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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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