Re: [Roll] [roll] #95: Why need stop flag? Is the receipt of DRO not sufficient to indicate completion of route discovery?

[Mukul Goyal <[e-mail]>]

Please see inline

Thanks
Mukul

#95: Why need stop flag? Is the receipt of DRO not sufficient to indicate completion of route discovery?

 Resolution: No because multiple DROs would be generated if multiple source  routes are being discovered.

 Discussion:

 p15 :  Stop (S): This flag, when set to one by a target, indicates that  the P2P-RPL route discovery is over.

 [Cedric]
 Is this bit really usefull ? My guess is that it will be always set to 1.
 If you hear a DRO, this indeed means that the RDO has reached the target,  so you could just stop processing RDO when you hear a DRO.
 Do we really need a flag to stop RDO processing or the hearing of a DRO  type message could do the job ?

 [Mukul]
 A P2P-RPL invocation might involve discovery of multiple source routes. In  that case, receipt of a DRO does not mean route discovery is over. Only  when the target sets the stop flag in the DRO, a node could be sure that  the route discovery is over.

[Cedric2]
OK fo multiple discovery.
But if I want to discover a route to a multicast group of target. I set a multicast adress in the target field of the RDO. Then, do I received as many DRO message as the number of target reached ? In that case, would the first DRO with a "S" flag stop the RDO propagation to reach all the target included in the multicast group ?

[Mukul2]
A target cannot set the S flag to one in the DRO if the target address in the P2P-RDO specified a multicast address. See the following text at the end of page 21 in P2P-RPL-9:

"The target MAY set the stop flag inside the DRO message to one if



Goyal, et al.           Expires September 7, 2012              [Page 21]
 
Internet-Draft         draft-ietf-roll-p2p-rpl-09             March 2012


   o  this router is the only target specified in the corresponding DIO,
      i.e., the corresponding DIO specified a unicast address of the
      router as the Target inside the P2P-RDO with no additional targets
      specified via RPL Target Options; and

" 

[Cedric3] 
So how do you stop the RDO flooding when the target adress is mulicast ? 

[Mukul3]

Stop flag cannot be used when the target address is multicast or when multiple unicast targets are there. The DIO generation will stop when the DAG dies. In the meanwhile, trickle algorithm would hopefully avoid unnecessary message generation. Note that the draft recommends a very small value for the redundancy constant.

-- 
-----------------------------------+---------------------
 Reporter:  jpv@…                  |      Owner:  mukul@…
     Type:  defect                 |     Status:  new
 Priority:  major                  |  Milestone:
Component:  p2p-rpl                |    Version:
 Severity:  Submitted WG Document  |   Keywords:
-----------------------------------+---------------------

Ticket URL: <http://trac.tools.ietf.org/wg/roll/trac/ticket/95>
roll <http://tools.ietf.org/wg/roll/>

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