In this paper, we propose to use the open industrial-strength eXtensible Messaging and Presence Protocol (XMPP) to build a network substrate for Peer-to-Peer Grid computing called Orbweb. We describe how to employ XMPP to tackle domain-specific challenges, including high scalability, support for volatility, NAT/Firewall traversal, and protocol efficiency. Where XMPP fails to meet these requirements, we contribute pertinent extensions. In particular, we boost the scalability of XMPP by taking load of the XMPP servers through dynamically negotiated direct Peer-to-Peer communication channels between XMPP peers. We pave the way for scalable group membership management by substituting the existing XMPP Multi-User Chat protocol for one that does not suffer from limitations imposed by a everyone knows everyone visibility model and allows for selecting a membership model that matches the requirements of a given application. As efficient multicasting is an essential prerequisite for many distributed algorithms and the centralized XMPP multicast is of limited scalability, we adapt the well-known Bimodal Multicast protocol to work in a highly volatile Peer-to-Peer Grid computing environment. Finally, we show how to improve the protocol efficiency of XMPP by leveraging a standardized binary encoding of the XML Information Set for XMPP packet transmission. To substantiate the applicability of our approach and the effectiveness of our extensions, we describe how some important higher-level services used in Peer-to-Peer Grid Computing can be implemented on top of Orbweb and provide a detailed experimental analysis.