A couple years back, we posted on TechNet Wiki an older article that was originally published on the TechNet Library by Steve Riley, when he worked at Microsoft. The article is Active Directory Replication over Firewalls. Since publishing it on TechNet Wiki, this article has been updated many times by different members of the community.
The purpose for this post is just to build awareness of it and encourage community members to continue adding their own knowledge and experiences in order to improve it. We originally published it to TechNet Wiki because there was some risk that it could be retired on the TechNet Library, and our AD doc team did not have resources to update and republish it. Rather than lose the article altogether, we copied it to the wiki in hopes the community could revise and extend it. Thanks to all the community members who have helped to achieve that.
Justin [Active Directory Documentation Team]