Jim Henderson wrote:
> I was simply pointing out that multicast (in particular) isn't 
> necessarily point-to-point from an IP standpoint. 

Sure. And it's not point-to-point from an ISDN or even POTS perspective 
either. Dialing into a conference bridge is the equivalent of putting the 
destination address as the multicast address in IP. It's lower-layer 
technology that actually does the multicasting, not IP itself.

>>>> You need to duplicate the packets at least once per physical wire.
>>>> Same as any network. IP doesn't force you to duplicate anything less.
>>> True.
>> That's my point. IP is still point-to-point, by necessity, when you
>> don't have a broadcast network.
> 
> This is where I think you're talking at the physical or network layer, 
> not the transport layer.

The transport layer isn't involved in multicast in IP, per se. Multicast 
transport layer is just UDP. The messages maintaining the spanning three 
modify things in the network layer.

> and the network hardware understands which ports 
> to send the multicast data to, 

Then it's not broadcast, and the switch is duplicating the data once for 
each subscriber. That's what I'm saying. *If* there's any bandwidth savings 
(i.e., more than one NIC reads the same packet), it's because every NIC in 
the broadcast area is reading the same packet and some of them are throwing 
it away.

>> The ISDN router (known
>> as the conference bridge) is doing the duplication, just like in IP.
> 
> Depends on whether you're talking unicast to *each* destination node or 
> multicast IP.  Therein lies the difference (and the bandwidth savings).  
> In a multicast scenario, at the transport layer, the switch isn't 
> "duplicating" the packet, it's forwarding it (which may be a semantic 
> debate).

It's duplicating it for each outward link the packet goes over.  If you have 
a router with someone upstream sending a multicast session and three routers 
downstream receiving it, how much bandwidth, total, is that taking up?

>>> the switches also typically have logic so they don't have to fill their
>>> buffers with multiple copies of the same data (for those that have a
>>> buffer).
>> 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. :-)

> Yes, physical layer (or network layer), not transport layer.  It feels 
> like you're conflating the two.

No. I'm pointing out that talking about the bandwidth savings, or "point to 
point" ness of IP implicitly is talking about the network and physical 
layers. Multicast *is* point to point, *unless* you're on a physical 
broadcast network. Multicast is point-to-point between any two elements of 
the network where there's a non-broadcast cable between them.

>> The only place that multicast saves bandwidth is on a unicast network
>> (like between routers) where nobody downstream of the receiver wants to
>> receive the data. If you have a separate wire to each PC with bandwidth
>> dedicated to that PC, multicast is 1:1 on any given segment of the
>> network.
> 
> 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?

> There again, though, mixing up physical/network layer with transport 
> layer.

Only because I'm trying to understand in what possible sense IP is 1:many 
and ISDN isn't. Sure, if a conference bridge is 1:many then so is an IP 
multicast. But I don't think you can say an IP multicast is 1:many and an 
ISDN conference bridge is 1:1, because it's doing the identical thing in the 
identical way.

>> Yes, the kind that carry IP via broadcast. Something like IP over CDMA
>> or GSM is much closer to having a wired connection than a broadcast
>> connection, since nearby nodes aren't listening to you.
> 
> Well, if it's DSS then that's true, but otherwise, the other nodes can 
> cause interference with you, so if they're doing CSMA-CD, then they are 
> listening to you, if only to see if the channel is clear to transmit.

I said CDMA, not CSMA. I'm not sure what DSS is here, but in CDMA everyone 
talks at the same time, and in GSM (afaik) nobody talks at the same time, so 
there's no interference (of the type we're talking about) in either one.

> 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. If 
you're on a network where each connection is 1:1, then it's the same as an 
ISDN conference bridge.


>> However, I strongly suspect that none of this discussion has anything to
>> do with whatever Andrew meant when he said IP isn't 1:1 like ISDN is.
> 
> Well, quite possibly, unless he was talking about multicast as a reason.

I don't think so...

-- 
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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