[6tisch] Questions on TSCH CSMA-CA retransmission algorithm in IEEE 802.15.4-2015

[Yasuyuki Tanaka <[e-mail]>]

Hello all,

Could anyone help me understand correctly the TSCH CSMA-CA
retransmission algorithm described in Section 6.2.5.3 of IEEE
802.15.4-2015, starting from page-64?

I have some questions...:

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[1] What does it mean exactly by "the backoff window"?

There is no "backoff window" found in Figure 6-6 nor in the latter
part of Section 6.2.5.3...

I think that "the backoff window"indicates BE (Backoff Exponent), but
it's unclear...


[2] Should we reset BE every time retransmission process starts?

Two conditions to reset the backoff window (BE?) are mentioned in page-64:

- "A successful transmission in a shared link resets the backoff window
  to the minimum value."

- "The backoff window is reset to the minimum value if the
  transmission in a dedicated link is successful and the transmit
  queue is then empty."

In page-65, I see another condition if the backoff windows and BE are
the same thing:

- "A device upon encountering a transmission failure in a shared link
  shall initialize the BE to macMinBe."

It seems the former conditions and the latter condition conflict each
other.

I'm not sure, after all the retransmissions for the last frame in
shared links failed, the next retransmission process should reset BE or
not. What about the case when the transmission for the last frame in a
dedicated link was successful but the transmission queue was NOT
empty?


[3] Errors in Figure 6-6

I don't think Figure 6-6 is correct...

- In the frist transmission attempt (the right-most branch):

  o Boxes of "transmit frame" and "transmission acknowledged?" are
    missing after "N" to "TschCca=On?"
    
  o It says transmission fails when NB exceeds macMaxFrameRetries; but
    I think, it would enter the retransmission process instead of
    "Failure". Which is correct?
    
- In the retransmission process without PCA (the middle branch):

  o "NB" is not initialized; should "NB=0" be there along with
    "BE=macMinBe"?
    
  o BE should be updated by "BE=min(BE+1, macMaxBe)" instead of
    "BE=min(BE-1,macMinBe)", shouldn't it?

I didn't look closer the left-most branch (retransmission with PCA), by the
way.


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Thank you for taking the time to read my email!

Best,
Yatch