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

From: Harlan Stenn
Subject: Re: [gpsd-dev] Updated docs on NTP segment management
Date: Thu, 26 Feb 2015 20:24:17 +0000

"Gary E. Miller" writes:
> On Thu, 26 Feb 2015 20:07:34 +0000
> Harlan Stenn <address@hidden> wrote:
> > 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.

If we start using a non-zero version number in the mode field that gets

And putting the "version" in the mode field is a case where I wonder if
that should be in the per-unit data or in the preamble.  There are
pros/cons with each choice.

> > 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?

The version of the SHM protocol/data structure, for example.

I don't understand the issue with multiple senders on this one.

> > 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.  :-)

Then that would be a "no" :)

This goes to multiple senders, or even a single sender that wants to
send V1 and V2 data to clients.

> > - 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.

This is another candidate for a "preamble" data area - "how many units
are on this SHM segment?"


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