Until when does NetworkStream.Write block?

Question

I can think of these possible answers:

• Until the data is written to some internal buffer in the IP stack.
• Until the data is sent over the wire.
• Until a confirmation of reception is received from the other machine.

Answer 1

Until data is written to the send buffer on the sender side.
So if buffer is full, it will block.

The send buffer can be full if it didn't transmit data yet, because of network issues or because receive buffer is full on the receiver side.

There is an experiment you can conduct: make a sender and receiver, set sender's socket send buffer to something small and receiver's receive buffer to something small to.

Start sending, accept connection on the receiver side, but don't receive. The socket will be blocked when the sent bytes number is about SenderSendBuffer + ReceiverReceiveBuffer.

Answer 2

NetworkStream does not buffer data. Ultimately, a call to NetworkStream.Write translates into socket send function call. MSDN article for this function says:

The successful completion of a send function does not indicate that the data was successfully delivered and received to the recipient. This function only indicates the data was successfully sent.

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