Re: Re: More xpl communication problems

[Kevin Hawkins <lists@xxxxxxxxxxxxxxxxx>]

Mal Lansell wrote:

>It does beg one question, though.  Why the need for more than one
>hub?  Doesn't the heartbeat broadcast supply all the information
>required to establish the link between a device and a single hub?
>After all, if the hub sent the config.list request to the IP and port
>I gave it in the heartbeat, everything would have worked despite
>being on different IPs.  I know there is probably a good reason (and
>I'll probably kick myself when I hear it) - I'm just curious to know
>what it is ;-)
>
>
Hi Mal,

Only one application can succesfully acquire and receive UDP
data from a specific port (under Windows) -  xPL packets although
'broadcast' are sent to a specific port (3865) and hence on any PC only
one application can receive them . This could be any single xPL
application, however if more than one application needs to listen then a
hub is required.  Transmit only applications are not effected.
The hub (which must be the first xPL application launched) acquires
the xPL specific port of  3865 and then acts as a relay agent for all
subsequent xPL applications launched on that machine. The applications
transmit directly to the broadcast  address but listen via the hub. So
if you later  launch an application that uses say port 40123 then the
hub notices that (from the heartbeat info) and relays incoming data it
receives on port 3865 out again on port 40123 for the specific benefit
of that application. I think, (although I could be wrong with xPL) that
it does this using the loopback address of 127.0.0.1 and that the ports
that subsequent (hub client) applications acquire should be in the
ephemeral range (32768 upwards).
The typical effect of two or more xPL applications being launched on
the same machine without a hub is that only the first application
receives xPL messages and all the rest appear 'deaf' - although they can
transmit OK,   As John says if you have only one application then there
will be no problem as long as that utilises the xPL 3865 port but two or
more applications will require a hub on most , if not all O/S's.

K


( I hope that info is right for xPL)