[Tinyos-help] [TinyOs 2.x] ACK Bug?

[Michael Schippling]

Hmm...interesting. Why would #2 happen? Message overrun?

If it was only #1 it would seem that you would have at worst
a 1::2^16 chance of a falsely correct CRC and then an even
smaller chance of it being the type field that was corrupted.

That said, the chance of missing a valid ACK is much greater
than getting a false one.

MS

Philip Levis wrote:
> On Dec 23, 2008, at 2:00 AM, Pratibha S wrote:
> 
>> Hi All,
>>
>> We are running an algorithm in TOSSIM over 200-nodes and we are  
>> experiencing packet losses due to improper ACKs. i.e. The sender  
>> would receive the packet.wasAcked() signal, but the receiver wouldnt  
>> have got the message. Is this a bug? Any help on this issue would be  
>> appreciated.
>>
> 
> This isn't a bug.
> 
> Experimentally, I've found that motes receive false positive  
> acknowledgements. This is rare, but it does happen. There are two  
> possible reasons:
> 
> 1) The packet was corrupted, but passed CRC. If the AM type is  
> different, then unless you have a handler for that type the stack will  
> just discard it.
> 
> 2) The packet is read properly by the radio and passes CRC, but the  
> software stack flushes the receive memory before reading it out.
> 
> Because net2 has observed this to happen experimentally on pretty much  
> every testbed we've used, I put it into TOSSIM. That way, your code  
> doesn't make incorrect assumptions about the behavior of hardware.  
> Remember the end-to-end argument: you can't depend on link-layer acks  
> to be sure that the packet arrived successfully at a network layer.
> 
> IIRC, TOSSIM issues false positive acks .1% of the time (1 in 1000),  
> which is approximately what net2 observed in CTP tests on telosb and  
> micaz hardware (CC2420).
> 
> Phil
> _______________________________________________
> Tinyos-help mailing list
> Tinyos-help at millennium.berkeley.edu
> https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help

-- 
Platform: WinXP/Cygwin
TinyOS version: 1.x, Boomerang
Programmer: MIB510
Device(s): Mica2, MicaZ, Tmote
Sensor board: homebrew