The IP Over NBMA (ION) Archive

[Tim Dwight <0006078043@mcimail.com>]

>> In the ATM Forum's TM 4.0 specification, standardization of ABR was 
>> limited to pt-pt as a practical consideration, in order to complete 
>> the specification.  There is no doubt that it will be extended to 
>> pt-mpt.  There was considerable work done in this area which did not 
>> make the final edit of the TM specification.  All agree that it is a
>> solvable problem, but more than one solution exists and we did not 
>> reach agreement on all the details, in time to make the document's 
>> publication date.
>
>From the best effort IP multicast point of view, if, among
>1,000 receivers, 999 can accept 6Mbps and 1 can't accept
>more than 64Kbps, what should the rate of the sender be?
>What if, it's 800 vs 200? If it's not a sender but an
>intermediate router at the edge of ATM network, what can
>it do?
>
>The answer, I think, is, it depends on the application and the
>sender controls everything.
>
>So, while ATM forum may have it's own policy of bandwidth
>management, any bandwidth control suggested by the network
>is not useful for IP.


I think that's a rather sweeping statement.  Is control over loss
probability not useful to IP?

I think pt-mpt ABR can be useful for IP, but I agree with your concern
about the non-usefulness (or non-scalability, anyway) of any service 
which constrains the throughput of the multicast tree according to the
rate of the slowest branch.  I hope it was clear from my previous mail,
that I do not believe this to be a necessary charactistic of pt-mpt ABR.
I only meant to say that due to the incomplete standardization, some
early implementations may "get away with" such behaviour.

I think it may be difficult to have a productive discussion of this
subject, until either the specification is clarified (so that the 
required service characteristics are clear) or we have experience with
actual implementations.


best regards,

tim


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