1---
2c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
3SPDX-License-Identifier: curl
4Title: CURLOPT_TCP_FASTOPEN
5Section: 3
6Source: libcurl
7See-also:
8  - CURLOPT_SSL_FALSESTART (3)
9---
10
11# NAME
12
13CURLOPT_TCP_FASTOPEN - TCP Fast Open
14
15# SYNOPSIS
16
17~~~c
18#include <curl/curl.h>
19
20CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);
21~~~
22
23# DESCRIPTION
24
25Pass a long as parameter set to 1L to enable or 0 to disable.
26
27TCP Fast Open (RFC 7413) is a mechanism that allows data to be carried in the
28SYN and SYN-ACK packets and consumed by the receiving end during the initial
29connection handshake, saving up to one full round-trip time (RTT).
30
31Beware: the TLS session cache does not work when TCP Fast Open is enabled. TCP
32Fast Open is also known to be problematic on or across certain networks.
33
34# DEFAULT
35
360
37
38# PROTOCOLS
39
40All
41
42# EXAMPLE
43
44~~~c
45int main(void)
46{
47  CURL *curl = curl_easy_init();
48  if(curl) {
49    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
50    curl_easy_setopt(curl, CURLOPT_TCP_FASTOPEN, 1L);
51    curl_easy_perform(curl);
52  }
53}
54~~~
55
56# AVAILABILITY
57
58Added in 7.49.0. This option is currently only supported on Linux and macOS
5910.11 or later.
60
61# RETURN VALUE
62
63Returns CURLE_OK if fast open is supported by the operating system, otherwise
64returns CURLE_NOT_BUILT_IN.
65