Hi Russ,
I’m trying to identify the differences between interactions with routing
protocols in I2RS and what is purely conflicts between clients. Currently I see
too many issues overlapping and I fear that the trees are not letting us see
the forest.
So my take on routing protocols and wedgies might have been too compact :-) Let
me give it a second try: Stepping outside the I2RS problem space, there is a
lot of work that shows that the origin for BGP-4 instability is that our
beloved route-maps create metrics that are not monotonically increasing or
decreasing and that makes the routing protocols meta-stable. (BTW, I’m the
first culprit when it comes to the use of them, I have created more than one
wedgie :-P ) Acknowledging that this is a significant (and quite complex)
problem for the Internet in general, I feel that it should be treated somewhere
else (GROW?).
The perspective I would like to take here is:
- assume that we have two or more clients that produce perfectly sound routing
information (no wedgies there)
- assume they talk to the same agent
- now my questions
1.- is there any way to detect whether the clients produce
contradicting/conflicting information?
2.- is there any way to resolve these contradictions/conflicts?
BR, /PA
---
Dr. Pedro A. Aranda Gutiérrez
Technology Exploration -
Network Innovation & Virtualisation
email: pedroa d0t aranda At telefonica d0t com
Telefónica, Investigación y Desarrollo
C/ Zurbarán,12
28010 Madrid, Spain
Fragen sind nicht da, um beantwortet zu werden.
Fragen sind da, um gestellt zu werden.
Georg Kreisler
-----Mensaje original-----
De: Russ White <[email protected]>
Fecha: lunes, 23 de noviembre de 2015, 14:06
Para: paag <[email protected]>, 'Jeffrey Haas' <[email protected]>
CC: "[email protected]" <[email protected]>, "'Joel M. Halpern'"
<[email protected]>, 'Andy Bierman' <[email protected]>, Sue Hares
<[email protected]>
Asunto: RE: [i2rs] Conversation on Priority and Panes
>
>> Re the metric 'problem', just to be more precise in what would be needed,
>> we are looking a metric that grows or decreases monotonically across the
>> whole network.
>
>I assume you mean in the routing space, and not in the controller/client
>space, correct? In terms of a distributed protocol? So, you're saying the
>delay could use "metrics" between 11 and 20, while the bandwidth could use
>"metrics" between 21 and 30, etc? And then you just add them all together?
>That's what IS-IS has done for years... Even BGP really only has one "metric,"
>with following "tie breakers..." So if you have something like weight/local
>pref/etc, such that they occupy different "spaces" in a bit pattern (something
>suggested, btw, in the original wide community work, and in other places, as
>well), you're actually just building a single metric.
>
>You've not "solved" the multiple metric problem -- just done something a
>little different than EIGRP's K values to combine them into a single metric,
>which is where you need to be to have the ability to build a single stable
>SPT/DAG.
>
>> The theoretical grounds for this are in T. Griffin’s and
>> Sobrinho’s work on BGP-4 and that lies already a couple of years ago and that
>> makes the analysis much ‘easier’ (no worries about np(complete) problems,
>> etc.). This could be applied in I2RS at the routing protocol level. So, we
>> could
>> discuss where that sits (should be the clients, right?) and I don’t know if
>> that’s been already done, since I’m quite new to this list.
>
>This doesn’t apply to the problem at hand, though... You seem to be saying
>(clarify if wrong) --
>
>1. Give each client some set of bits out of a 128 bit number space (or larger,
>etc.)
>2. Each client can have different "metrics" within their own number space
>3. When a client attempts to add some ephemeral state, they use a metric
>within their "space"
>4. The agent can just use the straight number as a relative priority for each
>potential piece of state
>
>This doesn't do anything different than the current concept of priority
>between clients, only in the current plan each client only has one priority,
>rather than multiples. I don't see where this relates to the problem at hand.
>
>> Now, having “solved” that part of the equation :-) , the part that interests
>> me
>> more is the conflicting clients problem, because this could be generalised to
>> other problem spaces in the SDN area. I do agree that agents should be able
>> to catch offending state before installing it and I’m looking for ways to
>> specify
>> a minimal set of features that need to be supported at protocol level.
>>
>> Anyone else interested?
>
>This is precisely where the problem lies. And this is where you're going to
>hit the CAP theorem in full force. There are only a few choices --
>
>1. Make the database eventually consistent
>2. Shut down accessibility during changes
>3. ??
>
>(1) is the idea of either having the agent call back to all the clients when
>state they installed is overwritten or allowing the agent to locally store
>some state where it already has the information in hand and install it locally
>-- the only real difference between these two solutions is the "balance of
>complexity versus speed..." The entire discussion here is how much additional
>complexity are we actually adding by doing "panes of glass," which are just
>locally stored state which isn't currently installed. I'm arguing that there's
>minimal complexity added that you're not already going to have in the system
>to allow the agent to store information locally _if_ it chooses to. Jeff is
>arguing (I think) that the "panes of glass" concept is a coherent way of
>looking at this problem that will help us understand and manage the complexity
>in a way that makes sense. Joel is arguing (I think) that this sort of
>solution is out of the WG charter, and it's too complex. I _think_ I have the
>three general perspectives right, but I don't know if I really have so... :-)
>
>(2) is the idea of locking the database while you're changing it. This is
>explicitly outside the scope of I2RS entirely, given we're trying to design
>something that's atomic/restful. There are a lot of techniques you can use to
>speed up locking -- row locking, sharding, etc. -- but none of these are
>interesting from the I2RS perspective.
>
>:-)
>
>Russ
>
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