In the Network framework "header" file I have found the following doc comment for NWConnection.startDataTransferReport: /// Start a new pending data transfer report on a connection. Multiple /// reports may be created for a single NWConnection. The report begins /// capturing data when the connection moves to the .ready state, or when /// the report is created (whichever occurs last). So it seems that not capturing the TLS handshake is the intended behavior. Too bad! I also noticed that TLS alert messages are not included in the data report.

Thanks for checking with the network team so quickly! I have filed a bug: FB22181144

Yes, I already tested that method. But unfortunately, sentTransportByteCount and receivedTransportByteCount don’t include the Handshake payload even when I call startDataTransferReport before starting the connection. So I get, for example, 35/31 Bytes instead of 1674/3070 Bytes for a simple "send 'Hello' and cancel" TLS connection.

[quote='817942022, DTS Engineer, /thread/770707?answerId=817942022#817942022'] And Safari? [/quote] It seems that it is a problem with Chromium-based UDP flows. For Chromium, Chrome, Brave, Opera I get the "The peer closed the flow" errors, for Safari and Firefox not.

With Safari flows it works as well.