Fix compatibility with old system usrsctp v0.9.5.0

src/impl/sctptransport.cpp

@@ -88,6 +88,9 @@ void SctpTransport::Init() {
 	usrsctp_init(0, SctpTransport::WriteCallback, SctpTransport::DebugCallback);
 	usrsctp_sysctl_set_sctp_pr_enable(1);  // Enable Partial Reliability Extension (RFC 3758)
 	usrsctp_sysctl_set_sctp_ecn_enable(0); // Disable Explicit Congestion Notification
+#ifndef SCTP_ACCEPT_ZERO_CHECKSUM
+	usrsctp_enable_crc32c_offload(); // We'll compute CRC32 only for outgoing packets
+#endif
 #ifdef SCTP_DEBUG
 	usrsctp_sysctl_set_sctp_debug_on(SCTP_DEBUG_ALL);
 #endif
@@ -267,13 +270,15 @@ SctpTransport::SctpTransport(shared_ptr<Transport> lower, const Configuration &c
 		throw std::runtime_error("Could not disable SCTP fragmented interleave, errno=" +
 		                         std::to_string(errno));
 
-	// When using SCTP over DTLS, the data integrity is ensured by DTLS. Therefore, there's no need
-	// to check CRC32c additionally when receiving.
-	// See https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-sctp-zero-checksum
+#ifdef SCTP_ACCEPT_ZERO_CHECKSUM // not available in usrsctp v0.9.5.0
+	// When using SCTP over DTLS, the data integrity is ensured by DTLS. Therefore, there's no
+	// need to check CRC32c additionally when receiving. See
+	// https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-sctp-zero-checksum
 	int edmid = SCTP_EDMID_LOWER_LAYER_DTLS;
 	if (usrsctp_setsockopt(mSock, IPPROTO_SCTP, SCTP_ACCEPT_ZERO_CHECKSUM, &edmid, sizeof(edmid)))
 		throw std::runtime_error("Could set socket option SCTP_ACCEPT_ZERO_CHECKSUM, errno=" +
 		                         std::to_string(errno));
+#endif
 
 	int rcvBuf = 0;
 	socklen_t rcvBufLen = sizeof(rcvBuf);
@@ -964,6 +969,15 @@ void SctpTransport::UpcallCallback(struct socket *, void *arg, int /* flags */)
 int SctpTransport::WriteCallback(void *ptr, void *data, size_t len, uint8_t tos, uint8_t set_df) {
 	auto *transport = static_cast<SctpTransport *>(ptr);
 
+#ifndef SCTP_ACCEPT_ZERO_CHECKSUM
+	// Set the CRC32 ourselves as we have enabled CRC32 offloading
+	if (len >= 12) {
+		uint32_t *checksum = reinterpret_cast<uint32_t *>(data) + 2;
+		*checksum = 0;
+		*checksum = usrsctp_crc32c(data, len);
+	}
+#endif
+
 	// Workaround for sctplab/usrsctp#405: Send callback is invoked on already closed socket
 	// https://github.com/sctplab/usrsctp/issues/405
 	if (auto locked = Instances->lock(transport))