When I successfully write data to a TCP/IP socket, as I understand it, I am only guaranteed the data gets to the TCP/IP stack's buffer. However, a successful write doesn't guarantee that the data actually gets to the recipient. Since data can linger in the TCP/IP stack's buffer "indefinately," it is quite possible that the TCP/IP socket be closed before the buffered data is ever delivered.

Of course, since the recipient's side is also buffered, I realize the sender's TCP/IP stack can successfully flush its buffer, presumably meaning it entered the recipient's buffer, but the recipient never actually has to read it. The recipient's failure to read the data will, long term, cause their buffer to fill. As their buffer fills the packets will be acked in such a way to cause the sender's TCP/IP window reduce until, ultimately, the recipient's buffer fills entirely and the sending stack is blocked. At which point, since the sending stack can't flush, the sender's buffer will fill up, which will then cause subsequent writes, to a blocking socket, to block or, to a non-blocking socket, to return -1 setting errno to EAGAIN.

So...I suppose that's a long way of saying that buffering in the two TCP stacks make it hard to figure out, just by a successful call to write, whether or not the data written actually got to the recipient. Further, without an acknowledgment from the recipient, knowing the data was actually read is impossible. This leads me to my question.

Assuming I can't have the recipient acknowledge the response (protocol limitation), the best I could presume to do is verify the data got to the recipient's TCP/IP buffer. Since the two TCP/IP stacks will ack the packets under the hood, checking the sender's TCP buffer capacity should be sufficient to tell me how much data has been undelivered. If this is true, is there any good way to accomplish it? I ask because, given I can't modify the protocol to contain an acknowledgement, knowing the data got to the recipient's buffer is superior to "well, I stuck it the TCP/IP buffer (write succeeded), it must have got there."

The O/S is AIX 5.3 if it matters. Also, feel free to correct any misunderstandings I may have about the process.

Thanks!