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[ssm] building multi-sender apps with ssm



[Was Re: Re: Re: [ssm] Re: last call comments on ssm-arch doc ]

Hopefully these are the last tweaks.

Hitoshi pointed out that this sentence of what I wrote

>     SSM is particularly well-suited to dissemination-style applications
>     with a single sender.  

can be interpreted to mean that SSM isn't well-suited to applications
with more than one sender.  I think he is right and furthermore, I
realized that I'm missing a fundamental aspect in this paragraph: The
key attribute of an SSM-based application that introduces the need for
application-layer rendezvous is not the fact that there are multiple
senders but the fact that the senders are not all known in advance, so
I ought to say that.

Furthermore, my example application of a backup data source is more in
the category one of an application with two sources that are known in
advance, so I moved it to follow the text that describes that
application model.

So... this is hopefully my last try on this paragraph.  

  SSM is particularly well-suited to dissemination-style applications
  with one or more senders whose identities are known before the
  application begins.  For instance, a data dissemination application
  that desires to provide a secondary data source in case the primary
  source fails over might implement this by using one channel for each
  source and advertising both of them to receivers.  It can be used to
  build multi-source applications where all participants' identities are
  not known in advance, but the multi-source "rendezvous" functionality
  does not occur in the network layer.  Just like in an application that
  uses unicast as the underlying transport, this functionality can be
  implemented by the application or by an application-layer library.

  Peer-to-peer multicast resource discovery of the form in
  which a client sends a multicast query directly to a "service
  location group" to which servers listen is not directly supported
  by SSM

-Hugh
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