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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?
http://queue.acm.org/detail.cfm?id=2717320
> > 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
easier.
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?"
H
- Re: [gpsd-dev] Updated docs on NTP segment management, (continued)
- Re: [gpsd-dev] Updated docs on NTP segment management, Hal Murray, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Eric S. Raymond, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Terje Mathisen, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Gary E. Miller, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Harlan Stenn, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Gary E. Miller, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management,
Harlan Stenn <=
- Re: [gpsd-dev] Updated docs on NTP segment management, Gary E. Miller, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Harlan Stenn, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Gary E. Miller, 2015/02/25
- Re: [gpsd-dev] Updated docs on NTP segment management, Eric S. Raymond, 2015/02/25
- Re: [gpsd-dev] Updated docs on NTP segment management, Harlan Stenn, 2015/02/25
- Re: [gpsd-dev] Updated docs on NTP segment management, Eric S. Raymond, 2015/02/26
- Re: [gpsd-dev] Updated docs on NTP segment management, Hal Murray, 2015/02/28
- Re: [gpsd-dev] Updated docs on NTP segment management, Hal Murray, 2015/02/18
- Re: [gpsd-dev] Updated docs on NTP segment management, Gary E. Miller, 2015/02/18
- Re: [gpsd-dev] Updated docs on NTP segment management, Harlan Stenn, 2015/02/18