Subject: Re: mboned: PIM-DM and reliance on data plane etc.

From: Pekka Savola <[email-masked]>
Date: Tue, 4 Jul 2006 10:41:34 +0300 (EEST)

On Fri, 30 Jun 2006, John Zwiebel wrote:
> On Jun 30, 2006, at 1:53 AM, Pekka Savola wrote:
>> Based on these and John Z.'s comments, my working version now reads:
>>
>>                <t>PIM-DM was used as a first step in transitioning away 
>> from DVMRP.  It also became apparent that most networks would not have 
>> receivers for most groups, and to avoid the bandwidth and state overhead, 
>> the flooding paradigm was abandoned.  Replaced PIM-DM with PIM-SM was easy 
>> as PIM-SM was designed to use compatible packet formats. PIM-DM is no 
>> longer in widespread use.</t>
...
>> Is that accurate?  Suggestions for changes?
>
> It doesn't quite suggest that DM and SM existed in parallel for a 
> fairly long time.  In fact, sparseness or denseness used to be a 
> choice made by each interface that the mpacket went through.  It 
> didn't take too long to correct that idea maybe 18 months.  But it 
> wasn't until 2000 or maybe later that we started dissing dense-mode.
>
> Perhaps its the word "abandoned" that gives it the wrong flavor.  No one
> wants to abandon a child.  :-)

I added "gradually" before "abandoned", that might probably convey 
that it didn't happen overnight.

> And DM wasn't 'replaced' since it was developed in parallel with SM.

I made that 'Transitioning from PIM-DM to PIM-SM ...' which is 
probably a bit better.

Thanks!

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
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---

From: Beau Williamson <[email-masked]>
Date: Tue, 11 Jul 2006 07:53:18 -0500

At 6/30/2006 03:53 AM, Pekka Savola wrote:

>Sure. But PIM-SM also includes data-driven events (e.g., register 
>messages), so my question would be, how is PIM-DM worse than PIM-SM wrt 
>data plane reliance?

Simple, PIM-SM makes use of explicit Join messages to build and maintain 
the multicast trees.  PIM-DM doesn't and hence has a very non-deterministic 
behavior.

While there are some data-driven events left in PIM-SM, they don't result 
in multicast loops and blackholes like PIM-DM does.

Beau Williamson