[Din] draft-mazieres-dinrg-scp-04 (page 16) Potential improvement to balloting protocol, setting cCounter

[Piers Powlesland <[e-mail]>]

I think it may be possible to improve the chances of nodes being able
to accept ballots as committed during the prepare phase.

The following instructions are given to update cCounter.

> * If either "(prepared > c && prepared.value != c.value)" or
>   "(preparedPrime > c && preparedPrime.value != c.value)", then
>   reset "c = NULL".
>
> * If "c == NULL" and "hCounter == ballot.counter" (meaning
>    "ballot" is confirmed prepared), then set "c" to "ballot".


If we consider an example where a node has the following state.

ballot = (8,x)
prepared = (8,x)
preparedPrime = (3,y)
cCounter = 4
hCounter = 8

If the node accepts prepared (4,z) then using the update steps above c
will be set to NULL and then c will be set to ballot. This means that
the node will transition from voting to commit ballots with value == x
and counters 4...8 to just the single ballot (8,x).

Out of the ballots that the node was previously voting to commit only
(4,x) has been accepted aborted due to setting preparedPrime = (4,z),
so it seems that it would still be legal to vote to commit ballots
with value == x and counters 5...8.

So I am proposing that the first part of the above update step is
changed to the following, to allow for that.

> * If c != NULL set c.counter to the counter of the lowest ballot with value ==
>   h.value and not accepted aborted according to p and p'. If c.counter >
>   hCounter, then set c=NULL