Subject: [Roll] PathDigest question

From: Mukul Goyal <[email-masked]>
Date: Fri, 23 Oct 2009 11:24:02 -0500 (CDT)

Hi Pascal and Tim

I had a doubt about PathDigest.

Sec 5.3.4.2 of rpl-3 draft says that when "The node receives a modified RA-DIO message from a DAG parent", its trickle timer is reset.

Does it mean that the trickle timer is reset if a parent RIO shows modified PathDigest?

Now, a modified PathDigest from the parent means that the parent's parent set changed. I was wondering why should it be an inconsistency for me. If information in this RIO does not force me to change my rank/parent_set/routing_metrics, I need not reset my trickle timer.

Now, parent's changed PathDigest may be used to trigger NA-DAOs to this parent (as the draft already mentions). But I am not sure why should it cause resetting of trickle timer governing RA-DIOs.

Thanks
Mukul

---

From: "Anders Jagd" <[email-masked]>
Date: Fri, 23 Oct 2009 12:51:12 -0400

Also, PathDigest seems a bit expensive. I have previously suggested "D" bit
might suffice, but Tim correctly replied that the message with "D" bit set
could be lost, whilst a change in PathDigest would eventually be observed.

Let me therefore modify my suggestion as follows: Remove PathDigest, but
replace "D" bit with say D[3:0]. A change would be signaled by incrementing
the current value of D[3:0]. D[3:0] would be local to each node, so
basically a node would check for changes in parent node D[3:0] setting, and,
when so detected, increase its own D[3:0] value - signaling this new value
(indicating request for update of NAs) downstream.

/Anders

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Mukul Goyal
Sent: Friday, October 23, 2009 12:24 PM
To: Pascal Thubert (pthubert)
Cc: roll
Subject: [Roll] PathDigest question

Hi Pascal and Tim

Sec 5.3.4.2 of rpl-3 draft says that when "The node receives a modified
RA-DIO message from a DAG parent", its trickle timer is reset.

Does it mean that the trickle timer is reset if a parent RIO shows modified
PathDigest?

Now, a modified PathDigest from the parent means that the parent's parent
set changed. I was wondering why should it be an inconsistency for me. If
information in this RIO does not force me to change my
rank/parent_set/routing_metrics, I need not reset my trickle timer.

Now, parent's changed PathDigest may be used to trigger NA-DAOs to this
parent (as the draft already mentions). But I am not sure why should it
cause resetting of trickle timer governing RA-DIOs.

Thanks
Mukul
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---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Fri, 23 Oct 2009 20:09:01 +0200

Hi Mukul:

A path digest tells that something has changed in the inwards path of this node and suggest that DAOs should be sent.
This is mostly useful for source route. For stateful, all you need to know is that your parents change.
Also, this is mostly meaningful if the DAOs do not fan out since this is the digest of the preferred path. 

I think the current draft suggests to send DAOs over one parent (as opposed to fan out) but does not impose anything on which that parent is (preferred or not). 

WRT trickle, if there's no source route, I agree with you and I do not see why that would require to do anything.
If there's source route, then the exponential backoff of RAs might help ensure that all the children reconstruct their path faster.

What do you think? 

Pascal
>From: Mukul Goyal [mailto:[email-masked]]
>Sent: vendredi 23 octobre 2009 18:24
>Subject: PathDigest question
>
>Hi Pascal and Tim
>
>Now, a modified PathDigest from the parent means that the parent's parent set changed. I was wondering why
>should it be an inconsistency for me. If information in this RIO does not force me to change my
>Now, parent's changed PathDigest may be used to trigger NA-DAOs to this parent (as the draft already
>mentions). But I am not sure why should it cause resetting of trickle timer governing RA-DIOs.
>
>Thanks
>Mukul

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Fri, 23 Oct 2009 20:28:44 +0200

Hi Anders,

This works fine. Likewise, Mathilde opened the discussion on the size of
the prefix. I trust we'll need a general pass on compression though at
some point it might be like 6LoWPAN a local optimization for a given
sort of link. 

I know that it's critical for implementations and Interop to discuss the
formats early though the formats are only relevant as long as we know
what we do with them.

OTOH, we see more discussions on the formats than on the base
mechanisms, and we end up making decisions, like the removal of the Dag
Hop Timer and the DAG split / merge just announced, with almost no vote
on the list. That concerns me.

Cheers,

Pascal

>From: Anders Jagd [mailto:[email-masked]]
>Sent: vendredi 23 octobre 2009 18:51
>To: 'Mukul Goyal'; Pascal Thubert (pthubert)
>Cc: 'roll'
>Subject: RE: [Roll] PathDigest question
>

---

From: "Mathilde Durvy (mdurvy)" <[email-masked]>
Date: Mon, 26 Oct 2009 10:37:23 +0100

Hi All,

As I mention in my previous email, I  don't see a clear justification
for the PathDigest in the current draft. The only place where the
PathDigest is mentioned (outside of the DIO format) is Section 5.9.2.1.
"Destination advertisements may also be scheduled for sending when the
PathDigest of the RA-DIO message has changed". 

