[Ntp] Public NTP servers already responds to NTPv5

[g16 <[e-mail]>]

Hi, NTP WG and Miroslav !

I'm yuki.

I am interested in NTPv5 and ossification.

So I surveyed how the currently public NTP servers respond to NTPv5 packets.

As a result, as you may know, some NTP servers respond to NTPv5 packets.
Surprisingly, the version field of some responses were 5.

## Survey method
I sent two packets to about 70 public NTP servers

- NTPv4 format packet that version field is 5
- NTPv5 format packet (defined in draft-mlichvar-ntp-ntpv5-00)

the tool is here: https://github.com/flano-yuki/mock-ntpv5-client

## Result
> NTPv4 format packet that version field is 5
- 25% response: timeout
- 65% response: NTPv4 format packet that version field is 5
- 10% response: NTPv4 or NTPv3

> NTPv5 format packet (draft-mlichvar-ntp-ntpv5-00)
- 10% response: NTPv4 format packet that version field is  5
- 90% response: timeout

The result depends on the date of Transmit Timestamp and the existence of
Extension Field.

full result list:
https://gist.github.com/flano-yuki/5a587ab0197b23fe647d148a087b82ef

## Is this a problem?
I'm not sure. It may confuse implementations that support nptv5.

I know some WG suffered from anomalous behavior in tcp-fast-open and TLS
1.3 deployments.
- https://mailarchive.ietf.org/arch/msg/tls/i9blmvG2BEPf1s1OJkenHknRw9c/
- https://archive.nanog.org/sites/default/files/Paasch_Network_Support.pdf

but, it may be not matter with following reason in NTP WG
- This response is not a problem for the client, because it can be filtered
- It will be fixed as NTPv5 develops/deploys

##  Who should solve the problem?
Yes, this is an implementation issue.  RFC5905 (9.2) saids
"Format checks require correct field length and alignment, acceptable
version number (1-4), and correct extension field syntax, if present."

Even if it is fixed, I think it will take some time for all public ntp
servers to be replaced. Meanwhile, we may be able to mitigate this problem
with the v5 design that does not cause incorrect response.

I welcome any comments.

Thank you for reading the entire email.

Best regards