Re: [gpsd-dev] [PATCH v2 1/1] gps2udp: add a 'pack' mode

[Ferry Huberts]

On 03/01/14 21:53, Gary E. Miller wrote:
> Yo Ferry!
>
> On Fri, 03 Jan 2014 21:25:03 +0100
> Ferry Huberts <address@hidden> wrote:
>
> > > Seems like a lot of work for zero gain.  gpsd tries to keep the
> > > servers as simple as possible.  If the client wants to group things
> > > the client should do so.
> >
> > Well, I have a use-case for it. My use-case involves low-bandwidth
> > networks on which every byte counts.
>
> How low?  How many bytes does this really save?  A UDP header is 8
> bytes, is this really noticeable in your application?  Is it worth
> the added compleixty and latency?

How low?
- Our use-case is MANET networks, which can have bandwidths from ~1Kbps 
to over 100Mbps, fluctuating rather quickly and wildly, depending on 
circumstances.

How many bytes does this really save?
- (IP header + UDP header) * (number of sentences per pack - 1)

Is it worth the added compleixty and latency?
- Oh yes! This is also about saving on medium access. On our MANETs it's 
way more efficient to burst data in a single packet than to sent out 
many small packets.

As an example, our GPS hardware puts out these sentences every second, 
always in the same order:
> ---> [22:01:34] -- $GPRMC,.....
> ---> [22:01:34] -- $GPVTG,.....
> ---> [22:01:34] -- $GPGGA,.....
> ---> [22:01:34] -- $GPGSA,.....
> ---> [22:01:34] -- $GPGSV,.....
> ---> [22:01:34] -- $GPGSV,.....
> ---> [22:01:34] -- $GPGSV,.....
> ---> [22:01:34] -- $GPGLL,.....

Packing the sentences makes a huge difference on our MANETs.

It also simplifies our clients because now the received gps information 
is already packed, and they don't need to do that themselves.


> > Also, this pack mode
> > communicates to a listener an as-complete as possible GPS state in a
> > single packet.
>
> gpsd data goes stale really quickly.  Every time in the past we
> have attempted to accumulate state we have had to revert.
>
> > Furthermore, from my point of view gpsd is the server, gps2udp is a
> > simple client program of gpsd that performs a simple transformation
> > and sends out the result.
>
> Yes, gps2udp is the client to gpsd, but it is the server to the
> udp clients.  And you are making the transformation less simple.
>
> > I'm trying very hard to do the right thing here and upstream work we
> > have.
>
> Yes, and thank you for that.  On our end we want to be sure changes
> to git head suit all users.  As you have seen, if you work with us
> a little we often get to a good, if delayed, solution.
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
>         address@hidden  Tel:+1(541)382-8588

--
Ferry Huberts