An alpha version of an open source implementation of the ARC protocol has been released. Congratulations to all involved in bringing this project to an initial release.
ARC allows forwarders and mailing lists to convey message authentication results that would otherwise be lost, which can present difficulties to some DMARC adopters. OpenARC v0.1.0 can be found at https://github.com/trusteddomainproject/OpenARC/releases.
An overview of how the ARC protocol works, and in what situations it would be deployed, is available in the presentation ARC Protocol Overview from October 2016. While there have been some changes to the protocol since then, the concepts and overall functions remain the same.
The site http://arc-spec.org is dedicated to development of the ARC protocol, and may have additional resources for those interested.