Reading Notes – TCP tuning

TCP tuning

What is delay ack

TCP is a reliable protocol because of sending ack after receiving the request.

TCP delayed acknowledgment is a technique used by some implementations of the Transmission Control Protocol in an effort to improve network performance. In essence, several ACK responses may be combined together into a single response, reducing protocol overhead.

In one word, if delay ack is on, the ack of one request can be sent within another request together. Or if there are two ack packages, it will also be sent. Or if it’s over 40ms, the single ack will be sent.


The logic of Nagle is like:

if there is new data to send
  if the window size >= MSS and available data is >= MSS
        send complete MSS segment now
    if there is unconfirmed data still in the pipe
          enqueue data in the buffer until an acknowledge is received
          send data immediately
    end if
  end if
end if

If the data size is larger than MSS, send it directly. Otherwise, see if there are any requests haven’t received ack. if not, we can wait until we receive the previous ack.

If the client is using Nagle and the Server side is using delay ack

Sometimes, we have to wait until the request reaches the delay ack timeout. example

Some HTTP requests will divide the header and content into two packages which can trigger this issue easier.


Found online

struct curl_slist *list = NULL;
list = curl_slist_append(list, "Expect:");  

if (CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_URL, oss.str().c_str())) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_TIMEOUT_MS, timeout)) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &write_callback)) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L)) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_POST, 1L)) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, pooh.sizeleft)) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback)) &&
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_READDATA, &pooh)) &&                
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1L)) && //1000 ms curl bug
        CURLE_OK == (code = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, list))                
        ) {

        //这里如果是小包就不开delay ack,实际不科学
        if (request.size() < 1024) {
                code = curl_easy_setopt(curl, CURLOPT_TCP_NODELAY, 1L);
        } else {
                code = curl_easy_setopt(curl, CURLOPT_TCP_NODELAY, 0L);
        if(CURLE_OK == code) {
                code = curl_easy_perform(curl);

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