Are there limits to the number of global IPv6 multicast addresses in use at a time?

Question

The address space for IPv6 multicast addresses is very large (something like 2^112) and so I'm imagining it might not be so hard to get my hands on a million or so of them at some point, and try using the internet as a pub/sub broker between some thousands of devices.

I'm pretty new to multicast but it doesn't seem like it was designed for something like this, and the feeling I get from browsing around the topic is that there's not much confidence in the multicast implementations in the internet networks.

I'm also generally wondering what public multicast can be used for at the moment and whether the use case above would be abusing the system.

Answer 1

You can easily get your hands on a /48 IPv6 prefix. this will allow you to have 65,536 /64 subnets, each subnet with a possible 18,446,744,073,709,551,616 addresses. ISPs will not advertise any prefix longer than /48, so your subnets will be aggregated into that single /48 address summary on the Internet. This means that, from the perspective of the Internet, all your subnets are at the same location, even if you have multiple locations with private links between them. At any point where your sites connect to the Internet, you would need to advertise the /48 prefix since the ISP for each point will not advertise any prefix longer than that. Basically, you would need to get a /48 prefix for each site you have, but fortunately, that is really not a problem. Most companies get smaller prefixes (/44, /40, /36, or /32) and chop them into longer prefixes for different sites and advertisement to the Internet.

Multicast requires multicast routing on every router in the path. This is not enabled on the Internet routers, so you can multicast within you company, but you cannot multicast across the Internet. You would need to create a tunnel between sites in order to get multicast from one location to another.

IPv6 has a dedicated multicast range (ff00::/8) with hard restrictions based on flags (third nibble) and scopes (fourth nibble). You need to take these flags and scopes into account when deciding on which multicast address(es) to use. Still, you have a ridiculously large number of multicast addresses which may be used. It is important for your company to define a unified, consistent multicast policy, otherwise it can get out of hand pretty quickly.

Answer 2

The problem with multicast is that it requires routers to keep a lot of additional state.

Each "multicast group" for which traffic passes through a router must be tracked in the routers memory along with a list of what ports the router should forward traffic from that multicast group to.

Most ISPs consider that too much of a burden. So while global-scope multicast addresses do exist they are only likely to work over specialist networks that explicitly support them, not over the internet in general.