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Does multicast work over the itnernet?

A

AMD_Gamer

Fully [H]
Joined
Jan 20, 2002
Messages
18,287
I was learning about IGMP in school and was wondering if ip multicast is used a lot over the internet? or is it just for LAN's? I understand what it is and does but I was reading something on google that says most ISP's do not allow IP Multicast? now I am confused?

do things like streaming video and games use ip multicast over the internet?
 
ISPs filter mutlicast you can't join a multicast stream over the internet.

Not to mention how do you think ISPs like Cox, Comcast, TWC move video traffic around their networks :)
 
Think about how people typically access content on the internet; on demand. There isn't a demand for multicast friendly traffic on the internet ( for your generic end user ).
 
What?

Routers can be rendezvous points when using multicast with PIM sparse mode.

You use IGMP to join mutlicast streams.

If you use PIM multicast the routes are decided based on whatever IGP you're running.


You can't use multicast over the internet because it would have to be routed AS to AS via BGP.
 
Multicast is for better or worse my specialty. I get sick of it every now and then :)

It exists in main 3 phases in the world:
IPTV
Financial networks (my jurisdiction)
video/audio conferencing in a corp

For IPTV, think of how much of a waste it would be to unicast channels from a video server.

For video/audio conferencing.. same thing, good luck scaling it with unicast to all your clients

For financial markets, think of how the stock market works. You want the latest price for all stocks. To do this efficiently (we deal with it at the microsecond level) it wouldn't work if you had unicast all over the place. Some trading firms would get it the updated price quicker than others based on their wait in line. Not fair.

There are 3 main aspects of multicast.
IGMP (who wants the group? who still wants it?)
PIM (multicast routing protocol -- sparse, dense, and sparse-dense modes)
Unicast routing (yes ...)

A router that wants to deal with multicast elects itself as an IGMP querier for a particular vlan. It is responsible to ensure that its clients get the groups asked for and that they still want it after some time. A switch running IGMP snooping can limit a vlan to a port-by-port basis on who receives the stream. Otherwise, it will flood/broadcast the whole vlan with multicast if there is only one in that vlan who subscribed to it.

PIM dense mode is the easiest PIM mode. You don't need a rendezvous point. As soon as a router receives sources coming in, it will flood out all interfaces running dense mode (except the one received on). If one or more of those downstream routers have no IGMP joins for that stream, they will send back a prune message to stop the traffic. After some time (60sec I believe) it will repeat the process and the flooding will take place again. Yes you can optimize it by sending prunes faster than the timeout, but at that point, just use sparse.

PIM sparse mode is the only one used in financial markets. You statically elect RPs to be responsible for multicast groups. When an IGMP join is received by a router running SM, he will look at his RP mappings and if he has a match, he will send a PIM join to the RP. The RP will build a shared tree if it hasn't done so already, and he will start forwarding the multicast down the SPT if there are incoming sources available. For sources, they will start sending to the group. The router on that local segment running SM, if not the RP, will send unicast registration messages to the RP and "register" the source. In order to do so, RPF check must not fail (incoming interface of the stream must match the unicast routing interface back to the source). Again, you can optimize the streams so that instead of getting all multicast from the RP, you can switch to a shorter source tree instead of a further away shared tree.

Don't worry about sparse-dense mode yet.. it's commonly used for auto-rp (automatic discovery of what RPs are responsible for which groups via announce/discovery messages) but you need to do some reading before you're up to that point. :) As well as MSDP, anycast RP, etc...
 
Sparse mode sounds complicated however in real life once you get it working its really not that bad...

I will say this though a lot of the companies that make video equipment for cable companies had to switch all of their gear to multicast and I had no idea about networking.

Also a lot of companies Cisco, Juniper, and Alcatel follow the RFCs for multicast traffic differently its funny to see things that work on one and not the other.
 
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