Le 15/05/2010 22:02, Darren New nous fit lire :
> 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).

>>>> 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.
> 
[snip]
> 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.
> 

And then came in play the (G)MPLS system(s), allowing to reserve
bandwidth, having "routers" to perform split/duplication of data, and
putting in place minimalist-covering trees for optimal diffusion (and
even worse: you can have 1:1 protection, or even 1+1 !) adapting in
nearly realtime to the destinations interested in the "channel" and
reacting to the network events too.

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