Re: Use of SAP in a PIM SSM Network

[Marshall Eubanks <tme@21rst-century.com>]

Toerless Eckert wrote:
> 
> > I am trying to understand in a pure SSM network without any shared tree,
> > how would SAP operate? My understanding is that SAP is transmitting on a
> > shared tree. Does it mean that we always stuck with a PIM-SM shared
> > tree, even in a pure SSM network?
> 
> I think you have a network without a shared tree and without SAP. You can
> of course use SSM to get SAP information, but the receivers need to know
> the address of the sources in the first place, which is a bit beyond the
> point of what SAP was meant for in the first place. SAP is nice to have
> in limited environments, but simply trying to broadcast directory information
> on a world-wide scale is not a scaling solution. Already today lots of
> SDP sessions are just made available via the web instead of SAP.
> 
> -- Toerless

I am not sure that I agree with this. We would certainly be interested
in hosting a directory service using SSM, and I suspect others would as
well. I would, in fact, claim that in some ways a multicast directory
service scales better than web pages FOR AGGREGATED INFORMATION.

Suppose that one fine day there are 50,000 SSM groups in permanent operation.
Clearly, a web page covering ALL of these groups would basically be a search
engine - just a list would not be useful. If each group was announced
by a sap type packet with a size (say) of 200 bytes, to go through the entire
list would require 10 megabytes. Even if 8 kilobits/sec were allocated to
a multicast directory service, it would still take > 3 hours to go through
all of these announcements. If, however, the groups were divided into 100
classes, each with ~500 members, and each class getting its own SSM group
for directory announcements, all the sessions in a class could be
discovered in
only 100 seconds, about the time it takes the program announcements on Channel
1 of my cable TV service to cycle through. I think that such a service
would be
useful; therefore I think that, while SAP may die, multicast session announcements
will live on.



                                   Regards
                                   Marshall Eubanks


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