[Tinyos-devel] [Tinyos Core WG] TEP105: LowPowerListening DutyCycles

[Janos Sallai]

>> >> I think TEP 105 should redefine the semantics of cancel, without
the
>> >>
>> >> clause that requires that the packet is not transmitted yet (or
with
>> >>
>> >> an error code different than ECANCEL in sendDone?).
>> >
>> >[..]
>> >If it
>> >is using XMAC-style cyclic sending, then it must return FAIL on
>> >cancel(), but can signal an early sendDone() with SUCCESS. The
packet
>> >did go out over the air; the chances the receiver got it are much
>> >lower than if the complete send cycle occurred, but they are
non-zero.
>>
>> Yes, this is in sync with TEP 116. But this implies that a cancel
that
>> returns FAIL does have an effect on the behavior of the radio stack,
>> which is a bit counterintuitive. Anyway, this appears to be the best
>> solution so far.
>>
> To keep the semantics of the TEP, we would have to predict 
> the future whether we will get an ACK and depending on this prediction
return 
>either SUCCESS or FAIL to the cancel call.

No. If there has been at least one packet that went out, cancel() will
return FAIL, since there is a non-zero probability of the message having
been delivered. But, as a side effect of this failed cancel, the mote
stops transmitting subsequent packets and signals a sendDone() with
SUCCESS.

This is, in fact, in compliance with the TEP, because the TEP does not
say that a failed cancel must not have any side effects with respect to
transmission. It's a different question whether this is the best way to
implement cancel() in LPL mode...

Janos