curl / libcurl / API / curl_easy_setopt / CURLOPT_TCP_FASTOPEN

CURLOPT_TCP_FASTOPEN explained

Name

CURLOPT_TCP_FASTOPEN - TCP Fast Open

Synopsis

#include <curl/curl.h>
 
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);

Description

Pass a long as parameter set to 1L to enable or 0 to disable.

TCP Fast Open (RFC 7413) is a mechanism that allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, saving up to one full round-trip time (RTT).

Beware: the TLS session cache does not work when TCP Fast Open is enabled. TCP Fast Open is also known to be problematic on or across certain networks.

Default

Protocols

This functionality affects tcp only

Example

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    curl_easy_setopt(curl, CURLOPT_TCP_FASTOPEN, 1L);
    curl_easy_perform(curl);
  }
}

Notes

This option is only supported on Linux and macOS 10.11 or later.

Availability

Added in curl 7.49.0

Return value

Returns CURLE_OK if fast open is supported by the operating system, otherwise returns CURLE_NOT_BUILT_IN.