[AG-TECH] IGMPv3, Source Specific Multicast, fully supported in Windows XP

Bill Nickless nickless at mcs.anl.gov
Fri Aug 16 11:15:33 CDT 2002

At 10:49 AM 8/16/2002 -0500, Jeffrey Eschbach wrote:
>I don't think IGMPv3 actually is involved on the
>source side... the router just needs to know that it should not forward
>traffic in the SSM range unless it has received a join.  (For ASM range,
>traffic would go to the RP and then receive a "register stop" to keep
>the traffic from leaving the network.)

Close.  You're right on with the SSM mechanism.  But in the ASM case, the 
traffic goes to the RP and then a subset gets converted into MSDP Source 
Active messages and are flooded.  When a remote PIM domain notices the 
active source and sends source-specific joins, the traffic leaves the local 
PIM domain and gets sent towards the receiver.

In my opinion, this is one of the fundamental flaws of the current Internet 
multicast architecture.  The architectural assumption is that the Internet 
is analogous to an old-fashioned 10Base5 Ethernet segment--nothing needs to 
be done before putting a packet on the wire.

Since RFC 1112 laid down that architecture, the Internet community has 
learned that the best known way to route multicast traffic is via 
source-rooted distribution trees.  So there's a race condition--the 
source-rooted distribution tree can't be created until the network knows 
there's a source.  But if the network only discovers a source when it puts 
a packet on the wire, it's too late to create those source-rooted 
distribution trees.

SSM gets around this problem by making the receivers identify the source to 
the network before the source goes active.

MSDP gets around this problem by flooding the knowledge of active 
receivers, and the first packet of the source's traffic, independent of the 
source-rooted distribution trees.

Bill Nickless    http://www.mcs.anl.gov/people/nickless      +1 630 252 7390
PGP:0E 0F 16 80 C5 B1 69 52 E1 44 1A A5 0E 1B 74 F7     nickless at mcs.anl.gov

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