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Re: [RRG] A Late Response to Questions on Six/One Router
On Jul 11, 2008, at 5:33 AM, Christian Vogt wrote:
Comrades,
previous discussions around tunneling vs. address rewriting have shown
that some of the concepts of Six/One Router are unclear in the
community. This is, of course, my fault because the papers that I
have so far published on Six/One Router do not go into sufficient
detail to dispel these unclarities. I have done my homework now and
would like to point you to the following paper, which motivates and
explains the design of Six/One Router more thoroughly:
http://users.piuha.net/chvogt/pub/2008/vogt-2008-six-one-router-design.pdf
better later than never: I have 3 questions on this writeup; Robin
already brought up the first one in his long review comment (dated
July 30, 2008 10:25:39 AM PDT), but I have not seen the rest been
asked (or maybe I read Robin's long msg too quickly and missed).
1/ fig-4 in the above pdf file shows an example of backward
compatibility packet exchanges between an upgraded and a legacy edge
network, however note that both are single-homed in this example.
If the legacy net L is multihomed, everything still works, as (I
assume) L still has its prefix injected into the global routing table.
However if the upgraded network U is multihomed, the communication
with L can only use the address of one of U's providers, right?
(this would be a dis-incentive for U to do upgrading...)
2/ how does six/one handle transient failures?
Take Figure-3 for example: if the link between provider-1 and six/one
router fails, since the other end sends to the specific address block
of this six/one router (as explained in fig-2), what causes the other
end to change the translation to the address belonging to the right-
side six/one router?
Or there is some "local repair" done between provider-1 and provider-2?
(the latter is hinted by the following pragraph quoted from the paper:
Six/One Router
should permit fast re-establishment of packet exchanges in
Unilateral mode upon failure of the provider via which they
used to be routed. Six/One Router achieves this by making the
providers of a multi-homed edge network responsible for
connectivity to disjoint and complementary subsets of the
transit address space, while having all of them provide
connectivity to the complete remote edge address space.
Providers back up each other’s routes to remote transit
addresses.
I don't fully understand exactly what it means by "responsible for
connectivity to disjoint and complementary subsets of the transit
address space", or
how providers "back up each other’s routes to remote transit
addresses"..)
3/ the paper shows that six/one deployment is at edge sites--how does
this help align cost with incentives?
I would like to especially thank Robin Whittle, who has posed a number
of excellent questions that helped me understand what is unclear about
Six/One Router, and thus address these unclarities in the above paper.
Before responding to Robin's question more carefully in a separate
email, let me here just emphasize two main concepts of Six/One Router.
Likely, this will resolve most of the existing vagueness:
- Mapping between edge addresses and the transit addresses from a
given provider is one-to-one. This is why no packet encapsulation
is necessary: The edge/transit address after rewriting is
unambiguously determined by the transit/edge address prior to
rewriting. This also makes Six/One Router function without
per-host state, like tunneling, but unlike NAT boxes.
- Six/One Router has two components, which are independent of each
other even though they both use address rewriting: The first
component uses bilateral address rewriting for communications
between two upgraded edge networks; the second component uses
unilateral address rewriting on the border of an upgraded edge
network for communications with a legacy edge network. Since
these components are independent, it is possible to replace either
of them with a corresponding component from tunnel-based
approaches: Bilateral address rewriting can be combined with
proxies for backwards compatibility. Tunneling can be combined
with unilateral address rewriting for backwards compatibility.
I will follow up with a more careful response to Robin's previous
questions about Six/One Router.
- Christian
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