Re: Explicit measurability in the QUIC wire image (was Re: Packet number encryption)

[Christian Huitema <[e-mail]>]

On 2/7/2018 1:10 AM, Kazuho Oku wrote:
> The pros of using a sequence that randomly distributes 0 and ones
> (i.e. PI or xorshift+) is that you can extract good loss / reordering
> information if the error rate is low.
>
> OTOH, it is fragile to high error rate or certain patterns of error.
> Let me explain two examples.
>
> Consider the case where the packet loss rate is 50%. I doubt if you
> could use pattern matching against those sequences to figure out the
> loss rate. This is because the power among the sequence is mostly high
> frequencies. If you use a sequence that distributes power among lower
> frequencies (like square wave or sin wave that I discussed), you can
> calculate the loss rate in such cases as well.

Actually, it should be possible to build a maximum likelihood decoder
for an xorshift sending pattern. You are mostly concerned with erasure
and permutations, and you roughly know the range of probability, so you
can see likelihood of 1, 2, 3, etc. erasures, or permutations in some
range. Obviously, the range that you can manage depends on the length of
the code. If you can keep the decoder synchronized, you will be able to
measure the erasure rate. If it looses synchronization, you know that
the error rate is truly horrendous, and that's information too. And
xorshift codes are self synchronizing, so you will regain
synchronization quickly.

-- Christian Huitema