Source Specific Multicast question

Unanswered Question
Jan 12th, 2008


I am reading about Source Specific Multicast, and I just don't get the point.... if the end result of using SSM is that one broadcaster or sender makes a shortest path tree directly one receiver, what is the advantage at that point of having multicast enabled, over unicast?

Maybe I'm totally missing the point....



I have this problem too.
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Edison Ortiz Sat, 01/12/2008 - 07:02

Hi Lisa,

I want to keep it simple to avoid any confusion. Multicast is one-to-many while Unicast is one-to-one. With SSM, the source still sends to a multicast group. Receivers need to join that group to obtain that feed. With unicast, the source sends to a specific receiver. If you have multiple receivers, the source needs to send this packet exponentially.

In other words, if you have 50 receivers, it needs to send the packet 50 times. While with Multicast, the source only sends the packet once to the group and it gets distributed to every receiver that has joined the group.




Konstantin Dunaev Mon, 01/14/2008 - 01:32


if I correctly understood your question, you're basicly asking what is the difference between "usual" multicusting and the SSM?

if so, then the main difference is SSM makes possible for receivers to define from which source they want to get the traffic (with help of IGMPv3 ).

And it's correct, the SSM uses "shortest-path" tree which means that the tree is built up with sender at the root.

lgontarsk Mon, 01/14/2008 - 06:08


Once you have a SPT instead of a shared tree, I guess I don't see the difference between multicasting and unicasting. There's still one stream per receiver, even if its a multicast stream. The only difference is that the destination of the packet is a multicast addr.

Thanks for both of your answers. Its a pleasure to see people responding on these discussion groups.

Lisa G

Edison Ortiz Mon, 01/14/2008 - 06:24

There is also a stream per receiver on a shared tree. The only difference is the stream flows up from the sender, to the RP, down to the receiver.

Again, the beauty of multicasting is that the sender does not need to send multiple packets if there are x amount of receivers.

Konstantin Dunaev Mon, 01/14/2008 - 08:28

hm, I think here there is some misuderstanding:

SPT (shortest-path tree) and "shared tree" are both types of multicusting trees. They differ from each other in a way how they are built up and how then the multicast packets are flowing.

In shared-tree the root is the RP and the source first send the packet to RP and then RP send the packets down the the tree to recievers. If you get the second source you can use the same RP and the same tree, and it's exactly what the name tells us - many sources can share the same tree to deliver the packet.

In the shortest-path tree there is a separate tree for each source , source in this case is the root and source simply sends the packets down the tree to recievers.

the main difference between unicast and mulitucast (in any form) that in multicast the source sends only ONE packet and it doesn't know how many recievers are getting this packet, the routers and switches make the copy of it if necessary.

in unicast the source sends the number of packets equally to number of recievers.

lgontarsk Mon, 01/14/2008 - 08:45

"In the shortest-path tree there is a separate tree for each source , source in this case is the root and source simply sends the packets down the tree to recievers."

Right... exactly. think we agree.


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