Incoming SACK-Chunks are allowed when NR-SACK-Chunks are listed at the supported-extensions

[TheAomx]

When both endpoints have listed NR-SACK-Chunks at the supported extensions in the INIT and INIT-ACK-Chunks incoming SACK-Chunks are accepted by the FreeBSD-Kernel-Implementation. At the internet draft "Load Sharing for the Stream Control Transmission Protocol (SCTP)" i was not be able to find any information on how to handle those situations (see https://tools.ietf.org/html/draft-tuexen-tsvwg-sctp-multipath-12#page-6). There is although one paragraph stating:

Once both endpoints indicate during association establishment that
they support the NR-SACK extension, each endpoint SHOULD acknowledge
received DATA chunks with NR-SACK chunks, and not SACK chunks. That
is, throughout an SCTP association, both endpoints SHOULD send either
SACK chunks or NR-SACK chunks, never a mixture of the two.

See the following test cases fore more details:

• https://github.com/nplab/PR_SCTP_Testsuite/blob/master/nr-sack/handshake-with-nr-sack/handshake-with-nr-sack-7.pkt
• https://github.com/nplab/PR_SCTP_Testsuite/blob/master/nr-sack/handshake-with-nr-sack/handshake-with-nr-sack-8.pkt

[tuexen]

We also need a clarification in the specification...