Dear all,
I have a couple of doubts regarding the LOAD specification. Just in case I am
missing something, I would like to expose them in the list. My doubts are:
1) WEAK_LQI_VALUE
*****************
In Section 3, page 4, the latest LOAD draft states that "The approach taken by
LOAD avoids a route which contains weak links whose LQI is below certain
threshold value (i.e. WEAK_LQI_VALUE)". On the other hand, the proposed
default value for the WEAK_LQI_VALUE parameter is 8.
However, according to the datasheet of the CC2400 radio chip (i.e. the one
included in Crossbow's MICAz motes), LQI values range between 50 (bad quality)
and 110 (good quality). We (and also other researchers) have conducted
experiments which confirm this range of values. In consequence, it seems
difficult that a link could have an LQI value below 8 (at least with MICAz
motes).
Is it possible that proposing WEAK_LQI_VALUE equal to 8 comes from experiences
with other platforms (i.e. not the MICAz ones)? Do you think a different
default value for WEAK_LQI_VALUE could be proposed by default?
2) Sequence numbers
*******************
LOAD avoids usage of sequence numbers for the sake of simplicity, forcing that
only the destination is allowed to generate RREP messages in order to avoid
routing inconsistencies (loops, etc.). However, I would like to expose the
following doubt.
In Section 6.3, LOAD spec. states that: "When the destination receives a RREQ,
it tries to find the entry of the same originator address and RREQ ID pair in
the route request table. If the entry is found, the destination compares the
route cost of the RREQ with the forward route cost of the entry. If the cost
of the RREQ is better than (i.e. less than) that of the entry, the destination
updates the reverse route to the originator in the routing table and generates
a RREP in reply. If the cost of the RREQ is not less than that of the entry,
the destination just discards the RREQ".
Let's consider the following network topology, where A transmits data to C.
Let's assume all links are equal and have the same quality:
B
/ \
A C
\ /
D - E
According to the 'minimum hop while avoiding weak links' metric, the chosen
path will be ABC.
Now, if the AB link is broken, A starts a new Route Discovery. There is one
available path for transmitting data from A to C (ADEC) but since this path
has a higher cost than the former (let's assume information about the old
route hasn't yet expired at C), then C discards the RREQ according to Section
6.3. In consequence, it would not be possible to use the available path.
Am I missing something? Would it be necessary to use sequence numbers to take
into account, not only the routes cost but also their freshness?
Many thanks in advance.
With best regards,
Carles Gómez
Wireless Networks Group
Technical University of Catalonia (UPC)
-------------------------------------------------
This mail sent through IMP: http://horde.org/imp/
_______________________________________________
6lowpan mailing list
[email protected]
https://www1.ietf.org/mailman/listinfo/6lowpan
