Thank you for your response. The program we have created is currently working as expected with QUIC Stream for the most part. Originally, we wanted to see how it would behave when communicating with DATAGRAM, but before we could see that, we first got an error sending the message, which is why we posted this topic. On another platform (Ubuntu Linux), when client/server programs using msquic and C Language communicate with each other, sending without wait may slow down the process but does not cause an error, However, in the communication between the iOS app created this time and the server using msquic above, many errors occurred shortly after the start of transmission. After that, when the send method of NWConnectionGroup was made to be executed serially using NSLock, the errors stopped occurring, but the CPU load became extremely high. (50-60% without NSLock, 100-130% with NSLock) The situation has changed a bit, but as you said, I will find time to create a simple project and send it to you as a report. Thank you very much.