[Roll] Closure text for ticket #91

[Mukul Goyal <[e-mail]>]

#91: Is it possible for an origin to get an error message in case the P2P-RPL route discovery fails.

Resolution:
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No. Because no intermediate router knows whether a DIO reached the target or not.


 Discussion:
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 [Cedric]
 On big question that rise my mind is, what happened if the route discovery  fail ?
 Some protocols sends out an error message when the route discovery fails  or get stuck.
 Do authors think that it could be relevant to add a "discovery-error"
 message as defined in other route discovery protocols ?

 [Mukul]
 I dont think it is possible to detect the failure of a P2P-RPL route  discovery. No node knows if a P2P-RPL route discovery has failed.

 P2P-RPL forms a temporary DAG and the route discovery (well, at least the  first half) succeeds when a target joins the DAG. Only the target knows  whether it joined the DAG or not. So, no node knows if the (first half of
 the) route discovery failed.

 Second half involves the target sending DRO to the origin. If the DRO does  not reach the origin, (the second half of) the route discovery fails. The  target can ensure (or at least increase the probability of) success by  asking for DRO-ACK and retransmitting the DRO if the DRO-ACK is not  received within certain time duration. DRO message travels by multicast,  so an intermediate router, that forwards a DRO further, has no idea  whether the next hop on the route received the DRO or not. Again, no node  knows if the (second half of the) there is no one to generate the  discovery-error message.

 I think an origin might infer the route discovery to have failed, if the  DAG's life time has expired but no DRO is received. But I am not sure we  should mandate this to be the way failure is inferred. We have just 4  values for the DAG life time. So, I think we should leave it to origin how  much to wait for a DRO before admitting failure.

[Cedric2]

I was thinking about an error message if the delivery of a message fails when using a route established by the P2P-RPL mechanism.
When a node included in the discovered route cannot be reached, then an error message could initiate a new route discovery using the P2P-RPL mechanism.

[Mukul2]
P2P-RPL routes are used in exactly the same manner as core RPL routes, that is you use an RPL Source Routing Header (RFC6554) or an RPL Option (RFC6553) to send a packet to its destination. These RFCs specify what ICMP error messages could be generated if the route is broken.

[Cedric3]
Ok.
If the route error detection is alredy handled, I think an additional error message is not necessary.