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Re: IPv6 RA vs DHCPv6 - The chosen one?
From: Cameron Byrne <cb.list6 () gmail com>
Date: Wed, 28 Dec 2011 07:50:20 -0800

On Wed, Dec 28, 2011 at 7:28 AM, TJ <trejrco () gmail com> wrote:
2011/12/28 Masataka Ohta <mohta () necom830 hpcl titech ac jp>

Valdis.Kletnieks () vt edu wrote:



<SNIP>

In this case, the following statement in RFC1883:
   If the minimum time for rebooting the node is known (often more than
   6 seconds),
is the wrong assumption which made RA annoying.

Oddly enough, a lot of us are running on networks where assuming this
about end
user gear is perfectly reasonable.

That is because, as I wrote already in the previous mail,

      Network configuration was mostly stationary

For example, IPv6 might work well, if most of your end users
are not moving rapidly between small mobile cells.

However, assuming you change the cells every 100m in average
and you are moving at 100km/h, you must change the cells every
3.6 seconds in average, which means you must be able to change
the cells frequently, which means each cell change take a lot
less than 3.6 seconds.


To me, that sounds like an argument in favor of SLAAC.  SLAAC is noticeably
faster in my experience than DHCP (v4 or v6).  Also, RAs can be sent in the
ms range - for environments that expect that type of attachment-point-churn
...

Also:
Isn't 100m an arbitrarily tight range for a cel tower?
And for cellular, isn't the real churn happening more at the Layer2 side
... no L3/IPv6/IPv4 interaction?



Correct.  Cellular network mobility at the cell site level is all L1
and L2 magic.  GSM / UTMS / LTE will never engage in SLAAC churn as a
result of a normal mobility event.


We haven't seen many consumer-grade
Windows, Macs, or Linux boxes that are able to reboot in much under 6
seconds.

IPv6 is wrongly architected, not because it assumes nodes are
able to reboot in much under 6 seconds, but because it assumes
new configurations necessary only at boot time.


Boot time, or anytime a change in network attachment point is detected ...
is that not sufficient?


Yes, I know you can do it with careful tuning and throwing SSDs and other
hardware at it - doesn't mean it's common.

Obviously, the IPv6 committee and you are assuming computers
of immobile main frame computers or, at least, immobile
workstations.

However, in the real world, commonly available mobile phones
are IP capable computers which wake up from dormant state
within a second and needs handover often within a second.


Again, if we are arguing about simple speed of address attainment - SLAAC
wins.



                                                Masataka Ohta



/TJ


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