Jim Henderson wrote:
> Arguably, it would be a technology - not a protocol - implementation.

Yeah, basically.

> I may need to read more on this.

Yep. There's (IIRC) ICMP messages to control the routing (via expressing 
interest), and just UDP with multicast destination addresses to deliver the 
data.

> But that isn't the case, necessarily.  The switch is allowing the packet 
> to get to those subscribers who've requested it.  Whether that's 
> duplication or filtering is really just semantics.

Sure. As is "1:1" vs "1:many" when every pair of receivers had an 
independent connection, which was my point. :-)

> That depends on a lot of different factors.  If there's 10 stations at 
> the far end of the third router hop, the amount of bandwidth between 
> routers is still a constant 1:1; 

Yep. And if there's 10 people on the conference call, my phone is only 
sending my voice once. I completely agree with you. I'm simply stating that 
ISDN and all those other network technologies work the same way, so IP 
multicast isn't a distinguishing feature.

>>>> True, but now we're talking about something far below the level of IP.
>>> Not in a Layer3 switch we're not. ;-)
>> I'm pretty sure IP says nothing about how you implement copying a packet
>> from one NIC to another. :-)
> 
> That's because the switch provides the implementation details; the 
> protocol doesn't implement itself.

Hmmm. You seem to be disagreeing with yourself. Unless you were joking and 
saying a layer 3 switch is only a little below IP and not far below.

> I think that made sense to me.

That's all I've been trying to express. IP multicast is equivalent to ISDN 
conference bridge, where the bridge serves as the router doing the routing 
to multiple outgoing segments.

>>> The destination "1" is a single multicast address that the specific
>>> targets subscribe to - that's what makes it not 1:1, but 1:many.
>> Yes. And 858-102-1137 is a single multicast address that hosts a
>> conference bridge. Is that not 1:many, if many phones call into that
>> address?
> 
> It is, sure.  Each phone is a "subscriber" to the bridge.  The "data" 
> isn't sent to phones that aren't subscribers.

OK. That was what I was trying to express. Saying IP multicast is 1:many and 
ISDN conference bridge isn't is not a reasonable comparison.

> I don't think I was saying that.  I haven't commented on ISDN, only on IP 
> multicast.

Fair enough. That's where this came up, and why I'm talking about layers 
below IP. Otherwise, I think we both understand how IP handles multicast.

>>> Not with multicast IP; again, if you don't subscribe to it, you don't
>>> receive it, at least that's my understanding (http://
>>> www.multicasttech.com/faq/ provides a good explanation).
>> If you're on a broadcast network, you have no choice but to receive it.
> 
> True, because you see everything, addressed to your or not.  That's why 
> hubs are used for sniffing, generally.

Exactly my point. That's why I said multicast in that case doesn't "save" 
bandwidth.

>>> Well, quite possibly, unless he was talking about multicast as a
>>> reason.
>> I don't think so...
> 
> In which case the discussion has been an 'evolution'. ;-)

 From his other message, I think he doesn't realize that an ISDN connection 
has an address at all, or something.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

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