Re: [gpsd-dev] Updated docs on NTP segment management

[Harlan Stenn]

"Gary E. Miller" writes:
> The mode word can be used as a version.

Yes, and folks who did not check for "unused bits must be zero" will
lose, and that's OK in my book.  I would have rather seen an explicit
version field in the mode bits, and I'm game to add that now declaring
the first (old) version bits are 0.

Adding to that, in my ACM article about NTP one of the first things I do
is point folks at Dan Geer's Blackhat 2014 talk, which covers this point
nicely.

One thing to think about is whether or not the SHMID should be different
for each version where there is a change in size of shmTime.

I'm also wondering if the V2 SHM interface should include a preamble
structure before the per-unit structures, as that seems to be a more
efficient way to go.  Duplicating data in the per-unit structures seems
wasteful to me.

I also wonder if there is a good way (and a good reason) to support:

- a mix of V1 and V2 SHM areas on the same shmid

- more than 4 units' data structures in an SHM segment

That's the list off the top of my head.

H