Re: [shim6] Exit selection [New Version Notification - draft-mrw-nat66-08.txt]
Brian E Carpenter <brian.e.carpenter@gmail.com> Sun, 13 March 2011 22:31 UTC
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Date: Mon, 14 Mar 2011 11:32:25 +1300
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
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To: Geoff Huston <gih@apnic.net>
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Cc: "S.P.Zeidler" <spz@serpens.de>, shim6-wg <shim6@ietf.org>, Scott Brim <scott.brim@gmail.com>, Ralph Droms <rdroms.ietf@gmail.com>
Subject: Re: [shim6] Exit selection [New Version Notification - draft-mrw-nat66-08.txt]
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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. > > 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. > 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. 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 > > > _______________________________________________ shim6 mailing > list shim6@ietf.org > https://www.ietf.org/mailman/listinfo/shim6 >
- [shim6] Exit selection [New Version Notification … Brian E Carpenter
- Re: [shim6] Exit selection [New Version Notificat… marcelo bagnulo braun
- Re: [shim6] Exit selection [New Version Notificat… Sébastien Barré
- Re: [shim6] Exit selection [New Version Notificat… Brian E Carpenter
- Re: [shim6] Exit selection [New Version Notificat… Rémi Després
- Re: [shim6] Exit selection [New Version Notificat… Scott W Brim
- Re: [shim6] Exit selection [New Version Notificat… Brian E Carpenter
- Re: [shim6] Exit selection [New Version Notificat… Scott W Brim
- Re: [shim6] Exit selection [New Version Notificat… Rémi Després
- Re: [shim6] Exit selection [New Version Notificat… Geoff Huston
- Re: [shim6] Exit selection [New Version Notificat… Sander Steffann
- Re: [shim6] Exit selection [New Version Notificat… Brian E Carpenter
- Re: [shim6] Exit selection [New Version Notificat… Geoff Huston
- Re: [shim6] Exit selection [New Version Notificat… Brian E Carpenter
- Re: [shim6] Exit selection [New Version Notificat… Geoff Huston
- Re: [shim6] Exit selection [New Version Notificat… Fred Baker
- Re: [shim6] Exit selection [New Version Notificat… Brian E Carpenter
- Re: [shim6] Exit selection Rémi Després
- Re: [shim6] Exit selection [New Version Notificat… Rémi Després