curl-w32/docs/libcurl/opts/CURLOPT_TCP_FASTOPEN.md

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---
c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_TCP_FASTOPEN
Section: 3
Source: libcurl
See-also:
- CURLOPT_SSL_FALSESTART (3)
---
# NAME
CURLOPT_TCP_FASTOPEN - TCP Fast Open
# SYNOPSIS
~~~c
#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
0
# PROTOCOLS
All
# EXAMPLE
~~~c
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);
}
}
~~~
# AVAILABILITY
Added in 7.49.0. This option is currently only supported on Linux and macOS
10.11 or later.
# RETURN VALUE
Returns CURLE_OK if fast open is supported by the operating system, otherwise
returns CURLE_NOT_BUILT_IN.