Shim6, was: Re: filtering /48 is going to be necessary

Masataka Ohta mohta at
Mon Mar 12 18:29:20 CDT 2012

William Herrin wrote:

> When I ran the numbers a few years ago, a route had a global cost
> impact in the neighborhood of $8000/year. It's tough to make a case
> that folks who need multihoming's reliability can't afford to put that
> much into the system.

The cost for bloated DFZ routing table is not so small and is
paid by all the players, including those who use DFZ but do
not multihome.

Those who can't pay the cost silently give up to be multihomed,
which is why you overlooked them.

Even those who pays the cost are not using full routing
table for IGP, which makes their multihoming less capable.

> A *working* multi-addressed end user system (like shim6 attempted)

Shim6 is too poorly designed that it does not work.

> Often overlooked is that multihoming through multi-addressing could
> solve IP mobility too.


What is often overlooked is the fact that they are orthogonal

> Carry
> your voip call uninterrupted from your home wifi on the cable modem to
> your cell provider in the car to your employer's wired ethernet and
> back.

Use mobile IP implemented long before shim6 was designed.

> Unfortunately, shim6 didn't work in some of the boundary cases. Since
> single-homing works pretty well in the ordinary case, there's not much
> point to a multihoming protocol that fails to deliver all the boundary
> cases.

Just like NAT, shim6 is an intelligent intermediate entity trying
to hide its existence from applications, which is why it does not
work sometimes just as NAT does not work sometimes.

The only end to end way to handle multiple addresses is to let
applications handle them explicitly.

						Masataka Ohta

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