In addition its current definition is not very clear to me. It says that
the PathDigest is "the result of a CRC-32c computation on a bit string
obtained by appending the received value and the ordered set of DAG
parents of the LLN Node". Which received value? The one from the
'prefered' parent? As mentioned by Mukul, does a change of PathDigest
necessarily affects the receiving node? (it might not require any change
locally)...

I would be happy to know if someone as a clear example highlighting the
use of the PathDigest. Otherwise I propose to add the removal of the
PathDigest to the list of possible simplifications.

Best,
Mathilde

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Pascal Thubert (pthubert)
Sent: vendredi, 23. octobre 2009 20:09
To: Mukul Goyal
Cc: roll
Subject: Re: [Roll] PathDigest question

Hi Mukul:

 A path digest tells that something has changed in the inwards path of
this node and suggest that DAOs should be sent.
This is mostly useful for source route. For stateful, all you need to
know is that your parents change.
Also, this is mostly meaningful if the DAOs do not fan out since this is
the digest of the preferred path. 

I think the current draft suggests to send DAOs over one parent (as
opposed to fan out) but does not impose anything on which that parent is
(preferred or not). 

WRT trickle, if there's no source route, I agree with you and I do not
see why that would require to do anything.
If there's source route, then the exponential backoff of RAs might help
ensure that all the children reconstruct their path faster.

What do you think? 

Pascal

>Now, a modified PathDigest from the parent means that the parent's 
>parent set changed. I was wondering why should it be an inconsistency 
>for me. If information in this RIO does not force me to change my

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Mon, 26 Oct 2009 21:50:57 +0100

Hi Mathilde;

A new path digest indicates a change in the preferred tree within the
DAG that leads to this node.
The source route path follows that tree. 
So if the digest has changed, then the source route path is obsolete and
must be reconstructed by sending a DAO.

Pascal

>From: Mathilde Durvy (mdurvy)
>Sent: lundi 26 octobre 2009 10:37
>To: Pascal Thubert (pthubert); Mukul Goyal; Anders Jagd
>Cc: roll
for the PathDigest in the current draft.
>The only place where the PathDigest is mentioned (outside of the DIO
format) is Section 5.9.2.1.  "Destination
>advertisements may also be scheduled for sending when the PathDigest of
the RA-DIO message has changed".
>
>In addition its current definition is not very clear to me. It says
that the PathDigest is "the result of a
>CRC-32c computation on a bit string obtained by appending the received
value and the ordered set of DAG
>parents of the LLN Node". Which received value? The one from the
'prefered' parent? As mentioned by Mukul,
>does a change of PathDigest necessarily affects the receiving node? (it
might not require any change
>I would be happy to know if someone as a clear example highlighting the
use of the PathDigest. Otherwise I
>propose to add the removal of the PathDigest to the list of possible
simplifications.
> A path digest tells that something has changed in the inwards path of
this node and suggest that DAOs should
>be sent.
>I think the current draft suggests to send DAOs over one parent (as
opposed to fan out) but does not impose
>anything on which that parent is (preferred or not).
>
see why that would require to do
>anything.
ensure that all the children
>reconstruct their path faster.
>
>What do you think?
>
>Pascal
rank/parent_set/routing_metrics, I need not
>reset my trickle timer.
>>parent (as the draft already mentions). But I am not sure why should
it cause resetting of trickle timer
>governing RA-DIOs.

---

From: "Mathilde Durvy (mdurvy)" <[email-masked]>
Date: Tue, 27 Oct 2009 09:09:03 +0100

Hi Pascal,

Yes, I understand that. Two questions:
- Can we be sure that packets follow a 'preferred path'? It seems to me
that the only case where this is true is when pure source routing is
used.  
- The 'D' flag achieves the same effect but less reliably as pointed by
Anders. Is that an acceptable tradeoff?

Best,
Mathilde  

-----Original Message-----
From: Pascal Thubert (pthubert) 
Sent: lundi, 26. octobre 2009 21:51
To: Mathilde Durvy (mdurvy); 'Mukul Goyal'; 'Anders Jagd'
Cc: 'roll'
Subject: RE: [Roll] PathDigest question

Hi Mathilde;

Pascal

>format) is Section 5.9.2.1.  "Destination advertisements may also be
scheduled for sending when the PathDigest of the RA-DIO message has
changed".
>that the PathDigest is "the result of a CRC-32c computation on a bit 
>string obtained by appending the received value and the ordered set of 
>DAG parents of the LLN Node". Which received value? The one from the 

>opposed to fan out) but does not impose anything on which that parent
is (preferred or not).

>>Sec 5.3.4.2 of rpl-3 draft says that when "The node receives a 
>>modified RA-DIO message from a DAG parent", its trickle timer is
reset.
>>Now, parent's changed PathDigest may be used to trigger NA-DAOs to 
>>this parent (as the draft already mentions). But I am not sure why 
>>should it cause resetting of trickle timer
>governing RA-DIOs.
>>
>>Thanks
>>Mukul