multicast at the data layer (layer 2) non-flooding ?

Hi,

I am looking into multicast in a private network at the data layer.
Basically what I need is multicast which is not flooded within a LAN.
Therefore, if there are 5 hosts connected to a router, I would like to
do layer 2 multicast such that 4 hosts in the network for example can
be in a multicast group without host 5 hearing any of the traffic. I
believe multicast at the data layer does this.

This is a general network question and not related to linux, but I
could not find a general networking group on usenet that is active and
seems to have intelligent readers!

Thank you!
George

George Nychis wrote:
> Hi,
>
> I am looking into multicast in a private network at the data layer.
> Basically what I need is multicast which is not flooded within a LAN.
> Therefore, if there are 5 hosts connected to a router, I would like to
> do layer 2 multicast such that 4 hosts in the network for example can
> be in a multicast group without host 5 hearing any of the traffic. I
> believe multicast at the data layer does this.
>
> This is a general network question and not related to linux, but I
> could not find a general networking group on usenet that is active and
> seems to have intelligent readers!

Well, you're asking in a Linux ng, so expect examples and links to be
Linux related. If you want general multicasting info, use google :-)

http://www.tldp.org/HOWTO/Multicast-HOWTO.html#toc1
http://www.tldp.org/HOWTO/Multicast-HOWTO-2.html#ss2.1
[look down to Mapping of IP Multicast Addresses to Ethernet/FDDI
addresses.]
http://www.linuxjournal.com/article/3041
http://www.linuxjournal.com/article/6070
http://www.oser.org/~oser/ds/node27.html
http://www.linuxfocus.org/English/Ja...ticle144.shtml
http://www.faqs.org/rfcs/rfc1112.html < Host extensions for IP
multicasting

As to what I think your question is (ie., how to multicast without
unnecessary traffic on the wire) you do, indeed, need some basic info
on multicasting.

At the link layer, when you join a multicast group, you inform your
kernel (or whatever contols the TCP/IP stack in your OS of choice) that
you want to listen for certain traffic, traffic that will have a
defined data link address. It's sorta like adding an additional MAC
address to your system. So to get a better idea of how it works, try
the first link -- it has a short explanation of how multicast link
layer addresses are formed.

The other links are programming examples.

If you plan to multicast beyond the local link, you will also need to
get up to speed on multicast routing. The first link will barely get
you started. Check its refs. Then check their refs. Or google.

good luck,
prg

Thanks for the response, but I think you're missing my main question

you say:
At the link layer, when you join a multicast group, you inform your
kernel (or whatever contols the TCP/IP stack in your OS of choice) that
you want to listen for certain traffic, traffic that will have a
defined data link address.

I understand this about multicasting, however what I am wondering is
controlling the multicast data from being flooded in the network.

What you're saying is you tell your kernel you want to listen for
certain traffic... which is of course the multicast traffic. However,
what I am wondering is what kind of technology is out there at the
router to prevent those not in a multicast group from getting the
multicast flooded messages.

For instance, you join a multicast group, and I do not. However if we
are on the same router, as you said, the multicast message gets flooded
and we both get it. You told your kernel you want to receive those
messages, so you receive it. I did not, so i drop the message. The
message still came to my link though. I want to prevent it from ever
being put on my link, i do not even want to hear it.

I think I found an article as to what I was talking and asking about
though and an answer:
http://www.intelligraphics.com/artic...2_article.html

George Nychis wrote:
> Thanks for the response, but I think you're missing my main question
>
> you say:
> At the link layer, when you join a multicast group, you inform your
> kernel (or whatever contols the TCP/IP stack in your OS of choice) that
> you want to listen for certain traffic, traffic that will have a
> defined data link address.
>
> I understand this about multicasting, however what I am wondering is
> controlling the multicast data from being flooded in the network.

Multicast routers only forward traffic out an interface from which a
join request/membership is active. If no one requests membership, then
no traffic is forwarded. This is _one_ of the main functions of the
multicast routing protocols. The other is building the trees that lead
back from the membership request, through intermediate multicast
routers, to the closest source (or rendevous point). The routing
protocols can get really dumbfounding.

> What you're saying is you tell your kernel you want to listen for
> certain traffic... which is of course the multicast traffic. However,
> what I am wondering is what kind of technology is out there at the
> router to prevent those not in a multicast group from getting the
> multicast flooded messages.
>
> For instance, you join a multicast group, and I do not. However if we
> are on the same router, as you said, the multicast message gets flooded
> and we both get it. You told your kernel you want to receive those
> messages, so you receive it. I did not, so i drop the message. The
> message still came to my link though. I want to prevent it from ever
> being put on my link, i do not even want to hear it.
>
> I think I found an article as to what I was talking and asking about
> though and an answer:
> http://www.intelligraphics.com/artic...2_article.html

Yes, I have the article. Beware that it is Cisco protocols (and GARP)
and switched equipment that is being discussed. If you have all Cisco
gear, this is a good start on what you have available. Note also that
the problems discussed are directly related to switching technology,
not to the layer 2 aspects of multicasting per se. If yours is a
wholly switched network then this is what you need. If you have
routers in the path, you will have to catch up on the multicast routing
protocols as well.

For switches you might also want to check this:
http://www.cisco.com/univercd/cc/td/...oc/ipmulti.htm

again, good luck,
prg

George Nychis wrote:
> Hi,
>
> I am looking into multicast in a private network at the data layer.
> Basically what I need is multicast which is not flooded within a LAN.
> Therefore, if there are 5 hosts connected to a router, I would like to
> do layer 2 multicast such that 4 hosts in the network for example can
> be in a multicast group without host 5 hearing any of the traffic. I
> believe multicast at the data layer does this.
>
> This is a general network question and not related to linux, but I
> could not find a general networking group on usenet that is active and
> seems to have intelligent readers!

George, the most economical solution to your problem is a DLink 3226
switch, which can be had for $300 - $400.

http://support.dlink.com/products/vi...tid=DES%2D3226

I use them and they are great. One caveat... If you arrange them in a
hierarchy (i.e. more than 1 switch in your net), then you can only see
multicast from the other switch if someone on that switch has joined
the group. Sounds weird but I haven't been able to configure them
around that flaw. You won't find a cheaper IGMP-V2 managed switch
anywhere. If you do, then let me know.