[Tinyos-devel] retransmissions in networking protocols

[Andreas Koepke]

> and read the discussion with subject:
> 
> "L2 retx and CTP FE"
> 
> 
> 
> - om_p

Thanks, done. I understand that the decision was postponed. Just for the
records: the tda5250 MAC protocols had atomic ACKs and retries since the
first working CSMA, if I remember correctly. So it is not only the
CC2420 that can do it. I added this because of the PacketAcknowledgment
interface: If I have to support it, I can also handle the retries. In
addition, I guessed that there is a reason why many wireless standards
like 802.11 and 802.15.4 define retries as part of the MAC.

At MAC level the random retransmission delay can be computed without
much problems, they can also be adapted more easily to the duty cycle
than in the network protocol. Of course the network protocol can
retransmit more or less immediately and trust the fairness mechanisms of
the MAC to introduce a sensible amount of delay. I don't care, as long
as the performance does not suffer too much when the ACK status in the
packet is a "false negative".

Best, Andreas

> 
>>I'd vote for including hop by hop filtering if we have hop by hop
>>retransmissions in the network protocols. Or remove retransmissions from
>>the network and let the MAC/LinkLayer do this kind of stunts.
>>
>>Best, Andreas
>>
>>Matt Welsh wrote:
>>
>>>Actually these protocols perform retransmissions at the *link* layer,
>>>not the network layer (the latter would imply end-to-end).
>>>
>>>I'm firmly in the camp that link-layer ARQ is a necessity with these
>>>kinds of radios, due to interference and collisions (the MAC could
>>>perhaps be tuned to help avoid the latter).  Better route selection -
>>>choosing only 'symmetric' links - can cut down on duplicate packets. One
>>>could imagine a node detecting many duplicates from its children telling
>>>those children to reduce their ARQ threshold. Even so I think filtering
>>>out duplicates hop-by-hop or at the base station is perfectly
>>>acceptable.
>>>
>>>On Thu, 2007-10-04 at 14:02 +0200, Andreas Koepke wrote:
>>>
>>>
>>>>Hi guys in the net group,
>>>>
>>>>looks like MultihopLqi and Ctp all do retransmissions on the network
>>>>level if the MAC reports a transmission failure. So whenever the MAC did
>>>>not receive an ACK the packet is duplicated. This happens quite often in
>>>>our testbed, because the TX power of the nodes seems to have a
>>>>siginficant variance.
>>>>
>>>>What is your experience, should we do something about it?
>>>>
>>>>Best, Andreas
>>>>
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>>>
>>>
>>>
>>
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> 
>