Re: The first octet

<alexandre.ferrieux@orange.com> Wed, 08 August 2018 13:22 UTC

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Subject: Re: The first octet
To: Martin Thomson <martin.thomson@gmail.com>
CC: Kazuho Oku <kazuhooku@gmail.com>, QUIC WG <quic@ietf.org>
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From: <alexandre.ferrieux@orange.com>
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On 08/08/18 12:05, Martin Thomson wrote:
> 
> I know you discussed taking more bits and even bytes, but the case
> hasn't been made in this working group to my knowledge.  I certainly
> don't think that's justified; it improved fidelity of measurements,
> but the spin bit appears to be pretty good at that without the extra
> overheads.

Fidelity is indeed the criterion to decide 1 (spin bit) vs 3 (spin bit + VEC) 
for RTT measurements, right.

However, measurements also include packet loss, which is typically more 
widespread than delay in big pipes (which don't have enough RAM to keep a packet 
for too long at line rate). And spin+VEC do not address packet loss at all, 
hence our 7-bit spin counter proposal shown in London.

I admit that 7 is hefty if you're starting from Christian's songle spin bit ; 
however:

  - I assume nobody will emit any doubt about the criticality of 
non-delay-related (big pipe) loss location. Unless everybody here has perfect 
networks and ideal reporting, that is.

  - Stick the spin bit on top of these 7, make that an extra easurement byte and 
you've got your three spare bits again in the first byte.


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