[Tinyos-devel] CRC bug

[Razvan Musaloiu-E.]

Hi!

On Fri, 7 Dec 2007, Vlado Handziski wrote:

> My recollection is that his has been done in HW on the CC2420, but this is
> before the new stack was introduced.

The CC2420 chip verifies the CRC but it doesn't do any filtering based on 
this. David, can you please confirm this? :-)

Below is a patch that does the filtering in CC2420ActiveMessageP.nc.

diff --git a/tos/chips/cc2420/CC2420ActiveMessageP.nc b/tos/chips/cc2420/CC2420ActiveMessageP.nc
index 2cd35c8..4e1a452 100644
--- a/tos/chips/cc2420/CC2420ActiveMessageP.nc
+++ b/tos/chips/cc2420/CC2420ActiveMessageP.nc
@@ -164,6 +164,10 @@ implementation {

    /***************** SubReceive Events ****************/
    event message_t* SubReceive.receive(message_t* msg, void* payload, uint8_t len) {
+    cc2420_metadata_t* header = call CC2420PacketBody.getMetadata(msg);
+    if (header->crc == 0) {
+      return msg;
+    }
      if (call AMPacket.isForMe(msg)) {
        return signal Receive.receive[call AMPacket.type(msg)](msg, payload, len - CC2420_SIZE);
      }

Happy holidays to everyone!
Razvan ME

> On Dec 7, 2007 10:31 PM, Kevin Klues <klueska at gmail.com> wrote:
>
>> Does anyone know why the crc footer bit is never checked anywhere in
>> the entire tinyos-2.x tree?  In tinyos-1.x the radio stack passed the
>> packet up to the AM layer (AMPromiscuous.nc, AMStandard.nc) which
>> checked to see if  (footer->crc == 1) before passing the packet up the
>> stack.  The AM layer contains no such functionality in T2.
>> Because of this, a disconcerting number of packets (at least on motes
>> that use a cc1000) obtained by the application layer end up bing
>> corrupted.
>>
>> This is obviously a bug, the question is where this filtering should
>> actually be done.  As I mentioned before, in tinyos-1.x it was done at
>> the AM layer.  Is that where we want to keep it, or should it be
>> pushed down into the radio stacks themselves?  In what cases will we
>> ever want to receive a packet if it has a bad crc?  I can think of
>> situations where this might be useful (i.e. doing forward error
>> correction or something), but if such functionality were implemented
>> it should exist below the AM layer.
>> I guess my real question then, is where should we put this filtering
>> NOW so taht corrupted packets dont get sent up to the application.
>>
>> --
>> ~Kevin
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>>
>>
>>
>