Re: [TLS] Efficiency of ACKing scheme

[Hannes Tschofenig <[e-mail]>]

Hi Thomas, Hi Ekr,

It seems we need to fix Figure 11 of Section 7.1 of
https://www.ietf.org/id/draft-ietf-tls-dtls13-37.html<https://www.ietf.org/id/draft-ietf-tls-dtls13-37.html#section-7.1>
(ACK[1] should actually be ACK[])

It should follow the “empty ACK” description of the last paragraph of the same section.

Good catch!

Ciao
Hannes

From: TLS <[e-mail]> On Behalf Of Eric Rescorla
Sent: Thursday, April 9, 2020 4:33 PM
To: Thomas Fossati <[e-mail]>
Cc: [e-mail]
Subject: Re: [TLS] Efficiency of ACKing scheme



On Thu, Apr 9, 2020 at 7:28 AM Thomas Fossati <[e-mail]<mailto:[e-mail]>> wrote:
On 09/04/2020, 15:18, "Eric Rescorla" <[e-mail]<mailto:[e-mail]>> wrote:
> > On Thu, Apr 9, 2020 at 6:59 AM Thomas Fossati <[e-mail]<mailto:[e-mail]>> wrote:
> > On 09/04/2020, 14:20, "Eric Rescorla" <[e-mail]<mailto:[e-mail]>> wrote:
> > > Assuming I understand Hanno's proposal, I do not believe that this
> > > is in fact an improvement, as it does not cover the important case
> > > where the record containing the SH is lost and then the rest of
> > > the messages from the server are uninterpretable.
> >
> > I don't want to speak for Hanno here but the refinement proposed in
> > [1], specifically the bit that says:
> >
> >   [...] They may also proactively retransmit parts of a flight early
> >   if an ACK message indicates a gap.
> >
> > should cover the case you mention I think.
>
> But this requires being able to send an empty ACK that means "I got
> nothing". In which case, I don't see how it's really different from
> the current text except that it gives the sender less guidance.

Not sure there's an "empty ACK" in Hanno's proposal.  This is how I
interpret it in the context of your example: in the second flight, if
rec#0 (containing SH) is lost and rec#1 gets through, the receiver sends
ACK {1}.  From that the sender can infer the gap and retransmit rec#0.

You can't send ACK{1} because you can't  decrypt it because it is out of order with respect to the DH key. This is the point of the empty ACK.

-Ekr



(But again, I'm not him and that's why I suggested collecting all the
pieces of this discussion together in one PR.)

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