[tcpm] <SYN,FIN> in SYN-SENT (793bis)

"Scheffenegger, Richard" <rs.ietf@gmx.at> Sun, 30 May 2021 11:00 UTC

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To: "tcpm@ietf.org" <tcpm@ietf.org>
From: "Scheffenegger, Richard" <rs.ietf@gmx.at>
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Date: Sun, 30 May 2021 13:00:25 +0200
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Subject: [tcpm] <SYN,FIN> in SYN-SENT (793bis)
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Hi,

I would like to gather some feedback around a recently observed
modification in one TCP stack:

The 793(bis) TCP FSM does not have any direct state-transition from
SYN-SENT to CLOSE-WAIT or FIN_WAIT_1, which would be the expected
logical transition, if a <SYN,FIN> is received in that state.

Rather, the 793bis prescribes, that the FSM transitions only to
SYN-RECEIVED (or ESTABLISHED), acking on the SYN-bit, but is silent on
how to treat a FIN bit observed in this state...


Logically, the receipt of a <SYN,FIN> could be broken into the SYN
processing (first) and FIN processing second.

Any thoughts if such an additional, explicit direct state transition /
FIN processing in SYN-SENT should be added to 793bis?

(only ACKing the SYN bit should cause the other end to retransmit the
FIN in a subsequent segment, ending in either of the two states
mentioned above - depending if the ACK bit is set too).

Best regards,
   Richard