listen fastopen

[Maxim Dounin]

Hello!

On Thu, May 04, 2017 at 04:10:33AM -0400, beatnut wrote:

> Hello,
> Does anybody know what this warning which i found in the docs
> http://nginx.org/en/docs/http/ngx_http_core_module.html#listen  in the
> context of fastopen means?
> 
> "Do not enable this feature unless the server can handle receiving the  same
> SYN packet with data more than once."
> 
> My kernel is 4.9.9  and cat /proc/sys/net/ipv4/tcp_fastopen is 3 so this
> feature is supported. Is this enought ?

No, this note is not about what your kernel can handle, but rather 
about further processing by nginx and backend servers.

Main problem with fastopen is that it allows unintentional request 
duplication: the same request may be received (and answered by 
nginx) more than once.  This is usually ok for static files, but 
may not be ok for various dynamic scripts and so on.

-- 
Maxim Dounin
http://nginx.org/