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My internet has an upload limit of 1 Mb/s. If I am in a UDP multicast group with 10 people and I send something to the group at a rate of .1 Mb/s, will my upload rate be 0.1 Mb/s or 1.0 Mb/s? Assume all people are on the public internet.

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    You can't multicast on the public Internet. Also, the number of receivers doesn't affect how much bandwidth multicast uses. – Ron Maupin Jun 30 '15 at 19:05
  • You can't? Not even if your router supports multicast and the other person's router also supports multicast? – Sacha T Red Jun 30 '15 at 19:09
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    No, you can't. Your ISP will drop packets with a multicast destination address since it has no place to send them. Multicast packets are not routed the same way unicast packets are. The networks in between the sender and receiver need to be multicast capable, aware, and have multicast routing enabled. The Internet is not set up this way. – Ron Maupin Jun 30 '15 at 19:25
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    In general, ISP networks are not setup to handle user multicast streams. (a: it's a mess. b: nobody needs it. and c: script kiddies.) Uverse has a multicast video distribution network, but it's one way / to the user – Ricky Beam Jul 1 '15 at 0:17
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 11 '17 at 17:28
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Multicast is a special case of broadcast. It is a one-to-many mechanism. The sender sends a single packet and the network copies it everywhere it needs to go. To a switch or WiFi access-point, it's handled just like broadcast traffic[1], 'tho multicast filtering may apply. To a router that isn't configured for multicast routing (which is basically every router, ever), it's random broadcast noise.

For this to work, the various routers between you and your targets would have to be configured for multicast routing. This is unlikely. While it is, in theory, possible within a single ISP network, across the internet would be unmanageable. (not to mention immediately abused)

[1] This is why Uverse uses a dedicated AP for their wireless set-top boxes. Those APs run special software that handles the multicast video stream(s) like unicast. Wifi broadcasts go out at a basic rate (a minimum every associated client can handle) that's just far too slow.

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The sender will not duplicate the content so your rate will remain .1. You will need to tunnel your multicast traffic across the Internet. Specifically, you may want to tunnel multicast over a GRE tunnel.

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  • This assumes that the receivers are all on the same LAN. If the ten receivers are all on different LANs, the OP may as well unicast to each, avoiding all the headaches of tunnels and multicasting, and it will still take multiples of bandwidth to send. – Ron Maupin Jun 30 '15 at 21:26
  • That depends on where the mrouter is. If he's the tunnel junction for 10 spokes, then he will, indeed, be sending 10 packets for every 1. (PC sends 1, router copies it into 10 tunnels) – Ricky Beam Jul 1 '15 at 0:39

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