User Domains gathers and combines resources from multiple sources to create a per-user geographically distributed heterogeneous virtualization platform where user-provided virtual machines can be executed in user mode. To address the challenge of building a global computing infrastructure it is necessary to provide a communication overlay able to deal with the existence of computing facilities located behind NAT devices. In this work we present SPM, a simple peer-to-peer messaging system based on ICE and XMPP technologies, to efficiently interconnect remote user domains. Given that User Domains' performance is highly dependent on the network characteristics and there is little work regarding NAT traversal performance, we evaluated SPM performance in multiple scenarios of interest. Results show that the use of SPM doesn't inflict a large performance penalty confirming the expectable usability both at high bandwidth and at home computing facilities.