Re: [trill] RtgDir review of draft-ietf-trill-directory-assist-mechanisms-07.txt
Donald Eastlake <d3e3e3@gmail.com> Sun, 17 April 2016 02:03 UTC
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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Sat, 16 Apr 2016 22:03:22 -0400
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To: "Joel M. Halpern" <jmh@joelhalpern.com>
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Subject: Re: [trill] RtgDir review of draft-ietf-trill-directory-assist-mechanisms-07.txt
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Hi Joel, On Fri, Apr 15, 2016 at 11:46 PM, Joel M. Halpern <jmh@joelhalpern.com> wrote: > If by the connectivity check to the directory server, you mean the > underlying IS-IS routing reporting connectivity, then say that. OK. > While that > is not actually interchangeable with real connectivity, it is perfectly > reasoanble for the WG to deem it sufficient. I think it would only take a > sentence or two to clarify for the reader that what is meant is apparent > topological connectivity, as distinct from verified communication. The phrase usually used in TRILL (See RFC 7780) is "data reachable". Thanks, Donald ============================= Donald E. Eastlake 3rd +1-508-333-2270 (cell) 155 Beaver Street, Milford, MA 01757 USA d3e3e3@gmail.com > Yours, > Joel > > > On 4/15/16 11:12 PM, Donald Eastlake wrote: >> >> Hi Joel, >> >> On Fri, Apr 15, 2016 at 11:51 AM, Joel M. Halpern <jmh@joelhalpern.com> >> wrote: >>> >>> Thank you Donald. Points of agreement elided, some responses to try to >>> clarify my observations. I will note that from your comments about 3.1, >>> I >>> believe my concerns, now moved to 3.7, are larger, as I had assumed that >>> the >>> magic was in some other protocol, and you now say it is not defined >>> there. >>> >>> Yours, >>> Joel >>> >>> On 4/15/16 11:23 AM, Donald Eastlake wrote: >>>> >>>> >>>> Hi Joel >>>> >>>> Thanks for your thorough review and comments. See below >>>> >>>> On Wed, Apr 13, 2016 at 4:47 PM, Joel M. Halpern <jmh@joelhalpern.com >>>> <mailto:jmh@joelhalpern.com> wrote: >>>> >>> ... >>> >>>>> Major Issues: >>>>> In the state machine transitions in section 2.3.3 >>>>> for push servers, it appears that if the event indicating that the >>>>> server is being shut down occurs while the server is already Going >>>>> Stand-By or Uncompleting, the transitions indicate that this >>>>> "going >>>>> down" event will be lost. A strict reading of this would seem to >>>>> mean that the "go Down" event would need to recur after the >>>>> timeout >>>>> condition. This would seem to be best addressed by a new state >>>>> "Going-Down" whose timeout behavior is to move to down state. >>>> >>>> >>>> I understand your point but "going down" and the like are called >>>> "events or conditions" in this draft, not just events. >>>> The problem with adding a single "Going-Down" state is that >>>> transition >>>> to that state would lose the information as to whether or not the >>>> Push >>>> Directory had been advertising that it was pushing complete >>>> information or not. The reason to remember this is that you would >>>> want >>>> to behave a differently if the "going down" condition was revoked >>>> before it completed. This information could be preserved in a >>>> Boolean >>>> pseudo variable but the current style of state machine in this draft >>>> avoids such pseudo variables and encodes all of the relevant push >>>> directory's state into the state machine state. Thus, I can see >>>> three >>>> possible responses to your comment: >>>> >>>> 1) Change wording to emphasize that these "events or conditions" can >>>> be conditions that cause a state transition some substantial time >>>> after they become true. >>>> >>>> 2) Add two new states: (1) going down - was complete; (2) going down >>>> - >>>> was incomplete. >>>> >>>> 3) Change the style of state machine to admit pseudo variables which >>>> can be set and testing as part of the state machinery. >>>> >>>> Option 1 is just some minor wording changes but adopting either >>>> options 2 or 3 involves more extensive changes so I would prefer to >>>> avoid them. >>> >>> >>> From what I have seen, trying to build a state machine with conditions >>> rather than events is fraught with problems and tends to lead to errors >>> in >>> implementation. It amounts to hiding pseudo-variables inside the states, >>> but not describing them. >>> Thus, I would much prefer solution 2, but it is of course up to the WG. >> >> >> Well, option 2 wouldn't be too hard. Option 3 would probably involve the >> most >> change. >> >>> ... >>> >>>>> Minor Issues: >>>>> In section 2.3.3 describing the state transitions for push >>>>> servers, there is an event (event 1) described as "the server was >>>>> Down but is now Up." The state transition diagram describes this >>>>> as >>>>> being a valid event that does not change the servers state if the >>>>> server is in any state other than "Down." In one sense, this is >>>>> reasonable, saying that such an event is harmless. I would >>>>> however >>>>> expect some sort of logging or administrative notification, as >>>>> something in the system is quite confused. >>>> >>>> >>>> Again, I see your point but it seems to me to be a matter of state >>>> machine style. Note that the "event" is described as a condition, so >>>> from that point of view, it is true anytime the state is other than >>>> Down. On the other hand, if you view it as strictly an event, you >>>> are >>>> left with the question of what to put at the intersection of a state >>>> and event in the table when it is impossible for that event to occur >>>> in that state. Some people note this with an "N/A" (not applicable) >>>> entry. In fact, previous TRILL state diagrams such as in RFC 7177 >>>> use >>>> "N/A" so it would probably be simplest to change to that for >>>> consistency. >>> >>> >>> I think N/A would be good. >> >> >> OK. >> >>> ... >>> >>>>> Text in section 3.2.2.1 on lifetimes and the information >>>>> maintenance in section 3.3 imply that the clients and servers must >>>>> maintain a connection. Presumably, this is required already by the >>>>> RBridge Channel protocol, and I understand that we should not >>>>> repeat >>>>> the entire protocol here. It would seem to make readers life MUCH >>>>> simpler if the text noted that the RBridge Channel protocol >>>>> requires >>>>> that there be a maintained connection between the client and the >>>>> server, and that these mechanisms leverage the presence of that >>>>> connection. >>>> >>>> >>>> The basic RBridge Channel protocol [RFC7178] is a datagram protocol >>>> rather than a connection protocol. So there is no guaranteed >>>> continuity of connection between RBridges that have previously >>>> exchanged RBridge Channel messages. But connection would only be >>>> lost >>>> if the network partitions since RBridge Channel messages look like >>>> data packets to any transit RBridges and will get forwarded as long >>>> as >>>> there is any route. Network partition is immediately visible in the >>>> link state database to the RBridges at both ends of an RBridge >>>> Channel >>>> exchange. Section 3.7 provides that if a Pull Directory is no >>>> longer >>>> reachable (i.e., RBridge Channel protocol packets would no longer >>>> get >>>> through), then all pull responses from that Pull Directory MUST be >>>> discarded since cache consistency update messages can't get through. >>>> Perhaps a reference to Section 3.7 should be added to Section 3.3. >>> >>> >>> I don't think a reference to 3.7 is sufficient, although it is helpful. >>> If the protocol is a datagram protocol, and if it is important to discard >>> data from unreachable pull servers, then I think 3.7 NEEDS to say more >>> than >>> just ~if you happen to magically figure out you can't reach the server, >>> discard data it has given you.~ From the rest of the text, this is an >>> important and unspecified protocol mechanism. >> >> >> Figuring out whether/how you can reach other RBridges is a basic >> function of TRILL IS-IS based routing, not something "magical". >> Whenever their is a topology change, an RBridge MUST determine routes >> to all data reachable RBridges in the new topology. If there was an >> RBridge previously reachable but no longer reachable, as would be the >> case for all RBridges on the other side of a network partition, this >> MUST be noticed so that, for example, all MAC reachability information >> associated with each of the no longer reachable RBridges can be discarded. >> It does not seem like much of a stretch to believe that an RBridge would >> keep track of the Pull Directory or Directories it was using, each of >> which will be some other RBridge, and notice when a topology change >> makes any of them inaccessible. But I have no problem adding some >> wording to make this clearer. >> >>> ... >>> In the flooding flag and behavior, (long text elided) I don't think there >>> is >>> anything wrong with the intended behavior. It is just that the very >>> brief >>> description of the FL flag leads the reader to an incorrect expectation. >>> Yes, it gets sorted out, but that is not good. What I would suggest is >>> when >>> the flag is defined (with whatever name you choose) note that "for the >>> qtypes 2,3,and 4, the flag indicates that the server should flood its >>> response." >> >> >> We can work on clarifying the wording. >> >> Thanks, >> Donald >> ============================= >> Donald E. Eastlake 3rd +1-508-333-2270 (cell) >> 155 Beaver Street, Milford, MA 01757 USA >> d3e3e3@gmail.com >> >
- [trill] RtgDir review of draft-ietf-trill-directo… Joel M. Halpern
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Susan Hares
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Alia Atlas
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Donald Eastlake
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Joel M. Halpern
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Donald Eastlake
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Joel M. Halpern
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Donald Eastlake
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Donald Eastlake
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Joel M. Halpern
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Joel M. Halpern
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Donald Eastlake
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Joel M. Halpern
- Re: [trill] RtgDir review of draft-ietf-trill-dir… Donald Eastlake
- Re: [trill] [RTG-DIR] RtgDir review of draft-ietf… Joel M. Halpern