Acceptable patent license for free software?

[Simon Josefsson]

Hi!  I'm writing guidelines for IETF document authors on how to achieve
free software compatible copyright and patent licenses [1].  The
copyright area is rather well understood, I think; pick any liberal
license that is compatible with all (reasonable) free software licenses
and you are done.

However, the patent license area seems less clear to me.

Does anyone have some references to any patent license that have been
deemed acceptable to some free software projects?

Going further, is there some kind of patent license that free software
would _prefer_ (as opposed to just _accept_)?

To give a concrete example of what I'm thinking of:

There is one patent license in <https://datatracker.ietf.org/ipr/942/>:

  Subject to the terms and conditions of this License, Google hereby
  grants to You a perpetual, worldwide, non-exclusive, no-charge,
  royalty-free, irrevocable (except as stated in this License) patent
  license for patents necessarily infringed by implementation (in whole
  or in part) of this specification. If You institute patent litigation
  against any entity (including a cross-claim or counterclaim in a
  lawsuit) alleging that the implementation of the specification
  constitutes direct or contributory patent infringement, then any
  patent licenses for the specification granted to You under this
  License shall terminate as of the date such litigation is filed.

This patent license is a reciprocal license, but is fairly limited in
scope.

Would that be acceptable to free software projects?

/Simon

[1] http://josefsson.org/bcp78broken/draft-josefsson-free-standards-howto.html