The core Protocol Buffers technology is provided courtesy of Google.
At the time of writing, this is released under the BSD license.
Full details can be found here:

http://code.google.com/p/protobuf/

The protobuf-net logo is Copyright 2021 onwards Marc Gravell; you must
not use this logo (or any derivative) in your own works without explicit
permission.

This .NET implementation is Copyright 2008 onwards Marc Gravell

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.