Re: [TLS] Efficiency of ACKing scheme

[Thomas Fossati <[e-mail]>]

On 06/04/2020, 02:49, Martin Thomson <[e-mail]> wrote:
> I think that you are assuming a lot about how the loss recovery part
> of the sender is operating here.
>
> If you receive ACK { 1, 3 }, then it might be reasonable to assume
> that 2 got lost, but it seems to me that assuming anything about 4 is
> premature.

The spec as currently written says that the signal "ACK {1, 3}" means
"retransmit {2, (3, end]}":

   Upon receipt of an ACK for only some messages from a flight, an
   implementation SHOULD retransmit the remaining messages or
   fragments.

This works as expected only if the sender can assume that the receiver
has let enough time pass before generating ACK {1, 3}, but this is not
strongly stated in the description of receiver behaviour.

We should keep in mind that DTLS implementers are not necessarily
transport experts, and that warrants a bit more care in what we say as
well as what we don't say.  In particular, we should try hard to avoid
expert bias.

One example where we can improve: we have a nice sentence about bunching
ACKs at 25% of current RTO halfway through bullet 2 of Section 7.1.  In
fact, avoiding knee-jerk ACKing is one of the basic things to understand
here.  Incidentally, this is how one's solves Hanno's conundrum above
(at least in non-pathological cases) because "retransmit {2, (3, end]}"
would be generated when the receiver has got some confidence about 4 &
co. being actually lost.  Unfortunately, the sentence is oddly placed
and one tends to overlook it and not giving it the high and general
relevance it actually has.

> The sender can also make some adjustments, without necessarily
> adhering to a strict interpretation of the recommendations in the
> spec.

Agree again, but deciding to violate a SHOULD should be an exception,
not the rule.  If I have to implement this, especially on very
low-bandwidth paths that fragment a lot, I probably would never follow
the recommendation: I'd first re-send 3 and sit back for a while waiting
for further ACKs before resending (3, end].

> The draft is imprecise about this logic intentionally.  It recommends
> that the sender send missing data, which will likely work, and be
> fast.  For large flights, yes, this will be suboptimal, but we are
> also assuming that this data is not subject to congestion control
> limits.

Just a note: "large flights" is relative to the available bandwidth so
(very) low-bandwidth would always have suboptimal recovery even if HS
data is not congestion controlled.  And that's the environment where you
absolutely want to have a reliability scheme that is bandwidth
efficient.

> If we go to PQ schemes with large amounts of data, then that requires
> a different set of assumptions.  It might also require that
> implementations take extra steps to avoid the resulting
> inefficiencies.

True that.

Cheers!

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