[tcpinc] TCP's treatment of data in SYN packets

[Kyle Rose <[e-mail]>]

In the absence of a TFO cookie, what is the behavior of TCP in the presence
of data in the SYN packet? The ability of ENO to send early data on session
resumption to servers that potentially don't understand ENO depends on the
servers throwing this data away so it can be replaced in the following ACK.

RFC 4987 section 3.5 (https://tools.ietf.org/html/rfc4987#section-3.5)
states that:

q( If data accompanies the SYN segment, then this data is not
   acknowledged or stored by the receiver, and will require
   retransmission. )

But I have read elsewhere that the server might queue it up and wait for
the 3WHS to complete (though if it is conformant, it will ACK only a single
byte for the SYN and have the blob retransmitted by the client anyway).

I guess I'm interested in both normative and observed behavior. I'm happy
to inquire elsewhere if someone has a suggestion. (tcpm?)

Kyle