no cache from varnish if max-age=0 + meaning of insert_pass

[jean-marc pouchoulon]

Helo all,

I set max-age=0 and s-max-age=600 in the zope caching policy manager and 
varnish don't cache any content ( except if I force
obj.ttl = 10m in vcl_fetch.)

if max-age=1 varnish is caching the content.

is it the considering behaviour for varnish ? when varnish is supposed 
to cache a content ?

can someone explain the meaning of insert_pass , the difference between 
pass and pipe ?

thanks

jean-marc