as they will get classified as re-transmits.