[tcpm] review of draft-zimmermann-tcpm-reordering-detection

[Wesley Eddy <[e-mail]>]

I reviewed the reordering detection algorithm document:
https://tools.ietf.org/html/draft-zimmermann-tcpm-reordering-detection-02

The document is very well written.  The specification in the draft seems
quite clear and like it would be easy to implement directly from the
document.

The purpose of section 4.7 ("Placeholder for Response Algorithm") isn't
clear to me, especially in relation to the separate reaction draft
(draft-zimmermann-tcpm-reordering-reaction).

One thing that was unclear to me, and seems useful, is a discussion of
the potential sources of error.  In other words, what events would
cause the computed metrics to be wrong and how significantly wrong?
There are a number of different events and potential corner cases that
are already addressed in the algorithm, and it wasn't clear to me
"what's left" or if it's supposed to just work perfectly in all cases.
(I'm sure it's not!)

Obvious things that occurred to me were:
- different compositions of loss and reordering "features" in the
  forward path:
  - lossiness followed by reordering
  - reordering followed by lossiness
  - reordering followed by reordering somewhere else (and do we even
    care about this, since it's the state of the packets at the
    receiver that matters, and not how many times the order has been
    shuffled)
- loss and reordering in the ACK path

The sensitivity to these things, I think, matters in understanding the
utility of the computed metrics, and their feedback into a response
function.


-- 
Wes Eddy
MTI Systems