Information in nr_gap-Blocks in NR-SACK-Chunks are not used optimally

[TheAomx]

When an association uses NR-SACK-Chunks and support for PR-SCTP is enabled when sending three user messages where the first and third have a ttl of 500ms and the second user message must be delivered, the FreeBSD-Implemention does not use the information of an incoming NR-SACK-Chunk where in the nr_gap-Block the second user message is acknowledged non regenable.

It should ideally send an FORWARD-TSN with cum_tsn=3. See the following test cases for more details:

• https://github.com/nplab/PR_SCTP_Testsuite/blob/master/nr-sack/with-forward-tsn/with-forward-tsn-2.pkt
• https://github.com/nplab/PR_SCTP_Testsuite/blob/master/nr-sack/with-forward-tsn/with-forward-tsn-3.pkt