International science collaboration and data mobility rely heavily on network infrastructure to transfer data between sites, which involves multiple network administrative domains. Ensuring that things are operating well, on an end-to-end basis, is critical. While monitoring within a single domain is a common and accepted practice; cross-domain performance monitoring is difficult to do with traditional tools. Within the research and education networking community, the perfSONAR toolkit has been broadly deployed to monitor and ensure network performance, with over 2,000 publicly accessible test points worldwide.