Re: [RRG] A Late Response to Questions on Six/One Router

[Lixia Zhang <lixia@CS.UCLA.EDU>]

sorry folks, this msg went out by mistake, as I had not finished  
typing yet :(
let me add at least one missing part to my 2nd question:


> > 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...)

I replace the original question 2/ with the following:

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 paper touched this issue in a super brief way:) in the following 2  
paragraphs:

for bilateral case (= both ends are upgraded):

   Active
   packet exchanges can therefore be redirected between the
   providers of a multi-homed edge network only if the resulting
   change in transit addresses is handled in a manner transparent
   to hosts. Bilateral mode achieves this because the rewriting of a
   host’s multiple transit addresses into a single edge address
   hides transit address changes from the host.

I agree that rewriting is easily done (this is equivalent to say that,  
with an encap solution, one can easily encap packets with a different  
ETR address)
the hard question is how a six/one router at the source site border  
learn about the failure promptly to do the redirection.

For unilateral case (i.e. one end upgraded, one end not)

   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?



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