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If a wireless modem router has the option Allow multicast from Broadband Network, what does this mean exactly?

What risks are there to leaving it enabled and why would you have it enabled?

I understand that a broadcast is sending a packet to every device on the network and a multicast is send a packet to a specified set of addresses. A multicast packet can cross routers whereas a broadcast cannot although I don't quite understand how these "set of addresses are specified"

PeanutsMonkey
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3 Answers3

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IP multicast is a method by which data can be broadcasted to a select group of hosts using a single transmission. These days, it's primarily used for IPTV and satellite connections. In theory, it could be used for any kind of streaming broadcast-type media (e.g. internet TV, internet radio, teleconferencing, and other Video-over-IP applications), and the more hosts on a network that subscribe to a multicast stream, the more efficient it becomes.

However, the only consumer-level implementation of it that I know of is IPTV. In this case, the user's set-top box is the multicast/IGMP client, and their home router would be the multicast/IGMP router. If you only have a single set-top box, I don't think you need it enabled, but if you have more than one, then it allows the upstream server to send only one transmission to your router to serve all of your set-top boxes. So if you had 4 set-top boxes at home, and each set-top box was turned onto the same channel, you could reduce the total bandwidth usage by 75%.

The nice thing about multicast vs. broadcast is that the multicast router knows to send out each multicast transmission only to the devices that have subscribed to it. So your LAN won't be flooded with useless broadcast traffic.

There aren't really any risks to leaving it enabled. But for now you're not likely to get any benefit from it unless your home/office is subscribed to IPTV or some other IP multicast service.

Lèse majesté
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  • [mDNS](http://en.wikipedia.org/wiki/Multicast_DNS) anyone? – Ignacio Vazquez-Abrams Apr 12 '12 at 06:38
  • @Ignacio: mDNS uses IP multicast, but it's purely within your LAN. There'd be no reason to enable multicast from broadband for it. – Lèse majesté Apr 12 '12 at 06:51
  • You're assuming that the broadband part is connected to the Internet. – Ignacio Vazquez-Abrams Apr 12 '12 at 06:53
  • @Ignacio: I suppose you're right. That would be another application scenario where you'd want to enable the feature. – Lèse majesté Apr 12 '12 at 07:08
  • @Lèse majesté - Thanks. Sorry for being a n00b at this but I don't quite follow. From what you mentioned, I understand the basic premise of a multicast broadcast i.e. sending a single transmission to multiple clients who subscribe to it. What I don't quite understand is what role does the router play if the multicast broadcast is say from a host streaming internet TV or radio. So for example, if my router does not have multicast broadcast enabled or doesn't support it, how this affect me listening to internet radio? – PeanutsMonkey Apr 12 '12 at 07:52
  • @Lèse majesté - What role does it play when my modem and router are the one and same device connected to internet? Also when you say same channel, what do you mean exactly? – PeanutsMonkey Apr 12 '12 at 07:54
  • @Lèse majesté - Having come across a post at http://uk.answers.yahoo.com/question/index?qid=20071106231645AAhS4Ik, the author seems to suggest it could be a security risk although doesn't quite explain why. – PeanutsMonkey Apr 12 '12 at 07:57
  • @Lèse majesté - Lastly how does it become more efficient if there are more subscribers or clients? – PeanutsMonkey Apr 12 '12 at 08:05
  • @PeanutsMonkey: Like most Yahoo! Answers posters, that guy has no idea what he's talking about. And the reason you need it enabled in a router is because IP multicasting is typically done through a signally protocol like IGMP or MLD, and these are processed by your router so that it can keep track of which clients have subscribed to which multicast group. If it doesn't keep track of this, then it won't know which clients it should or shouldn't forward a particular multicast transmission to. – Lèse majesté Apr 12 '12 at 08:31
  • Multicast is more efficient with more hosts because the upstream server always only has to send each packet once to a multicast network whereas it'd have to send it once for each host on a unicast network. So on a 2-host network, multicast is 200% as efficiency as unicast; 3-host network is 300% efficiency; 4-host network is 400% efficiency... And "same channel" means same TV channel. IPTV is digital TV transmitted over the internet. If every client is receiving different channels, then you can't multicast. But if 2+ clients are receiving the same channel, then it can be multicasted. – Lèse majesté Apr 12 '12 at 08:43
  • @Lèse majesté - Thanks. Okay so please correct me if I am wrong. If I have allow multicast broadcast enabled on my router it effectively knows which clients have subscribed to receive the appropriate packets. What I don't get is how does an upstream provider know if I have multicast broadcast enabled? When you say upstream are you referring to my ISP or the provider of the content? Just to be absolutely clear on channels, I take it you mean a group of people watching the same TV channel i.e. channel 5 at the same time. Is that correct? – PeanutsMonkey Apr 13 '12 at 00:07
  • @PeanutsMonkey: Both your ISP and the content provider are upstream to your router. But that's the beauty of IP multicast. The upstream clients don't need to know this information. Your multicast router handles all the IGMP information for the upstream servers and does the multicasting for them. All the content server has to do is address their packets to a multicast IP. Your set-top boxes simply tell your router that they want to subscribe to this multicast IP, and that puts them into the multicast group. – Lèse majesté Apr 13 '12 at 02:52
  • @PeanutsMonkey: And yes, that's what I mean by a channel. And by set-top box I mean something like what you use for digital cable or satellite TV at home. Except (consumer) IPTV specifically means TV services like AT&T's U-Verse service and CenturyLink's Prism service. But IPTV is more popular in Germany, France and South Korea than in the U.S. – Lèse majesté Apr 13 '12 at 03:03
  • @Lèse majesté - Thanks. Do the upstream servers need to know about the multicast routers though or does it seem send the IGMP packets when it is requested? – PeanutsMonkey Apr 13 '12 at 07:51
  • @PeanutsMonkey: The upstream servers don't actually use IGMP, which is only used for group management between local routers and connected local hosts. E.g. a video server sends UDP/RTP packets to a remote router; that remote router happens to support multicast, so it announces this new content source/multicast group to your local router via PIM (similar to IGMP, but used between routers); if local hosts join to the group via IGMP, your local router will then also join it via PIM. That tells the remote router to start forwarding multicast datagrams for this group to your local router. – Lèse majesté Apr 13 '12 at 08:48
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I just found a reason to activate that option even for domestic use. Hosts on the wireless and cable interfaces will not see IGMP requests to 224.0.0.1 although they are all assigned to the same network segment.

