Official mirrors are mirrors that
a) have a FreeBSD.org DNS entry (usually a CNAME).
b) are listed as an official mirror in the FreeBSD documentation (like handbook).
So far to distinguish official mirrors. Official mirrors are not necessarily Tier-1-mirrors. However you probably will not find a Tier-1-mirror, that is not also official.
It is not so easy to state requirements for all official mirrors, since the project is sort of tolerant here. It is more easy to say, what official tier-1 mirrors are required to. All other official mirrors can consider this a big should.
Note: The following applies mainly to the FTP fileset, since a CVS repository should always be mirrored completely, and the web pages are a case of its own.
Tier-1 mirrors are required to:
carry the complete fileset
allow access to other mirror sites
provide FTP and rsync access
Furthermore, admins should be subscribed to the FreeBSD mirror sites mailing lists. See this link for details, how to subscribe.
Important: It is very important for a hub administrator, especially Tier-1 hub admins, to check the release schedule for the next FreeBSD release. This is important because it will tell you when the next release is scheduled to come out, and thus giving you time to prepare for the big spike of traffic which follows it.
It is also important that hub administrators try to keep their mirrors as up-to-date as possible (again, even more crucial for Tier-1 mirrors). If Mirror1 does not update for a while, lower tier mirrors will begin to mirror old data from Mirror1 and thus begins a downward spiral... Keep your mirrors up to date!
We are not accepting any new mirrors at this time.