Jim Henderson wrote:
> On Sat, 15 May 2010 13:02:58 -0700, Darren New wrote:
> 
>> Jim Henderson wrote:
>>>> If you have one cable coming into the university campus, and a network
>>>> for each building, the router is going to have to send the packets to
>>>> each building, duplicating the packets, regardless of how "aware"
>>>> anyone is.
>>> Yes, but that's not a 1:1 transmission (compared to the receiving
>>> clients).
>> I don't know what you mean.
> 
> It's not one sender to one receiver.  It's one sender to many receivers.

Right. The question isn't how many senders and receivers there are. The 
question is whether you're duplicating packets.

>> For one thing, modern ethernet (the kind that goes over CAT5 instead of
>> coax) is indeed 1:1 on every part of the network. Nothing I send from my
>> computer to yours does not go through the router.
> 
> Not necessarily true if it's a local subnet. 

Uh, yes. There's nothing that's going to go over this cat5 wire that doesn't 
go to the other end of the wire. If your PC is plugged into your 
router/switch, and your other PC is plugged into your router/switch, then 
talking between the PCs goes thru the router/switch.

The only difference would be if you have (a) only two PCs on the network, at 
which point multicast is irrelevant, or (b) you have a hub, at which point 
the hub is still duplicating packets for every wire, except it's doing it 
indiscriminately.


> But even if it is across 
> routers, so what?  The router is just setting up a multicast domain for 
> the downstream network, which may include another router that does the 
> same thing.

I think at this point you may have forgotten why we're discussing this.

The contention was that things like ATM and ISDN are "point to point" while 
IP somehow isn't. My contention is that any network physically wired 
anything close to what ATM and ISDN does is also point to point, and that 
most IP networks other than wireless are of the same sort of topology as ATM 
and ISDN. (And maybe wireless, too. I don't know enough to know whether one 
node can send to the other without the AP repeating it.)

>> 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.

When you *do* have a broadcast network, unless you do something really funky 
(unlike, say, coax ethernet), all the nodes can be conceptually treated as 
one node, because they can't talk at the same time, they all see the same 
data, they don't get all the bandwidth to themselves, etc. And that hasn't 
anything to do with IP. IP multicast doesn't route packets to a specific 
machine. It routes them to a specific broadcast subnet, if there is one. If 
I'm on a segment with someone receiving multicast, I have no way *not* to 
receive that, and it can saturate my bandwidth even tho I'm uninterested.

>> Granted, if you're broadcasting from one point, the broadcaster only has
>> to send one packet per physical connection regardless of the number of
>> recipients. But that's the same with ISDN and all those technologies
>> too. You call into a conference bridge and the bridge duplicates the
>> data as necessary. You're not sending out the same data once for each
>> receiver there either.
> 
> That's also the point; the amount of traffic sent by the sender is 
> significantly reduced;

When I'm on a conference call via ISDN, I'm only sending one copy of each 
byte. I'm not the one doing the duplication. The ISDN router (known as the 
conference bridge) is doing the duplication, just like in IP.

> 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.

>> Firstly, if your subnet isn't broadcast, it doesn't save you any
>> bandwidth. The router still needs to duplicate all the packets. If your
>> subnet *does* support broadcast (like coax ethernet, or alohanet, or
>> something like that) then sure, you can broadcast to everyone, but
>> that's because *any* transmission goes to everyone, taking up your
>> bandwidth even if it's not addressed to you.
> 
> I think there may be a conflation of "broadcast" and "multicast" here.

Nope. I'm saying there are two physical networking possibilities: broadcast, 
and unicast. Either everyone gets the signal on a broadcast network, or only 
one person gets the signal on a unicast network.  Coax ethernet is 
broadcast. CAT5 ethernet is unicast.

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.

> 
>> Broadcast networks tend to be very small, and they tend to not allow two
>> conversations at once. You're not saving bandwidth by broadcasting.
>> You're just using it more efficiently. A targeted communication on a
>> broadcast network still takes up everyone's bandwidth. They just ignore
>> it. Multicast on a broadcast network isn't saving bandwidth as much as
>> it's not wasting bandwidth.
> 
> Small relative to the size of the Internet, sure.  Small relative to the 
> size of a given network, maybe not so much.
> 
> That last sentence is just semantics, though.

By that I mean that multicast on a broadcast network doesn't needlessly send 
the same packet multiple times. But neither does something like ATM or ISDN.

>> Wireless is about the only common broadcast IP-based network still
>> around, tho, exactly because it's so bandwidth-inefficient and
>> impossible to squelch when something goes wrong.
> 
> I assume you're talking about 802.11 networks; 

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.

> some wireless networking 
> technologies implement CSMA-CA while others do CSMA-CD; the latter is 
> much more efficient but requires hardware that is capable of listening; 
> in a spread-spectrum implementation (which most 802.11 networks are), 
> CSMA-CA is more common but the DSS implementations mean that there's much 
> less chance of a collision (that's part of the point of that type of 
> implementation).

Sure. But if I have a 10Mbps coax, or a 54Mbps 802.11g, I'm sharing that 
with everyone else on that network. If I can hear everything coming out of 
the router and I have to wait for everyone else to stop transmitting before 
I can, then having multicast not transmit duplicates isn't "saving" 
bandwidth. It's taking up my bandwidth whether I'm listening to it or not. 
If I have a 9Mbps stream coming in, my coax ethernet network is going to be 
slow whether I'm watching video or not. What it *does* do is not send it 
twice when everyone interested has already heard it. That's what I was 
talking about.

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.

-- 
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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