Subject: [magma] Multipoint-to-multipoint PIM-SM

From: "Tiago Duarte (Ext_PDM&FC LDA)" <[email-masked]>
Date: Thu, 18 Nov 2004 11:00:50 -0000

Hello,

I´m wondering if PIM-SM is capable of multipoint-to-multipoint...
In the RFC 2362 is says that:
"If the data rate warrants it, routers with
   local receivers can join a source-specific, shortest path,
   distribution tree, and prune this source's packets off of the shared
   RP-centered tree"
This just make multipoint-to-multipoint impractible, because the receiver just chose one source!

Am i correct? How can multipoint-to-multipoint multicast work with PIM-SM?
If PIM-SM isn´t suitable for this kind of multicast, does PIM-Bidir completely resolves the problem?
What is the best soluction to multicast a video-conference, where there are multiple sources and multiple receivers? Create a group specific for each source ?!? Create many (S1,G), (S2,G), (S3,G) ? Or just have one group (*,G) ?

Could any one respond to my questions? Or at least some of them?

Thanks

Tiago Duarte
...................................................................................
System Engineering - R&D Wide Optical Networks
SIEMENS

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From: Marshall Eubanks <[email-masked]>
Date: Thu, 18 Nov 2004 08:58:38 -0500

Hello;

In practice, most PIM domains immediately switch over to a shortest path tree.
Groups with hundreds and at times thousands of active members have
been maintained
in inter-domain multicast, so it certainly can support pretty large
groups. So, from one stand-point,
PIM-SM (ASM or SSM) can certainly handle large groups. There can be
problems, however, with sporadic group traffic.

BiDir was created to support large many to many groups, reduce the
amount of state requried, and address the sporadic traffic problem. In
BiDir, there is a special Rendezvous Point Address (RPA), which
handles all traffic for the BiDir group. Note that as yet there is no
interdomain BiDir in IPv4 (and it is unclear if there ever will be).

I think that the answer to your specific question is, it depends. If
you are in IPv4, and if you can
remain in one domain, BiDir may serve for you. I might point out,
though, that a 50 end-point video conference would be a very big one,
and either ASM or SSM PIM-SM can handle this. (In ASM,
there would be one or more (*,G), in SSM lots of (S,G).) Since every
party in a videoconference may
well want to NOT receive every transmission from every other - you may
want to receive one site only, or receive audio only, or have multiple
bit rate groups available for the same video feeds - an SSM like
approach with lots of separate groups is probably best.

Regards
Marshall Eubanks

On Thu, 18 Nov 2004 11:00:50 -0000, Tiago Duarte (Ext_PDM&FC LDA)
<[email-masked]> wrote:
>  
>  
> 
> Hello, 
> 
> I´m wondering if PIM-SM is capable of multipoint-to-multipoint… 
> What is the best soluction to multicast a video-conference, where there are
> multiple sources and multiple receivers? Create a group specific for each
> source ?!? Create many (S1,G), (S2,G), (S3,G) ? Or just have one group (*,G)
> ? 
>

---

From: Stig Venaas <[email-masked]>
Date: Thu, 18 Nov 2004 15:44:27 +0100

> Am i correct? How can multipoint-to-multipoint multicast work with PIM-SM?

You got one answer, but maybe not to this specific question.

The process of building the source specific shortest path tree and
pruning the source off the shared tree is done on a per-source basis.
So if there are multiple sources you'll get multiple source specific
trees. So in theory this works just fine with many sources.

The end result is actually similar to using SSM and joining each (S,G).
The difference is that you won't have a shared tree to begin with.

Stig

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From: Gorry Fairhurst <[email-masked]>
Date: Thu, 18 Nov 2004 15:41:39 +0000

There's a couple of other "things" I could add from another viewpoint, based
on the application layer usage.

1) You could have continuous presence for all speakers - that is everyone
sees everyone else all of the time.

In PIM_SM (ASM), all you need is one group address (*,G), and each site
simply sends their video bit stream to their upstream router with the
address of the Group and Join the Group to receive. Receivers join to the
Group, and then as the network discovers the sources for each group, joins
each in turn (Sn,G). Initially data is sent from you via network to the RP -
and later via an optimised (shortest-path) tree to the set of other
receivers. From the sender or receiver on a LAN, you don't know this has
happened, and the receiver does not need to worry about how this occurs.

In PIM-SM with SSM, you also have to know the set of addresses being used
(which is a configuration/discovery problem). Again, the receiver doesn't
know how the routers choose to get the traffic, and senders do not worry
about how many receivers there are.

2) You could have a single video stream which combines the video from all
speakers - that is everyone sends to a specific address in the network
(could be unicast) and at this address a multipoint-control-unit merges the
bit-streams in some sensible fashion. It could select a source, e.g. Based
on who is speaking, on a floor-control protocol, or insert one video stream
into a window in another using transcoding  - this is an application, you
may also wish to do this to mute the mike of other participants, do
echo-cancellation, etc. This would be much more attractive for large groups.

The video stream from the device could use ASM, or equally SSM.

Gorry

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From: Marshall Eubanks <[email-masked]>
Date: Thu, 18 Nov 2004 11:16:35 -0500

And, of course, PIM-SM (ASM) actually allows you NOT to switch over,
or to switch over after so much traffic has been received. In practice, this
is rarely if ever done, and so a source tree is set up after the first packet is
received down the shared tree.

Regards
Marshall