On 29/08/2023 16:34, Scheffenegger, Richard via tcpdump-workers wrote:
> And some initial discussions which aren't yet reflected on the mailing list:
> -----Original Message-----
> From: Scheffenegger, Richard 
> Sent: Freitag, 18. August 2023 14:01
> To: Francois-Xavier Le Bail <devel.fx.leb...@orange.fr>; 
> tcpdump-workers@lists.tcpdump.org
> Cc: Denis Ovsienko <de...@ovsienko.info>; Guy Harris <ghar...@sonic.net>; 
> [...] ; Michael Richardson <m...@sandelman.ca>; [...]
> Subject: RE: Accurate ECN support in tcpdump/libpcap
> (Adding everyone who has participated in the discussion so far, since it 
> seems that tcpdump-workers mailman is not working, as well as a number of 
> additional recent committers and participants)


> I do not think there is ambiguity here.

Hi Richard,

I think there's an ambiguity here.
Using "A" as the new TCP flag for tcpdump is a mistake because it sounds like 
"ACK" (even if tcpdump prints "." for "ACK").
Thus my proposal to use "N", last letter of "Accurate ECN", as already 
suggested in some PR comments and a previous message.
Mnemonic: "Accurate EC[N]" or "[N]ew TCP scheme/mechanism".
- "M" from "[M]ore Accurate Explicit Congestion Notification", from your draft 
- "G" from "TCP Pra[G]ue", perhaps less mnemonic.

> Tcpdump - any every tool afterwards - has been using "." for ACKs.

This is incorrect. Wireshark, for example, does not use "." for ACKs.

> "N" had been in (very rare) use for the NS bit (RFC3540), which is now 
> obsolete 
> (https://datatracker.ietf.org/doc/status-change-ecn-signaling-with-nonces-to-historic/).
>  While this is the same bit (and arguably, the authors of RFC3540 could have 
> made more effort to get that bit adopted in tooling back in 2001-2003), the 
> semantics are dramatically different than the current use.
> "A" has never been in use before, as a single character abbreviation,

This is incorrect. Wireshark uses "A" for "ACK".
(e.g. TCP Flags: ····C··A··S·)

> and is in use for the last 2+ years in other tooling around packet mangling, 
> tracing, decoding and forging such as wireshark and packetdrill...
> (and yes, I did let the ball drop with tcpdump for a couple months, when all 
> the other tools were updated reflecting the "A" char change there, which was 
> completely uncontentious in those communities).

s/for a couple months/for 2 years and half/
Your Proposal to packetdrill: Jan 18, 2020. Your PR to tcpdump: Jul 27, 2022. 
Denis questions: Oct 23, 2022. No answer until Jul 23, 2023.

> I suspect it would create more confusion, if tcpdump was using a different 
> mapping, than other tools

There are already some different mapping between, for example, Wireshark and 
ACK: "." for tcpdump, "A" for Wireshark.
CWR: "W" for tcpdump, "C" for Wireshark.

If you think this is a problem for the new flag, have you considered modifying 
the two tools you mention with two small patches?

Wireshark, for example, prints "AE" without -V, e.g. "[SYN, ECE, CWR, AE]". No 
"A" alone here.
And the only place where the new "A" appears is with -V:
TCP Flags: ···ACE····S·
It appears with:
  "...1 .... .... = Accurate ECN: Set"

If it's a problem, it could easily be changed to e.g. "N", with a little patch.

Same for packetdrill.

> (just like with the "." for Ack, which is in common use throughout those very 
> same tools).

And again « the "." for Ack » is not used by Wireshark, one of « those very 
same tools ».

> Another interesting aspect which I would be keen on learning, if per-session 
> stateful decoding is something that the tcpdump community would entertain. 
> Once the AccECN handshake (in a SYN) was seen, subsequently the AE, ECE and 
> CWR bits together form the ACE counter. In packetdrill and also wireshark 
> this gets then decoded numerically (0..7) instead of having one character per 
> bit...

This per-session stateful session decoding could be examined later...

> Best regards,
>   Richard

Best regards,

> -----Original Message-----
> From: Francois-Xavier Le Bail <devel.fx.leb...@orange.fr> 
> Sent: Freitag, 18. August 2023 13:20
> To: Scheffenegger, Richard <richard.scheffeneg...@netapp.com>; 
> tcpdump-workers@lists.tcpdump.org
> Subject: Re: Accurate ECN support in tcpdump/libpcap
> NetApp Security WARNING: This is an external email. Do not click links or 
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> On 18/08/2023 09:55, Scheffenegger, Richard wrote:
>> Hi,
>> I’ve been asked to reach out to this mailing list, to gather some feedback 
>> around the support for tcpdump to properly decode the upcoming (WGLC) 
>> Accurate ECN signalling enhancement, which is part of the L4S (low loss, low 
>> latency, scalable) effort, ultimately culminating in a variant of TCP called 
>> TCP Prague:
>> Here is the change to tcpdump – adding the additional “A” TCP header flag 
>> bit; Since the ACK bit has traditionally been mapped to “.”, and the 
>> refurbished former Nonce bit #9 in the 12-bit TCP header flags field has 
>> been named “AE” (Accurate ecn Enable), the code changes to other tools such 
>> as Wireshark and Packetdrill are using the mapping to the character “A” (or 
>> decode Accurate ECN stateful into the ACE counter value of 0..7)
> Hi,
> Even if tcpdump prints the "ACK" flag with a ".", "A" is ambiguous precisely 
> because of "ACK".
> We should use "N" for the new flag, last letter of "Accurate ECN", to avoid 
> this ambiguity, as already suggested in a PR comment.
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