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

[Fabrice Théoleyre <[e-mail]>]

Hello Yatch,


> [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…

the backoff window is selected randomly between 0 and 2^BE-1. 

> [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."

If it corresponds to a retransmission, this means that the previous transmission has failed. 
So, this case does not correspond here.

> - "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."

dedicated links > no collision / no backoff. 
You transmit it without delay 
(NB: you may have the first transmission during a shared link, and its retransmission in a dedicated one)

> 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."

According to figure 6.6, yes with TSCH: the packet has been dropped, it corresponds to a novel packet, and it enters in the condition "PCA=Y" after the first transmission  

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

they don’t actually correspond to the same thing.
You must make a distinction between shared and dedicated links, and you may use a succession of shared and dedicated links for the same packet (i.e. its different retransmissions)

> 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?

If I understand correctly the standard, in that case, the dedicated link should not impact the BE value: the next packet will be picked in the queue, and transmitted with the same BE value as previously. 

Probably to combat unfairness: a transmitter with dedicated links would else succeed to transmit packets in its dedicated links => it would reset more frequently its BE  => more bandwidth than the nodes without dedicated links. 

> [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?

This part seems strange. It seems mixing the case "first transmission" and "no ack required". 
First transmission -> no backoff (the current description is correct)
no ack -> the first transmission is considered successful (the transmitter cannot have any feedback). Thus:
     => the box "transmission acknowledged?" is missing
     =>  the arrow after "NB > ** ?" should point to "retransmission?" instead of "Wait for the next TX link"
 
> - In the retransmission process without PCA (the middle branch):
> 
>  o "NB" is not initialized; should "NB=0" be there along with
>    "BE=macMinBe"?

no, this is a retransmission so, NB cannot be equal to 0 (to my mind)
"NB is the number of times the CSMA-CA algorithm was required to back off while attempting the current transmission"

>  o BE should be updated by "BE=min(BE+1, macMaxBe)" instead of
>    "BE=min(BE-1,macMinBe)", shouldn't it?

+1

I have also the same doubts as you for the figure 6.6….

Best regards,
Fabrice