Misconceptions, was: IPv6 RA vs DHCPv6 - The chosen one?
rps at maine.edu
Thu Dec 29 20:18:35 CST 2011
OK, this is getting ridiculous.
Let's assume that we have a model where host systems receive the
global routing table from service providers. The stated reason for
this is so that they could make their own routing decisions when
multi-homed environment. Presumably with each ISP connected to a L2
device; let's say an Ethernet Switch.
So now we have ISP A, B, and let's even add C. Each are providing all
available routes in the global table, and path information, allowing
the host to make intelligent routing decisions.
Unfortunately, for bi-directional communication, the prefix assigned
to the customer must be provider independent. It's a residential
service so maybe they have a single 64-bit prefix. So we now need ISP
A, B, and C to be announcing that prefix.
Congratulations, you have just created a model where every customer
would have their own entry in the global table (since route
summarization and aggregation is now impossible), one that is
exponentially greater than the production IP global table today.
But that is only the case if you let customers have a PI prefix (which
I think is really required in a purist end-to-end model, but for the
sake of argument...).
We could, of course, only provide customers with the global table, and
have each ISP give hosts a different prefix. This would allow for the
selection of the best path by the host, but once that communication is
established it would be locked into that path. The remote host would
have no knowledge of other available prefixes (even if there is a path
change, and a different path becomes favorable). To get around this
we could develop another message that allows a host to announce
alternative addresses to other hosts; which means systems now also
need to keep a table of alternate peer addresses, and have the ability
to quickly detect changes in path availability and quality.
The result is still a very large amount of overhead. You will still
experience significant change propagation delay (slower than change
propagation under the current model as it would be more complex,
involve exponentially more peers, paths, etc, and be performed on
This all would be significantly more complex than IPv6 (which is
already claimed, by the proponents of the beloved end-to-end model, to
be too complex and have too much overhead), it would introduce even
more delay (another complaint) than IPv6 due to that complexity.
Limitations aside. We now know that it takes well over a decade for
people to move to a protocol, even when it is almost operationally
identical to its predecessor. Getting buy-in for such a fundamental
shift in how the Internet is build would be almost impossible at this
point. To be frank, you would have to build a completely new and
parallel network and hope you could somehow convince people to adopt
it (when everything they want works "just fine" on the "old" network).
I honestly can't believe we're even having this discussion. It's
2011/12/29 Masataka Ohta <mohta at necom830.hpcl.titech.ac.jp>:
> Valdis.Kletnieks at 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.
> Developing more complicated IGP proxy makes the
> incompleteness and the incorrectness not disappear but
> more complicated.
> Masataka Ohta
> Note that the paper was written in 1984, where as RFC791
> was written in 1981.
Epic Communications Specialist
Phone: +1 (207) 561-3526
Networkmaine, a Unit of the University of Maine System
More information about the NANOG