I-FORWARD-TSN-Chunk is used although support has not been indicated by both endpoints

[TheAomx]

When an endpoints wants to use the user message interleaving and activates it by calling:

+0.0 setsockopt(3, IPPROTO_SCTP, SCTP_FRAGMENT_INTERLEAVE, [2], 4) = 0
+0.0 setsockopt(3, IPPROTO_SCTP, SCTP_INTERLEAVING_SUPPORTED, {assoc_value=1}, 8) = 0

after receiving an INIT-Chunk like:

INIT[flgs=0, tag=1, a_rwnd=1500, os=16, is=16, tsn=1, FORWARD_TSN_SUPPORTED[], SUPPORTED_EXTENSIONS[types=[FORWARD_TSN, I_DATA]]]

and completing the handshake, it uses the I-DATA-Chunk, which should be acceptable, because of section 2.2.1. of sctp-ndata internet draft:

A sender MUST NOT send an I-DATA chunk unless both peers have
indicated its support of the I-DATA chunk type within the Supported
Extensions Parameter as defined in [RFC5061]. If I-DATA support has
been negotiated on an association, I-DATA chunks MUST be used for all
user messages and DATA-chunk MUST NOT be used. If I-DATA support has
not been negotiated on an association, DATA chunks MUST be used for
all user messages and I-DATA chunks MUST NOT be used.

If one outstanding user message is abandoned it uses the I-FORWARD-TSN-Chunk that the other endpoint does not support. It should use the FORWARD-TSN-Chunk i assume. See the following test-case for more details:

• https://github.com/nplab/PR_SCTP_Testsuite/blob/master/i-forward-tsn/handshake-with-i-forward-tsn/handshake-with-i-forward-tsn-9.pkt
• https://github.com/nplab/PR_SCTP_Testsuite/blob/master/i-forward-tsn/handshake-with-i-forward-tsn/handshake-with-i-forward-tsn-14.pkt

[tuexen]

You can't use the FORWARD-TSN chunk in combination with the I-DATA chunk. The text in
draft-ietf-tsvwg-sctp-ndata-08.txt doesn't seem to explicitly cover this situation. You might want to bring this question up on the tsvwg@ietf.org mailing list. The ID is currently in WG-last call.

[TheAomx]

Yeah, FORWARD-TSN-Chunk does not make sense because there is no ssn value with the I-DATA-Chunk right?

Should I expect an ERROR-Chunk instead? I have never sent questions to the tsvwg@ietf.org mailing list. Do I have to write a mail to tsvwg@ietf.org with the subject WG-last call to do so?

[tuexen]

You can subscribe to the list and then send an e-mail asking about how an implementation should behave when it received an INIT (or INIT-ACK) chunk indicating I-DATA and only FORWARD-TSN support or only DATA and I-FORWARD-TSN.

I guess I would expect I-FORWARD-TSN only be used if both sides indicate support and also indicate I-DATA support. If both sides support I-DATA and one side only FORWARD-TSN, then not PR-SCTP can be used. This is what I would suggest. But we need clear text in the spec.