On 20 mrt 2009, at 14:40, Brian E Carpenter wrote:
NAT does not offer ANY multihoming benefits whatsoever, in fact, NAT
breaks multihoming because after a rehoming event, the addresses are
translated differently.
It's correct that NAT changeovers break existing sessions. But your
blanket
statement isn't true. NAT-based multihoming has the major benefit that
the number of extra BGP4 routes caused by a multihomed site is exactly
zero.
No. What you're talking about is multiaddress multihoming.
Then you add NAT to hide the changes to addresses from the hosts. But
IPv6 hosts can work with multiple addresses anyway (well, there's the
ingress filtering issue) so NAT is largely orthogonal to the
multihoming.
Also, shim6 gives you actual multihoming where sessions survive rather
than the watered down thing where you only get to reestablish new
sessions.
Also, NAT-based multihoming has value for large international
corporate
networks with dozens or hundreds of interconnection points to
the public network. It basically solves their address management
problem when dealing with multiple ISPs in multiple locations. That's
running code today.
People run whatever they can get away with. Doesn't mean it's a good
idea.
However, I do agree that it's useful to have stable internal
addressing when external connectivity is subject to change. That is a
legitimate advantage of NAT (66) which we haven't managed to make work
without NAT. We could though, by making sure that ULAs are used for
local connectivity regardless of the external connectivity.
On 21 mrt 2009, at 16:07, Brian E Carpenter wrote:
Suppose you're operating a large international network with (to take
a random example) IPv4 1/8 as its PI prefix.
You can't just advertise 1/8 in BGP4, because in fact it is split
up into many longer prefixes for various kinds of use and various
geographies.
Then what is the point of having a single prefix?
So how do you connect your internal users to the Internet?
Same way as everyone else, return the /8.
You have (I'm making this up) 100 different interconnects to the
public Internet around the world, across a variety of ISPs. If you
advertise longer prefixes out of 1/8 through those ISPs, life gets
highly complex if you want multihoming. Certainly you won't be able
to advertise *all* those prefixes through *all* those ISPs, so
you'll need
a complex worldwide management system just for your BGP4
advertisements,
to decide which prefixes are advertised where, and what the desired
backup
paths are. It can be done, but the OPEX is high.
Cost for the community is also high because a single organization puts
a whole bunch of prefixes in the routing table.
So instead, you run NAT at every ISP connection.
Ok, I said they didn't need the /8 before, but now you've completely
lost me. What is the point of having that prefix now??
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