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


From: Gary E. Miller
Subject: Re: [gpsd-dev] Updated docs on NTP segment management
Date: Thu, 26 Feb 2015 12:13:35 -0800

Yo Harlan!

On Thu, 26 Feb 2015 20:07:34 +0000
Harlan Stenn <address@hidden> wrote:

> "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.

+1

> 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.

Got a URL?

> 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.

That also works, but spawns complexity.  I would rather avoid.

> 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.

What is duplicated?  And how to handle multiple senders?

> 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

Yes, but not at the same time.  :-)
 
> - more than 4 units' data structures in an SHM segment

Absolutely.  207 supported now.  With linuxptp and soon more senders
talking NTP SHM this could grow large in the lab.


RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588



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