RE: Measurement bit(s) or not

["Lubashev, Igor" <[e-mail]>]

I am a bit sceptical about the square. With TCP, packet trace shows loss of any packet, except for SYN. The "square wave" whould show no loss at all for short connections (fewer than a "wavelength" of packets). Moreover, the square wave can only detect loss after up to the wave length of subsequent packets has been transmitted. If there is an application pause in transmission, loss may not become evident.

Basically, a spin bit or a bit pattern (like a square wave) may help in troubleshooting, but it will be more uncertain and hokey than what people come to expect of tcp. We have be very clear in acknowledging it.

-----Original Message-----
From: [e-mail] [[e-mail]]
Received: Saturday, 10 Feb 2018, 12:34PM
To: [e-mail] [[e-mail]]
Subject: Measurement bit(s) or not

On 07/02/2018 14:34, Brian Trammell (IETF) wrote:
 > hi Jana,
 >
 >> 3. Some sequencing information -- a few bits of the packet number perhaps
 >> -- should be revealed (for monitoring. Number of bits TBD.)
 >
 > This is the crux of the argument. On one side we have the risk of misuse and
 > ossification (well, not ossification -- these bits are *meant* for the path
 > -- rather the risk that we'll figure out later that we specified the wrong
 > thing), on the other side we have the loss of visibility into how QUIC
 > traffic interacts with the network as compared to TCP, with a side question
 > of whether or not this visibility is really the transport layer's problem
 > despite the evolution the practice of diagnostics and troubleshooting using
 > TCP information.
 >
 > If we can come to agreement on this question, everything else falls into
 > place. I have my arguments here, but as you said, this subthread is not the
 > place for them. :)

The crux indeed. So what about settling it first ?

With the troubleshooting hat, I can only stress the need for measurement bits,
for the benefit of everybody, since s**t happens, networks are imperfect, and
nifty encapsulations-with-seqnum will simply not be where you need them.

Now to the exact nature of these measurement bits:

Thanks to the detailed exchanges on this thread, it is by now clear that a
simple gapless counter, even nonzero-based and XORed, is not acceptable. The
4-bit SSN comes pretty close but is not enough when things go really wrong (and
they will - and that's where we need the tool).

Then Kazuho's square signal and Mikkel's Pi (or any other consensual
self-synchronizing sequence) ramification came up. They are both appealing for
their elegance and low complexity on QUIC endpoints. Beyond their quirks
acknowledged here, here are a few considerations for troubleshooting:

  (1) Since reordering is less of a concern to QUIC than to TCP, it becomes a
secondary goal. This is nice, because the square doesn't see it, and the
self-synchronizing sequence will only tolerate a mild one, and never see its
detail like cycle length etc.

  (2) There's of course a huge difference between them in complexity for the
midpoint: square is trivial, Pi is hefty.

Given these, a benevolent, troubleshooting-minded passive midpoint will clearly
vote for the square. Now the obvious question is: is this acceptable, or deemed
too easy for a Murphy, Inc. active middlebox to see upstream losses and
benevolently wreak havoc by delaying packets ?

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