Subject: [pim] Query on PIMDM repeated Flooding/Pruning

From: "Krishna Mohan" <[email-masked]>
Date: Thu, 29 Nov 2007 16:03:57 +0530

This is regarding the flood and prune behavior in PIM-DM [RFC3973].

I am actually thinking of ways to reduce this flood and prune behavior
in networks that are not State Refresh capable.

I am assuming the following:
> Timer Name: Prune Timer (PT(S,G,I)) - From message (Default: 60 secs)
> Timer Name: Prune Limit Timer (PLT(S,G)) - Default: 210 secs

Consider the following scenario:
         +------+     +------+
         |      |     |      |
Source --+  R1  +-----+  R2  |
         |      |     |      |
         +------+     +------+
R2 doesn't have any neighbors/listeners and R2 Prunes its Upstream
interface by setting the PLT for 210 secs.
Upon Prune reception, R1 Prunes its Downstream interface by setting
the PT for (3*60=180) secs.

R1's PT expires and R1 resorts to flooding again until subsequently
Pruned again by R2 (which it does after its PLT expiry i.e., after 30
secs).
As per my understanding, the drawback here is that unwanted traffic
floods the network unnecessarily for 30 secs and this is repeated
every 180 secs.

Below is one probable solution that I can think of here to tackle
this(minimize the repeated flooding/pruning):
1. Reduce the interval of PLT(S,G) to make it less that PT(S,G,I).
   (or)
   Increase the interval of PT(S,G,I) so that it is greater than
PLT(S,G), which means increasing the Prune Holdtime.
2. Upon PLT(S,G) expiry, if no downstream listeners are present, Send
a Prune immediately.

If this is done, R2's PLT expires much before than R1's PT so that R2
sends a Prune.

Did I miss any point here that it can't be done?
But one problem that I can see with this is the age-out of entries.

Also, Is there any rationale behind the default value of 210 secs for
PLT(S,G) and 60 secs for Prune Holdtime?

Any comments on this are welcome.

Thanks & Regards,
Krish...

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