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Re: why multicasting is working?

--- Oleg Butorin <lists@mega-casino.ru> wrote:

> Mike Mestnik wrote:
> >>>I'm not an expert on MC, but I'd think would be routed to
> >>>      
> >>>
> >>your
> >>    
> >>
> >>>default route.  Then the pkt would get multicasted and you would
> receve
> >>>multiple responces.
> >>>      
> >>>
> >>Yes, but I received responces from the systems where multicasting 
> >>disabled in the kernel.
> >>    
> >>
> >There is no difference on the end points, only the routing.  In MC the
> >routers must(MAY) take one packet and pass on multiple packets.  If a
> >  
> >
> No, the great feature of multicasting is to send only ONE packet to 
> multicast address and
> other computers, who "subscribed" to this address, will receive this 
> packet!
I don't see what your saying/mean.  This subscription happens in place X,
if it happens on the recever then how dose the sender know who should get
the packet.  If it happens on the sender, why is there a MC ip.  Given all
the avalible info only the routers need to know anything about
subscriptions.  The router takes the MC packet and sends a copy to each of
it's directly upstream routers(or clients) that have subscriptions.

The sender just sends a PKT to the MC ip, no special software is needed. 
The recever needs only to subscribe, this is no specialer then sending an
MC paket.  So it's clear that only the routers need special software, for
the end points there is nothing special about them.

> For example (only in theory, I didn't set up this), I have 3 computers 
> in my LAN: two workstations
> and one router. Multicasting disabled on all computers.
> 1. I send packet to address
> 2. This is multicast address, that's why on the network interface layer 
> packet will have special destination MAC
No, this would mean all interface would have to be in promiscus mode. 
This would not be secure!!!

Unless you mean MAC BC or MAC unicast, but not a MAC that isn't your own.

> 3. All hosts will receive this packet. There is no work for router in 
> this case. Multicast packet will be delivered like broadcast.
I don't think MC packets will use BC, only unicast pkts should be
deliverd.  If you want todo BC filetransfers there are other programs for

> 4. In this stage I have a question. If system has multicasting enabled, 
> it will unpack the packet with the special MAC and
> then will decide to answer to this packet or deliver it to the 
> application. But I have multicasting disabled, and don't understand
> why kernel receive the packet with the special MAC and multicast IP.
The NIC will drop the special MAC and not forward it to the PCI(or system)
bus unless in promiscuos mode.  When a router sends an MC packet to more
then one host on the same subnet, a single unicast pkt is deliverd to
each.  The only saving here is the trip from the internet to the route and
only if it supports MC.

There is no ETHERNET support for MC, none, nada, zip.  MC is a newer proto
then ethernet and I don't think any replacement for eth will solve this

> 5. If router has option to route multicast packets, then the packet can 
> be routed to the another network or multicast enabled router.
> >normal router gets a MC packet it's likely just going to pass it to the
> >default route.
> >  
> >
> Normal router does nothing with the multicast packets.
> >>>IIRC kernel level MC support is only for if you want to be on Mbone,
> >>>      
> >>>
> >>not
> >>    
> >>
> >>>if you want to use it as a client/server.
> >>>
> >>>      
> >>>
> >>But the option called "IP: multicasting" and help:
> >>
> >>This is code for addressing several networked computers at once,
> >>enlarging your kernel by about 2 KB...
> >>
> >>    
> >>
> >It also says if you wish to participate in Mbone.  The code dose allow
> for
> >your computer to send more then one MC packet, but a normal kernel will
> >still send pakets to ANY IP -
> >  
> >
> Sorry, I don't understand, what do you mean "to send more then one MC 
> packet"?
> All kernels can send packets to anywhere, the question is:
> why it receive the alien packet???
> >>And Mbone is the standard network, that supports multicasting
> (routers, 
> >>computers...).
> >>As I understand, there is no special support for Mbone, this is
> support 
> >>for Multicasting.
> >>
> >>    
> >>
> >That's correct, but without MC enabeled routers your one packet will
> end
> >up going to a router with no default route and then you will get a
> >netunreach ICMP.
> >  
> >
> I don't speak about option "IP: multicast routing". It is clear, that 
> this option for routing!
> Best regards,
> Oleg.

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