Re: TCP Header Flags

[Francois-Xavier Le Bail <devel.fx.lebail () orange fr>]

On 18/02/2023 21:51, Denis Ovsienko wrote:
> On Sat, 18 Feb 2023 17:06:29 +0100
> Francois-Xavier Le Bail <devel.fx.lebail () orange fr> wrote:
>
> > Hello,
> >
> > https://www.rfc-editor.org/rfc/rfc9293 states:
> > "Control bits:
> >
> >     The control bits are also known as "flags". Assignment is managed
> > by IANA from the "TCP Header Flags" registry [62]. The currently
> > assigned control bits are CWR, ECE, URG, ACK, PSH, RST, SYN, and FIN."
> >
> > (All on three characters.)
> >
> > To be in sync with it, we could use 'tcp-psh' in addition to
> > 'tcp-push' in libpcap scanner.l, and in pcap-filter.7 and tcpdump.1
> > man pages.
>
> That's an interesting point.  Adding "tcp-psh" would certainly restore
> consistency with the registry on one hand.  On the other, for backward
> compatibility reasons "tcp-push" would have to remain a valid alias for
> who knows how many years.

We could keep "tcp-push" indefinitely...

> I wonder if there would be any other incurred future maintenance.
>
> The fact tcpdump(1) prose says "PUSH" instead of "PSH" may be related
> to the origin of this discrepancy.  Perhaps this is the only part that
> can be fixed immediately without unwanted side effects.

There is not even consistency in the tcpdump man page (PUSH and PSH):
$ git grep -En '\<(PSH|PUSH)\>' tcpdump.1.in
tcpdump.1.in:1322:F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
tcpdump.1.in:1389:The PUSH flag is set in the packet.
tcpdump.1.in:1415:.I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
tcpdump.1.in:1486:left, so the PSH bit is bit number 3, while the URG bit is number 5.


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