[Xastir-Dev] my aprsdigi multicasting

[Alan Crosswell]

If you are on a part of the Internet that participates in global multicast 
(generally this only applies to higher ed and r&d sites -- see 
http://multicast.internet2.edu/) then you simply run a client that joins the 
multicast group.  The internals of redisributing multicast are handled by "the 
network" which builds a shortest path tree to distribute content only to those 
who have indicated they want it.  This is called "Protocol Independent Multicast 
- Sparse Mode" (PIM-SM) so the traffic does not in fact get broadcast to the 
whole world but could if everyone in the whole world wanted to see it.  It is 
actually much more efficient than the multiple-unicast model used by typical 
streaming media or even APRS-IS.  The main down side is that global multicast 
connectivity is still in its infancy outside the research and education 
networks.  However, the R&E networks could be used to hub out multicast which is 
then multiple-unicast (or gated to RF).

For the local RLAN case, it is just a broadcast and is simply a means of using 
IP but recognizing that the medium is in fact a broadcast medium and is more 
efficiently used that way for this kind of data.  Imagine if all APRS over AX.25 
was connection-oriented instead of connectionless and you had to connect to 
these central nodes even if your station and your neigbhors could both directly 
hear the traffic.  (Kinda like packet clusters:-)
/a

Curt Mills, WE7U wrote:
> On Tue, 25 Mar 2003, Gerry Creager N5JXS wrote:
> 
> 
>>Woo-HOOOO!
>>
>>gerry
>>
>>Alan Crosswell wrote:
>>
>>>I'm about to release a new version of aprsdigi.  aprs.w2aee.columbia.edu
>>>is currently running it and multicasting to 233.0.14.99/12345.  Might be
>>>cool if xastir could handle multicast UDP (IPv4 or v6).  You can
>>>multicast back to 233.0.14.99 but you need to tell me you are doing it
>>>so I can add you to the budlist.  I envision using UDP multicast via I2
>>>as well as more locally within an RLAN....
> 
> 
> Ok, so tell the rest of us what this means.  I have a very fuzzy,
> grey, fog-like definition in my mind of what you're talking about.
> 
> As I understand it, multicast allows you to broadcast 1-to-many, but
> I don't know whether individual clients have to "register" somewhere
> to be able to receive these broadcasts.  I would expect so, as
> broadcasting these things everywhere on the 'net at once would clog
> up the 'net awfully fast (with lots of broadcasters at once).
> 
> So what's the procedure for a client?
>