[Tinyos-devel] handling FTSP packets with errors

[Branislav Kusy]

committed the fix...

Philip Levis wrote:
> On Feb 27, 2009, at 3:11 PM, Omprakash Gnawali wrote:
> 
>> If a packet seems inconsistent (ENTRY_THROWOUT_LIMIT), FTSP will not
>> only discard that packet, but ignore rest of the packets with the same
>> root+sequence number, regardless of the packet origin. The latter
>> seems overly aggressive unless there is an evidence that inconsistency
>> is correlated in sequence number even across the nodes. If there is no
>> evidence of that, it might be better to allow the possibility of
>> "good" packets following "bad" packets with the same sequnence number.
>> Consider the possibility of running FTSP with TS_TIMER_MODE, there is
>> one node with some problem, and due to phase of the timer, this node
>> might always send the first "bad" beacon in a neighborhood for every
>> sequence number...
> 
> I don't think this is a major issue. But there is a bug in FTSP. In  
> addNewEntry:
> 
>      uint8_t numErrors=0;
>      void addNewEntry(TimeSyncMsg *msg)
>      {
>          int8_t i, freeItem = -1, oldestItem = 0;
>          uint32_t age, oldestTime = 0;
>          int32_t timeError;
> 
>          tableEntries = 0;
> 
>          // clear table if the received entry's been inconsistent for  
> some time
>          timeError = msg->localTime;
>          call GlobalTime.local2Global((uint32_t*)(&timeError));
>          timeError -= msg->globalTime;
>          if( (is_synced() == SUCCESS) &&
>              (timeError > ENTRY_THROWOUT_LIMIT || timeError < - 
> ENTRY_THROWOUT_LIMIT))
>          {
>              if (++numErrors>3)
>                  clearTable();
>          }
>          else
>              numErrors = 0;
> 
>          // Code for adding an entry follows
> 
> the bug is that a bad reading is incorporated into the table. So if  
> you hear one bad reading (++numErrors), you put it in the table. Then  
> subsequent good readings can seem bad. So a bad reading needs to not  
> be added. Furthermore, as this code counts the number of entries in  
> the table, you need to move that reset later in the function. The  
> function should be:
> 
>      uint8_t numErrors=0;
>      void addNewEntry(TimeSyncMsg *msg)
>      {
>          int8_t i, freeItem = -1, oldestItem = 0;
>          uint32_t age, oldestTime = 0;
>          int32_t timeError;
> 
>          // clear table if the received entry's been inconsistent for  
> some time
>          timeError = msg->localTime;
>          call GlobalTime.local2Global((uint32_t*)(&timeError));
>          timeError -= msg->globalTime;
>          if( (is_synced() == SUCCESS) &&
>              (timeError > ENTRY_THROWOUT_LIMIT || timeError < - 
> ENTRY_THROWOUT_LIMIT))
>          {
>              if (++numErrors>3)
>                  clearTable();
>              return; // Don't incorporate a bad reading
>          }
> 
>          tableEntries = 0; // Don't reset table size unless you're  
> recounting
>          numErrors = 0;
> 
> 
> Om and I have tested this fix and it makes FTSP maintain  
> synchronization much better. Happy to provide experimental data if  
> desired.
> 
> Phil
> 
> 
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