[AusNOG] TCP state

Mark Smith markzzzsmith at gmail.com
Fri Nov 13 18:09:45 EST 2020

On Fri, 13 Nov 2020, 14:33 Alex Samad, <alex at samad.com.au> wrote:

> Hi
> Quick question is it normal or acceptable to finish a TCP session or I
> should say start to finish session with a fin ack packet. When I google TCP
> state I get it's just a fin packet not fin/ack

Yes. The Ack is acknowledging the Fin.

A full close involves two pairs of FINs and matching ACKs in both
directions. It's actually possible therefore to have a half closed TCP
connection, where only one end has sent the FIN and it's been ACK'd, but
that hasn't happened in the other direction, and doesn't have to
immediately - and data can still be sent in the still half opened
direction. (Although I'd guess firewalls or NATs might stop that working
these days.)

The book "TCP/IP Illustrated", Volume 1 (2nd Edition now) is great for
explaining, and of course illustrating, this stuff.


> Alex
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