RFC 3262, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", June 2002

Errata ID: 4604
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Christer Holmberg
Date Reported: 2016-01-25
Verifier Name: Ben Campbell
Date Verified: 2016-01-25

Section 4 says:

Handling of subsequent reliable provisional responses for the same
initial request follows the same rules as above, with the following
difference: reliable provisional responses are guaranteed to be in
order.  As a result, if the UAC receives another reliable provisional
response to the same request, and its RSeq value is not one higher
than the value of the sequence number, that response MUST NOT be
acknowledged with a PRACK, and MUST NOT be processed further by the
UAC.  An implementation MAY discard the response, or MAY cache the
response in the hopes of receiving the missing responses.

It should say:

Subsequent reliable provisional responses for the same initial
request are guaranteed  to have been generated by the UAS in the
order of their RSeq values and must be acknowledged in that order.
As a result, if the UAC receives another reliable provisional
response to the same request, and its RSeq value is one higher than
the value of the previously received RSeq value in the dialog (or 
early dialog), then the new RSeq value is saved and the response is
handled as described above. If the RSeq value is not one higher than
the value of the sequence number, that response MUST NOT be
acknowledged with a PRACK, and MUST NOT be processed further by the
UAC. An implementation MAY discard the response, or MAY cache the
response to be processed (and acknowledged) after receiving the
missing responses.

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