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Re: Misconceptions, was: IPv6 RA vs DHCPv6 - The chosen one?
From: Steven Bellovin <smb () cs columbia edu>
Date: Thu, 29 Dec 2011 18:37:20 -0500


On Dec 29, 2011, at 5:30 16PM, Masataka Ohta wrote:

Valdis.Kletnieks () vt edu wrote:

IGP snooping is not necessary if the host have only one next
hop router.

You don't need an IGP either at that point, no matter what some paper from
years ago tries to assert. :)

IGP is the way for routers advertise their existence,
though, in this simplest case, an incomplete proxy of
relying on a default router works correctly.

Beyond that, if there are multiple routers, having a default
router and relying on the default router for forwarding to
other routers and/or supplying ICMP redirects stops working
when the default router, the single point of failure, goes
down, which is the incompleteness and/or incorrectness
predicted by the paper of the end to end argument.

Considering that the reason to have multiple routers
should be for redundancy, there is no point to use
one of them as the default router.

VRRP?  The Router Discovery Protocol (RFC 1256).  But given
how much more reliable routers are today than in 1984, I'm
not convinced it's that necessary these days.

Developing more complicated IGP proxy makes the
incompleteness and the incorrectness not disappear but
more complicated.

                                      Masataka Ohta

PS

Note that the paper was written in 1984, where as RFC791
was written in 1981.

There was a lot less understanding of the difference between hosts
and routers in 1984 than there is today -- if nothing else, note
how 4.2BSD and 4.3BSD considered all multihomed machines to be
routers.  



                --Steve Bellovin, https://www.cs.columbia.edu/~smb







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