如何测量从客户端发送数据到服务器所花费的时间

Question

I am sending data from the client to a server. I want to accurately calculate the time taken for the data to reach the destination. The client and server are on different PCs, so I think it is unwise to use the system time as they are on different systems.

Please advise the most accurate method for measuring the time taken for the data to reach the destination.

Answer 1

close will block until all data is sent, so if you start time at write , and end after close , you will have the send time. For UDP, the data will have been transmitted, and for TCP it will have been acked. If you are sending to a Mars rover, then the time will be round trip, but normally the ack time is small.

start_time = someTimeFunc();
char *foo = buf;
while (size) {
    ssize_t sent = send(sock,foo,size,flags);
    if (sent <= 0)
        fail;
    foo += sent;
    size -= sent;
}
close(sock);
end_time = someTimeFunc();

Answer 2

When an app "sends" data, the data is not actually transmitted right away. It is put in a queue in the OS kernel, and then control is returned to the app. The kernel will transmit the queued data in the background on its own time. There is no way to know when the kernel actually transmits, or how long the data takes to arrive as the destination. And even then, the data is put into another kernel queue at the receiving end, waiting for the receiving app to extract the data from that queue.

So, the only way to do what you are asking for is to have the receiver explicitly send an acknowledgement back to the sender, specifying when the data was actually read by the receiver. The sender and receiver will have to agree on a time scale, like UTC, and also have their clocks synced to a common source. However, this will NOT be a true reflection of the transmission time, but it will likely be close if both sender and receiver are handling their respective socket I/O's in a timely manner.