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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.


> 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.

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

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