What is backoff for? It's not to prevent overloading nodes; nodes reject
requests pre-emptively. What it is for is this:
If we don't backoff slow nodes, then these slow nodes will have to
reject (or occasionally timeout) the requests that we do forward to
them. Those failures will be propagated back to the request senders, who
will have to reduce their send rate. In other words, if we don't avoid
sending requests to the slowest nodes, the requests we do send to them
will be rejected, and those rejections will cause a universal slowdown
in request rates.
Thus, I don't think that the current mechanism is sensible. Nodes can be
expected to take care of themselves, now that we have pre-emptive
rejection. I don't think the ethernet analogy works 100%; we don't
really have a shared medium.
What we are looking at here is simply a way to ensure that requests are
not forwarded to nodes who will inevitably reject them and thus cause an
overall slowdown at the requester end. This is completely different to
the case in 0.5, where requesters never slowed down.
So what am I proposing?
For each node, we track the proportion of requests which are accepted.
We calculate a median. For any node above the median, all requests are
forwarded to it: if most of our nodes are slow, that's not our problem,
it's our peers who want to forward requests through us who have to deal
with it (by not sending requests to us). For nodes below the median, we
artificially narrow their specializations:
While routing:
public double distance(PeerNode node, double target) {
double distance;
if(node.acceptedFrac >= medianAcceptedFrac) {
distance = rawDistance(node.location, target);
} else {
distance = rawDistance(node.location, target) *
medianAcceptedFrac / node.acceptedFrac;
}
return distance;
}
Now, to some degree, this is alchemy, in the sense that it hasn't been
simulated. However it will be some time before the new rate limiting
mechanism is available, and it appears a solid argument to me: If all
our peers are slow, that DOES NOT mean we shouldn't send requests to
them. Because we have load propagation, it is entirely legitimate to
send requests ANYWAY, and let upstream nodes deal with the slowness -
either the entire network is slow, in which case we reduce all the
send rates, or this particular node is slow, in which case we reduce
the number of requests sent to it! Note also that slavishly following
backoffs reduces our anonymity quite noticeably.
Comments?
--
Matthew J Toseland - toad at amphibian.dyndns.org
Freenet Project Official Codemonkey - http://freenetproject.org/
ICTHUS - Nothing is impossible. Our Boss says so.
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