nanog mailing list archives

RE: IPv6 /64 links (was Re: ipv6 book recommendations?)


From: "Tony Hain" <alh-ietf () tndh net>
Date: Tue, 12 Jun 2012 15:26:39 -0700

Masataka Ohta
Tony Hain wrote:

It is because you avoid to face the reality of MLD.

MLD != ND
MLD == IGMP

OK.

ND ~= ARP

Wrong, because ND requires MLD while ARP does not.

Note the ~ ...  And ARP requires media level broadcast, which ND does not.
Not all media support broadcast. 


ND is less overhead on end systems than ARP

Today, overhead in time is more serious than that in processor load.

As ND requires MLD and DAD, overhead in time when addresses are
assigned is very large (several seconds or more if multicast is not very
reliable), which is harmful especially for quicking moving mobile hosts.

So leveraging broadcast is why just about every implementation does a
gratuitous ARP-and-wait multiple times, which is no different than DAD
timing? MLD does not need to significantly increase time for address
assignment. If hosts are moving quickly the fabric needs to be able to keep
up with that anyway, so adding a new multicast member needs to be fast
independent of IPv6 address assignment.


because it is only received by
nodes that are subscribed to a specific multicast group rather than
broadcast reception by all.

Broadcast reception by all is good because that's how ARP can detect
duplicated addresses without DAD overhead in time.

BS ... Broadcasts are dropped all the time, so some nodes miss them and they
need to be repeated which causes further delay. On top of that, the
widespread practice of a gratuitous ARP was the precedent for the design of
DAD. 


Multicast group management is inherently noisy,

Thus, IPv6 is inherently noisy while IPv4 is not.

but a few more bits on the
wire reduces the load on the significantly larger number of end
systems. Get over it ...

First of all, with CATENET model, there is no significantly large number
of end
systems in a link.

Clearly you have never looked at some networks with > 64k nodes on a link.
Not all nodes move, and not all networks are a handful of end systems per
segment.


Secondly, even if there are significantly large number of end systems in a
link, with the end to end principle, network equipments must be dumb while
end systems must be intelligent, which means MLD snooping is unnecessary
and end systems must take care of themselves, violation of which results
in
inefficiencies and incompleteness of ND.

MLD snooping was a recent addition to deal with intermediate network devices
that want to insert themselves into a process that was designed to bypass
them. That is not a violation of the end systems taking care of themselves,
it is an efficiency issue some devices chose to assert that isn't strictly
required for end-to-end operation. 

Just because you have never liked the design choices and tradeoffs made in
developing IPv6 doesn't make them wrong. I don't know anybody that is happy
with all aspects of the process, but that is also true for all the bolt-on's
developed to keep IPv4 running over the last 30 years. IPv4 had its day, and
it is time to move on. Continuing to complain about existing IPv6 design
does nothing productive. If there are constructive suggestions to make the
outcome better, take them to the IETF just like all the constructive changes
made to IPv4.

Tony




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