Zero checksum support for SCTP

[voluntas]

https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-sctp-zero-checksum-04

I would like to add to libdatachannel an RFC Draft support for zero checksum for SCTP.

WebRTC does not need to calculate the SCTP checksum (CRC32c) since SCTP works over DTLS.

Also supported by usrsctp
sctplab/usrsctp@ca70d3a

Support is also about to be added to dcsctp in libwebrtc
https://bugs.chromium.org/p/webrtc/issues/detail?id=14997&sort=-modified

Are there any negatives to adding this one?

[paullouisageneau]

libdatachannel already has an optimization not to verify SCTP checksums by leveraging CRC32 offloading as it's indeed unnecessary over DTLS:

libdatachannel/src/impl/sctptransport.cpp

Line 105 in 5145ba8

usrsctp_enable_crc32c_offload(); // We'll compute CRC32 only for outgoing packets

I see no issue with enabling SCTP_ACCEPT_ZERO_CHECKSUM, it should provide a significant performance increase as computing CRC32 is the second biggest CPU cost after encryption.

[voluntas]

Thanks for the quick response!

I see that there was already a mechanism in place to not calculate the checksum of SCTP packets received by usrsctp.

If that is the case, it seems that there is no problem to enable the function to set the checksum of SCTP packets to 0 if the Zero Checksum Acceptable Chunk Parameter handshake is successful. Now let's consider a Pull-Request!