NSD 3: slave too agressive

[Stephane Bortzmeyer]

I test NSD on a machine which is secondary for ~ 500 zones, many of
them lame (for instance, the master no longer replies). 

When it occurs, nsd keeps asking and asking again and logs every
attempt:

[1166301627] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301627] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301628] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301638] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301638] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301639] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301649] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301649] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301650] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301662] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301662] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3
[1166301663] nsd[21603]: error: xfrd: zone 202.16.192.in-addr.arpa received error code REFUSED from 137.39.1.3

Three requests, every ten seconds, and it does not stop even after
several hours! Is it normal?