When that option was activated then Wuala, on Laptop connected through WIFI, found the NAS connected in the gigabit ethernet port. I guess that the complex configuration of briges changes with this configuration.

m4ktub
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Unless the broadband side is a larger network, turn it off. Multicast is used in specific situations to let multiple clients receive a packet without having to explicitly send it to each IP address.

Ignacio Vazquez-Abrams
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  • What do you mean by `unless broadband is a large network`? What do you mean by `without explicitly sending it to each IP address? – PeanutsMonkey Apr 12 '12 at 07:47
  • Err... they both mean exactly what I wrote. You're going to have to be more specific about what you don't understand. – Ignacio Vazquez-Abrams Apr 12 '12 at 07:51
  • The broadband side is always a larger network isn't it and if so, why turn it off? Also my understanding is that although multicast allows multiple clients receive the same packet, it is explicitly sending it to a group of subscribers and not every single IP address on the network. – PeanutsMonkey Apr 12 '12 at 18:59
  • Well, I'm not counting "the Internet" as a network per se, since it's public. – Ignacio Vazquez-Abrams Apr 12 '12 at 19:00
  • Thanks. Are you able to give me real world examples of when Multicast is used? – PeanutsMonkey Apr 13 '12 at 00:08
  • [mDNS](http://en.wikipedia.org/wiki/Ghost_%28software%29) uses it to find and announce services. [Ghost](http://en.wikipedia.org/wiki/Ghost_%28software%29) can use it to feed an image to multiple clients at once. [IPTV](http://en.wikipedia.org/wiki/IPTV) uses it for live streams. – Ignacio Vazquez-Abrams Apr 13 '12 at 03:01
  • Sorry for being a complete n00b here but if I wish to subscribe to say a multicast stream wouldn't my router need to have multicast enabled? Why would I disable it? – PeanutsMonkey Apr 13 '12 at 07:53
  • If it was on the broadband side, then yes. Multicast is already open on the LAN side. – Ignacio Vazquez-Abrams Apr 13 '12 at 12:45