Re: [Roll] Some confusions in RFC 6719 (MRHOF)

Duong Ngoc Nguyen <nduong@purdue.edu> Thu, 29 November 2012 13:45 UTC

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Date: Thu, 29 Nov 2012 08:45:14 -0500 (EST)
From: Duong Ngoc Nguyen <nduong@purdue.edu>
To: Philip Levis <pal@cs.stanford.edu>
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Cc: roll@ietf.org
Subject: Re: [Roll] Some confusions in RFC 6719 (MRHOF)
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Thanks.

Sincerely,
Duong Nguyen

----- Original Message -----
> From: "Philip Levis" <pal@cs.stanford.edu>;
> To: "Duong Ngoc Nguyen" <nduong@purdue.edu>;
> Cc: roll@ietf.org
> Sent: Wednesday, November 28, 2012 4:27:25 PM
> Subject: Re: [Roll] Some confusions in RFC 6719 (MRHOF)
> Phil
> 
> On Nov 7, 2012, at 11:55 AM, Duong Ngoc Nguyen wrote:
> 
> > As I work with MRHOF, I find some statements are not consistent that
> > I cannot resolve by myself. I'm very grateful if someone can help me
> > overcome this confusion.
> >
> > 1. Which path cost is carried in Metric Container?
> >
> > Section 3.2.2 states:
> >
> >   Once the preferred parent is selected, the node sets its
> >   cur_min_path_cost variable to the path cost corresponding to the
> >   preferred parent. The value of the cur_min_path_cost is carried in
> >   the Metric Container corresponding to the selected metric when DIO
> >   messages are sent.
> >
> > ==> it suggests Metric Container carries the cost through the best
> > (i.e. preferred) parent.
> >
> > While section 3.4 states:
> >
> >   Once the preferred parent is selected, the node sets its
> >   cur_min_path_cost variable to the path cost corresponding to its
> >   preferred parent. It then calculates the metric it will advertise
> >   in
> >   its metric container. This value is the path cost of the member of
> >   the parent set with the highest path cost. Thus, while
> >   cur_min_path_cost is the cost through the preferred parent, a node
> >   advertises the highest cost path from the node to the root through
> >   a
> >   member of the parent set. The value of the highest cost path is
> >   carried in the metric container corresponding to the selected
> >   metric
> >   when DIO messages are sent.
> >
> > ==> it suggests Metric Container carries the cost through the worst
> > parent.
> 
> 3.4 is correct.
> 
> > 2. Which value is written to Rank field of DIO messsage's base
> > object in case ETX is used?
> >
> > When ETX is the metric, node must not include Metric container, thus
> > the rank field in DIO's base object is the only information to
> > advertise to neighbors.
> >
> > Section 3.4 states:
> >
> >   If ETX is the selected metric, a node MUST NOT advertise it in a
> >   metric container. Instead, a node MUST advertise an approximation
> >   of
> >   its ETX in its advertised Rank value, following the rules
> >   described
> >   in Section 3.3...
> >
> > ==> The phrase "in its advertised Rank value, following the rules
> > described in Section 3.3." means we would write the value of node
> > rank (computed by rules in section 3.3) into the rank field of DIO
> > message.
> >
> > On the other hand, section 3.5 states:
> >
> >   ... Once parent selection and rank computation is performed
> >   using the ETX metric, the node advertises the Rank and MUST NOT
> >   include a metric container in its DIO messages. While assigning
> >   Rank
> >   in this case, use the representation of ETX described in
> >   [RFC6551],
> >   i.e., assign Rank equal to ETX * 128.
> >
> > ==> The phrase "assign Rank equal to ETX * 128" suggests that we
> > should write value of ETX of path cost to the rank field of DIO
> > message.
> >
> 
> 3.4 is correct. The term "assign" is meant to describe the
> transformation between ETX and rank values, not the Rank in the DIO.
> 
> Phil