On Jul 25, 2008, at 11:28 AM, John Hendrie wrote:
> Hi,
>
> Is Drip designed so one node changes the variable and the rest
> listen or is it designed so that every node in the network can
> change the shared variable?
>
> I ask because of the way the sequence number is generated in
> DisseminatorP.nc:
>
> /* Increment the counter and append the local node ID. */
> seqno = seqno >> 16;
> seqno++;
> if ( seqno == DISSEMINATION_SEQNO_UNKNOWN ) { seqno++; }
> seqno = seqno << 16;
> seqno += TOS_NODE_ID;
>
> Doesn't this mean that a node with the highest TOS_NODE_ID will
> force other nodes in the network to ignore valid changes made by
> nodes with smaller IDs because it will always have the largest seqno?
You have the significant bits reversed. Dissemination needs to have
eventual consistency, such that in a connected network all nodes
converge on the same value. There's a race condition, where two nodes
that have version V both decide to update the value to version V+1.
This could lead to a problem where all nodes think they have the same
version but some have one value and others have a second value.
Drip and DIP solve this problem by making version numbers 32 bits,
where the least significant 16 bits are the node ID. That way, if two
nodes A and B both increment to version V+1, then in reality the
versions are
(((V + 1) << 16) + A) and
(((V + 1) << 16) + B)
Whichever (A or B) has a larger node ID will win the tie, and the
network will converge on a consistent value. However, as the node ID
is the least significant 16 bits, then if the node with a lower ID
increments the version number to V + 2, it will win over any V + 1.
Phil
_______________________________________________
Tinyos-help mailing list
[email protected]
https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
