TCP Fast Open

[Jeremy Harris]

Could I put in a request for better observability?  That is, for
an application to be able to tell (for both client and server cases)
if TFO was not tried / requested successfully / used with no data /
used with data ?

There is partial observability currently, via sockopts TCP_FASTOPEN
and TCP_INFO... and they are unreliable.

    [ I'll note here that Linux is no better ]


Also, a means for getting data on the SYN,ACK packet.  Currently
AFAIK we cannot; we can send data from the server (under TFO) before
the ACK of the SYN,ACK but only as a separate packet.

The use case in mind is SMTP, which is a server-first protocol.
Whether or not the facility would have to restrict to preloaded data,
or could manage an application interaction, is up for argument.
-- 
Cheers,
   Jeremy