I believe that determining whether two independent and internally
consistent sets of changes can, in combination, produce a wedgie or
other failure is a research problem.
As far as I know, the I2RS working group concluded that it was a
sufficiently hard problem that we did not expect the I2RS agent to get
involved in trying to detect, much less remediate, such cross-object
inconsistencies.
Yours,
Joel
On 11/24/15 10:32 AM, PEDRO ANDRES ARANDA GUTIERREZ wrote:
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 th
r
ee 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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