TCP Fast Open

Jeremy Harris jgh at
Thu Dec 5 19:58:31 UTC 2019

On 05/12/2019 19:43, Michael Tuexen wrote:
> Are you asking how to figure out whether TFO was used or not?
> In that case you can use 
> getsockopt(fd, IPPROTO_TCP, TCP_FASTOPEN, &on, &len);
> If on == 0, TFO was not used, on != 0, TFO was used.

I'd found that, but it's so far unclear what choices it
makes among
- didn't try TFO
- tried TFO but the server didn't take it up
- server accepted the TFO cookie, but not the data that was on the SYN
- server acked the data

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