Set SCTP zero checksum

[melpon]

Based on the discussion at #1012, where it was concluded that enabling SCTP_ACCEPT_ZERO_CHECKSUM is not an issue, I have implemented zero checksum.

[paullouisageneau]

I don't think this works, since checksums for outgoing packets will always be computed here, even if the remote side is willing to accept zero-checksum packets:

libdatachannel/src/impl/sctptransport.cpp

Line 977 in 283488d

*checksum = usrsctp_crc32c(data, len);

[melpon]

I see, it seems I misunderstood the specification. Thank you for the comment. I'll make the corrections.

[melpon]

I had been working on an implementation to eliminate the CRC calculation during transmission, but there were some issues.

The first issue is that it seems usrsctp users cannot be informed when SCTP_ZERO_CHECKSUM_ACCEPTABLE is received. As a result, it's not possible to determine within sctptransport.c whether there's a need to compute the CRC during transmission.

Thus, I decided to disable CRC offloading and let usrsctp handle the CRC computation. As long as we include usrsctp_setsockopt(IPPROTO_SCTP, SCTP_ACCEPT_ZERO_CHECKSUM), the CRC check during reception should not be conducted, similar to when CRC offloading is enabled.

The second issue is that, when usrsctp is connected using the AF_CONN address family, it appears to compute the CRC even if SCTP_ACCEPT_ZERO_CHECKSUM has been received from the server. This seems like a bug, so I've submitted a Pull Request addressing this at sctplab/usrsctp#690. However, it's uncertain if or when it will be merged.

If there's a better implementation approach, please let me know.

[paullouisageneau]

It looks good now, thank you! I hope the issue with AF_CONN sockets in usrsctp can be sorted out, thanks for submitting the PR.