Re: [EXT] Re: [ippm] New QUIC Packet Loss Measurement (draft-cfb-ippm-spinbit-measurements)

[Dmitri Tikhonov <[e-mail]>]

Hello Fabio,

On Thu, Apr 23, 2020 at 01:27:23PM +0000, Bulgarella Fabio (Guest) wrote:
> > Do you not agree that locating loss source is one of the probe's goals?
> 
> It is more interesting to know if a problem occurs in our domain or
> outside. And this is done by the Qbit (With 2 probes).

Consider the following setup:

    Client ... (OpA) ... (OpB) ... (OpC) ... Server

We are OpB, and Client's and Server's lawyers are banging at our door
with a legally binding SLA signed with our blood.  What do you think is
a realistic way of getting rid of the situation:

    a) Here are captures that prove that loss is in OpC, or in your
       premises, Mr Server.

    b) Here are captures that have no losses in OpA or OpB.  Have a
       good day.

Since a unidirectional QR capture does not include any evidence of
loss downstream, it would be dismissed by the Server lawyers as just
not capturing the right traffic.  An equivalent packet dump with TCP
would include evidence of loss in the form of packet retransmission,
and so would QL scheme.

Simultaneous captures by different organizations are often hard to
coordinate, especially for problems that do not reproduce all the time.
At any given point of the investigation, you depend only on your own
local resources and need to be able to provide evidence that makes sense
to everybody.

> However, it is not uncommon to be able to intercept the whole traffic
> of a connection by observing a single point.  In this case, all the
> necessary measurements can be made.

How common is "not uncommon:" 90%? 50%?  Here is a study that shows how
common symmetric flows are: out of six observations of four different
Internet links, the percentage of symmetric flows was 57%, 7%, 2%, 3%,
2%, and 3%:

  https://www.caida.org/research/traffic-analysis/asymmetry/

The way routing works is asymmetric in essence, as the destination decides
what it announces, which leads to the way to reach it.  When you revert
the arrow, the source cannot but just follow the routes.  The result is
a high prevalence of asymmetric routing.

  - Dmitri.