Re: [TLS] Efficiency of ACKing scheme

[Hanno Becker <[e-mail]>]

Hi Thomas, Hi Ekr,

> > * Receiving ACKs: Upon receipt of an ACK, implementations should note
> > which messages have been received and omit them from future
> > retransmissions. It is up to the implementation to decide when to
> > retransmit and what to retransmit, but it is recommended they
> > retransmit after a period of time during which no further ACK messages
> > have been received. They may also proactively retransmit parts of a
> > flight early if an ACK message indicates a gap (note, though, that in
> > this example one would only retransmit the gap, not the gap + tail as
> > before).
>
> Looks like a sound proposal to me.  The only problem I see with this is
> that recovery from tail loss is not efficient, which might or might not
> be a problem, depending on the loss pattern of your path.
>
> > By contrast. we do know that DTLS retransmission is too slow and given
> > that small flights are common, losing the ability to indicate that you
> > lost a tail seems undesirable.
>
> I agree this should be avoided if possible.

As far as I see, tail loss indication involves a timer in both cases:

- As it stands, tail loss recovery is triggered by the ACK resulting from the 'lack of progress' indicator of
disruption, described in the second bullet point of https://tools.ietf.org/html/draft- ietf-tls-dtls13-37#section-7.1<https://tools.ietf.org/html/draft-ietf-tls-dtls13-37#section-7.1>
In particular, it only occurs after a 'short' timer triggered on the receiver, where by 'short' I mean that is
has smaller threshold than the ordinary retransmission timer from DTLS 1.2, marking the bottom line
recovery time we want to improve upon.

- Likewise, there's short timer based recovery in the new proposal, but mirrored: The sender retransmits upon
noticing a gap in the ACKs, which too can be detected by a short timer as in the current proposal.

Best,
Hanno

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