One of the topics that came up in the architectural debate is that a few folk
made statements of the form that application developers assume that
applications only engage in bilateral communications. In fact one person went
so far that applications developers are not aware of the range of applications
protocols.
But more generally, some appear to have voiced the opinion that the IETF
transport area only serves the IETF applications area, not the Internet
application developer community which is many, many times the size of the IETF.
No examples were given of these non-application application protocols. So here
is why there can only be bilateral communications at the application layer.
From the point of view of an application there are only two significant
classes of object:
1) Itself
2) Anything that is not itself
Every communication at the application layer, in every protocol that I am aware
of boils down to a bilateral communication between those two groups. There are
thus four possibilities
1) Application talking to itself, not as unusual as it might seem. I was once
surprised to find that a DEC tape drive would not work unless RPC was turned on
despite having a direct SCSI connection to the machine.
2) Inbound communication
3) Outbound communication
4) Communication that does not involve the application directly but may affect
the application indirectly.
As far as an application is concerned, multicast is simply a communication with
a service that is not itself. There may be one or a million hosts in a
multicast session but the session itself is a single service. It is like being
in an IRC chat room, there is one chat room, but many hosts.
If multicast protocols appear to require applications to have direct knowledge
of an IP address, that is a layer violation. From my own point of view I am
really skeptical about any protocol that requires state to be maintained in the
Internet backbone to operate (as opposed to local state for optimization)
As it is however, the multicast protocols don't seem to be any different here.
As far as the receivers are concerned they might as well be the only receiver
in the entire world. As far as the source is concerned there is only the source
and 'rest of world'.
Similar arguments may be applied to local broadcast.
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