[Babel-users] Rate limiting [was: New Version Notification...]

[Juliusz Chroboczek]

> while this has been discussed elsewhere, perhaps the nature of the
> rate limiter for a full (source) routing request dump should wind up
> in the rfc?

It doesn't belong here, it should be discussed in the main spec.  But
really, it's an implementation detail.

Babeld (the implementation) has two rate limiters:

  (1) it has a token bucket for all packets, full at 200 packets,
      replenished at 40 packets/s; and

  (2) it has a limit for full updates, at most once per hello interval.

(1) never triggers in practice, since (2) triggers before that (it can
trigger if a whole network is rebooting at the same time).

> what happens [...] with loss

As usual -- Babel doesn't care.

> What if you had, say, a small city's worth of source specific routes?
> (say 10,000, which I think equates to about 125 full size packets?).

125 packets is fine, it fits in the bucket.  Double that, and you'll want
to increase the constants at the beginning of message.h.  I'll make them
into config variables as soon as you build a network with more than 10 000
nodes.

(At that scale, a full update is around 1MB of data.  I'd probably split
the network into a dozen subnets, assign a separate prefix to each subnet,
and perform aggregation at subnet boundaries.)

> Is the entire update processed at once?

No, each update is processed separately (except for prefix compression).
So if a packet contains 40 updates, each update results in a separate call
to update_route, with no interaction between the updates.

To make things clearer, there's nothing preventing you from sending
a packet that says

  update(p)
  retract(p)
  update(p)
  retract(p)

and update_route will be called four times, leading to p being updated,
retracted, updated, retracted.  Only the final retraction will cause an
update to be sent out, however.

> If a packet is dropped is the whole thing invalid?

If a packet is dropped, all the other packets are still processed
normally.  There's no interaction between packets whatsoever.

-- Juliusz