Thanks Robert,
I am guessing you are referring to the MAX_RA_DELAY_TIME .5 seconds?
The problem I am thinking about goes a little beyond this randomization…
The way I see it, in LLNs hidden nodes are a common case, so if two
responding routers are not within direct connectivity, their packets will
collide if randomization resolution is too small. Unless the randomization
resolution is not in the order of magnitude of L2 frame transmissions
(including overheads such as preambles, backoffs, etc.) their packets are
very likely to collide.
This is why I am concerned with the value of 0.5 seconds being a little
small, and also wonder whether defining the resolution of the randomization
is important for 6LoWPAN-ND.
Trying to illustrate below.
Consider the following topology:
R1 --> H <-- R2
+----+
Query: | RS |
+----+
+--------+
R1 Resp: | RA1 |
+--------+
+---------+
R2 Resp: | RA2 |
+---------+
---------------------------------->t
Above you see the responses collide, even though R1 and R2 randomized a
different value, because their L2 could not separate their transmissions (as
they cannot hear each other).
Had the randomization was using resolution at least the size of the response
packet and R1 and R2 randomized two different values, the collision would
have been avoided.
Of course in realistic large and dense networks, you’re likely to see more
than two routers respond, which may escalate the problem.
I hope this better clarifies my concern.
Thanks,
Yoav
*From:* Robert Cragie [mailto:[email protected]]
*Sent:* Sunday, September 26, 2010 4:45 PM
*To:* Yoav Ben-Yehezkel
*Cc:* 6lowpan
*Subject:* Re: [6lowpan] congestion of responses to multicast RS query from
an ND host
Hi Yoav,
There is already a mechanism specified in rfc4861 to randomize the
transmission of RAs - see section 6.2.6. This would apply to 6lowpan-nd as
well. The downside of this approach is the lengthening of the whole RS/RA
procedure.
Robert
Robert Cragie (Pacific Gas & Electric)
Gridmerge Ltd.
89 Greenfield Crescent,
Wakefield, WF4 4WA, UK
+44 1924 910888
+1 415 513 0064
http://www.gridmerge.com
On 26/09/2010 1:32 PM, Yoav Ben-Yehezkel wrote:
Hi,
I have a question on 6LoWPAN-ND. I tried looking for this issue in the
mailing list, but couldn’t find anything on this.
Does the WG see a need for some kind of regulation of RA responses to host
multicast of a RS query in 6LoWPAN-ND? Unless regulated, in a large and
dense networks, this could cause excessive traffic for a meaningful period
of time.
Best regards,
Yoav
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