Re: [shim6] Exit selection [New Version Notification - draft-mrw-nat66-08.txt]

Le 13 mars 2011 à 23:32, Brian E Carpenter a écrit :

> On 2011-03-14 10:14, Geoff Huston wrote:
>> On 13/03/2011, at 3:32 AM, Rémi Després wrote:
>> 
>>> Le 11 mars 2011 à 20:13, Scott W Brim a écrit :
>>> 
>>>> Shim6 itself cannot select an exit.  In all but trivial
>>>> cases the network has the intelligence and must be
>>>> involved (e.g. the exit points).  Given that one has to
>>>> involve the network, then an exit selection mechanism is
>>>> not just for Shim6 but for anything that can take
>>>> advantage of multiple src+dst pairs, and it should not be
>>>> Shim6-specific. Therefore whatever needs doing, I
>>>> recommend that it not be done in Shim6 but somewhere more
>>>> general -- IntArea WG looks like a good place.
>>> +1
>>> 
>> 
>> wg co-chair hat OFF
>> 
>> I appreciate that in the context of the IETF saying "we
>> should not do this" is invariably like standing in front of a
>> bulldozer, and like the bulldozer, it rarely has any effect -
>> but nevertheless I want to share with you a personal opinion
>> that we should not do this! :-)
> 
> If by "we" you mean the shim6 WG, I'm inclined to agree,
> beyond documenting that this problem needs to be solved
> generically for IPv6. I don't believe shim6 is really a
> special case: once you have prefixes from and exits to several
> different ISPs, you have a potential problem.
> 
>> 
>> I have two problems with this issue of "exit selection".
>> 
>> Firstly, I recall the demise of the site-local scoped address
>> in IPv6. The compelling argument for me was one of inability
>> of self-awareness of context. i.e. it was never clear where
>> the edge of a site actually lay. So let me replay the same
>> problem statement here - what is an "exit" and how does a
>> packet know when it has gone through it? And how does a
>> router "know" its the exit router and not an interior router?
>> In the context of nested sites (e.g. workgroup, department,
>> faculty) whats _the_ exit?
> 
> The host and the departmental router cannot know the answer to
> that question, I fear, without erecting some very complicated
> machinery.

Complicated, not that I know if its involving hosts is accepted (see below)

>> Secondly, its not just the problem of scope of a site here.
>> Its also trying to clearly understand the nature of the
>> problem and place that into juxtaposition with the various
>> proposed "solutions." From memory the various exit selection
>> solutions proposed in the context of shim6 were all various
>> forms of source-address based forwarding, and for me its
>> getting the cart well and truly before the horse. Hop-by-hop
>> stateless destination-based forwarding is an incredibly
>> powerful architectural concept in packet networks, and any
>> other forwarding mechanism tends to invoke copious quantities
>> of complexity and state, and should only be contemplated as a
>> last resort of the desperate rather than the first line
>> solution. 
> 
> I fully agree. That would be baroque, and despite having started
> this conversation, I am not in favour of a host-based or even
> departmental-router-based solution.

Well, if host based is the only complete solution, it shouldn't IMHO be taboo. 

> 
>> So why was exit selection an issue in shim6? Surely
>> if routing is working, and shim6 does not assume broken
>> routing, if a route to a destination is offered, then its a
>> valid route. So why select exits that are not being offered
>> by routing as being on the best path? From my memory I
>> believe it was because some folk  thought that a) unicast
>> reverse route filtering would be prevalent in IPv6 and that
>> b) clients could not negotiate filters with their provider.
>> But is a) true? and is b) really true? 
> 
> Well, it seems from other responses that both are real problems,
> but not specific to shim6. draft-v6ops-multihoming-without-nat66
> talks about this, and at first sight, a valid approach for
> the problems described in that draft would work just fine for
> shim6 too. Maybe shim6 people should read that draft carefully.

I agree that it is a valid approach.
However, AFAIK, it doesn't deal with ULA's:
- it assumes that hosts have their global IPv6 addresses
- it doesn't say how global addresses would traverse domains in which ULA's are used.
In this respect, the SAM approach is complementary.

RD

> 
>    Brian
> 
>> Frankly I'm pretty
>> sceptical that this is the case and it concerns me that this
>> is a case of over solving.
>> 
>> my 2c anyway
>> 
>> Geoff
>> 
>> 
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