Re: [Roll] question about DAO-ACK

Michael Richardson <mcr+ietf@sandelman.ca> Sat, 19 July 2014 18:35 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: Routing Over Low power and Lossy networks <roll@ietf.org>
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Subject: Re: [Roll] question about DAO-ACK
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<CHAIR HAT OFF>

Pascal Thubert (pthubert) <pthubert@cisco.com> wrote:
    >> Or is it enough that B says, "okay, I got you", and then takes
    >> responsibility for sending reachability information on its own?

    > [PT] The design was the latter, that it was OK for B to say "I got
    > you".  Today it is mostly an alternate to MAC layer Ack, but the design
    > is richer than that since a status code is provisioned though not used.
    > A negative status code could say things like "I'm overloaded with DAO
    > state already" or "I'm out of battery".

Yes... so if the sender does set the K bit, they might still get the MAC
layer ACK, which tells them the packet was received, but can not tell them
anything about being overloaded, etc.   What reason would there be not
to set the K bit?

In non-storing mode, if you set the K bit, can you expect to ever get a
negative reply,  if the root is overloaded, or otherwises refuses to accept
your DAO?
(You can't get a reply, because the root can't route to you, since it didn't
accept your DAO...)

    >> The problem I have with not waiting is that if C thinks it is on the
    >> network when it gets the DAOACK, it can start sending traffic upwards,
    >> but downward packets would get lost for awhile.  Further, it is going
    >> to start sending DIOs, and start attracting traffic from other nodes
    >> when really, it isn't ready to do so.

    > [PT] This could effectively happen; mostly if you are considering the
    > join process.  This suggests that leveraging a tunnel from a joined
    > parent could effectively be a good idea there (eg piggy backing join
    > info to system/security manager on DAR/DAC message).  Also that
    > piggybacking security info to the DAO could effectively improve the
    > trust in the DAO content and ensure that the next phases of join
    > process can be responded to.

I was not thinking about any of the (initial) join process; it could just be
a node rejoining after a movement, an RF change reveals a new parent, or
just a cold restart of the node.

    >> 3) in the non-storing case, if a parent (B) of C, in fact had multiple
    >> interfaces, node C might not see the interface that is closer to A,
    >> and can only, in the Transit option, indicate the interface that it
    >> does see.  This implies that root A, may have to do two lookups to
    >> figure out where Bdown is, as it likely only has route to Bup.

    > [PT] You seem to imply that BDown would be advertised as reachable via
    > BUp.

Yes, that's right.  The root would have a (source) route to Bup (whether
direct or via other layers of the mesh), and another source route to Bdown
(via Bup)
The important thing is that C can not mention Bup, it can only mention Bown.

--
Michael Richardson <mcr+IETF@sandelman.ca>ca>, Sandelman Software Works
 -= IPv6 IoT consulting =-