There are no strict rules of how and what URLs you should specify for
your tarballs. But it is highly advisable to get rid of single point of
failures. Hosts may be down. They may have connectivity problems.
Network splits may occur.

Here we use the following precedence preference:

* balancers > mirrors > CDNs > homepage > GitHub
* all links should use HTTP as higher priority than HTTPS
* distcache.FreeBSD.org and cdn.NetBSD.org/pub/pkgsrc/distfiles
  are used as least priority fallback
* be aware that codeload.github.com downloads have no guarantees
  of format/compression stability (their hashes may change)

There are special country codes for identifying and specifying CDNs:
    xa -> Akamai
    xc -> Cloudflare
    xf -> Fastly
    xg -> GitHub
    xm -> Amazon

Example list of URLs:

    1|dk|http://mirrors.dotsrc.org/gcrypt/gnupg/gnupg-2.5.17.tar.bz2
    1|gb|http://mirrorservice.org/sites/ftp.gnupg.org/gcrypt/gnupg/gnupg-2.5.17.tar.bz2
    2|dk|https://mirrors.dotsrc.org/gcrypt/gnupg/gnupg-2.5.17.tar.bz2
    2|gb|https://mirrorservice.org/sites/ftp.gnupg.org/gcrypt/gnupg/gnupg-2.5.17.tar.bz2
    3|de|https://gnupg.org/ftp/gcrypt/gnupg/gnupg-2.5.17.tar.bz2
    20||http://distcache.FreeBSD.org/ports-distfiles/gnupg-2.5.17.tar.bz2
    21|eu|http://distcache.eu.FreeBSD.org/ports-distfiles/gnupg-2.5.17.tar.bz2
    21|us|http://distcache.us-east.FreeBSD.org/ports-distfiles/gnupg-2.5.17.tar.bz2
    21|us|http://distcache.us-west.FreeBSD.org/ports-distfiles/gnupg-2.5.17.tar.bz2

Here we see two HTTP mirrors or the same priority. Then their HTTPS
alternative. Then the main project's download area. FreeBSD's distcache
as a fallback, having balancer entrypoint.

List of known mirrors:
    => http://download-mirror.savannah.gnu.org/releases/00_MIRRORS.html
    => http://mirror.planetunix.net/
    => http://mirrors.cpan.org/
    => http://www.openbsd.org/build/mirrors.dat
    => https://archlinux.org/mirrors/
    => https://cran.r-project.org/mirrors.html
    => https://crux.nu/Main/Download#Mirrors
    => https://ctan.org/mirrors/
    => https://docs.freebsd.org/en/books/handbook/mirrors/#mirrors
    => https://gcc.gnu.org/mirrors.html
    => https://gnupg.org/download/mirrors.html
    => https://launchpad.net/ubuntu/+cdmirrors
    => https://mirrors.almalinux.org/
    => https://mirrors.alpinelinux.org/
    => https://mirrors.opensuse.org/
    => https://mirrors.slackware.com/mirrorlist/
    => https://sourceforge.net/p/forge/documentation/Mirrors/
    => https://sourceware.org/mirrors.html
    => https://status.artixlinux.org/mirrors/
    => https://www.debian.org/mirror/list
    => https://www.gentoo.org/downloads/mirrors/
    => https://www.gnu.org/prep/ftp.html
    => https://www.netbsd.org/mirrors/
    => https://www.openbsd.org/ftp.html
    => https://www.openssh.org/portable.html
    => https://www.ruby-lang.org/en/downloads/mirrors/
    => https://www.x.org/wiki/Releases/Download/
    => https://xmirror.voidlinux.org/