Re: [Ntp] Fwd: Re: Antwort: Re: TLS records vs NTS-KE

[Miroslav Lichvar <[e-mail]>]

On Wed, Jun 27, 2018 at 12:38:19PM +0200, Martin Langer wrote:
> But what can we do if we need requests larger than 1K (maybe in the future)?
> For example: A new record type needs parameters/key material/certificates
> and the size of this records is over 1K.
> ....but ok. Actually it is not a problem, I guess.

Do you think there might be a case where we would need to exchange
keys or certificates as application data? I'd expect that to be
handled by TLS itself.

If it does turn out later that 1K is too small for a request, in the
worst case I think we can always define "ntske/2" as a new ALPN name
for the same protocol with larger limits.

> cookie: 256
> 
> The definition of the cookie size is a bit ...tricky. ...and do you mean the
> cookie itself or the whole record?

Just the cookie, i.e. the data the client has to save in order to send
a valid NTP+NTS packet after NTS-KE is finished.

> But the NTP connection can be interessting:
> - max. space for NTS extension fields:  1500 bytes MTU (Ethernet) - 20 bytes
> IPv4 header - 8 bytes UDP header - 48 bytes NTPv4 header = 1424 bytes
> - UniqueID_ExtField and AuthAndEncExt_ExtField need around 80 bytes --> so
> we have 1344 bytes for cookies EFs
> - current guideline: max. 8 cookies or placeholder  ---> 1344/8 = 168 bytes
> (without IP fragmentation)
> ....but I think this is not a problem. With a limit of 255 bytes we can
> transmit 5 cookies/placeholder without  IP fragmentaion

That's a good point. We should also not forget that clients may use
extension fields for other things than NTS.

-- 
Miroslav Lichvar