Re: [Isis-wg] Requesting WG adoption of draft-baker-ipv6-isis-dst-src-routing-06
David Lamparter <equinox@diac24.net> Fri, 09 December 2016 11:53 UTC
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Date: Fri, 09 Dec 2016 12:44:41 +0100
From: David Lamparter <equinox@diac24.net>
To: Tony Przygienda <tonysietf@gmail.com>
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Subject: Re: [Isis-wg] Requesting WG adoption of draft-baker-ipv6-isis-dst-src-routing-06
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Hi Tony, sorry - this got slightly lost in the IETF Seoul haze at my end :( - more inline. On Thu, Oct 27, 2016 at 07:02:43PM -0700, Tony Przygienda wrote: > My first comment a good while ago on the mike when you were showing > your first proposals was that you'll on a slippery slope to redo MT in > topology #0 ;-) Stupid is too strong a word, it's simply one of those > problems that looks like a "small addition" needed initially until it > sucks you treacherously underwater. Yep, let's call that a fun excursion, but now we're on the serious track ;) > I favor adoption as well as an elegant application of MT with the > following necessary fixes ;-) > a) you prescribe 2 different things in the draft ;-) > > a.1 you want to forklift all the routers in section 2.3. In fact, > MT-ISIS has been proposed to prevent such fork-lifting need. > Forklifting is sufficient but not necessary as you observe > later in the "correctness considerations". That's not my intention to express, as you say MT's purpose is to avoid a need to forklift. I'm not sure where I worded this as forklifting, do you mean "Therefore, all routers participating in D/S routing MUST implement M-ISIS and participate in the appropriate D/S topology [...]"? If so, I need to reword that to make more clear that you *can* mix D/S and non-D/S routers, but all D/S routers need to consistently implement MT. (That's what I tried to say with "participating", I guess that's a poor choice of words...) > a.2 in "correctness considerations" you allow to mix the routers which > is correct but the logical chain doesn't hold completely. > First "axiom" is clear (i.e. forwarding lookup). The second > "pilar" is confused by Oh - I'm missing a step there. So, the unwritten assumption that I need to write out there is: there are continuous "subdomains" of D/S routing. Normally, I would hope there is only a single such subdomain of D/S routers, and ideally this is your entire IS-IS domain if every router supports dst-src. If you lose connectivity and this subdomain splits into two, the case described there can occur - D/S reachabilities for which no path inside the MT topology can be found, even though there is non-D/S connectivity. *However* this is a transitive property. If a router A can't find a path for a D/S reachability, neither can its direct neighbors. To be completely correct and account for max-metric too, "neither can its direct neighbors who are farther away". What the proof really relies on is: "if your SPF calculation found a path for D/S reachability (D,S) via IS-IS adjacency N, then N will also find a path for (D,S) [with a lower path metric]". > *"If, for case (b1), the system knows of the existence of a more* > * specific D/S route, but cannot calculate a valid path, it may* > * either apply non-D/S routing (i.e. not install any route) or* > * discard the packet (i.e. install a discard route). The next hop* > * will either be a non-D/S system, or a D/S system with the same* > * link-state information (and thus again unable to calculate a valid* > * path -- or, more specifically, won't calculate a path that* > * includes the previous router)."* > > That seems to contradict *"**Packets will transit into D/S > routing but not out of it"* It shouldn't; see below. If the packet is forwarded on D-only, it hasn't entered D/S routing yet, even if a D/S router forwarded it. Once a D/S router forwards it using D/S information, it's safe to assume (and neccessary for correctness) that the next router will be a D/S router that also successfully applies D/S forwarding. > In short, with your axiom (prefer routes with source match over > 0/0 source entry) you have two choices for the 2nd axiom (both > lead to correct routing but exclude each other). The reason is > that you either "enter semantically richer topology and never > leave" _OR_ "exit semantically richer semantics and never enter > it again". Less formally that translates into > > Either > > a) a packet may enter from non D/S into D/S and any D/S MUST NOT > forward it to a non D/S next-hop, i.e. you can't install > default topology entries into FIB as "union". This holds EVEN > when you received the packet from a D/S neighbor since the > packet MUST NOT jump out into the non-D/S topo again (it may > have came from here upstream). Otherwise you'll find looping > scenarios (Let me add: "If you are able to calculate a D/S path, you MUST install and use it.") > or > > b) a packet may exit from D/S into non-D/S, i.e. no source-specific > FIB match is found and you match on 0/0 source (topology #0). > Once it's out in the non-DS world, it MUST stay there, i.e. > you must not as D/S router advertise topology #0 information > since a non-DS router may use that to jump into the D/S again > not understanding it's doing it. NB: the edits I have pending for Chris' comments also clarify that the D/S topology is NOT intended to contain duplicates of topology #0 information; in fact it should not contain any reachability with S=0/0 (meaning both implicit [no Sub-TLV] or explicit [Sub-TLV with S=0/0]). > Again, in short you either enter D/S once and only once and > never leave D/S or you exit D/S once and only once and never > enter D/S again. This still holds true: once a system is forwarding a packet based on a successful SPF calculation in the D/S topology, it can be assumed that the next router can also make that successful SPF calculation in the D/S topology. In reverse, if a D/S system is unable to calculate a path in the D/S topology, it can assume that the system it received the packet from wasn't able to do so either. It may have been a non-D/S router or a D/S router with no SPF result, but either way the packet has /not/ entered a D/S forwarding path yet. (The exception on this, of course, are microloops while information is being flooded, but that's a generic problem, not dst-src...) > b) section 3.1: I would explain what is the multiplicity of this TLV and I > expect it to be 1 and only 1. Omitting it will collide with MTID#0 > and having two has no semantics (unless specified). Right -- this is an open "existence" question where my assumption was that it'd always be "exactly 1" but I'd left it to wait and see if someone sees a use for it that I don't :) (It could be "more than 1" which could signal more than one D/S route with same D but different S in a single TLV, but I think that's a counterproductive micro-optimisation that has significant negative complexity impact.) I've received no other input and will change it to "MUST ==1". --Thanks for your extensive input! -David P.S.: meta-question - this is still running in WG adoption call; should I push a new draft anyway or wait? I also have some edits queued to adress Chris' comments... though I wouldn't say any of these edits are "substantial" changes to the idea.
- [Isis-wg] Requesting WG adoption of draft-baker-i… David Lamparter
- Re: [Isis-wg] Requesting WG adoption of draft-bak… Fred Baker
- Re: [Isis-wg] Requesting WG adoption of draft-bak… Chris Bowers
- Re: [Isis-wg] Requesting WG adoption of draft-bak… David Lamparter
- Re: [Isis-wg] Requesting WG adoption of draft-bak… Les Ginsberg (ginsberg)
- Re: [Isis-wg] Requesting WG adoption of draft-bak… Tony Przygienda
- Re: [Isis-wg] Requesting WG adoption of draft-bak… David Lamparter
- Re: [Isis-wg] Requesting WG adoption of draft-bak… Tony Przygienda