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.

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.

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.

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.

>>> The difference between, for example,
>>> using unicast to push an image down to 15 workstations on a subnet and
>>> using multicast to push an image down to 15 workstations on a subnet is
>>> a significant reduction in overall network traffic.
>> Only for that one subnet.
> 
> The router is part of the subnet, though.  It retransmits upstream if the 
> TTL hasn't expired (and it's configured to do so).

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.

>> It's better than a unicast stream to each destination, yes. That doesn't
>> mean IP isn't point-to-point, if you factor in the broadcast address as
>> meaning "one subnet is the point."
> 
> That's a bit like saying shouting is point-to-point, though, because the 
> target "point" is a group.

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.

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.

-- 
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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