[Olsr-dev] ETX mambo jambo
Mon May 10 21:24:55 CEST 2010
Probably a lot on this topic has already been written but ETX is
really badly conceptualized and it really does not properly influence
For example, we should have first two kind of links: full-duplex (or
all in the neighborhood can talk with a very low cost at the same
time) or half-duplex (or all in the neighborhood have to be quiet when
one speaks). And then packet loss for those links. And then costs for
routing decisions should take into account this. And if there is a
full-duplex link without packet loss (like ethernet cable) there
should be no cost associated with it). And this would be the reason
why ethernet links would be less costly than one hop over half-duplex
link. And not that we have a "special" flag for ethernet links.
And then if OLSR would support asymmetric links it could take this
half-duplex information and even better do routing.
Also hops does not really add much on full duplex links (except for
latency which is not the main performance factor in comparison to
others in wireless links) in comparison with half duplex links.
So a crude way would be to multiply ETX costs on full duplex links and
add on half duplex (in comparison with just adding).
I really do not understand why ETX costs are summed in the first
place. If we measure packet loss this is a probability of packet
getting through over that particular hop and probability of packet
getting through over the whole path is a product of all this
probabilities for one hop.
The only reason why summing is reasonable is because there is a hidden
cost of transferring packets over half-duplex (wireless) links to
other links in vicinity which have to be quiet at that time. But all
this is a bad probability model for what is happening and this is why
discrepancies between the model and real world occur.
Because now, as we use a lot of ethernet links and VPN links, routing
decisions are very bad sometimes.
(And all this is without taking into the account bandwidth limits.)
More that I think now more I see it is a wrong approach. What routing
protocols are? They are prediction tools which try to evaluate past
behavior to predict behavior and performance of one route over
another. As such first there should be a good model which would the
networks be are talking about and what we would like to predict -
performance (bandwidth, latency...) and what influences performance
(packet loss, selected wifi link speed, limits on VPN tunnel) and how
to get measurements which would be used for filling this model with
data for prediction (with of course side-effects of active
measurements on the performance).
And what are current routing protocols differ is how they measure data
and how they compute shortest paths. I do not understand why exactly
do we need new routing protocol for every this combination? Why we
cannot make a modular routing protocol where you would be able to
define how to measure links (which messages you send and such) and
then some prediction tool which you can vary to evaluate all this data
(of which a good approach to its computation is just one part) and
then what you do once you evaluate/predict paths performance - do you
use it for L2 or L3 links, routes? If all this would be modular we
would be able to interchange things. Like if somebody makes a good L3
module for adding/removing IPv4 and IPv6 routes once then everybody
would be able to use it. Then we could play with different data
collections and feed data into different predictive models. And if we
collect data with OLSR or B.A.T.M.A.N. messages - it is not really
Currently everybody is trying to make everything from the scratch.
Their way of measuring data, evaluating it and then pushing changes to
network stack. This is crazy. Probably because most work originates
from academia where author is concerned with testing/evaluating
his/her own combination of things mentioned above. But if we would
have such general "routing stack" it would be useful for everybody.
For networks because we would be able to all work on same codebase
(and have interchangeable modules) and for academia because they would
only need to write a new, for example, prediction model and try it
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