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UDP Multicasting


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#1 Rich76   Members   -  Reputation: 178

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Posted 08 December 2010 - 02:34 PM

Am I not understanding multicasting?

If I want to multicast live audio at 128Kbps to 5000 people, I'd only need to use 128Kbps of my bandwidth, opposed to 128Kbps x 5000?

Seems too good to be true..

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#2 Rich76   Members   -  Reputation: 178

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Posted 08 December 2010 - 03:15 PM

I listen to a Verizon streaming audio that said with Multicasting you'd use only a fraction of bandwidth you'd normal use. I still have no idea how to calculate bandwidth usage using multicasting..

#3 hplus0603   Moderators   -  Reputation: 5163

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Posted 08 December 2010 - 04:10 PM

It sounds too good to be true, because it is.

In multicasting, you "join" a multicast group. (There is a limited number of these, but that's not the biggest problem).
Then, anyone else who wants to listen to your station also needs to join the same group.
Unfortunately, this means that every router on the internet must know about your multicast group (and all multicast groups).
That doesn't actually work at internet scale :-)
Hence, pretty much all routers and ISPs filter multicast groups and prevent them from going across the internet.

Multicast works in limited cases, where you control all the routers involved -- typically, when sending on a LAN, or across a few interconnected LANs.


#4 Washu   Senior Moderators   -  Reputation: 4992

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Posted 08 December 2010 - 04:13 PM

Multicasting, over the internet, doesn't work.

Well, in general anyways.

Over a private network where the routers can be properly configured to allow it, you might use multicasting to reduce traffic costs.

However, the internet is a very large non-homogenous set of interconnected ASes that each have their own standards for what constitutes good and bad traffic, internal routing processes, and even supported network protocols... such as the multicast advertisement protocol.

Many ISPs just don't support multicast advertisements, and so they won't be properly propagated to all of the hosts that might wish to be attached to your application. They especially tend to not accept such advertisements over their consumer connections, which are usually extremely heavily filtered.

In the end, multicasting is a great idea... but a poor implementation. As such you can just ignore it as a protocol for use in most network aware applications.

In time the project grows, the ignorance of its devs it shows, with many a convoluted function, it plunges into deep compunction, the price of failure is high, Washu's mirth is nigh.
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#5 Rich76   Members   -  Reputation: 178

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Posted 08 December 2010 - 04:18 PM

Dang it! I knew it. :)

Thank you both for the great information.

#6 elFarto   Members   -  Reputation: 206

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Posted 09 December 2010 - 09:55 AM

Quote:
Original post by hplus0603
...this means that every router on the internet must know about your multicast group (and all multicast groups)...

With IPv6 Embedded RP (RFC3956) this is no longer true.

Of course it still won't work over the internet as we don't really have IPv6 yet.

Regards
elFarto

#7 hplus0603   Moderators   -  Reputation: 5163

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Posted 09 December 2010 - 12:43 PM

Well, for multicast to *actually* work, you still need each participating router to learn about the multicast. This is a section ripe for DOS attacks. My money is on multicast not being useful within the next five years.


#8 elFarto   Members   -  Reputation: 206

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Posted 10 December 2010 - 10:23 AM

Quote:
Original post by hplus0603
This is a section ripe for DOS attacks.

Oh hell yeah. All the multicast stuff is from time where the internet was full of fluffy bunny rabbits and folks greeting you with "Good day kind sir", now it's more burnt out cars and "Gimme all your money".

Regards
elFarto



#9 Antheus   Members   -  Reputation: 2397

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Posted 10 December 2010 - 10:30 AM

IPTV cable providers use multicast. Predictable numbers, control over subscribers, high bandwidth. And subscription model actually makes sense.

Quote:
I listen to a Verizon streaming audio that said with Multicasting you'd use only a fraction of bandwidth you'd normal use.

If that is their service and you're connecting through one of their endpoints, then that is very likely.




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