Subject: SPT bit and intermediate routers.

From: Sudhir Cheruathur <[email-masked]>
Date: Wed, 7 Apr 1999 11:56:55 -0700

Hi,

A question on PIM-SM.

Consider the following network

    G ----a-R1-b---------a-R2-b--------a-R3-b-------a-R4-b-- S

    Receiver joins G at last hop router R1
    Sender S's last hop router is  R4
    R2 is the RP.

    Say G joins first

     So we have the following states
     Router     State      iif          oif            flags
     R1          (*,G)        b           a             WC,RPT
     R2          (*,G)       nil           a             WC,RPT
     
     S starts to send to G

     R4          (S,G)      b        nil-Reg.         ----
     
     Now R2 decides to build SPT between S and itself

     R2          (S,G)      b(ie RPF to S)     a(copy of  *,G)         ----
     R3          (S,G)      b(ie RPF to S)     a (incoming J/P if)     -----
     R4          (S,G)      b                        a
-----

     R2 starts to recv packets from both register and native.
    
    According to 3.4.2.b,2 
             "The Join/Prune message is being sent to the RPF
              neighbor toward the RP, there exists a (S,G) entry with the
              RPT-bit flag cleared and SPT-bit set, and the incoming
              interface toward S is different than the incoming interface
              toward the RP, or "

    since iifs are different  {(*,G) - nil, (S,G) - b }
    the SPT bit is set. And since R2 is itself the RP no prune is sent.
Therefore 

    R2           (S,G)     b                        a                SPT.

    According to 3.3.2.1.1

             "If a (S,G) entry with cleared (0) SPT bit exists, and
             the received Register does not have the Null-Register-Bit
             set to 1, the packet is forwarded; and the SPT bit is left
             cleared (0). If the SPT bit is 1, the packet is dropped,
             and Register-Stop messages are triggered.  Register-Stops
             should be rate-limited (in an implementation-specific
             manner) so that no more than a few are sent per round trip
             time. This prevents a high datarate stream of packets from
             triggering a large number of Register-Stop messages between
             the time that the first packet is received and the time
             when the source receives the first Register-Stop.  "

    Register-Stops are sent to R4.

    From the above we see that R3 belongs to the SPT but does not have the
SPT
    bit set. Is this correct or did I miss something ?

    Thanks in advance,

    Regards
    Sudhir

---

From: [email-masked] (Billy Ng)
Date: Wed, 7 Apr 1999 15:56:31 -0400

> From [email-masked] Wed Apr  7 15:08:58 1999
> 
> Hi,
> 
> A question on PIM-SM.
> 
> 

R3 is just an intermediate router.  SPT is only set if a router has a
(*,G) and the iif is different from the iif of (S,G).  In the R3
case, it only has (S,G), thus no need to set the SPT bit.

--Billy