Subject: [pim] Receiving register packets at the RP from a (*,G) interface

From: "Christophe Jelger" <[email-masked]>
Date: Fri, 23 Nov 2001 12:01:57 +0100

Hello,

I have a specific question for the following case: when a source S for group
G appears at a router that is already on the (*, G) tree.

Example :

            RP
            |
            R (*,G)
            |
       S--R (*,G)
            |
            R (*,G)

I am not really sure on what's supposed to happen at the RP and at
intermediate routers (on the way to the RP) with (*,G) state.

1. The source S starts sending in native multicast to G.
2. The DR of S starts encapsulating S's packets in register messages that
are sent to the RP.
3. The RP receives the packets from an interface that has a (*,G) state ...

What happens next ?

4. The RP starts sending (S,G) join messages towards the source.

And what happens at intermediate routers with (*,G) state ?

5. The router creates a new (S,G) entry and forwards the packet to the
downstream router.

So now intermediate routers have a (*,G) state and a (S,G) state with
"opposite interfaces" ... I mean the router may have :

            (*,G) : iif = i1, oif = i2        and
            (S,G) : iif = i2, oif = i1

Now packets sent from S immediatly reach the RPT and can shortcut down the
stream. But they also match the (S,G) (that is not a SPT, it's just the
branch from S to the RP) so they should be forwarded to the RP but in this
case they flow up the stream of the RPT back to the RP.

Well I feel very confused with that case. If anybody could please give me
some clues on that I would really appreciate.

Thanks

Christophe

---

From: "Prashant Jhingran" <[email-masked]>
Date: Sat, 24 Nov 2001 19:39:07 +0530

Well the scenario is quite simple....

 "The RP receives the packets from an interface that has a (*,G) state ...

What happens next ?"
Solution: The Rp shall receive Register packet from the Source DR,RP tries
to find whether there is any downstream receiver present in the RPT or not.
if not then it should send a unicast a Register Stop towards the Source DR.
And if yes then it should decapsulate the register packet & should fwd in
the RPT.It should also start the computation of register rate threshold.
This should continue till the register threshold rate exceeds a particular
(configurable ) limit.
once this limit is crossed , the RP should start the formation of the SPT
towards the Source DR by sending (S,G) join towards the Source DR.
Once the SPT is completely formed, the RP should send a register stop
towards the Source DR.

Thus now the RP has S,G as well as *,G state...the SG entry should contain
all the inherited oif's from the *,G entry...
hence the RP should now receive & transmit the packet using the SG entry.

In case of IR, the same would have happened, but the only additional thing
would have been, the trigger of SGrpt Prune towards RP once the SPT is
formed.(meaning the SPT bit have been set in the SG entry).

See the basic is(All except RP's) once you switch to SPT(and form it
completely),you should trigger a SGrpt Prune towards the upstream router
towards the RP so to receive packets in SPT "ONLY" !!.

Hope the above clears your confusion.......

regards,
Prashant

"As the different streams having their sources in different paths which men
take through different tendencies, various though they appear, crooked or
straight, all lead to Thee."

Prashant Jhingaran
Future Software
480-481 , Mount road ,
Nanadnam ,
Chennai - 600 035
Ph# +91-44-433 0550 Ext. 413

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of Christophe
Jelger
Sent: Friday, 23 November 2001 4:32 PM
To: PIM
Subject: [pim] Receiving register packets at the RP from a (*,G)
interface

Hello,

Example :

            RP
            |
            R (*,G)
            |
       S--R (*,G)
            |
            R (*,G)

What happens next ?

Thanks

Christophe